Faculty Profiles - SUDA Jun

Papers - SUDA Jun

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Fabrication of GaN cantilever on GaN substrate by photo-electrochemical etching

Yamada, T; Ando, Y; Watanabe, H; Furusawa, Y; Tanaka, A; Deki, M; Nitta, S; Honda, Y; Suda, J; Amano, H

APPLIED PHYSICS EXPRESS Vol. 14 ( 3 ) 2021.3

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

Mg-implanted bevel edge termination structure for GaN power device applications Open Access

Matys, M; Ishida, T; Nam, KP; Sakurai, H; Narita, T; Uesugi, T; Bockowski, M; Suda, J; Kachi, T

APPLIED PHYSICS LETTERS Vol. 118 ( 9 ) 2021.3

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

Effect of Nitrogen Plasma Treatment on Characteristics of GaN Trench MOSFETs

NAM KyungPil, Ishida Takashi, Matys Maciej, Uesugi Tsutomu, Kachi Tetsu, Suda Jun

JSAP Annual Meetings Extended Abstracts Vol. 2021.1 ( 0 ) page： 2403 - 2403 2021.2

Design guidelines suppressing dynamic punch-through in GaN vertical MOSFETs by considering the Poole-Frenkel effect

Rokuno, S; Suda, J

APPLIED PHYSICS EXPRESS Vol. 14 ( 2 ) 2021.2

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

Nitrogen-displacement-related electron traps in <i>n</i>-type GaN grown on a GaN freestanding substrate Open Access

Horita, M; Narita, T; Kachi, T; Suda, J

APPLIED PHYSICS LETTERS Vol. 118 ( 1 ) 2021.1

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

Impact of gamma-ray irradiation on capacitance-voltage characteristics of Al<sub>2</sub>O<sub>3</sub>/GaN MOS diodes with and without post-metallization annealing

Aoshima, K; Horita, M; Suda, J; Hashizume, T

APPLIED PHYSICS EXPRESS Vol. 14 ( 1 ) 2021.1

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

Enhanced activation of Mg ion-implanted GaN at decreasing annealing temperature by prolonging duration

Nakashima, T; Kano, E; Kataoka, K; Arai, S; Sakurai, H; Narita, T; Sierakowski, K; Bockowski, M; Nagao, M; Suda, J; Kachi, T; Ikarashi, N

APPLIED PHYSICS EXPRESS Vol. 14 ( 1 ) 2021.1

Enhanced activation of Mg ion-implanted GaN at decreasing annealing temperature by prolonging duration

Nakashima T., Kano E., Kataoka K., Arai S., Sakurai H., Narita T., Sierakowski K., Bockowski M., Nagao M., Suda J., Kachi T., Ikarashi N.

Applied Physics Express Vol. 14 ( 1 ) 2021.1

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

Improvement of channel property of GaN vertical trench MOSFET by compensating nitrogen vacancies with nitrogen plasma treatment

Ishida, T; Nam, KP; Matys, M; Uesugi, T; Suda, J; Kachi, T

APPLIED PHYSICS EXPRESS Vol. 13 ( 12 ) 2020.12

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

Impact of Film Stress of Field-Plate Dielectric on Electric Characteristics of GaN-HEMTs

Ando, Y; Takahashi, H; Ma, Q; Wakejima, A; Suda, J

IEEE TRANSACTIONS ON ELECTRON DEVICES Vol. 67 ( 12 ) page： 5421 - 5426 2020.12

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

Effects of ultra-high-pressure annealing on characteristics of vacancies in Mg-implanted GaN studied using a monoenergetic positron beam Open Access

Uedono, A; Sakurai, H; Narita, T; Sierakowski, K; Bockowski, M; Suda, J; Ishibashi, S; Chichibu, SF; Kachi, T

SCIENTIFIC REPORTS Vol. 10 ( 1 ) 2020.10

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

Why do electron traps at<i>E</i><sub>C</sub>-0.6 eV have inverse correlation with carbon concentrations in n-type GaN layers?

Narita, T; Horita, M; Tomita, K; Kachi, T; Suda, J

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 59 ( 10 ) 2020.10

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

Depth profiling of surface damage in n-type GaN induced by inductively coupled plasma reactive ion etching using photo-electrochemical techniques

Yamada, S; Takeda, K; Toguchi, M; Sakurai, H; Nakamura, T; Suda, J; Kachi, T; Sato, T

APPLIED PHYSICS EXPRESS Vol. 13 ( 10 ) 2020.10

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

Progress on and challenges of p-type formation for GaN power devices

Narita, T; Yoshida, H; Tomita, K; Kataoka, K; Sakurai, H; Horita, M; Bockowski, M; Ikarashi, N; Suda, J; Kachi, T; Tokuda, Y

JOURNAL OF APPLIED PHYSICS Vol. 128 ( 9 ) 2020.9

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

Impacts of high temperature annealing above 1400<sup>°</sup>C under N<inf>2</inf>overpressure to activate acceptors in Mg-implanted GaN

Sakurai H., Narita T., Hirukawa K., Yamada S., Koura A., Kataoka K., Horita M., Ikarashi N., Bockowski M., Suda J., Kachi T.

Proceedings of the International Symposium on Power Semiconductor Devices and ICs Vol. 2020-September page： 321 - 324 2020.9

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

Redistribution of Mg and H atoms in Mg-implanted GaN through ultra-high-pressure annealing

Sakurai, H; Narita, T; Omori, M; Yamada, S; Koura, A; Iwinska, M; Kataoka, K; Horita, M; Ikarashi, N; Bockowski, M; Suda, J; Kachi, T

APPLIED PHYSICS EXPRESS Vol. 13 ( 8 ) 2020.8

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

Identification of origin of<i>E</i><sub>C</sub>-0.6 eV electron trap level by correlation with iron concentration in n-type GaN grown on GaN freestanding substrate by metalorganic vapor phase epitaxy

Horita, M; Narita, T; Kachi, T; Suda, J

APPLIED PHYSICS EXPRESS Vol. 13 ( 7 ) 2020.7

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

Dual-color-sub-bandgap-light-excited isothermal capacitance transient spectroscopy for quick measurement of carbon-related hole trap density in n-type GaN Open Access

Kanegae, K; Narita, T; Tomita, K; Kachi, T; Horitata, M; Kimoto, T; Suda, J

JAPANESE JOURNAL OF APPLIED PHYSICS Vol. 59 ( SG ) 2020.4

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

Electron traps formed by gamma-ray irradiation in homoepitaxial <i>n</i>-type GaN and their annealing behavior Open Access

Aoshima, K; Kanegae, K; Horita, M; Suda, J

AIP ADVANCES Vol. 10 ( 4 ) 2020.4

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

Defect evolution in Mg ions implanted GaN upon high temperature and ultrahigh N<sub>2</sub> partial pressure annealing: Transmission electron microscopy analysis

Iwata, K; Sakurai, H; Arai, S; Nakashima, T; Narita, T; Kataoka, K; Bockowski, M; Nagao, M; Suda, J; Kachi, T; Ikarashi, N

JOURNAL OF APPLIED PHYSICS Vol. 127 ( 10 ) 2020.3

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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