Faculty Profiles - AMANO, Hiroshi

Papers - AMANO, Hiroshi

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Observation of 2D-magnesium-intercalated gallium nitride superlattices. Reviewed Open Access

Wang J, Cai W, Lu W, Lu S, Kano E, Agulto VC, Sarkar B, Watanabe H, Ikarashi N, Iwamoto T, Nakajima M, Honda Y, Amano H

Nature Vol. 631 ( 8019 ) page： 67 - 72 2024.7

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

Since the demonstration of p-type gallium nitride (GaN) through doping with substitutional magnesium (Mg) atoms1,2, rapid and comprehensive developments, such as blue light-emitting diodes, have considerably shaped our modern lives and contributed to a more carbon-neutral society3–5. However, the details of the interplay between GaN and Mg have remained largely unknown6–11. Here we observe that Mg-intercalated GaN superlattices can form spontaneously by annealing a metallic Mg film on GaN at atmospheric pressure. To our knowledge, this marks the first instance of a two-dimensional metal intercalated into a bulk semiconductor, with each Mg monolayer being intricately inserted between several monolayers of hexagonal GaN. Characterized as an interstitial intercalation, this process induces substantial uniaxial compressive strain perpendicular to the interstitial layers. Consequently, the GaN layers in the Mg-intercalated GaN superlattices exhibit an exceptional elastic strain exceeding −10% (equivalent to a stress of more than 20 GPa), among the highest recorded for thin-film materials12. The strain alters the electronic band structure and greatly enhances hole transport along the compression direction. Furthermore, the Mg sheets induce a unique periodic transition in GaN polarity, generating polarization-field-induced net charges. These characteristics offer fresh insights into semiconductor doping and conductivity enhancement, as well as into elastic strain engineering of nanomaterials and metal–semiconductor superlattices13.

DOI： 10.1038/s41586-024-07513-x

15 GHz GaN Hi-Lo IMPATT Diodes With Pulsed Peak Power of 25.5 W Reviewed

Kawasaki, S; Kumabe, T; Deki, M; Watanabe, H; Tanaka, A; Honda, Y; Arai, M; Amano, H

IEEE TRANSACTIONS ON ELECTRON DEVICES Vol. 71 ( 3 ) page： 1408 - 1415 2024.3

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

The p+-n-n - n+ structure, known as Hi-Lo structure, was investigated in gallium nitride (GaN) single-drift-region (SDR) impact ionization avalanche transit-time (IMPATT) diodes to improve the output power and efficiency. The 15 GHz GaN Hi-Lo IMPATT diode was designed according to the Scharfetter and Gummel model under realistic conditions, suppressing the tunneling current (<10-4 cm2) and breakdown voltage (< 400 V). Even in such conditions, the calculated efficiency was higher than that of the p+-n abrupt junction structure and the improvement of RF characteristics was expected. The fabricated GaN Hi-Lo IMPATT diodes showed a clear avalanche breakdown and a pulsed microwave oscillation in the frequency range from 15 to 17 GHz. The maximum peak output power of 25.5 W and the efficiency of 2% were achieved, showing the highest values on microwave band GaN IMPATT diodes, and we confirmed that the Hi-Lo structure is effective for the high-power and high-efficiency operation of GaN IMPATT diodes.

DOI： 10.1109/TED.2023.3345822

Demonstration of AlGaN-on-AlN p-n Diodes With Dopant-Free Distributed Polarization Doping Reviewed Open Access

Kumabe T., Yoshikawa A., Kawasaki S., Kushimoto M., Honda Y., Arai M., Suda J., Amano H.

IEEE Transactions on Electron Devices 2024

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

Nearly ideal vertical Al<inline-formula> <tex-math notation="LaTeX">$_{\textit{x}}$</tex-math> </inline-formula>Ga<inline-formula> <tex-math notation="LaTeX">$_{\text{1}-\textit{x}}$</tex-math> </inline-formula>N (<inline-formula> <tex-math notation="LaTeX">$\text{0.7} \leq \textit{x} < \text{1.0}$</tex-math> </inline-formula>) 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 M<inline-formula> <tex-math notation="LaTeX">$\Omega$</tex-math> </inline-formula> cm<inline-formula> <tex-math notation="LaTeX">$^{\text{2}}$</tex-math> </inline-formula>, 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<inline-formula> <tex-math notation="LaTeX">$^{-\text{1}}$</tex-math> </inline-formula>, 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

Junction Diameter Dependence of Oscillation Frequency of GaN IMPATT Diode Up to 21 GHz Reviewed

Kawasaki, S; Kumabe, T; Ando, Y; Deki, M; Watanabe, H; Tanaka, A; Honda, Y; Arai, M; Amano, H

IEEE ELECTRON DEVICE LETTERS Vol. 44 ( 8 ) page： 1328 - 1331 2023.8

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

An experimental study on the effects of junction capacitance and current density on the oscillation characteristics of GaN single-drift-region (SDR) impact ionization avalanche transit-time (IMPATT) diodes were carried out using GaN p+-n abrupt junction diodes of various diameters, 200, 150, and 100 μ m , with a depletion layer width of 2 μ m. The fabricated diodes showed a clear avalanche breakdown at 315 V and a pulsed microwave oscillation with a peak output power exceeding 30 dBm. The oscillation frequency depended on junction diameter and current density. It was widely modulated from 8.56 to 21.1 GHz with decreasing junction diameter and increasing current density. The highest oscillation frequency was obtained with a current density of 13.8 kA/cm2 and a junction diameter of 100 μ m. A numerical calculation based on Read-type small-signal theory was carried out and found to well explain the experimental results.

DOI： 10.1109/LED.2023.3285938

Reverse Leakage Mechanism of Dislocation-Free GaN Vertical p-n Diodes Reviewed

Kwon, W; Kawasaki, S; Watanabe, H; Tanaka, A; Honda, Y; Ikeda, H; Iso, K; Amano, H

IEEE ELECTRON DEVICE LETTERS Vol. 44 ( 7 ) page： 1172 - 1175 2023.7

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

The reverse leakage mechanism of threading dislocation (TD)-free gallium nitride (GaN) vertical p-n diode was investigated in various temperature range, and it was compared with that of the p-n diode having a threading dislocation density (TDD) of around 10^6 cm -2. The reverse leakage current was increased markedly by increasing the temperature from 400 K, the dominant mechanism was explained by thermionic and Poole-Frenkel emissions for TD-free and high-TDD p-n diodes, respectively. At high temperatures and electric fields, the leakage current of the high TDD p-n diode showed 2 times higher than the TD-free p-n diode. These results indicate that the performance of vertical GaN devices, especially when employed at high temperatures and electric fields, can be enhanced by removing TDs.

DOI： 10.1109/LED.2023.3274306

Hole mobility limiting factors in dopant-free p-type distributed polarization-doped AlGaN Reviewed Open Access

Kumabe, T; Kawasaki, S; Watanabe, H; Honda, Y; Amano, H

APPLIED PHYSICS LETTERS Vol. 122 ( 25 ) 2023.6

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

We investigated the hole mobility limiting factors in dopant-free p-type distributed polarization-doped (DPD) AlGaN layers by an experimental method. p-DPD AlGaN exhibited a higher hole mobility than GaN:Mg with a similar room temperature hole concentration across all temperature ranges owing to the absence of ionized impurity scattering. In addition, unlike in n-DPD AlGaN, alloy scattering was not always critical in p-DPD AlGaN. The extracted alloy scattering potential was only 0.3 eV, which resulted in a reduced effect of alloy scattering and originated from a small valence band offset in the GaN/AlN heterojunction system. The results suggest that DPD principally enables the fabrication of low-resistance p-type nitride semiconductor thin films as a result of the high hole concentration and high hole mobility.

DOI： 10.1063/5.0155363

High In content nitride sub-micrometer platelet arrays for long wavelength optical applications Reviewed

Cai, WT; Furusawa, Y; Wang, J; Park, JH; Liao, YQ; Cheong, HJ; Nitta, S; Honda, Y; Pristovsek, M; Amano, H

APPLIED PHYSICS LETTERS Vol. 121 ( 21 ) 2022.11

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

We demonstrate high, up to 30% In content InGaN sub-micrometer platelets on GaN by metalorganic vapor phase epitaxy. These InGaN platelets were selectively grown on flat GaN seeds formed in sub-micrometer-scale openings in a SiNx mask. The platelets were highly uniform without any dislocations or pits, with an atomically flat (0001) surface. The typical height was ∼120 nm, which significantly exceeded the normal critical layer thickness of a c-plane InGaN film. The strain state was comprehensively characterized by microbeam x-ray diffraction and transmission electron microscopy. Due to a gradual elastic relaxation of strain, the In content increased almost linearly from bottom to top because of the strong strain-dependent In incorporation. These platelets can serve as high-quality strain-relaxed templates for long wavelength micro-light-emitting diodes.

DOI： 10.1063/5.0120723

Tuning the p-type doping of GaN over three orders of magnitude via efficient Mg doping during halide vapor phase epitaxy Reviewed Open Access

Ohnishi, K; Fujimoto, N; Nitta, S; Watanabe, H; Lu, S; Deki, M; Honda, Y; Amano, H

JOURNAL OF APPLIED PHYSICS Vol. 132 ( 14 ) 2022.10

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

The precise control of Mg concentration ([Mg]) in p-type GaN layers from 2.3 × 1016 to 2.0 × 1019 cm-3 was demonstrated by halide vapor phase epitaxy (HVPE) on n-type GaN (0001) freestanding substrates. [Mg] in GaN layers could be controlled well by varying the input partial pressure of MgCl2 formed by a chemical reaction between MgO solid and HCl gas under the thermodynamic equilibrium condition. In the sample with [Mg] of 2.0 × 1019 cm-3, a step-bunched surface was observed because the surface migration of Ga adatoms was enhanced by the surfactant effect of Mg atoms. The samples show high structural qualities determined from x-ray rocking curve measurements. The acceptor concentration was in good agreement with [Mg], indicating that almost all Mg atoms act as acceptors. The compensating donor concentrations in the samples were higher than the concentrations of Si, O, and C impurities. We also obtained the Mg acceptor level at a sufficiently low net acceptor concentration of 245 ± 2 meV. These results show that the HVPE method is promising for fabricating GaN vertical power devices, such as n-channel metal-oxide-semiconductor field-effect transistors.

DOI： 10.1063/5.0122292

Laser slice thinning of GaN-on-GaN high electron mobility transistors Reviewed Open Access

Tanaka, A; Sugiura, R; Kawaguchi, D; Wani, Y; Watanabe, H; Sena, H; Ando, Y; Honda, Y; Igasaki, Y; Wakejima, A; Ando, Y; Amano, H

SCIENTIFIC REPORTS Vol. 12 ( 1 ) page： 7363 2022.5

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

As a newly developed technique to slice GaN substrates, which are currently very expensive, with less loss, we previously reported a laser slicing technique in this journal. In the previous report, from the perspective of GaN substrate processing, we could only show that the GaN substrate could be sliced by a laser and that the sliced GaN substrate could be reused. In this study, we newly investigated the applicability of this method as a device fabrication process. We demonstrated the thinning of GaN-on-GaN high-electron-mobility transistors (HEMTs) using a laser slicing technique. Even when the HEMTs were thinned by laser slicing to a thickness of 50 mm after completing the fabrication process, no significant fracture was observed in these devices, and no adverse effects of laser-induced damage were observed on electrical characteristics. This means that the laser slicing process can be applied even after device fabrication. It can also be used as a completely new semiconductor process for fabricating thin devices with thicknesses on the order of 10 mm, while significantly reducing the consumption of GaN substrates.

DOI： 10.1038/s41598-022-10610-4

Continuous-wave lasing of AlGaN-based ultraviolet laser diode at 274.8 nm by current injection Reviewed Open Access

Zhang, ZY; Kushimoto, M; Yoshikawa, A; Aoto, K; Schowalter, LJ; Sasaoka, C; Amano, H

APPLIED PHYSICS EXPRESS Vol. 15 ( 4 ) 2022.4

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

We demonstrated continuous-wave lasing of an AlGaN-based ultraviolet laser diode, fabricated on a single-crystal AlN substrate when operating at 5 °C. The threshold current density and device series resistance were reduced by improvements to the epitaxial structure and electrode arrangement. A peak wavelength of 274.8 nm was observed for lasing at a drive current over 110 mA, which corresponded to a threshold current density of 3.7 kA cm-2. The operating voltage at the threshold current was as low as 9.6 V.

DOI： 10.35848/1882-0786/ac6198

Ohmic contact on low-doping-density p-type GaN with nitrogen-annealed Mg Reviewed Open Access

Lu, S; Deki, M; Wang, J; Ohnishi, K; Ando, Y; Kumabe, T; Watanabe, H; Nitta, S; Honda, Y; Amano, H

APPLIED PHYSICS LETTERS Vol. 119 ( 24 ) 2021.12

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

We have demonstrated a fabrication process for the Ohmic contact on low-doping-density p-type GaN with nitrogen-annealed Mg. An Ohmic contact with a contact resistance of 0.158 ω cm2 is realized on p-GaN ([Mg] = 1.3 × 1017 cm-3). The contact resistance of p-type GaN with higher Mg concentration ([Mg]=1.0 × 1019 cm-3) can also be reduced to 2.8 × 10-5 ω cm2. A localized contact layer is realized without any etching or regrowth damage. The mechanism underlying this reduced contact resistance is studied by scanning transmission electron microscopy with energy dispersive x-ray spectroscopy and secondary ion mass spectrometry, representing a mutual diffusion of Ga and Mg atoms on the interface. Reductions in the barrier height and surface depletion width with the nitrogen-annealed Mg layer are confirmed by XPS and Hall effect measurements qualitatively.

DOI： 10.1063/5.0076764

Smart-cut-like laser slicing of GaN substrate using its own nitrogen Reviewed Open Access

Tanaka, A; Sugiura, R; Kawaguchi, D; Yui, T; Wani, Y; Aratani, T; Watanabe, H; Sena, H; Honda, Y; Igasaki, Y; Amano, H

SCIENTIFIC REPORTS Vol. 11 ( 1 ) page： 17949 2021.9

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

We have investigated the possibility of applying lasers to slice GaN substrates. Using a sub-nanosecond laser with a wavelength of 532 nm, we succeeded in slicing GaN substrates. In the laser slicing method used in this study, there was almost no kerf loss, and the thickness of the layer damaged by laser slicing was about 40 µm. We demonstrated that a standard high quality homoepitaxial layer can be grown on the sliced surface after removing the damaged layer by polishing.

DOI： 10.1038/s41598-021-97159-w

SiN interlayer to improve external quantum efficiency and reduce sidewall recombination for blue (micro) light-emitting diodes

Pristovsek M., Park J.H., Kwon W., Cheong H., Amano H.

Applied Physics Letters Vol. 127 ( 7 ) 2025.8

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

We demonstrate that inserting a fractional monolayer of SiN below the first InGaN quantum well (QW) does not change the In content but increases the external quantum efficiency (EQE) of light-emitting diodes (LEDs) and reduces the sidewall recombination, together with a small redshift and slight broadening of the emission. At low currents, we could directly measure the reduced emission from the sidewall of 80 μm diameter circular LEDs by inserting an SiN interlayer. This points to a reduced lateral diffusion of carriers in the QWs via an enhanced composition fluctuation indicated by the broadening of x-ray diffraction satellite peaks in addition to the formation of V-defects. Thus, the SiN interlayer increases the carrier localization in the QWs, which increases the EQE of LEDs due to fewer carriers reaching the sidewall as well as dislocations and point defects. The lateral localization of carriers could also explain the large scattering of the reported size dependencies of micro LEDs.

DOI： 10.1063/5.0272825

Advancements in ohmic contact technology for AlGaN/GaN high-electron-mobility transistors

Kim H.Y., Horng R.H., Amano H., Seong T.Y.

Progress in Quantum Electronics Vol. 102 2025.7

Language：English Publishing type：Research paper (scientific journal) Publisher：Progress in Quantum Electronics

AlGaN/GaN-based high electron mobility transistors (HEMTs) hold significant technological importance due to their applications in power electronics, radio frequency (RF) amplifiers, and microwave communication systems. A critical factor affecting the performance of AlGaN/GaN HEMTs is the formation of high-quality ohmic contacts to the source and drain, which facilitates efficient carrier injection from metal electrodes to the semiconductor. Therefore, various approaches have been employed to achieve the formation of high-quality ohmic contacts. This review presents recent advancements in ohmic contact technology for AlGaN/GaN HEMTs. Specifically, we introduce and discuss contact technologies focusing on multilayer schemes under different annealing conditions, Au-free metallization schemes, surface treatments, non-traditional annealing processes, recess etching, selective area regrowth, and ion implantation.

DOI： 10.1016/j.pquantelec.2025.100578

Role of Sidewall Conditions in the External Quantum Efficiency of InGaN-Based Micro-LEDs Open Access

Park, JH; Pristovsek, M; Han, DP; Seong, TY; Amano, H

PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS Vol. 19 ( 6 ) 2025.6

Language：English Publishing type：Research paper (scientific journal) Publisher：Physica Status Solidi Rapid Research Letters

This study aims to investigate the external quantum efficiency (EQE) of InGaN-based blue micro light-emitting diodes (μLEDs) depending on sidewall conditions and current stress. To this end, this study prepares two sets of InGaN blue μLEDs with sizes of 10 × 10 μm<sup>2</sup>, 20 × 20 μm<sup>2</sup>, and 40 × 40 μm<sup>2</sup>: 1) μLEDs with and without a 10 nm-thick ALD-SiO<inf>2</inf> interlayer and 2) μLEDs with varying atomic layer deposition (ALD)-Al<inf>2</inf>O<inf>3</inf> passivation layer thickness and tetramethylammonium hydroxide (TMAH) treatment time. The results of set 1 demonstrate that the ALD-SiO<inf>2</inf> interlayer layer has a significant effect on the EQE of μLEDs only at low current densities, as the interlayer effectively protects against subsequent plasma damage. The results of current stress show that the ALD-SiO<inf>2</inf> interlayer plays an important role at low current densities. The results of set 2 demonstrate that a sufficiently thick ALD passivation layer should be deposited to minimize sidewall interface states, and a short TMAH treatment time of 1 min is sufficient. The results in this study highlight that sidewall conditions have a substantial impact on the EQE at low current densities and indicate that optimizing ALD thickness and TMAH treatment time can reduce processing time and cost.

DOI： 10.1002/pssr.202500042

Comparison of chip size effects of thin film GaN-based μLEDs fabricated by plasma etching and ion implantation processing Open Access

Wan T.C., Tsai P.H., Lin H.W., Lin C.C., Wuu D.S., Amano H., Seong T.Y., Horng R.H.

Applied Surface Science Advances Vol. 27 2025.6

Language：English Publishing type：Research paper (scientific journal) Publisher：Applied Surface Science Advances

We investigated the effects of chip size and N-polar n-GaN surface roughening on the performance of conventional thin-film blue-light micro-light-emitting diodes (TFFC μLEDs) and As<sup>+</sup> ion isolated TFFC blue μLEDs. For this, TFFC μLEDs with two different sizes of 10 × 10 μm<sup>2</sup> and 25 × 25 μm<sup>2</sup> pixel array were fabricated. In all samples, N-face n-GaN surfaces were etched using a 4 M KOH solution after laser lift-off (LLO) process. A 2 min-etching resulted in the formation of pyramids (size: ∼90 – ∼270 nm), while the 4 min-etching produces pyramids (∼370 – ∼780 nm). Regardless of treatments, all samples exhibit similar forward bias characteristics. For all samples, the light output power increased after n-GaN surface roughening. Before the LLO process, the 10 μm-μLEDs showed higher EQE than the other samples with the implanted μLEDs showing the lowest EQE. All samples exhibited their highest EQE after optimal 4-min etching. Unlike before LLO, after 4 min-etching, the 10 μm- and 25 μm-μLEDs show almost similar EQEs, while the implanted μLEDs reveal a slightly lower EQE than the mesa μLEDs, but produce up to 74 % EQE improvement. Furthermore, the 10 μm-μLEDs showed the shortest photoluminescence (PL) decay, while the 25 μm-μLEDs gave the longest PL decay. The 10 μm μLEDs showed the highest ideality factor and the As<sup>+</sup> ion-implanted μLEDs gave the lowest value. Based on time resolved PL and ideality factor, the size and etching time dependence of the EQE characteristics of all samples are described and discussed.

DOI： 10.1016/j.apsadv.2025.100776

Electron Spin Resonance and Photoluminescence Studies of Carbon-Induced Point Defects in GaN: Influence of Doping Concentration and Method Open Access

Honda, A; Watanabe, H; Kato, T; Honda, Y; Amano, H

PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS 2025.5

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

The potential to modify the substitution of carbon (C) atom for gallium (Ga) or nitrogen (N) or the occupancy of the interstitial site for the different methods of C doping in gallium nitride is investigated. Herein, two types of samples with C doping concentrations of 1 × 10<sup>17</sup>, 1 × 10<sup>18</sup>, and 1 × 10<sup>19</sup> cm<sup>−3</sup> are prepared by supplying acetylene (C<inf>2</inf>H<inf>2</inf>) gas during growth and C autodoping. Based on the analysis of electron spin resonance and photoluminescence, it appears that the point defect generated by C substitution for the N site (C<inf>N</inf>) is formed in the C-doped sample with C<inf>2</inf>H<inf>2</inf> gas at C concentration of 1 × 10<sup>17</sup> cm<sup>−3</sup>, and the defect changes to the C interstitial (C<inf>i</inf>) at C concentrations of 1 × 10<sup>18</sup>–1 ×10<sup>19</sup> cm<sup>−3</sup>. In contrast, the C<inf>i</inf> defect is formed in the C autodoped samples at C concentrations of 1 × 10<sup>17</sup>–1 × 10<sup>18</sup> cm<sup>−3</sup> and changed to the C<inf>N</inf> defect at C concentration of 1 × 10<sup>19</sup> cm<sup>−3</sup>. These results indicate the possibility that the formation mechanism of yellow luminescence band is different depending on the C doping method even at the same C concentration.

DOI： 10.1002/pssb.202500012

Characteristics of 2DEG generated at the heterointerface of an AlN/GaN structure grown on an AlN substrate using metal organic vapor phase epitaxy Open Access

Yoshikawa, A; Kumabe, T; Sugiyama, S; Arai, M; Suda, J; Amano, H

JOURNAL OF APPLIED PHYSICS Vol. 137 ( 19 ) 2025.5

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

A pseudomorphic AlN/GaN structure was grown on an AlN substrate using metalorganic vapor phase epitaxy (MOVPE). The Hall effect characteristics of two-dimensional electron gas induced at the interface between AlN and GaN and the properties of high-electron-mobility transistors (HEMTs) fabricated using the proposed AlN/GaN structures were investigated. The Hall effect measurements indicated that the electron mobility at room temperature was 294 cm<sup>2</sup> V<sup>−1</sup> s<sup>−1</sup>, the sheet carrier density was 1.1 × 10<sup>13</sup> cm<sup>−2</sup>, and the sheet resistance was 1930 Ω/sq. Cross-sectional transmission electron microscopy images revealed an extremely abrupt interface between the AlN and GaN layers. By contrast, surface atomic force microscopy measurements indicated that GaN exhibited a clear step-terrace surface and the AlN barrier layer exhibited the signs of transitioning to island-like growth. Furthermore, the HEMT devices exhibited clear transistor characteristics with a source-drain breakdown voltage of 2.3 kV and an ON/OFF ratio of >10<sup>7</sup>. These findings demonstrate the potential of MOVPE for fabricating AlN/GaN on AlN substrates, showing the inherent potential of AlN as a substrate.

DOI： 10.1063/5.0255068

Quantum efficiency characteristics of low-threading-dislocation-density InGaN photocathode grown on GaN substrate

Idei, M; Sato, D; Koizumi, A; Nishitani, T; Honda, Y; Amano, H

JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B Vol. 43 ( 3 ) 2025.5

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Vacuum Science and Technology B

The dependence of the quantum efficiency (QE) of a Cs/O-activated negative-electron-affinity (NEA) state InGaN photocathode on threading dislocation density (TDD) was investigated. InGaN photocathodes with different TDDs were grown on sapphire and GaN substrates by metal-organic vapor-phase epitaxy (MOVPE). The TDDs were 1 × 10<sup>9</sup> cm<sup>−2</sup> on the sapphire substrate and less than 5 × 10<sup>6</sup> cm<sup>−2</sup> on the GaN substrate. After the activation of the NEA state of InGaN photocathode surfaces by Cs/O, the QEs were 1.1% on the sapphire substrate and 0.91% on the GaN substrate. Despite a TDD difference of approximately two orders of magnitude, the QEs were comparable. The results show that the QE is not limited by the high TDD in the InGaN photocathode.

DOI： 10.1116/6.0004199

Modeling and Designing a GaN-Growth Reactor With Halogen-Free Vapor Phase Epitaxy: NH<sub>3</sub> Decomposition at the Catalytic Surface of Components to Replicate Parasitic Polycrystal Formation Open Access

Shimazu, H; Nishizawa, SI; Nitta, S; Amano, H; Nakamura, D

IEEE TRANSACTIONS ON SEMICONDUCTOR MANUFACTURING Vol. 38 ( 2 ) page： 311 - 323 2025.5

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE Transactions on Semiconductor Manufacturing

Achieving long-duration, large bulk GaN growth is crucial to supply low-cost, high-quality GaN. Halogen-free vapor phase epitaxy (HF-VPE) is a promising method for bulk GaN growth but faces challenges due to severe polycrystals deposition on reactor components, such as the source-gas nozzles, which impedes stable, extended growth. In this study, we developed models to simulate the polycrystal deposition in HF-VPE-GaN growth conditions by including surface reactions of GaN formation and NH3 decomposition. Moreover, we devised conditions for controlling gas flow and interdiffusion to suppress polycrystal deposition around the source-gas nozzles. Experimental results aligned with simulations, showing that increasing the distance between Ga and NH<inf>3</inf> nozzles and replacing the sheath gas from H<inf>2</inf> to N<inf>2 </inf>effectively minimized polycrystal formation. The findings confirm that reducing NH<inf>3</inf> concentration through catalytic surface decomposition on refractory components is crucial to polycrystal suppression. Optimizing nozzle dimensions and gas species synergistically controls the gas flow and interdiffusion. The constructed models contribute to advancing the design of polycrystal suppressive structures and conditions for long-duration bulk GaN growth.

DOI： 10.1109/TSM.2025.3558328

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