Applied Physics Express

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.

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.

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.

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.

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.

Nanodiamonds (NDs) are quantum sensors that enable local temperature measurements, taking advantage of their small size. Though model-based analysis methods have been used for ND quantum thermometry, their accuracy has yet to be thoroughly investigated. Here, we apply model-free machine learning with the Gaussian process regression (GPR) to ND quantum thermometry and compare its capabilities with the existing methods. We prove that GPR provides more robust results than them, even for a small number of data points and regardless of the data acquisition methods. This study extends the range of applications of ND quantum thermometry with machine learning.

Optogenetics enables precise neural control but is limited by conventional optical fibers in complex networks. We present a hybrid device integrating multi-point micro-light-emitting diodes (MicroLEDs) with neural electrodes for localized light stimulation and simultaneous neural recording. Fabricated via direct bonding, it ensures optimal alignment for high spatial-temporal resolution. The thin MicroLED probes minimize invasiveness while maintaining optical performance. Validated in mouse brain models, the system achieves selective neural activation and recording with minimal thermal effects. This scalable, flexible tool overcomes integration challenges, advancing optogenetic research and opening avenues for exploring neural dynamics and targeted neurological therapies.

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.

The simultaneous analysis of the molecular positions and their recognition functions of immobilized molecules presents a significant challenge. We investigated an analytical method using a Co–saloph complex as a model molecule. By employing frequency modulation atomic force microscopy with an amino-terminated tip, we examined the positions and their recognition functions at the single-molecular level. The results reveal that the specific coordination bond formations between the amino group on the tip and the Co–saloph complex is detectable as an increase in the dissipation signal, which occurs concurrently with the detection of the positions of the Co–saloph complexes.

The development of tunable photonic devices is strategic for miniaturized optical instrumentation and sensing systems. Exploiting the birefringence variation of liquid crystals (LCs) instead of MEMS actuation in such devices could bring better spectral stability and lower power consumption. However, aligning LCs inside a III–V semiconductor device is tricky. We demonstrate that self-assembled gallium arsenide (GaAs) quantum dashes (QDHs) could serve as direct planar aligners for LC nematic molecules. The alignment quality and birefringence variation of a LC-microcell embedding QDHs are shown to be similar to those of a polymer nanograting-based reference, with the added advantage of better electrical performance.

Bias voltage and temperature dependence of positive shift of threshold voltage (V_TH) by applying positive bias stress to GaN planar-gate metal-oxide-semiconductor field-effect transistors with gate dielectric (SiO₂) formed by remote-plasma-assisted chemical vapor deposition were investigated. The V_TH shift increased with increasing bias voltage. Stress-time dependence implies trapping of electrons by near-interface traps via tunneling process. V_TH shift was theoretically calculated by using a proposed tunneling model considering thermal distribution of electrons in the channel as well as energetic distribution of traps. By good fitting analysis, trap energy level of ${E}_{{\rm{C}}}$ + 0.78 eV and trap concentration of 2–8.5 × 10¹² cm⁻³ were obtained.

In this study, we report the application of a p-Al_0.45Ga_0.55N contact layer in far ultraviolet-C light-emitting diodes (LEDs) on AlN substrates to enhance wall-plug efficiency (WPE). The LEDs achieved an optical power of 9.3 mW and a voltage of 6.4 V at 100 mA current, corresponding to a 1.7-fold improvement in WPE (1.4%) compared to conventional p-GaN-based LEDs. Lifetime tests suggest that this efficiency gain arises from both light extraction and carrier injection. Finally, Hall-effect measurements of bulk Mg-doped Al_0.45Ga_0.55N demonstrated low resistivity, small activation energy, and inverted carrier type.

We herein report a method for fabricating near-infrared multiple-wavelength surface-emitting light sources. After growing a GaAs layer containing stacked InAs quantum dots (QDs) on a GaAs/AlAs distributed Bragg reflector by molecular beam epitaxy, additional GaAs layers were grown via selective-area growth using a rotational metal-mask. The GaAs thickness variation generated different vertical cavity (VC) modes that resonated with different emission wavelengths within the broadband emission from the InAs QDs. Photoluminescence measurements confirmed monolithic VCs formation in the selected areas. This method offers a simple fabrication process and extends the emission wavelength range of near-infrared multiple-wavelength surface-emitting light sources.

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.

Nonvolatile and reversible control of perpendicular magnetic properties of thin Co_0.2Fe_0.6B_0.2 layers formed on a MgO layer was demonstrated by ferroelectric switching of the stacked Al_0.88Sc_0.12N and Ta₂O₅ layers. A change in the coercivity (H_c) of the 1.3-nm-thick Co_0.2Fe_0.6B_0.2 layer from 30 to 141 Oe was confirmed by depleting electrons at the interface, modifying the magnetic domain wall energy. Saturation magnetization showed a slight decrease toward depletion condition, presumably presenting a dead layer at the interface. The ferroelectric polarization-induced control of magnetism has high potential for magnetic memory applications.

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.

It is known that the orientation characteristics of ferroelectric nematic liquid crystals (FNLCs) were completely different from those of paraelectric NLCs due to the presence of macroscopic polarization. Moreover, the conventional FNLC orientation is strongly dependent on the alignment film and alignment process. In this study, we focused on two key parameters given by the alignment process: pretilt and the fluorophilic/fluorophobic properties of the alignment film. We clarified that these parameters commonly affect the polarization orientation characteristics of FNLC under both the rubbing method and the photo-alignment method. Furthermore, we successfully achieved uniform vertical polarization orientation without an external field.

We investigated the impact of In within barriers on the gate leakage current in InAlGaN/GaN high-electron-mobility transistors (HEMTs). Results revealed that the gate leakage current in the (In)AlGaN barriers depends solely on the two-dimensional electron gas density, regardless of the presence of In atoms. Furthermore, the inclusion of In atoms reduces tensile strain within the high-Al-composition barrier, suppressing crack formation. This makes InAlGaN suitable for high-output-power HEMTs with high-Al-content barriers. Moreover, we demonstrate that terminating dislocations using an AlN spacer and an amorphous AlN cap is an effective method for suppressing the gate leakage current in InAlGaN/GaN HEMTs.

In this work, we report the experimentally observed nonlinear dependence of Hall resistance on current in a Pt/RuO2 bilayer structures. Temperature-dependent measurements reveal that the Hall resistance is highly sensitive to temperature variations. The first-principles calculations suggest that the nonlinear dependence of Hall resistance on current may arise from changes in the Berry curvature induced by the electric field. Based on Pt/RuO2(101) films, we developed a temperature sensor with a wide range, high precision, and excellent reliability. This work provides strategies for temperature sensing and promotes the future application of Hall temperature sensors.

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.

ScAlN is a promising barrier material for next generation RF high electron mobility transistors, outperforming AlGaN thanks a higher 2-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 1.6x10¹³ cm^-2 with a mobility μ ∼ 621 cm²/V.s. A 75-nm gate length transistor exhibits a drain current density of 1.35 A/mm, a transconductance of ~284 mS/mm, a current gain cutoff frequency of 82 GHz and a maximum oscillation frequency of 112 GHz.

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.