Anomalously large heat generation phenomena that cannot be explained by any known chemical processes were discovered: Ni-based nano-structured multilayer metal composites were preloaded with hydrogen gas and heated rapidly to diffuse hydrogen and trigger the heat generation reaction. Maximum energy released per total hydrogen absorption was over 10 keV H–1 and no gamma rays or neutrons, which are harmful to the human body, were observed. It is possible to intentionally induce the heat burst phenomenon, which can increase the amount of heat generated without any new energy input. This can be applied to reaction control as well as to improving the accuracy of heat generation evaluation. A common feature, that regions of very high oxygen concentrations are observed in places, was observed in the heat-producing samples. At this time, however, the discussion of this oxygen concentration as nuclear in origin must exclude the possibility of many chemical processes.
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Yasuhiro Iwamura et al 2024 Jpn. J. Appl. Phys. 63 037001
Harry J. Levinson 2022 Jpn. J. Appl. Phys. 61 SD0803
High-NA extreme ultraviolet (EUV) lithography is currently in development. Fabrication of exposure tools and optics with a numerical aperture (NA) equal to 0.55 has started at ASML and Carl Zeiss. Lenses with such high NA will have very small depths-of-focus, which will require improved focus systems and significant improvements in wafer flatness during processing. Lenses are anamorphic to address mask 3D issues, which results in wafer field sizes of 26 mm × 16.5 mm, half that of lower NA EUV tools and optical scanners. Production of large die will require stitching. Computational infrastructure is being created to support high-NA lithography, including simulators that use Tatian polynomials to characterize the aberrations of lenses with central obscurations. High resolution resists that meet the line-edge roughness and defect requirements for high-volume manufacturing also need to be developed. High power light sources will also be needed to limit photon shot noise.
Ruizhe Zhang and Yuhao Zhang 2023 Jpn. J. Appl. Phys. 62 SC0806
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.
Tsunenobu Kimoto 2015 Jpn. J. Appl. Phys. 54 040103
Power semiconductor devices are key components in power conversion systems. Silicon carbide (SiC) has received increasing attention as a wide-bandgap semiconductor suitable for high-voltage and low-loss power devices. Through recent progress in the crystal growth and process technology of SiC, the production of medium-voltage (600–1700 V) SiC Schottky barrier diodes (SBDs) and power metal–oxide–semiconductor field-effect transistors (MOSFETs) has started. However, basic understanding of the material properties, defect electronics, and the reliability of SiC devices is still poor. In this review paper, the features and present status of SiC power devices are briefly described. Then, several important aspects of the material science and device physics of SiC, such as impurity doping, extended and point defects, and the impact of such defects on device performance and reliability, are reviewed. Fundamental issues regarding SiC SBDs and power MOSFETs are also discussed.
Makoto Kambara et al 2023 Jpn. J. Appl. Phys. 62 SA0803
Low-temperature plasma-processing technologies are essential for material synthesis and device fabrication. Not only the utilization but also the development of plasma-related products and services requires an understanding of the multiscale hierarchies of complex behaviors of plasma-related phenomena, including plasma generation in physics and chemistry, transport of energy and mass through the sheath region, and morphology- and geometry-dependent surface reactions. Low-temperature plasma science and technology play a pivotal role in the exploration of new applications and in the development and control of plasma-processing methods. Presently, science-based and data-driven approaches to control systems are progressing with the state-of-the-art deep learning, machine learning, and artificial intelligence. In this review, researchers in material science and plasma processing, review and discuss the requirements and challenges of research and development in these fields. In particular, the prediction of plasma parameters and the discovery of processing recipes are asserted by outlining the emerging science-based, data-driven approaches, which are called plasma informatics.
Kohei Nakajima 2020 Jpn. J. Appl. Phys. 59 060501
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.
Yuan Qin et al 2023 Jpn. J. Appl. Phys. 62 SF0801
Benefitted from progress on the large-diameter Ga2O3 wafers and Ga2O3 processing techniques, the Ga2O3 power device technology has witnessed fast advances toward power electronics applications. Recently, reports on large-area (ampere-class) Ga2O3 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 Ga2O3 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 Ga2O3 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.
Kazuhito Hashimoto et al 2005 Jpn. J. Appl. Phys. 44 8269
Photocatalysis has recently become a common word and various products using photocatalytic functions have been commercialized. Among many candidates for photocatalysts, TiO2 is almost the only material suitable for industrial use at present and also probably in the future. This is because TiO2 has the most efficient photoactivity, the highest stability and the lowest cost. More significantly, it has been used as a white pigment from ancient times, and thus, its safety to humans and the environment is guaranteed by history. There are two types of photochemical reaction proceeding on a TiO2 surface when irradiated with ultraviolet light. One includes the photo-induced redox reactions of adsorbed substances, and the other is the photo-induced hydrophilic conversion of TiO2 itself. The former type has been known since the early part of the 20th century, but the latter was found only at the end of the century. The combination of these two functions has opened up various novel applications of TiO2, particularly in the field of building materials. Here, we review the progress of the scientific research on TiO2 photocatalysis as well as its industrial applications, and describe future prospects of this field mainly based on the present authors' work.
Zhe Zhuang et al 2022 Jpn. J. Appl. Phys. 61 SA0809
InGaN-based LEDs are efficient light sources in the blue–green light range and have been successfully commercialized in the last decades. Extending their spectral range to the red region causes a significant reduction in LED efficiency. This challenge hinders the integration of red, green, and blue LEDs based on III-nitride materials, especially for full-color micro-LED displays. We review our recent progress on InGaN-based red LEDs with different chip sizes from hundreds to tens of micrometers, including the epitaxial structures, device fabrication, and optical performance (peak wavelength, full-width at half-maximum, light output power, efficiency, temperature stability, and color coordinates).
Norio Nakamura et al 2023 Jpn. J. Appl. Phys. 62 SG0809
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.
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Jing Liang et al 2024 Jpn. J. Appl. Phys. 63 06SP05
In this paper, the memory characteristics of In-Ga-Zn-O (IGZO)-channel ferroelectric FETs (FeFETs) with stackable vertical channel-all-around structure are investigated by technology computer-aided design (TCAD) simulation. The simulated drain current–gate voltage (IDS–VGS) curves of the IGZO FeFET show an on–off ratio of up to 107 and a memory window of 1.76 V, proving that ferroelectric hafnium oxide (FE-HfO2) is suitable for a 2T0C transistor. To solve the potential current-sharing problem of the 2T0C dynamic random access memory (DRAM) array, an advanced operation design methodology is proposed, which utilizes the bipolar polarization characteristics of FE-HfO2. This solution shows a remarkable current ratio between data "1" and data "0", not only demonstrating the feasibility of the IGZO-based FeFET on 2T0C DRAM memory cells, but also providing an array design guideline for highly reliable 2T0C memory applications.
Joseph Baki Kaore et al 2024 Jpn. J. Appl. Phys. 63 061002
A systematic study was carried out to observe possible boosts in the performance of poly-(3-hexyl thiophene):phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) based organic solar cell via synergistic coupling of plasmonic effects. Various volume concentration ratios of gold nanoparticles and nanorods ranging from 2%, 4%, 6%, and 8% were used to determine an optimum condition. An indicative 2% optimum condition led to synergistic tests. The synergistic coupling of gold nanorods with backcontact grating revealed a power conversion efficiency (PCE) of 4.88%. Since post-thermal annealing process results in optimum interfacial surface morphology and structural reorganization, a post-process hypothesis was performed to observe the behavior of the devices at RT with further comparisons at 40 °C. A further increase in the initial performance of the devices was observed with an optimum PCE of 5.43%. The behavior in the performances revealed stable measurements mostly attributed to conditions below the glass transition temperature of P3HT:PC61BM.
Takahiro Goya et al 2024 Jpn. J. Appl. Phys. 63 06SP04
Indium phosphide (InP) has been focused on as one of the emerging materials that can be implemented in advanced semiconductor devices. We proposed optical and electrical characterization methods to evaluate plasma-induced physical damage (PPD)—ion bombardment damage—to InP substrates. By introducing a native oxide phase in an interfacial layer, we proposed an optical model of the damaged structure applicable for in-line monitoring by spectroscopic ellipsometry. Gas species dependence was obtained, which suggested that the H2 plasma exposure formed a thicker damaged layer than Ar. Impedance spectroscopy (IS) under various biases (Vb) was implemented to reveal the nature of damaged structures. Capacitive and conductive components assigned by the IS were confirmed to depend on incident species from plasma, indicating the difference in the energy profile of created defects. The presented methods are useful to characterize and control PPD in designing future high-performance InP-based devices.
Masataka Katsuumi and Tetsuya Akasaka 2024 Jpn. J. Appl. Phys. 63 065501
GaN films were grown on sapphire substrates using liquid phase epitaxy under an atmospheric pressure nitrogen ambience, employing molten Ga and Fe3N as a source mixture. Single-crystal GaN (0001) films were successfully grown on sapphire (0001) substrates within a growth temperature (Tg) range of 750 °C–900 °C. When varying the Fe3N concentration in the range of 0.05–3 mol%, lower iron nitride resulted in high crystallinity of GaN (0001) films. The incorporation of iron atoms in GaN can negatively impact crystal quality. Parameterizing Tg at a concentration of 0.1 mol% Fe3N showed that higher Tg led to a reduction in the peak width of GaN (0002) X-ray rocking curves. However, at 3 mol%, elevating Tg resulted in the degradation of the crystallinity of GaN. This degradation may be attributed to the increased solubility of iron atoms in GaN with increasing Tg.
Takanobu Kuroyama et al 2024 Jpn. J. Appl. Phys. 63 06SP02
Acoustic cavitation bubbles under ultrasonic horn in water emit acoustic cavitation noise, which consists of spherical shockwaves. This study theoretically derived the spatial coherence of acoustic cavitation noise or, more precisely, the spectral degree of coherence. The acoustic cavitation noise was found to have spatial coherence characteristics similar to the "thermal light" in optics, unlike ultrasound generated by general transducers, which are analogous to "laser" with high coherence. The experiments validated the derived theory and showed that the spectral degree of coherence of the acoustic cavitation noise depends on the product between the distribution width of the shockwave origin, proportional to the horn diameter, and the angle between the hydrophones viewed from the horn. The lower the product gives, the higher the spectral degree of coherence at a higher frequency range.
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Naritaka Kobayashi 2024 Jpn. J. Appl. Phys. 63 050808
Organic molecule crystalline materials have been widely utilized for various applications. Controlling their crystallization and morphology is important for improving their performance and functionality. We have been investigating fundamental mechanisms of the crystal growth process of organic molecules from the viewpoint of hydration structure formed at an interface between a crystal surface and a solution. In this review, we introduce our recent studies on comparison of hydration structure between organic crystal polymorphs and between organic crystal planes measured by frequency modulation atomic force microscopy combined with three-dimensional atomic force microscopy, discussing the relationship of hydration structure with crystal growth process.
Kai E. Thomenius 2024 Jpn. J. Appl. Phys. 63 050807
From its earliest appearance in the 1950s, ultrasound has received much continuous attention by the research community. In this review paper, the evolution of the field will be discussed throughout its various hardware and software implementations with the goal of establishing the state-of-the-art for the present. This supplies a convenient launching point to consider possible directions for future research. A useful tool for this assessment is an analysis of the focus areas of various disciplines at medical ultrasound conferences and their relative frequencies. The assumption behind this methodology is that each topic has received much attention from academic faculties, technical program committees, journal editorial boards, and grant review processes. This evaluation suggests that ultrasound beamformation is becoming increasingly based on computational methods more along the lines of computed tomography or magnetic resonance imaging. As part of the process, select traditional challenges are starting to be translated into clinical practice.
Yasuyuki Yokota 2024 Jpn. J. Appl. Phys. 63 050806
In recent years, electrochemical devices have become increasingly important, and atomic- and molecular-scale understanding of the electronic and ionic transfers and chemical reactions at the electrode/electrolyte interface is required. While electrochemical scanning tunneling microscopy (EC-STM) has long enabled atomic-resolution observations in real space, it is difficult to identify reaction products and evaluate their electronic states at the interface in the electrochemical environment because of various limitations imposed by the presence of electrolyte solutions in the measurement. In this perspective review, we present our recent progresses with in situ (EC-STM combined with near-field spectroscopy) and ex situ (precise measurements in ultrahigh vacuum after electrode emersion) experiments for elucidating the microscopic properties of the electrochemical interfaces. Current issues and future perspective of both techniques are also discussed in detail.
Lingke Xu et al 2024 Jpn. J. Appl. Phys. 63 050805
Famous for their two-dimensional magnetism, the transition-metal halides with significant anisotropy and correlated d-electrons have been reduced to a low dimension and caught substantial attention in recent years. At the same time, owing to the excellent capability of discerning various degrees of freedom in solid-state systems, a scanning tunneling microscope greatly advances the understanding of low-dimensional transition-metal halides and their heterostructures by providing key results regarding structural, electronic, and magnetic properties. Here, we review the key insights about the fabrication methods, crystallography, strongly correlated electronic structures, and magnetic orders of low-dimensional revealed by scanning tunneling microscope, and introduce the latest discoveries of emergent physics under the interplay between dimensionality confinement, many-body correlation, and quantum-coupling mechanisms.
Toshiki Ito et al 2024 Jpn. J. Appl. Phys. 63 050804
Field-by-field-type UV nanoimprint lithography equipped with an on-demand inkjet dispense system, known as jet and flash imprint lithography (JFIL), has been developed. In JFIL, the inkjet resist drops still remain independent of each other when imprinting a mold, so that the ambient gas is trapped among the resist drops to generate bubbles. It takes time for the trapped bubbles to disappear, and the bubbles sometimes remain in the cured resist film to cause open defects. The waiting time for the disappearance of the gas results in low throughput and the remaining bubbles cause defect problems in JFIL. The fast disappearance of trapped bubbles was demonstrated in the case when carbon dioxide gas was used as the ambient gas. On the basis of fluid mechanics, combined-drop JFIL and its resist material was developed, in which the resist drops were combined with each other prior to imprinting to minimize trapped gas volume.
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Takeuchi et al
We would like to improve detection sensitivity by making photoelectron transmission window (SiNx membrane) of liquid cell ultra-thin for liquid measurement using XPS or X-ray PEEM at UHV. In this study, thinning of the membrane using gas cluster ion beams (GCIB) was demonstrated and the burst pressure was compared with those thinned with atomic 400 eV Ar+ ions. It was shown that SiNx membranes thinned by GCIB was 2.5 times higher burst pressure than the Ar+ ions. In addition, improvement of sensitivity of characteristic X-ray from liquid-water induced by low-energy electrons was investigated. By using 4.5 nm thick SiNx membrane etched by GCIB, the X-ray intensity became 1.6 times higher than those from 11 nm thick pristine membrane at electron beam energy of 1.5 keV. This result showed good agreement with Monte Carlo simulation results of the electron-beam-induced X-ray emission from liquid-water beneath SiNx membrane.
Hanai et al
Using a atomic force microscope-based nanomechanics to measure the micromechanical properties of different polymer blends, we found that the miscibility of the blend system affects the phase structure and micromechanical properties at the nanoscale, which further affects macroscopic mechanical properties of materials. In the immiscible polypropylene / ethylene propylene diene rubber blends, the microscopic phase structure is connected to the macroscopic mechanical properties of the material, since the microscopic modulus of elasticity of the phases remains constant even if the blend ratio is changed. On the other hand, in the partial miscible polypropylene / styrene-ethylene-butylene-styrene block copolymer blends, the macroscopic mechanical properties of the material are determined by the combination of the microscopic phase structure and the microscopic elastic modulus, because the microscopic elastic modulus of each phase changes with the blending ratio.
Wongcharoen et al
Methylammonium lead iodide (CH3NH3PbI3; MAPbI3) films were fabricated from sputtered lead sulfide (PbS) films prepared at various substrate temperatures according to the Thornton structural zone model. PbS films were converted to lead iodide (PbI2) and finally to MAPbI3 in a two-step gas-phase reaction. The increase in substrate temperature caused the morphology to change to fibrous interconnected grains, which played an important role in improving the optoelectrical properties of perovskite films. Moreover, enhanced charge transport of MAPbI3 films was observed owing to the fibrous interconnected PbI2 precursor, which was confirmed by higher absorption coefficient and longer carrier lifetime.
Floriduz et al
In this work, we demonstrate that GaN can be directly grown at high temperature on Si(111) substrates by metalorganic chemical vapor deposition without using any intentional AlN buffer, by simply employing a trimethylaluminum (TMAl) preflow. We found that n-GaN layers directly grown on n-Si with a TMAl preflow not only present a better crystalline quality compared to the use of thin AlN buffers, but also exhibit orders-of-magnitude improvement in vertical current conduction between GaN and Si, thanks to the absence of highly resistive AlN layers. Our proposed technique opens a new pathway for the effective realization of fully-vertical GaN-on-Si devices.
Sugisaki et al
The biological human brain-mimicking neuromorphic computing systems have drawn great attention recently. Synaptic elements of the neuromorphic computing systems are required to have high integration capability consumption , low power, and low cost. We have realized a memristor characteristic of a Ga-Al-O/Ga-Sn-O/Ga-Al-O stack device using mist-chemical vapor deposition (mist CVD). The mist CVD method is a thin film fabrication technology with a safe, simple equipment configuration, and low-cost environmental impact. It is achieved that hysteresis I-V curves of memristor characteristics were certainly obtained, and electric resistance for the high resistance state (HRS) and the low resistance state (LRS) were stably repeated at least 500 times. The results suggest a possibility that Ga-Sn-O thin films by mist CVD methods can be a key component of neuromorphic computing systems.
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Alessandro Floriduz et al 2024 Jpn. J. Appl. Phys.
In this work, we demonstrate that GaN can be directly grown at high temperature on Si(111) substrates by metalorganic chemical vapor deposition without using any intentional AlN buffer, by simply employing a trimethylaluminum (TMAl) preflow. We found that n-GaN layers directly grown on n-Si with a TMAl preflow not only present a better crystalline quality compared to the use of thin AlN buffers, but also exhibit orders-of-magnitude improvement in vertical current conduction between GaN and Si, thanks to the absence of highly resistive AlN layers. Our proposed technique opens a new pathway for the effective realization of fully-vertical GaN-on-Si devices.
Igor Prozheev et al 2024 Jpn. J. Appl. Phys.
We report positron annihilation results on in-grown and proton-irradiation-induced vacancy defects in AlN single crystals grown by physical vapor transport. The samples were irradiated with 100 keV H+ ions to varying fluences in the range of 5×1014 - 2×1018 ions/cm2. Doppler broadening of annihilation radiation was recorded in as-grown and irradiated samples with a slow positron beam with varying implantation energy. Doppler results combined with first principles theoretical calculations show that the 100 keV H+ irradiation introduces isolated VAl on the ion track and VN-rich vacancy clusters at the end of the ion range. The results suggest that the excess amount of detected VN originates from a high concentration of in-grown VN. So far, these defects have been considered to be unidentified negative ion-like defects in AlN.
Bingzhuo WANG et al 2024 Jpn. J. Appl. Phys.
Air gap discharge is one of the basic scientific problems in the field of high voltage engineering. The homogeneous electric field 1.5 mm air gap negative streamer at overvoltage and atmospheric pressure is observed by a high-speed 4-channel framing camera. The ultra-high temporal resolution images of a single negative stream are captured (exposure time is 5 ns, and inter-frame delay is no more than 0.1 ns). It is observed that the negative streamer formed in the middle of the air gap and growth bidirectionally towards both electrodes. At the same time, the electrical measurement is also carried out.
Soomin Kim and Seongjae Cho 2024 Jpn. J. Appl. Phys. 63 054002
In advanced MOSFET design, a vertical-channel structure provides the advantages of a smaller footprint of the transistor cell and stronger immunity against short-channel effects by introducing higher freedom in determining the channel length. For these reasons, vertical devices are still predicted to be an upcoming solution in the most recent technology roadmap. However, due to the cell-to-cell or wafer-to-wafer processing deviation that inevitably exists, it can be quite challenging to locate the gate edges at the exact positions that maximize the device performance. In this work, a series of technology computer-aided design (TCAD) device simulations have been carried out to investigate the effects of gate underlap and overlap structures on the device performance of vertical-channel MOSFETs. The device characterizations were conducted from the aspects of both DC and HF operations for higher completeness of this work, since both are not usually optimized at the same time under the same structural and processing conditions. Under the underlap condition, slight degradation in the on-state current (Ion) drivability was observed. On the other hand, a noticeable off-state current (Ioff) increase was witnessed under the underlap conduction. It is explicitly demonstrated that excessive gate underlap results in non-ideal effects, including degradation of the subthreshold swing (S), worsening of drain-induced barrier lowering, and lowering of the maximum transconductance (gm,Max). In the HF analyses, although fT and fmax remained high under overlap and gate–drain alignment conditions, it was observed that both were likely to deteriorate under underlap conditions. As a result, a processing margin in the anisotropic etching of the gate can be obtained for the optimization of the DC and HF performance of vertical-channel MOSFETs, paving the way for a wide variety of low-power and high-speed analog and digital applications.
Tatsuya Kitazawa et al 2024 Jpn. J. Appl. Phys. 63 055508
This study investigates the effects of sulfur atomic defects and crystallinity on the thermal conductivity of MoS2 thin films. Utilizing scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), and Raman spectroscopy, we examined MoS2 films, several nanometers thick, deposited on Si/SiO2 substrates. These films were prepared via a combination of RF magnetron sputtering and sulfur vapor annealing (SVA) treatment. Structural analyses, including cross-sectional STEM and in-plane and out-of-plane XRD measurements, revealed an increase in the S/Mo ratio and grain size of the MoS2 films following SVA treatment. Notably, the in-plane thermal conductivity of MoS2 films treated with SVA was found to be at least an order of magnitude higher than that of films without SVA treatment. This research suggests that the in-plane thermal conductivity of MoS2 thin films can be significantly enhanced through crystallinity improvement via SVA treatment.
Gisya Abdi et al 2024 Jpn. J. Appl. Phys. 63 050803
Reservoir computing is an unconventional computing paradigm that uses system complexity and dynamics as a computational medium. Currently, it is the leading computational paradigm in the fields of unconventional in materia computing. This review briefly outlines the theory behind the term 'reservoir computing,' presents the basis for the evaluation of reservoirs, and presents a cultural reference of reservoir computing in a haiku. The summary highlights recent advances in physical reservoir computing and points out the importance of the drive, usually neglected in physical implementations of reservoir computing. However, drive signals may further simplify the training of reservoirs' readout layer training, thus contributing to improved performance of reservoir computer performance.
Konrad Seidel et al 2024 Jpn. J. Appl. Phys. 63 050802
In this work the integration of ferroelectric (FE) devices for advanced in-memory computing applications is demonstrated based on the FeMFET memory cell concept. In contrast to FeFET having the FE layer directly embedded in the gate-stack, the FeMFET consists of a separated ferroelectric capacitor which can be integrated in the chip-interconnect layers. Optimization of the FE material stack under such lower thermal budget constraints will be discussed as well as the significant performance improvement and reduction of variability by application of superlattice FE-stacks and further optimization knobs. The low memory state variability is important for accurate multiply-accumulate (MAC) operation. Such improvements are demonstrated on a memory array test chip including functional verification of MAC operation along a FeMFET-based array column with good accuracy over high dynamic current range.
Jun Wu et al 2024 Jpn. J. Appl. Phys. 63 056501
This paper reports on a fabrication process suitable for ultra-low resonant frequency inertial MEMS sensors. The low resonant frequency is achieved by electrically tunable springs and a heavy mass formed by through-silicon deep reactive-ion etching (DRIE) applied to a silicon-on-glass. A thermal issue of through-silicon DRIE (TSD) stemming from the low-resonant-frequency structure is circumvented by two methods: introducing cooling time between the DRIE steps, and adopting a metal hard mask. A blade dicing method suited for this process is also presented. To monitor the verticality of TSD, a non-destructive taper detection method that utilizes a capacitance–voltage (CV) curve is proposed and verified.
Shogo Matsuda and Shigeki Matsuo 2024 Jpn. J. Appl. Phys. 63 052001
In this study, we used femtosecond laser-assisted etching (FLAE) to drill through glass vias (TGVs) in 0.3 mm thick non-alkali glass substrates. In FLAE, the focus of the femtosecond laser pulses is scanned to modify the material along a preprogrammed pattern, and the modified region is preferentially removed by chemical etching. We found that the scanning strategy affected the etching rate along the laser-modified lines. Among four types of scanning strategies tested, the strategy 〈du〉—that is, scanning in a downward direction followed by an upward direction—obtained the highest etching rate. In this case, the etching rate along the laser-modified line was approximately 10 times larger than that of the unmodified region.
Akihiko Teshigahara et al 2024 Jpn. J. Appl. Phys. 63 055501
A ScAlN thin film is one of the key materials of MEMS and high-frequency filters used in new-generation communication devices. Piezoelectricity can be improved by increasing Sc concentration. However, abnormal grains often appear at high Sc concentrations, degrading crystallinity and piezoelectricity. Herein, we demonstrated that underlayer roughness considerably affects the emergence of abnormal grains in a Sc0.4Al0.6N thin film formed via reactive DC sputtering. Dry etching with Ar plasma can effectively reduce the surface roughness of amorphous SiN and polycrystalline Si. Sc0.4Al0.6N thin films deposited on amorphous SiN and polycrystalline Si with sufficient flat surfaces exhibited a low density of abnormal grains, high crystallinity and piezoelectricity, and low loss tangent. Moreover, such high-quality thin films were obtained on a borophosphosilicate glass flattened using a reflow process without Ar etching. Therefore, underlayer roughness played an important role. The findings can help enable the large-scale production of highly doped ScAlN thin films.
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Junpei Igarashi et al 2024 Jpn. J. Appl. Phys. 63 05SP17
A hydrogen gas sensor based on a silicon microring resonator (MRR) with a Pt–SiO2 thin film as a hydrogen-sensitive film is proposed and investigated to realize a high-sensitivity hydrogen sensor. The sensor detects hydrogen on the basis of the resonant wavelength shift caused by the reaction heat generated in the Pt–SiO2 film. In the hydrogen exposure measurement, resonant wavelength shifts of approximately 5.0 and 2.4 nm were observed at hydrogen concentrations of 4.0 and 0.4 vol%, respectively, showing the high sensitivity of the proposed sensor. In addition, an MRR sensor with an upper Al2O3 cladding layer is proposed and its higher sensitivity is theoretically demonstrated.
Kei Sato et al 2024 Jpn. J. Appl. Phys. 63 04SP76
In this study, precious metal/tungsten trioxide (WO3) composite particles in which palladium (Pd) and platinum (Pt) were loaded on WO3 particles were synthesized via the ultrasonic reduction method. The surface observation of the synthesized composite materials was performed and their photocatalytic performance under visible light irradiation was evaluated from the decomposition rate of methylene blue in aqueous solution. From the TEM image, it was found that the Pd/WO3 composite particles synthesized by the ultrasonic reduction method had a structure in which Pd nanoparticles were supported on WO3 particles. The photocatalytic performance of Pd/WO3 and Pt/WO3 increased with increasing contents of Pd and Pt. When synthesizing Pd(0.5 wt%)/WO3 particles by ultrasonic reduction method, the photocatalytic activity was improved by feeding Pd equivalent to 0.17 wt% per feed three times at regular time intervals, rather than by feeding 0.5 wt% of Pd at a time.
Hideaki Numata et al 2024 Jpn. J. Appl. Phys. 63 04SP73
A 100 nm wide superconducting niobium (Nb) interconnect was fabricated by a 300 mm wafer process for Cryo-CMOS and superconducting digital logic applications. A low pressure and long throw sputtering was adopted for the Nb deposition, resulting in good superconductivity of the 50 nm thick Nb film with a critical temperature (Tc) of 8.3 K. The interconnects had a titanium nitride (TiN)/Nb stack structure, and a double-layer hard mask was used for the dry etching process. The exposed area of Nb film was minimized to decrease the effects of plasma damage during fabrication and atmosphere. The developed 100 nm wide and 50 nm thick Nb interconnect showed good superconductivity with a Tc of 7.8 K and a critical current of 3.2 mA at 4.2 K. These results are promising for Cryo-CMOS and superconducting digital logic applications in the 4 K stage.
Keisuke Yamamoto et al 2024 Jpn. J. Appl. Phys. 63 04SP32
Ge-on-Insulator (GOI) is considered to be a necessary structure for novel Ge-based devices. This paper proposes an alternative approach for fabricating GOI based on the Ge-on-Nothing (GeON) template. In this approach, a regular macropore array is formed by lithography and dry etching. These pores close and merge upon annealing, forming a suspended monocrystalline Ge membrane on one buried void. GOI is fabricated by direct bonding of GeON on Si carrier substrates, using an oxide bonding interface, and subsequent detachment. The fabricated GOI shows uniform physical properties as demonstrated using micro-photoluminescence measurements. Its electrical characteristics and cross-sectional structure are superior to those of Smart-CutTM GOI. To demonstrate its application potential, back-gate GOI capacitors and MOSFETs are fabricated. Their characteristics nicely agree with the theoretically calculated one and show typical MOSFET operations, respectively, which indicates promising Ge crystallinity. This method, therefore, shows the potential to provide high-quality GOI for advanced Ge application devices.
Kohei Iino and Tomohiro Kita 2024 Jpn. J. Appl. Phys. 63 04SP21
We developed a compact thermo-optic Mach–Zehnder interferometer switch with a direct heating heater using multimode interference and achieved a sufficiently low thermal crosstalk performance. Large-scale switch systems, such as optical neural networks, require thermo-optical switches with low power consumption, fast switching speed, compact size, and low thermal crosstalk. This switch is equipped with a heater that directly heats the Si core waveguide, which is a structure that connects non-doped Si wires between phase shifters and a heatsink. As a result, a significant miniaturization with a phase shifter length of approximately 7 μm, low π-phase shift power consumption of less than 20 mW, and fast switching in sub-microseconds were achieved. The improved phase shifter showed a very small figure of merit of 8.89 mWμs. Simultaneously, transmission spectrum measurements of nearby ring resonators show that the thermal crosstalk is significantly reduced even at a distance of only 30 μm. This device can contribute to the overall circuit performance and footprint reduction in large-scale optical integrated circuits and optical neural network configurations.
Haruki Matsuo et al 2024 Jpn. J. Appl. Phys. 63 04SP19
Two metal-induced lateral crystallization (MILC) methods are proposed as candidate techniques to enhance cell current in future ultra-high-density NAND-type 3D flash memory devices. The channel crystallinity differs depending on the MILC method. In a single MILC, the channel is composed of single-crystal Si, whereas in a regional MILC, the channel comprises multiple crystal grains that are larger than those of the conventional polycrystalline Si. Using transmission electron microscopy, the inhibiting factor of MILC was modeled to reveal that the two MILC approaches result in different cell current distributions that are related to their degree of crystallinity. A comparison of these two cell current distributions in a 3D flash memory with over 900 word-line stacks showed that the single MILC delivers a higher median cell current with outliers on the lower side. In contrast, the regional MILC delivers a lower median cell current without outliers on the lower side.
Naoko Misawa et al 2024 Jpn. J. Appl. Phys. 63 03SP83
This paper comprehensively analyses dual integration of approximate random weight generator (ARWG) and computation-in-memory for event-based neuromorphic computing. ARWG can generate approximate random weights and perform multiply-accumulate (MAC) operation for reservoir computing (RC) and random weight spiking neural network (SNN). Because of using device variation to generate random weights, ARWG does not require any random number generators (RNGs). Because RC and random weight SNN allow approximate randomness, ARWG only needs to generate approximate random weights, which does not require error-correcting code to correct weights to make the randomness accurate. Moreover, ARWG has a read port for MAC operation. In this paper, the randomness of random weights generated by the proposed ARWG is evaluated by Hamming distance and Hamming weight. As a result, this paper reveals that the randomness required for ARWG is much lower than that for physically unclonable functions and RNGs, and thus the proposed ARWG achieves high recognition accuracy.
Kaori Yamamoto et al 2024 Jpn. J. Appl. Phys. 63 03SP14
By modulating a ζ potential of graphene FET (G-EFT), the sensitivity of G-FET could be enhanced than that without modulation. Therefore, 1 × 107 FFU ml−1 SARS-CoV-2 was detected using G-FET modified with the ζ potential modulator which is the cation polymer with the positive charge. This method is based on the relationship between the surface charge and the sensitivity, in which the highest sensitivity is obtained when the ζ potential is 0 and/or the surface charge is almost 0. In this study, the microfluidic channel was installed on G-FET to get the precise result because it could wash away the free-floating virus and the physical adsorbed virus. 32 G-FETs including the reference FETs were integrated on the silicon substrate and the precise results were obtained by subtracting the noise terms.
Naoko Misawa et al 2024 Jpn. J. Appl. Phys. 63 03SP05
This paper proposes a design methodology for a compact edge vision transformer (ViT) Computation-in-Memory (CiM). ViT has attracted much attention for its high inference accuracy. However, to achieve high inference accuracy, the conventional ViT requires fine-tuning many parameters with pre-trained models on large datasets and a large number of matrix multiplications in inference. Thus, to map ViT to non-volatile memory (NVM)-based CiM compactly for edge applications (IoT/Mobile devices) in inference, this paper analyses fine-tuning in training, clipping, and quantization in inference. The proposed compact edge ViT CiM can be optimized by three design methods according to use cases considering the required fine-tuning time, ease of setting memory bit precision, and memory error tolerance of ViT CiM. As a result, in CIFAR-10, the most compact type successfully reduces the total memory size of ViT by 85.8% compared with the conventional ViT. Furthermore, the high accuracy type and high error-tolerant type improve inference accuracy by 4.4% and memory-error tolerance by more than four times compared with convolutional neural networks, respectively.
Sung-Won Youn et al 2024 Jpn. J. Appl. Phys. 63 03SP06
Plasmonic color is a structural color generated via preferential light absorption and scattering in dielectric nanostructures. In this study, a large plasmonic color image was successfully fabricated by an electron beam lithography (EBL) system. A software program, referred to as P-color in this study, was developed to facilitate the conversion of a desired color bitmap image to a GDS file composed of multiple nano-patterns to realize plasmonic color. The relationship between the color, width, and pitch of the pattern structures was investigated under different area-dose conditions during EBL as basic data for plasmonic color image design. After establishing conversion techniques for both the large-capacity GDS and EBL files, a plasmonic color image sample with a size of 60 mm × 40 mm area (which is difficult to fabricate using a conventional point-type EBL system) was successfully fabricated.