Awards

We demonstrated continuous-wave lasing of an AlGaN-based ultraviolet laser diode, fabricated on a single-crystal AlN substrate when operating at 5 °C. The threshold current density and device series resistance were reduced by improvements to the epitaxial structure and electrode arrangement. A peak wavelength of 274.8 nm was observed for lasing at a drive current over 110 mA, which corresponded to a threshold current density of 3.7 kA cm⁻². The operating voltage at the threshold current was as low as 9.6 V.

Room-temperature pulsed oscillation with a laser wavelength of 290 nm and a threshold current density of 35 kA cm⁻² was achieved by fabricating a UV-B laser diode on a thick AlGaN film formed on a 1 μm periodic concavo–convex pattern AlN (PCCP-AlN) on a sapphire substrate. The advantage of this method using PCCP-AlN is that it promotes the nucleation of AlGaN crystals. Planarization of this growth nucleus with AlGaN reduces the threading dislocation density at the top of the AlGaN growth layer while suppressing the formation of giant micrometer-sized hillocks and V-shaped pits that appear irregularly.

Regarding deep-ultraviolet optical device applications, face-to-face annealed sputter-deposited AlN (FFA Sp-AlN) is a promising alternative to the conventional metalorganic vapor phase epitaxy (MOVPE)-prepared AlN templates on sapphire substrates. However, FFA Sp-AlN tends to exhibit AlGaN growth-related hillock generation and surface morphology deterioration. In this study, we optimized the sputter-deposition conditions for AlN and MOVPE growth conditions for AlGaN to respectively reduce hillock density and size. After confirming AlGaN surface-flattening, we fabricated 263 nm wavelength UV-C LEDs on the FFA Sp-AlN and achieved maximum external quantum efficiencies of approximately 4.9% and 8.0% without and with silicone encapsulation, respectively.

Thermal neutron (∼25 meV) beam is a powerful tool for investigating the structure and properties of materials used in science and technology. A laser-driven neutron source generating 10¹⁰ neutrons within 1 ns duration is utilized to a single shot radiography with a dual beam of thermal neutrons and X-rays. As a proof of principle, we show the non-destructive inspection of hazardous substances (Cadmium) contained in a typical battery, when the cadmium anode thickness is evaluated from the transmittance of thermal neutrons. The fact that the neutron inspection above is performed with a single laser shot, i.e. with a single bunch of neutrons leads to a safer neutron source that is optically controlled on/off, and provides a novel tool for science and engineering.

Angular distributions of high energy neutrals and ions, impinging on an RF-biased electrode in a 13.56 MHz capacitively-coupled argon plasma were investigated. Ions and neutrals were introduced into a drift chamber that was directly connected to the RF electrode. A two-dimensional beam image was measured by a micro-channel plate. Neutral and ion beams were separated by an electrostatic deflector in the drift chamber. Angular distribution widths for ion and neutral were less than 1° at self-bias voltages above 300 V and monotonically decreased with increasing the self-bias voltage. Neutral angular distribution width was larger than that of ion, irrespective of self-bias voltage.

The three-dimensional distribution of oxygen atoms segregated at Σ9{114} grain boundaries (GBs) in Czochralski-grown silicon ingots is analyzed within a high spatial resolution of less than 0.5 nm by atom probe tomography combined with a focused ion beam (FIB) operated at −150 °C. The analysis reveals a segregation of oxygen atoms within a range of 2.5 nm across the GB plane, which is much narrower in comparison with the previous reports obtained using a conventional FIB. The oxygen concentration profile accurately reflects the distribution of the segregation sites, which exist at bond-centered sites under tensile stresses above 2 GPa, as calculated by ab initio local stress calculations.

Stable crystal orientation (CO) for lateral growth of Si thin film sandwiched by SiO₂ was evidenced to be only {001} in normal direction (ND {001}) and 〈100〉 ±5° in scanning direction (SD 〈100〉). Crystal with ND{001} is quasi-stable when angle θ between inplane 〈110〉 and SD is among 15° ≤ θ < 40° and is unstable when θ is θ < 15°. CO other than the stable CO will rotate spontaneously toward the stable CO, i.e. ND{001} with SD〈100〉 ±5°. Most ND{001} crystal was ended by twinning before the CO come to the stable CO. The twinning was triggered by gas ejection or particles, so suppressing of these phenomena would be the key for increasing ND{001}SD〈100〉 crystal occupations. These results have been verified for crystal growth velocity among 0.04–45 mm s⁻¹.

Terahertz flat optics based on our originally developed low-reflection metasurface with a high refractive index can offer attractive two-dimensional optical components for the manipulation of terahertz waves. However, it remains to be shown whether a planar collimating metalens made with our original metasurface could be mounted on a resonant tunneling diode with a short distance. Here, we demonstrate that a collimating metalens with a distance of 1.0 mm from the RTD enhances the directivity to 3.0 times at 0.312 THz. The proposed metalens would be integrated with various terahertz continuous-wave sources for emerging industry such as 6 G (beyond 5 G) communications.

The effects of a process that minimizes oxidation of SiC on the channel mobility of heavily doped 4H-SiC (0001), (112̄0) and (11̄00) metal-oxide-semiconductor field-effect transistors (MOSFETs) were investigated. High field-effect mobilities were obtained for these MOSFETs even when the acceptor concentration of the p-body (N_A) exceeded 1 × 10¹⁸ cm⁻³. The field-effect mobility for the (0001) MOSFETs reached 25 cm² V⁻¹ s⁻¹ (N_A = 1 × 10¹⁸ cm⁻³). The fabricated (11$\bar{2}$0) and (1$\bar{1}$00) MOSFETs showed very high channel mobilities of 125 cm² V⁻¹ s⁻¹ (N_A = 1 × 10¹⁸ cm⁻³) and 80 cm² V⁻¹ s⁻¹ (N_A = 5 × 10¹⁸ cm⁻³), respectively.

This paper reports AlN barrier Al_0.5Ga_0.5N high electron mobility transistors (HEMTs) with heavily Si-doped degenerate GaN contacts prepared by pulsed sputtering deposition. Selectively regrown n-type GaN contacts exhibit typical degenerate properties with the electron concentration and mobility of 2.6 × 10²⁰ cm⁻³ and 115 cm² V⁻¹ s⁻¹, respectively, resulting in a record low contact resistance R_C of 0.43 Ω mm for the AlN/Al_0.5Ga_0.5N HEMTs. The AlN/Al_0.5Ga_0.5N HEMTs displayed a remarkable DC output characteristic with a maximum drain current density of 250 mA mm⁻¹, a transconductance of 32 mS mm⁻¹, and an On/Off ratio >10⁶. The present results show potential overcoming challenges in ohmic contact formation for high-power and high-frequency AlGaN electron devices with high Al composition.

We fabricated and characterized a single-crystal silicon phonon waveguide structure with lead zirconate titanate (PZT) piezoelectric transducers. The compressive stress in a silicon-on-insulator wafer causes a membrane waveguide to buckle, leading to the quadratic nonlinearity. The PZT transducer integrated in an on-chip configuration enables us to excite high-intensity mechanical vibration, which allows the characterization of nonlinear behavior. We observed a softening nonlinear response as a function of the drive power and demonstrated the mode shift and frequency conversion. This is the first report of the nonlinear behavior caused by the quadratic nonlinearity in a buckled phonon waveguide structure. This study provides a method to control the sign and the order of nonlinearity in a phonon waveguide by utilizing the internal stress, which allows the precise manipulation of elastic waves in phononic integrated circuits.

We have proposed a method for quantitative capacitance measurements using frequency modulation electrostatic force microscopy (EFM) with a dual bias modulation method and demonstrated it on n- and p-type Si samples. First, we theoretically derived a conversion formula from a frequency shift of cantilever resonance in EFM into a capacitance value based on the parallel plate capacitor model, by which a pair of an EFM tip and a semiconductor sample is expected to be equivalently represented. Then the capacitance measurements were experimentally conducted on the n- and p-type Si substrates, and the acquired capacitance–voltage curves indicated that the obtained capacitance values were consistent with the expected ones and that the carrier densities evaluated from the depletion capacitances were also in good agreement with those evaluated by the conventional Hall effect measurements. From those results, the validity of our quantitative evaluation method has been well confirmed.

Aluminum alloy contains intermetallic compounds, which contribute to the improvement of strength properties. However, when it is exposed a to a corrosive environment, the area around the compounds is dissolved preferentially, resulting in the formation of pitting corrosion. Although this dissolution reaction is presumed to be caused by the potential difference (ΔV) between the matrix and the compounds, it has not been quantitatively clarified how ΔV is generated. In this article, we present our study on the effects of the compound composition on ΔV by using the technique of machine learning. The results showed that ΔV and the elemental concentration of the compounds have a linear relationship.

We investigated the effect of the sidewall passivation by hydrogen plasma on the InGaN green micro-LED performance. Hydrogen passivation deactivates the surface region of p-GaN around the perimeter of the device mesa. Thus, hole injection is suppressed in this region, where etching-caused material degradation results in leakage current, decreasing device efficiency. We have confirmed the hydrogen passivation effect on LED square pixels with sizes of 20 and 100 μm. For smaller LEDs, the reverse leakage current has reduced more than tenfold, and the external quantum efficiency of LEDs was enhanced 1.4-times due to the suppression of the non-radiative recombination.

Understanding the fundamental relationships between physics and its information-processing capability has been an active research topic for many years. Physical reservoir computing is a recently introduced framework that allows one to exploit the complex dynamics of physical systems as information-processing devices. This framework is particularly suited for edge computing devices, in which information processing is incorporated at the edge (e.g. into sensors) in a decentralized manner to reduce the adaptation delay caused by data transmission overhead. This paper aims to illustrate the potentials of the framework using examples from soft robotics and to provide a concise overview focusing on the basic motivations for introducing it, which stem from a number of fields, including machine learning, nonlinear dynamical systems, biological science, materials science, and physics.

The fascinating point of 2D and layered materials is that they can be assembled into van der Waals (vdW) heterostructures, in which atomic layers are integrated by vdW force. There are almost infinite potential combinations in vdW heterostructures owing to the multiple degrees of freedom, i.e., the choice of materials, stacking order, and lateral orientation angle at the interfaces. In this article, we review the fabrication technique of vdW heterostructures, which has played an essential role in the development of the 2D materials research field. First, we describe the primary technique of mechanical exfoliation to fabricate and identify high-quality atomic layers. We then discuss the assembly of atomic layers into vdW heterostructures. Finally, we introduce the recent advancement of fabrication techniques using autonomous robotic assembly. We hope this article would help the readers to acquire basic knowledge of vdW assembly and motivate them to fabricate vdW heterostructures.

Plasma processing plays an important role in manufacturing leading-edge electronic devices such as ULSI circuits. Reactive ion etching achieves fine patterns with anisotropic features in metal-oxide-semiconductor field-effect transistors (MOSFETs). In contrast, it has been pointed out over the last four decades that plasma processes not only modify the surface morphology of materials but also degrade the performance and reliability of MOSFETs as a result of defect generation in materials such as crystalline Si substrate and dielectric films. This negative aspect of plasma processing is defined as plasma (process)-induced damage (PID) which is categorized mainly into three mechanisms, i.e. physical, electrical, and photon-irradiation interactions. This article briefly discusses the modeling of PID and provides historical overviews of the characterization techniques of PID, in particular, by the physical interactions, i.e. ion bombardment damage.