Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

Free electron mobility (μ free) in 4H-SiC(0001) MOSFETs with gate oxides annealed in NO or POCl3 was calculated in a wide range of effective normal field (E eff) from 0.02 to 2 MV cm−1, taking account of scattering by fixed charges and trapped electrons. The present calculation indicates that the Hall mobility in the high-E eff region experimentally obtained for NO-annealed MOSFETs (14 cm2 V−1 s−1 at 1.1 MV cm−1) is much lower than that for POCl3-annealed MOSFETs (41 cm2 V−1 s−1) due to severe Coulomb scattering by electrons trapped at a very high density of interface states.

DOI： 10.35848/1882-0786/ad63ef

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

For rooting the development of GaN-based optoelectronic devices, understanding the roles of midgap recombination centers (MGRCs), namely, nonradiative recombination centers and deep-state radiative recombination centers, on the carrier recombination dynamics is an essential task. By using the combination of time-resolved photoluminescence and positron annihilation spectroscopy (PAS) measurements, the origins of major MGRCs in the state-of-the-art GaN epilayers, bulk crystals, and Mg-implanted layers were identified, and their concentrations were quantified for deriving the capture coefficients of minority carriers. In this article, potential standardization of the room-temperature photoluminescence lifetime for the near-band-edge emission ( τ PL RT ) as the concentration of major MGRCs well below the detection limit of PAS is proposed. For n-GaN substrates and epilayers grown from the vapor phase, τ PL RT was limited by the concentration of carbon on N sites or divacancies comprising a Ga vacancy (VGa) and a N vacancy (VN), [VGaVN], when carbon concentration was higher or lower, respectively, than approximately 1016 cm−3. Here, carbon and VGaVN act as major deep-state radiative and nonradiative recombination centers, respectively, while major MGRCs in bulk GaN crystals were identified as VGa(VN)3 vacancy clusters in Na-flux GaN and VGa or VGaVN buried by a hydrogen and/or VGa decorated with oxygen on N sites, VGa(ON)3-4, in ammonothermal GaN. The values of τ PL RT in n-GaN samples are compared with those of p-GaN, in which τ PL RT was limited by the concentration of VGa(VN)2 in Mg-doped epilayers and by the concentrations of VGaVN and (VGaVN)3 in Mg-implanted GaN right after the implantation and after appropriate activation annealing, respectively.

DOI： 10.1063/5.0201931

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE Transactions on Electron Devices  

Nearly ideal vertical AlxGa-{{1}-{x}}text{N} ({0.7} leq {x} < {1.0} ) p-n diodes are fabricated on an aluminum nitride (AlN) substrate. Distributed polarization doping (DPD) was employed for both p-type and n-type layers of the p-n junction, instead of conventional impurity doping, to overcome the major bottleneck of AlN-based material: the control of conductivity. Capacitance-voltage measurements revealed that the net charge concentration agreed well with the DPD charge concentration expected from the device layer structure. The fabricated devices exhibited a low turn-on voltage of 6.5 V, a low differential specific on-resistance of 3 text{M}Omega cm2, electroluminescence (maximum at 5.1 eV), and an ideality factor of 2 for a wide range of temperatures (room temperature - 573 K). Moreover, the breakdown electric field was 7.3 MV cm-1, which was almost twice as high as the reported critical electric field of GaN at the same doping concentration. These results clearly demonstrate the usefulness of DPD in the fabrication of high-performance AlN-based power devices.

DOI： 10.1109/TED.2024.3367314

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE Transactions on Electron Devices  

This article reports a comparative study on the effect of dopants in semi-insulating GaN substrates grown by hydride vapor phase epitaxy (HVPE) on the electrical characteristics of AlGaN/GaN high-electron-mobility transistors (HEMTs). GaN-HEMTs were fabricated on GaN substrates doped with iron (Fe), carbon (C), or manganese (Mn), and their electrical characteristics from 300 to 600 K were compared. Similar on-state characteristics were observed at room temperature, regardless of the substrate dopant. However, in the off-state breakdown characteristics, the devices on the C- and Mn-doped substrates showed slightly lower gate leakage currents than those on the Fe-doped substrate. Pulsed {I} -{V} characteristics showed that all devices had a small current collapse of approximately 10% or less, although the devices on the C- and Mn-doped substrates had slightly larger current collapse than that on the Fe-doped substrate. Furthermore, the temperature dependence of the off-state breakdown characteristics showed that the devices on the C- and Mn-doped substrates had a breakdown voltage ({V}-{text {BD}} ) that exceeded 100 V, even at 600 K, while that for the device on the Fe-doped substrate was less than 20 V. Arrhenius plots of leakage current suggested that for the devices on the Fe-doped substrate, current conduction through the substrate occurred above 350 K, which resulted in a low {V}-{text {BD}}. The devices on the C- and Mn-doped substrates had low leakage through the substrate due to the high resistivity of the substrate. These results indicate the feasibility of C- or Mn-doped GaN as HEMT substrates for use in high-voltage and high-temperature applications.

DOI： 10.1109/TED.2024.3375837

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE Transactions on Electron Devices  

AlGaN/GaN gated-anode diodes (GADs) have been proposed as microwave rectifying devices for microwave wireless power transmission (WPT) systems. However, achieving the desired turn-on voltage (Von) has posed a critical challenge due to the requirement for precise etching control. This article introduces a novel GAD structure aimed at enhancing productivity. In this new configuration, a GaN etch endpoint detection layer (GaN marker layer) is incorporated into the AlGaN layer, positioned approximately 4.5 nm away from the channel interface. During gate recess etching, the residual thickness of AlGaN is monitored using an optical interferometer. Etching is halted successfully upon detection of the GaN marker layer, signaled by an abrupt change in interference. Fabricated GADs consistently achieved the target Von of +0.20 V, exhibiting improved uniformity compared to prior GADs lacking a GaN marker layer. These devices typically demonstrated a maximum forward current of 0.69 A/mm and a reverse breakdown voltage exceeding 100 V. Additionally, a bridge-type rectifier circuit employing four GADs was simulated using a SPICE model constructed from measured pulse I-V characteristics and S-parameters. A conversion efficiency of 84% was predicted with an input power of 25.6 W (8 W/mm) at 5.8 GHz.

DOI： 10.1109/TED.2024.3381570

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

We developed a low-temperature ohmic contact formation process for N-polar GaN surfaces. Specific-contact-resistances of 9.4 × 10−5 and 2.0 × 10−5Ω cm2 were obtained using Ti/Al metal stacks on heavily-germanium-doped GaN films, which were deposited at 500 °C and 600 °C using a radical-assisted reactive sputtering method, respectively. The electrode sintering temperature was as low as 475 °C. Carrier concentrations for the 500 °C and 600 °C samples were 2.6 × 1020 and 1.8 × 1020cm−3, respectively. These results suggest that this method is highly effective in reducing the contact resistance of GaN devices with low thermal budgets.

DOI： 10.35848/1882-0786/ad2783

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

Threshold voltage instability (shift) due to positive bias stress in GaN planar-gate MOSFETs was investigated. Gate dielectric (SiO2) was formed by remote-plasma-assisted CVD on homoepitaxial Mg-doped p-type GaN layers with Si-implanted n-type source and drain regions. The threshold voltage shift of 5.8 V was observed after a stress voltage of 30 V for a sample without post-deposition annealing (PDA). The threshold voltage shift was significantly reduced to 1.4 V for a sample with PDA (800 °C for 30 min). Stress time dependences up to 6000 s were measured, revealing that the main origin of the threshold voltage shift is electron trapping into near interface traps (NITs). These results suggest that PDA is effective for the reduction of the NITs.

DOI： 10.35848/1347-4065/ad0c43

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

To address the issue of the high cost of GaN substrates, a recycling process for GaN substrates using a laser slicing technique was investigated. The channel properties of lateral MOSFETs and the reverse characteristics of vertical PN diodes, which represent the main components of vertical power devices, exhibited no degradation either before and after laser slicing or due to the overall GaN substrate recycling process. This result indicates that the proposed recycling process is an effective method for reducing the cost of GaN substrates and has the potential to encourage the popularization of GaN vertical power devices.

DOI： 10.35848/1882-0786/ad269d

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

In this study, a novel post-deposition annealing (PDA) technique employing ultra-high pressure was demonstrated for the first time. A 40 nm thick AlSiO gate insulator was deposited using atomic layer deposition (ALD) on n-type gallium nitride (GaN) epitaxial layers grown on free-standing GaN substrates. These PDA techniques were performed at 600 °C in a nitrogen ambient under 400 MPa, with normal pressure conditions used as the references. The annealing duration varied within the range of 10, 30, 60, and 120 min. For normal pressure annealing, the flat-band voltage of capacitance-voltage curves exhibited a shift towards the positive bias direction as the annealing time increased. Conversely, for the 400 MPa annealing, the flat-band voltage approached the ideal curve as the annealing time extended. For 400 MPa and 120 min, low interface state density of ∼5 × 1011 cm−2 eV−1 or less at E c −0.20 eV was obtained. These results suggest that post-deposition annealing under ultra-high pressure could be a viable method for improving the interfacial characteristics of AlSiO/GaN.

DOI： 10.35848/1882-0786/ad0ba5

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Letters  

Impact of carbon impurities on the electrical properties of lightly doped n-type GaN [electron concentration ∼( 1 - 2 ) × 10 15 cm − 3 at room temperature] was investigated using temperature-dependent Hall effect measurements. GaN crystals with a threading dislocation density of ( 1 - 3 ) × 10 6 c m − 2 were grown by our originally developed quartz-free hydride vapor phase epitaxy method, which enabled the background Si, O, and C concentrations to be suppressed to below the mid-1014 cm−3 range. We prepared three samples with different C concentrations ([C]) by intentional C doping. The C incorporation induced severe mobility collapse at temperatures greater than 60 K, where the measured mobility decreased and deviated from the theoretical value as [C] increased. The mobility collapse was eliminated for the purest GaN crystal with [C] ∼ 1.4 × 10 14 cm − 3 , exhibiting a record high room-temperature mobility of 1480 cm2/(V·s), as well as a record high maximum mobility of 14 300 cm2/(V·s) at 62 K. The latter was almost double the previous record. We found that the overall mobility behavior can be well reproduced by adding an empirical [C]-dependent mobility component expressed as μ UNK = K / T n UNK with 1 ≤ n UNK ≤ 2 and K ∝ C − 1 to the conventional mobility theory (phonon and impurity scattering). Although the mechanism of the component remains uncertain, our findings provide insight into the unsolved issue of mobility collapse.

DOI： 10.1063/5.0178086

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

Recombination centers originating from point defects generated by the displacement of N atoms in n-type and p-type GaN were investigated by analyzing Shockley-Read-Hall (SRH) recombination currents in homoepitaxial GaN p-n junctions. These defects were intentionally generated by electron beam (EB) irradiation at 137 keV. The net doping concentrations in p+-n junction diodes were not changed following irradiation although the levels in p-n+ junction diodes decreased as the EB fluence was increased. The SRH recombination current also increased with increases in the fluence. This work additionally evaluated the relationship between recombination lifetimes and trap concentrations obtained by deep level transient spectroscopy.

DOI： 10.35848/1882-0786/ad16ad

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Technical Digest - International Electron Devices Meeting, IEDM  

The polarization in a III-nitride system enhances conductivity in a MOS channel, while reduces the threshold voltage (Vth) and causes its temperature dependence. To simultaneously obtain a high channel mobility and a stable Emode operation, the gate insulator composed of an amorphous AlSiO and a crystalline AlN are applied to the mplane GaN channel using a trench gate structure. By inserting a 1.1 nm thick AlN, interface traps on a m-plane MOS channel are minimized, resulting in the effective mobility over 180 cm2 V–1s–1. Different from a MOSFET on a c-plane, the Vth value does not depend on the AlN thickness and is approximately 1 V for the acceptor doping at 1017 cm–3. Furthermore, the Vth on the m-plane GaN channel exhibits little temperature dependence in the range of 223 to 473 K because of no pyroelectric effect.

DOI： 10.1109/IEDM50854.2024.10873525

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

In this review, we briefly summarize the major challenges and our recent progress in the development of GaN Junction Barrier Schottky (JBS) diodes using selective-area p-type doping with ion implantation and ultra high-pressure annealing (UHPA) process. As a starting point, we discuss the properties of Schottky contacts in the context of UHPA and provide design principle for a high performance JBS diode. Next, we propose a JBS diode having p-type regions formed by channeled ion implantation. This kind of device can provide ultra-low leakage currents and a much better trade-off between on-resistance (R ON) and breakdown voltage (BV). Finally, we demonstrate our high-performance JBS diodes which exhibited the superior electrical characteristics (record low R ON from 0.57 to 0.67 mΩ cm2 and high BV from 660 to 675 V) and nondestructive breakdown.

DOI： 10.35848/1347-4065/acec69

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

Mist CVD was applied to grow the β-Ga2O3 channel layer of a MESFET on a semi-insulating β-Ga2O3 (010) substrate. The mobility and carrier concentration of the channel layer were 80 cm2 V-1 s-1 and 6.2 × 1017 cm−3, respectively. The device exhibited a pinch-off characteristic with a threshold gate voltage of −9 V, and the maximum drain current was 240 mA mm−1. The maximum transconductance was 46 mS mm−1 and the on-resistance was 30 Ω mm. This device performance suggests that mist CVD is a potential growth technology capable of providing low-cost devices in the future.

DOI： 10.35848/1882-0786/acefa5

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physica Status Solidi (A) Applications and Materials Science  

AlGaN/GaN high-electron-mobility transistor (HEMT)-based gated-anode diodes (GADs) for a 5.8 GHz rectenna application are proposed. An anode of the GAD is formed by connecting a gate and a drain of a normally off GaN HEMT. Herein, a wide recessed HEMT structure reported in the previous article is modified to a buried-type recessed gate HEMT structure. The gate-to-cathode distance is optimized to maximize device performance. Typical direct current (DC) characteristics of the HEMTs are a threshold voltage (Vth) of +0.4 V and a maximum drain current (Imax) of 450 mA mm−1. GADs using the HEMTs show the characteristics of maximum forward current (If) of 600 mA mm−1 and reverse breakdown voltage (BVr) of more than 100 V. The GAD using the recessed gate structure HEMT is shown to significantly improve If and BVr simultaneously compared to the previous work using the wide recessed gate structure. The rectifying performance of a bridge-type rectifier simulated with SPICE model of the GAD shows radio frequency (RF)–DC conversion efficiency of 84% and RF input power of 13 W at 5.8 GHz using four GADs with each gate width (Wg) of 400 μm.

DOI： 10.1002/pssa.202200837

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

Hall effect measurements were conducted for MOSFETs with and without post-oxidation-annealing (POA) fabricated on the p-body doping in a wide doping range to vary the effective normal field (E eff). The Hall mobility (μ Hall) in the high-E eff region of the MOSFETs annealed in phosphoryl chloride (μ Hall = 41 cm2 V−1 s−1 at E eff = 1.1 MV cm−1) is much higher than that of MOSFETs annealed in nitric oxide (NO) (μ Hall = 14 cm2 V−1 s−1 at E eff = 1.1 MV cm−1), suggesting that the trapped electrons act as strong Coulomb scattering centers for the MOSFETs annealed in NO and without POA.

DOI： 10.35848/1882-0786/ace150

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Electronics Letters  

This article reports radio frequency characteristics of 150-nm gate aluminum gallium nitride (AlGaN)/gallium nitride (GaN) high electron mobility transistors (HEMTs) fabricated using i-line stepper lithography and a thermal reflow technique. The authors have developed two different gate structures that were a field-plated gate using the lift-off process and a Y-shaped gate using the ion-milling process. Fabricated HEMTs using these different gate structures exhibited nearly equivalent DC characteristics. The field-plated gate device showed a unity current gain cutoff frequency (fT) of 35 GHz and a maximum oscillation frequency (fmax) of 106 GHz, while the Y-shaped gate device showed fT of 36 GHz and fmax of 115 GHz. The equivalent circuit analysis indicated a decrease in the gate-drain capacitance and the drain conductance is responsible for the improved fmax in the Y-shaped gate device.

DOI： 10.1049/ell2.12798

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Letters  

This work examined the intentional generation of recombination centers in GaN p-n junctions on freestanding GaN substrates. Irradiation with a 4.2 MeV proton beam was used to create a uniform distribution of vacancies and interstitials across GaN p+/n- and p-/n+ junctions through anode electrodes. With increasing proton dose, the effective doping concentrations were found to be reduced. Because the reduction in the doping concentration was much higher than the hydrogen atom concentration, this decrease could not be attributed solely to carrier compensation resulting from interstitial hydrogen atoms. In fact, more than half of the electron and hole compensation was caused by the presence of point defects. These defects evidently served as Shockley-Read-Hall (SRH) recombination centers such that the SRH lifetimes were reduced to several picoseconds from several hundred picoseconds prior to irradiation. The compensation for holes in the p-/n+ junctions was almost double that for electrons in the p+/n- junctions. Furthermore, the SRH lifetimes associated with p-/n+ junctions were shorter than those for p+/n- junctions for a given proton dose. These differences can be explained by variations in the charge state and/or the formation energy of intrinsic point defects in the p-type and n-type GaN layers. The results of the present work indicate the asymmetry of defect formation in GaN based on the fact that intrinsic point defects in p-type GaN readily compensate for holes.

DOI： 10.1063/5.0141781

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

The electrical properties of semi-insulating GaN substrates doped with iron (Fe), carbon (C), or manganese (Mn) grown by hydride vapor phase epitaxy are presented. Hall effect measurements were performed at temperatures ranging from 300 to 800 K. At all of the investigated temperatures, the Mn-doped samples exhibited the highest resistivity. The Fe-doped samples showed n-type conduction, whereas the C-doped samples and the Mn-doped sample with a Mn concentration of 1 × 1019 cm-3 showed p-type conduction. A detailed analysis of the temperature dependence of the carrier concentration showed that all of the impurities formed acceptor levels at EC -(0.59-0.61) eV for Fe, at EV +(0.90-1.07) eV for C, and at EV +1.55 eV for Mn. The Mn-doped sample with a Mn concentration of 8 × 1017 cm-3 showed a negative Hall coefficient (suggesting n-type conduction) at high temperatures, contradicting the formation of acceptor levels by Mn. We successfully explained the negative value by considering the conduction of both holes and electrons with different mobilities. On the basis of the results, we calculated the relationship between the resistivity and doping concentration for each dopant. The calculations indicated that the highest resistivity can be realized in Mn-doped GaN with an optimized doping concentration (depending on the residual donor concentration). All of the dopants can effectively realize high resistivity at room temperature. Mn is an effective dopant for attaining high resistivity, especially at high temperatures (e.g., 800 K).

DOI： 10.1063/5.0131470

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

We investigated the effect of H2 addition in halide-vapor-phase epitaxy of GaN on Ga-polar GaN(0001) using an external GaCl3 supply method. To overcome the problem of the very low growth rate on GaN(0001) using GaCl3, we intentionally added H2 to convert GaCl3 to GaCl in the reactor. Using this approach, we successfully increased the growth rate, and also improved the surface morphology of the grown layer.

DOI： 10.35848/1347-4065/acb97b

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Letters  

Production rate (PR = trap concentration/incident fluence) of traps formed by energetic particles is important for predicting device degradation caused by radiation when developing radiation-resistant devices. We demonstrate a clear correlation between non-ionizing energy loss (NIEL) and PR of an electron trap at about 0.12-0.20 eV below the conduction band edge [EC - (0.12-0.20) eV] for various types of energetic particles in gallium nitride (GaN). NIEL values in GaN for electrons, protons, and α-rays were calculated using a screened-relativistic treatment, and NIEL values for gamma-rays were calculated by simulating slowed-down spectra due to shielding material. To obtain the PRs of the electron trap, 60Co gamma-rays with an average photon energy of 1.25 MeV and electron beams with energies from 137 keV to 2 MeV were irradiated onto n-type GaN Schottky barrier diodes. We measured the concentration of an electron trap at EC - (0.13-0.14) eV using deep-level transient spectroscopy. We also used the PRs of electron traps with similar energy levels of EC - (0.12-0.20) eV from previous studies on electrons, protons, and α-rays irradiated on GaN. All the trap PRs were proportional to the NIEL in a range of eight orders of magnitude, which confirms that the energy levels formed by various energetic particles have the same origin of being generated by atomic displacements. The obtained relationship coefficient between the NIEL and PRs of the trap is useful for predicting the degradation of GaN-based devices due to traps formed by various kinds of radiation.

DOI： 10.1063/5.0128709

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

To evaluate the impact of gate structures on the switching performance (R on Q g) and cost (required chip size, proportional to R on A) of GaN vertical MOSFETs, we calculated the R on AR on Q g of trench-gate structures with and without a countermeasure to reduce the electric field applied to the gate insulator, as well as a planar structure with various cell pitches, channel mobilities, and blocking voltages. When the blocking voltage was 600 V, the planar-gate structure achieved the lowest R on AR on Q g owing to its low Q g/A, despite the high R on A. However, when the blocking voltage was 1800 V, a trench-gate structure without the countermeasure achieved the lowest R on AR on Q g owing to its low R on A and optimal cell pitch. The R on AR on Q g of a trench-gate structure with a countermeasure and planar-gate structure became close with increasing channel mobility. This indicates that high channel mobility is the most important factor, rather than the selection of the device structure.

DOI： 10.35848/1347-4065/aca266

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1117/12.2646233

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Technical Digest - International Electron Devices Meeting, IEDM  

Nearly ideal AlN-based vertical p-n diodes are demonstrated on an AIN substrate utilizing dopant-free distributed-polarization doping (DPD). Capacitance-voltage measurements revealed that the effective doping concentration agreed well with the designed DPD charge concentration. The fabricated devices exhibited a low tum-on voltage of 6.5 V, a low differential specific ON-resistance of 3 mO cm2, and an ideality factor of 2 for a wide range of temperatures (room temperature-573 K). Moreover, the breakdown electric field was 7.3 MV/cm, which was almost twice as high as the reported critical electric field of 4H-SÌC and GaN. These results clearly demonstrate the usefulness of DPD in the fabrication of high-performance AlN-based power devices.

DOI： 10.1109/IEDM45741.2023.10413866

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Technical Digest - International Electron Devices Meeting, IEDM  

Polarization engineering by AIN interlayers (AlN-ILs) deposited via plasma-enhanced atomic layer deposition was demonstrated in AlSiO/p-type GaN MOSFETs. Transmission electron microscopy observations revealed that the AlN-ILs were grown on GaN epitaxially and therefore could induce polarization charges, similar to AlGaN/GaN. The decrease in the threshold voltage (Vth) with increasing AIN-IL thickness corresponded to the polarization charge density. In addition, insertion of the AIN-IL suppressed the positive bias instability by less than 0.05 V. By controlling the AIN-IL thickness and the channel p-type doping, we controlled the Vth the range from -3 to 5 V and achieved enhanced channel mobility compared with that for the corresponding MOSFET without an AIN-IL.

DOI： 10.1109/IEDM45741.2023.10413714

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE International Reliability Physics Symposium Proceedings  

We focus on reliability issues of gate oxides and p-n junctions to realize vertical GaN metal-oxide-semiconductor field-effect transistors (MOSFETs). An annealed AlSiO gate oxide on GaN displayed a lifetime of over 20 years at 150 °C and suppressed positive bias instability in MOSFETs. The key to high channel mobility and stability under positive gate bias is the interface structure designed to minimize oxide border traps. We also evaluated the reliability of GaN p-n diodes (PNDs) on freestanding GaN substrates with different threading dislocation densities. The reverse leakage for PNDs involving threading dislocations was explained by variable-range hopping, while the reverse leakage for dislocation-free PNDs was dominated by band-to-band tunneling. The fabricated PNDs demonstrated excellent robustness under high-temperature reverse bias. However, after continuous forward current stress, reverse leakage pathways were formed at threading screw dislocations, which should be minimized in future GaN substrates.

DOI： 10.1109/IRPS48203.2023.10118047

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

Si ions were implanted into homoepitaxial n-type GaN at a peak concentration of 3 × 1014 cm−3 with subsequent annealing, and the associated formation of electron traps was investigated in detail using deep-level transient spectroscopy. A major electron trap was identified as 0.26 eV below the conduction band minimum and this trap concentration increased with increasing post-implantation annealing temperature, to a value of 6-8 × 1015 cm−3. Significant increases in the net donor concentration (N D) were also observed within the implanted region. The profile of the electron trap concentration was correlated with these increases in N D, suggesting that these traps acted as donors.

DOI： 10.35848/1882-0786/aca45d

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Letters  

Vertical GaN junction barrier Schottky (JBS) diodes with superior electrical characteristics and nondestructive breakdown were realized using selective-Area p-Type doping via Mg ion implantation and subsequent ultra-high-pressure annealing. Mg-ion implantation was performed into a 10 μm thick Si-doped GaN drift layer grown on a free-standing n-Type GaN substrate. We fabricated the JBS diodes with different n-Type GaN channel widths Ln = 1 and 1.5 μm. The JBS diodes, depending on Ln, exhibited on-resistance (RON) between 0.57 and 0.67 mω cm2, which is a record low value for vertical GaN Schottky barrier diodes (SBDs) and high breakdown (BV) between 660 and 675 V (84.4% of the ideal parallel plane BV). The obtained low RON of JBS diodes can be well explained in terms of the RON model, which includes n-Type GaN channel resistance, spreading current effect, and substrate resistance. The reverse leakage current in JBS diodes was relatively low 103-104 times lower than in GaN SBDs. In addition, the JBS diode with lower Ln exhibited the leakage current significantly smaller (up to reverse bias 300 V) than in the JBS diode with large Ln, which was explained in terms of the reduced electric field near the Schottky interface. Furthermore, the JBS diodes showed a very high current density of 5.5 kA/cm2, a low turn-on voltage of 0.74 V, and no destruction against the rapid increase in the reverse current approximately by two orders of magnitude. This work demonstrated that GaN JBS diodes can be strong candidates for low loss power switching applications.

DOI： 10.1063/5.0106321

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

P-type doping in selected areas of gallium nitride (GaN) using magnesium (Mg)-ion implantation and subsequent ultra-high-pressure annealing (UHPA) are investigated to improve the performance of vertical GaN power devices. UHPA allows a high-temperature process without decomposition of the GaN surface and virtually complete activation of the implanted Mg ions in GaN. In the present paper, we provide an overview of recent challenges in making UHPA more realistic as an industrial process. Instead of UHPA at more than 1400 °C for a short duration, prolonged UHPA at 1300 °C demonstrates a comparable acceptor activation of Mg-ion-implanted GaN. This can reduce the annealing pressure to approximately 300 MPa and enlarge the processable wafer diameter. The second challenge is controlling the doping profiles in the lateral and vertical directions. We demonstrate fine patterning of the p-type regions, which indicates the limited lateral diffusion of Mg through UHPA. However, controlling the vertical doping profile is challenging. The nitrogen vacancies formed by ion implantation reduce the effective acceptor concentration near the surface, which can be compensated for by sequential nitrogen ion implantation. Defect-assisted Mg diffusion to the deeper region causes a redistribution of the Mg atoms and should be considered in the design of a device. Such anisotropic diffusion of Mg to the c-axis has potential applications in the fabrication of unique vertical device structures such as super junctions.

DOI： 10.1063/5.0107921

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physica Status Solidi (B) Basic Research  

Herein, the annealing behaviors of defects in ion-implanted GaN are studied by positron annihilation, cathodoluminescence, scanning transmission electron microscopy, and atom probe tomography. Si or Mg ions are implanted into GaN to obtain 300 nm deep box profiles of the impurities. The samples are annealed up to 1480 °C under a N2 pressure of 1 GPa. For as-implanted GaN, the major defect species is identified as Ga-vacancy-type defects. After annealing above 1000 °C, vacancy clusters are introduced, and they remain even after 1480 °C annealing. For Mg-implanted GaN with the Mg concentration ([Mg]) ≤ 1018 cm−3, no large change in the depth distribution of Mg is observed before and after annealing at 1400 °C. For the sample with [Mg] = 1019 cm−3, however, Mg diffuses into the bulk, which is attributed to the over-doping of Mg and their vacancy-assisted diffusion. The Mg diffusion is suppressed, and the donor–acceptor pair emission is enhanced by sequential N-implantation, which is attributed to the reaction between Mg and vacancies under a N-rich condition. For the samples annealed at 1480 °C, an accumulation of Mg around dislocation loops and Mg clustering are enhanced by the N-implantation.

DOI： 10.1002/pssb.202200183

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

Phosphorus treatment, which can substantially reduce the interface state density (D it), was used to investigate the impact of D it on effective channel mobility (μ eff) of 4H-SiC (0001) metal-oxide-semiconductor field-effect transistors (MOSFETs). A high μ eff of 126 cm2 V-1 s-1, which exceeds the reported phonon-limited mobility of 83 cm2 V-1 s-1 determined from Hall mobility of nitridation-treated MOSFETs, at a high effective normal field of 0.57 MV cm-1 was obtained in MOSFETs fabricated on a high-purity semi-insulating 4H-SiC substrate at room temperature. This high mobility may be caused by the difference of the density of electrons trapped at the interface states.

DOI： 10.35848/1347-4065/ac87e4

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

We carried out atomic-scale observations of Mg-ion-implanted GaN by transmission electron microscopy (TEM) and atom probe tomography (APT) to clarify the crystallographic structures of extended defects and Mg agglomerations that form during post-implantation annealing. The complementary TEM and APT analyses have shown that Mg atoms agglomerate at dislocations that bound extended defects. The concentration of Mg is higher at the dislocations with a larger Burgers vector. This indicates that Mg agglomeration is caused by the pressure at the dislocations. Mg concentration in highly Mg-rich regions is 1 at. %, which exceeds the solubility limit of Mg in GaN. We investigated isothermal and isochronal evolution of the defects by TEM, cathodoluminescence analysis, and positron annihilation spectroscopy. The results indicated that the intensity of donor-acceptor pair emission increases with the annealing temperature and duration and reaches a maximum after elimination of the extended defects with highly Mg-rich regions. These results strongly suggest that such extended defects reduce the acceptor formation and that they as well as the previously reported compensating centers, such as N-related vacancies, can inhibit the formation of p-type GaN. The mechanism by which the extended defects reduce acceptor formation is discussed.

DOI： 10.1063/5.0097866

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

One critical issue hindering high-quality, high-speed growth of GaN is cluster formation in the gas phase. We investigated cluster formation in tri-halide vapor phase epitaxial growth of GaN. The growth system is equipped with an external GaCl3 gas supply system. We observed cluster formation under certain growth conditions experimentally. A simulation was also carried out to reveal the critical conditions for cluster formation. We propose that increasing the gas temperature is an effective way to suppress cluster formation, and thus achieve a higher growth rate with a flat surface morphology.

DOI： 10.35848/1347-4065/ac7a7a

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

Si ions were implanted at a dose of 1 × 1010 cm-2 into a homoepitaxial n-type GaN layer with a net donor concentration (N D) of 3-8 × 1015 cm-3. The N D in the implanted region increased by 1-3 × 1015 cm-3 after annealing at a temperature greater than 900 °C compared with that for the as-grown homoepitaxial layer. The increase in N D was considerably larger than the peak concentration of implanted Si ions (3 × 1014 cm-3). No increase in N D was observed for an as-grown sample after annealing. These results clearly suggest that donor-like defects were introduced by implantation of Si ions and a subsequent annealing process.

DOI： 10.35848/1882-0786/ac7433

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

We present SiO2/GaN interfaces with a low interface state density and a high breakdown electric field. The SiO2 films were deposited by plasma-enhanced atomic layer deposition (ALD) using bis(diethylamino)silane and O2 plasma at 300 °C on n-type GaN (0001) homoepitaxial layers. An interface state density of less than 1011 cm-2 eV-1 at 0.3 eV below the conduction band edge was confirmed by the conductance method. The value is much lower than those of previously reported ALD-SiO2/GaN interfaces (1012-1013 cm-2 eV-1). A low fixed charge density at the SiO2/GaN interface of 3.7 × 1011 cm-2 and a high dielectric breakdown field of ∼10 MV cm-1 were obtained. Moreover, the interface state density and current-voltage characteristics were further improved by post-deposition annealing at 400 °C in N2 ambient. Scanning transmission electron microscopy with energy-dispersive X-ray analysis revealed the existence of a GaO x interlayer between SiO2 and GaN. The unintentionally formed interlayer could be one of the reasons for the improvement of interface properties at ALD-SiO2/GaN.

DOI： 10.35848/1347-4065/ac4f79

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Letters  

This work investigated deep levels in p-type GaN originating from intrinsic point defects, using deep level transient spectroscopy (DLTS) to examine homoepitaxial GaN p+-p-n+ junction diodes grown via metalorganic vapor-phase epitaxy. Following exposure to an electron beam with an energy of 137 keV that generated nitrogen vacancies (VN) and nitrogen interstitials (NI), a peak due to EHa hole traps (at 0.52 eV) was observed in DLTS spectra. The injection of minority carriers resulting from applying a forward bias generated signals for EHb (0.5 eV) and EHc (0.8 eV) hole traps while decreasing the EHa signal and increasing the net accepter concentration. The generation of EHa traps can likely be attributed to VN (3+/+) or NI (2+/+) defects based on the results of first-principles calculations. The EHb and EHc hole traps may have been associated with complex defects, including those that generated EHa traps, because these two traps appeared as the concentration of EHa traps decreased.

DOI： 10.1063/5.0086535

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Scientific Reports  

Power devices are operated under harsh conditions, such as high currents and voltages, and so degradation of these devices is an important issue. Our group previously found significant increases in reverse leakage current after applying continuous forward current stress to GaN p–n junctions. In the present study, we identified the type of threading dislocations that provide pathways for this reverse leakage current. GaN p–n diodes were grown by metalorganic vapor phase epitaxy on freestanding GaN(0001) substrates with threading dislocation densities of approximately 3 × 105 cm−2. These diodes exhibited a breakdown voltage on the order of 200 V and avalanche capability. The leakage current in some diodes in response to a reverse bias was found to rapidly increase with continuous forward current injection, and leakage sites were identified by optical emission microscopy. Closed-core threading screw dislocations (TSDs) were found at five emission spots based on cross-sectional transmission electron microscopy analyses using two-beam diffraction conditions. The Burgers vectors of these dislocations were identified as [0001] using large-angle convergent-beam electron diffraction. Thus, TSDs for which b = 1c are believed to provide current leakage paths in response to forward current stress.

DOI： 10.1038/s41598-022-05416-3

Open Access

Web of Science

Scopus

PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE Electron Device Letters  

Breakdown characteristics in homoepitaxial GaN p-n junction diodes with p+-n and p-n+ junctions with relatively heavy doping concentrations are systematically investigated. The devices have vertical deep mesa etch termination, which enables uniform (nearly ideal) avalanche breakdown without electric field (E-field) crowding at the device edge. For p+-n junction, breakdown E-field of 3.0, 3.3 and 3.8 MV/cm and breakdown voltage (BV) of 340, 207 and 128 V were achieved at the donor concentrations of 7.5 × 1016, 1.5 × 1017, 3.1× 1017 cm-3, respectively. For p-n+ junction, breakdown E-field of 3.2, 3.3 and 4.0 MV/cm and BV of 235, 180 and 110 V were achieved at the acceptor concentrations of 1.3 × 1017, 1.8 × 1017, 4.1 × 1017 cm-3, respectively. No significant difference of the breakdown characteristics between n-type and p-type voltage-blocking layers was observed. These results are consistent with numerical simulations using impact ionization coefficients (IICs) in GaN reported in our previous studies.

DOI： 10.1109/LED.2021.3125328

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE Transactions on Electron Devices  

This article reports a systematic study on the effects of the epitaxial layer structure on the electrical characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) fabricated on freestanding GaN substrates. First, GaN-on-GaN HEMTs were fabricated comprising channel and buffer layers unintentionally doped with Fe atoms those were diffused from the substrate. Their electrical characteristics were compared with GaN-on-SiC HEMTs. The tradeoff relation between maximum drain current and the breakdown characteristics was improved in GaN-on-GaN devices than in GaN-on-SiC devices. A small current collapse was observed in GaN-on-GaN devices despite a high Fe density in the channel. This suggested an influence of Fe diffusion on the frequency dispersion to be comparatively limited. Second, GaN-on-GaN HEMTs were fabricated using a thin undoped channel layer grown on Fe-doped GaN substrate directly or through an Fe diffusion stopper layer. Without the stopper layer, the buffer leakage was significantly increased by reducing the channel thickness, and a 100-nm-thick channel device showed nonpinch-off characteristics. On the other hand, samples with the stopper layer exhibited no buffer leakage and a complete pinchoff up to 200 V. In addition, a sample with an intentional C doping to the buffer layer showed a relatively large current collapse, suggesting a major contribution of C-doping to the frequency dispersion of GaN-on-GaN HEMTs.

DOI： 10.1109/TED.2021.3126270

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Technical Digest - International Electron Devices Meeting, IEDM  

Current status and future prospects of GaN vertical power devices fabricated on GaN substrates are presented. Fundamental material properties of GaN related to vertical power devices, progress of GaN bulk substrate developments and device fabrication processes such as epitaxial growth, ion implantation and MOS interface are reviewed. Future challenges toward commercialization of GaN vertical devices are also discussed.

DOI： 10.1109/IEDM45625.2022.10019514

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

We performed an isothermal annealing study on Mg-implanted GaN at 1300 °C in an ultra-high-pressure (1 GPa) nitrogen ambient. Annealing for more than 30 min resulted in a high acceptor activation ratio and a low compensation ratio that were comparable to those obtained with annealing at 1400 °C for 5 min. We also performed annealing at 1300 °C in a reduced nitrogen pressure of 300 MPa which makes us possible to expand the inner diameter of annealing equipment in the future. High electrical activation, similar to one obtained by annealing at 1 GPa, was successfully obtained.

DOI： 10.35848/1882-0786/ac39b0

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

This work examined reverse leakage currents in GaN p-n junctions nearly free of dislocations. Diodes with shallow bevel mesas and breakdown voltages (BVs) in the range of 130-1000 V exhibited avalanche breakdown at the designed voltages. Significant leakage currents were observed in response to reverse bias values far below the BVs and a weak effect of temperature was also evident. The data were explained based on direct band-to-band tunneling (BTBT). The BTBT current was dominant in those devices having BVs of several hundred volts but was far below the detection limit in the case of a 1000 V class diode.

DOI： 10.35848/1882-0786/ac2a03

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

The sequential implantation of Mg and N ions into GaN was investigated using conventional rapid thermal annealing and ultra-high-pressure annealing (UHPA). In cathodoluminescence, the green luminescence related to nitrogen vacancies (VNs) was mostly suppressed at the Mg/N ratio of 0.5-1.0, whereas the donor-acceptor pair (DAP) emission as a signature of Mg acceptors was maintained high. The excess N implantation reduced the DAP emission through the formation of nonradiative recombination centers. The combined process of optimal Mg/N implantation and UHPA at 1673 K improved ohmic contacts by increasing Mg concentration and suppressing VNs near the surface.

DOI： 10.35848/1882-0786/ac2ae7

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AIP Advances  

Deep-level transient spectroscopy (DLTS) using Schottky barrier diodes (SBDs) is widely used for quantitative analysis of deep levels. This study focuses on the dependence of Schottky barrier height on apparent time constants and concentrations of electron traps in n-type GaN. DLTS using SBDs with various barrier heights was carried out. Experimental data show that large reverse leakage currents due to low barrier heights resulted in underestimation of time constants and concentrations. Theoretical calculations considering the impact of leakage currents reproduced experimental results well. Based on the calculations, we suggest a minimum required barrier height where accurate time constants and concentrations can be evaluated.

DOI： 10.1063/5.0073747

Open Access

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Electronics Letters  

This article reports a high throughput 150-nm-gate AlGaN/GaN high electron mobility transistor (HEMT) process using i-line stepper lithography and a thermal reflow technique. Optimizing thermal reflow conditions, fabrication of a 150-nm gate structure was successfully realized with the initial resist opening of 0.7 μm. AlGaN/GaN field-plated HEMTs were fabricated on a semi-insulating SiC substrate by using this process. In spite of unoptimized structures, fabricated 150-nm gate devices exhibited the maximum drain current of 0.65 A/mm and the gate-drain breakdown voltage exceeding 200 V. Based on cold HEMT extraction measurements, the average gate length of 187 nm and the standard deviation of 30 nm were obtained on a quarter 4-in. wafer.

DOI： 10.1049/ell2.12303

Open Access

Web of Science

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

<p>Fundamental knowledge of point defects is essential for developing epitaxial growth, device processes and device design and characterization. However, the knowledge with regard to GaN is quite limited compared to Si and GaAs. The authors have carried out an extensive investigation of point defects in GaN by using deep-level transient spectroscopy (DLTS). In this paper, the origin of the E3 trap, the development of quantitative measurement techniques for carbon-related hole traps in GaN by using sub-bandgap photoexcitation and studies of nitrogen-displacement-related point defects (nitrogen vacancy and interstitial) intentionally formed by electron beam irradiation are reviewed.</p>

DOI： 10.11470/oubutsu.90.10_628

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Electronics Letters  

Here, a gated-anode diode (GAD) is proposed where an anode electrode is formed by connecting a gate electrode and an ohmic electrode of a normally-off GaN HEMT for a 5.8 GHz rectenna. A wide recessed gate GaN GADs were prepared and the recess length dependence of their electrical characteristics was investigated. Typical DC characteristics of the HEMTs are a threshold voltage (Vth) of +0.3 V and a maximum drain current (Imax) of 300 mA/mm. The GADs showed the characteristics of maximum forward current (If) of 350 mA/mm, reverse breakdown voltage (BVr) of 40 V, and off-state capacitance (Coff) of 0.28 pF/mm by using optimized recess length. We constructed SPICE model of the GADs. The SPICE simulation predicted a rectifier efficiency of 81% and a DC output power of 10 W for bridge type 5.8 GHz rectifier using four GADs with each gate width of 0.8 mm.

DOI： 10.1049/ell2.12269

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

Electron traps generated during the reactive ion etching (RIE) process in n-type 4H-SiC are investigated using the deep-level transient spectroscopy technique and isothermal capacitance transient spectroscopy (ICTS) technique. Two electron traps of the Z1/2 center are detected in the RIE-etched sample by ICTS measurement at 300 K. A method is proposed to determine the depth profiles of the electron traps that are localized near the etched surface, whereby a depth profile is extracted from the dependence of averaged trap density on the depletion layer width. An exponential distribution is assumed as the depth profile of the electron traps generated during the RIE process. The extracted depth profile was confirmed to be consistent with that determined by the double-correlation method. An appropriate function for the depth profile of carrier traps is assumed and the dependence of the averaged trap density on the depletion layer width is analyzed, which enables the extraction of a depth profile that has both higher depth resolution and higher resolution in the carrier trap density with the proposed method than that with the double-correlation method.

DOI： 10.1063/5.0059588

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

To simultaneously evaluate the switching performance and cost (required chip size) of GaN vertical-trench metal-oxide-semiconductor field-effect transistors, we calculated RonARonQg considering the Miller effect with various cell pitches, channel mobilities, and blocking voltages. When the blocking voltage was 600 V, optimized cell pitches of 8 and 12 μm minimized RonARonQg with channel mobilities of 100 and 200 cm2 V-1 s-1, respectively. Moreover, a wide range of cell pitches could maintain a low RonARonQg with a channel mobility of 200 cm2 V-1 s-1. This indicates that a channel mobility of 100 cm2 V-1 s-1 or higher, particularly 200 cm2 V-1 s-1, is desirable for a good switching performance and low cost.

DOI： 10.35848/1882-0786/ac18af

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

The ratio of the photoionization cross sections (σo/n σ o/p) of carbon substituting at the nitrogen site [CN (0/-)] in n-type GaN, which is detected as a hole trap H1 (EV + 0.85 eV) under sub-bandgap-light irradiation (390 nm), is determined with isothermal capacitance transient spectroscopy (ICTS). The current-injection ICTS and the sub-bandgap-light-excited ICTS were compared for the same p+-n junction diode, whereby the hole occupancy ratio (fT) was obtained. Analysis of the dependence of fT on the temperature gave σo/n σ o/p of 3.0 was then used to estimate the charge state of CN (0/-) under sub-bandgap-light irradiation.

DOI： 10.35848/1882-0786/ac16ba

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

A nearly-ideal edge termination for GaN p-n junctions was designed and demonstrated using Mg-ions implanted field limiting rings (FLRs). The FLRs were fabricated via the ultra-high-pressure annealing process after implanting Mg-ions into the etched n-type region outside the main p-n junction. The results of the technology computer-aided design simulation indicate that by optimizing the space and width of the rings, the breakdown voltage (BV) can be increased by over 90% of the ideal parallel plane BV (973 V). Accordingly, the fabricated diodes exhibited low leakage current and a BV of 897 V (92% of the ideal BV).

DOI： 10.35848/1882-0786/ac0b09

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Letters  

Reliability tests involving the application of high electrical stresses were employed to assess GaN-based vertical p-n junctions fabricated on freestanding GaN substrates with threading dislocation densities less than 104 cm−2. Electric field crowding at the device edges was eliminated by employing a shallow bevel mesa structure, thus allowing an evaluation of the reliability of the internal p-n junctions. The p-n diodes exhibited reproducible avalanche breakdown characteristics over the temperature range of 25-175 °C. No degradation was observed even during tests in which the devices were held under a reverse bias near the breakdown voltage. Despite this high degree of reliability in response to reverse bias stress, a small number of diodes were degraded during continuous forward current tests, although the majority of diodes remained unchanged. The reverse leakage current exhibited by degraded diodes was increased with an increase in the forward current density within the range of 50-500 A/cm2, while the breakdown voltages were unchanged in response to current stress. The leakage level increased exponentially with an increase in the total amount of injected carriers but eventually plateaued. In the degraded p-n diode, a luminous point in an emission microscope corresponded to one of the threading dislocations observed in the synchrotron x-ray topography, indicating that a specific dislocation played as a leakage path after injecting carriers.

DOI： 10.1063/5.0053139

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

Avalanche multiplication characteristics in a reverse-biased homoepitaxial GaN p-n junction diode are experimentally investigated at 223-373 K by novel photomultiplication measurements utilizing above- and below-bandgap illumination. The device has a non-punch-through one-side abrupt p--n+ junction structure, in which the depletion layer mainly extends to the p-type region. For above-bandgap illumination, the light is absorbed at the surface p+-layer, and the generated electrons diffuse and reach the depletion layer, resulting in an electron-injected photocurrent. On the other hand, for below-bandgap illumination, the light penetrates a GaN layer and is absorbed owing to the Franz-Keldysh effect in the high electric field region (near the p-n junction interface), resulting in a hole-induced photocurrent. The theoretical (non-multiplicated) photocurrents are calculated elaborately, and the electron- and hole-initiated multiplication factors are extracted as ratios of the experimental data to the calculated values. Through the mathematical analyses of the multiplication factors, the temperature dependences of the impact ionization coefficients of electrons and holes in GaN are extracted and formulated by the Okuto-Crowell model. The ideal breakdown voltage and the critical electric field for GaN p-n junctions of varying doping concentration are simulated using the obtained impact ionization coefficients, and their temperature dependence and conduction-type dependence were discussed. The simulated breakdown characteristics show good agreement with data reported previously, suggesting the high accuracy of the impact ionization coefficients obtained in this study.

DOI： 10.1063/5.0050793

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

Isochronal annealing was performed on Mg-ion-implanted GaN under 1 GPa N2 ambient pressure for 5 min at temperatures of 1573-1753 K. Secondary ion mass spectrometry showed diffusion of Mg atoms and introduction of H atoms during annealing. Deeper diffusion was observed with increasing temperature. From Hall-effect measurements, p-type conductivity was found even for the sample with the lowest annealing temperature of 1573 K. For this sample, the acceptor activation ratio was 23% and the compensation ratio was 93%. The acceptor activation ratio increased to almost 100% and the compensation ratio decreased to 12% with increasing annealing temperature.

DOI： 10.35848/1882-0786/abf4f3

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Letters  

A trench-gate metal-oxide-semiconductor field-effect transistor (T-MOSFET) has great potential for use in gallium nitride (GaN)-based vertical power switching devices owing to its high blocking voltage and high current capability. To form an optimal trench shape that has highly vertical sidewalls and rounded corners, we developed a dry-etching technique using inductively coupled plasma reactive ion etching (ICP-RIE). A highly vertical trench was obtained by including SiCl4 reactive gas mixed with Cl2 gas in the ICP-RIE process, where Si-related byproducts suppressed the etching of the sidewall and allowed selective etching in the vertical direction. We found that the optimization of the bias power was a key to suppress the formation of subtrenches and to avoid an isotropic etching mode. The optimal etching condition leads to natural formation of rounded corners at the trench bottom. In addition, a multistep-bias etching technique was applied to reduce etching-induced damage. Cross-sectional transmission electron microscopy images revealed that lattice distortion on the sidewall surface was eliminated by multistep-bias etching. Based on the rectification properties of the Schottky barrier diodes formed on the trench sidewalls, the Schottky barrier height was comparable to the not-etched surfaces. This indicates that the gap states caused by etching-induced damage can almost be eliminated in the multistep-bias process. The proposed technique is suitable for GaN-based vertical T-MOSFETs.

DOI： 10.1063/5.0040920

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

Photo-electrochemical (PEC) etching is a promising technique for fabricating GaN microelectromechanical systems devices. In this study, we demonstrate the fabrication of GaN cantilevers by the bandgap-selective PEC etching of an InGaN superlattice sacrificial layer. By using an InGaN superlattice as a sacrifice layer, we found the PEC etching rate became higher than using a normal InGaN layer. As a result, the InGaN superlattice was completely etched and we fabricated GaN-based cantilevers whose resonance characteristics were measured. The Young’s modulus of GaN was determined from the resonance characteristics of GaN cantilevers to be the same as the highest value reported previously.

DOI： 10.35848/1882-0786/abe657

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Letters  

Herein, we propose and demonstrate the edge termination for GaN-based one-sided abrupt p-n junctions. The structure is comprised of a combination of a shallow negative bevel mesa and selective-area p-type doping under the mesa. Based on the Technology Computer Aided Design (TCAD) simulation, the maximum electric field at the junction edge is markedly reduced to approximately 1.3 times that of the parallel-plane electric field in the proposed structure, which is almost half of the unimplanted diode. The TCAD simulation also shows that the shallow mesa angle of 6 ° effectively reduces the optimum acceptor concentration (Na) in the implanted region and enhances the breakdown voltage. The optimum Na value can be covered by the proposed technology based on the Mg-ion implantation and subsequent ultra-high-pressure annealing (UHPA). Using the formation of the shallow bevel mesa, the Mg-ion implantation, and the UHPA process, we experimentally demonstrate the p-n diodes with a breakdown voltage over 600 V, which is in good agreement with the TCAD simulation. The proposed method can be applied to a vertical trench-gate metal-oxide-semiconductor field-effect transistor with a high figure-of-merit.

DOI： 10.1063/5.0039183

Open Access

Web of Science

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

DOI： 10.11470/jsapmeeting.2021.1.0_2403

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

Due to the large ionization energy of Mg acceptors in GaN, dynamic punch-through will occur in vertical GaN MOSFETs. To avoid this, higher doping and/or a thicker p-body region should be utilized. However, this increases the channel resistance. In this letter, we suggest that the Poole-Frenkel (P-F) effect has significant impact on dynamic punch-through because of the high electric field in the depletion region under a large bias voltage. Systematic TCAD simulations of simplified vertical GaN MOSFET structures were carried out. We show that the device design considering the P-F effect results in a reduction in the increase in channel resistance.

DOI： 10.35848/1882-0786/abd960

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Letters  

Energy levels due to intrinsic point defects are identified by deep-level transient spectroscopy (DLTS). Electron-beam (EB) irradiation created nitrogen vacancies (VN) and nitrogen interstitials (NI) in n-type GaN layers grown via metalorganic vapor phase epitaxy on freestanding GaN substrates, where the irradiation energies were selected to be within 100-401 keV to displace only nitrogen atoms in GaN. Two electron traps, EE1 (0.13 eV) and EE2 (0.98 eV), were observed in the DLTS spectra. The production rates of EE1 and EE2 were 0.093 and 0.109 cm-1 under 401 keV irradiation, which were nearly equal values. In the DLTS spectra recorded for EB-irradiated samples at the energy ranging from 100 to 401 keV, EE1 and EE2 were found to appear simultaneously at an irradiation energy of 137 keV and were observed at energies greater than 137 keV. On the basis of a comparison with the results of recent first-principles calculations, we attributed the EE1 and EE2 peaks to nitrogen vacancies VN (+/0) and nitrogen interstitials NI (0/-), respectively. Furthermore, annealing led to reductions of the densities of these traps at the same rate. The reduction of the densities of EE1 and EE2 can be explained by the migration of NI and the subsequent recombination with VN. The displacement energy of 21.8 eV for nitrogen in GaN was obtained from the irradiation-energy dependence of EE1.

DOI： 10.1063/5.0035235

Open Access

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

Atomic layer deposited Al2O3/GaN metal-oxide-semiconductor (MOS) diodes with and without post-metallization annealing (PMA) were irradiated with gamma-rays. Capacitance-voltage measurements were made before and after irradiation to investigate trap formation in Al2O3 films and interface states between Al2O3 and GaN. Negative flat-band voltage shifts were observed. The flat-band voltage shift depends on the Al2O3 thickness, showing different distributions of gamma-ray-induced positive charges for samples with and without PMA. The interface state density of the PMA sample slightly increased after irradiation, but was lower than that of the sample without PMA before irradiation.

DOI： 10.35848/1882-0786/abd71a

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.35848/1882-0786/abd308

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

Defect time-evolution was investigated in Mg ion-implanted GaN after annealing at 1573 K for an unprecedentedly long duration. Transmission electron microscopy directly revealed that annealing for over 30 min reduced defects inhibiting Mg activation, just like annealing at 1753 K for a short duration. The cathodoluminescence intensity of donor-acceptor pair originating from Mg acceptors increased as the duration increased, and the intensity after annealing for 60 min was higher than after short-duration annealing at 1753 K. These show the potential of lowering the annealing temperature by prolonging the duration, which would lead to practical annealing technology for Mg ion-implanted GaN.

DOI： 10.35848/1882-0786/abd308

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

The electrical properties of vertical GaN trench MOSFETs without drift layers were evaluated to investigate the effect of nitrogen plasma treatment on the trench sidewalls. It is demonstrated that nitrogen plasma treatment improves the channel property of the vertical GaN trench MOSFET. The possible mechanism of this improvement is the supply of nitrogen atoms from nitrogen plasma treatment to the trench surfaces, and the compensation of the nitrogen vacancies near the trench surfaces by the nitrogen atoms during gate oxide annealing. The temperature dependence and the limiting factors of the channel property are also discussed.

DOI： 10.35848/1882-0786/abcdbb

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE Transactions on Electron Devices  

This article reports a systematic study focused on the mechanical stress effect of field-plate dielectric film on the electric characteristics of AlGaN/GaN high-electron mobility transistors (HEMTs). AlGaN/GaN HEMTs were fabricated on SiC substrates, where the stress of a SiN field-plate dielectric film ranged from -252 (compressive) to +26.5 (tensile) MPa. Si-rich and compressive SiN films exhibited a significant increase in the isolation leakage. On the other hand, relatively N-rich and tensile SiN films showed a significant increase in the gate leakage current of HEMTs. In addition, pulsed ${I}$ - ${V}$ measurements showed the suppression in the current collapse by increasing the tensile stress. Consequently, small current collapse and small gate leakage current were obtained simultaneously with good isolation, as the film stress was optimized.

DOI： 10.1109/TED.2020.3029540

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Scientific Reports  

Vacancy-type defects in Mg-implanted GaN were probed by using a monoenergetic positron beam. Mg ions were implanted into GaN to obtain 0.3-μm-deep box profiles with Mg concentrations of 1 × 1019 cm−3. The major defect species in an as-implanted sample was determined to be Ga-vacancy related defects such as a complex between Ga and N vacancies. The sample was annealed under a nitrogen pressure of 1 GPa in a temperature range of 1000–1480 °C without a protective capping layer. Compared with the results for Mg-implanted GaN annealed with an AlN capping layer, the defect concentration was decreased by the cap-less annealing, suggesting that the surface of the sample was an effective sink for vacancies migrating toward the surface. Depth distributions of Mg after annealing above 1300 °C were influenced by the presence of residual vacancies at this temperature. Hydrogen atoms were unintentionally incorporated into the sample during annealing, and their diffusion properties were also affected by both vacancies and Mg.

DOI： 10.1038/s41598-020-74362-9

Open Access

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Scopus

PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

We found that the graphite elements in a metalorganic vapor phase epitaxy reactor are a significant source of iron atoms, and lead to the formation of E3 electron traps at an energy E C-0.6 eV in n-type GaN layers. Thus, the previously reported inverse correlation between the E3 trap and carbon concentrations can be explained in terms of the correlation between the growth parameters. A high V/III ratio reduced carbon incorporation and simultaneously decreased the growth rate, resulting in increased iron incorporation in the layer grown. A high temperature also reduced carbon incorporation, while iron incorporation and the formation of E3 traps were both increased by enhanced etching of the graphite elements. By optimizing the growth conditions, we successfully suppressed both E3 trap formation and carbon incorporation in n-type GaN layers on 2 inch wafers.

DOI： 10.35848/1347-4065/abb9ca

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

Depth profiling of the dry-etching damage in n-type GaN induced by inductively coupled plasma reactive ion etching was carried out by cyclical electrochemical impedance spectroscopy (EIS) measurements and damage-free photo-electrochemical (PEC) etching. The GaN samples were dry-etched under different etching bias power (Pbias) conditions, and PEC etchings were conducted in increments of 10 nm after EIS measurements. The damage depth was determined to be less than 50 nm for the sample corresponding to Pbias = 30 W and was less than 10 nm for the two-step etching sample corresponding to Pbias = 30 W + 5 W.

DOI： 10.35848/1882-0786/abb787

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

The fabrication processes of p-type regions for vertical GaN power devices are investigated. A p-type body layer in a trench gate metal-oxide-semiconductor field-effect transistor requires precise control of the effective acceptor concentration, which is equal to the difference between the Mg acceptor concentration (Na) and the compensating donor concentration (Nd). The carbon atoms incorporated during growth via metalorganic vapor phase epitaxy substitute nitrogen sites (CN) and function as donor sources in a p-type GaN layer. Since interstitial H atoms (H i) also compensate holes, their removal from an Mg-doped layer is crucial. Extended anneals to release H atoms cause the formation of extra hole traps. The p+ capping layer allows effective and rapid removal of H atoms from a p-type body layer owing to the electric field across the p+/p- junction. On the other hand, selective area p-type doping via Mg ion implantation is needed to control the electrical field distribution at the device edge. Ultrahigh-pressure annealing (UHPA) under a nitrogen pressure of 1 GPa enables post-implantation annealing up to 1753 K without thermal decomposition. Cathodoluminescence spectra and Hall-effect measurements suggest that the acceptor activation ratio improves dramatically by annealing above 1673 K as compared to annealing at up to 1573 K. High-temperature UHPA also induces Mg atom diffusion. We demonstrate that vacancy diffusion and the introduction of H atoms from the UHPA ambient play a key role in the redistribution of Mg atoms.

DOI： 10.1063/5.0022198

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Proceedings of the International Symposium on Power Semiconductor Devices and ICs  

In this study, we clarify the impact of annealing pressure and temperature of the ultra-high-pressure annealing (UHPA) as a post-implantation-annealing (PIA) on the acceptor activation for Mg-ions-implanted GaN samples. The pressure to prevent the thermal decomposition is absolutely determined by the equilibrium N2 partial pressure in the phase diagram of GaN-Ga-N2 system, so that the pressure below the critical pressure always leads to the serious surface decomposition of GaN. In low temperature cathodoluminescence examinations, the samples processed at 1400°C or above exhibited more intense emissions in the near band edge and donor-acceptor pair band with a suppression of green luminescence related to point defects. Temperature dependent Hall-effect measurement was allowed for the UHPA samples annealed at 1400°C or above, whereas free holes were not observed for the 1300°C-UHPA. Moreover, the acceptor concentrations (Na) for samples annealed at 1400°C or above were close to Mg concentrations. We thus revealed the key process parameters in UHPA, which is at more than 1400°C under the N2 overpressure exceeding the equilibrium partial pressure. These findings can play as a key role in the selective area doping using UHPA.

DOI： 10.1109/ISPSD46842.2020.9170174

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

Diffusion in a magnesium (Mg)-implanted homoepitaxial GaN layer during ultra-high-pressure annealing (UHPA, in ambient nitrogen, under 1 GPa) was investigated. Annealing at 1573 K resulted in Mg-segregation at the edge of the implanted region, which was suppressed using a higher temperature of 1673 K. Hydrogen (H) atoms were incorporated during the UHPA, resulting in the Mg and H developing the same diffusion profile in the deeper region. The diffusion coefficient of the Mg-implanted sample was 3.3 10-12 cm2 s-1 at 1673 K from the annealing duration dependence, 30 times larger than that of the epitaxial Mg-doped sample, originating from ion implantation-induced defects.

DOI： 10.35848/1882-0786/aba64b

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

The origin of E3 electron traps at E C -0.58 eV in GaN was investigated using Si-doped n-type GaN layers grown on freestanding GaN substrates using MOVPE. These layers contained impurity Fe at various concentrations depending on the growth conditions and the position within the wafer. Twenty E3 concentrations (N T,E3) determined by deep-level transient spectroscopy were plotted against the corresponding Fe concentrations ([Fe]) obtained from secondary ion mass spectrometry. A correlation was evident between N T,E3 and [Fe] in the range (0.4-12) 1015 cm-3, suggesting that the E3 level in MOVPE-grown homoepitaxial GaN originates from the substitution of Fe atoms at Ga sites.

DOI： 10.35848/1882-0786/ab9e7c

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

A quick method is proposed for measurement of the carbon-related hole trap (H1: +0.87 eV) density in an n-type GaN homoepitaxial layer using dual-color-sub-bandgap-light-excited isothermal capacitance transient spectroscopy. Shorter wavelength (390 nm) light irradiation is employed to cause the hole traps to be in the hole-occupied state. Longer wavelength (660 nm) light irradiation is then used to emit the hole from the trap to the valence band. The photoemission of holes is much quicker than the thermal emission, which reduces the measurement time. The trap density can be calculated from the capacitance transient.

DOI： 10.35848/1347-4065/ab6863

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AIP Advances  

Gamma-ray irradiations of up to 500 kGy on homoepitaxial n-type GaN layers were carried out, and the formation of electron traps was investigated by deep-level transient spectroscopy (DLTS) using Ni Schottky barrier diodes (SBDs). Before performing DLTS, current-voltage (I-V) and capacitance-voltage (C-V) measurements of the SBDs were performed and it was found that there was no change in the net donor concentration, ideality factor, and Schottky barrier height after irradiation. In the DLTS measurements, two new peaks, labeled G1 and G2, were observed after irradiation. The filling pulse width dependence of G1 revealed that the peak consists of two electron trap levels, labeled G1a (EC - 0.13 eV) and G1b (EC - 0.14 eV). Isothermal capacitance transient spectroscopy measurements of samples with different Schottky barrier heights showed that the G2 peak is a complex peak consisting of at least three electron traps, labeled G2a (EC - 0.80 eV), G2b (EC - 0.98 eV), and G2c (EC - 1.08 eV). The production rates (formation rates of traps by gamma-ray irradiation) for each trap were obtained. Finally, we investigated the annealing behavior of each trap and found that G1b and G2b decreased by the same amount with increasing annealing temperature, suggesting that the behavior originates from a recombination of vacancy-interstitial (Frenkel) pairs.

DOI： 10.1063/1.5144158

Open Access

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

Defects in Mg ion-implanted GaN epitaxial layers formed after annealing at 1573 K and at 1753 K were analyzed by transmission electron microscopy. Degradation of the GaN surface, which occurs at temperatures higher than about 1573 K, was avoided by ultra-high-pressure annealing under a N2 atmosphere at 1 GPa. Annealing for damage recovery in ion-implanted compound semiconductors generally requires temperatures at about two-thirds of their melting point, which is reportedly 2518 K or higher for GaN. Thus, defect analysis in ion-implanted GaN annealed at temperatures higher than 1573 K is necessary to understand the defect recovery. Atomic-resolution transmission electron microscopy analysis showed that interstitial-type extended defects and inversion domains with Mg segregation were formed during the annealing at 1573 K. These defects were not observed in a sample annealed at 1753 K; instead, vacancy-type extended defects were formed. Such evolution of defects can be explained by previous theoretical studies that indicated that the migration energy of vacancy-type defects is higher than that of interstitial-type defects. Moreover, the formation of vacancy-type extended defects implies a reduction in the concentrations of vacancies and their complexes. Since the vacancies and their complexes can compensate for Mg acceptors, their reduced concentration should increase the acceptor activation efficiency. Also, the disappearance of Mg segregation during high-temperature annealing should increase the activation efficiency since the segregated Mg atoms are electrically inactive. It is thus concluded that the evolution of defects caused by high-temperature annealing above 1573 K increases the activation efficiency of acceptors in Mg ion-implanted GaN.

DOI： 10.1063/1.5140410

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physica Status Solidi (B) Basic Research  

The effect of increasing the dosage on the electrical properties of Mg-ion-implanted GaN before activation annealing is investigated to obtain knowledge on the defect levels generated by ion implantation. To probe the near-surface region, GaN metal-oxide-semiconductor (MOS) structures with Al2O3 are used. Two kinds of MOS diodes with Mg-ion dosages of 1.5 × 1011 and 1.5 × 1012 cm−2 implanted at 50 keV are prepared. Although anomalous capacitance–voltage (C–V) characteristics are observed for the low-dosage sample, they are improved by annealing at 600 °C for 3 h. However, for the high-dosage sample, more severe and persistent frequency dispersion is observed in the C–V characteristics, which is not improved by the same annealing. On the basis of the detailed analysis of capacitance–frequency (C–f) characteristics, it is concluded that the discrete interface trap at 0.2–0.3 eV below the conduction band is responsible for the frequency dispersion observed for the high-dosage sample. Combined with the results of deep-level transient spectroscopy, it is highly likely that the bulk deep levels affect the C–V and C–f characteristics. The possibility that the dominant deep levels are changed by the increase in Mg-ion dosage is discussed.

DOI： 10.1002/pssb.201900367

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

Sources of carrier compensation in n-type and p-type GaN layers grown by metalorganic vapor phase epitaxy were quantitatively identified by a combination of Hall-effect analysis and deep level transient spectroscopy. For n-type GaN, we identified three electron compensation sources: Residual carbon atoms likely sitting on nitrogen sites (CN), an electron trap at the energy level of E C-0.6 eV (the E3 trap), and self-compensation appearing with increasing donor concentration. We showed that the CN also play a key role in hole compensation in p-type GaN by forming donor-like charged states. We also investigated the reduction of acceptor concentrations (N a) in highly Mg-doped GaN. Atomic-resolution scanning transmission electron microscopy revealed that electrically inactive Mg atoms of 3/2 atomic layers are segregated at the boundary of pyramidal inversion domains. The N a reduction can be explained by this Mg segregation.

DOI： 10.7567/1347-4065/ab4610

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

Plasma-induced damage was reduced by multistep-bias etching that involved a stepwise decrease of the etching bias power (P bias) and subsequent annealing. The depth of damage at P bias = 60 W was determined to be 60 nm from the capacitance-voltage characteristics of Ni/Al2O3/etched-GaN metal-oxide-semiconductor diodes. The damaged layer was removed by subsequent etching at P bias = 5 W and 2.5 W. The residual and shallow damage induced by the low P bias was then recovered by subsequent annealing at 400 °C. The multistep-bias etching of inductively coupled plasma reactive ion etching was thus confirmed to be effective for achieving a high etching rate with low damage.

DOI： 10.7567/1882-0786/ab5ffe

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Technical Digest - International Electron Devices Meeting, IEDM  

We propose a novel method to extract impact ionization coefficients of electrons and holes using above-and sub-bandgap illuminations for a p-/n+ junction diode. For above-bandgap illumination, the light is absorbed near p-GaN surface. Then, generated minority carriers diffuse and reach the edge of the depletion layer, resulting in an electron-injected photocurrent. On the other hand, for sub-bandgap illumination, the light is selectively absorbed near the p-n junction interface by the Franz-Keldysh effect, resulting in a hole-injected photocurrent. The electron- and hole-initiated multiplication factors are obtained as the ratios of the measured photocurrents to the calculated unmultiplicated photocurrents. By analyzing the electron- and hole-initiated multiplication factors, the impact ionization coefficients of electrons and holes in GaN are extracted separately.

DOI： 10.1109/IEDM19573.2019.8993438

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

To investigate Mg diffusion during ultra-high-pressure annealing, which activates Mg acceptors in GaN, GaN samples with p-n junctions prepared via epitaxial growth were annealed at 1573 K under 1 GPa. The profiles of Mg diffusion toward the underlying n-type layer cannot be explained by a simple diffusion model. We found that H atoms diffused along with Mg atoms. By considering the suppressed diffusion of positively charged interstitial H atoms due to the electric field in the depletion layer, we could better reproduce the Mg-H diffusion profiles, suggesting that H atoms play a key role in the Mg diffusion process.

DOI： 10.7567/1882-0786/ab4934

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IEEE Transactions on Electron Devices  

SiC bipolar junction transistors (BJTs) were fabricated based on the design criterion proposed in our previous study, which quantitatively proved the importance of decreasing a base spreading resistance. To reduce the base spreading resistance, Al+-implantation was performed in the parasitic base region. No negative influences due to the implantation damage on the current gain were confirmed when the implantation is performed sufficiently apart from the emitter mesa sidewall, the distance of which is longer than 3μ m. Since the fabricated BJTs satisfied the design criterion, clear conductivity modulation was achieved, resulting in a reduced collector-resistance, that is, 50% of the unipolar resistance. In addition, we experimentally demonstrated that the conductivity modulation in SiC BJTs could be enhanced by decreasing the base spreading resistance.

DOI： 10.1109/TED.2019.2941884

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Letters  

Avalanche multiplication characteristics of GaN p-n junction diodes (PNDs) with double-side-depleted shallow bevel termination, which exhibit nearly ideal avalanche breakdown, were investigated by photomultiplication measurements using sub-bandgap light. In GaN PNDs under reverse bias conditions, optical absorption induced by the Franz-Keldysh (FK) effect is observed, resulting in a predictable photocurrent. The avalanche multiplication factors were extracted as a ratio of the measured values to the calculated FK-induced photocurrent. In addition, the temperature dependences of the avalanche multiplications were also investigated.

DOI： 10.1063/1.5114844

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Letters  

A high activation ratio of acceptors to Mg ions implanted into a homoepitaxial GaN layer was achieved through an ultra-high-pressure annealing (UHPA) process. Capless annealing under a nitrogen pressure of 1 GPa in a temperature range of 1573-1753 K activated acceptors without thermally decomposing the GaN layer. Conventional rapid thermal annealing leads to a serious decomposition at 1573 K, even with an AlN protective cap. The sample annealed at 1673 K under UHPA exhibited very intense cathodoluminescence in near-band edge and donor-acceptor-pair band emissions. The intensities were over one order of magnitude higher than those of the sample treated by conventional annealing. A Hall-effect measurement was obtained in the temperature range of 275-500 K for the UHPA sample. The obtained hole concentration and mobility at 300 K were 3.6 × 1016 cm-3 and 24.1 cm2 V-1 s-1, respectively. The mobility value was close to that of an epitaxial p-type GaN with the same doping concentration. An Arrhenius plot of hole concentrations showed that the acceptor concentration and ionization energy were separately estimated to be (2.6 ± 0.8) × 1018 cm-3 and 212 ± 5 meV, respectively. By comparing the Mg concentrations obtained from secondary ion mass spectrometry, the acceptor activation ratio (acceptor concentration/Mg concentration) of the UHPA samples exceeded 70%. These results suggest that the UHPA process as a postimplantation annealing technique is promising for the fabrication of GaN-based power devices with selective area doping.

DOI： 10.1063/1.5116866

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

Electric-field dependence of optical absorption induced by Franz-Keldysh (FK) effect strongly depends on the value of the reduced effective mass along electric field. In this study, reverse current-voltage characteristics of 4H-SiC{1120} p-n junction diodes under sub-bandgap illumination were investigated. Under a reverse bias condition, a photocurrent induced by FK effect was observed and increased with the reverse voltage. We calculated a photocurrent with consideration of phonon-assisted optical absorption induced by FK effect in a depletion region using the reduced effective mass perpendicular to the c-axis (μ⊥ = 0.26 m0), and the calculated values showed good agreement with the experimental values. This result indicates that the anisotropy of optical absorption induced by FK effect in 4H-SiC is small, since the reduced effective mass perpendicular to the c-axis (μ⊥ = 0.26 m0) is close to that parallel to the c-axis (μ∥ m = 0.28 m0).

DOI： 10.7567/1347-4065/ab3873

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Letters  

We studied deep levels in quartz-free hydride-vapor-phase epitaxy (QF-HVPE)-grown homoepitaxial n-type GaN layers within which three electron and eight hole traps were detected. The dominant electron and hole traps observed in the QF-HVPE-grown GaN layers were E3 (EC - 0.60 eV) and H1 (EV + 0.87 eV), respectively. We found that the E3 trap density of QF-HVPE-grown GaN (∼1014 cm-3) was comparable with that of MOVPE-grown GaN layers, whereas the H1 trap density of QF-HVPE-grown GaN (∼1014 cm-3) was much smaller than that of an MOVPE-grown GaN layer with a low-residual-carbon growth condition. A detailed analysis of the QF-HVPE-grown GaN layers revealed that the H1 trap density is almost equal to the carbon impurity concentration and other impurities that compensate the Si donors besides the carbon impurity were hardly detected in the QF-HVPE-grown GaN layers.

DOI： 10.1063/1.5098965

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

The Shockley-Read-Hall (SRH) lifetime in homoepitaxial p-GaN (N a = 1 × 1017 cm-3) is investigated by analyzing forward current-voltage (I-V) characteristics in GaN-on-GaN p-n+ junction diodes with mesa-isolation structure. The ideality factor around 2 due to recombination current was obtained in the 1.8-2.7 V window, which is different from the characteristic of a p+-n- junction involving considerable diffusion current. The recombination current was proportional to the junction area, indicating that the recombination current is a bulk component, not a mesa-surface component. Analyzing the recombination current with consideration of the SRH recombination rate in the depletion layer, we obtained an SRH lifetime of 46 ps at 298 K. The temperature dependence of the I-V characteristics was also investigated and the SRH lifetimes were extracted in the range of 223-573 K. The SRH lifetime in homoepitaxial p-GaN followed the empirical power law of = 1.2 × 10-16 × T 2.25 (s).

DOI： 10.7567/1347-4065/ab07ad

Open Access

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.7567/1347-4065/ab1411

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE Electron Device Letters  

We report on homoepitaxial GaN p-n junction diodes with a negative beveled-mesa termination. The electric field distribution in a beveled-mesa was investigated using TCAD simulation, and the devices were designed using currently available GaN growth techniques. Shallow-angle (ca. 10°) negative bevel GaN p-n junction diodes were fabricated with various Mg acceptor concentrations in the p-layers. The suppression of electric field crowding and improvement of the breakdown voltage were observed, as the Mg concentration was decreased. The parallel-plane breakdown field of 2.86 MV/cm was obtained for a device with the breakdown voltage of 425 V.

DOI： 10.1109/LED.2019.2912395

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Device Research Conference - Conference Digest, DRC  

The vertical structure of the power device has advantages such as small chip size, easy wiring, and high breakdown voltage. Furthermore, wideband gap semiconductors have the greatest feature of low on-resistance. GaN is a material having the ability to fully exhibit these properties and in recent years development of GaN vertical devices has been accelerated. For example, GaN vertical devices with over lkV breakdown voltage have been reported recently1-5). Moreover, over 3kV pn diodes were also reported6,7). Therefore, ability of GaN for high voltage devices has been proven. Next issues are developments of fabrication process technologies which make devices stable operation. In this paper, we will report recent advances in process technology for GaN vertical devices.

DOI： 10.1109/DRC46940.2019.9046467

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Proceedings of the International Symposium on Power Semiconductor Devices and ICs  

A valanche multiplication characteristics in GaN p-n junction diodes (PNDs) under high reverse bias conditions were investigated. The GaN-on-GaN PNDs with double-side-depleted shallow bevel termination, which showed low reverse leakage current and excellent avalanche capability, were used for the measurements. Under sub-bandgap light illumination, the photocurrents induced by Franz-Keldysh (FK) effect, which can be well reproduced by the theoretical calculations of the optical absorption, and their avalanche multiplications were observed. The multiplication factors were extracted as the ratios of the experimental photocurrents to the calculated FK-induced photocurrent. Under an assumption of equal impact ionization coefficients of electrons and holes, the electric-field dependence of an impact ionization coefficient in GaN were estimated.

DOI： 10.1109/ISPSD.2019.8757676

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Technical Digest - International Electron Devices Meeting, IEDM  

We report homoepitaxial GaN p-n junction diodes with novel beveled-mesa structures. The n-layers and p-layers, the doping concentrations of which are comparable, were prepared. We found that electric field crowding does not occur in the structure using TCAD simulation. The fabricated devices showed the breakdown voltages of 180-480 V, small leakage currents, and excellent avalanche capabilities. The breakdown voltages increased at elevated temperature. At the breakdown, nearly uniform luminescence in the entire p-n junctions was observed in all the devices. These results are strong evidences that the uniform avalanche breakdowns occurred in the devices. We carefully characterized the depletion layer width at the breakdown, and the parallel-plane breakdown electric fields of 2.8-3.5 MV/cm were obtained, which are among the best of the reported non-punch-through GaN vertical devices.

DOI： 10.1109/IEDM.2018.8614669

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/IEDM19573.2019.8993438

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：19th International Workshop on Junction Technology, IWJT 2019  

For the solution to global energy issues, highly-efficient energy conversion using next-generation power devices is required. Gallium nitride (GaN) having superior properties such as high breakdown electric field (2.8-3.75 MV/cm) is a powerful candidate for next-generation high-power semiconductor devices. [1] , [2] The selective area doping makes it possible to precisely engineer high-power devices with complex structures, allowing formation of low-resistivity region for contacting electrodes and optimization of the electric field configuration in the edge termination represented by the field limiting ring (FLR) and in the junction barrier Schottky (JBS) structures, as used in Si and SiC power devices.

DOI： 10.23919/iwjt.2019.8802621

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IEEE Transactions on Electron Devices  

Surface recombination velocity on mesa sidewalls of SiC p-n diodes with various surface passivation conditions was evaluated from the device-size-dependent preexponential factor of recombination current (J0rec). The diodes passivated by SiO2 with postoxidation nitridation were dipped into HF to eliminate a shunt current, which is evoked by the nitrided SiO2 layer and disturbs the analysis of the recombination current. For accurate determination of the surface recombination velocity, an effective recombination zone width was precisely derived taking account of the distribution of carrier density in the depletion layer. The surface recombination velocity of the diodes without any passivation and with the postoxidation nitridation (NO annealing at 1250 °C for 70 min) was determined as 1.2 × 107 and 6.0 × 105 cm/s, respectively, which indicates that the postoxidation nitridation can reduce the surface recombination by a factor of about 20. We confirmed that TCAD simulation could reproduce the current-voltage characteristics by utilizing the extracted parameters. In addition, an evaluation method was proposed to determine the surface recombination velocity from the high-current region, where diffusion current is dominant.

DOI： 10.1109/TED.2018.2867545

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Physics D: Applied Physics  

Recent progress and current understanding of carrier lifetimes and avalanche phenomena in silicon carbide (SiC) are reviewed. The acceptor level of carbon vacancy (V C), called the Z 1/2 center, has been identified to be the primary carrier lifetime killer in SiC. The V C defects can be eliminated by the introduction of excess carbon atoms followed by carbon diffusion in the bulk region. The true bulk lifetime after V C elimination was estimated to be approximately 110 μs. The doping dependence of carrier lifetimes in n- and p-type SiC is also presented. The impact ionization coefficients of electrons and holes were extracted in the temperature range of 298 to 423 K. The intrinsic critical electric field strength of SiC0 0 0 1 was determined to be 2.0, 2.5, and 3.3 MV cm-1 for doping densities of 1 × 1015, 1 × 1016, and 1 × 1017 cm-3, respectively, at room temperature; it slightly increased at elevated temperature. The obtained set of impact ionization coefficients has enabled us to accurately predict the breakdown voltage of SiC devices, including its temperature dependence. Due to the unusually low impact ionization coefficient of electrons, the breakdown voltage of a SiC p+n junction is about 6%-9% higher than that of an n+p junction with a given doping density.

DOI： 10.1088/1361-6463/aad26a

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.7567/APEX.11.091302

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

Temperature dependence of resistivity from 250 to 900K in p-type 4H-SiC with various doping concentrations (5.8 × 1014-7.1 × 1018cm-3) was presented. The resistivity was obtained by the van der Pauw method in samples, whose doping concentrations were precisely determined in our previous work. From the experimental results, coefficients for a fitting formula with polynomial approximation were derived. We confirmed that the fitting formula can accurately estimate the resistivity of p-type SiC with wide-range doping concentrations.

DOI： 10.7567/JJAP.57.088002

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.7567/APEX.11.071002

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.7567/JJAP.57.070309

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IEEE Transactions on Electron Devices  

Impacts of finger numbers on ON-state characteristics in multifinger 10-kV-class SiC bipolar junction transistors (BJTs), whose base spreading resistance is sufficiently reduced by using aluminum ion implantation, were investigated by performing TCAD simulations. Common-emitter current-voltage characteristics of the BJTs with different base current densities and carrier lifetimes in the collector layer were analyzed. The simulation results exhibited that the BJTs with fewer finger numbers could achieve superior characteristics owing to an expansion of a conductivity-modulated region and to a higher current density per finger. In addition, we showed that BJTs with a punchthrough structure have a potential to achieve superior characteristics suitable for power device applications under a certain condition, where strong conductivity modulation occurs. The presented results indicate that the appropriate finger numbers should be designed for a better performance of the multifinger SiC BJTs.

DOI： 10.1109/TED.2018.2834354

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

The temperature dependencies of hole density and hole mobility of p-type 4H-SiC obtained by Hall effect measurement were theoretically analyzed taking account of its anisotropic valence band structure. The experimental Hall factor, which was derived from the ratio of theoretical hole density to experimental Hall hole density, was reproduced by theoretical Hall factor computed using the valence band structure and relaxation times of scattering mechanisms. The product of the theoretical Hall factor and drift mobility computed by the same transport model agreed well with the experimental Hall mobility. Based on analyses of the results, it was revealed that the temperature dependence of Hall factor can be explained by considering the anisotropic valence band structure along with consideration of anisotropic relaxation times. The contribution of each scattering mechanism was also discussed, and empirical formulas for Hall and drift mobilities are presented.

DOI： 10.1063/1.5025776

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.5031215

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.7567/JJAP.57.04FR04

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.7567/APEX.11.041001

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

The quasi-ballistic hole transport in germanium and silicon nanowires was theoretically investigated by solving the Boltzmann transport equation taking account of phonon scattering in an atomistic framework. Comparison of quasi-ballistic hole transport capabilities between germanium and silicon nanowires showed that the transmission coefficients for the two materials are similar. Then, the behavior of forward and backward current fluxes was analyzed focusing on the impact of energy relaxation process. The slower energy relaxation of holes in germanium nanowires leads to a longer distance where backscattering enables holes to return to the source. This cancels the benefit of the longer mean free path of holes in germanium nanowires, resulting in similar transmission coefficients in germanium and silicon nanowires.

DOI： 10.1063/1.5010052

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Letters  

AlN/GaN short-period superlattices (SPSLs) were grown on 4H-SiC(112-0) substrates with a 4H-AlN template layer. The thickness of the GaN layers was controlled to be less than 1 nm to suppress the generation of extended defects. The stacking fault and threading dislocation density were found to be 3 × 105 cm-1 and 6 × 107 cm-2, respectively, which were as low as those for a 4H-AlN single layer. The superlattice replicated the polytype of the underlying 4H-AlN layer (4H-SiC substrate), meaning that a 4H-AlN/4H-GaN SPSL was grown. Roomerature cathodoluminescence measurements revealed that the 4H-AlN/4H-GaN SPSLs exhibited intense luminescence in the deep ultraviolet region of 4.4-5.0 eV, depending on the GaN layer thickness. The emission wavelengths of the SPSLs fairly agreed with the estimation based on the Kronig-Penney model.

DOI： 10.1063/1.5006435

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Materials Science Forum  

Effects of a parasitic region in SiC BJTs on conductivity modulation and a forced current gain (βF) were investigated by using TCAD simulation with various device structures. By introducing an Al+-implanted region below the base parasitic region, βF can be improved because the implanted region can reduce the base spreading resistance, leading to alleviation of debiasing effect. βF in devices with various parasitic areas, whose base spreading resistances were reduced by the Al+-implantation, were compared. We found that βF can be enhanced by expanding the parasitic area if the base spreading resistance is sufficiently reduced. The higher βF is attributed to an expanded conductivity-modulated region. The collector current spreading in the collector layer and hole injection from the parasitic region as well as from the intrinsic region can play a role to evoke conductivity modulation. Thus, the larger parasitic region can expand the conductivity-modulated region, which results in expansion of an active area and the enhancement of βF, though a higher base voltage is required.

DOI： 10.4028/www.scientific.net/MSF.924.629

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/IEDM.2018.8614669

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CS MANTECH 2018 - 2018 International Conference on Compound Semiconductor Manufacturing Technology  

Thanks to superior materials properties, GaN has great potential as the next-generation power devices. The author will discuss challenges for GaN vertical devices by referring SiC technologies, which has over 20 years history.

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：2017 Silicon Nanoelectronics Workshop, SNW 2017  

The quasi-ballistic hole transport capabilities of Ge and Si NWs were calculated using atomistic electron-phonon coupling and Boltzmann transport equation. Analyzing the forward and backward current fluxes, it was shown that the positive impact of high mobility of Ge is canceled by its slower energy relaxation, which results in comparable transmission coefficients and current transport capabilities between Ge and Si NWs.

DOI： 10.23919/SNW.2017.8242284

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Electronic Materials  

We investigated limiting factors of carrier lifetimes and their enhancement by post-growth processes in lightly-doped p-type 4H-SiC epitaxial layers (NA ∼ 2 × 1014 cm−3). We focused on bulk recombination, surface recombination, and interface recombination at the epilayer/substrate, respectively. The carrier lifetime of 2.8 μs in an as-grown epilayer was improved to 10 μs by the combination of VC-elimination processes and hydrogen annealing. By employing surface passivation with deposited SiO2 followed by POCl3 annealing, a long carrier lifetime of 16 μs was obtained in an oxidized epilayer. By investigating carrier lifetimes in a self-standing p-type epilayer, it was revealed that the interface recombination at the epilayer/substrate was smaller than the surface recombination on a bare surface. We found that the VC-elimination process, hydrogen annealing, and surface passivation are all important for improving carrier lifetimes in lightly-doped p-type epilayers.

DOI： 10.1007/s11664-017-5677-4

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：International Conference on Simulation of Semiconductor Processes and Devices, SISPAD  

The quasi-ballistic hole transport capabilities of Ge and Si nanowire pMOSFETs were analyzed based on a quantum-corrected Boltzmann transport equation. A new formalism of quantum-correction potential was proposed, and using this model, the current drive capabilities of Ge and Si nanowire pMOSFETs were compared. Though the ON-current was larger in the Ge nanowire pMOSFET, the transmission coefficients are similar between Ge and Si, because the higher hole mobility of Ge is canceled by its slower energy relaxation. Thus, the larger current of the Ge nanowire pMOSFET was attributed to its larger injection current. The impact of device geometry on the performance was also investigated, and the [110]-oriented Ge nanowire pMOSFET with a 15 nm gate length exhibited the highest performance among the devices considered in this study.

DOI： 10.23919/SISPAD.2017.8085318

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IMFEDK 2017 - 2017 International Meeting for Future of Electron Devices, Kansai  

Hall-effect measurements for n-type and p-type GaN with low doping concentration are presented. The GaN layers were grown by metal-organic vapor phase epitaxy on hydride-vapor-phase-epitaxy-grown free-standing GaN substrates. For n-GaN, the origin of acceptor which compensating donor is not only C but also native defects for the Si doping concentration of 1016 cm-3 level. The electron mobility is mainly limited by ionized impurity scattering or polar optical phonon scattering in the temperature less or higher than 200 K, respectively. For p-GaN, lightly Mg doping of mid 1016 cm-3 was achieved, which shows the donor concentration of 3.2×1016 cm-3 and the mobility of 31 cm2/Vs at 300 K.

DOI： 10.1109/IMFEDK.2017.7998055

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IEEE Transactions on Electron Devices  

An origin of shunt current, a ledge in forward current-voltage ( ${I}$ - ${V}$ ) characteristics, in 4H-SiC mesa p-n diodes was investigated by adopting various surface passivation processes. Experimental results indicated that the shunt current path is located along the mesa sidewall and the shunt current is enlarged with increasing NO-annealing period and temperature. Based on these results, we qualitatively explain that nitrogen-related positive charges near the SiC/SiO2 interface, which are formed by postoxidation nitridation, induce band bending and lowering of the diffusion potential along the mesa sidewall, resulting in occurrence of the shunt current.

DOI： 10.1109/TED.2017.2700336

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

Phosphorus or aluminum ions were directly implanted into semi-insulating 4H-SiC substrates with no epitaxial layers to form n- or p-type layers, respectively, with doping densities in the range from 1017 to 1019 cm-3. The electrical properties of these implanted layers annealed at 1650°C were characterized by Hall effect measurements in the temperature range of 160-900K. The electrical activation ratios of implanted dopants were 88-98%. The density of compensating defects was higher in Al+-implanted layers than in P+-implanted ones. The mobilities of the implanted layers were mostly comparable to those of epitaxial layers in the doping range investigated.

DOI： 10.7567/JJAP.56.070306

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IEEE Transactions on Electron Devices  

It is empirically known that the ON-resistance (voltage) of the SiC bipolar junction transistors (BJTs) with a thin-base layer is prone to be larger than the resistance of a voltage-blocking collector layer. In this paper, we explain the mechanism of this degradation of ON-characteristics by focusing on a high base spreading resistance and a parasitic diode existing below the base contact. An equivalent circuit model of the SiC BJT was proposed, and SPICE simulation was performed.In addition, TCAD simulation confirmed the validity of the model well. Based on the model, a design criterion to avoid the unwanted increase of the ON-resistance is proposed.

DOI： 10.1109/TED.2017.2684181

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

The temperature dependence of barrier height in a Ni/n-GaN Schottky barrier diode fabricated on a GaN homoepitaxial layer was investigated by capacitance-voltage, current-voltage, and internal photoemission measurements in the range of 223-573 K. The barrier height obtained by these methods linearly decreased with increasing temperature. The temperature coefficient was %(1.7-2.3) ' 10-4 eV/K, which is about half of the temperature coefficient of the band gap reported previously. This indicates that the decrease in the barrier height may mainly reflect the shrinkage of the band gap (lowering of the conduction band edge) in GaN with increasing temperature.

DOI： 10.7567/APEX.10.051002

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

We investigated electrical characteristics of nitric oxide (NO)-annealed silicon carbide (SiC) (0001 ), (11 2 ¯ 0 ), and (1 1 ¯ 00 ) metal-oxide-semiconductor field effect transistors (MOSFETs) with heavily doped p-bodies (NA = 1 × 1017-3 × 1018 cm−3). Regardless of crystal faces or off-direction, the channel mobility decreased for higher acceptor density. We evaluated the interface state density (Dit) very near the bottom edge of 2-dimensional density of states (2D-DOS) in the conduction band of SiC from the low-temperature subthreshold slope of the MOSFETs. When the acceptor density of the p-body of the MOSFET is increased, the energy levels of 2D-DOS increase due to a stronger quantum confinement effect. Accordingly, the carriers in the heavily doped channel are influenced by the interface states located at higher energy levels. In the SiC MOS structures, the Dit values significantly increase near the conduction band edge (EC). Thus, MOSFETs on heavily doped p-bodies are affected by the higher density of Dit, leading to substantially lower mobility.

DOI： 10.1063/1.4981127

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：AIP Advances  

We found that post-oxidation Ar annealing at high temperature is effective in reducing the interface state density (Dit) near the conduction band edge (EC) of SiC (0001) MOS structures. The Dit reduction effect is comparable to that of nitridation process (annealing in nitric oxide (NO)) which has been a standard in SiC MOS technologies, without introducing any foreign atoms into the interface/oxide. The generation of fast interface states, which have been pointed out as a problem of nitridation process, is suppressed in the case of Ar annealing. In the proposed method, the final Dit values are mainly determined by the Ar annealing temperature rather than the initial oxidation temperature. The Dit values are not sensitive to the cooling speed, which means that rapid cooling is not necessary in the proposed method.

DOI： 10.1063/1.4980024

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IEEE Transactions on Electron Devices  

In this paper, ultrahigh-voltage (UHV) SiC devices with hybrid unipolar/bipolar operation are introduced and demonstrated. As the first step of such a device, a merged p-i-n Schottky (MPS) diode with an epitaxial p+-anode layer is proposed to reduce the conduction loss of a bipolar device in the low current region. A 'snapback' phenomenon is intensively investigated by analytical modeling, device simulation, and experiment and a design guideline of snapback-free hybrid operating MPS diodes is presented. Using the design guideline, snapback-free MPS diodes are fabricated and forward characteristics are investigated. By using a proper edge termination structure, a UHV SiC MPS diode with breakdown voltage of 11.3 kV is demonstrated.

DOI： 10.1109/TED.2016.2636573

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.7567/JJAP.56.031001

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IEEE Transactions on Nanotechnology  

In this paper, high-field hole transport in germanium nanowires was studied by using Boltzmann's transport equation in an atomistic framework. The scattering mechanisms taken into account are phonon and surface roughness. The hole drift velocities of [110], [111], and [112]-oriented germanium nanowires showed negative-differential characteristics, while that of the [001] nanowire did not. The behavior of hole drift velocity was analyzed based on the highly nonparabolic and orientation-dependent valence band structure. High-field hole transport properties in silicon nanowires were also calculated, and the differences between germanium and silicon nanowires were discussed, focusing mainly on momentum and energy relaxation times. The [110], [111], and [112] silicon nanowires showed faster hole drift velocity at high field than the germanium nanowires with the same orientation. This was attributed to faster energy relaxation in silicon nanowires, which mitigates the negative differential mobility in silicon nanowires compared to germanium nanowires.

DOI： 10.1109/TNANO.2016.2635110

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：AIP Advances  

Phonon frequencies of a high-quality AlN layer coherently grown on a 6H-SiC (0001) substrate are investigated by Raman scattering. Owing to the largest strain in our coherent AlN layer among heteroepitaxially grown AlN layers ever reported, phonon frequencies of the E2 (low), E2 (high), and A1 (LO) modes are considerably shifted to 244.5 (−3.3, compared with bulk AlN), 672.1 (+16.3), and 899 (+11)cm−1, respectively. Full widths at half maximum of the phonon modes in the coherent AlN are almost equal to those of high-quality bulk AlN, clearly indicating its high crystalline quality and uniform strain. We discuss accuracy of phonon deformation potentials reported by several other groups thorough comparing our experimental results.

DOI： 10.1063/1.4974500

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/IMFEDK.2017.7998055

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Publishing type：Research paper (scientific journal)  

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Philosophical Magazine  

Shockley-type stacking faults expanded in 4H–SiC epilayers induced by ultraviolet illumination were investigated using a photoluminescence imaging method, a photoluminescence mapping method and X-ray topography. After ultraviolet illumination, more than 30 patterns of Shockley-type stacking faults which expanded from perfect basal plane dislocations were observed by photoluminescence imaging. The initial basal plane dislocations were crystallographically classified, and individual shapes of expanded Shockley-type stacking faults were predicted. The correspondence between the predicted shapes and observed ones was discussed.

DOI： 10.1080/14786435.2017.1350788

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Physics: Conference Series  

The impact of how to model phonon scattering on hole transport in Si nanowires was studied based on Boltzmann's transport equation. Boundary conditions for atomistic description of phonons in nanowires and approximation by bulk acoustic and optical phonons were analyzed in terms of their impacts on high-field hole transport. The boundary conditions for phonons influence the drift velocity and momentum relaxation time, especially at low electric field, but the energy relaxation time hardly depends on the boundary conditions. The impacts by the change of boundary conditions can be approximated by the change of the strength of acoustic phonon scattering in bulk phonon picture, though the behavior of energy relaxation and distribution function of holes can not be reproduced by bulk phonon approximation.

DOI： 10.1088/1742-6596/864/1/012046

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Publishing type：Research paper (scientific journal)  

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：WiPDA 2016 - 4th IEEE Workshop on Wide Bandgap Power Devices and Applications  

We investigate an impact of annealing temperature on surface passivation of SiC epitaxial layers with deposited SiO2 followed by POCl3 annealing. The POCl3 annealing process consists of two steps: (i) thermal annealing in POCl3 and (ii) subsequent thermal annealing in pure N2. We find that the annealing temperature of the subsequent N2 annealing is important to reduce surface recombination on SiC. For surface passivation, N2 annealing at high temperature at 1000° C is necessary.

DOI： 10.1109/WiPDA.2016.7799944

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Superlattices and Microstructures  

A carbon vacancy defect is one of the most abundant point defects in SiC (as-grown, irradiated, annealed) and of technological importance because the acceptor-like level of a carbon monovacancy (Z1/2 center: EC – 0.63 eV) works as the primary carrier-lifetime killer in 4H–SiC. The carbon vacancy defects can be preferentially generated by either low-energy electron irradiation or high-temperature treatment in an inert gas ambient. On the other hand, the carbon vacancy defects can be almost eliminated by either a carbon-ion implantation process or thermal oxidation. By combination of these techniques, the density of carbon vacancy defects can be controlled in the wide range from 1011 cm−3 to 1015 cm−3 or even higher.

DOI： 10.1016/j.spmi.2016.03.029

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Energies  

Although various silicon carbide (SiC) power devices with very high blocking voltages over 10 kV have been demonstrated, basic issues associated with the device operation are still not well understood. In this paper, the promise and limitations of high-voltage SiC bipolar devices are presented, taking account of the injection-level dependence of carrier lifetimes. It is shown that the major limitation of SiC bipolar devices originates from band-to-band recombination, which becomes significant at a high-injection level. A trial of unipolar/bipolar hybrid operation to reduce power loss is introduced, and an 11 kV SiC hybrid (merged pin-Schottky) diodes is experimentally demonstrated. The fabricated diodes with an epitaxial anode exhibit much better forward characteristics than diodes with an implanted anode. The temperature dependence of forward characteristics is discussed.

DOI： 10.3390/en9110908

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.7567/JJAP.55.119201

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

The reduced effective masses (μhh-e and μlh-e) were incorrect in the original manuscript. Table I and Fig. 4 should be replaced with the table and the figure shown below. The description "using a reduced effective mass of 0.14m0, which is calculated from the masses reported for electrons and holes in GaN" in line 44, column 1, page 1 should be replaced with "using the effective masses reported for electrons and holes in GaN, which are listed in Table I", and "a reduced effective mass of GaN (μ∥ = 0.14m0) was used" in line 34, column 2, page 3 should be replaced with "calculation was done by the two-band-model using the effective masses listed in Table I". The calculations of the absorption coefficient and the photocurrent was done by the two-hole-band-model using me, mhh, and mlh. The correction does not affect the results and conclusions in this paper.

DOI： 10.7567/APEX.9.109201

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：2016 IEEE Silicon Nanoelectronics Workshop, SNW 2016  

High-field hole transport in Ge nanowires was studied using Boltzmann's transport equation. The behavior of drift velocity was analyzed based on the highly non-parabolic valence band structure. High-field hole transport properties in Si nanowires were also calculated, and the differences between Ge and Si nanowires were discussed.

DOI： 10.1109/SNW.2016.7578046

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Solid-State Electronics  

The ballistic hole transport properties in rectangular cross-sectional germanium nanowire transistors with various geometries were studied based on the “Top of the Barrier” model. Then, by an extension of this model, the quasi-ballistic hole transport was discussed taking into account phonon and surface roughness scattering in the channel and source-to-drain direct tunneling. Among several nanowire geometries targeted in this study, the [1 1 0]-oriented nanowire with large height along [11¯0] ([1 1 0]/(1 1¯ 0) NW) exhibited the largest ballistic current. This was understood from its large density of states and resulting high hole density. Large density of states, however, enhances backscattering in the channel. An approximation analysis of quasi-ballistic transport suggested that the [1 1 0]/(0 0 1) NW with higher mobility can outperform [1 1 0]/(1 1¯ 0) NW when scattering and tunneling are considered.

DOI： 10.1016/j.sse.2016.04.015

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

The photocurrent of GaN vertical Schottky barrier diodes was investigated under sub-bandgap wavelength light irradiation. Under a low reverse bias voltage, the photocurrent is induced by internal photoemission, while under a high reverse bias voltage, the photocurrent increases significantly with the bias voltage. This is due to sub-bandgap optical absorption in a depletion region due to the Franz-Keldysh effect. The voltage and wavelength dependences of the photocurrent are successfully explained quantitatively.

DOI： 10.7567/APEX.9.091002

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Device Research Conference - Conference Digest, DRC  

Wide-bandgap (WBG) semiconductors have attracted great attention as materials for the next-generation power devices since they have superior material properties compared to silicon (Si). The most advanced WBG semiconductor for power devices is silicon carbide (SiC). In 1987, the growth technology called 'step-controlled epitaxy', which enables single-phase (polytype) growth, was developed. In 1993-1994, SiC Schottky-barrier diodes (SBDs) which exceeds the Si material limit was demonstrated. In 2001, SiC SBDs were commercialized. Key technologies for SiC SBDs were edge termination to obtain an ideal breakdown voltage and a junction barrier Schottky (JBS) structure to suppress reverse leakage current. For power MOSFETs, it took longer time due to low channel mobility at SiO2/SiC and oxide reliability issues. Channel mobility was much improved by post-oxidation nitridation in NO or N2O ambient. Now, channel mobility and reliability are well controlled (balanced). SiC power MOSFETs as well as power modules with SiC MOSFETs and SiC SBDs, are commercially available. Last 5 years, the implementation of SiC devices into electronic vehicles and railway trains were extensively investigated, demonstrating a significant improvement of power efficiency.

DOI： 10.1109/DRC.2016.7548292

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：2016 Compound Semiconductor Week, CSW 2016 - Includes 28th International Conference on Indium Phosphide and Related Materials, IPRM and 43rd International Symposium on Compound Semiconductors, ISCS 2016  

Electrical characterization of low-doped n-type and p-type GaN layers grown by metal-organic vapor phase epitaxy are presented. The GaN layers were grown on hydride-vapor-phase-epitaxy-grown free-standing GaN substrates. For n-type GaN, the impact of electron trap on electrical characterization is discussed. Charging of trap in a depletion layer affects C-V characteristics. Correction is needed for depth profiling of net donor concentration. For p-type GaN, hole mobility for various Mg doping concentrations at various temperatures are presented. At 300 K, 33 cm2/Vs is obtained for p-type GaN with Mg doping concentration of mid 1016 cm-3.

DOI： 10.1109/ICIPRM.2016.7528835

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：2016 16th International Workshop on Junction Technology, IWJT 2016  

Electrical activation of implanted dopants and defect generation in SiC have been investigated. A nearly perfect (> 95%) electrical activation can be obtained including the implant tail region after annealing at 1650-1700 °C. The majority of point defects generated in implanted SiC can remarkably be reduced by thermal oxidation. The high activation ratio of implanted Al acceptors is a key factor for fabricating effective junction termination structures in high-voltage SiC devices. Recent high-quality semi-insulating SiC wafers offer the opportunity of high-temperature SiC integrated devices, which can be fabricated by only ion implantation without an epitaxial growth process.

DOI： 10.1109/IWJT.2016.7486673

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

We investigated the carrier lifetime and Z1/2 center density of thick n-type 4H-SiC epilayers, which were oxidized and subsequently annealed in Ar at high temperatures. The Z1/2 center density decreased below the detection limit in the region to, at least, a 130μm depth by thermal oxidation. After subsequent high-temperature annealing, the Z1/2 center density increased with increasing annealing temperature, while the distribution of the Z1/2 center density was nearly uniform to a 130μm depth. The carrier lifetime could be controlled from 26 to 2.4 μs by changing the annealing temperature from 1600 to 1800 °C.

DOI： 10.7567/APEX.9.061303

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

Mg-doped p-type gallium nitride (GaN) layers with doping concentrations in the range from 6.5 × 1016cm-3 (lightly doped) to 3.8 × 1019cm-3 (heavily doped) were investigated by Hall-effect measurement for the analysis of hole concentration and mobility. p-GaN was homoepitaxially grown on a GaN free-standing substrate by metalorganic vapor-phase epitaxy. The threading dislocation density of the p-GaN was 4 × 106cm-2 measured by cathodoluminescence mapping. Hall-effect measurements of p-GaN were carried out at a temperature in the range from 160 to 450 K. A low compensation ratio of less than 1-was revealed. We also obtained the depth of the Mg acceptor level of 235 meV considering the lowering effect by the Coulomb potential of ionized acceptors. The hole mobilities of 33 cm2V-1 s-1 at 300K and 72cm2V-1 s-1 at 200K were observed in lightly doped p-GaN.

DOI： 10.7567/JJAP.55.05FH03

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Physica Status Solidi (B) Basic Research  

Abstractauthoren We propose partially relaxed ultrathin GaN interlayer for strain control of AlN on SiC substrates. According to the degree of relaxation of the GaN interlayer, the lattice constant of the interlayer changes from that of SiC to that of bulk GaN, which leads to strain control of an AlN top layer grown on the interlayer. Growth of these layers is conducted by plasma-assisted molecular beam epitaxy. Before growing the interlayer, an AlN layer coherently grown on SiC is used as a template layer. The interlayers that have different degrees of relaxation are successfully obtained by changing the interlayer thickness. As a result, strain values of the AlN top layer grown on the interlayers are widely controlled from compressive (-0.53%) to tensile (+ 0.07%). TEM observation revealed the relaxation is induced by U-shaped half-loop dislocations originating from the GaN interlayer.

DOI： 10.1002/pssb.201552649

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

We investigated surface passivation on 4H-SiC epitaxial layers with deposited or thermally grown SiO2 followed by POCl3 annealing. The measured carrier lifetime in a p-type epilayer with deposited SiO2 was limited to 0.5 μs and it was improved to 3.0 μs after POCl3 annealing. In an n-type epilayer, a measured carrier lifetime of 5.8 μs was improved to 12 μs after POCl3 annealing. We found a clear relationship between the measured carrier lifetime and the interface state density at SiO2/n-SiC after POCl3 annealing, suggesting that the reduction in interface state density lowered the surface recombination velocity on the 4H-SiC.

DOI： 10.7567/APEX.9.051301

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Letters  

Interface properties of heavily Al-doped 4H-SiC (0001) (Si-face), (1120) (a-face), and (1100) (m-face) metal-oxide-semiconductor (MOS) structures were characterized from the low-temperature gate characteristics of metal-oxide-semiconductor field-effect transistors (MOSFETs). From low-temperature subthreshold slopes, interface state density (Dit) at very shallow energy levels (ET) near the conduction band edge (Ec) was evaluated. We discovered that the Dit near Ec (Ec - 0.01 eV < ET < Ec) increases in MOS structures with higher Al doping density for every crystal face (Si-, a-, and m-face). Linear correlation is observed between the channel mobility and Dit near Ec, and we concluded that the mobility drop observed in heavily doped MOSFETs is mainly caused by the increase of Dit near Ec.

DOI： 10.1063/1.4946863

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Physical Review B  

The authors present a calculation model of surface roughness scattering (SRS) in nanowires (NWs) based on atomistic description of electronic states by an sp3d5s∗ tight-binding scheme, and then this model is applied to hole transport in rectangular cross-sectional germanium (Ge) NWs. In this SRS model, the change of electronic band structures due to width or height reduction is first computed, and then it is expressed using an equivalent potential near the surface. The perturbation corresponding to a surface roughness is calculated from this equivalent potential. Using the aforementioned SRS model, hole mobility in Ge NWs was computed taking into account phonon scattering and SRS. The impacts of SRS on hole mobility in Ge NWs were analyzed, focusing on the valence band structure and hole states of NWs. The main results are as follows. At low hole density, the impacts of SRS are strongly dependent on NW geometry, and Ge NWs with high phonon-limited hole mobility, such as rectangular cross-sectional [110]-oriented NWs with large height along the [001] direction and square cross-sectional [111]-oriented NWs, tend to be less affected by SRS. At high hole density, however, the geometry dependence of hole mobility becomes weaker. These are understood from the nature of hole states and the valence band structure.

DOI： 10.1103/PhysRevB.93.155303

Web of Science

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

The Hall scattering factor (γH) in p-type 4H-SiC with various aluminum doping concentrations of 5.8 ' 1014-7.1 ' 1018cm%3 was investigated from 300 to 900 K. γH was determined by comparing the Hall coefficient with the theoretical carrier concentration derived from acceptor and donor concentrations obtained from secondary ion mab spectrometry and capacitance-voltage measurements. γH decreased with increasing temperature or doping concentration; γH = 1-0.4 for the doping concentration of 5.8 ' 1014cm%3 and γH = 0.5-0.2 for the doping concentration of 7.1 ' 1018cm%3. The dependence might be caused by the anisotropic and nonparabolic valence band structure of 4H-SiC.

DOI： 10.7567/APEX.9.041301

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Scopus

Publishing type：Research paper (scientific journal)   Publisher：Semiconductor Science and Technology  

DOI： 10.1088/0268-1242/31/3/030301

Web of Science

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

The initial Al/N ratio for AlN growth of plasma-assisted molecular-beam epitaxy without plasma stabilization is investigated. The in situ growth rate of AlN gradually increased and its temporal variation corresponded to that of nitrogen atoms, which indicated that the initial Al/N ratio was excessively Al-rich. For AlN growth, such a high-Al/N-ratio condition resulted in a three-dimensional growth mode in the initial stage of the growth, and AlN with high threading dislocation density was obtained. By controlling the initial Al/N ratio by introducing a short standby time, the resulting two-dimensional initial growth mode leads to high-quality growth of AlN.

DOI： 10.7567/APEX.9.025502

Web of Science

Scopus

Publishing type：Research paper (scientific journal)  

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Materials Science Forum  

We studied the hydrogen passivation/depassivation of four types of intrinsic defects (EI5/6, HEI7/8, HEI9/10, and P6/7) in p-type and semi-insulating 4H-SiC by means of electron spin resonance (ESR) for examining the origin of carrier-lifetime-killing defects. We suggest that the HEI7/8 and P6/7 centers are the strongest candidate for the origin of the lifetime-killing defects.

DOI： 10.4028/www.scientific.net/MSF.858.318

Scopus

Publishing type：Research paper (scientific journal)  

Web of Science

Publishing type：Research paper (scientific journal)  

Web of Science

Publishing type：Research paper (scientific journal)  

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)  

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：2014 Silicon Nanoelectronics Workshop, SNW 2014  

The phonon-limited hole mobility in rectangular cross-sectional [001], [110], [111], and [112]-oriented germanium nanowires was calculated and the hole transport characteristics were compared. The calculation revealed that [110] germanium nanowires with larger height along [001] show high hole mobility and are favorable for p-channel FETs.

DOI： 10.1109/SNW.2014.7348616

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

We investigated majority and minority carrier traps in lightly doped n-and p-type 4H-SiC epitaxial layers before and after thermal oxidation using deep level transient spectroscopy and minority carrier transient spectroscopy. We detected oxidation-induced new minority carrier traps, HO1 (EV + 0.36 eV) and HO2 (EV + 0.54 eV) for n-type 4H-SiC, and EO1 (EC % 0.59 eV) and EO2 (EC % 0.84 eV) for p-type 4H-SiC after thermal oxidation at 1300 °C. After subsequent Ar annealing at 1550 °C, the HO1, HO2, and EO1 centers disappeared, whereas the concentration of the EO2 center increased. The properties of these carrier traps are discussed.

DOI： 10.7567/APEX.8.111301

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IEEE Transactions on Electron Devices  

A temperature dependence of impact ionization coefficients in 4H-SiC was studied in a wide range of electric field toward the accurate designing of ultrahigh-voltage devices. The photomultiplication measurement was conducted for various photodiodes with different multiplication layer structures to obtain multiplication factors and ionization coefficients in a wide range of electric field strength. Especially, using multiplication layer structure with low doping concentration, the hole impact ionization coefficient was extracted at low electric field of 1 MV/cm. In high-temperature measurement, the hole ionization coefficient decreased with the increase of temperature, as observed in other semiconductor materials. For the electron ionization coefficient, however, its temperature dependence was very small and values obtained at room temperature could be used, at least up to 150 °C.

DOI： 10.1109/TED.2015.2466445

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：2015 Silicon Nanoelectronics Workshop, SNW 2015  

The hole mobility in rectangular cross-sectional germanium nanowires was calculated taking into account phonon and surface roughness scattering (SRS). SRS was modeled based on atomistic description of electronic states, and the impacts of SRS on hole mobility were analyzed.

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

Forward characteristics of ultrahigh-voltage 4H-SiC p-i-n diodes having four different n--layer (i-layer) thicknesses from 48 to 198μm were investigated in the temperature range from room temperature to 573 K. After enhancement of carrier lifetimes in i-layers, nearly ideal forward characteristics (differential on-resistance = 1.1-5.5mΩcm2 at 100A/cm2) were obtained at room temperature. The forward voltage drop decreased with temperature, which is consistent with the temperature dependence of junction voltage. The differential on-resistance exhibited a slight increase at elevated temperatures, which can mainly be ascribed to the increase in substrate resistance.

DOI： 10.7567/JJAP.54.098004

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IMFEDK 2015 - 2015 International Meeting for Future of Electron Devices, Kansai  

We fabricated 〈100〉, 〈110〉, 〈111〉, and 〈112〉 p-channel gate-all-around Si nanowire (SiNW) MOSFETs, cross sections of which are rectangles with various widths, and investigated the hole mobility of the SiNW MOSFETs using the double Lm method. Measured hole mobilities of SiNW MOSFETs were about 80-140 cm2/Vs at surface carrier density of 1 × 1013 cm-2. The dependences of the hole mobility on orientations and cross-sectional shapes of nanowires were analyzed. The orientation and geometry dependences can be explained by the band structures calculated by tight-binding approximation.

DOI： 10.1109/IMFEDK.2015.7158572

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：公益社団法人 応用物理学会  

<p>1949年，赤﨑先生が京都大学に在学中，湯川秀樹先生がノーベル物理学賞を受賞されました．若き赤﨑先生は「湯川先生のお仕事というのは雲の上のことだけれど，自分も何か小さいことでもいいから，今までにない何かをやりたい」と強く思ったそうです．そして，17年後の1966年，松下電器東京研究所の室長としてGaAsP系赤色LEDの開発を進めていた赤﨑先生は，「青がない」ことに気が付きます．それを実現することこそが「何か」だと確信し，窒化物の研究を始められました．その後，20年間におよぶ地道な研究を経て，1986年，ついに低温バッファ層を用いて高品質GaNの結晶成長に成功し，次いで1989年，GaN pn接合型青色／紫外LEDの作製を実現．今日のGaN光・電子デバイスの基盤が確立されることとなりました．赤﨑先生にノーベル賞受賞発表後の様子から，若い頃の思い出，研究の苦労と喜び，応用物理学会や若手会員へのメッセージを伺いました．</p>

DOI： 10.11470/oubutsu.84.6_492

CiNii Research

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

The temperature dependence of current gain from 140 to 460K in 4H-SiC bipolar junction transistors (SiC BJTs) was investigated. The current gain increased from 110 to 1200 with decreasing temperature from 460 to 200 K. The high current gain at the low temperature can be ascribed to the enhanced incomplete ionization of aluminum acceptors in the base layer, resulting in the increase in injection efficiency. However, when the temperature was further reduced from 200 to 140 K, the current gain decreased from 1200 to 515, which is caused by high injection condition in the base layer, because of a very low hole concentration below 200 K.

DOI： 10.7567/JJAP.54.04DP13

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Technical Digest - International Electron Devices Meeting, IEDM  

UHV (> 15 kV) SiC PiN diodes and IGBTs with improved on-state performance are presented. Through enhancement of carrier lifetime and optimization of junction termination, a breakdown voltage over 26.9 kV and a differential on-resistance of 9.7 mΩcm2 were achieved for PiN diodes. Flip-type n-channel IGBTs with a chip size of 8 mm × 8 mm exhibited a breakdown voltage over 16 kV, and 6.5 kV - 60 A switching at 250°C was demonstrated.

DOI： 10.1109/IEDM.2014.7046967

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IEEE Transactions on Electron Devices  

Interface properties of 4H-SiC (1120) and (1100) metal-oxide-semiconductor (MOS) structures annealed in nitric oxide are characterized by conductance, high-low, and C-Ψs methods. Compared with 4H-SiC (0001) MOS structures, generation of very fast interface states by nitridation is much smaller in 4H-SiC (1120) and (1100). The effective mobility of planar MOSFETs fabricated on Al+-implanted p-body doped to 1×1017 cm-3 is 103 cm2/Vs on (1100), 92 cm2/Vs on (1120), and 20 cm2/Vs on (0001). The mobility-limiting factors are discussed on the basis of experimental results. The high channel mobilities for (1120) and (1100) MOSFETs can be correlated with the lower density of fast interface states generated by nitridation.

DOI： 10.1109/TED.2014.2352117

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IEEE Transactions on Electron Devices  

Silicon carbide (SiC) p-i-n diodes having five different n--layer ( i-layer) thicknesses from 48 to 268 μm are fabricated. The forward characteristics of SiC p-i-n diodes are significantly improved by carrier-lifetime enhancement. After this improvement, the differential on-resistance is inversely proportional to the square root of current density for all the diodes with different thicknesses of n--layer. As a result, the forward current density-voltage characteristics can be approximately expressed by a parabolic function, as in the case of Si p-i-n diodes. Using a 268- μm-thick n--layer, the lifetime enhancement, and an improved space-modulated junction termination extension structure, a very high blocking voltage over 26.9 kV and low differential on-resistance of 9.7 mΩ·cm2 are achieved.

DOI： 10.1109/TED.2014.2352279

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Materials Science Forum  

We fabricated electrostatically-excited single-crystalline 4H-SiC microcantilever resonators with various thicknesses and lengths. Their resonant characteristics were investigated from room temperature (RT) up to 600°C. The resonant frequency of the cantilevers decreased with increasing temperature. From the results, the temperature dependence of Young’s modulus of single-crystalline 4H-SiC was obtained, i.e., 3% decrement with increasing temperature from RT to 600°C. The cantilevers with different thicknesses showed different temperature dependences of the quality factor. A 2-μm-thick cantilever exhibited a high quality factor (Q) (250,000) at RT and the Q decreased to 6,000 at 600°C, which can be explained by thermoelastic damping. On the other hand, a Q of a 0.45-μm-thick cantilever was still high (50,000) even at 600°C.

DOI： 10.4028/www.scientific.net/MSF.821-823.914

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Materials Science Forum  

It was discovered that the oxidation rate for SiC depended on the conduction type. The oxidation was performed for SiC(0001) with nitrogen doping (n-type) in the range from 2×1016 cm-3 to 1×1019 cm-3, and aluminum doping (p-type) in the range from 2×1015 cm-3 to 1×1019 cm-3, exhibiting a clear dependence. For n-type SiC the oxide thickness increases for higher doping density, and for p-type the thickness decreases. Note that in the case of Si oxidation, there exists very little difference of oxidation rate between the conduction types in such low doping density, and the dependence is peculiar to SiC.

DOI： 10.4028/www.scientific.net/MSF.821-823.456

Scopus

Publishing type：Research paper (scientific journal)  

Web of Science

Publishing type：Research paper (scientific journal)  

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

We calculated the phonon-limited hole mobility in rectangular cross-sectional [001], [110], [111], and [112]-oriented germanium nanowires, and the hole transport characteristics were investigated. A tight-binding approximation was used for holes, and phonons were described by a valence force field model. Then, scattering probability of holes by phonons was calculated taking account of hole-phonon interaction atomistically, and the linearized Boltzmann's transport equation was solved to calculate the hole mobility at low longitudinal field. The dependence of the hole mobility on nanowire geometry was analyzed in terms of the valence band structure of germanium nanowires, and it was found that the dependence was qualitatively reproduced by considering an average effective mass and the density of states of holes. The calculation revealed that [110] germanium nanowires with large height along the [001] direction show high hole mobility. Germanium nanowires with this geometry are also expected to exhibit high electron mobility in our previous work, and thus they are promising for complementary metal-oxide-semiconductor (CMOS) applications.

DOI： 10.1063/1.4904844

Web of Science

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

We discovered that the oxidation rate of SiC depends on the conduction type. The oxidation was faster in n-type SiC than in p-type SiC. The linear rate constant B/A was higher for n-type SiC than for p-type SiC, which may be caused by the stability of Si vacancies. The parabolic rate constant B was nearly constant for n-type SiC, whereas it decreased dramatically for heavily doped p-type SiC. Secondary-ion mass spectrometry measurements revealed that a high density of Al atoms was incorporated in the oxide of p-type SiC, which may hinder the diffusion of O2 or COx.

DOI： 10.7567/APEX.7.121301

Web of Science

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

The impacts of the ambipolar diffusion constant and surface recombination in carrier lifetime measurements of p- and n-type 4H-SiC epilayers are investigated in detail by comparing a numerical simulation based on a diffusion equation and the measurement of microwave photoconductance decay (μ-PCD) curves measured from 4H-SiC epilayers. The simulation reveals that the shapes of decay curves of excess carrier concentration in epilayers, which defines the effective carrier lifetime, are different between p- and n-type 4H-SiC under a low-level injection condition, even when the bulk lifetime and the surface recombination velocity are fixed to the same values for p- and n-type epilayers. In experiments, the shapes of the microwave photoconductance decay curves measured from p- and n-type 4H-SiC epilayers show a similar tendency to the simulation results under a low-level injection condition. This is attributed to the difference in the dependence of the ambipolar diffusion constant on the excess carrier concentration for p- and n-type 4H-SiC. The comparison of μ-PCD decay curves obtained from 50-μm-thick epilayers with different surface passivation indicates that the surface recombination velocity on the epilayer passivated with deposited SiO2 followed by NO annealing is about one order of magnitude lower than that of the epilayer passivated with the dry oxide.

DOI： 10.7567/JJAP.53.111301

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

Optical absorption coefficients of 4H- and 6H-SiC were measured at cutoff wavelengths from a value of 0 to 400-500cm-1 at various temperatures that is from room temperature to 300 °C. The redshift of the absorption edge with increasing temperature was observed. It is caused by a decrease in bandgap energy and a change in phonon occupation with increasing temperature. By considering TA-, LA-, and TO-mode phonon-assisted indirect transitions, the measured data are well fitted using a theoretical model. The obtained parameters are reported.

DOI： 10.7567/JJAP.53.108003

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Letters  

We grew AlN layers on 6H-SiC (0001) substrates with three Si-C bilayer high (0.75nm) steps. In the AlN layers, most of the threading dislocations (TDs) were arranged in rows. The TD row consisted of arrays of a half-loop dislocation, which was formed by an AlN/SiC interfacial dislocation along the step edges of the SiC substrate surfaces and a TD pair at both ends. The configuration of the interfacial dislocation was highly relevant with two-dimensional AlN nuclei at the initial stage of growth. We concluded that the half-loop dislocation arrays were generated in the AlN nucleus coalescence over the SiC step edges. © 2014 AIP Publishing LLC.

DOI： 10.1063/1.4892807

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

We investigated the enhancement of carrier lifetime in lightly Al-doped p-type 4H-SiC epilayers (NA ≃ 2 × 1014cm-3) by postgrowth processing. A carrier lifetime of 2.8 ìs in an as-grown epilayer is increased to 5.1 μs by carbon vacancy elimination, i.e., thermal oxidation at 1400 °C for 48 h. It reaches 10 μs by subsequent hydrogen annealing at 1000 °C for 10 min. The carrier lifetime in the as-grown epilayer is also increased to 4.0 μs by only hydrogen annealing. These results suggest that, in addition to carbon vacancy, there is another lifetime killer in p-type SiC, which cannot be eliminated by thermal oxidation but can be passivated by hydrogen annealing. © 2014 The Japan Society of Applied Physics.

DOI： 10.7567/APEX.7.085501

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IMFEDK 2014 - 2014 International Meeting for Future of Electron Devices, Kansai  

The conduction-type dependent thermal oxidation rate in SiC was discovered. The oxidation was performed for SiC(0001) with nitrogen doping (n-type) in the range from 2.0×1016cm-3to 1.0×1019cm-3, and aluminum doping (p-type) in the range from 2.0×1015cm-3to 1.0×1019cm-3, exhibiting a clear dependence. For n-type SiC the oxide thickness increases for higher doping density, and for p-type the thickness decreases. Note that in the case of Si oxidation, there exists very little difference of oxidation rate between the conduction types in such low doping density, and the dependence is peculiar to SiC. The authors speculate the difference originates from the difference in carrier (electron/hole) density during the oxidation, which can reasonably explain the difference in the oxidation rate between Si and SiC.

DOI： 10.1109/IMFEDK.2014.6867060

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IEEE Transactions on Electron Devices  

The phonon-limited electron mobility in rectangular cross-sectional germanium (Ge) nanowires (NWs) with various orientations was theoretically investigated. The electronic states were calculated by a tight-binding model and the phononic states were calculated by a valence force field model. Then, transition probability was calculated by Fermi's golden rule, and Boltzmann's transport equation was solved for calculating lowfield mobility. The electron mobility of Ge NWs strongly depends on the wire orientations and cross-sectional shapes, and this dependence can be explained by the conduction band structure of Ge NWs. Among several geometries investigated in this paper, [110]-oriented NWs with wider width along [001] showed the highest electron mobility at low carrier concentration, and [112] NWs with wider width along [110] showed the highest electron mobility at high carrier concentration. This result indicates that these kinds of Ge NWs are suitable as n-channel material. © 1963-2012 IEEE.

DOI： 10.1109/TED.2014.2318896

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

In this study, to reveal the origin of the Z1/2 center, a lifetime killer in n-type 4H-SiC, the concentrations of the Z1/2 center and point defects are compared in the same samples, using deep level transient spectroscopy (DLTS) and electron paramagnetic resonance (EPR). The Z1/2 concentration in the samples is varied by irradiation with 250 keV electrons with various fluences. The concentration of a single carbon vacancy (VC) measured by EPR under light illumination can well be explained with the Z1/2 concentration derived from C-V and DLTS irrespective of the doping concentration and the electron fluence, indicating that the Z1/2 center originates from a single VC. © 2014 AIP Publishing LLC.

DOI： 10.1063/1.4871076

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IEEE Electron Device Letters  

4H silicon carbide (4H-SiC) metal-insulator-semiconductor field-effect transistors (MISFETs) with 4H aluminum nitride (4H-AlN) gate insulators have been demonstrated. The 4H-AlN layers are isopolytypically grown on 4H-SiC (1120) by molecular-beam epitaxy. Gate controlled transistor operation was realized using the AlN/SiC MISFETs. The MISFETs exhibit a low gate leakage current (<10-10~A) and normally on characteristics with a threshold voltage of approximately-10~V and a field-effect mobility of 0.5 cm 2V-1s-1. Capacitance-voltage measurements of AlN/SiC MIS capacitors reveal a large negative flat band shift of-10.9~V, which is consistent with the normally on characteristics. © 2014 IEEE.

DOI： 10.1109/LED.2014.2299557

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

GaN/SiC heterojunction bipolar transistors (HBTs) with an ultrathin AlN spacer layer at the n-GaN/p-SiC emitter junction are proposed for the control of the electronic properties of GaN/SiC heterojunctions. The insertion of an AlN spacer is found to be promising in terms of improving electron injection efficiency owing to the reduced potential barrier (0.54 eV) to electron injection and reduced recombination via interface traps. We also investigated the effect of pre-irradiation of active nitrogen atoms (N*) prior to AlN growth for the control of the electronic properties of GaN/ AlN/SiC heterojunctions. We found that the potential barrier was further reduced to 0.46 eV by N* pre-irradiation. The HBT structure was successfully fabricated using our newly developed process featuring ion implantation and Pd ohmic contacts to obtain a low contact resistivity to a p-SiC base at a temperature as low as 600 °C. A fabricated HBT without an AlN layer showed a low current gain (α ∼ 0.001), whereas the GaN/ AlN/SiC HBT showed improved current gains of 0.1 in the case of using a 1-nm-thick AlN spacer without N* pre-irradiation and 0.2 in the case of using a 2-nm-thick AlN spacer with N* pre-irradiation. © 2014 The Japan Society of Applied Physics.

DOI： 10.7567/JJAP.53.034101

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

The conduction band structure and electron effective mass of GaAs nanowires with various cross-sectional shapes and orientations were calculated by two methods, a tight-binding method and an effective mass equation taking the bulk full-band structure into account. The effective mass of nanowires increases as the cross-sectional size decreases, and this increase in effective mass depends on the orientations and substrate faces of nanowires. Among [001], [110], and [111]-oriented rectangular cross-sectional GaAs nanowires, [110]-oriented nanowires with wider width along the [001] direction showed the lightest effective mass. This dependence originates from the anisotropy of the Γ valley of bulk GaAs. The relationship between effective mass and bulk band structure is discussed. © 2014 AIP Publishing LLC.

DOI： 10.1063/1.4864490

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

Dislocations in n- and p-type substrates as well as in epitaxial layers (epilayers) were clearly identified using a photoluminescence (PL) imaging technique. Dislocations in epilayers show large/small bright spots or lines in infrared PL images, which correspond to threading screw/edge dislocations (TSDs/TEDs) or basal plane dislocations (BPDs), respectively. In contrast, dislocations in substrates exhibit large/small dark spots or dark lines in infrared PL images, corresponding to TSDs/TEDs or BPDs, respectively. These different features (bright/dark contrast) of dislocations may originate from the different densities of point defects or impurities. © 2014 The Japan Society of Applied Physics.

DOI： 10.7567/JJAP.53.020304

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

The etching-limiting step in slow Si etching with HCl/H2 at atmospheric pressure was investigated. The etching was performed at a low etching rate below 10nm/min in the temperature range of 1000-1100 °C. In the case of bare Si etching, it was confirmed that the etching rate showed little temperature dependence and was proportional to the equilibrium pressure of the etching by-product SiCl2 calculated by thermochemical analysis. In addition, the etching rates of Si(100) and (110) faces were almost the same. These results indicate that SiCl2 diffusion in the gas phase is the rate-limiting step. In the etching of the Si surface with SiO2 mask patterns, a strong loading effect (mask/opening pattern dependence of the etching rate) was observed. The simulation of the diffusion of gas species immediately above the Si surface revealed that the loading effect was attributed to the pattern-dependent diffusion of SiCl2.© 2014 The Japan Society of Applied Physics.

DOI： 10.7567/JJAP.53.016502

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Publishing type：Research paper (scientific journal)  

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Materials Science Forum  

We fabricated electrostatically actuated single-crystalline 4H-SiC microcantilever resonators. To realize a narrow gap between cantilevers and substrate, we etched a thin p-type SiC layer in n/p/n multilayer structure by doping-selective electrochemical etching. The resonant characteristics of the fabricated 4H-SiC microcantilevers were investigated under a vacuum condition. Electrostatic actuation of microcantilevers was successfully performed by applying 10 mVrms ac voltage with 20 mV dc bias. The quality factor of 4H-SiC microcantilevers was above 100,000, which is about ten times higher than the quality factor of Si cantilevers with the same structure. Resonant characteristics were almost identical for mechanical actuation and electrostatic actuation. © (2014) Trans Tech Publications, Switzerland.

DOI： 10.4028/www.scientific.net/MSF.778-780.780

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Materials Science Forum  

In freestanding n-type 4H-SiC epilayers irradiated with low-energy (250 keV) electrons at room temperature, the electron paramagnetic resonance (EPR) spectrum of the negative carbon vacancy at the hexagonal site, V-C(h), and a new signal were observed. From the similarity in defect formation and the spin-Hamiltonian parameters of the two defects, the new center is suggested to be the negative C vacancy at the quasi-cubic site, V-C (k). The identification is further supported by hyperfine calculations. © (2014) Trans Tech Publications, Switzerland.

DOI： 10.4028/www.scientific.net/MSF.778-780.285

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：2014 International Workshop on Junction Technology, IWJT 2014  

Silicon carbide (SiC) is a newly-emerging wide bandgap semiconductor, by which high-voltage, low-loss power devices can be realized owing to its superior properties. Because of its strong bonding energy and thermal stability, however, special cares must be paid to form high-quality junctions by ion implantation. This paper reviews present status and remaining issues of ion implantation technology in SiC. Requirements of hot implantation and high-temperature annealing are discussed in terms of electrical activation, defect generation, and junction characteristics. Furthermore, recent progress in junction termination for high-voltage SiC devices by using ion implantation is described. © 2014 IEEE.

DOI： 10.1109/IWJT.2014.6842018

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Publishing type：Research paper (scientific journal)  

Web of Science

Publishing type：Research paper (scientific journal)  

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Proceedings of 2014 4th IEEE International Workshop on Low Temperature Bonding for 3D Integration, LTB-3D 2014  

We have developed 100mm in diameter 4H-SiC/poly-SiC bonded wafer by SAB method. The SiC bonded wafer demonstrated an excellent thermal stability against device processing temperature. SBDs fabricated on the SiC bonded wafer exhibited good I-V characteristics. These results suggest that it is a promising alternative wafer for SiC power device. © 2014 IEEE.

DOI： 10.1109/LTB-3D.2014.6886195

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)  

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)  

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Physical Review B - Condensed Matter and Materials Physics  

The carbon vacancy (VC) has been suggested by different studies to be involved in the Z1/Z2 defect-a carrier lifetime killer in SiC. However, the correlation between the Z1/Z2 deep level with VC is not possible since only the negative carbon vacancy (VC-) at the hexagonal site, VC-(h), with unclear negative-U behaviors was identified by electron paramagnetic resonance (EPR). Using freestanding n-type 4H-SiC epilayers irradiated with low energy (250 keV) electrons at room temperature to introduce mainly VC and defects in the C sublattice, we observed the strong EPR signals of VC-(h) and another S = 1/2 center. Electron paramagnetic resonance experiments show a negative-U behavior of the two centers and their similar symmetry lowering from C3v to C1h at low temperatures. Comparing the 29Si and 13C ligand hyperfine constants observed by EPR and first principles calculations, the new center is identified as VC-(k). The negative-U behavior is further confirmed by large scale density functional theory supercell calculations using different charge correction schemes. The results support the identification of the lifetime limiting Z1/Z2 defect to be related to acceptor states of the carbon vacancy. © 2013 American Physical Society.

DOI： 10.1103/PhysRevB.88.235209

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

The growth, electrical characterization, and electroluminescence (EL) of GaN/SiC heterojunction bipolar transistors (HBTs) are presented. GaN grown on off-axis SiC by molecular beam epitaxy showed step bunching owing to the large off-angle of SiC substrates, which contributed to the annihilation of edge dislocations. We investigated the impact of base doping concentration and SiC polytype (4H and 6H) on the characteristics of GaN/SiC heterojunction diodes. By utilizing a reduced doping concentration of 1 × 1018 cm -3 instead of 1 × 1019 cm-3, we suppressed the tunneling current via interface traps, resulting in an improved rectifying behavior in the diodes. Capacitance-voltage (C-V) and EL characteristics revealed that the band lineup of GaN/SiC is of type II, and 6H-SiC is better for electron injection. In accordance with diode characteristics, the fabricated GaN/SiC HBTs showed an improved common-base current gain of 0.03 by employing a reduced base doping concentration of 1 × 1018 cm-3 and 6H-SiC, whereas a current gain below 1 × 10-4 was obtained in the HBTs with a base doping concentration of 1 × 1019 cm-3. © 2013 The Japan Society of Applied Physics.

DOI： 10.7567/JJAP.52.124102

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

Carrier lifetimes in a highly Al-doped p-type epilayer (NA = 1 × 1018 cm-3) are investigated by differential microwave photoconductance decay (μ-PCD) measurements. A carrier lifetime of 310 ns in the as-grown p-type epilayer decreases to 90 ns by thermal treatment in Ar, O2, or N2 atmospheres (>700 °C), and recovers to 300 ns by H2 annealing (>750 °C). Hydrogen is detected at a concentration of (2-3) × 1015 cm-3 in the H2-annealed epilayer. These results suggest that a lifetime killer exists in the p-type epilayer, limiting the carrier lifetime to 90 ns and is passivated by hydrogen annealing, resulting in the improved carrier lifetime of 300 ns. © 2013 The Japan Society of Applied Physics.

DOI： 10.7567/APEX.6.121301

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IMFEDK 2013 - 2013 International Meeting for Future of Electron Devices, Kansai  

The conduction band structure of GaAs nanowires with various cross-sectional shapes and orientations was calculated by a tight-binding model and an effective mass equation taking the bulk full-band structure into account. The effective mass of nanowires increases as the cross-sectional size decreases. However, the amount of the increase in mass is strongly dependent on the wire orientations and substrate faces of nanowires, which originates from the anisotropy of Γ valley of bulk GaAs. © 2013 IEEE.

DOI： 10.1109/IMFEDK.2013.6602267

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IEEE Transactions on Electron Devices  

Growth and electrical characterization of aluminum gallium nitride (AlGaN)/SiC heterojunction bipolar transistors (HBTs) featuring AlN/GaN short-period superlattice as a quasi-AlGaN emitter are presented. The AlN/GaN superlattice emitter was grown by molecular beam epitaxy on off-axis SiC, which showed adequate structural and electronic properties as the emitter of the HBTs. We investigated the impact of Al composition in the emitter on the transport characteristics and current gain of the HBTs. Using Al composition of over 0.5, we achieved type-I band alignment in AlGaN/SiC, and suppressed the tunneling current via interface traps, resulting in an improved current gain of up to 2.7. Toward further improvement of current gain, we also investigated the effect of n-SiC spacer between n-AlGaN and p-SiC and p-SiC base width. Using 200-nm-thick n-SiC spacer and 250-nm-thick p-SiC base layer, we achieved an improved current gain of 13 owing to the reduced interface and bulk recombination. © 1963-2012 IEEE.

DOI： 10.1109/TED.2013.2273499

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Sensors and Actuators, A: Physical  

Single-crystalline 4H-SiC micro cantilevers were fabricated by doping-type selective electrochemical etching of 4H-SiC. Using this method, n-type 4H-SiC cantilevers were fabricated on a p-type 4H-SiC substrate, and resonance characteristics of the fabricated 4H-SiC cantilevers were investigated under a vacuum condition. The resonant frequencies agreed very well with the results of numerical simulations. The maximum quality factor in first-mode resonance of the 4H-SiC cantilevers was 230,000. This is 10 times higher than the quality factor of conventional 3C-SiC cantilevers fabricated on an Si substrate. © 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.sna.2013.04.014

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

To obtain a high-crystalline-quality AlN/GaN short-period superlattice with higher average GaN mole fraction, the effects of the thicknesses of AlN barrier and GaN well layers on the superlattice growth were investigated. Coherent growth with an average GaN mole fraction of 20% was realized by reducing the AlN barrier layer thickness to 8 bilayers (BL) while keeping the GaN well layer thickness at 2 BL. Further reduction in the AlN barrier layer thickness resulted in lattice relaxation and degradation of the crystalline quality. Grown layers with various well and barrier thicknesses were investigated by transmission electron microscopy, X-ray diffraction, and atomic force microscopy. © 2013 The Japan Society of Applied Physics.

DOI： 10.7567/JJAP.52.08JE21

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

Ultrahigh-voltage SiC PiN diodes with an improved junction termination extension (JTE) structure and improved forward characteristics are presented in this paper. An improved space-modulated JTE (SM-JTE) structure was designed by device simulation, and a breakdown voltage of over 17 kV was obtained in a wider range of JTE dose with the improved SM-JTE. In addition, a lifetime enhancement process (thermal oxidation) was performed to improve the forward characteristics. The on-resistance of the SiC PiN diodes with the lifetime enhancement process was reduced to 13m°Cm2 at 150°C compared with that of the SiC PiN diodes with the conventional process (32m°Cm2). © 2013 The Japan Society of Applied Physics.

DOI： 10.7567/JJAP.52.070204

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

The optical properties of wurtzite AlN under large compressive strain are investigated by photoluminescence and optical reflectivity measurements with two different geometries. The AlN layer was coherently grown on 6H-SiC(0001), resulting in strains of ∈xx = ∈yy = -9.6 × 10-3 and ∈zz = 5.12 × 10-3, as confirmed by high-resolution X-ray diffraction. Free exciton transitions were clearly observed. The transition energy of A free exciton (with Γ1 symmetry) was estimated to be 6.246 eV at 10 K. The large energy shift of the free exciton transition with respect to the transition in unstrained AlN was well explained by the reported deformation potentials of AlN. © 2013 The Japan Society of Applied Physics.

DOI： 10.7567/APEX.6.062604

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

The Z1/2 center (EC - 0:67 eV), which is a lifetime killer in n-type 4H-SiC epilayers, is reduced by thermal oxidation. The oxidation, however, simultaneously generates other deep levels: ON1 (E C - 0:84 eV) and ON2 (EC - 1:1 eV) centers. From the behaviors (generation condition, thermal stability, and change in the depth profiles) of the ON1 and ON2 centers in samples (i) oxidized in O2, (ii) implanted with C+ or Si+ atoms, and (iii) oxidized in N2O (or NO), we suggest that these defects may originate from the same defect in different charge states, related to both carbon interstitials and N atoms. © 2013 The Japan Society of Applied Physics.

DOI： 10.7567/APEX.6.051301

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Letters  

The Z1/2 center in n-type 4H-SiC epilayers-a dominant deep level limiting the carrier lifetime-has been investigated. Using capacitance versus voltage (C-V) measurements and deep level transient spectroscopy (DLTS), we show that the Z1/2 center is responsible for the carrier compensation in n-type 4H-SiC epilayers irradiated by low-energy (250 keV) electrons. The concentration of the Z1/2 defect obtained by C-V and DLTS correlates well with that of the carbon vacancy (VC) determined by electron paramagnetic resonance, suggesting that the Z1/2 deep level originates from VC. © 2013 American Institute of Physics.

DOI： 10.1063/1.4796141

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IEEE Transactions on Electron Devices  

Effects of nitric oxide (NO) and nitrous oxide (N2O) annealing on 4H-SiCmetal-oxide-semiconductor field-effect transistors (MOSFETs) fabricated on the (0001), (000-1), and (11-20) faces are investigated. MOSFETs on (11-20) exhibited high channel mobility (108 cm2/V s) compared to those on (0001) (29-37 cm2/V s) and (000-1) (39-46 cm2/V s). The MOSFET characteristics are discussed in terms of oxidation taking place during the nitridation annealing and crystal faces.

DOI： 10.1109/TED.2012.2236333

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IEEE Transactions on Electron Devices  

The electron transport properties of square and rectangular cross-sectional germanium nanowire (GeNW) field-effect transistors (FETs) with [001], [110], [111], and [112] crystal orientations are investigated. The electronic states of GeNWs are calculated by using an sp^{3}d^{5}s\ast tight-binding model coupled to a Poisson equation self-consistently. A semiclassical ballistic FET model is used to evaluate the electron transport characteristics. For the square cross section, electron injection velocity dominates the drive current in GeNW FETs because the inversion electron density in the GeNW channels is mainly determined by the capacitance of the gate insulator, and a [110] GeNW FET achieves the highest drive current of all the orientations. In the case of rectangular cross section, the electron density in GeNWs is dependent on their orientations and cross-sectional geometries due to the small quantum capacitance, and the difference of the density of states of GeNWs significantly affects the drive current. A [112] GeNW FET on a (\hbox{1}\bar{\hbox{1}}\hbox{0} ) face exhibits the highest injection velocity of all the calculated FETs but low drive current because of its insufficient density of states. As a result, a [110] GeNW FET on a (001) face, which has both large density of states and high injection velocity, achieves the highest drive current. © 1963-2012 IEEE.

DOI： 10.1109/TED.2013.2237779

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

Thermal oxidation is an effective method to reduce deep levels, especially the Z1/2-center (EC - 0.67 eV), which strongly suppresses carrier lifetimes in n-type 4H-SiC epilayers. The oxidation, however, simultaneously generates other deep levels, HK0 (EV + 0.79 eV) and HK2 (EV + 0.98 eV) centers, within the lower half of the bandgap of SiC, where the HK0 center is a dominant deep level with a concentration of about 1 × 1013 cm-3 after oxidation. By comparing deep levels observed in three sets of p-type 4H-SiC: oxidized, electron-irradiated, and C+- or Si+-implanted samples, we find that the HK0 and HK2 centers are complexes including carbon interstitials such as the di-carbon interstitial or di-carbon antisite. Other defects observed in p-type 4H-SiC after electron irradiation or after C+/Si+ implantation are also studied. © 2013 American Institute of Physics.

DOI： 10.1063/1.4776240

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

We investigated the photoconductivity decay characteristics of p-type 4H-SiC bulk crystals by differential microwave photoconductance decay (μ-PCD) measurements using a 349-nm laser as an excitation source. The decay time at room temperature was 2600 μs, which is much longer than that of n-type 4H-SiC bulk crystals (40 ns). Decay time decreased with increasing temperature, resulting in 120 μs at 250 °C, and the activation energy of decay time was determined to be 140 ± 10 meV. Long decay characteristics were also observed by below-band-gap excitation at 523 nm. © 2013 The Japan Society of Applied Physics.

DOI： 10.7567/JJAP.52.010202

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Extended Abstracts of the 13th International Workshop on Junction Technology 2013, IWJT 2013  

In electric power conversion systems of power infrastructures, electric vehicles, and power supplies, Si-based power semiconductor devices are employed as a key hardware. Reduction of power dissipation in the conversion systems is strongly required for energy saving. In particular, ultrahigh-voltage power converters with high efficiency are essential to realize a stable and highly efficient electric power network by optimizing the use of solar power and wind-generated power in the future. The efficiency of power converters/inverters strongly relies on the performance of power semiconductor devices employed in the power electronic systems. Silicon carbide (SiC) is a newly-emerging wide bandgap semiconductor, by which high-voltage, low-loss power devices can be realized owing to its superior properties [1-3]. The major features of SiC power devices include high-voltage blocking capability, low on-state resistance, fast switching speed, and high-temperature operation. © 2013 IEEE.

DOI： 10.1109/IWJT.2013.6644492

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Publishing type：Research paper (scientific journal)  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IEEE Electron Device Letters  

We report here 20-kV-class small-area (0.035 mm 2 ) 4H-SiC bipolar junction transistors. We implemented edge termination techniques featuring two-zone junction termination extension and space-modulated rings. on-state characteristics showed a current gain of 63 and a specific on-resistance of 321 mΩ cm 2, which is slightly below the SiC unipolar limit. We achieved the open-base blocking voltage of 21 kV at a leakage current of 0.1 mA/cm 2 , which is the highest blocking voltage among any semiconductor switching devices. © 2012 IEEE.

DOI： 10.1109/LED.2012.2215004

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

The temperature dependence of the refractive indices of 4H-SiC, GaN, and AlN were determined in a wavelength range from the near band edge (392 nm for 4H-SiC, 367nm for GaN, and 217nm for AlN) to infrared (1700 nm) and a temperature range from room temperature to 500 °C. Optical interference measurements with vertical incidence along a c-axis configuration were employed to precisely evaluate ordinary refractive indices. Near the band-edge region, the temperature dependence of the refractive index mainly originates from the temperature change of the bandgap. At 450 nm, the thermo-optic coefficients of 4H-SiC, GaN, and AlN were measured to be 7:8 × 10 -5, 1:6 × 10 -4, and 3:6 × 10 -5 K -1, respectively. © 2012 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.51.112101

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Physical Review Letters  

Using electron paramagnetic resonance (EPR), energy levels of the carbon vacancy (V C) in 4H-SiC and its negative-U properties have been determined. Combining EPR and deep-level transient spectroscopy we show that the two most common defects in as-grown 4H-SiC-the Z 1/2 lifetime-limiting defect and the EH 7 deep defect-are related to the double acceptor (2-|0) and single donor (0|+) levels of V C, respectively. © 2012 American Physical Society.

DOI： 10.1103/PhysRevLett.109.187603

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：2012 IEEE Silicon Nanoelectronics Workshop, SNW 2012  

We calculated the conduction band structure of GeNWs by a tight-binding model and obtained the fundamental understanding of electron transport characteristics in [001], [110], [111], and [112] GeNW FETs. The simulation of ballistic electron transport revealed that [110] GeNW FETs on the (001) face achieve high drive current as well as high injection velocity, being the best choice for n-channel FETs. © 2012 IEEE.

DOI： 10.1109/SNW.2012.6243360

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IEEE Transactions on Electron Devices  

15-kV-class 4H-SiC PiN diodes with various junction termination structures have been experimentally investigated. Employment of the space-modulated junction termination extension (SM-JTE) and the two-zone JTE have realized a breakdown voltage over 15 kV, corresponding to 93% of the parallel-plane breakdown voltage. The window of the implanted JTE dose to achieve the ultrahigh voltage has been enlarged, which indicates the robustness to the deviation of effective JTE dose. From the comparison of the experimental JTE-dose dependence of breakdown voltage with the numerical device simulation, a shift toward the heavier JTE-dose region was observed. To explain the phenomenon, effects of the charges at the SiO 2/SiC interface are discussed. © 2012 IEEE.

DOI： 10.1109/TED.2012.2210044

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

A longest carrier lifetime of 33.2 μs was achieved by eliminating the Z 1/2 center via thermal oxidation at 1400 °C for 48 h and subsequent surface passivation with a nitrided oxide on a 220-μm-thick n-type 4H-SiC epilayer. By deep-level elimination, photoluminescence (PL) in the infrared region (wavelength: 700-950 nm) was remarkably enhanced at locations of threading dislocations. A threading screw dislocation exhibited much stronger infrared PL than a threading edge dislocation. The present results indicate that carrier recombination at extended defects becomes pronounced through the elimination of the Z 1/2 center in the epilayers. © 2012 The Japan Society of Applied Physics.

DOI： 10.1143/APEX.5.101301

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

We report on the critical thickness of AlN on SiC(0001). AlN was directly grown on 6H-SiC(0001) at 650 °C by rf-plasma-assisted molecular beam epitaxy. The growth layer had a relatively low threading dislocation density of 4 × 10 8-4 × 10 9 cm -2. Although the critical thickness of AlN on SiC(0001) is estimated to be 3.5nm using a Matthews-Blakeslee model, the critical thickness in our experiment was over 700 nm. Low dislocation density, a layer-by-layer growth mode, and low growth temperature may contribute to such a large critical thickness. Sharp and intense free exciton emission was observed in low-temperature PL measurements of the AlN layer. © 2012 The Japan Society of Applied Physics.

DOI： 10.1143/APEX.5.105502

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

Enhancement and control of carrier lifetimes in p-type 4H-SiC have been investigated. In this study, thermal oxidation and carbon ion implantation methods, both of which are effective for lifetime enhancement in n-type SiC, were attempted on 147-μm thick p-type 4H-SiC epilayers. Effects of surface passivation on carrier lifetimes were also investigated. The carrier lifetimes in p-type SiC could be enhanced from 0.9 μs (as-grown) to 2.6 μs by either thermal oxidation or carbon implantation and subsequent Ar annealing, although the improvement effect for the p-type epilayers was smaller than that for the n-type epilayers. After the lifetime enhancement, electron irradiation was performed to control the carrier lifetime. The distribution of carrier lifetimes in each irradiated region was rather uniform, along with successful lifetime control in the p-type epilayer in the range from 0.1 to 1.6 μs. © 2012 American Institute of Physics.

DOI： 10.1063/1.4748315

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

4H-SiC pn photodiodes were fabricated and the temperature dependence of the photoresponse was measured for various wavelengths (280-365 nm) as a function of reverse-bias voltage. A temperature-independent photoresponse was obtained at 280nm illumination from room temperature to 300 °C under zero-bias condition. By applying reverse-bias voltage up to 150 V, the wavelength of the temperature-independent photoresponse was varied from 280 to 300 nm. The temperature-independent photoresponse was explained by the temperature dependence of optical absorption coefficient together with surface recombination effect. © 2012 The Japan Society of Applied Physics.

DOI： 10.1143/APEX.5.094101

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Proceedings of the International Symposium on Power Semiconductor Devices and ICs  

Ultrahigh-voltage 4H-SiC PiN diodes with improved junction termination extension (JTE) structures have been investigated. Breakdown characteristics of 4H-SiC PiN diodes with conventional single-zone JTE was shown to be severely affected by the charge near the SiO 2/SiC interface from experiment and device simulation. Taking the effect of the interface charge into account, and by using "Space-Modulated" JTE structure with a wide optimum JTE-dose window to tolerate the impact of interface charge, we achieved a breakdown voltage of 21.7 kV (81 % of the ideal breakdown voltage calculated from the epilayer structure), which is the highest breakdown voltage among any semiconductor devices ever reported. © 2012 IEEE.

DOI： 10.1109/ISPSD.2012.6229101

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IMFEDK 2012 - 2012 International Meeting for Future of Electron Devices, Kansai  

15 kV-class 4H-SiC PiN diodes with various junction termination extension (JTE) structures have been experimentally investigated. JTE-dose dependence of the breakdown voltage for conventional single and two-zone JTE showed a narrow window of optimum JTE-dose to obtain high breakdown voltage. To widen this window, space-modulated JTE (SM-JTE) was introduced. 4H-SiC PiN diodes with SM-JTE showed a highest breakdown voltage of 15 kV, and a widening of the optimum JTE-dose window to obtain ultrahigh-voltage was realized at the same time. © 2012 IEEE.

DOI： 10.1109/IMFEDK.2012.6218579

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

Ultrahigh-voltage 4H-SiC mesa PiN diodes are fabricated and characterized. An original space-modulated two-zone junction termination extension (SM-two-zone JTE) has realized a laterally tapered profile of the JTE dose, which enlarged the tolerance to the deviation of effective JTE dose compared with a conventional two-zone JTE. We demonstrate a SiC PiN diode with a breakdown voltage of 21.7 kV (81% of the ideal breakdown voltage calculated from the epilayer structure), which is the highest breakdown voltage among any semiconductor devices ever reported. © 2012 The Japan Society of Applied Physics.

DOI： 10.1143/APEX.5.064001

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

Lattice parameters of high-dose ion-implanted 4H-SiC were investigated with reciprocal space mapping (RSM). N, P, Al, or (CSi) ions were implanted into lightly doped epilayers to form a (330-520) nm-deep box profile with concentrations of 10 19-10 20 atoms/cm 3. After activation annealing at 1800 °C, RSM measurements were conducted. The RSM images for (0008) reflection revealed that high-dose ion implantation causes c-lattice expansion in implanted layers, irrespective of ion species. In addition, crystallographic tilt was observed after high-dose ion implantation. The tilt direction is the same for all the samples investigated; the c-axis of the implanted layers is inclined toward the ascending direction of the off-cut. The c-lattice mismatch and the tilt angle increased as the implantation dose increases, indicating that the implantation damage is responsible for the lattice parameter change. From these results and transmission electron microscopy observation, the authors conclude that the c-lattice mismatch and the crystallographic tilt are mainly caused by secondary defects formed after the ion-implantation and activation-annealing process. © 2012 American Institute of Physics.

DOI： 10.1063/1.4720435

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

We demonstrate the coherent growth of AIN/GaN short-period superlattice (SPSL) on 6H-SiC(0001) substrates by molecular beam epitaxy. A high-quality 5-nm-thick AIN layer was grown on SiC as a template layer, followed by the growth of AIN (12BL)/GaN (2BL) SPSL, which consists of 40 periods (total thickness: 140nm). The SPSL was coherently grown on SiC, and its threading dislocation density (TDD) was as low as 8 × 10 8cm -2. The SPSL, which had 3-BL-thick GaN layers, was relaxed, and the TDD increased to 8 × 10 10cm -2. © 2012 The Japan Society of Applied Physics.

DOI： 10.1143/APEX.5.051002

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

The current transport characteristics of quasi-Al xGa 1-xN/SiC heterojunction bipolar transistors (HBTs) with various band discontinuities were investigated in a low-current range using a Gummel plot. In the low-current range, the base currents of the HBTs were dominated by recombination currents. The collector current characteristics of the HBTs in the low-current range were almost the same in spite of the various band discontinuities, and the ideality factor n was 1.0. The band discontinuities at the heterojunction had no effect on electron injection in the lowcurrent range. This is because the collector currents were dominated by diffusion process in the base region rather than by injection process at the AlGaN/SiC interface. © 2012 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.51.04DP09

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

Two trap-reduction processes, thermal oxidation and C + implantation followed by Ar annealing, have been discovered, being effective ways for reducing the Z 12 center (E C - 0.67 eV), which is a lifetime killer in n-type 4H-SiC. In this study, it is shown that new deep levels are generated by the trap-reduction processes in parallel with the reduction of the Z 12 center. A comparison of defect behaviors (reduction, generation, and change of the depth profile) for the two trap-reduction processes shows that the reduction of deep levels by thermal oxidation can be explained by an interstitial diffusion model. Prediction of the defect distributions after oxidation was achieved by a numerical calculation based on a diffusion equation, in which interstitials generated at the SiO 2SiC interface diffuse to the SiC bulk and occupy vacancies related to the origin of the Z 12 center. The prediction based on the proposed analytical model is mostly valid for SiC after oxidation at any temperature, for any oxidation time, and any initial Z 12-concentration. Based on the results, the authors experimentally achieved the elimination of the Z 12 center to a depth of about 90 μm in the sample with a relatively high initial-Z 12-concentration of 10 13 cm -3 by thermal oxidation at 1400°C for 16.5 h. Furthermore, prediction of carrier lifetimes in SiC from the Z 12 profiles was realized through calculation based on a diffusion equation, which considers excited-carrier diffusion and recombination in the epilayer, in the substrate, and at the surface. © 2012 American Institute of Physics.

DOI： 10.1063/1.3692766

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

In this paper, the impact of high-temperature annealing of 4H silicon carbide (SiC) on the formation of intrinsic defects, such as Z 1/2 and EH 6/7, and on carrier lifetimes was studied. Four nitrogen-doped epitaxial layers with various initial concentrations of the Z 1/2- and EH 6/7-centers (10 11 - 10 14 cm -3) were investigated by means of deep level transient spectroscopy and microwave photoconductance decay. It turned out that the high-temperature annealing leads to a monotone increase of the Z 1/2- and EH 6/7- concentration starting at temperatures between 1600°C and 1750°C, depending on the initial defect concentration. In the case of samples with high initial defect concentration (10 14 cm -3) a distinct decrease in Z 1/2- and EH 6/7-concentration in the temperature range from 1600°C to 1750°C was observed, being consistent with previous reports. For higher annealing temperatures (T anneal 1750°C), the defect concentration is independent of the samples' initial values. As a consequence, beside the growth conditions, such as C/Si ratio, the thermal post-growth processing has a severe impact on carrier lifetimes, which are strongly reduced for samples annealed at high temperatures. © 2012 American Institute of Physics.

DOI： 10.1063/1.3681806

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

200-nm-thick N-polar AlN layers were grown on 6H-SiC(0001̄) substrates with 6-bilayer-high steps by molecular-beam epitaxy. During N-polar AlN growth, multinucleation growth occured easily, increasing the surface roughness of AlN. By reducing supersaturation (nucleation probability), the surface roughness was improved. The FWHMs of (0002) and (011̄2̄) ω-scan diffraction peaks of the AlN layer were 120 and 210 arcsec, respectively. The formation of stacking-mismatch boundaries (SMBs) was successfully suppressed by step-height control of the SiC substrate and the initial layer-by-layer growth. Most of the threading dislocations (TDs) were generated at the step edges of the SiC surfaces. The density of TDs in the AlN layers was 2 × 10 9 cm -2. © 2012 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.51.02BH02

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IEEE Transactions on Electron Devices  

An edge termination method, referred to as space-modulated junction termination extension (SMJTE) combined with a mesa structure, is presented for ultrahigh-voltage p-i-n diodes in 4H-SiC. Numerical device simulations have been performed for over 15-kV-class 4H-SiC p-i-n diodes with the proposed edge termination. The structure exhibits a high breakdown capability with an improved tolerance for the deviation of impurity dose in the JTE region. Unlike conventional multi-implantation, the proposed termination technique utilizes a single-step implantation with a single mask. A desired laterally tapered doping profile is achieved by fragmenting a conventional JTE region using relatively wide spaces. The simple process of the proposed edge termination makes it applicable to fabrication of various high-voltage devices in 4H-SiC. © 2006 IEEE.

DOI： 10.1109/TED.2011.2175486

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Publishing type：Research paper (scientific journal)  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：ECS Transactions  

Fast epitaxial growth, defect reduction, device designing, and process development in SiC toward ultrahigh-voltage (> 10 kV) bipolar devices are investigated. 100∼200μ-thick 4H-SiC epilayers with a low background doping concentration in the low 1013 cm-3 can be grown at a growth rate greater than 50μ/h. Impacts of extended defects on carrier recombination are clarified in photoluminescence mapping measurements. Generation and reduction of Z1/2 center, the dominant lifetime killer, are summarized. After Z1/2 elimination by thermal oxidation at 1400°C, the carrier lifetime can be enhanced to 25 s or even longer. By utilizing space-modulated junction termination extension, a 21.7 kV PiN diode is demonstrated. Through unique process development, the current gain in bipolar junction transistors is increased to 250∼330. © The Electrochemical Society.

DOI： 10.1149/05003.0025ecst

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Materials Science Forum  

Breakdown characteristics of 4H-SiC PiN diodes with various JTE structures have been investigated. By combining two-zone JTE and Space-Modulated JTE (SM-JTE), a breakdown voltage over 15 kV, corresponding to about 93 % of the parallel-plane breakdown voltage, was realized. The window of optimum JTE dose to obtain high breakdown voltage was widened, which indicates the robustness to the deviation of JTE dose. By comparing the breakdown voltage obtained by simulation and experimental results, impacts of the charge near the SiO 2/SiC interface are discussed. © (2012) Trans Tech Publications.

DOI： 10.4028/www.scientific.net/MSF.717-720.973

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Materials Science Forum  

We demonstrate 4H-SiC bipolar junction transistors (BJTs) with an enhanced current gain over 250. High current gain was achieved by utilizing optimized device geometry as well as optimized surface passivation, continuous epitaxial growth of the emitter-base junction, combined with an intentional deep-level-reduction process based on thermal oxidation to improve the lifetime in p-SiC base. We achieved a maximum current gain (β) of 257 at room temperature and 127 at 250°C for 4H-SiC BJTs fabricated on the (0001)Si-face. The gain of 257 is twice as large as the previous record gain. We also demonstrate BJTs on the (000-1)C-face that showed the highest β of 439 among the SiC BJTs ever reported. © (2012) Trans Tech Publications.

DOI： 10.4028/www.scientific.net/MSF.717-720.1117

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Materials Science Forum  

It has been clarified that the Z1/2 center, a well known deep level as a lifetime killer, can be reduced to the concentration below 10 11 cm-3 by thermal oxidation or C+ implantation plus Ar annealing. In this study, the authors investigate the trap-reduction phenomena systematically (experimentally), and propose a model to analyze the phenomena. Furthermore, prediction of the defect distributions is realized by solving a diffusion equation in accordance with the trap reduction model. This analytical model can explain almost all experimental data: oxidation-temperature dependence, oxidation-time dependence, and initial-Z1/2- concentration dependence of the defect reduction. Based on these results, the authors achieved the elimination of the Z1/2 center to a depth of ∼100 μm in a sample with a relatively high initial-Z1/2- concentration of 1013 cm-3 by thermal oxidation at 1400°C for 16.5 h. © (2012) Trans Tech Publications.

DOI： 10.4028/www.scientific.net/MSF.717-720.241

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Materials Science Forum  

The c- and a-lattice constants of nitrogen-doped 4H-SiC were measured in the wide temperature range (RT - 1100°C). The samples used in this study were heavily doped substrates and lightly-doped free-standing epilayers. The lattice constants at room temperature are almost identical for all the samples. However, the lattice contraction by heavy nitrogen doping was clearly observed at high temperatures, which indicates that the thermal expansion coefficients are dependent on the nitrogen concentration. The lattice mismatch (Δd/d) between a lightly-doped free-standing epilayer (Nd = 6×10 14 cm-3) and a heavily-doped substrate (Nd = 2×1019 cm-3) was calculated as 1.7×10 -4 at 1100°C. The authors also investigated lattice constants of high-dose N+, P+, and Al+-implanted 4H-SiC. Reciprocal space mapping (RSM) was utilized to investigate the lattice mismatch and misorientation. The RSM images show the c-lattice expansion and c-axis tilt of the ion-implanted layers, irrespective of ion species. The authors conclude that the lattice expansion is not caused by heavy doping itself, but by secondary defects formed after the ion-implantation and activation-annealing process. © (2012) Trans Tech Publications.

DOI： 10.4028/www.scientific.net/MSF.717-720.481

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Materials Science Forum  

In this paper the impact of high temperature annealing on the formation of intrinsic defects in 4H-SiC such as Z1/2 and EH6/7 is examined. Three epitaxial layers with various initial concentrations of the Z1/2- and EH6/7-centers (1011 - 1013 cm-3) are investigated by means of deep level transient spectroscopy (DLTS). It turns out that depending on the initial defect concentration, the high temperature annealing leads to a monotonic increase of the Z1/2- and EH6/7-concentration in a temperature range from 1600 to 1750°C. For higher temperatures, the resulting defect concentration is independent of the sample's initial values. Therefore, beside the growth conditions such as C/Si ratio, the thermal post-growth processing has a severe impact on the carrier lifetime and must be taken into account during device fabrication. © (2012) Trans Tech Publications.

DOI： 10.4028/www.scientific.net/MSF.717-720.247

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Proceedings of the International Symposium on Power Semiconductor Devices and ICs  

We demonstrate 4H-SiC bipolar junction transistors (BJTs) with record current gains. Improved current gain was achieved by utilizing optimized device geometry as well as optimized surface passivation and continuous epitaxial growth of the emitter-base junction, combined with an intentional deep-level-reduction process based on thermal oxidation to improve the lifetime in p-SiC base. Current gain (β) of 257 was achieved for 4H-SiC BJTs fabricated on the (0001)Si-face. The gain of 257 is twice as large as the previous record gain. We also demonstrate, for the first time, BJTs on the (000-1)C-face that showed the highest β of 335 among the SiC BJTs ever reported. © 2011 IEEE.

DOI： 10.1109/ISPSD.2011.5890848

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of the Vacuum Society of Japan  

Recent progress in fast epitaxial growth and defect control of silicon carbide (SiC) toward development of high-voltage power devices is reviewed. In chemical vapor deposition of 4H-SiC on off-axis (0001), a high growth rate of 85 μm/h and a low background doping of 1 × 10 13 cm -3 are achieved. Conversion of basal-plane dislocations to threading edge dislocations and generation of stacking faults during epitaxial growth are discussed. Deep levels in as-grown n-type and p-type 4H-SiC epitaxial layers have been investigated. A lifetime-killing defect, Z 1/2 center, can be almost eliminated by thermal oxidation, which leads to significant increase in carrier lifetimes. The obtained carrier lifetimes are long enough to fabricate 10 kV-class bipolar devices. Control of carrier lifetimes by lowenergy electron irradiation is demonstrated.

DOI： 10.3131/jvsj2.54.362

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

In this paper, we have investigated reliability of n- and p-type 4H-SiC(0001) metal-oxide-semiconductor (MOS) devices with N2O-grown oxides and deposited oxides annealed in N2O. From the results of time-dependent dielectric breakdown (TDDB) tests, it is revealed that the N 2O-grown oxides have relatively-high reliability (4-30Ccm -2 for n- and p-MOS structures). In addition, the deposited SiO 2 on n- and p-SiC exhibited a high charge-to-breakdown of 70.0 and 54.9Ccm-2, respectively. The n/p-MOS structures with the deposited SiO2 maintained a high charge-tobreakdown of 19.9/15.1Ccm -2 even at 200 °C. The deposited SiO2 annealed in N2O has promise as the gate insulator for n- and p-channel 4HSiC(0001) MOS devices because of its high charge-to-breakdown and good interface properties. © 2011 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.50.090201

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

Threading dislocations (TDs) in 4H-SiC epilayers have been investigated by means of micro-photoluminescence (-PL) mapping at room temperature. Enhanced nonradiative recombination at TDs was confirmed experimentally, resulting in a reduced local PL emission intensity in the -PL intensity map performed at 390 nm (near band-edge emission). The behavior of nonradiative recombination at TDs depends on the dislocation type: the screw type of TDs shows stronger effect on the nonradiative recombination activity than the edge type, evidencing a larger local reduction of PL emission intensity. Furthermore, the contrast of TDs in the μ-PL intensity map greatly depends on the carrier lifetimes of the 4H-SiC epilayers. Lifetimes longer than 0.5 s are essential to obtain a discernible contrast for the individual TDs. © 2011 American Institute of Physics.

DOI： 10.1063/1.3622336

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IEEE Electron Device Letters  

We demonstrate 4H-SiC bipolar junction transistors (BJTs) with record current gains. An improved current gain was achieved by utilizing optimized device geometry and continuous epitaxial growth of the emitterbase junction, combined with an intentional deep-level-reduction process based on thermal oxidation to improve the lifetime in p-SiC base. A current gain (β) of 257 was achieved for 4H-SiC BJTs fabricated on the (0001) Si face. A gain of 257 is twice as large as the previous record gain. We also demonstrate BJTs on the (0001) C face that showed the highest β of 335 among the SiC BJTs ever reported. © 2011 IEEE.

DOI： 10.1109/LED.2011.2142291

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Physica Status Solidi (A) Applications and Materials Science  

Crystalline orientation dependence of Ga incorporation in growth of high Al-content AlGaN was investigated. Growth was carried out by molecular-beam epitaxy (MBE) using elemental Al, Ga, and rf-plasma-excited nitrogen under various V/III ratios. 6H-SiC (0001), 4H-SiC (1√100) and 4H-SiC (11√20) were used as substrates. Ga incorporation increased with increase of V/III ratio in the layers grown on (0001) and (1√100) planes. On the other hand, Ga was not incorporated in the layer grown on (11√20) plane even when the layer was grown under a nitrogen rich condition, indicating much lower Ga incorporation on (11√20) plane than those of other planes. AlGaN with good quality was successfully grown on (1√100) plane. Utilization of (1√100) plane is suitable in MBE growth of AlGaN-based deep-ultraviolet light emitting devices. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

DOI： 10.1002/pssa.201001033

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

Impacts of reduction of deep levels and surface passivation on carrier lifetimes in p-type 4H-SiC epilayers are investigated. The authors reported that the carrier lifetime in n-type epilayers increased by reduction of deep levels through thermal oxidation and thermal annealing. However, the carrier lifetimes in p-type epilayers were not significantly enhanced. In this study, in order to investigate the influence of surface passivation on the carrier lifetimes, the epilayer surface was passivated by different oxidation techniques. While the improvement of the carrier lifetime in n-type epilayers was small, the carrier lifetime in p-type epilayers were remarkably improved by appropriate surface passivation. For instance, the carrier lifetime was improved from 1.4 s to 2.6 s by passivation with deposited SiO2 annealed in NO. From these results, it was revealed that surface recombination is a limiting factor of carrier lifetimes in p-type 4H-SiC epilayers. © 2011 American Institute of Physics.

DOI： 10.1063/1.3583657

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Publishing type：Research paper (scientific journal)   Publisher：Silicon Carbide  

DOI： 10.1002/9783527629053.ch10

Scopus

Publishing type：Research paper (scientific journal)   Publisher：Silicon Carbide  

DOI： 10.1002/9783527629077.ch10

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

Si-nanowire (Si-NW) MOSFETs, the cross-sectional size (square root of the cross-sectional area of NWs) of which was changed from 18 to 4 nm, were fabricated and characterized. Both n- and p-channel MOSFETs have shown a nearly ideal subthreshold swing of 63 mV/decade. The threshold voltage of n-/p-channel MOSFETs has gradually increased/decreased with decreasing the cross-sectional size. The bandgap shift from bulk Si has been derived from the threshold-voltage shift. The bandgap of Si-NWs was calculated by a density functional theory, tight binding method, and effective mass approximation. The calculated bandgap shows good agreement with that derived from threshold voltage. The theoretical calculation indicates that the bandgap is dominated by the cross-sectional size (area) and is not very sensitive to the shape within the aspect-ratio range of 1.0-2.5. © 2011 American Institute of Physics.

DOI： 10.1063/1.3559265

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

The authors theoretically studied the valence band structure and hole effective mass of rectangular cross-sectional Si nanowires (NWs) with the crystal orientation of [110], [111], and [001]. The E-k dispersion and the wave function were calculated using an sp3d5s tight-binding method and analyzed with the focus on the nature of p orbitals constituting the subbands. In [110] and [111] nanowires, longitudinal/transverse p orbitals are well separated and longitudinal component makes light (top) subbands and transverse component makes heavy subbands. The heavy subbands are located far below the top light band when NW has square cross-section, but they gain their energy with the increase in the NW width and come near the band edge. This energy shift of heavy bands in [110] NWs shows strong anisotropy to the direction of quantum confinement whereas that in [111] NWs does not have such anisotropy. This anisotropic behavior and the difference among orientations are understandable by the character of the wave function of heavy subbands. Regarding the [001] nanowires, the top valence state is formed by the mixture of longitudinal/transverse p orbitals, which results in heavy effective mass and large susceptibility to lateral-size variation. The correlation of the wave function of hole states between nanowires and bulk is also discussed briefly. © 2011 American Institute of Physics.

DOI： 10.1063/1.3552593

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IEEE Electron Device Letters  

We report the improvement of current gain in 4H-SiC bipolar junction transistors (BJTs) by utilizing deposited oxides as a surface passivation layer. Various postdeposition annealing processes, including annealing ambient (N 2, N2O, and NO) and annealing time, were investigated. We successfully demonstrate SiC BJTs with high current gains β of 73 and 102 using deposited oxides annealed in N2O and NO, respectively, whereas BJTs having conventional thermally grown oxides showed a current gain of 50. © 2006 IEEE.

DOI： 10.1109/LED.2010.2101575

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

Molten KOH etching of 6H- and 4H-SiC{0001} on-axis substrates was investigated. After molten KOH etching, etch pits originating from threading dislocations (TDs) and basal-plane dislocations (BPDs) were observed on (0001) surfaces. On the other hand, large and small hillocks were observed on (0001̄) surfaces. The etch hillocks consist of SiC, indicating slower etching at TDs. By comparing the (0001) side and (0001̄) side of the same substrate, it was found that large hillocks correspond to edge-type TDs, while small hillocks correspond to screw-type TDs. © 2011 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.50.038002

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

300-nm-thick AlN layers were grown directly on 6H-SiC(0001) with six Si-C bilayer-height (1.5 nm) steps by rf-plasma-assisted molecular-beam epitaxy (MBE). To avoid unintentional active-nitrogen exposure, AlN was grown just after the nitrogen plasma ignition. By combining optimized Ga pre-deposition and no active-nitrogen exposure, layer-by-layer growth was realized from the first layer of AlN. Screw-type and edge-type threading dislocation densities in the AlN layer were reduced to 6 × 104 and 4 × 108 cm-2, respectively. Most of the edge-type dislocations were located at the step edge of the SiC substrate. The dislocation density of the AlN grown on the terrace of the SiC substrate was as low as 8 × 107 cm-2. © 2011 The Japan Society of Applied Physics.

DOI： 10.1143/APEX.4.025502

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

Dependencies of temperature and injection level on carrier lifetimes in 50 μm thick p-type and n-type 4H-SiC epilayers have been investigated. The carrier lifetimes have been measured by differential microwave photoconductance decay measurements at various injection levels and temperatures. In both p-type and n-type epilayers, the carrier lifetimes gradually increased with increasing the injection level, which were naturally expected from the Shockley-Read-Hall (SRH) model, and after taking a maximum, the lifetimes dropped at the very high-injection level. In contrast, the carrier lifetimes exhibited continuous increase with elevating the temperature for both epilayers. In addition, the impact of thermal oxidation process on the carrier lifetimes has been also investigated. The thermal oxidation process, by which the Z1/2 and EH6/7 centers were remarkably reduced that had been observed in n-type 4H-SiC in our previous work, led to the improvement of the carrier lifetimes especially for n-type epilayers. The carrier lifetime reached 4.1 μs in p-type and 6.1 μs in n-type epilayers at 250 °C with an injection level of 1.8×1016 cm-3 through the thermal oxidation processing. © 2011 American Institute of Physics.

DOI： 10.1063/1.3524266

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Publishing type：Research paper (scientific journal)  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Proceedings of SPIE - The International Society for Optical Engineering  

An electrostatic-actuated suspended bridge structure composed by single-crystalline silicon carbide (SiC) is fabricated. The structure is entirely made of homoepitaxially grown single-crystalline 4H-SiC. Electrical isolation between the suspended bridge and the base plate is established with a pnp junction formed by multiple ion implantation. The structure is fabricated by a combination of reactive ion etching (RIE) and doping-selective photoelectrochemical (PEC) etching. The suspended bridge is actuated by applying a voltage between the bridge and the base plate.

DOI： 10.1117/12.874543

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Proceedings of SPIE - The International Society for Optical Engineering  

The temperature dependence of the refractive indices of 4H-SiC, GaN, and AlN were investigated in a wavelength range from the near band edge (392 nm for SiC, 367 nm for GaN, and 217 nm for AlN) to infrared (1700 nm) and a temperature range from room temperature to 512°C. Optical interference measurements with vertical incident configuration were employed to precisely evaluate ordinary refractive indices. In visible region, the thermo-optic coefficient of GaN has the largest value in these materials. Optical simulation of GaN-based tunable band-pass filter with AlGaN/GaN distributed Bragg reflectors (DBRs) was also carried out by using the obtained thermo-optic coefficients. It revealed that 9 nm red-shift can be obtained from room temperature to 500°C.

DOI： 10.1117/12.874531

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：2010 Silicon Nanoelectronics Workshop, SNW 2010  

The present tight-binding study of rectangular SiNWs along [001], [110], and [111] revealed that the hole m* of [001] and [110] NWs on the {001} basal face has strong dependence on the width. Because this nature may make the design of devices difficult, these NWs are considered to be unfavorable for p-channel devices. In contrast, rectangular [111] NWs on both (112̄) and (1̄10) basal faces are favorable for p-channel devices because they have the smallest hole m* and its value is very resistant to the variability of the width.

DOI： 10.1109/SNW.2010.5562567

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

After remarkable reduction in the Z1/2 center in n-type 4H-SiC epilayers, the measured carrier lifetimes can be severely affected by other recombination paths. Impacts of carrier recombination at the surface as well as in the substrate are investigated in detail by using numerical simulation based on a diffusion equation. The simulation reveals that a very thick (>100 μm) epilayer is required for accurate measurement of carrier lifetimes if the bulk lifetime in the epilayer is longer than several microsecond, due to the extremely short lifetimes in the substrate. The fast decay often observed at the initial stage of decay curves can be explained by fast recombination at the surface and in the substrate. In experiments, the carrier lifetime is improved from 0.69 to 9.5 μs by reducing the Z1/2 center via two-step thermal treatment (thermal oxidation and Ar annealing) for a 148-μm -thick n-type epilayer. This lifetime must be still, to large extent, affected by the recombination at the surface and in the substrate, and the real bulk lifetime may be much longer. The carrier recombination paths and their impacts on the decay curves are discussed. © 2010 American Institute of Physics.

DOI： 10.1063/1.3498818

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of the Optical Society of America B: Optical Physics  

We have determined all the three independent components of quadratic nonlinear-optical coefficients of GaN by highly accurate Maker-fringe measurements on high-quality bulk samples combined with theoretical analysis taking account of the multiple-reflection effects in slightly misoriented optically anisotropic samples. Especially, the d33 coefficient is determined with sufficient accuracies for the first time, to our knowledge, by using (112̄0)-oriented samples. The obtained values of quadratic nonlinear-optical coefficients are d31 =2.5±0.1 pm/V, d 15=2.5±0.1 pm/V, and d33=-3.8±0.2 pm/V at the fundamental wavelength of 1.064 μm. © 2010 Optical Society of America.

DOI： 10.1364/JOSAB.27.002026

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Language：Japanese   Publishing type：Research paper (scientific journal)  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IEEE Electron Device Letters  

We report here on the fabrication and characterization of new aluminum gallium nitride (AlGaN)/silicon carbide heterojunction bipolar transistors (HBTs). In the HBTs, AlN/GaN short-period superlattice (quasi-AlGaN) was employed as the widegap emitter. We have successfully demonstrated band-offset control and the first commonemitter-mode operation (β ∼ 2.7) in the HBTs. © 2010 IEEE.

DOI： 10.1109/LED.2010.2052012

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

The authors have investigated effects of thermal oxidation on deep levels in the whole energy range of the band gap of 4H-SiC by deep level transient spectroscopy. The deep levels are generated by ion implantation. The dominant defects in n-type samples after ion implantation and high-temperature annealing at 1700 °C are IN3 (Z1/2: EC-0.63 eV) and IN9 (EH 6/7: EC-1.5 eV) in low-dose-implanted samples, and IN8 (EC-1.2 eV) in high-dose-implanted samples. These defects can remarkably be reduced by thermal oxidation at 1150 °C. In p-type samples, however, IP8 (HK4: EV +1.4 eV) survives and additional defects such as IP4 (HK0: EV +0.72 eV) appear after thermal oxidation in low-dose-implanted samples. In high-dose-implanted p-type samples, three dominant levels, IP5 (HK2: EV +0.85 eV), IP6 (EV +1.0 eV), and IP7 (HK3: EV +1.3 eV), are remarkably reduced by oxidation at 1150 °C. The dominant defect IP4 observed in p-type 4H-SiC after thermal oxidation can be reduced by subsequent annealing in Ar at 1400 °C. These phenomena are explained by a model that excess interstitials are generated at the oxidizing interface, which diffuse into the bulk region. © 2010 American Institute of Physics.

DOI： 10.1063/1.3456159

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Electronic Materials  

Successive reactive-ion etching and microphotoluminescence (PL) intensity mapping have been performed in order to investigate the sources of epitaxial growth-induced stacking faults (SFs) in thick 4H-SiC epilayers. Three kinds of SFs, i.e., 4SSFs, 3SSFs, and 2SSFs, have been identified in the samples. Two of these (3SSFs and 2SSFs) show similar nucleation behaviors, and their formation may be due to stress within the epitaxial layer. In contrast, 4SSFs nucleate at the epilayer-substrate interface and might be related to an unknown dislocation, which shows a rounded-shape etch pit in the substrate. © 2010 TMS.

DOI： 10.1007/s11664-010-1192-6

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Applied Physics  

In this study, the authors investigate deep levels, which are induced by reactive ion etching (RIE) of n-type/p-type 4H-SiC, by deep level transient spectroscopy (DLTS). The capacitance of a Schottky contact fabricated on as-etched p-type SiC is abnormally small due to compensation or deactivation of acceptors extending to a depth of ∼14 μm, which is nearly equal to the epilayer thickness. The value of the capacitance can recover to that of a Schottky contact on as-grown samples after annealing at 1000 °C. However, various kinds of defects, IN2 (EC-0.30 eV), EN (EC -1.6 eV), IP1 (EV +0.30 eV), IP2 (EV +0.39 eV), IP4 (HK0: EV +0.72 eV), IP5 (EV +0.85 eV), IP7 (EV +1.3 eV), and EP (EV +1.4 eV), remain at a high concentration (average of total defect concentration in the region ranging from 0.3 μm to 1.0 μm:∼5× 1014 cm-3) even after annealing at 1000 °C. The concentration of all these defects generated by RIE, except for the IP4 (HK0) center, remarkably decreases by thermal oxidation. In addition, the HK0 center can also be reduced significantly by a subsequent annealing at 1400 °C in Ar. © 2010 American Institute of Physics.

DOI： 10.1063/1.3460636

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Materials Science Forum  

In this paper, nitrided insulators such as N2O-grown oxides, deposited SiO2 annealed in N2O, and deposited SiN x/SiO2 annealed in N2O on thin-thermal oxides have been investigated for realization of high performance n- and p-type 4H-SiC MIS devices. The MIS capacitors were utilized to evaluate MIS interface characteristics and the insulator reliability. The channel mobility was determined by using the characteristics of planar MISFETs. Although the N 2O-grown oxides are superior to the dry O2-grown oxides, the deposited SiO2 and the deposited SiNx/SiO2 exhibited lower interface state density (n-MIS: below 7×1011 cm-2eV-1 at EC-0.2 eV, p-MIS: below 6×1011 cm-2eV-1 at EV+0.2 eV) and higher channel mobility (n-MIS: over 25 cm2/Vs, p-MIS: over 10 cm2/Vs). In terms of reliability, the deposited SiO2 annealed in N2O exhibits a high charge-to-breakdown over 50 C/cm 2 at room temperature and 15 C/cm2 at 200°C. The nitrided-gate insulators formed by deposition method have superior characteristics than the thermal oxides grown in N2O. © (2010) Trans Tech Publications, Switzerland.

DOI： 10.4028/www.scientific.net/MSF.645-648.825

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Physica Status Solidi (C) Current Topics in Solid State Physics  

300-nm-thick AlN layers without a nucleation layer were grown on 6H-SiC (0001) vicinal substrates with 3-bilayer-height steps by rf-plasma-assisted molecular-beam epitaxy. A Ga beam was supplied for 0 to 30 seconds just before growth of AlN. The Ga pre-irradiation for 7 seconds (~1.6 ML) was found to be effective for realization of the AlN layer-by-layer growth mode at an earlier stage of the growth and reduction of threading dislocation densities (TDD) in the AlN layers. The screw-type and edge-type TDDs were ∼106 cm-2 and 6×108 cm-2, respectively. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA.

DOI： 10.1002/pssc.200983579

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

Carrier lifetimes in n-type 4H-SiC epitaxial layers are limited by several factors such as deep levels, surface recombination, and recombination in the substrate. In this study, the carrier lifetime is significantly improved from 0.68 to 13.1 μs by eliminating deep levels and by improving surface passivation. Deep levels can be almost eliminated by two-step annealing as reported before, and the surface recombination can be reduced by passivating the surface with a deposited oxide annealed in nitric oxide at 1300 °C. Major recombination paths are discussed based on numerical simulation. © 2010 The Japan Society of Applied Physics.

DOI： 10.1143/APEX.3.121201

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

Influence of effective fixed charges, which are located near the metal-oxide-semiconductor (MOS) interface, on short-channel effects in 4H-SiC MOS field-effect transistors (FETs) has been investigated. The relationship between the threshold voltage and the channel length was theoretically calculated by using an original charge-share model, which takes the effective fixed charges into account. As a result, it was revealed that the effective fixed charges, which are attributed to electron trapping at the interface states, affect the relationship. The threshold voltage is decreased by reducing the channel length in the relatively-long channel region when the effective fixed charges exist. The theoretical relationship between the threshold voltage and the channel length calculated by using the proposed model agrees very well with the experimental results. © 2010 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.49.024204

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Publishing type：Research paper (scientific journal)  

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

The current-voltage characteristics of Schottky barrier diodes formed on GaN(0001) free-standing substrates with net donor concentrations of 7:6 × 1015-1:4 × 1017 cm-3 are discussed. The substrates were grown by hydride vapor phase epitaxy. Ni Schottky contacts were directly formed on chemical-mechanical-polished Ga-polar faces of the substrates. Nearly ideal characteristics for both directions were obtained. The ideality factors for forward characteristics are 1.02 -1.05, very close to unity. The reverse characteristics agree well with calculations based on thermionicfield emission theory without any fitting parameter. © 2010 The Japan Society of Applied Physics.

DOI： 10.1143/APEX.3.101003

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Materials Science Forum  

Temperature and injection level dependencies of carrier lifetimes in p-type and n-type 4H-SiC epilayers have been investigated. The carrier lifetimes have been measured by differential microwave photoconductance decay measurements at various injection levels and temperatures. In both p-type and n-type epilayers, the carrier lifetimes gradually increased with increasing the injection level except for the very high injection condition. And the carrier lifetimes exhibited continuous increase with elevating the temperature for both epilayers. The carrier lifetime reached 3.3 μs in p-type and 4.2 μs in n-type epilayers at 250°C and an injection level of 1.8×1016 cm-3. © (2010) Trans Tech Publications.

DOI： 10.4028/www.scientific.net/MSF.645-648.199

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Applied Physics Express  

Nonpolar 4H-polytype AlN/AlGaN multiple quantum well (MQW) structures were grown on 4H-SiC(11̄00) substrates by molecular-beam epitaxy. Layer-by-layer growth of MQW layers is observed and abrupt AlN/AlGaN interfaces have been achieved. Generation of additional extended defects is not observed at the AlN/AlGaN interfaces and the crystal structure of 4H-polytype is preserved throughout the entire MQW structure. In cathodoluminescence spectra, a band-edge emission peak is observed at 5.41 eV (229 nm). The band edge peak position is not blue-shifted when the irradiation beam current is increased. © 2010 The Japan Society of Applied Physics.

DOI： 10.1143/APEX.3.051001

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

Dislocations in 4H-SiC epilayers were imaged nondestructively by means of micro photoluminescence (μ-PL) mapping at room temperature. The one-to-one correspondence between the individual dislocations and the μ-PL mapping contrast has been consistently obtained. By analyzing the reduction of the intensity in the μ-PL mapping image performed at 390nm (near band-edge emission), we were able to distinguish threading screw dislocations and threading edge dislocations. Furthermore, it was found that a basal plane dislocation dissociates into a single Shockley stacking fault during the measurement. © 2010 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.49.090201

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Materials Research Society Symposium Proceedings  

The extended defects, such as dislocations and in-grown stacking faults (IGSFs), in 4H-SiC epilayers have been detected and visualized by a non-destructive method, the micro photoluminescence (μ-PL) intensity mapping method, at room temperature. The one-to-one correspondence between the extended defects and the μ-PL mapping contrast has been successfully obtained. A threading dislocation corresponds to a dark circle with the reduced intensity in the μ-PL mapping image performed at 390 nm, while a basal plane dislocation dissociates into a single Shockley SF during the measurements. Three kinds of IGSFs have been identified in the samples. Each kind of IGSF shows the distinct PL emission located at 460 nm, 480 nm, and 500 nm, respectively. The shapes and distributions of IGSFs have also been profiled by μ-PL intensity mapping. © 2010 Materials Research Society.

DOI： 10.1557/PROC-1246-B03-02

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Physica B: Condensed Matter  

The optical properties of major in-grown stacking faults (IGSFs) in 4H-SiC epilayers have been characterized by micro-photoluminescence (micro-PL) spectroscopy and its intensity mapping. Strong PL emissions from the IGSFs are observed even at room temperature. Three kinds of IGSFs have been identified in the samples based on the micro-PL spectra. Each kind of IGSF shows the distinct PL emission peak located at 460, 480, and 500 nm, respectively. The micro-PL intensity mapping at the emission band of each IGSF has been performed to spatially profile the IGSF. The shapes, distributions, and densities of IGSFs in the epilayers are then presented. The microstructure of each IGSF has been revealed by high-resolution transmission electron microscopy observations. The stacking sequences of three IGSFs are determined as (4,4), (3,5), and (6,0) in the Zhdanov's notation, respectively, which apparently differ from the perfect 4H-SiC, (2,2). Three identified IGSFs are then classified as quadruple Shockley SFs, triple Shockley SFs, and double Shockley SFs, respectively, based on the shear formation model. © 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2009.08.189

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Device Research Conference - Conference Digest, DRC  

SiC bipolar-junction transistors (BJTs) are attractive candidates for high-power switching devices due to their high breakdown voltage and low on-resistance. However, SiC BJT has so far suffered from the limited current gain. An alternative device structure would be heterojunction bipolar transistors (HBTs). Because it is impossible to grow SixC 1-x solid solutions with x near 0.5, HBTs cannot be fabricated within group-IV semiconductors. Heteroepitaxial growth of wider bandgap group-III nitride (Al)GaN on SiC is one possible way to realize bandgap engineering in SiC devices. The fabrication of (Al)GaN/SiC HBTs was first reported by Pankove et al. [1] followed by several groups [2]. But any HBTs did not show common-emitter mode operation due to the large leakage at the emitterjunction. © 2009 IEEE.

DOI： 10.1109/DRC.2009.5354933

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Journal of Applied Physics  

An electrostatic-actuated suspended ribbon structure composed of single-crystalline SiC is presented. All the main parts of the structure, namely, a suspended ribbon, posts, base plate, and electrical connections, are made of homoepitaxially grown single-crystalline 4H-SiC with selective ion implantations. Electrical isolation between the ribbon and the base plate is established with a pnp junction. The structure is fabricated by a combination of reactive ion etching (RIE) and doping-selective photoelectrochemical (PEC) etching. The suspended ribbon is actuated by applying a voltage between the ribbon and the base plate. © 2009 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.48.111101

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：IEEE Transactions on Electron Devices  

4H-SiC (0001) metal-oxide-semiconductor field-effect transistors (MOSFETs) with a 3-D gate structure, which has a top channel on the (0001) face and side-wall channels on the {1120} face, have been fabricated. The 3-D gate structures with a 1-5-μm width and a 0.8- μm height have been formed by reactive ion etching, and the gate oxide has been deposited by plasma-enhanced chemical vapor deposition and then annealed in N2O ambient at 1300 °C. The fabricated MOSFETs have exhibited good characteristics: The ION/IOFF ratio, the subthreshold swing, and VTH are 109, 210 mV/decade, and 3.5 V, respectively. The drain current normalized by the gate width is increasing with decreasing the gate width. The normalized drain current of a 1-μm-wide MOSFET is 16 times higher than that of a conventional planar MOSFET. © 2009 IEEE.

DOI： 10.1109/TED.2009.2030437

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of the Optical Society of America B: Optical Physics  

The second-order nonlinear optical coefficients of 4H-SiC and 6H-SiC have been measured by use of two second-harmonic generation methods, the rotational Maker-fringe and wedge techniques, at the fundamental wavelength of 1.064 μm. Measurements on high-quality (0001) and (112̄0) plane samples as well as rigorous analyses taking into account the multiple-reflection effects allowed us to accurately determine the magnitudes of the nonlinear optical coefficients. The obtained values are d31 = 6.7 pm/V, d15 = 6.5 pm/V, and d33 = -12.5 pm/V for 6H-SiC; and d31 = 6.5 pm/V, d15 = 6.7 pm/V, and d33 = -11.7 pm/V for 4H-SiC. © 2009 Optical Society of America.

DOI： 10.1364/JOSAB.26.001892

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