Applied Physics Express

An AlGaN/GaN high-electron-mobility transistor (HEMT) on a free-standing GaN substrate achieved impressive power-added and drain efficiencies of 85.2% and 89.0%, respectively, at 2.45 GHz. We improved the GaN channel quality by reducing the C concentration and eliminated the buffer leakage path by removing the residual Si at the substrate-epitaxial layer interface. These improvements, combined with the reduction in dislocation density and the elimination of the nucleation layer by using a free-standing GaN substrate, contributed to the enhanced efficiency. To the best of our knowledge, the achieved efficiency represents the highest reported for GaN-based discrete HEMTs in this frequency band.

A rutile germanium dioxide (r-GeO₂) vertical Schottky barrier diode (SBD) was fabricated. An unintentionally doped n-type single-phase r-GeO₂ epitaxial layer and Sb-doped n⁺-type graded Ge_ySn_1−yO₂ buffer layers were grown on an Nb-doped n⁺-type (001) TiO₂ substrate via mist chemical vapor deposition. A Ni Schottky electrode and a Ti/Al Ohmic contact were formed on the surface and the backside, respectively. Capacitance–voltage and current–voltage characteristics of the Ni/r-GeO₂ vertical SBD revealed clear rectification behavior and a significantly low on-resistance. These findings contribute to the advancement of r-GeO₂-based electronic devices.

Major features of silicon carbide (SiC) power devices include high blocking voltage, low on-state loss, and fast switching, compared with those of the Si counterparts. Through recent progress in the material and device technologies of SiC, production of 600–3300 V class SiC unipolar devices such as power metal-oxide-semiconductor field-effect transistors (MOSFETs) and Schottky barrier diodes has started, and the adoption of SiC devices has been demonstrated to greatly reduce power loss in real systems. However, the interface defects and bulk defects in SiC power MOSFETs severely limit the device performance and reliability. In this review, the advantages and present status of SiC devices are introduced and then defect engineering in SiC power devices is presented. In particular, two critical issues, namely defects near the oxide/SiC interface and the expansion of single Shockley-type stacking faults, are discussed. The current physical understanding as well as attempts to reduce these defects and to minimize defect-associated problems are reviewed.

We proposed a strain-compensated distributed Bragg reflector (DBR) with high reflectivity and a practical growth time. The DBR consists of AlGaN/GaN multilayer as the low refractive index layer and InGaN as the high refractive index layer. The DBR was grown on c-plane GaN substrate as an asymmetric structure with InGaN thicker than λ/4. By growing each layer at ≥1 μm h⁻¹ and minimizing interruptions, a 60.5-period DBR with a center wavelength of 441.8 nm and nearly 100% peak reflectivity was achieved in about 5 h. A VCSEL with the strain-compensated DBR was fabricated, and the clear threshold characteristic was confirmed by optical pumping.

Aluminum Scandium Nitride (AlScN) is an attractive material for use as a lattice-matched epitaxial barrier layer in GaN high-electron mobility transistors (HEMTs). Here we report the device fabrication, direct current (DC) and radio frequency (RF) characteristics of epitaxial AlScN/AlN/GaN HEMTs on SiC substrates with regrown ohmic contacts. These devices show record high on-current of over 4 A/mm, high cutoff frequency (f_T) of 92.4 GHz and maximum oscillation frequency (${f}_{{\rm{MAX}}}$) of 134.3 GHz.

This review describes the progress of research on gallium oxide as a material for power devices, covering the development of bulk crystal growth through to epitaxial growth, defect evaluations, device processes, and development, all based on the author's research experiences. During the last decade or so, the epi-wafer size has been expanded to 4–6 inches, and Schottky barrier diodes and field-effect transistors capable of ampere-class operations and with breakdown voltages of several kV have been demonstrated. On the other hand, challenges to the practical application of gallium oxide power devices, such as the cost of epi-wafers, killer defects, purity of epitaxial layer, etc., have also become apparent. This paper provides a comprehensive summary of the history of these developments, including not only papers but also patents and conference presentations, and gives my personal views on the prospects for this material's continued development.

400 metal-oxide-semiconductor field-effect transistors (MOSFETs) were fabricated on a half-inch diamond substrate. The performance of each device was evaluated, and an H-terminated diamond MOSFET chip was created by connecting over 300 of the well performing MOSFETs in parallel, resulting in a gate width of 32 cm. This chip was used for double pulse testing, with its switching characteristics being evaluated at 2.5 A. The results show a fall/rise time of 19/32 ns, respectively, and switching losses during turn-off/turn-on of 4.65/1.24 μJ. This study demonstrated switching operation at large currents in diamond power MOSFETs.

A full-color monolithic micro-light-emitting diode (LED) display based on InGaN quantum wells is demonstrated. We stacked red, green, and blue (RGB) light-emitting layers and selectively removed and regrew a p-type layer to create distinct areas on a single chip that emitted RGB colors. Subsequently, we fabricated a full-color monolithic micro-LED chip with a pixel pitch of 30 μm and pixel number of 96 × 96. Each color subpixel emits light with a single peak. We obtained a full-color image by driving the chip using a microcontroller. The proposed semiconductor process-based method enables the fabrication of low-cost and high-resolution microdisplays.

ScAlN is a promising barrier material for next generation RF high electron mobility transistors, outperforming AlGaN thanks a higher two-dimensional electron gas (2DEG) density and a thinner barrier with a lower lattice mismatch with GaN. A sub-10 nm barrier ScAlN/GaN heterostructure, grown by ammonia-source molecular beam epitaxy on Si (111), is processed into transistors. The 2DEG density is 1.6 × 10¹³ cm⁻² with a mobility μ ∼ 621 cm² V⁻¹ s⁻¹. A 75 nm gate length transistor exhibits a drain current density of 1.35 A mm⁻¹, a transconductance of ∼284 mS mm⁻¹, a current gain cutoff frequency of 82 GHz and a maximum oscillation frequency of 112 GHz.

We have developed a measurement system for soft X-ray ambient pressure photoelectron spectroscopy at BL08U of NanoTerasu. An excitation light of 800 eV was introduced by a vacuum tube terminated with a SiN window. Photoelectrons were detected through a ϕ24 μm aperture at the electron lens entrance of a differentially pumped analyzer. The Au 4f core-level spectra of a Au film were measured up to atmospheric pressure (1 bar) with He, to 1 bar with H₂, and to 0.4 bar with N₂. The system extends applications of operando experiments for the real functional materials.

To improve the blocking capability of U-shaped trench gate MOSFETs (UMOSFETs) based on nitrogen-implanted current blocking layer (CBL), the post-implantation annealing atmosphere is optimized. Compared to the UMOSFET annealed in nitrogen, the oxygen-annealed device shows an improved breakdown voltage, increasing from 830 V to 1330 V, while maintaining similar specific on-resistance. By analyzing the leakage mechanisms of CBLs, the lower leakage current in the oxygen-annealed device is attributed to the reduced concentration of certain deep donors and the change in trap energy level involved in the Poole–Frenkel (PF) emission. This work validates oxygen annealing as an effective approach to optimizing nitrogen-implanted β-Ga₂O₃ power MOSFETs.

We report the successful integration of low-temperature polycrystalline all-around diamond as heat spreaders with 150 nm gate length N-polar GaN MISHEMT platform to improve power efficiencies for X-band applications. With an all-around integration scheme, the CVD-grown diamond reduces channel's peak temperature, improving device performance and reliability. A combination of optimized low-temperature diamond growth at 500 °C with the thermally stable molybdenum gate metal and MOCVD-grown SiN_x gate-dielectric was utilized for the successful integration. The fabricated device exhibited a I_DSS of 0.96 A mm⁻¹ and an ON-to-OFF ratio of 10⁵. This marks the first post-process diamond integration on a RF GaN HEMT device.

The total internal reflection at the fluorescent glass–air interface severely limits the external quantum efficiency (EQE) of fluorescent-glass-based light sources. In this work, micro-patterning of the light incident and exit interface of fluorescent glasses was employed to enhance the light coupling efficiency between LED chip and fluorescent glass, as well as the efficiency of light emission into the external space. A highly efficient white light emission with 41% of EQE was obtained by integrating double-sided micro-patterned Ce³⁺/Dy³⁺ co-doped fluorescent glass with 310 nm LED excitation chip, increasing by 31.5% and 22.53% respectively, as compared to the ones with planar fluorescent glass and single-sided patterned fluorescent glass.

Feedback-controlled electromigration (FCE) enables precise regulation of atomic migration by carefully optimizing multiple experimental parameters. However, manually fine-tuning these parameters poses significant challenges. This study investigated the feasibility of autonomously fabricating Au atomic junctions through gate-based quantum computing using a noisy intermediate-scale quantum (NISQ) device, which effectively approximates solutions to combinatorial optimization problems. We compared the computational accuracy of the NISQ device against a previously reported D-Wave quantum annealer. The results indicate that the NISQ device achieved lower residual energies and produced higher-quality approximate solutions for large-scale problems than the quantum annealing system.

This review describes the progress of research on gallium oxide as a material for power devices, covering the development of bulk crystal growth through to epitaxial growth, defect evaluations, device processes, and development, all based on the author's research experiences. During the last decade or so, the epi-wafer size has been expanded to 4–6 inches, and Schottky barrier diodes and field-effect transistors capable of ampere-class operations and with breakdown voltages of several kV have been demonstrated. On the other hand, challenges to the practical application of gallium oxide power devices, such as the cost of epi-wafers, killer defects, purity of epitaxial layer, etc., have also become apparent. This paper provides a comprehensive summary of the history of these developments, including not only papers but also patents and conference presentations, and gives my personal views on the prospects for this material's continued development.

During the last two decades, rapid advancements in RT oscillators that use resonant tunneling diodes (RTDs) have been reported, with operations approaching the limits of electronic device oscillators. Although RTD devices are known for HF operation, milliwatt-level high-output powers have been recently obtained using a single device. Moreover, interesting operations using feedback and injection locking phenomena are also emerging. This paper outlines the basic oscillation principles, oscillation characteristics, and applications of RTD devices. Unlike previous reviews, the basic parts include harmonic signal generation, the construction of resonators and antennas, and bias circuits, which have been newly summarized. A graphical method for determining oscillation is introduced, and the oscillator characteristics are summarized in terms of new indicators, such as power density. This paper also includes the modulation characteristics of the intrinsic part of the device, spectral changes owing to feedback, and the characteristics of the RTD device as a receiver.

Electronic states at the boundaries of crystals, such as surfaces, interfaces, edges, hinges, corners, and extremities, play crucial roles in emerging quantum materials, such as graphene and similar monatomic-layer materials, van der Waals crystals, and topological insulators. Electronic states at such boundaries are different from those inside the three- or two-dimensional crystals, not only because of the truncation of crystal lattices but also because of space-inversion-symmetry breaking and difference in topology in band structures across the boundaries. Such quantum materials are expected to advance energy-saving/-harvesting technology as well as quantum computing/information technology because of exotic phenomena, such as spin–momentum locking of an electron, pure spin current, dissipation-less charge current, nonreciprocal current, and possible Majorana fermions. In this review, their fundamental concepts are introduced from the viewpoint of surface physics, in which atomic and electronic structures, as well as charge/spin transport properties, are directly probed using state-of-the-art techniques.

An emerging concept of "nanoarchitectonics" has been proposed as a way to apply the progress of nanotechnology to materials science. In the introductory parts, we briefly explain the progress in understanding materials through nanotechnology, the overview of nanoarchitectonics, the effects of nanoarchitectonics on the development of functional materials and devices, and outline of nanoarchitectonics intelligence as a main subject of this review paper. In the following sections, we explain the process of constructing intelligent devices based on atomic switches, in which the behavior of atoms determines the device functions, by integrating them with nanoarchitectonics. The contents are categorized into (i) basic operation of atomic switch, (ii) artificial synapse, (iii) neuromorphic network system, (iv) hetero-signal conversion, (v) decision making device, and (vi) atomic switch in practical uses. The atomic switches were originally relatively simple ON/OFF binary-type electrical devices, but their potential as multi-level resistive memory devices for artificial synapses and neuromorphic applications. Furthermore, network-structured atomic switches, which are complex and have regression pathways in their structure and resemble cranial neural circuits. For example, A decision-making device that reproduces human thinking based on a principle different from brain neural circuits was developed using atomic switches and proton-conductive electrochemical cells. Furthermore, atomic switches have been progressively developed into practical usages including application in harsh environments (e.g. high temperature, low temperature, space). Efforts toward information processing and artificial intelligence applications based on nanoarchitectonics tell remarkable success stories of nanoarchitectonics, linking the control of atomic motion to brain-like information control through nanoarchitecture regulations.

Nanopores are cost-effective digital platforms, which can rapidly detect and identify biomolecules at the single-molecule level with high accuracy via the changes in ionic currents. Furthermore, nanoscale deoxyribonucleic acid and proteins, as well as viruses and bacteria that are as small as several hundred nanometers and several microns, respectively, can be detected and identified by optimizing the diameters of a nanopore according to the sample molecule. Thus, this review presents an overview of the methods for fabricating nanopores, as well as their electrical properties, followed by an overview of the transport properties of ions and analyte molecules and the methods for electrical signal analysis. Thus, this review addresses the challenges of the practical application of nanopores and the countermeasures for mitigating them, thereby accelerating the construction of digital networks to secure the safety, security, and health of people globally.

By using tetramethylsilane (TMSi) as a Si dopant source, thick intentionally Si-doped β-Ga₂O₃ homoepitaxial layers were grown by low-pressure hot-wall metalorganic vapor phase epitaxy. The Si concentration was linearly controlled by varying the TMSi supply, and a RT electron density nearly equal to the Si concentration was achieved in the range from 1.8 × 10¹⁶ to 1.3 × 10¹⁹ cm^–3. For the layer with a Si concentration and RT electron density of 1.8 × 10¹⁶ cm^–3, the RT electron mobility of 136 cm² V^–1 s^–1 was found to be limited by polar optical phonon scattering.

During the lithography development process, the resist films absorb developer molecules and transiently swell before dissolving. This swelling layer is crucial for resist pattern formation. However, an effective method for analyzing the transient swelling layer is lacking. In this study, we report a novel method for analyzing a transient swelling layer based on a quartz crystal microbalance (QCM). By analyzing the frequency and impedance charts of the QCM using diffusion theory, the proposed method enables the estimation of the temporal change in the weight of the developer molecules contained in the polymer film.

Exciton dissociation into free electron-hole carriers is a central issue in optoelectronic devices. Despite recent progress in free-carrier generation due to an admixture of organic compounds, strategies to prolong the lifetime of photo-induced carriers are limited. Herein, we present a proof-of-concept of efficient exciton dissociation via heterogeneous device engineering; an insulating polymer layer adjacent to organic semiconductors (OSCs) in conjunction with an application of electrical voltage can capture photogenerated electrons selectively, and the resulting semipermanent built-in electric field gives rise to an extraordinarily persistent photo-induced carriers, which lasts for more than half a year in single-crystalline OSCs.

GaN-on-GaN epitaxial growth on 4- and 6-inch wafers was demonstrated using a new mass-production-type quartz-free hydride vapor phase epitaxy (QF-HVPE) system. The thickness, effective donor density, and near-band-edge photoluminescence peak intensity were confirmed to be uniform for 4-inch wafers. In addition, a new QF-HVPE system enabled the growth of extremely pure GaN crystals with a C concentration lower than 1×1014 /cm3, enabling a wide range of doping control from 1×1014 to 1×1018 /cm3. The resultant GaN wafers were free from C-induced mobility collapse and exhibited record-high room-temperature and maximum mobilities of 1591 cm2/(V·s) and 18,175 cm2/(V·s) at 35K, respectively.

Based on a 2.0-cm-long Tm³⁺-doped fiber (TDF) and in-band pumped configuration, an efficient long-wavelength single-frequency distributed Bragg reflector (DBR) fiber laser at 2097 nm is demonstrated experimentally. With delicate optimization of the TDF length and the pump wavelength to consider the effective cavity length and pump absorption, a stable single-longitudinal-mode laser output of 10.4 mW is achieved under 1610 nm in-band pumping. To the best of our knowledge, this is the first demonstration of a single-frequency DBR Tm³⁺-doped fiber laser at a long operation wavelength of 2.1 μm.

400 metal-oxide-semiconductor field-effect transistors (MOSFETs) were fabricated on a half-inch diamond substrate. The performance of each device was evaluated, and an H-terminated diamond MOSFET chip was created by connecting over 300 of the well performing MOSFETs in parallel, resulting in a gate width of 32 cm. This chip was used for double pulse testing, with its switching characteristics being evaluated at 2.5 A. The results show a fall/rise time of 19/32 ns, respectively, and switching losses during turn-off/turn-on of 4.65/1.24 μJ. This study demonstrated switching operation at large currents in diamond power MOSFETs.

Magneto-thermal switching (MTS) is a key technology for efficient thermal management. Recently, large MTS with nonvolatility has been observed in Sn-Pb solders [H. Arima et al. Commun. Mater. 5, 34 (2024)] where phase separation, the different superconducting transition temperatures (T_c) of Sn and Pb, and magnetic-flux trapping are the causes of the nonvolatile MTS. To further understand the mechanism and to obtain the strategy for enhancing switching ratio, exploration of new phase-separated superconductors with nonvolatile MTS is needed. Here, we show that the In52-Sn48 commercial solder is a phase-separated superconducting composite with two T_c and traps vortices after field cooling. A clear signature of nonvolatile MTS was observed at T = 2.5 K. From specific heat analyses, we conclude that the vortices are mainly trapped in the lower-T_c phase (γ-phase) after field cooling, which is evidence that vortex trapping also works on achieving nonvolatile MTS in phase-separated superconducting composites.

We report on the preparation of vanadium dioxide (VO₂) ultrathin films on hexagonal boron nitride (hBN), which is a typical two-dimensional material, to show clear metal–insulator transition owing to weak van der Waals interaction at their surface. It is confirmed that VO₂ films on hBN with thicknesses ranging from 10 to 40 nm exhibit bulk like metal–insulator transition without degradation using Raman scattering spectroscopy and electric transport measurements. These results demonstrate the importance of the 2D material nature of hBN for producing strain-free oxide thin films.