研究者詳細 - 須田　淳

論文 - 須田　淳

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Hole traps related to nitrogen displacement in p-type GaN grown by metalorganic vapor phase epitaxy on freestanding GaN

Endo, M; Horita, M; Suda, J

APPLIED PHYSICS LETTERS 120 巻 ( 14 ) 2022年4月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Identification of type of threading dislocation causing reverse leakage in GaN p-n junctions after continuous forward current stress Open Access

Narita, T; Kanechika, M; Kojima, J; Watanabe, H; Kondo, T; Uesugi, T; Yamaguchi, S; Kimoto, Y; Tomita, K; Nagasato, Y; Ikeda, S; Kosaki, M; Oka, T; Suda, J

SCIENTIFIC REPORTS 12 巻 ( 1 ) 2022年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Breakdown Electric Field of GaN p<SUP>+</SUP>-n and p-n<SUP>+</SUP> Junction Diodes With Various Doping Concentrations

Maeda, T; Narita, T; Yamada, S; Kachi, T; Kimoto, T; Horita, M; Suda, J

IEEE ELECTRON DEVICE LETTERS 43 巻 ( 1 ) 頁： 96 - 99 2022年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Dependence of Electrical Characteristics on Epitaxial Layer Structure of AlGaN/GaN HEMTs Fabricated on Freestanding GaN Substrates

Ando, Y; Makisako, R; Takahashi, H; Wakejima, A; Suda, J

IEEE TRANSACTIONS ON ELECTRON DEVICES 69 巻 ( 1 ) 頁： 88 - 95 2022年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Current Status and Future Prospects of GaN-on-GaN Vertical Power Devices

Suda, J

2022 INTERNATIONAL ELECTRON DEVICES MEETING, IEDM 2022-December 巻頁： 3571 - 3574 2022年

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Effect of annealing time and pressure on electrical activation and surface morphology of Mg-implanted GaN annealed at 1300 °C in ultra-high-pressure nitrogen ambient

Sumida, K; Hirukawa, K; Sakurai, H; Sierakowski, K; Horita, M; Bockowski, M; Kachi, T; Suda, J

APPLIED PHYSICS EXPRESS 14 巻 ( 12 ) 2021年12月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Analysis of intrinsic reverse leakage current resulting from band-to-band tunneling in dislocation-free GaN p-n junctions

Shoji, T; Narita, T; Nagasato, Y; Kanechika, M; Kondo, T; Uesugi, T; Tomita, K; Ikeda, S; Mori, T; Yamaguchi, S; Kimoto, Y; Kojima, J; Suda, J

APPLIED PHYSICS EXPRESS 14 巻 ( 11 ) 2021年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Effect of Schottky barrier height on quantitative analysis of deep-levels in n-type GaN by deep-level transient spectroscopy Open Access

Aoshima, K; Horita, M; Suda, J

AIP ADVANCES 11 巻 ( 11 ) 2021年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Effects of the sequential implantation of Mg and N ions into GaN for p-type doping

Sakurai, H; Narita, T; Kataoka, K; Hirukawa, K; Sumida, K; Yamada, S; Sierakowski, K; Horita, M; Ikarashi, N; Bockowski, M; Suda, J; Kachi, T

APPLIED PHYSICS EXPRESS 14 巻 ( 11 ) 2021年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Fabrication of 150-nm AlGaN/GaN field-plated High Electron Mobility Transistors using <i>i</i>-line stepper Open Access

Ando, Y; Makisako, R; Takahashi, H; Wakejima, A; Suda, J

ELECTRONICS LETTERS 57 巻 ( 24 ) 頁： 948 - 949 2021年11月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

GaN縦型パワーデバイス実現に向けた点欠陥評価

須田淳, 堀田昌宏, 鐘ヶ江一孝

応用物理 90 巻 ( 10 ) 頁： 628 - 631 2021年10月

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：公益社団法人応用物理学会

<p>GaN縦型パワーデバイスのエピタキシャル成長，デバイスプロセスの開発，あるいは，デバイス設計に点欠陥の知見は不可欠であるが，SiやGaAsに比べると，GaNの点欠陥の理解はまだまだ不足している．議論が分かれていた電子トラップの起源特定，n型GaN中の正孔トラップの簡便な定量評価方法の開発や測定の高速化によるウェーハマッピング，電子線照射により意図的に窒素原子のみを変位させることにより，窒素空孔や格子間窒素の作るトラップ準位，アニール挙動など，筆者らが取り組んできた研究について紹介する．</p>

DOI： 10.11470/oubutsu.90.10_628

Electrical characteristics of gated-anode diodes based on normally-off GaN HEMT structures for rectenna applications

Takahashi, H; Ando, Y; Tsuchiya, Y; Wakejima, A; Hayashi, H; Yagyu, E; Kikkawa, K; Sakai, N; Itoh, K; Suda, J

ELECTRONICS LETTERS 57 巻 ( 21 ) 頁： 810 - 812 2021年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Depth profiles of electron traps generated during reactive ion etching in n-type 4H-SiC characterized by using isothermal capacitance transient spectroscopy

Kanegae, K; Okuda, T; Horita, M; Suda, J; Kimoto, T

JOURNAL OF APPLIED PHYSICS 130 巻 ( 10 ) 2021年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Impact of channel mobility on design optimization of 600-3300 V-class high-speed GaN vertical-trench MOSFETs based on TCAD simulation

Ishida, T; Sakao, K; Kachi, T; Suda, J

APPLIED PHYSICS EXPRESS 14 巻 ( 9 ) 2021年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Photoionization cross section ratio of nitrogen-site carbon in GaN under sub-bandgap-light irradiation determined by isothermal capacitance transient spectroscopy

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

APPLIED PHYSICS EXPRESS 14 巻 ( 9 ) 2021年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Design and demonstration of nearly-ideal edge termination for GaN p-n junction using Mg-implanted field limiting rings

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

APPLIED PHYSICS EXPRESS 14 巻 ( 7 ) 2021年7月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Increase of reverse leakage current at homoepitaxial GaN p-n junctions induced by continuous forward current stress

Narita, T; Nagasato, Y; Kanechika, M; Kondo, T; Uesugi, T; Tomita, K; Ikeda, S; Yamaguchi, S; Kimoto, Y; Kosaki, M; Oka, T; Kojima, J; Suda, J

APPLIED PHYSICS LETTERS 118 巻 ( 25 ) 2021年6月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Impact ionization coefficients and critical electric field in GaN

Maeda, T; Narita, T; Yamada, S; Kachi, T; Kimoto, T; Horita, M; Suda, J

JOURNAL OF APPLIED PHYSICS 129 巻 ( 18 ) 2021年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Isochronal annealing study of Mg-implanted p-type GaN activated by ultra-high-pressure annealing

Hirukawa, K; Sumida, K; Sakurai, H; Fujikura, H; Horita, M; Otoki, Y; Sierakowski, K; Bockowski, M; Kachi, T; Suda, J

APPLIED PHYSICS EXPRESS 14 巻 ( 5 ) 2021年5月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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

Formation of highly vertical trenches with rounded corners via inductively coupled plasma reactive ion etching for vertical GaN power devices

Yamada, S; Sakurai, H; Osada, Y; Furuta, K; Nakamura, T; Kamimura, R; Narita, T; Suda, J; Kachi, T

APPLIED PHYSICS LETTERS 118 巻 ( 10 ) 2021年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：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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