Akio Yamamoto

Effects of the ultrathin 1 nm thick AlGaN regrown layer before gate insulator deposition on the performance of ZrO2/AlGaN/GaN metal‐insulator‐semiconductor (MIS) structures are investigated. In comparison with the reference ZrO2/AlGaN/GaN MIS high‐electron‐mobility transistors (MIS‐HEMTs), MIS‐HEMTs with AlGaN regrown layer exhibit increased maximum drain current (>1050 mA mm−1) and broader full‐width at half maximum (FWHM) of transconductance profile (11 V). Moreover, the gate leakage current in the forward direction is reduced by about four orders of magnitude while the hysteresis of the transfer curves is cut‐down to more than 40%. Compared with the control MIS‐capacitors (MIS‐caps), MIS‐caps with regrowth show highly evident sudden increase in capacitance known as “spill‐over” in the capacitance–voltage (C–V) profiles, suggesting improved ZrO2/AlGaN interfaces. Extracted …

We have compared the electrical performance of a proposed normally-off GaN-based MIS-HEMTs employing ultrathin AlGaN barrier layer in the channel with those of conventional recessed-gate structure MIS-HEMTs. The proposed device exhibited much less density of interface states extracted from the measured capacitance-voltage characteristics, suggesting improved Al2O3/AlGaN interface. For corresponding three-terminal transistors, while the conventional reference device exhibited poor control of gate-to-source voltage on drain current with about 3 V hysteresis in the transfer curves, the proposed device showed well-behaved subthreshold characteristics with only 0.8 V hysteresis. Furthermore, the proposed device showed a much higher VTH of+ 5 V compared to+ 1 V of the conventional reference device.

In this paper, we provide a comprehensive review of our studies on the metal–organic-vapor-phase-epitaxial (MOVPE) growth of AlGaN directly on reactive-ion-etched (RIE) GaN surfaces and its application to the fabrication of vertical and lateral enhancement (E)-mode AlGaN/GaN metal–insulator–semiconductor (MIS) high-electron-mobility transistors (HEMTs). First, the peculiarities and challenges of MOVPE growth of AlGaN on RIE-GaN surfaces and their countermeasures are described and discussed. Next, the fabrication of vertical trench E-mode MIS-HEMTs using AlGaN regrowth and their output characteristics are demonstrated. Finally, the recent results on lateral recessed-gate E-mode MIS-HEMTs are described.

We report on the impact of the 3 nm thick ex situ AlGaN regrown layer prior to insulator deposition on the interfacial properties of Al 2 O 3/AlGaN/GaN metal–insulator–semiconductor (MIS) structures. MIS-capacitors (MIScaps) with regrown AlGaN layer exhibited anomalously excessive threshold voltage shift compared to reference sample without regrown AlGaN, suggesting highly reduced interface states density (D it). Moreover, MIScaps with regrown AlGaN layer exhibited" spill-over" in the capacitance–voltage profiles, further evidencing the improved Al 2 O 3/AlGaN interfaces. Fabricated three-terminal MIS-HEMTs with regrown AlGaN showed less hysteresis in transfer curves, enhanced maximum drain current, and increased linearity over the reference device.

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We have fabricated and characterized ZrO2/AlGaN/GaN MIS-HEMTs with "dry-etching free" gate recess structures realized using Selective Area Growth (SAG) of AlGaN barrier on the access regions. The transfer characteristics revealed threshold voltage -0.5 V, suggesting quasi-normally-off operation. In comparison with reference planar HEMTs, the threshold voltage of SAG-HEMTs was shifted by about +5 V towards the positive voltage direction. Moreover, SAG-HEMTs showed almost parallel shift of transfer curves with respect to that of the reference planar devices, suggesting no considerable mobility degradation.

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We have fabricated and characterized ZrO2/AlGaN/GaN MIS-HEMTs with a 1-nm-thick AlGaN layer regrown on top of the original AlGaN barrier layer. Capacitance-voltage characteristics revealed spill-over phenomenon, suggesting good quality of the resulting ZrO2/regrown-AlGaN interface. From the transfer characteristics, maximum drain current of 1100 mA/mm and maximum transconductance of 102 mS/mm were obtained.

The superconductivity in InN is the foundation for the all III-V semiconductor based superconductor/semiconductor integration. To study the flux pinning properties of InN superconductor, the I-V relationships, R-B transitions, and R-T transitions are investigated. The scaling results of I-V curves indicate there is a vortex glass-liquid transition. The R-T curves are well fitted by thermally activated flux flow (TAFF) model. The TAFF activated energy satisfies a power-law relationship with magnetic field,but it has two different exponents under the low magnetic field and high magnetic field. We explain it as the result of a transition from single flux pinning to collective flux pinning which also leads to the rapid attenuation of critical current as the magnetic field increases. By analyzing the temperature dependence of critical current, we found the dominant delta L-pinning mechanism. Furthermore, the dependence of pinning force …

Si tandem solar cells are attractive for new applications such as photovoltaic-powered vehicles because of their high-efficiency and low-cost potential. In particular, III-V/Si tandem solar cells have higher efficiency potential compared to perovskite/Si and other Si tandem solar cells. Although the direct growth of III-V layers on Si is very attractive for cost reduction and simple processing potential, high-quality growth of the III-V thin-film layer on Si is necessary. The paper discusses the effectiveness of the low-temperature growth of III-V layer on Si substrates for realizing low-density dislocations on Si substrates. Low dislocation density of less than 3× 10 5 cm− 2 in GaAs-on-Si by low-temperature growth is demonstrated in this study. According to our analytical results, this low dislocation density shows high potential efficiency of more than 33% and 38% for III-V/Si 2-junction and 3-junction tandem solar cells, respectively.

Herein, the first successful fabrication of enhancement (E)‐mode AlGaN/GaN vertical trench metal–insulator–semiconductor (MIS) high‐electron‐mobility transistors (HEMTs) using n+‐GaN/p‐GaN/n−‐GaN epistructures on free‐standing n+ substrates is reported. A trench with smooth semipolar planes (sidewalls) with angles of 45° and 135° from the c‐plane is formed by reactive ion etching. Using metalorganic vapor‐phase epitaxy, a uniform thickness of the AlGaN layer is regrown in the trench. Devices fabricated without Mg activation treatment for p‐GaN show depletion (D)‐mode operation. The operation mode is changed from D to E when Mg activation annealing temperature exceeds 700 °C. A high drain current (ID) ≥ 0.8 A mm−1 is obtained in the devices with a relatively low Mg concentration (≤1 × 1018 cm−3), whereas a threshold voltage (VTH) as high as 22 V is obtained in the devices with a …

We report on an Al 2 O 3 /AlGaN/GaN metalinsulator-semiconductor high-electron-mobility transistor (MIS-HEMT) with recessed-gate structure and regrown AlGaN barrier. After analyzing the possibility of obtaining high threshold voltage (V th ) within the framework of Tapajna and Kuzmik model from preliminary experiments using MIS-diode structures, we fabricated a MIS-HEMT with the same materials and structures. The transistor exhibited a highVth value of +2.3 V determined at the drain current criterion of 10 μ A/mm together with a maximum drain current density (I Dmax ) of 425 mA/mm. We believe that the adoption of a technology, i. e., AlGaN regrowth on dryetched GaN surface, previously demonstrated by our group in planar device, is the main key for achieving such desirable performance.