The 50 most-cited articles in Japanese Journal of Applied Physics, 2017

Abstract Traditional research on thermoelectric materials focused on improving the figure-of-merit z T to enhance the energy conversion efficiency. With further growth and commercialization of thermoelectric technology beyond niche applications, other factors such as materials availability, toxicity, cost, recyclability, thermal stability, chemical and mechanical properties, and ease of fabrication become important for making viable technologies. Several silicide alloys were identified that have the potential to fulfill these requirements. These materials are of interest due to their abundancy in earth’s crust (e.g., silicon), non-toxicity, and good physical and chemical properties. In this paper, an overview of the silicide thermoelectrics from traditional alloys to advanced material structures is presented. In addition, some of the most effective approaches as well as fundamental physical concepts for designing and developing efficient thermoelectric materials are presented and future perspectives are discussed.

Abstract CMOS miniaturization is now approaching the sub-10 nm level, and further downscaling is expected. This size scaling will end sooner or later, however, because the typical size is approaching the atomic distance level in crystalline Si. In addition, it is said that electron transport in FETs is ballistic or nearly ballistic, which means that the injection velocity at the virtual source is a physical parameter relevant for estimating the driving current. Channel-materials with higher carrier mobility than Si are nonetheless needed, and the carrier mobility in the channels is a parameter important with regard to increasing the injection velocity. Although the density of states in the channel has not been discussed often, it too is relevant for estimating the channel current. Both the mobility and the density of states are in principle related to the effective mass of the carrier. From this device physics viewpoint, we expect germanium (Ge) CMOS to be promising for scaling beyond the Si CMOS limit because the bulk mobility values of electrons and holes in Ge are much higher…

Abstract As the largest manufacturer of Cu(In,Ga)(Se,S) 2 (CIGS) thin-film photovoltaic modules with more than 1 GW/year production volume, Solar Frontier K.K. has continuously improved module performance and small-area cell efficiencies in the laboratory. Because of our low-cost and environmentally-friendly process, Solar Frontier’s CIGS is a promising technology for the mass production of photovoltaic modules to fill ever-increasing demand. Recently we have achieved certified efficiencies of 22.3 and 22.0% on CdS-buffered and Cd-free buffered small-area cells, respectively, as well as 18.6% on a Cd-free mini-module. In this paper, a review of our CIGS technology and recent progress on the development of the module and the small-area cell is presented.

Abstract Plasma-activated water (PAW) is receiving much attention in biomedical applications because of its reported potent bactericidal properties. Reactive oxygen and nitrogen species (RONS) that are generated in water upon plasma exposure are thought to be the key components in PAW that destroy bacterial and cancer cells. In addition to developing applications for PAW, it is also necessary to better understand the RONS chemistry in PAW in order to tailor PAW to achieve a specific biological response. With this in mind, we previously developed a UV–vis spectroscopy method using an automated curve fitting routine to quantify the changes in H 2 O 2 , NO 2 − , NO 3 − (the major long-lived RONS in PAW), and O 2 concentrations. A major advantage of UV–vis is that it can take multiple measurements during plasma activation. We used the UV–vis procedure to accurately quantify the changes in the concentrations of these RONS and O 2 in PAW. However, we have not yet provided an in-depth commentary of how we perform the curve fitting procedure or its implications. Therefore, in this study, we…

Abstract Mg-doped p-type gallium nitride (GaN) layers with doping concentrations in the range from 6.5 × 10 16 cm −3 (lightly doped) to 3.8 × 10 19 cm −3 (heavily doped) were investigated by Hall-effect measurement for the analysis of hole concentration and mobility. p-GaN was homoepitaxially grown on a GaN free-standing substrate by metalorganic vapor-phase epitaxy. The threading dislocation density of p-GaN was 4 × 10 6 cm −2 measured by cathodoluminescence mapping. Hall-effect measurements of p-GaN were carried out at a temperature in the range from 130 to 450 K. For the lightly doped p-GaN, the acceptor concentration of 7.0 × 10 16 cm −3 and the donor concentration of 3.2 × 10 16 cm −3 were obtained, where the compensation ratio was 46%. We also obtained the depth of the Mg acceptor level to be 220 meV. The hole mobilities of 86, 31, 14 cm 2 V −1 s −1 at 200, 300, 400 K, respectively, were observed in the lightly doped p-GaN.

Abstract The electrical properties of Schottky barrier diodes (SBDs) on a (001) β-Ga 2 O 3 substrate were characterized and correlated with wet etching-revealed crystal defects below the corresponding Schottky contacts. The etching process revealed etched grooves and etched pits, indicating the presence of line-shaped voids and small defects near the surface, respectively. The electrical properties (i.e., leakage currents, ideality factor, and barrier height) exhibited almost no correlation with the density of the line-shaped voids. This very weak correlation was reasonable considering the parallel positional relation between the line-shaped voids extending along the [010] direction and the (001) basal plane in which the voids are rarely exposed on the initial surface in contact with the Schottky metals. The distribution of small defects and SBDs with unusually large leakage currents showed similar patterns on the substrate, suggesting that these defects were responsible for the onset of fatal leak paths. These results will encourage studies on crystal defect management of (001)…

Abstract Thick (20–30 µm) layers of highly pure GaN with device-quality smooth as-grown surfaces were prepared on freestanding GaN substrates by using our advanced hydride-vapor-phase epitaxy (HVPE) system. Removal of quartz parts from the HVPE system markedly reduced concentrations of residual impurities to below the limits of detection by secondary-ion mass spectrometry. Appropriate gas-flow management in the HVPE system realized device-quality, smooth, as-grown surfaces with an excellent uniformity of thickness. The undoped GaN layer showed insulating properties. By Si doping, the electron concentration could be controlled over a wide range, down to 2 × 10 14 cm −3 , with a maximum mobility of 1150 cm 2 ·V −1 ·s −1 . The concentration of residual deep levels in lightly Si-doped layers was in the 10 14 cm −3 range or less throughout the entire 2-in. wafer surface. These achievements clearly demonstrate the potential of HVPE as a tool for epitaxial growth of power-device structures.

Abstract Certified efficiency of 22.3% has been achieved for Cu(In,Ga)(Se,S) 2 solar cell. Compared to our previous record cell with 20.9% efficiency, the major breakthrough is due to the increased V oc , benefited from potassium treatment. A lower reverse saturation current and a longer carrier collection length deduced from electron-beam induced current indicate that the degree of carrier recombination at the heterojunction and depletion region for the 22.3% cell is lower. Further characterizations (capacitance–voltage profiling, temperature-dependent V oc , Suns– V oc ) and analysis indicate that the recombination coefficients at all regions were reduced, especially for the interface and depletion regions. Device simulation was performed assuming varying defect densities to model the current–voltage curve for the 22.3% cell. The best model was also used to estimate the achievable V oc if defect densities were further reduced. Furthermore, by using higher bandgap Cd-free buffer layers, a higher J sc was achieved which gives an in-house solar cell efficiency of 22.8%. Recombination…

Abstract Solution plasma is a new electrical discharge process where an atmospheric non-equilibrium plasma is generated, usually at room temperature, in a liquid environment, such as an aqueous solution or an organic compound. There are a large variety of combinations used in experiments among solutions, electrode materials, plasma configurations, volumes, and reactor geometries, as well as the characteristics of the power supply. The solution plasma process (SPP) combines gas discharge physics, fluid thermodynamic properties, and fluid interfaces reactions. In SPP, nanoparticles with various sizes, shapes, crystallinities, and compositions could be obtained. The synthesis of carbon and hetero-carbon nanomaterials proves that SPP is an efficient and rapid method for their production. The polymerization of benzene in SPP can form graphene. By simply changing the organic precursor, carbon-doped nanostructures can be synthesized with a controlled composition. This review demonstrates that SPP is a new reaction field for nanomaterial production.

Abstract Epitaxial ε-Ga 2 O 3 thin films with smooth surfaces were successfully grown on c -plane AlN templates by mist chemical vapor deposition. Using X-ray diffraction 2θ–ω and φ scans, the out-of-plane and in-plane epitaxial relationship was determined to be (0001) ε-Ga 2 O 3 ∥ (0001)AlN . The gallium/oxygen ratio was controlled by varying the gallium precursor concentration in the solution. While scanning electron microscopy showed the presence of large grains on the surfaces of the films formed for low concentrations of oxygen species, no large grains were observed under stoichiometric conditions. Cathodoluminescence measurements showed a deep-level emission ranging from 1.55–3.7 eV; however, no band-edge emission was observed.

Abstract We demonstrate an independently confirmed 25.0%-efficient interdigitated back contact silicon solar cell with passivating polycrystalline silicon (poly-Si) on oxide (POLO) contacts that enable a high open circuit voltage of 723 mV. We use n-type POLO contacts with a measured saturation current density of J 0n = 4 fA cm −2 and p-type POLO contacts with J 0p = 10 fA cm −2 . The textured front side and the gaps between the POLO contacts on the rear are passivated by aluminum oxide (AlO x ) with J 0AlO x = 6 fA cm −2 as measured after deposition. We analyze the recombination characteristics of our solar cells at different process steps using spatially resolved injection-dependent carrier lifetimes measured by infrared lifetime mapping. The implied pseudo-efficiency of the unmasked cell, i.e., cell and perimeter region are illuminated during measurement, is 26.2% before contact opening, 26.0% after contact opening and 25.7% for the finished cell. This reduction is due to an increase in the saturation current density of the AlO x passivation during chemical etching of the contact…

Abstract Dry etching behavior of unintentionally-doped β-Ga 2 O 3 has been studied in a BCl 3 /Ar chemistry using inductively-coupled-plasma reactive ion etching (ICP-RIE). The effects of various etch parameters like ICP and RIE powers, BCl 3 /Ar gas ratio and chamber pressure on etch rate are studied systematically. Higher ICP, RIE powers and lower pressure conditions are found to enhance the etch rate. A synergic etching mechanism between chemical and physical components is proposed and used to obtain fast Ga 2 O 3 etch rates more than 160 nm/min, nearly-vertical sidewalls and smooth etched surfaces. The findings of this work will enable Ga 2 O 3 vertical devices for power electronics.

Abstract A pixel array of vertical Schottky-barrier diodes (SBDs) was fabricated and measured on the surface of a β-Ga 2 O 3 single crystal. Subsequently, etch pits and patterns were observed on the same surface. Three types of etch pits were discovered: (1) a line-shaped etch pattern originating from a void and extending toward the [010] direction, (2) an arrow-shaped etch pit whose arrow’s head faces toward the [102] direction and, (3) a gourd-shaped etch pit whose point head faces toward the [102] direction. Their average densities were estimated to be 5 × 10 2 , 7 × 10 4 , and 9 × 10 4 cm −2 , respectively. We confirmed no clear relationship between the leakage current in SBDs and these crystalline defects. Such results are obtained because threading dislocations run mainly in the [010] growth direction and do not go through the sample plate.

Abstract The propagation properties of leaky surface acoustic waves (LSAWs) and longitudinal-type LSAWs (LLSAWs) on a LiNbO 3 (LN) or LiTaO 3 (LT) thin plate bonded to an AT-cut quartz or c -plane sapphire ( c -Al 2 O 3 ) substrate with a high phase velocity were investigated. It was theoretically revealed that when the LN or LT thin-plate thickness is less than one wavelength, the particle displacement of LLSAWs was concentrated in the thin plate and the electromechanical coupling factor ( K 2 ) was increased to two to three times that in the single substrate. Furthermore, for 36° Y -cut X -propagating LT/ c -Al 2 O 3 with an LT thin-plate thickness of 0.35 λ and X -cut 36° Y -propagating LN/ c -Al 2 O 3 with an LN thin-plate thickness of 0.19 λ, the values of K 2 for an LSAW and an LLSAW were experimentally found to increase from 5.6 and 10.4% in the single substrate to 11.5 and 19.7% in the thin-plate bonded structure, respectively.

Abstract The directional atomic layer etching (ALE) of GaN and AlGaN has been developed. The GaN ALE process consists of cyclic Cl 2 plasma chemisorption and Ar ion removal. The etch per cycle (EPC) was 0.4 nm within the self-limiting regime, which is 50 to 100 V. The root-mean-square surface roughness R RMS was 0.6 nm, which was improved from an initial roughness of 0.8 nm. For AlGaN ALE, BCl 3 was added to the chlorine step to obtain a smooth surface with R RMS of 0.3 nm and stoichiometry similar to the initial sample. The ultra smooth surface obtained by etching is promising for use in next-generation power devices.

Abstract Ideally ordered anodic porous alumina through-hole membranes were prepared by a combination of the pretexturing of Al and subsequent two-layer anodization. In the two-layer-anodized sample, the selective dissolution of a layer formed in concentrated H 2 SO 4 generated an ideally ordered through-hole membrane. The repetition of two-layer anodization and the detachment of the membrane allowed the high-throughput preparation of an ideally ordered anodic porous alumina membrane from a single pretextured Al substrate. The obtained ideally ordered through-hole membranes are expected to be used in various application fields that require ideally ordered through-hole membranes.

Abstract The increasing demand for higher performance of ULSI circuits requires aggressive shrinkage of device feature sizes in accordance with Moore’s law. Plasma processing plays an important role in achieving fine patterns with anisotropic features in metal–oxide–semiconductor field-effect transistors (MOSFETs). This article comprehensively addresses the negative aspect of plasma processing — plasma-induced damage (PID). PID naturally not only modifies the surface morphology of materials but also degrades the performance and reliability of MOSFETs as a result of defect generation in the materials. Three key mechanisms of PID, i.e., physical, electrical, and photon-irradiation interactions, are overviewed in terms of modeling, characterization techniques, and experimental evidence reported so far. In addition, some of the emerging topics — control of parameter variability in ULSI circuits caused by PID and recovery of PID — are discussed as future perspectives.

Abstract We investigated the charge state stability and coherence properties of near-surface single nitrogen vacancy (NV) centers in 12 C-enriched diamond for potential use in nanoscale magnetic field sensing applications. The stability of charge states in negatively charged NV centers (NV − ) was evaluated using one of the pulsed optically detected magnetic resonance measurements, Rabi oscillation measurements. During the accumulation of Rabi oscillations, an unstable shallow NV − was converted to a neutral state. As a result, the contrast of Rabi oscillations degraded, depending on charge state stability. We stabilized the NV − state of very shallow NV centers (∼2.6 ± 1.1 nm from the surface) created by 1.2 keV nitrogen ion implantation by diamond surface modification, UV/ozone exposure, and oxygen annealing. This improvement indicates that we can suppress the upward surface band bending and surface potential fluctuations through Fermi level pinning originating from oxygen-terminated diamond surfaces.

Abstract Single-layer, large-scale two-dimensional material growth is still a challenge for their wide-range usage. Therefore, we carried out a comprehensive study of monolayer MoS 2 growth by CVD investigating the influence of growth zone configuration and precursors ratio. We first compared the two commonly used approaches regarding the relative substrate and precursor positions, namely, horizontal and face-down configurations where face-down approach is found to be more favorable to obtain larger flakes under identical growth conditions. Secondly, we used different types of substrate holders to investigate the influence of the Mo and S vapor confinement on the resulting diffusion environment. We suggest that local changes of the S to Mo vapor ratio in the growth zone is a key factor for the change of shape, size and uniformity of the resulting MoS 2 formations, which is also confirmed by performing depositions under different precursor ratios. Therefore, to obtain continuous monolayer films, the S to Mo vapor ratio is needed to be kept within a certain range throughout the…

Abstract We investigate the efficiency of current-induced torque, i.e., the spin torque efficiency, in in-plane magnetized heavy metal/CoFeB/MgO heterostructures (heavy metals = Pt, W, and Ta) using the harmonic Hall technique and the spin Hall magnetoresistance. We find that the amplitude of the external magnetic field has a strong influence on the spin torque efficiency evaluation by the harmonic Hall measurements. This can be corrected by measuring the corresponding Hall resistance susceptibility. The sign and magnitude of the resulting Slonczewski-like spin torque efficiencies are in agreement with previous reports and the measurements utilizing the spin Hall magnetoresistance, except for the Pt underlayer films. The origin of the discrepancy for the Pt underlayer films is unclear. The field like torque efficiencies, upon subtracting the Oersted field contribution, are quite low or negligible. This is in significant contrast to what has been found for the field like torque in heterostructures with perpendicular magnetization. These results suggest that a more advanced model is…

Abstract AlGaN-based LEDs (λ < 300 nm) fabricated on n-AlGaN templates with a threading dislocation density larger than 5 × 10 8 /cm 2 , which were grown on (0001) sapphire with a 1.0° miscut relative to the m -plane, showed external quantum efficiencies (EQEs) of 3.5, 3.9, 6.1, and 6.0% at 266, 271, 283, and 298 nm, respectively. These EQE values reveal significantly high internal quantum efficiencies (IQEs). This performance was obtained using an uneven multiple quantum well (MQW) grown on the AlGaN template with macrosteps having height larger than the well thickness. The electroluminescence spectra of the fabricated LEDs using this MQW structure shifts to a longer wavelength compared with those on sapphire with a miscut angle of 0.3° relative to the m -plane. Furthermore, the LEDs with this MQW show no deleterious effect on the lifetime, broader electroluminescence spectral widths, and higher output powers when using sapphire with a miscut of 1.0°.

Abstract Advances in nanotechnology have intensified the need for tools that can characterize newly synthesized nanomaterials. A variety of techniques has recently been shown which combines atomic force microscopy (AFM) with optical illumination including tip-enhanced Raman spectroscopy (TERS), scattering-type scanning near-field optical microscopy (sSNOM), and photothermal induced resonance microscopy (PTIR). To varying degrees, these existing techniques enable optical spectroscopy with the nanoscale spatial resolution inherent to AFM, thereby providing nanochemical interrogation of a specimen. Here we discuss photoinduced force microscopy (PiFM), a recently developed technique for nanoscale optical spectroscopy that exploits image forces acting between an AFM tip and sample to detect wavelength-dependent polarization within the sample to generate absorption spectra. This approach enables ∼10 nm spatial resolution with spectra that show correlation with macroscopic optical absorption spectra. Unlike other techniques, PiFM achieves this high resolution with virtually no constraints…

Abstract In this paper, we report the formation of vertical GaN Schottky barrier diodes (SBDs) on a 2-in. free-standing (FS) GaN wafer, using CMOS-compatible contact material. By realizing an off-state breakdown voltage V BR of 1200 V and an on-state resistance R on of 7 mΩ·cm 2 , the FS-GaN SBDs fabricated in this work achieve a power device figure-of-merit of 2.1 × 10 8 V 2 ·Ω −1 ·cm −2 on a high quality GaN wafer. In addition, the fabricated FS-GaN SBDs show the highest I on / I off current ratio of ∼2.3 × 10 10 among the GaN SBDs reported in the literature.

To investigate current linearity and operation stability of metal–oxide–semiconductor (MOS) AlGaN/GaN high electron mobility transistors (HEMTs), we have fabricated and characterized the Al 2 O 3 -gate MOS-HEMTs without and with a bias annealing in air at 300 °C. Compared with the as-fabricated (unannealed) MOS HEMTs, the bias-annealed devices showed improved linearity of I D – V G curves even in the forward bias regime, resulting in increased maximum drain current. Lower subthreshold slope was also observed after bias annealing. From the precise capacitance–voltage analysis on a MOS diode fabricated on the AlGaN/GaN heterostructure, it was found that the bias annealing effectively reduced the state density at the Al 2 O 3 /AlGaN interface. This led to efficient modulation of the AlGaN surface potential close to the conduction band edge, resulting in good gate control of two-dimensional electron gas density even at forward bias. In addition, the bias-annealed MOS HEMT showed small threshold voltage shift after applying forward bias stress and stable operation even at high…

Abstract Lead-free ferroelectric and piezoelectric ceramics, (Bi 0.5 Na 0.5(1− x ) Li 0.5 x )TiO 3 and (1 − y )(Bi 0.5 Na 0.48 Li 0.02 )TiO 3 – y BaTiO 3 (abbreviated as BNLT100 x and BNLBT4-100 y , respectively), were fabricated by a quenching procedure after sintering, and then their electrical properties were investigated with the aim of increasing their depolarization temperature T d . From the measurements by a resonance–antiresonance method, the electromechanical coupling factors k 33 of BNLT100 x and BNLBT4-100 y were exactly the same after the quenching treatment as compared with the nonquenched one. The T d of all BNLT100 x quenched from 1100 °C reached around 240 °C, and they are almost 60 °C higher for x = 0, 0.02, 0.04, and almost 80 °C higher for x = 0.08 than those of nonquenched ceramics. We also found, from the measurement on BNLBT4-100 y quenched from 1100 °C, that the T d values were enhanced to higher temperature by the quenching treatment, even in the case of rhombohedral side (100 y < 6), tetragonal side (100 y > 6), and MPB composition (100 y = 6). Moreover, we…

An atomic layer etching process for silicon nitride (Si 3 N 4 ) has been developed in which ammonium fluorosilicate [(NH 4 ) 2 SiF 6 ] is formed and desorbed using infrared annealing. The cycle of forming and removing ammonium fluorosilicate was repeated, demonstrating that the Si 3 N 4 etching depth was accurately controlled with high selectivity to SiO 2 by changing the number of cycle. An X-ray photoelectron spectroscopy peak, which had been previously assigned as N–H bond of an ammonium salt, was observed after radical exposure, indicating that the ammonium fluorosilicate-based modified layer had formed. This peak disappeared after infrared annealing for 10 s, demonstrating desorption of the modified layer. In thermal desorption spectroscopy, NH 3 , HF, and SiF x were detected, providing further evidence for the formation of the ammonium fluorosilicate-based modified layer. In addition, this layer has a multilayer structure, protecting the Si 3 N 4 from exposure to reactive radicals.

Abstract We demonstrated n- and p-type carrier injections into a transition metal dichalcogenide (TMD) WSe 2 using van der Waals (vdW) contacts of two-dimensional (2D) materials: graphite for an n-type contact and NbSe 2 for a p-type contact. Instead of conventional methods such as the evaporation of metals on TMD, 2D metals were transferred onto WSe 2 in order to form van der Waals contacts. With these contacts, we demonstrated a small Schottky barrier height for both carrier polarities. Our finding reveals the potential of a high-performance vdW metal/semiconductor contact for use in electronics applications.

Abstract The present work reports on room-temperature ethanol sensing performance of ZnO nanospheres and nanorods prepared using pulsed laser ablation in water. Nanosecond and millisecond lasers were used to prepare ZnO nanomaterials with hexagonal wurtzite crystal structure. The two contrasting nanostructures were tested as gas sensors towards volatile compounds such as ethanol, ammonia, and acetone. At room temperature, devices based on both ZnO nanomaterials demonstrated selectivity for ethanol vapor. The sensitivity of nanospheres was somewhat higher compared to that of nanorods, with response values of ∼19 and ∼14, respectively, towards 250 ppm. Concentrations as low as 50 ppm could be easily detected.

Abstract In this review, we discuss the progress of emerging dry processes for nanoscale fabrication. Experts in the fields of plasma processing have contributed to addressing the increasingly challenging demands in achieving atomic-level control of material selectivity and physicochemical reactions involving ion bombardment. The discussion encompasses major challenges shared across the plasma science and technology community. Focus is placed on advances in the development of fabrication technologies for emerging materials, especially metallic and intermetallic compounds and multiferroic, and two-dimensional (2D) materials, as well as state-of-the-art techniques used in nanoscale semiconductor manufacturing with a brief summary of future challenges.

Abstract In this study, atomic layer etching (ALE) of silicon nitride (SiN) using a cyclic process with monofluoromethane chemistry was investigated. Results show that an appropriate desorption time must be chosen at a specific adsorption time to achieve SiN ALE. The results also show that the infinite selectivity of SiN over Si can be achieved using the cycle process. To further understand this behavior, the adsorption and desorption effects were also studied. The results revealed a mechanism to obtain a high Si selectivity and a dominant factor that causes the Si loss. To further understand the ALE capability, we studied and compared the etched profiles and resulting surface roughness obtained by both a conventional process and an ALE process. The results show that the ALE process can achieve a high Si selectivity and a non-detectable level of Si surface damage, compared with a conventional continuous etching process.

Abstract The noncontact acoustic inspection method focuses on the resonance phenomenon, and the target surface is measured by being vibrated with an airborne sound. It is possible to detect internal defects near the surface layer of a concrete structure from a long distance. However, it requires a fairly long measurement time to achieve the signal-to-noise (S/N) ratio just to find some resonance frequencies. In our method using the conventional waveform “single-tone burst wave”, only one frequency was used for one-sound-wave emission to achieve a high S/N ratio using a laser Doppler vibrometer (LDV) at a safe low power (e.g., He–Ne 1 mW). On the other hand, in terms of the difference in propagation velocity between laser light and sound waves, the waveform that can be used for high-speed measurement was devised using plural frequencies for one-sound-wave emission (“multitone burst wave”). The measurement time at 35 measurement points has been dramatically decreased from 210 to 28 s when using this waveform. Accordingly, 7.5-fold high-speed measurement became possible. By some…

Abstract Some photovoltaic module technologies use toxic materials. We report long-term leaching on photovoltaic module pieces of 5 × 5 cm 2 size. The pieces are cut out from modules of the four major commercial photovoltaic technologies: crystalline and amorphous silicon, cadmium telluride as well as from copper indium gallium diselenide. To simulate different environmental conditions, leaching occurs at room temperature in three different water-based solutions with pH 3, 7, and 11. No agitation is performed to simulate more representative field conditions. After 360 days, about 1.4% of lead from crystalline silicon module pieces and 62% of cadmium from cadmium telluride module pieces are leached out in acidic solutions. The leaching depends heavily on the pH and the redox potential of the aqueous solutions and it increases with time. The leaching behavior is predictable by thermodynamic stability considerations. These predictions are in good agreement with the experimental results.

Abstract The composition pulling effect in metalorganic vapor-phase InGaN epitaxy was theoretically investigated by thermodynamic analysis. The excess energies of biaxial-strained In x Ga 1− x N were numerically calculated using empirical interatomic potentials considering different situations: (i) coherent growth on GaN(0001), (ii) coherent growth on In 0.2 Ga 0.8 N(0001), and (iii) bulk growth. Using the excess energies, the excess chemical potentials of InN and GaN alloys were computed. Our results show that compressive strain suppresses In incorporation, whereas tensile strain promotes it. Moreover, assuming chemical equilibrium, the relationship between the solid composition and the growth conditions was predicted. The results successfully reproduced the typical composition pulling effect.

Abstract Utilizing spintronics-based nonvolatile memories in integrated circuits offers a promising approach to realize ultralow-power and high-performance electronics. While two-terminal devices with spin-transfer torque switching have been extensively developed nowadays, there has been a growing interest in devices with a three-terminal structure. Of primary importance for applications is the efficient manipulation of magnetization, corresponding to information writing, in nanoscale devices. Here we review the studies of current-induced domain wall motion and spin–orbit torque-induced switching, which can be applied to the write operation of nanoscale three-terminal spintronics devices. For domain wall motion, the size dependence of device properties down to less than 20 nm will be shown and the underlying mechanism behind the results will be discussed. For spin–orbit torque-induced switching, factors governing the threshold current density and strategies to reduce it will be discussed. A proof-of-concept demonstration of artificial intelligence using an analog spin–orbit torque…

Abstract To obtain low-dislocation-density c -plane AlN on sapphire, we have studied the annealing of AlN films at 1650 °C. We prepared an AlN film with a thickness of 20 nm by radio-frequency (RF) sputtering, on which an AlN epilayer with a thickness of 280 nm was grown by metalorganic vapor phase epitaxy (MOVPE) at different growth temperatures. Finally, the AlN film on sapphire was annealed at 1650 °C. The full width at half maximum (FWHM) of the X-ray rocking curve (XRC) for AlN(0002) was approximately 50 arcsec for all samples, whereas that for AlN( ) significantly decreased from 6500 to 350 arcsec after thermal annealing. A surface morphology with atomic steps and a terrace structure was clearly observed after thermal annealing. From these results, we concluded that a high-quality AlN film can be obtained by annealing a stacking structure of MOVPE-grown AlN (buffer layer)/sputtered AlN (nucleation layer)/sapphire substrate.

Abstract GaN-based ultraviolet-light-emitting diodes (UV LEDs) with 375 nm emission were grown on different-sized patterned sapphire substrates (PSSs) with ex situ 15-nm-thick sputtered AlN nucleation layers by metal–organic chemical vapor deposition (MOCVD). It was observed through in situ optical reflectance monitoring that the transition time from a three-dimensional (3D) island to a two-dimensional (2D) coalescence was prolonged when GaN was grown on a larger PSS, owing to a much longer lateral growth time of GaN. The full widths at half-maximum (FWHMs) of symmetric GaN(002) and asymmetric GaN(102) X-ray diffraction (XRD) rocking curves decreased as the PSS size increased. By cross-sectional transmission electron microscopy (TEM) analysis, it was found that the threading dislocation (TD) density in UV LEDs decreased with increasing pattern size and fill factor of the PSS, thereby resulting in a marked improvement in internal quantum efficiency (IQE). Finite-difference time-domain (FDTD) simulations quantitatively demonstrated a progressive decrease in light extraction efficiency…

Abstract High-purity hexagonal bismuth telluride (Bi 2 Te 3 ) nanoplates were prepared by a solvothermal synthesis method, followed by the fabrication of nanoplate thin films by the drop-casting technique. The Bi 2 Te 3 nanoplates exhibited a single-crystalline phase with a rhombohedral crystal structure. The nanoplates had a flat surface with edge sizes ranging from 500 to 2000 nm (average size of 1000 nm) and a thickness of less than 50 nm. The resulting Bi 2 Te 3 nanoplate thin films were composed of well-aligned hexagonal nanoplates along the surface direction with an approximate film thickness of 40 µm. To tightly connect the nanoplates together within the thin films, thermal annealing was performed at different temperatures. We found that the thermoelectric properties, especially the Seebeck coefficient, were very sensitive to the annealing temperature. Finally, the optimum annealing temperature was determined to be 250 °C and the Seebeck coefficient and power factor were −300 µV/K and 3.5 µW/(cm·K 2 ), respectively.

Abstract Cesium iodide (CsI) is attracting attention as a substitute for organic materials such as CH 3 NH 3 I. In this work, we fabricated sequential-vacuum-deposited planar heterojunction (PHJ) cesium lead iodide (CsPbI 3 ) perovskite solar cells with enhanced efficiencies by varying the annealing time (0.5, 1, 5, and 10 min). The effect of performance enhancement was investigated as a function of varying annealing time at 350 °C employing a hot plate. The best-performing device was obtained with an annealing time of 1 min, delivered photocurrent density ( J SC ) of 12.06 mA/cm 2 , voltage ( V OC ) of 0.71 V, and fill factor (FF) of 0.67, leading to a power conversion efficiency (PCE) of 5.71% at standard AM 1.5G solar illumination.

Abstract The display backlight generated using sharp β-sialon:Eu (green) and K 2 SiF 6 :Mn (red) phosphors shows a very wide color gamut that mostly covers the whole National Television System Committee (NTSC) triangle. In this work, an alternative green phosphor is investigated to further improve the display color gamut. γ-AlON:Mn,Mg is a green phosphor that shows a shorter and narrower emission spectrum than sharp β-silaon:Eu. The display color gamut in the blue-green region is thus widened greatly by substituting γ-AlON:Mn,Mg for sharp β-sialon:Eu. The color gamut of displays with γ-AlON:Mn,Mg and K 2 SiF 6 :Mn exceeds 100% of the NTSC standard both in the CIE 1931 and the CIE 1976 color spaces. Furthermore, the white LEDs with γ-AlON:Mn,Mg have excellent stability that is comparable to those of LEDs with sharp β-sialon:Eu.

Abstract We have developed a compact solar-pumped laser (µSPL) employing an off-axis parabolic mirror with an aperture of 76.2 mm diameter and an yttrium aluminum garnet (YAG) ceramic rod of φ1 mm × 10 mm doped with 1% Nd and 0.1% Cr as a laser medium. The laser oscillation wavelength of 1.06 µm, just below the optical absorption edge of Si cells, is suitable for photoelectric conversion with minimal thermal loss. The concept of laser beam power feeding to an electric vehicle equipped with a photovoltaic panel on the roof was proposed by Ueda in 2010, in which the electricity generated by solar panels over the road is utilized to drive a semiconductor laser located on each traffic signal along the road. By substituting this solar-electricity-driven semiconductor laser with a solar-pumped laser, the energy loss of over 50% in converting the solar electricity to a laser beam can be eliminated. The overall feasibility of this system in an urban area such as Tokyo was investigated.

Abstract The valence band offset at both SiO 2 /β-Ga 2 O 3 and HfSiO 4 /β-Ga 2 O 3 heterointerfaces was measured using X-ray photoelectron spectroscopy. Both dielectrics were deposited by atomic layer deposition (ALD) onto single-crystal β-Ga 2 O 3 . The bandgaps of the materials were determined by reflection electron energy loss spectroscopy as 4.6 eV for Ga 2 O 3 , 8.7 eV for Al 2 O 3 and 7.0 eV for HfSiO 4 . The valence band offset was determined to be 1.23 ± 0.20 eV (straddling gap, type I alignment) for ALD SiO 2 on β-Ga 2 O 3 and 0.02 ± 0.003 eV (also type I alignment) for HfSiO 4 . The respective conduction band offsets were 2.87 ± 0.70 eV for ALD SiO 2 and 2.38 ± 0.50 eV for HfSiO 4 , respectively.

Abstract Yttrium oxyfluoride (YOF) ceramics are expected to be one of the attractive plasma-resistant materials for semiconductor production equipment. In this study, dense YOF ceramics were fabricated by hot pressing using YOF powder, and their physical, mechanical, and thermal properties were characterized. Moreover, behavior against fluorine-plasma exposure was investigated. The results suggest that the YOF ceramics showed excellent mechanical and thermal properties, and superior resistance for fluorine-plasma exposure to Y 2 O 3 ceramics.

Abstract The current status of study on the phase diagram of boron is given from the theoretical viewpoint. Boron is the last elemental crystal whose phase diagram is missing. In the last decade, several new structures of boron allotropes were found, while some were disproven. Presently, even the number of allotropes of boron is uncertain. A simple reason for this is that there are many and complicated structures, and some are minimally different from the others. A theoretical study thus requires very high accuracy. The difficulty, however, is not merely a technical difficulty of computational scale. The physics involved is quite different from what is obtained by band theory, which is the most successful theory of solids. It is only recent that a fundamental problem of metal / insulator has been solved. We come to know that the interrelationships between nonstoichiometry, partially occupied sites, and the balance of intra / inter -icosahedral bonding, which were considered to be uncorrelated properties, inevitably determine the relative stability of various structures. The…

Abstract We proposed an Schottky barrier diode wall integrated trench MOSFET (SWITCH-MOS) for the purposes of shrinking the cell pitch and suppressing the forward degradation of the body diode. A trench Schottky barrier diode (SBD) was integrated into a trench gate MOSFET with a wide shielding p+ region that protected the trench bottoms of both the SBD and the MOS gate from high electrical fields in the off state. The SBD was placed on the trench sidewall of the plane ( m -face). Static and transient simulations revealed that SWITCH-MOS sufficiently suppressed the bipolar current that induced forward degradation, and we determined that the optimum Schottky barrier height (SBH) was from 0.8 to 2.0 eV. The SBH depends on the crystal planes in 4H-SiC, but the SBH of the m -face was unclear. We fabricated a planar m -face SBD for the first time, and we obtained SBHs from 1.4 to 1.8 eV experimentally with titanium or nickel as a Schottky metal.

Abstract There are global concerns about threat of pandemic caused by the human-infectious avian influenza virus. To prevent the oncoming pandemic, it is crucial to analyze the viral affinity to human-type or avian-type sialoglycans with high sensitivity at high speed. Graphene-FET (G-FET) realizes such high-sensitive electrical detection of the targets, owing to graphene’s high carrier mobility. In the present study, G-FET was functionalized using sialoglycans and employed for the selective detection of lectins from Sambucus sieboldiana and Maackia amurensis as alternatives of the human and avian influenza viruses. Glycan-functionalized G-FET selectively monitored the sialoglycan-specific binding reactions at subnanomolar sensitivity.

Abstract Seedling growth enhancement of sunflower seeds by DC atmospheric non-thermal Ar–O 2 plasma has been proposed. The plasma reactor was simply designed by the composition of multi-pin electrodes bonded on a solderable printed circuit board (PCB) anode. A stable plasma was exhibited in the non-periodical self-pulsing discharge mode during the seed treatment. The experimental results showed that non-thermal plasma treatment had a significant positive effect on the sunflower seeds. Ar–O 2 mixed gas ratio, treatment time and power source voltage are the important parameters affecting growth stimulation of sunflower sprouts. In this research, the sunflower seeds treated with liters per minute (LPM) of Ar–O 2 plasma at a source voltage of 8 kV for 1 min showed the best results in stimulating the seedling growth. The results in this case showed that the dry weight and average shoot length of the sunflower sprouts were 1.79 and 2.69 times higher and heavier than those of the untreated seeds, respectively.

Abstract As the first step toward understanding the electrical properties of SiO 2 /GaN systems, the interface was characterized using high-resolution scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS). An epitaxial crystalline intermediate layer with a thickness of ∼1.5 nm was observed at the SiO 2 /GaN interface. STEM-EDS analyses revealed that this intermediate layer contained gallium and oxygen and mostly comprised the ε-Ga 2 O 3 phase. The ε-Ga 2 O 3 /GaN interface was atomically smooth and free from misfit dislocations despite lattice mismatch of ∼8.0%, suggesting that the initial oxidation of GaN surfaces is crucial to achieve good interfacial properties.

Abstract The homoepitaxial growth of thick β-Ga 2 O 3 layers on 2-in.-diameter (001) wafers was demonstrated by halide vapor phase epitaxy. Growth rates of 3 to 4 µm/h were confirmed for growing intentionally Si-doped n-type layers. A homoepitaxial layer with an average thickness and carrier concentration of 10.9 µm and 2.7 × 10 16 cm −3 showed standard deviations of 1.8 µm (16.5%) and 0.5 × 10 16 cm −3 (19.7%), respectively. Ni Schottky barrier diodes fabricated directly on a 5.3-µm-thick homoepitaxial layer with a carrier concentration of 3.4 × 10 16 cm −3 showed reasonable reverse and forward characteristics, i.e., breakdown voltages above 200 V and on-resistances of 3.8–7.7 mΩ cm 2 at room temperature.

Abstract The inverter performance comparisons of L- and U-shaped channel tunneling field-effect transistors (LTFET and UTFET) are investigated by using Sentaurus TCAD tool. The RF figures of merit for these two TFETs are analyzed in terms of transconductance ( g m ), output conductance ( g ds ), gate capacitance ( C gg ), gate-to-source capacitance ( C gs ), gate-to-drain capacitance ( C gd ), unit-gain cut-off frequency ( f T ), the maximum frequency of oscillation ( f MAX ) and gain bandwidth product (GBW). The simulation results reveal that LTFET and UTFET have the similar DC characteristic due to the identical tunneling process. And the better RF performance LTFET provides due to it has the much smaller C gd than UTFET. Meanwhile, this work uses mixed device-circuit simulations to predict the performance of inverter circuit implemented with LTFET and UTFET for the first time. And the calculated results demonstrate LTFET is more optimization for inverter circuit design in the novel generation transistors. And all the simulation results in this paper can be used as a reference to…