The 50 most-cited articles in Japanese Journal of Applied Physics, 2021

Abstract 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).

Abstract Leakage current analysis on 50 nm thick ferroelectric Al 0.78 Sc 0.22 N films with TiN electrodes has been performed. The electron conduction followed Schottky emission with an initial Schottky barrier height ( ϕ B ) of 0.46 eV. During the initial switching, a gradual shift in the leakage current was observed, changing the ϕ B to 0.36 eV, and stayed constant for further switching cycles. From the extracted Richardson constant, the change in the ϕ B can be interpreted as the formation of a tunneling barrier due to the formation of nitrogen vacancies at the metal interface.

Abstract Compared to conventional DC poling (DCP) treatment, in 2014 a novel technology, AC poling (ACP) treatment, was found to be a low-cost, time-saving and smart-process method to significantly enhance the properties of relaxor-PbTiO 3 (PT) ferroelectric single crystal (SC) transducers. In total, over 35 papers have been reported since 2018 in which researchers achieved a piezoelectric coefficient of ( d 33 ) > 4000 pC/N and electromechanical coupling factors of ( k 33 ) > 96% for the ACP SCs. In this review, we introduce the history of ACP study on relaxor-PT SCs, and our research group’s ACP study during the past three years. In particular, we focus on ACP conditions and their mechanisms of binary (1- x )Pb(Mg 1/3 Nb 2/3 )O 3 - x PbTiO 3 (PMN-PT) and ternary (1- x - y )Pb(In 1/2 Nb 1/2 )O 3 - y Pb(Mg 1/3 Nb 2/3 )O 3 - x PbTiO 3 (PIN-PMN-PT) SCs near the morphotropic phase boundary (MPB). Furthermore, we provide insight for further studies on optimal ACP conditions and the reduction of spurious-mode vibrations (SMVs). This work offers promising prospects and practical value for…

Abstract This article addresses the impact of the buffer doping on the critical performance issues of current-collapse and dynamic R ON in GaN high electron mobility transistors. It focusses on the effect of carbon, either incorporated deliberately in GaN-on-Si power switches, or as a background impurity in iron doped RF GaN-on-SiC devices. The commonality is that carbon results in the epitaxial buffer becoming p-type and hence electrically isolated from the two-dimensional electron gas by a P–N junction. Simulations which incorporate a model for leakage along dislocations are used to show that a remarkably wide range of experimental observations can be explained including dynamic R ON and the complex time dependence of drain current transients in power switches. In RF GaN-on-SiC devices, the current-collapse, the drain current dynamics, kink effect, pulse-IV and electric field distribution in the gate-drain gap can all be explained.

Abstract In this paper, first, the surface acoustic wave (SAW) propagation mode and a method of analyzing the propagation property are introduced briefly. Then, typical composite substrate structures that have been developed to obtain high-performance SAW devices are reviewed. Furthermore, the recent results obtained by the author and research colleagues on the propagation and resonance properties of leaky SAW (LSAW) and longitudinal-type LSAW on dissimilar-material bonded structures comprising a LiTaO 3 (LT) or LiNbO 3 thin plate with a thickness of less than 1 λ ( λ ː wavelength) and a quartz substrate are described. The control of attenuation and the cause of large coupling factor of LSAWs by utilizing layered structures were also discussed. For the bonded 4 inch wafer of 36°YX-LT/AT90°X-quartz with a thin-plate thickness of 0.3 λ , an admittance ratio of 81 dB, a fractional bandwidth of 4.2%, and resonance and antiresonance factors of approximately 1500 with markedly improved properties compared with a single 36°YX-LT substrate were obtained experimentally at 2.2 GHz.

Abstract In recent years, cold atmospheric pressure plasma (CAP) applications in agriculture are rapidly increasing. This review covers the effect of CAP and plasma-treated liquid (PTL) on seeds. CAP treatment on seed alters the germination percentage, morphology (shoot height, root length, surface area, etc.), gene expression, and biochemical responses (changes in hormones, antioxidants, amino acids, total soluble sugar content, chlorophyll content, etc.). The changes were incorporated in yield-related parameters (fresh and dry weight of seedlings) and water absorption capacity after plasma treatment. Lastly, we discussed the current status of CAP and PTL use in agricultural land.

Abstract We have constructed the scintillation light yield evaluation system for the scintillator with millisecond decay time, which is based on the digital signal processing and the analog integration circuit with the decay time of 0.1 ms. The digital pulse shape discrimination technique is introduced to check and/or remove the influence of the pile-up events. As the first trial of the absolute scintillation light yield evaluation for scintillators with millisecond decay time, we attempt to acquire the scintillation light yield spectrum of the Tb:Sr 2 Gd 8 (SiO 4 ) 6 O 2 (Tb:SrGS). The pulse area spectrum obtained from the Tb:SrGS scintillator shows a clear peak corresponding to the full-energy absorption events of 662 keV gamma-rays from a Cs-137 source. The peak channels in the pulse area spectra are almost the same both for the Tb:SrGS and bismuthgermanate (BGO) scintillator. Considering the difference in the wavelength matching between scintillators and the photomultiplier tube, the Tb:SrGS scintillator has a 2.8 times higher scintillation light yield compared with the BGO. We…

Abstract AlN templates fabricated via a combination of sputtering deposition and post-deposition high-temperature face-to-face annealing can be applied to deep-ultraviolet (DUV) light-emitting devices because of their extremely low threading dislocation density (TDD) despite their low-cost and simple fabrication process. First, this paper summarizes the overview and essential crystalline characteristics of the face-to-face annealed sputter-deposited AlN template (FFA Sp-AlN). Thereafter, recent progress in the TDD reduction of the FFA Sp-AlN and the metalorganic vapor phase epitaxy of AlN and Al x Ga 1− x N on the FFA Sp-AlN have been reviewed. The TDD of approximately 4 × 10 7 cm −2 was obtained on the sapphire substrates with an AlN film thickness of 1.2 μ m. Finally, the potential of the low-TDD FFA Sp-AlN has been discussed by demonstrating the performances of DUV light-emitting diodes fabricated on the FFA Sp-AlN.

Abstract Thickness scaling on ferroelectric properties of sputter-deposited poling-free Al 0.78 Sc 0.22 N films has been examined. The c -axis oriented films were confirmed by X-ray rocking curve measurements with a film as thin as 10 nm. Ferroelectric-type hysteresis and poling-free behaviors are observed from the capacitance measurements, even with a thickness of 20 nm. The remnant polarization ( P r ) shows a gradual degradation when the thickness is less than 35 nm. The switching (SW) cycle test reveals a wake-up effect for the film, especially for thick films of over 35 nm. A longer SW cycle of over 10 5 times can be obtained with thinner Al 0.78 Sc 0.22 N films around 20 nm at the cost of P r .

Abstract The 300 GHz band, which has a continuous 44 GHz frequency band from 252 to 296 GHz, was identified for wireless applications in WRC-19. The 300 GHz band has the potential to achieve significantly higher transmission rates than conventional wireless communications. In addition, atmospheric attenuation is relatively low. Therefore, research on the 300 GHz band has been attracting a lot of attention. Although compound semiconductors with excellent high-frequency performance have been used for the realization of ultrahigh-frequency transceivers in the 300 GHz band, CMOS integrated circuits are advantageous for practical applications in the 300 GHz band. We have realized a 300 GHz single chip transceiver using CMOS integrated circuits by using parallel connections for multipliers and mixers. This paper presents an overview of the 300 GHz band CMOS transceiver. In addition, we discuss the challenges of using 300 GHz ultrafast communication for long-distance transmission.

Abstract AlGaN-based ultraviolet light-emitting diodes (UV-LEDs) are key components for the inactivation of viruses. Highly efficient and high-power UV-LEDs, capable of inactivating viruses in a short time, are in demand. For this purpose, the growth technologies of n-type AlGaN contact layers were developed from two points of view: first, to decrease the resistivity of n-type Al 0.62 Ga 0.38 N by minimizing the electron compensation, resulting in electronic degeneracy with metallic conduction; second, to improve the light emission uniformity in AlGaN multiquantum wells (MQWs) by controlling the morphology of the underlying n-type AlGaN layer to inhibit macrostep formation. A UV-LED module emitting at 275 nm was demonstrated with the developed growth technology, and illuminated with an irradiation power of 2.6 mW cm −2 on SARS-CoV-2 samples. Over 99.999 % of viruses were inactivated within 5 s owing to the high power of this module.

Abstract Harvesting of ambient renewable energy resources, such as indoor light, is a viable solution for the development of autonomous, “install-and-forget”, environmental nanosensors. In this work we fabricated and characterized photovoltaic cells based on AgBiI 4 rudorffites as promising indoor photovoltaic energy harvesters demonstrating photoconversion efficiency of 5.17% and power output of 1.76 μ W cm −2 measured under white LED light of 1000 lux. Considering that modern low-power wireless transmitters consume <1 μ J per bit for data transmission, the indoor rudorffite photovoltaic cell combined with a supercapacitor can be used for sensor readout and reliable intermittent data transmission.

Abstract Thermodynamic analysis and experimental demonstration of β -Ga 2 O 3 growth by metalorganic vapor phase epitaxy using triethylgallium (TEG) and oxygen (O 2 ) precursors were performed. Thermodynamic analysis revealed that the O 2 supplied is preferentially used for the combustion of hydrocarbons and H 2 derived from TEG. Therefore, the use of high growth temperatures and high input VI/III ratios is essential for the complete combustion of hydrocarbons and H 2 , and β -Ga 2 O 3 growth. The use of an inert gas as the carrier gas was also determined as necessary to grow β -Ga 2 O 3 at high temperatures. Based on these results, a ( 2 ¯ 01) oriented smooth β -Ga 2 O 3 layer could be grown on a c -plane sapphire substrate at 900 °C with a growth rate of 1.4 μ m h −1 at an input VI/III ratio of 100. The grown layer showed a clear optical bandgap of 4.84 eV, and impurity concentrations of hydrogen and carbon were below the background levels of the measurement system.

Abstract Four types of waveforms of AC poling (ACP) and DC poling (DCP) were applied on [001]-oriented 0.72Pb(Mg 1/3 Nb 2/3 )O 3 -0.28PbTiO 3 (PMN-28PT) ferroelectric single crystals (SCs) at 80 °C. High voltage (5 kV cm −1 ) ACP of all four waveforms and DCP SCs tend to show low piezoelectric coefficient ( d 33 ) less than 1660 pC/N and large spurious-mode vibration (SMV) at the thickness mode vibration (TMV) on impedance spectrum. The highest d 33 of 1830 pC/N was obtained by a triangle wave of 3 kV cm −1 SCs with tiny SMV. By contrast, low voltage (1.5 kV cm −1 ) ACP SCs with sine and triangle waveforms showed high d 33 of 1750 pC/N and 1670 pC/N without any SMV. We consider that the SMV is more easily caused by the over-poling of PMN-28PT SC. This information will help realize high piezoelectric properties and high production yield for ACP PMN-PT SC transducers for high-resolution medical ultrasound imaging equipment.

Abstract Reservoir computing (RC), a low-power computational framework derived from recurrent neural networks, is suitable for temporal/sequential data processing. Here, we report the development of RC devices utilizing Ag–Ag 2 S core–shell nanoparticles (NPs), synthesized by a simple wet chemical protocol, as the reservoir layer. We examined the NP-based reservoir layer for the required properties of RC hardware, such as echo state property, and then performed the benchmark tasks. Our study on NP-based reservoirs highlighted the importance of the dynamics between the NPs as indicated by the rich high dimensionality due to the echo state property. These dynamics affected the accuracy (up to 99%) of the target waveforms that were generated with a low number of readout channels. Our study demonstrates the great potential of Ag–Ag 2 S NPs for the development of next-generation RC hardware.

Abstract Flexible electronics with high-performance devices are crucial for transformative advances in several emerging and traditional applications. To address this need, herein we present p-type silicon (Si) nanoribbons (NR)-based high-performance field-effect transistors (FETs) developed using an innovative direct roll transfer stamping (DRTS) process. First, ultrathin Si NRs (∼70 nm) are obtained from silicon on insulator wafers using the conventional top-down method, and then the DRTS method is employed to directly place the NRs onto flexible substrates at RT. The NRFETs are then developed following the RT fabrication process which includes deposition of high-quality SiN x dielectric. The fabricated p-channel transistors demonstrate high linear mobility ∼100 ± 10 cm2 V−1 s−1, current on/off ratio >104, and low gate leakage (<1 nA). Further, the transistors showed robust device performance under mechanical bending and at a wide temperature range (15 °C–90 °C), showing excellent potential for futuristic high-performance flexible electronic devices/circuits.

Abstract Since their early stages of development, micro-electro-mechanical systems (MEMS) have shown potential for breakthroughs in the fabrication of medical tools. The miniaturization of various devices using MEMS technology has enabled minimally invasive treatments and in situ measurements. In this paper, we introduce two advancements in MEMS applications in the medical field: (1) microneedle devices for brain activity evaluation, a transdermal drug delivery system, and biological fluid sampling; and (2) miniaturized MEMS sensors for monitoring the conditions inside blood vessels and respiratory organs. In addition, we provide a summary of MEMS sensors used in developing new drugs, detecting vital signs, and other applications.

Abstract Nitrogen is a very common element, comprising approximately 78% of Earth’s atmosphere, and is an important component of various electronic devices while also being essential for life. However, it is challenging to directly utilize dinitrogen because of the highly stable triple bond in this molecule. The present review examines the use of non-equilibrium plasmas to generate controlled electron impacts as a means of generating reactive nitrogen species (RNS) with high internal energy values and extremely short lifetimes. These species include ground state nitrogen atoms, excited nitrogen atoms, etc. RNS can subsequently react with oxygen and/or hydrogen to generate new highly reactive compounds and can also be used to control various cell functions and create new functional materials. Herein, plasma-processing methods intended to provide RNS serving as short-lived precursors for a range of applications are examined in detail.

Abstract Over the past two decades, research and development using various atmospheric-pressure non-equilibrium plasmas has been conducted energetically for materials synthesis, surface treatment, environmental and energy applications, plasma medicine, plasma agriculture, and other applications. Precise understanding of the physics and chemistry of atmospheric-pressure non-equilibrium plasmas is indispensable for further development of these applications. In this paper, we review progress in the simulation of atmospheric-pressure non-equilibrium plasmas, especially streamer discharges and atmospheric-pressure glow discharges as the most fundamental discharge forms, using the fluid approximation model over the last two decades, and the current understanding of the discharge evolution process. In addition, we discuss recent findings on plasma–liquid interactions obtained from simulations of atmospheric-pressure plasmas in contact with liquids.

Abstract We experimentally characterize sub-threshold swing ( SS ) of Si n-channel MOSFETs with a substrate boron concentration of 2 × 10 16 cm −3 as a function of drain current ( I DS ) and temperatures from 4 to 300 K in order to verify the validity of the physical model of SS . The minimum SS are obtained experimentally at 4 K of around 4 mV dec −1 . The physical model including mobile band tail states and localized interface states with a Gaussian distribution, proposed by Beckers at el., is employed to represent the experimental SS from 4 to 300 K. The impact of each parameter included in the physical model on the SS behavior is examined by changing the value of the parameters in simulation. It is found that the proposed physical model can quantitatively represent experimental SS in a wide range of I DS and temperature under a given set of the parameters regarding the band tail states and the interface states. This finding indicates the validity of the present physical model and the correctness of the physical picture.

Abstract Based on our recent studies, we review our proposed experimental method for evaluating internal quantum efficiency, which is a critical parameter for semiconductor light-emitting device performance. We validated our evaluation method by comparing the results of the efficiency curve analysis using an exciton rate equation model with the results of radiative and nonradiative recombination dynamics obtained by time-resolved photoluminescence spectroscopy. We also review the characteristics of the stimulated emission observed from optically pumped Al x Ga 1− x N-based multiple quantum wells. The most important finding was the contribution of excitons to optical gain formation. The excitonic gain is expected to decrease the threshold carrier density for stimulated emission and lead to higher performance of semiconductor laser diodes.

Abstract Recently, α -Ga 2 O 3 has been attracting great attentions as a new wide bandgap semiconductor, however, the reason why metastable α -Ga 2 O 3 is grown by mist chemical vapor deposition (CVD) has not been understood. In this study, in order to elucidate growth mechanism of mist CVD-grown α -Ga 2 O 3 , growth processes in the initial stage were investigated by atomic force microscopy, transmission electron microscopy, and X-ray diffraction reciprocal space mapping. We found that the characteristics of mist CVD make the relaxation mechanisms of α -Ga 2 O 3 on (0001) sapphire different from those of molecular beam epitaxy, pulsed-laser deposition, and metalorganic chemical vapor deposition. We also proposed the growth procedure in the initial stage.

Abstract Piezoelectric-on-silicon MEMS resonator is very well known for its compatibility with CMOS integrated circuit technology. However, its quality factor ( Q ) is highly affected by acoustic energy loss through the supporting structures. In this study, a resonator with butterfly-like round edge resonating plate structure and deploying properly designed PnC arrays on the tether and the anchoring boundaries are proposed to scale up the quality factor by effectively reducing the energy loss. The finite element analysis simulation results reveal that the proposed topologies can efficiently change the displacement field in the resonating plate and reduce the anchor loss. Consequently, an unloaded quality factor Q u of the conventional resonator is raised from 29 899 to 51 503 (butterfly-like round edge resonating structure), 83 349 (Phononic crystal on the tether) and 83 899 (Phononic crystal on the anchor), representing 1.7 folds, 2.78 folds and 2.8 folds improvement respectively.

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

Abstract This review paper describes the historical development, current issues, and future expectations of UV-B laser diodes, which are expected to be adopted in various applications such as in microfabrication and biotechnology in the near future. To achieve RT operation of UV-B laser diodes, several challenges need to be addressed, including the development of a crystal growth technique for high-crystalline-quality AlGaN films that enables laser oscillation with a low excitation carrier density, and the development of a semiconductor layer structure for simultaneous formation of a desirable optical cavity and injection of high-density carriers (operation of high current density), allowing laser oscillation in the active layer. These challenges and the corresponding technologies that have overcome them are reviewed. The current status of device characteristics and future challenges are also discussed.

Abstract Development of antibiotic resistance is a major challenge for antibiotics as an effective treatment approach of infectious diseases and pathogenic microbes with resistance to antibiotics will become difficult to be treated. Therefore, a new therapy method, which is safe and can inactivate pathogenic microbes effectively without developing a resistance, is highly needed. Ultraviolet irradiation is well known for its ability of effective microbial inactivation and it is widely used in sterilization of inanimate objects based on conventional ultraviolet light sources. Meanwhile, applying ultraviolet irradiation in human disinfection application is an emerging and rapidly progressing field. This review focuses on recent studies in ultraviolet based disinfection methods including both animal and human studies. We will introduce different microbial inactivation mechanisms, which are associated with the ultraviolet irradiation wavelength. Relevant research work will be summarized with a focus on their microbial inactivation effect and safety issues.

Abstract Diamond films were grown on GaN high-electron mobility transistors (HEMTs) to improve thermal dissipation. We observed the temperature reduction in GaN HEMTs using Raman spectroscopy. The large-grain-size (over 1 μ m) diamond films grown under the 〈110〉 preferential growth condition exhibited a high thermal conductivity of over 200 W m −1 K −1 . These diamond films, which were grown at a high temperature of 700 °C, could be directly applied onto the GaN HEMT surface using a thermally stable SiN x metal insulator semiconductor gate structure. The maximum temperature and thermal resistance of the GaN HEMTs decreased by 100 K and from 12.7 mm K W −1 to 7.4 mm K W −1 , respectively, when microcrystalline diamond films were applied onto the HEMT surfaces. As a result, the current density and trans-conductance of the GaN HEMTs were improved after diamond-film growth.

Abstract High-voltage pulsed electric fields (PEF) and pulsed discharge plasmas have received a great deal of attention in the fields of biochemistry, medicine, agriculture and food industry. The destruction phenomenon of cell membranes has been confirmed by PEF and discharge plasma, and it can be used for inactivating microorganisms and extracting cell contents. In recent years, it has been reported that PEF changes the activity of proteins such as enzymes. This review describes how PEF and pulsed discharge plasma function in enzyme activity change and permeabilization in food processing. It is thought that PEF affects the three-dimensional structure of proteins, and research is underway. Here, we will introduce the latest research results on the mechanism of enzyme activity change by PEF. In addition, we will introduce the latest research on changes in cell permeability using shock waves caused by the pulsed discharge plasma.

Abstract Ga 2 O 3 is a wide bandgap semiconductor and an understanding of its bandgap tunability is required to broaden the potential range of Ga 2 O 3 applications. In this study, the different bandgaps of α -Ga 2 O 3 were calculated by performing first-principles calculations using the pseudopotential self-interaction correction method. The relationships between these bandgaps and the material’s hydrostatic, uniaxial, and equibiaxial lattice strains were investigated. The direct and indirect bandgaps of strain-free α -Ga 2 O 3 were 4.89 eV and 4.68 eV, respectively. These bandgap values changed linearly and negatively as a function of the hydrostatic strain. Under the uniaxial and equibiaxial strain conditions, the maximum bandgap appeared under application of a small compressive strain, and the bandgaps decreased symmetrically with increasing compressive and tensile strain around the maximum value.

Abstract Orienting semiconducting polymers (SCPs) using solution-processable techniques for organic electronic devices is essential for realizing a printable electronics device. Preparation of oriented films on liquid substrates and transferring on actual device substrate offers many advantages, including layer-by-layer coating, independent control of morphology, freedom to choose substrates, and device geometry. The floating films transfer method (FTM) has emerged as a unique method to prepare oriented films of SCP using solution processing. Here, we review recent progress in this method from the viewpoint of improvement in FTM to orient SCPs and their anisotropy in organic field effect transistors. In addition, the use of FTM films in sensing applications is also reviewed.

Abstract This article demonstrates the study of thermal energy with the rheology of Williamson liquid over a rotating surface. The correlations between nanoparticles and hybrid nanoparticles in the base fluid are considered to determine the comparative analysis in nanoparticles and hybrid nanostructures. The variable terms are thermal conductivity and thermal radiation, which are also inserted in the energy equation with the associations of nano and hybrid nanoparticles. The ordinary differential equations (ODEs) are generated from the modeled problem using transformation. In contrast, the least square method is used to simulate analytic solutions of required ODEs, and further results are compared with a numerical scheme. The simulations of temperature and motion of fluid particles are performed in graphs and tables. The reduction in motion of fluid particles is captured versus the variation of Williamson number and magnetic number. At the same time, Williamson’s number and magnetic parameter also reduce momentum boundary layer thickness. The hybrid nanoparticles play a vital impact…

Abstract New high-speed photonic technologies and co-integration with electronics are required to keep up with the demand of future optical communication systems. In this paper, plasmonics is presented as one of the most promising next-generation photonic technologies that already fulfils these requirements in proof-of-concept demonstrations. Plasmonics features not only modulators and detectors of highest speed, but also compactness, cost- and energy-efficiency, and compatibility with CMOS electronics. Recently, co-integration with electronics was demonstrated with record performances of 222 GBd in a hybrid InP electronic-plasmonic transmitter assembly and of 120 GBd with a monolithic BiCMOS electronic-plasmonic transmitter.

Abstract We demonstrated the growth of a single-domain κ -Ga 2 O 3 thin film on ε -GaFeO 3 by using an organic-free compound as a precursor for mist chemical vapor deposition. X-ray diffraction analysis revealed that an 86 nm thick κ -Ga 2 O 3 thin film was grown almost coherently with slight lattice relaxation. The surface morphology of the κ -Ga 2 O 3 thin film exhibited a step-terrace structure without island growth. Furthermore, plan-view TEM observations revealed that the κ -Ga 2 O 3 thin film grown on ε -GaFeO 3 had a single domain, whereas the previously reported κ -Ga 2 O 3 thin film grown on AlN template had a domain structure.

Abstract CuInSe 2 and its related I–III–VI 2 chalcopyrite compounds have been studied for about 50 years for application to absorbers in polycrystalline thin-film solar cells. Several research groups developed Cu(Ga,In)Se 2 , Cu(Ga,In)(S,Se) 2 , and (Cu,Ag)(Ga,In)Se 2 solar cells with conversion efficiencies higher than 20%. The bandgap energies of these absorber materials in high-efficiency solar cells are less than 1.2 eV. Currently, wide bandgap chalcopyrite compounds, such as Ga-rich Cu(Ga,In)Se 2 , Cu(Ga,In)S 2 , and Ga-rich (Cu,Ag)(Ga,In)Se 2 , have gained attention as absorber materials for the top cells in tandem structure solar cells. However, the conversion efficiency of these wide bandgap solar cells cannot reach 20%. I–III–VI 2 chalcopyrite compounds are composed of three or more elements and their crystal and electronic structures are more complicated than III–V or II–VI compound semiconductors. In particular, phase diagrams containing the I–III–VI 2 chalcopyrite compounds are complex and vary greatly depending on the material systems. This article provides an overview…

Abstract Polarization-inverted multilayers are promising for application in bulk acoustic wave (BAW) resonators, BAW transformers, surface acoustic wave (SAW) devices and nonlinear optics crystals (NLOs). However, is difficult to obtain a polarization-inverted multilayer by a conventional polarization control technique using a buffer layer. Recently developed ion beam-induced polarization inversion film growth is attractive for multilayer fabrication. Low-energy ion beam irradiation (several hundred electron volts) during film growth enables the growth of polarization-inverted (0001)/(000 1 ¯ ) c-axis normal ZnO, AlN and ScAlN piezoelectric films. These structures excite a thickness extensional mode (longitudinal wave). In contrast, high-energy ion beam irradiation (300–3000 eV) induces c-axis parallel film growth which allows the fabrication of c-axis horizontal inversion ZnO and AlN multilayers. These structures are suitable for thickness shear mode (TSM) film bulk acoustic resonators (FBARs), TSM liquid sensors and out-of-plane NLOs. This review introduces the unusual…

Abstract We investigated the channeling implantation of dopant ions into 4H-SiC in detail. The advantages of channeling implantation were clarified by both experimental and Monte Carlo simulation. Extremely abrupt deep profile with box-like can be formed by channeling implantation of Al and P ions. In the case of B and N ions, retrograde-like profile can be obtained by channeling implantation. The unique dopant profile can be realized by channeling implantation. Both vertical and lateral straggling becomes small by using channeling implantation for Al ions. This technique should contribute to improving the device performance such as gate pitch scaling and low specific on-resistance.

Abstract We have investigated the memory characteristics of InGaZnO (IGZO)-channel ferroelectric FETs (FeFETs) with 2D planar and 3D structures by TCAD simulation to improve the memory window (MW) with a floating-body channel for high-density memory applications. From the study on 2D planar FeFETs with a single gate and a double gate, the MW depends on channel length ( L ) and is enhanced with shorter L due to stronger electrostatic coupling from the source and drain to the center region of the IGZO layer. From the study on 3D structure FeFETs with macaroni (MAC) and nanowire (NW) structures, a large MW can be obtained especially in NW FeFETs due to the electric field concentration by Gauss’s law in the 3D electrostatics. Furthermore, we have systematically studied and discussed the device design of MAC and NW structure FeFETs in terms of the diameter and thickness for high-density memory applications. As the IGZO thickness and the outer diameter of the IGZO layer decrease, the MW increases due to the voltage divider and the electric field concentration. The device parameters that…

Abstract In this paper we review the recent studies on wide InGaN quantum wells (QWs). InGaN QWs are known to suffer from an extremely high built-in piezoelectric polarization, which separates the electron and hole wavefunctions and causes the quantum-confined Stark effect. We show both by means of modeling and experimentally, that wide InGaN QWs can have quantum efficiency superior to commonly used thin QWs. The high efficiency is explained by initial screening of the piezoelectric field and subsequent emergence of optical transitions involving the excited states of electrons and holes, which have a high oscillator strength. A high pressure spectroscopy and photocurrent measurements are used to verify the mechanism of recombination through excited states. Furthermore, the influence of QW width on the properties of optoelectronic devices is studied. In particular, it is shown how the optical gain forms in laser diodes with wide InGaN QWs.

Abstract Monolayer antimony (antimonene) is an emerging 2D material that has recently attracted considerable attention due to its intriguing structure and fascinating electronic properties as theoretically predicted. In order to extensively explore this material in both fundamental research and potential applications, it is important to first prepare high-quality monolayer antimony. In this review, we focus on the theoretical calculations and experimental achievements for both α - and β - phases of antimonene, particularly through the use of molecular beam epitaxy. Moreover, we also provide insight for further exploring the appealing properties of antimonene and other materials with analogous structure.

Phononic crystals (PnCs) have been shown to manipulate and amplify elastic waves. Using this characteristic of PnCs to assist energy harvesting has a remarkable effect. Generally, a defect occurs when a unit cell in a PnC is replaced by another cell with different geometric or material properties; then the output electric power of piezoelectric energy harvesting (PEH) devices will be significantly enhanced. In this study, a cross-hole-type PnC-assisted PEH device with a large defect is presented by replacing several adjacent cells with other cells. It is found that multiple peak voltages can be created within the bandgap and multimodal energy harvesting can be performed. Compared with the defect mode composed of a small defect, energy localization and amplification of the proposed PnC leads to substantial enhancement of harvesting power after tailoring the geometric parameters of a PEH device. This work will help in designing PnC-assisted PEH devices in a reasonable way.

Abstract This paper discusses the applicability of dielectric materials with high acoustic impedance Z a for use in A 1 Lamb mode solidly mounted resonator (SMR) with large electromechanical coupling coefficient k 2 . The study first shows that the use of metal as a reflector creates parasitic capacitance C p , which reduces k 2 significantly. Then, A 1 Lamb mode SMRs are designed using HfN, HfO 2 , WN, WO 3 etc., and achievable performances are compared. When either HfN, HfO 2 , WN, or WO 3 is employed, relative large k 2 up to 25% is achievable. For further k 2 enhancement, a hybrid reflector configuration is also examined, wherein HfN is applied only to the top high Z a layer and W is applied to the others. The result indicates that C p caused by the W layers is still significant, and k 2 becomes worse in total.

Abstract Colonoscopy is regarded as the gold standard in colorectal tumor diagnosis, but it is costly and time-consuming. Raman spectroscopy has shown promise for differentiating cancerous from non-cancerous tissue and is expected to be a new tool for oncological diagnosis. However, traditional Raman spectroscopy analysis requires tedious preprocessing, and the classification accuracy needs to be improved. In this work, a novel Raman spectral qualitative classification method based on convolutional neural network (CNN) is proposed for the identification of three different colon tissue samples, including adenomatous polyp, adenocarcinoma and normal tissues. Experimental results show that this CNN model has superior feature extraction ability. For the spectral data of new individuals, the trained CNN model presents much better classification performance than traditional machine learning methods, such as the k-nearest neighbor, random forest, and support vector machine. Raman spectroscopy combined with CNN can be used as an effective auxiliary tool for the early diagnosis of colon…

Abstract Chemically amplified resists will be used in the high numerical aperture (NA) tools of extreme ultraviolet lithography. However, stochastic defects are a serious problem for their application to the high NA tools. In this study, the stochastic defect generation was simulated on the basis of the sensitization mechanisms and analyzed to clarify the contribution of process and material parameters using machine learning. The half-pitch HP, the sensitivity s , the total sensitizer concentration C s , the effective reaction radius for deprotection R eff , and the initial standard deviation of the number of protected units per polymer molecule σ i were used as variables. As a result, the exponential function reproduced the simulation results well. s and HP had dominant effects in LER formation. For pinching, s and HP were dominant. σ i had a major effect. For bridging, s and HP were also dominant, the effect of σ i was not major and C s and R eff effects were major.

Abstract Aimed towards an application of ultrasound diagnosis using contrast agents, the dynamics of encapsulated bubbles has been theoretically investigated under the restriction of a single bubble. In this paper, we extend the theory for single bubble or some bubbles to that for many bubbles, and theoretically investigate weakly nonlinear propagation of ultrasound in an initially quiescent incompressible liquid, uniformly containing many microbubbles encapsulated by the shell as a viscoelastic body (Kelvin–Voigt model). As a result, we derived the Korteweg–de Vries–Burgers equation for a low-frequency long wave and clarified that the shell affects the advection, nonlinear, and dissipation (not dispersion) effects of ultrasound propagation. In particular, shell rigidity, surface tension, and shell viscosity increased the advection, nonlinear, and dissipation effects, respectively.

Abstract Previous studies have shown that evaluation results of shear wave elastography were unstable due to factors such as liver structure and complexity of physical properties. The present study attempts to verify the influence of liver microstructure (fat droplets and fibrous tissue) on the shear wave and shear wave velocity (SWV) evaluation using a shear wave propagation simulation by the elastic finite-difference time-domain method. It was found that disruption of the shear wave causes variations in the SWV of the liver around fat droplets, and the SWV of the fibrous tissue depends on the shear wave propagation direction and the tissue shape. In a nonalcoholic steatohepatitis liver, which contains fat and fiber, the influences of these two tissues are synergistically reflected in the SWV evaluation.

Abstract To develop a high-density and long-life lithium-ion battery, a technology is needed that allows non-destructive visualization of the spatial distribution of deteriorated parts after cycle test. In the present study, we measured the distribution of the magnetic field leaking from the lithium-ion battery during its operation. Based on the measurement results, we evaluated the spatial distribution of electric current density that corresponds to the reaction rate of the active material and the ion diffusion rate in the electrolyte solution inside a battery using the electric current reconstruction process. With respect to the changes in the internal state of the lithium-ion battery associated with cycle deterioration, we successfully visualized the part where the electrical conductivity has changed that is the deteriorated part causing the battery capacity to decrease inside the lithium-ion battery.

Abstract In quantum chemistry, it is important to estimate an energy gap between a ground state and an excited state of molecular Hamiltonians. In previous researches, it was necessary to measure the energy of the ground state and that of the excited state separately, and the energy gap was estimated from the subtraction between them. Here, we show a novel scheme to estimate such an energy gap in a more direct manner. We use a concept of a Ramsey type measurement in the quantum annealing (QA) for such a direct estimation of the energy gap. In our scheme, the measured signal oscillates with a frequency of the energy gap. To study the performance of our scheme, we perform numerical simulations. The results show our scheme is robust against non-adiabatic transitions between the ground state and first excited state. Our scheme paves the way for the practical application of QA in quantum chemistry.

Abstract Birefringence imaging is one of the powerful methods for non-destructive characterization of defects in the semiconductor crystals. However, due to the complicated and unclear contrasts of dislocations in the birefringence image, it was considered to be difficult to automatically detect the position of the dislocation contrasts by the conventional image processing. In the present study, we designed the automatic detection algorithm for the dislocation contrasts taking into account the characteristic feature of the dislocation contrasts, which were always pair of black and white contrasts. To detect the large change in the contrast level near the dislocation contrast, the automatic detection algorithm was constructed by using a variance filter. Finally, we succeeded in detecting the position of the dislocation contrasts with relatively high precision and sensitivity.

Abstract The thermal stability of α -(Al x Ga 1– x ) 2 O 3 films grown on c-plane sapphire substrates was investigated. The α -(Al x Ga 1– x ) 2 O 3 epitaxial films grown by mist chemical vapor deposition were annealed at temperatures in the range of 600 °C–1100 °C in an atmospheric furnace, and then the crystal structures of the films were characterized using X-ray diffraction and transmission electron microscopy. When the Al composition was less than 0.5, the α -(Al x Ga 1– x ) 2 O 3 films converted to the β -phase, which is the thermodynamically most stable phase for Ga 2 O 3 . The thermal stability was enhanced by increase in the Al composition, and α -(Al x Ga 1– x ) 2 O 3 with x = 0.45 maintained the corundum structure after annealing at 950 °C. On the other hand, the α -(Al x Ga 1– x ) 2 O 3 layers with Al contents higher than 0.6 were stable against the thermal treatment and did not show phase transformation to other phases upon high-temperature annealing at 1100 °C.

Abstract Stochastic simulations are performed to analyze the effects of acid diffusion and resist molecular size on pattern profiles for chemically amplified resists in EUV lithography. In the simulation, polymer chains are formed by random connection of monomer units. The acid diffusion and the acid reaction with polymers in the post exposure bake are introduced by the random-walk model. Simulation results show that the inhomogeneous acid generation and diffusion more largely affects the pattern roughness than polymer molecular size for both deprotection and cross-linking types of resists. At high doses, the line edge roughness (LER) increases with increasing polymerization degree of resist polymer. In such condition, the acid distribution becomes uniform and molecular size affects the LER. On the other hand, at low doses, the LER decreases with increasing polymerization degree. That is because the larger polymer molecule is less sensitive to inhomogeneous distribution of acid accompanying low doses.