The 50 most-cited articles in Japanese Journal of Applied Physics, 2023

Abstract Benefitted from progress on the large-diameter Ga 2 O 3 wafers and Ga 2 O 3 processing techniques, the Ga 2 O 3 power device technology has witnessed fast advances toward power electronics applications. Recently, reports on large-area (ampere-class) Ga 2 O 3 power devices have emerged globally, and the scope of these works have gone well beyond the bare-die device demonstration into the device packaging, circuit testing, and ruggedness evaluation. These results have placed Ga 2 O 3 in a unique position as the only ultra-wide bandgap semiconductor reaching these indispensable milestones for power device development. This paper presents a timely review on the state-of-the-art of the ampere-class Ga 2 O 3 power devices (current up to >100 A and voltage up to >2000 V), including their static electrical performance, switching characteristics, packaging and thermal management, and the overcurrent/overvoltage ruggedness and reliability. Exciting research opportunities and critical technological gaps are also discussed.

Abstract The growth of large-diameter high-resistivity β-Ga2O3 (010) substrates is important for the low-cost production of lateral Ga2O3 devices. We grew a 2 inch diameter Fe-doped high-resistivity β-Ga2O3 (010) single crystal by using the vertical Bridgman (VB) method, which is expected to grow large-diameter β-Ga2O3 crystals with various crystal orientations. Two-inch substrates were prepared from the obtained crystals, and their crystallinity, concentration of Fe dopants, and electrical properties were investigated. Consequently, a 2 inch β-Ga2O3 (010) substrate, which is comparable to the largest size of (010) substrate prepared using the Czochralski method, was successfully fabricated with the VB method. The in-plane distribution of the X-ray rocking curve from 020 diffraction of the fabricated 2 inch substrate showed that the full widths at half maximums were less than 35 arcsec at almost all measurement points, indicating high crystallinity and high in-plane uniformity. In addition, the crystals contain Fe concentrations in the range of 3.5 × 1018–1.9 × 1019 cm−3, indicating…

Abstract Halide perovskites are promising photovoltaic, solar cell, and semiconductor materials. Density-functional theory (DFT) models address compressive and tensile biaxial strain effects on APbCl3, where A = (K, Rb, and Cs). This research shows how A-cation impacts bandgap energy and band structure. The direct bandgap for KPbCl3, RbPbCl3, and CsPbCl3 is found 1.612, 1.756, and 2.046 eV, respectively; increases from A = K to Cs. When spin–orbital coupling (SOC) is introduced, bandgaps in KPbCl3, RbPbCl3, and CsPbCl3 perovskites are reduced to 0.356, 0.512, and 0.773 eV, respectively. More tensile strain widens the bandgap; compressive strain narrows it. Without SOC, the bandgaps of KPbCl3, RbPbCl3, and CsPbCl3 were tuned from 0.486 to 2.213 eV, 0.778 to 2.289 eV, and 1.168 to 2.432 eV, respectively. When the compressive strain is increased, the dielectric constant of APbCl3 decreases (redshift) and increases (blueshift) as the tensile strain is increased. Strain improves APbCl3 perovskite’s optical performance.

Abstract High aspect ratio (HAR) silicon nitride and silicon oxide (ONON) channel hole patterning in 3D NAND flash presents great challenges. This report summarizes some of the recent progress in patterning from the perspective of HAR etching and deposition-etch co-optimization (DECO). HAR etching mechanisms will be discussed, with a focus on how to reduce the aspect ratio-dependent etching (ARDE) effect. Highlights of the new low-temperature etch process will be presented, with significant improvement in the ARDE being observed. New simulation results from a Monte Carlo feature-scale model provide insights into ion scattering and mask interactions on the control of the channel hole profile. DECO is a new frontier to enable better control of the channel hole shape at HAR. Film tier optimization and carbon liner insertion results show improvement in channel hole profile control.

Abstract Recent progress in α-phase gallium oxide (α-Ga2O3) grown on sapphire for low-cost and practical device applications is reviewed. This review focuses on (i) dislocations formed by heteroepitaxy, (ii) p-type conductivity (a common issue with β-Ga2O3), and (iii) thermal instability due to the metastable phase of α-Ga2O3, and discusses efforts aimed at overcoming these issues. The results reveal guidelines for the dislocation density (<1 × 108 cm−2) so that the dislocation scattering is veiled in the electron transport, and for this purpose we mentioned buffer layers and epitaxial lateral overgrowth. Quasi-vertical Schottky barrier diodes (SBDs) show defect-insensitive behavior in current–voltage characteristics under a low current density. We also demonstrate the heterojunction pn diodes with α-phase iridium oxide (α-Ir2O3) or α-(Ir,Ga)2O3 and the ways to improve thermal stability of α-Ga2O3. The up-to-date device characteristics, that is, low on-resistance and large current SBDs, and high reverse voltage of 1400 V of a pn junction suggest promising development in…

Abstract In this work, we report on the realization of vertical (100) β-Ga2O3 FinFET devices for the use in power electronics applications. The experiments are carried out on structures consisting of highly conducting (100) β-Ga2O3 substrates with a doping concentration N D of 3 × 1018 cm−3, and epitaxially grown layers with N D of 5 × 1016 cm−3 for the drift and channel region. The fabricated FinFET devices feature enhancement-mode properties with a threshold voltage of +4.2 V and on/off-current ratio of 105. Moreover, breakdown measurements of these devices reveal an average breakdown strength of 2.7 MV cm−1. Additional device simulation indicates the presence of electric field peaks near the gate edge outside the active device as high as 7 and 5 MV cm−1 in the Al2O3 gate oxide and β-Ga2O3 semiconductor, respectively.

Abstract Breakdown voltage (BV) is arguably one of the most critical parameters for power devices. While avalanche breakdown is prevailing in silicon and silicon carbide devices, it is lacking in many wide bandgap (WBG) and ultra-wide bandgap (UWBG) devices, such as the gallium nitride high electron mobility transistor and existing UWBG devices, due to the deployment of junction-less device structures or the inherent material challenges of forming p-n junctions. This paper starts with a survey of avalanche and non-avalanche breakdown mechanisms in WBG and UWBG devices, followed by the distinction between the static and dynamic BV. Various BV characterization methods, including the static and pulse I–V sweep, unclamped and clamped inductive switching, as well as continuous overvoltage switching, are comparatively introduced. The device physics behind the time- and frequency-dependent BV as well as the enabling device structures for avalanche breakdown are also discussed. The paper concludes by identifying research gaps for understanding the breakdown of WBG and UWBG power devices.

Abstract Recently, glasses have gained great interest for use as scintillators owing to lots of industrial benefits such as ease of producing customizable shapes and low production cost. Herein, the Er 3+ -activated BaO–Nb 2 O 5 –TeO 2 glasses were fabricated for the development of NIR glass scintillators. The Er 3+ -activated BaO–Nb 2 O 5 –TeO 2 glasses exhibited efficient photoluminescence and scintillation that originated from the 4f→4f transition of Er 3+ . Their quantum yields in photoluminescence were 80% (0.1%Er 2 O 3 ), 81% (0.5%Er 2 O 3 ), and 61% (1.0%Er 2 O 3 ). Further, an almost linear correlation between an X-ray dose rate and NIR scintillation intensity was observed in the 0.5–5000 mGy h −1 dose rate range. Interestingly, the lowest detectable dose rate limit (0.5 mGy h −1 ) was lower than that of Er-doped Bi 4 Ge 3 O 12 and Nd-doped GdVO 4 single crystals. Further, afterglow levels of the non-doped and Er 3+ -activated BaO–Nb 2 O 5 –TeO 2 glasses were about 600 ppm. The observed NIR scintillation properties indicated that the Er 3+ -activated tellurite glasses should…

Abstract Pb-based ferroelectrics and piezoelectrics in the form of bulk polycrystalline and textured ceramics, single crystals, and composites, have been used in sensors, actuators, and other electromechanical devices. However, the toxicity of these materials has been a major concern around the globe for the past few decades. The report of high piezoelectric activity in the lead-free BaTiO3 (BT), (Bi0.5Na0.5)TiO3 (BNT), and (K0.5,Na0.5)NbO3 (KNN) and binary and ternary systems with other compounds has given high hopes for alternatives to Pb-based materials. Recent modifications of KNN-based compositions with BaZrO3 in combination with (Bi0.5,K0.5)HfO3 result in excellent electromechanical properties. Therefore, increased research and development in Pb-free materials brings hope for practical applications closer to reality. In this article, the recent developments on BT, BNT, and KNN reproducible soft and hard Pb-free piezoelectric compositions with a range of electromechanical properties for low- and high-power transducer applications will be reviewed. Several examples in the…

Abstract 29.2%-conversion efficiency of a two-terminal (2T) perovskite/crystalline Si heterojunction tandem solar cell using 145 μ m thick industrial Czochralski (CZ) Si wafer is obtained. The structural optimization, such as surface passivation of the perovskite layer and better light management techniques, improved power conversion efficiency (PCE). To our knowledge, this PCE is the best in 2T-tandem solar cells using CZ wafers. Towards industrialization, crucial issues with the 2T tandem solar cells with crystalline Si bottom cell are discussed. Four-terminal (4T) tandem solar cells are evaluated as an approach to avoid the crucial issues. Examining our base technologies which realize 22.2%-conversion efficiency perovskite single junction solar cell module and 26%-heterojunction back-contact solar cells, we clarified that the based technologies were ready to realize 30%-conversion efficiency 4T perovskite/heterojunction crystalline Si tandem solar cells with approximately quarter size of an industrial crystalline Si solar cell (∼64 cm 2 ).

Abstract The development of a high-power EUV light source is very important in EUV lithography to overcome the stochastic effects for higher throughput and higher numerical aperture (NA) in the future. We have designed and studied a high-power EUV free-electron laser (FEL) based on energy-recovery linac (ERL) for future lithography. We show that the EUV-FEL light source has many advantages, such as extremely high EUV power without tin debris, upgradability to a Beyond EUV (BEUV) FEL, polarization controllability for high-NA lithography, low electricity consumption, and low construction and running costs per scanner, as compared to the laser-produced plasma source used for the present EUV lithography exposure tool. Furthermore, the demonstration of proof of concept (PoC) of the EUV-FEL is in progress using the IR-FEL in the Compact ERL (cERL) at the High Energy Accelerator Research Organization. In this paper, we present the EUV-FEL light source for future lithography and progress in the PoC of the EUV-FEL.

Abstract Molecular doping of organic semiconductors has been widely utilized to modulate the charge transport characteristics and charge carrier concentration of active materials for organic electronics such as organic photovoltaics, organic light-emitting diodes, and organic field-effect transistors. For the application of molecular doping to organic electronics, the fundamentals of molecular doping should be thoroughly understood in terms of doping mechanism, host and dopant materials, doping methodologies, and post-doping properties such as doping-induced structural/energetic disorder and doping stability. In this report, the fundamental understanding of molecular doping, types of dopants, doping methods, and their practical applications as organic field-effect transistors, organic photovoltaics, and organic thermoelectric are reviewed. Finally, key strategies for efficient molecular doping may exceed the trade-off relation between device performance and structural disorder.

Absrtract In this work, we comprehensively investigate the development of unwanted parasitic particles in the MOVPE chamber while growing μm level films. The density of the parasitic particles is found to be pronounced at film thicknesses starting from >1.5 to 2 μm. These particles seem to induce structural defects such as twin lamellae, thereby harming the electrical properties of the grown film. The origin of the parasitic particle is attributed to the parasitic reactions within the chamber triggered by the promoted gas-phase reactions during the growth process, which can be largely reduced by increasing the total gas flow and decreasing the showerhead distance to the susceptor. A film thickness of up to 4 μm has been achieved after minimizing the density of parasitic particles. Thereby, RT Hall measurements reveal carrier mobilities of 160 cm2V−1s−1 at carrier concentrations of 5.7 × 1016 cm−3.

Abstract Solution-processed photoresists have been forerunners in semiconductor patterning for decades. Even with the drastic reduction in photolithography wavelength, traditional spin-on resists still support the fabrication of the most advanced, sub-5 nm node logic and memory devices using EUV lithography (EUVL) (λ = 13.5 nm). However, trade-off between resolution, sensitivity, and roughness in the conventional resists pose a critical challenge in the race towards device downscaling to 1 nm node. While great efforts are being made to improve spin-on EUV photoresist performance, there has been emergence of new approaches focused on developing novel resists via vapor-phase processing routes, such as atomic layer deposition (ALD) and its analogs. This review summarizes recent advances in EUVL photoresist development based on ALD and its derivative techniques, which include ALD-based inorganic–organic dry resists and hybrid resists synthesized by infiltrating conventional spin-on resists. Despite being in the early stage, initial studies have shown the great potential of ALD…

Abstract This study elucidates the dislocation responsible for the leakage current in a halide vapor phase epitaxial (001) β-Ga2O3 Schottky barrier diode. A high reverse leakage current of −0.98 μA was observed at −100 V as an emission pattern via ultrahigh-sensitive emission microscopy. This was attributed to the presence of dislocation. Moreover, the Burgers vector of this dislocation was determined to be ⫽ 〈1 3 ¯ 2〉 from the g·b invisibility criteria via synchrotron X-ray topography observation. This dislocation was speculated to be induced by the relaxation of the strain field surrounding the void and subsequently propagated to the surface. Cross-sectional scanning transmission electron microscopy revealed multiple threading dislocations along the (100) plane.

Abstract This study demonstrated homoepitaxial growth of Ge-doped β-Ga2O3 thin films on β-Ga2O3 substrates via mist chemical vapor deposition (CVD) using GeI4, a water-soluble Ge precursor. The carrier concentration of the Ge-doped β-Ga2O3 thin films was controlled by varying the Ge precursor concentration in the solution. A mobility of 66 cm2 V−1 s−1 was obtained at a carrier density of 3.4 × 1018 cm−3 using oxygen carrier gas. X-ray diffraction (XRD) scans 2θ-ω revealed that homoepitaxial Ge-doped β-Ga2O3 thin films were grown on β-Ga2O3 without phase separation. However, the XRD rocking curves revealed that the mist CVD- grown Ge-doped β-Ga2O3 was degraded compared to the substrate as the Ge concentration increased. The surface morphologies of the Ge-doped β-Ga2O3 exhibited atomically flat surfaces with a root mean square roughness of less than 1 nm. These results indicate that the Ge-doped β-Ga2O3 thin films prepared by mist chemical vapor deposition are promising for device applications.

Abstract A superjunction (SJ) structure in power devices is compatible with low specific on-resistance and high breakdown voltage. To fabricate the SJ structure in SiC power devices, the repetition of ion implantation and epitaxial growth processes is a practical method. However, the impact of ion implantation on device performance has rarely been reported. In this study, we measured the carrier lifetime distributions in a SiC MOSFET with an SJ structure using a microscopic free carrier absorption method. Furthermore, we observed the distribution of defects via cathodoluminescence and deep levels via deep-level transient spectroscopy. We observed that Al ion implantation induced defects and reduced the carrier lifetime in the SJ structure. However, N ion implantation does not significantly induce defects. Additionally, Al ion implantation at room temperature exhibited more significant effects than implantation at 500 °C. The results can aid in controlling the carrier lifetime in SiC SJ MOSFETs.

Abstract A 0.26 M tetramethylammonium hydroxide (TMAH) aqueous solution has been used as a standard developer for manufacturing semiconductor devices. Alternative developers have recently attracted much attention because the 0.26 M TMAH developer may be approaching its performance limit. In this study, we measured the dissolution and swelling behavior of resist polymers in tetraalkylammonium hydroxide (TAAH) aqueous solutions using a quartz crystal microbalance method to clarify the effects of the alkyl chain length of TAAH. The resist polymer was poly(4-hydroxystyrene), whose hydroxyl groups were partially protected with t -butoxycarbonyl groups. When the alkyl chain length of TAAH was increased from two (ethyl) to three (propyl), the dissolution mode markedly changed. The dissolution mode did not depend on the polymer polarity. The change in the dissolution mode is probably caused by the size effect of TAAH, considering the independence of polymer polarity and the discrete change in the dissolution mode.

Abstract We attempt to improve the light extraction of AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) by introducing a highly transparent p-type layer together with reflective Rh/Al p-type electrodes. The p-GaN contact layer is thinned to balance the Ohmic contact and DUV light transmittance, which helps the Rh/Al p-type electrodes realize high reflection as well as good electrical performance. After optimization, the Rh/Al reflective p-type electrodes present reflectance of greater than 70% and specific contact resistivity of 3.75 × 10−4 Ω·cm2. Due to the improvement in the light extraction efficiency, the highest wall-plug efficiency of 278 nm DUV-LEDs is improved by 57% compared to the conventional configuration with Ni/Au as the p-type electrodes.

Abstract High impedance ratio (Z-ratio), low temperature coefficient of frequency (TCF) and spurious free characteristic are three main requirements for surface acoustic wave (SAW) resonators. The reported LiTaO3/Quartz (Qz) hetero acoustic layer (HAL) resonator can satisfy them, but the bandwidth is too narrow. LiNbO3 (LN) is suitable for wideband resonators, but the TCF is too large due to its high coupling factor. In this work, we study wideband HAL SAW resonators combining LN and three kinds of substrates with positive TCF and small coefficient of thermal expansion, including Qz, ABC-G and ZERO glasses. By simulation, the Euler angles of (0°, 101°, 0°) for LN, (0°, 160°, 90°) for Qz, and the 0.1–0.2λ (λ is the wavelength) LN thickness are used. Results show LN/Qz, LN/ABC-G and LN/ZERO exhibit Z-ratio of 82.7 dB, 76.2 dB and 76.2 dB, respectively. LN/ABC-G shows the lowest TCF of −61 ppm C−1 at resonance frequency and −62 ppm C−1 at anti-resonance frequency.

Abstract In this paper, development of NuFlare Technology’s multi-beam (MB) mask writing system MBM-2000 series is reviewed, and future plans for the MBM series are discussed. The MB mask writing systems were designed on the basis of unique concepts suitable for high-volume production of leading-edge masks, i.e. high beam current density, a reliable beam blanking aperture array (BAA) system with a 50 keV single-stage acceleration optics, high-speed inline pixel-level dose correction, and a distinctive hardware system for charging effect reduction. The latest MB mask writing system, MBM-2000PLUS, achieves a high throughput of 8.7 h in a 104 × 130 mm2 writing area for a 150 μC cm−2 resist thanks to a high beam current density of 3.2 A cm−2. In addition, a global position accuracy of 1.2 nm, a local position accuracy of 0.5 nm, and a local critical dimension uniformity of 0.61 nm are achieved. The BAA system has shown long-term stable operation for more than one year. The high productivity and writing accuracy realized by NuFlare Technology’s concepts will contribute to further…

Abstract An exceptionally large free dielectric permittivity of 14200, piezoelectric coefficient (d 33) of 4800 pC/N and calculated electromechanical coupling factor k 33 of 95.7% were obtained from Pb(Mg1/3Nb2/3)O3 (PMN) - Pb(Zr,Ti)O3 (PZT) single crystals (SCs) under AC poling (ACP). The PMN −0.3PZT SCs grown by a solid-state crystal growth (SSCG) method with high composition uniformity showed a phase change temperature of 57 °C–72 °C and Curie temperature T c of 140 °C. The measured d 33 of 250 pC/N was obtained after a heat treatment at 250 °C, which is 110 °C higher than the T c of the SC. Microstructural observations revealed that the ACP SC showed a different 109° domain wall layers morphology due to the presence of some pores, which is dissimilar to the Bridgman ACP relaxor-PbTiO3 SCs. These outstanding piezoelectric properties, heat durability, lower acoustic impedance, and better machinability of ACP SSCG SCs are promising for future piezoelectric device applications.

Abstract β-Ga2O3 substrates and homoepitaxial films were characterized using the multiphoton excitation photoluminescence (MPPL) method. MPPL emission peaks at 3.2 and 3.36 eV were obtained with broad shoulders on the low energy side. MPPL images showed the presence of nanopipes in the substrate with a unique contrast. Three-dimensional MPPL imaging revealed the distribution and spatial location of the nanopipes. All the nanopipes were elongated along the [010] direction with diameters of approximately 0.4–1.0 μm and lengths of 10–30 μm. The nanopipes were aligned in the [100] and [001] directions with sub-micron spacing in the (0 1 ¯ 0)-oriented substrate. The nanopipes were located under the hillocks at the homoepitaxial surface. The nanopipes are suggested to trigger the hillock growth by transforming into threading dislocations in the homoepitaxial film.

Abstract The electrical properties of beta-gallium oxide ( β -Ga 2 O 3 ) and gallium arsenide semiconductors were characterized using the emerging terahertz time-domain ellipsometry (THz-TDE) technique. The dielectric and conductivity properties were obtained from the complex ratio of the measured p- and s-polarized THz pulses reflected from the samples. The carrier concentration and mobility were then deduced using the Drude model, and the results showed good accuracy. This work demonstrates THz-TDE as a promising tool for characterizing semiconductors, especially those with high carrier concentrations and significant absorption in the THz region.

Abstract The sub-threshold swing (SS) of Si n-MOSFETs is experimentally and systematically evaluated in a temperature range of 4–300 K with varying the substrate impurity concentration (N sub) from ~1016 to ~1018 cm−3, to obtain a physical understanding of SS at cryogenic temperatures. It is clarified that the temperature and drain current dependencies of SS in n-MOSFETs are well represented by a model composed of mobile tail states and localized interface states, irrespective of N sub. The densities of these states are found to increase with increasing N sub. A physical origin of band tail states is studied by experimentally examining the impact of substrate bias on these states, which can separate the effects of the N sub and the surface electric field. It has been clarified, as a result, that the band tail states can be explained by the impurity-induced model.

Abstract The resonance properties of leaky surface acoustic waves (LSAWs) and longitudinal LSAWs (LLSAWs) on bonded structures consisting of a LiNbO3 (LN) thin plate and a support substrate, with periodic voids exhibiting a rectangular cross-section, were simulated using the finite element method. The voids were formed below the electrode at the boundary between the LN thin plate and support substrate and presented the same pitch and period as the electrode. LN, quartz, glass, or Si were used as support substrates. Simulations revealed that by introducing periodic voids, resonance properties similar to those of the SH0-mode plate and S0-mode Lamb waves can be obtained. The values of the simulated fractional bandwidths of approximately 13 and 18% were found for LSAW and LLSAW, respectively, when the ratio of the width of the void to the pitch of the electrode was 0.7.

Abstract We investigated whether carboxylic acid-terpene-based deep eutectic solvents (DESs) can be used as skin permeation enhancers. We performed transdermal experiments, and their results showed that DESs with longer hydrocarbon chains can enhance the skin permeability of hydrophilic fluorescein sodium and hydrophobic meloxicam. Additionally, all DESs could improve the skin permeability of poorly soluble resveratrol. Subsequently, we evaluated the stratum corneum’s structural transition after applying DESs. The results showed that the l-menthol-based DESs with longer hydrocarbon chains disrupt lipid layers more than those with shorter hydrocarbon chains. Thymol-based DES extracted the lipids in the stratum corneum and permeated the corneocytes. Transepidermal water loss measurements showed that l-menthol-based DESs with longer hydrocarbon chains and thymol-based DESs damage skin as they disturb the inner parts of stratum corneum lipids. Overall, we concluded that DESs with longer hydrocarbon chains can be used as skin permeation enhancers for various drugs by reducing skin…

Abstract The recycling method for thermal decomposition of photovoltaic modules is a recycling method that can completely remove EVA, which is a sealing material, and can neatly separate the cells and glass. However, most back sheets are made of PET, and if they are simply thermally decomposed, a large amount of complex bonded carbides (“soot”) is generated. We succeeded in suppressing the generation of soot by heating resin components such as PET and EVA, melting and dropping them into a ceramic filter supported by a catalyst, promoting the oxidation reaction, and separating the glass and the cells cleanly. It was confirmed that the thermal decomposed PV glass can be recycled into flat glass by a float glass manufacturing company.

Abstract Ga vacancies and their unusual structure in β -Ga 2 O 3 have been studied in recent years by several experimental techniques as well as theoretical calculations. Theory predicts that the so-called split Ga vacancy configuration is the one with the lowest formation energy. Positron annihilation spectroscopy faces challenges with β -Ga 2 O 3 due to the colossal signal anisotropy and lack of a proper reference sample. Nevertheless, the majority of the data strongly suggests that all studied β -Ga 2 O 3 samples contain high concentrations of split Ga vacancies in a wide distribution of configurations. EPR, IR spectroscopy and scanning transmission electron microscopy experiments all agree on the split Ga vacancy configuration as being the most likely interpretation of the data. However, the exact structure of the split Ga vacancies remains to be solved in terms of complexes with O vacancies and hydrogen.

Abstract We report the growth of α-Ga2O3 on m-plane α-Al2O3 by conventional plasma-assisted molecular-beam epitaxy and In-mediated metal–oxide-catalyzed epitaxy (MOCATAXY). We report a growth rate diagram for α-Ga2O3( 10 1 ¯ 0 ), and observe (i) a growth rate increase, (ii) an expanded growth window, and (iii) reduced out-of-plane mosaic spread when MOCATAXY is employed for the growth of α-Ga2O3. Through the use of In-mediated catalysis, growth rates over 0.2 μm h−1 and rocking curves with full width at half maxima of Δω ≈ 0.45° are achieved. Faceting is observed along the α-Ga2O3 film surface and explored through scanning transmission electron microscopy.

Abstract The conceptualization of the lithography process as captured in models was long based primarily on classical physics. It is now essential to model the lithography process at the quantum level. Photon shot noise and molecular inhomogeneity in resists lead to line-edge roughness and stochastic defects. To contain photon shot noise while maintaining good scanner throughput for EUV lithography requires very powerful light sources. Resists in the future will need to be single-component, and molecular building blocks will need to be much smaller than 0.5 nm. This precludes the long-term use of chemically amplified resists for EUV lithography, at least for minimum line/space patterns. Because the radiation chemistry of EUV resists is driven by photoelectrons and secondary electrons, the range and stochastics of these electrons needs to be considered in the design of future resists. Invention and innovation in patterning materials will be needed to a greater degree than before.

Abstract Perovskite material is leading the photovoltaic (PV) research industry nowadays. But its stability issue and the toxic nature of widely used lead-based perovskite are the main concern for researchers. Recently chalcogenide perovskites (C-PSKs) have received attention as a potential alternative to halide perovskites in the PV industry. Especially barium zirconium sulfide (BaZrS 3 ) has attracted researchers for its superior optoelectronic properties and stability. In this research, the performance of BaZrS 3 -based perovskite solar cells (PSCs) with TiO 2 as an electron transport layer (ETL) has been intensively studied through numerical simulation for different hole transport layers (HTLs), i.e. Cu 2 O, CuSCN, CuSbS 2 , NiO, CuO, CuI and Spiro-OMeTAD. Among these HTLs, CuSbS 2 has shown the best performance. By selecting this HTL for the PSC configuration, the thickness and defect density of the absorber layer has been optimized for the best performance. Then interface defects, doping concentration in the absorber layer, ETL and HTL have been varied and performances of the…

Abstract In this review, we briefly summarize the major challenges and our recent progress in the development of GaN Junction Barrier Schottky (JBS) diodes using selective-area p-type doping with ion implantation and ultra high-pressure annealing (UHPA) process. As a starting point, we discuss the properties of Schottky contacts in the context of UHPA and provide design principle for a high performance JBS diode. Next, we propose a JBS diode having p-type regions formed by channeled ion implantation. This kind of device can provide ultra-low leakage currents and a much better trade-off between on-resistance (R ON) and breakdown voltage (BV). Finally, we demonstrate our high-performance JBS diodes which exhibited the superior electrical characteristics (record low R ON from 0.57 to 0.67 mΩ cm2 and high BV from 660 to 675 V) and nondestructive breakdown.

Abstract The gas-phase etching of SiO2 was examined using HF and methanol vapor at temperatures below 0 °C and at low pressure. The etching rate of SiO2 increased with decreasing temperature and showed a maximum around –30 °C. The obtained etching rate was a maximum of 40 nm min−1 at plasma-enhanced chemical vapor deposition SiO2. The etching rate of SiN examined for comparison was more than ten times smaller than that of SiO2 under the same condition. As a result, the etching selectivity of SiO2 to SiN was found to be over 20 at –40 °C. When utilizing a low temperature of less than –30 °C, gas-phase etching of SiO2 showing a high etching rate and selectivity was achieved.

Abstract Development (the dissolution of resist films in developer) is an important lithographic process. However, details of the dissolution of resist polymers remain unclarified. In this study, the viscosity of 2.38 wt% tetramethylammonium hydroxide (TMAH) aqueous solution with poly(4-hydroxystyrene) (PHS) was investigated using a quartz crystal microbalance method. The relationship between PHS concentration and the viscosity of 2.38 wt% TMAH aqueous solution with PHS was measured. The kinetics (temporal change) of viscosity and PHS concentration during development was clarified. The maximum PHS concentration achievable during development approximately corresponded to the TMAH concentration independently of the film thickness.

Abstract We investigated the anisotropic selective-area HCl-gas etching behavior of SiO2-masked (001) β-Ga2O3 and its dependence on the temperature T (548 °C–949 °C) and HCl partial pressure P 0(HCl) (25–250 Pa). The cross-sectional width-to-depth aspect ratio of the etched trenches formed under the striped window along [010] decreased with increasing T and decreasing P 0(HCl). Secondary-ion mass spectrometry revealed slight diffusion of Si into β-Ga2O3 at T = 949 °C, while no diffusion was detected at T = 750 °C. These results provide practical guidelines for the fabrication of desired three-dimensional structures, such as fins/trenches, for high-performance β-Ga2O3-based power devices.

Abstract We observed killer defects that served as reverse leakage current paths for halide vapor phase epitaxial (001) β-Ga2O3 Schottky barrier diodes, which resulted in a leakage current of −0.46 μA at reverse bias of −100 V. Synchrotron X-ray topography revealed comet-shaped contrasts extending along [010] which are induced from the strain field surrounding the defects. They consisted of perfect (100)-cracks and horizontal-oriented (001) cracks. The cross-sectional scanning transmission electron microscopy observation showed (100)-cracks on the surface and dislocations along [100] beneath the surface. This defect was speculated to be generated from damage during the chemical-mechanical polishing.

Abstract As a high-resolution 3D phased-array imaging method, a piezoelectric and laser ultrasonic system (PLUS) was proposed, which is a combination of a piezoelectric transmitter and an ultra-multiple scan of a laser Doppler vibrometer. Thus far, PLUS has used an identical mode for incident and scattered waves. On the other hand, the ultrasonic scattering phenomena can accompany the mode conversion, which may provide further information on defects. In this paper, we propose multi-mode PLUS to enhance its imaging capability and attain more robustness. After describing the principle of multi-mode PLUS, we validated the imaging algorithm and demonstrated the improvement in the image resolution using a dataset of simulated received waves for the longitudinal-wave incidence to scatterers. Furthermore, we experimentally demonstrated the efficacy of multi-mode PLUS in a specimen with branched stress corrosion cracking.

Abstract High uniformity of the thickness and net donor concentrations (N d-N a) in 100 mm diameter β-Ga2O3 (001) epitaxial wafer prepared by halide vapor phase epitaxy was demonstrated. An epitaxial wafer grown on a substrate with a miscut angle near 0° generated a specific epitaxial region with a lower growth rate and a higher N d-N a than the rest of the region, which deteriorated the wafer uniformity. In contrast, epitaxial wafers prepared on substrates with a miscut angle of –0.1° suppressed the occurrence of the specific epitaxial region, and an average thickness after chemical mechanical polishing of 10.6 μm with 5% tolerance and an average N d-N a without intentionally doping of 1.1 × 1016 cm−3 within 7% tolerance were successfully obtained.

Abstract This study investigates the heat dissipation structures for resonant tunneling diodes (RTDs). The n+-InGaAs conductive layer beneath the RTD double-barrier layer, which possesses low thermal conductivity and disrupting heat dissipation, has been replaced with n+-InP, which has high thermal conductivity. We manufactured simple RTD mesa structures with varying areas to analyze the impact of heat dissipation. Additionally, we conducted a study to explore the relationship between mesa area and power consumption at the RTD mesa under current–voltage measurements. The results clearly indicate that the proposed structure, incorporating an n+-InP layer, can function over an area twice as large without experiencing heat-induced destruction. By integrating this proposed structure, we successfully fabricated terahertz oscillators equipped with rectangular-cavity resonators. These oscillators achieved relatively high output power, approximately 0.2 mW was achieved at a frequency of 0.53 THz, all without any heat-induced damage, even within a large-area RTD device.

Abstract In this study, a physical reservoir computing system, a hardware-implemented neural network, was demonstrated using a piezoelectric MEMS resonator. The transient response of the resonator was used to incorporate short-term memory characteristics into the system, eliminating commonly used time-delayed feedback. In addition, the short-term memory characteristics were improved by introducing a delayed signal using a capacitance-resistor series circuit. A Pb(Zr,Ti)O3-based piezoelectric MEMS resonator with a resonance frequency of 193.2 Hz was employed as an actual node, and computational performance was evaluated using a virtual node method. Benchmark tests using random binary data indicated that the system exhibited short-term memory characteristics for two previous data and nonlinearity. To obtain this level of performance, the data bit period must be longer than the time constant of the transient response of the resonator. These outcomes suggest the feasibility of MEMS sensors with machine-learning capability.

Abstract This work proposes a simple simulation method for the optimization of n-i-p perovskite solar cell (PSCs) via SCAPS-1D and aims to achieve high-performance devices. Nowadays, the carrier recombination induced by heavy defects in bulk and interfaces is one of the main obstacles which restricts PSC efficiency and is also harmful to device stability. Here we modify the MAPbI3 device through a series of structural and basic optimizations, including the thickness of each layer, carrier diffusion length, interface recombination, doping concentration and overall series resistance. Through the modified simulation, a high-performance MAPbI3 device with suppressed recombination and optimized structure is realized, resulting in an encouraging power conversion efficiency of 20.09%, an enhanced V oc of 1.087 V, J sc of 22.56 mA cm−2 and an FF of 78.5%. These findings unveil the critical effect of defect suppression on PSCs and offer a simple method to achieve high-performance devices.

Abstract We conducted an investigation of line-shaped defects, extending in the 〈010〉 direction, in bulk β-Ga2O3 single crystals grown by the vertical Bridgman (VB) method. Parallelepiped cross-section samples with {010} polished surfaces and {100} cleavage planes were prepared and were then etched in phosphoric acid at 140 °C. Rhombic etch pits were observed on the (010) surface and they were similar in shape to those reported as nanometer-sized grooves or plate-like nanopipes in crystals grown by the edge-defined film-fed growth (EFG) method. Groove-like voids in the 〈010〉 direction were also observed on {100} cleavage planes below the etch pits observed on the {010} surface. Therefore, we concluded that line-shaped defects observed in VB-grown crystals are similar to defects observed in EFG-grown crystals. The size of these defects was considerably smaller than that observed in EFG-grown crystals, but with a density of 5 × 105 cm−2. Based on these results, possible formation mechanisms for such defects were discussed.

Abstract In this study, Sc concentration dependence of Sc x Al 1− x N/AlN/poly-crystalline diamond/Si surface acoustic wave (SAW) characteristics at high Sc from 23.8% to 44.3% was investigated by fabricating one-port SAW resonator at high frequency. 3.8 GHz one-port resonator fabricated on Sc 0.43 Al 0.57 N showed an excellent performance of electro-mechanical coupling coefficient ( K 2 ) as high as 6.34% for 2nd mode Sezawa wave, which enables a wide bandwidth in high frequency applications. The temperature coefficient of frequency was approximately −40 to −50 ppm deg −1 for the device fabricated with Sc concentration of 42.9%. This is a smaller value compared to conventional high K 2 bulk materials such as LiNbO 3 . As the result, a high K 2 6.34% material system at a higher Sc concentration of ScAlN/AlN/PCD was found to be possible at a high phase velocity of 7000 m s −1 . Combined with the extremely high-power durability of diamond based device, high-power durable wideband SAW device at high frequency can be expected.

Abstract This paper describes 4 and 7 GHz solidly-mounted structure bulk acoustic wave resonators (SM-BAWRs) using LiNbO3 (LN) thin plates. A Bragg reflector solidly supports the LN thin plate. The two kinds of SM-BAWRs use thickness extension mode in 0.81 and 0.5 μm thick 36°Y LNs. The Bragg reflector is made of Al or SiO2, and Ta films. The 0.81 μm thick LN SM-BAWRs with each acoustic layer thickness 0.23 wavelength (λ), which is almost optimum thickness, exhibited resonance frequency (f r) of 3.84 GHz, antiresonance frequency (f a) of 4.17 GHz, a bandwidth of 8.4%, an impedance (Z) ratio of 54 dB, and coupling factor k t 2 of 17.7%. Despite using an acoustic film 0.12λ much thinner than the optimum thickness, SM-BAWR using the 0.5 μm thick LN exhibited f r of 6.86 GHz, f a of 7.15 GHz, and Z ratio of 37 dB. Those results suggest a high potential for this device.

Abstract This paper reports on the fabrication and characterization of crossbar array memristors using amorphous gallium oxide (a-GaO x ) for implementing high-speed and wide-dynamic range artificial synaptic functions. The a-GaO x memristors were fabricated by pulsed laser deposition in an argon atmosphere using a platinum bottom electrode and an indium tin oxide (ITO) top electrode. We revealed that the interface engineering at a-GaO x /ITO is the key to demonstrating exemplary resistive switching operation. Stable counter figure-8 hysteresis loops were obtained by voltage application, leading to the successful demonstration of non-volatile retention over 104 s and the multi-level conductance modulation. Furthermore, spike-timing-dependent plasticity (STDP) was artificially implemented by applying pre- and post-spike voltages to the device. Consequently, significant weight-change rates were achieved in the asymmetric STDP imitation, which can be attributed to the reliable resistive switching properties of the device with an extensive dynamic range. These results indicate that the…

Abstract The study encompasses the idea to employ a single bandgap-graded transport layer in lieu of two different (transparent conducting oxide and electron transport layer) layers in the perovskite solar cell to increase the overall device functionality. Numerical simulation has been used to investigate the cell performance parameters. The thickness of the absorber layer has been altered in relation to the defect density to identify the optimal cell parameter values. Maximum power conversion efficiency (PCE) has been recorded as 22.17% at 1E13 cm−3 defect density in the absorber. These findings demonstrate the numerical modeling limitations for the relationship between defect mechanism and performance. The activation energy and effects of series resistance (R s) on solar cells have also been assessed. The temperature degradation gradient of the proposed structure GZO/CsGeI3/NiO/Au has been found to be 3% only with a PCE of 22.17%, which validates the concept of using a bandgap-graded transport layer and paves the way for a new era for perovskite research.

Abstract The development of high-numerical aperture exposure tools for EUV lithography is in progress. The development process (the dissolution of resist films) is the key to fine patterning. The dissolution dynamics of acidic polymers (the backbone polymers of chemically amplified resists) depend on various parameters related to molecular structures. In this study, the dissolution dynamics in tetraalkylammonium hydroxide (TAAH) aqueous solutions were classified into six classes on the basis of the frequency and impedance changes observed during the development process by a quartz crystal microbalance method. The relationship between class and physical parameters of materials was analyzed by decision tree and support vector machine methods. The feature values used were the alkyl chain length, molecular weight, and concentration of TAAH; the viscosity of the developer; and the protection ratio, molecular weight, contact angle, surface free energies of polymers, and film thickness. The classification accuracy was 0.80 for the validation data.

Abstract Experiments and first-principles calculations were performed to investigate the effects of Cu substitution in CH3NH3PbI3 perovskite crystals. The first-principles calculations indicated that the energy level of the Cu d orbital formed above the VB maximum would be an acceptor or defect level. The effect of Cu addition on device properties was investigated, and the device with added 2% Cu provided higher efficiencies than the standard device. On the other hand, the decrease in short-circuit current density with increasing Cu content would be attributed to the defect level of the Cu d orbitals. First-principles calculations and experimental results provided insight into the function of Cu in CH3NH3-based perovskite crystals.