Masataka Imura

We report the first resonance characterization of β-Ga ₂ O ₃ -based doubly-clamped-beam resonators elongated along [100] and fabricated directly on a (001) β-Ga ₂ O ₃ substrate by crystallographic gas etching. The beams exhibited clear fundamental-mode resonances with quality factors ( Q ) on the order of 10 ⁴ . Analysis of the beam-length dependence of the resonant frequency ( f ₀ ) yielded an effective Young’s modulus of 143.2 (+16.4/−14.0) GPa within the Euler–Bernoulli beam approximation, while axial-stress analysis indicated only a small residual tensile stress. The f ₀ Q values exceeded the highest reported values for transferred-flake-based doubly clamped beams, highlighting the potential advantage of crystallographic etching for fabricating β-Ga ₂ O ₃ microelectromechanical systems.

Electron paramagnetic resonance (EPR) spectroscopy was used to investigate paramagnetic impurity centers in commercially available MgO single crystals at room temperature. Three dominant signals with resolved hyperfine structures were assigned to Mn-, V-, and Si-related centers based on their hyperfine patterns, angular dependences, and spin-Hamiltonian analysis. Angular-dependent measurements showed negligible anisotropy in the dominant resonance fields and hyperfine interactions, whereas weak Mn-related fine-structure components were reproduced by a cubic spin-Hamiltonian model. Sequential line-shape fitting and double integration enabled evaluation of representative g values, hyperfine constants, semi-quantitative spin concentrations, and effective spin-density ratios. The Mn-related signal is consistent with substitutional Mn ²⁺ in a nearly cubic environment, while the V-related signal is consistent with a nearly isotropic V ²⁺ -like center. The Si-related defect shows a much smaller hyperfine interaction, suggesting a spatially extended electronic state. These results provide a systematic EPR characterization of residual impurity centers in MgO substrates.

The demand for noncontact temperature measurements is growing as the microelectronic features to be probed become progressively smaller. However, traditional thermal imaging with infrared cameras is fundamentally constrained by the micron-long wavelength of thermal emission. In this study, we explore a cost-effective, high-spatial-resolution thermography technique utilizing an uncooled silicon CMOS camera with an optical microscope setup. By detecting the integrated near-infrared (NIR, 700–1100 nm) thermal emission from Joule-heated metallic microstructures, our method successfully bypasses the traditional microbolometer’s diffraction limit and spatial resolution constraints. To ensure accurate temperature estimation, the camera is calibrated by leveraging the temperature-dependent Raman peak shift of hexagonal boron nitride flakes. Our optical microscope setup is in an oversampling condition, allowing us to exploit digital binning to achieve significant noise reduction and to directly visualize and map thermal emission at temperatures as low as 413 K for our titanium sample, which has an emissivity of around 0.4 in the NIR. This method provides a robust, highly capable tool for diagnosing localized thermal bottlenecks and mapping heat diffusion in high-density microelectronics.

We demonstrated fully vertical heterojunction diodes (HJDs) featuring a Mg-doped AlN layer directly grown on an n -type 4H-SiC substrate by metal-organic chemical vapor deposition. The electrical activation of the Mg-doped AlN layer was performed at 900 °C. At room temperature, the specific contact resistivity of the Mg-doped GaN/Mg-doped AlN structure was 6 × 10 ⁻³ Ω cm ² . In the AlN-on-SiC HJD, rectifying characteristics were obtained from 25 °C to 500 °C, exhibiting a rectification ratio exceeding 10 ² . Reducing trap states and dislocations in the AlN layer would improve the device performance of the fully vertical AlN-based diodes.

We grew GaN/InxGa1−xN superlattices (SLs) on GaN/sapphire substrates by atmospheric-pressure metal-organic vapor phase epitaxy (MOVPE) and determined the InxGa1−xN growth rate using high-resolution x-ray diffraction analysis. The use of SL structures substantially reduced the influence of strain relaxation, phase separation, and mixed 2D/3D growth modes, enabling reliable extraction of the effective GaN and InN growth rates within the alloy. By maintaining identical temperature, pressure, and gas-flow balance, we precisely estimated the individual growth rates and proposed a model that explicitly separates incorporation and desorption processes. For low-In composition (xIn < 0.25), the GaN and InN rates exhibit negligible mutual interaction and depend solely on temperature under constant pressure and flow balance. The model successfully predicts InxGa1−xN growth rates using independent incorporation and desorption terms, and its applicability is confirmed for In vapor-phase ratios <0.7 and sufficiently high V/III ratios, while limitations are identified under excess-In or low-temperature conditions. These results provide practical guidelines for optimizing MOVPE growth, contributing to the efficient design of active and SL layers for high-power light-emitting diodes and green laser diodes.

We fabricated a GaN particle detector with a low-gain avalanche diode (LGAD) structure. The GaN detectors showed an ideality factor of 2.0 under forward bias, leakage currents less than 100 pA at −150 V, a differential specific on-resistance of 7 mΩ cm ² , and the breakdown voltage of 473 V. For below-bandgap illumination, the gains at 50 °C at −100 V and −210 V were 2 and 25, respectively. Although avalanche multiplication in the gain layer was not dominant in the reverse current, we observed signals from an alpha particle and a Xe swift-heavy ion using the GaN detectors with LGAD structure, showing the excellent potential of GaN LGAD radiation detectors for applications in high-energy particle physics.

Hydrogen-terminated diamond field-effect transistors (FETs) using a hexagonal boron nitride (h-BN) gate insulator were fabricated on a diamond surface with reduced surface roughness in the direction of source/drain electrodes. The diamond surface was prepared on a mesa structure using chemical vapor deposition with a low methane concentration. The hydrogen-terminated surface was laminated with the h-BN gate insulator without air exposure to prevent the adsorption of atmospheric surface acceptors. The hydrogen-terminated diamond FET exhibited a high mobility of ≈1000 cm2 V−1 s−1 at room temperature. We performed theoretical analysis on the temperature and carrier density dependences of mobility, which suggested that Coulomb and surface roughness scattering were effectively reduced. The high mobility obtained in this study indicates the high potential of diamond as a semiconducting material. This study can contribute to the future development of diamond devices.

Infrared near-field spectroscopy, or nano-FTIR, offers nanoscale resolution in three dimensions to probe the chemical and physical properties of samples, making it a unique characterization tool. This nanoscopic resolution in three dimensions is particularly suitable to probe a two-dimensional electron gas (2DEG) where a 2DEG has an effective thickness of a few nanometers and exists a few tens of nanometers below the capping layer. This work employs nano-FTIR spectroscopy to noninvasively probe the 2DEG of AlGaN/GaN heterostructures, which are crucial for high-power electronic devices and sensing applications. Higher harmonic amplitude and phase of the nano-FTIR spectra are sensitive enough to the carrier concentration of the 2DEGs, which is supported by analytical calculations based on the finite dipole model. A comparative analysis confirms that incorporating the 2DEG layer into the model is essential to matching spectral features with experimental observations. Furthermore, hyperspectral imaging of a cross-sectional sample provides a visual representation of the 2DEG. The findings demonstrate that nano-FTIR enables the characterization of 2DEG in AlGaN/GaN heterostructures with nanometric resolution under ambient conditions, hence expanding its applicability in the study of such systems.

We have been developing a CIGS detector for particle detection with high radiation tolerance. We irradiated the CIGS detector with a ¹³²Xe⁵⁴⁺ ion beam, delivering a total ionizing dose (TID) of 0.6 MGy at the HIMAC, and confirmed the recovery of leakage current and collected charge from radiation damage with heat annealing. To investigate higher radiation tolerance of the CIGS semiconductor, we irradiated CIGS solar cells with a 70 MeV proton beam with a non-ionizing energy loss (NIEL) for 10¹⁶n_eq· MeV/cm² at the RARiS. The V_In and In_Cu defects created by 70 MeV proton irradiation were observed by deep level transient spectroscopy (DLTS). During thermal annealing at around 100°C, the Cu⁺ and V^- _Cu ions are excited to react with defects. These ions interact with V_In and In_Cu defects, creating electrical neutrality: 2Cu⁺ + V^2- _In → Cu₂V_In and 2V^- _In + In²⁺ _Cu → 2V_In In_Cu. By these reaction process, it is confirmed that both V_In and In_Cu defects were reduced by the thermal annealing at 130°C for two hours.

We fabricated arrays of circular, square, and triangular air holes in GaN by the conventional method using the electron beam lithography and reactive ion etching. The vertex radius of curvature for the square and triangular holes was reduced by shape modification. The minimum vertex radius of curvature for the 100 nm size square and triangular holes were 17 and 11 nm, respectively.