The 50 most-cited articles in Japanese Journal of Applied Physics, 2016

Abstract β-Ga 2 O 3 bulk crystals were grown by the edge-defined film-fed growth (EFG) process and the floating zone process. Semiconductor substrates containing no twin boundaries with sizes up to 4 in. in diameter were fabricated. It was found that Si was the main residual impurity in the EFG-grown crystals and that the effective donor concentration ( N d − N a ) of unintentionally doped crystals was governed by the Si concentration. Intentional n-type doping was shown to be possible. An etch pit observation revealed that the dislocation density was on the order of 10 3 cm −3 . N d − N a for the samples annealed in nitrogen ambient was almost the same as the Si concentration, while for the samples annealed in oxygen ambient, it was around 1 × 10 17 cm −3 and independent of the Si concentration.

Abstract Gallium oxide (Ga 2 O 3 ) is an emerging wide-bandgap semiconductor for high-power, low-loss transistors and diodes by virtue of its excellent material properties and suitability for mass production. In this paper, we begin by discussing the material properties of Ga 2 O 3 that make it an attractive alternative to not only Si but also other wide-bandgap materials such as SiC and GaN. State-of-the-art Ga 2 O 3 -based devices that have been fabricated to date demonstrate the performance potential for power electronics applications.

Abstract The recent progress and development of corundum-structured III-oxide semiconductors are reviewed. They allow bandgap engineering from 3.7 to ∼9 eV and function engineering, leading to highly durable electronic devices and deep ultraviolet optical devices as well as multifunctional devices. Mist chemical vapor deposition can be a simple and safe growth technology and is advantageous for reducing energy and cost for the growth. This is favorable for the wide commercial use of devices at low cost. The III-oxide semiconductors are promising candidates for new devices contributing to sustainable social, economic, and technological development for the future.

Abstract We have structurally evaluated β-Ga 2 O 3 crystals grown by edge-defined film-fed growth process using etch pitting, focused ion beam scanning ion microscopy, transmission electron microscopy, and related techniques. We found three types of defects: arrays of edge dislocations corresponding to etch pit arrays on -oriented wafers, platelike nanopipes corresponding to etch pits revealed on the (010)-oriented wafers, and twins including twin lamellae.

Abstract The electron mobility in depletion-mode lateral β-Ga 2 O 3 (010) metal–oxide–semiconductor field-effect transistors (MOSFETs) with an n-channel formed by Si-ion (Si + ) implantation doping was extracted using low-field electrical measurements on FET structures. An undoped Ga 2 O 3 buffer layer protected the channel against charge compensation by suppressing outdiffusion of deep Fe acceptors from the semi-insulating substrate. The molecular beam epitaxy growth temperature was identified as a key process parameter for eliminating parasitic conduction at the buffer/substrate growth interface. Devices with a resistive buffer showed room temperature channel mobilities of 90–100 cm 2 V −1 s −1 at carrier concentrations of low- to mid-10 17 cm −3 , with small in-plane mobility anisotropy of 10–15% ascribable to anisotropic carrier scattering.

Abstract In this paper, we give an overview of the recent progress in GaN-based high-electron-mobility transistors (HEMTs) developed for mainstream acceptance in the power electronics field. The comprehensive investigation of AlGaN/GaN HEMTs fabricated on a free-standing semi-insulating GaN substrate reveals that an extracted effective lateral breakdown field of approximately 1 MV/cm is likely limited by the premature device breakdown originating from the insufficient structural and electrical quality of GaN buffer layers and/or the GaN substrate itself. The effective lateral breakdown field is increased to 2 MV/cm by using a highly resistive GaN substrate achieved by heavy Fe doping. Various issues relevant to current collapse are also discussed in the latter half of this paper, where a more pronounced reduction in current collapse is achieved by combining two different schemes (i.e., a prepassivation oxygen plasma treatment and a field plate structure) for intensifying the mitigating effect against current collapse. Finally, a novel approach to suppress current collapse is…

Abstract We achieved the successful fabrication of Sn-doped α-Ga 2 O 3 thin films with higher electron mobility and wider conductivity controls by improving the crystal quality. α-Ga 2 O 3 films showed n-type conductivity with a maximum electron mobility of 24 cm 2 V −1 s −1 . The carrier concentration was successfully controlled in the range of 10 17 –10 19 cm −3 . Crystal defects such as dislocations severely compensate the free carriers in α-Ga 2 O 3 films and restrict the mobility at low carrier concentrations. Therefore, to achieve further conductivity control and higher mobility, improving the crystallinity of α-Ga 2 O 3 films is necessary.

Abstract Homoepitaxial single-crystal beta gallium oxide (β-Ga 2 O 3 ) films were fabricated by the mist chemical vapor deposition method. The crystallinity of the films grown markedly depended on growth temperature, and the optimum growth temperatures were found to be 700–800 °C. Using unintentionally doped β-Ga 2 O 3 films grown on Sn-doped β-Ga 2 O 3 (010) substrates, the fabrication of Schottky barrier diodes was demonstrated. Furthermore, we fabricated electrically conductive Sn-doped β-Ga 2 O 3 films on semi-insulating Fe-doped β-Ga 2 O 3 (010) substrates. The carrier concentrations were between 1 × 10 18 and 5 × 10 20 cm −3 . The Hall mobility was 45 cm 2 V −1 s −1 at the carrier concentration of 1 × 10 18 cm −3 .

Abstract In this study, epitaxial ε-Ga 2 O 3 thin films are successfully grown on cubic (111) MgO and (111) yttria-stablized zirconia (YSZ) substrates by mist chemical vapor deposition. Pure-phase hexagonal ε-Ga 2 O 3 thin films are grown on the two substrates with a c -axis orientation determined by X-ray diffraction (XRD) 2θ–ω scanning. XRD pole figure measurements reveal that the in-plane orientation relationship between the (0001) of ε-Ga 2 O 3 and the (111) of the two substrates is ε-Ga 2 O 3 ∥ substrates . Using (111) MgO substrates with a 2.5% lattice mismatch, the epitaxial ε-Ga 2 O 3 films are successfully grown at a low temperature of 400 °C. The optical direct and indirect bandgaps of pure ε-Ga 2 O 3 thin films are estimated as 5.0 and 4.5 eV, respectively.

Abstract Cr 2 Ge 2 Te 6 (CGT), a layered ferromagnetic insulator, has attracted a great deal of interest recently owing to its potential for integration with Dirac materials to realize the quantum anomalous Hall effect (QAHE) and to develop novel spintronics devices. Here, we study the uniaxial magnetic anisotropy energy of single-crystalline CGT and determine that the magnetic easy axis is directed along the c -axis in its ferromagnetic phase. In addition, CGT is an insulator below the Curie temperature. These properties make CGT a potentially promising candidate substrate for integration with topological insulators for the realization of the high-temperature QAHE.

Abstract An InP/InGaAs heterobarrier rectifier called a Fermi-level managed barrier (FMB) diode was developed for attaining broadband and low-noise terahertz (THz) wave detection. The barrier height was controlled by doping in an InGaAs layer, achieving a very small barrier-height of 70 meV. The intrinsic zero-biased differential resistance obtained was 110 Ω for an FMB diode with a 0.4 µm 2 junction area. The fabricated device assembled in a quasi-optical module detected THz-wave signals at frequencies from 200 GHz to 1 THz at room temperature. The peak current and voltage sensitivities were respectively 3.7 A/W and 1110 V/W at 300 GHz for a zero-biased condition. An FMB diode module with an integrated preamplifier was also developed for characterizing its performance in practical use. Signal detection at frequencies from 160 GHz to 1.4 THz was attained with a high sensitivity of 3.2 MV/W and a low noise-equivalent power of 3.0 pW/ at 300 GHz.

Abstract We fabricated Schottky barrier diodes (SBDs) on the entire surface of a β-Ga 2 O 3 single crystal, and investigated the leakage current in both forward and reverse directions. Subsequently, we investigated the distribution of dislocation and void etch pits on the entire surface. The dislocation etch pit density on the surface ranged from <1 × 10 3 to 6 × 10 4 , and its average was 1.1 × 10 4 cm −2 . The void etch pit density on the surface ranged from <5 × 10 2 to 7 × 10 3 , and its average was 6 × 10 3 cm −2 . From a comparison between the SBD leakage current and the dislocation and void etch pit densities, we found that dislocations are closely related to the SBD reverse leakage current, and that not all voids produce the leakage current.

Abstract Lead-free ferroelectric and piezoelectric ceramics, (Bi 0.5 Na 0.5 )TiO 3 (BNT), were fabricated by a quenching procedure after sintering, and then their electrical properties were investigated with the aim to increase their depolarization temperature T d . From the measurement of the temperature dependence of dielectric properties, T d increased with increasing quench temperature. The T d of a BNT sample quenched from 1100 °C was 223 °C, which was almost 50 °C higher than that prepared by the ordinary cooling process. From the measurement of P – E hysteresis loops, both the remanent polarization P r and the coercive field E c of BNT samples prepared by ordinary firing were almost the same as those quenched from 1100 °C. Additionally, from the measurements by a resonance–antiresonance method, the electromechanical coupling factor k 33 of ordinarily fired BNT was 0.45, and that of the quenched BNT was 0.46. From these results, it is clarified that the quenching procedure is an effective way to increase the T d of BNT ceramics without deteriorating ferroelectric and…

Abstract We report on the impact of TiN interfaces on the ferroelectricity of nondoped HfO 2 . Ferroelectric properties of nondoped HfO 2 in TiN/HfO 2 /TiN stacks are shown in capacitance–voltage and polarization–voltage characteristics. The Curie temperature is also estimated to be around 500 °C. The ferroelectricity of nondoped HfO 2 clearly appears by thinning HfO 2 film down to ∼35 nm. We directly revealed in thermal treatments that the ferroelectric HfO 2 film on TiN was maintained by covering the top surface of HfO 2 with TiN, while it was followed by a phase transition to the paraelectric phase in the case of the open surface of HfO 2 . Thus, it is concluded that the ferroelectricity in nondoped HfO 2 in this study was mainly driven by both of top and bottom TiN interfaces.

Abstract The characteristics of structural defects observed on (100) wafers in β-Ga 2 O 3 single crystals grown by directional solidification in a vertical Bridgman furnace were studied in terms of crystal growth conditions. No high-dislocation-density regions near the wafer periphery were observed owing to the lack of adhesion between the as-grown crystal ingot surface and the crucible inner wall, and directional solidification growth in a crucible with a very low temperature gradient resulted in β-Ga 2 O 3 single crystals with a low mean dislocation density of 2.3 × 10 3 cm −2 . Line-shaped defects up to 150 µm long in the [010] direction were detected at a mean density of 0.5 × 10 2 cm −2 , which decreased with decreasing growth rate. The line-shaped defect structure and formation mechanism were discussed.

Abstract Etch pits of various shapes were observed on etched β-Ga 2 O 3 (010) single crystals and classified into types A–F according to shape. Type-A etch pits changed in shape in the order of types B, C, and D by etching. Groove-shaped pits observed on as-grown β-Ga 2 O 3 (010) single crystal surfaces [K. Hanada et al., Jpn. J. Appl. Phys. 55 , 030303 (2016) ] were classified into type G. Type-G pits, which were determined to be void defects because of three-dimentional spaces in single crystals, existed before etching and changed to type A after etching. Therefore, after etching, void defects must change in shape as follows: Type G → A → B → C → D. The exposed facets change with etching time. Types-E and -F etch pits were observed to be parallelograms and hexagons, respectively. Types-E and -F etch pits must include dislocations along the [010] direction because they did not change in shape after etching.

Abstract This work reports on three important aspects of threshold-voltage instability in SiC power MOSFETs: (1) the threshold-voltage bias-temperature instability observed in commercial devices from two leading manufacturers, (2) a summary of the basic mechanisms driving this instability, and (3) the need for an improved test method for evaluating these devices. Even under significant overstress conditions, no negative threshold-voltage shift was observed in the most-recent-vintage commercial devices from one of the manufacturers during a −15 V, 175 °C negative-bias temperature stress lasting 120 h.

Abstract Edge-defined fed-grown β-Ga 2 O 3 single crystals with high electron concentration of 3.9 × 10 18 cm −3 at 300 K were characterized by Hall effect measurement, and Schottky barrier diodes have been demonstrated. Electron mobility was as high as 74 cm 2 /(V·s) at 300 K regardless of the high doping concentration. The electron concentration did not change substantially in the low temperature below 160 K. This properties can be explained by the two-band model due to the inter-band conduction. On the Schottky barrier diodes, the rectification characteristics were clearly observed, and the current density of 96.8 A/cm 2 at the forward voltage of 1.6 V was obtained.

Abstract We report a feasibility study of a terahertz imaging system with resonant tunneling diodes (RTDs) that oscillate at 0.30 THz. A pair of RTDs acted as an emitter and a detector in the system. Terahertz reflection images of opaque samples were acquired with our RTD imaging system. A spatial resolution of 1 mm, which is equal to the wavelength of the RTD emitter, was achieved. The signal-to-noise ratio (SNR) of the reflection image was improved by 6 dB by using polarization optics that reduced interference effects. Additionally, the coherence of the RTD enabled a depth resolution of less than 3 µm to be achieved by an interferometric technique. Thus, RTDs are an attractive candidate for use in small THz imaging systems.

We prepared CuInSe 2 and Cu-poor Cu–In–Se (CIS) phases such as CuIn 3 Se 5 and CuIn 5 Se 8 in the composition of (1 − x )Cu 2 Se–( x )In 2 Se 3 with 0.5 ≤ x ≤ 1.0. The crystal structure of the sample changed from chalcopyrite-type CuInSe 2 to hexagonal CuIn 5 Se 8 through stannite-type CuIn 3 Se 5 with increasing x (decreasing Cu/In ratio). The band-gap energies of Cu-poor CIS samples, i.e., CuIn 3 Se 5 (1.17 eV) and CuIn 5 Se 8 (1.22–1.24 eV), are larger than that of chalcopyrite-type CuInSe 2 (0.99 eV). The energy levels of the valence band maxima (VBMs) were estimated from the ionization energy by photoemission yield spectroscopy (PYS) measurements. The energy levels of the VBMs of the Cu-poor CIS samples decrease rapidly with decreasing Cu/In ratio. The ionization energy of stannite-type CuIn 3 Se 5 is 0.4 eV larger than that of chalcopyrite-type CuInSe 2 . The ionization energy of CuIn 5 Se 8 is 0.1–0.3 eV larger than that of CuIn 3 Se 5 . These results show that the energy position of the VBM from the vacuum level of Cu-poor CIS phases, such as CuIn 3 Se 5 and CuIn 5 Se 8 , is…

Abstract Crystalline interfaces have a tremendous impact on the properties of materials. Determination of the atomic structure of the interface is crucial for a comprehensive understanding of the interface properties. Despite this importance, extensive calculation is necessary to determine even one interface structure. In this study, we apply a technique called kriging, borrowed from geostatistics, to accelerate the determination of the interface structure. The atomic structure of simplified coincidence-site lattice interfaces were determined using the kriging approach. Our approach successfully determined the most stable interface structure with an efficiency almost 2 orders of magnitude better than the traditional “brute force” approach.

Abstract We have recently implemented a new version of the quasiparticle self-consistent GW (QSGW) method in the ecalj package released at http://github.com/tkotani/ecalj . Since the new version of the ecalj package is numerically stable and more accurate than the previous versions, we can perform calculations easily without being bothered with tuning input parameters. Here we examine its ability to describe energy band properties, e.g., band-gap energy, eigenvalues at special points, and effective mass, for a variety of semiconductors and insulators. We treat C, Si, Ge, Sn, SiC (in 2H, 3C, and 4H structures), (Al, Ga, In) × (N, P, As, Sb), (Zn, Cd, Mg) × (O, S, Se, Te), SiO 2 , HfO 2 , ZrO 2 , SrTiO 3 , PbS, PbTe, MnO, NiO, and HgO. We propose that a hybrid QSGW method, where we mix 80% of QSGW and 20% of LDA, gives universally good agreement with experiments for these materials.

Abstract This paper presents a review on state-of-the-art of thin-film transistor (TFT) technology and its wide range of applications, not only in liquid crystal displays (TFT-LCDs), but also in sensing devices. The history of the evolution of the technology is first given. Then the standard applications of TFT-LCDs, and X-ray detectors, followed by state-of-the-art applications in the field of chemical and biochemical sensing are presented. TFT technology allows the fabrication of dense arrays of independent and transparent microelectrodes on large glass substrates. The potential of these devices as electrical substrates for biological cell applications is then described. The possibility of using TFT array substrates as new tools for electrical experiments on biological cells has been investigated for the first time by our group. Dielectrophoresis experiments and impedance measurements on yeast cells are presented here. Their promising results open the door towards new applications of TFT technology.

Abstract In this study, we developed a higher manganese silicide (HMS) that possesses a high dimensionless figure of merit ZT exceeding unity. HMSs containing a larger amount of Re than its solubility limit were prepared by the liquid quenching technique, and the obtained metastable HMSs showed good thermal stability to enable pulse current sintering at 1240 K. The lattice thermal conductivity was effectively reduced with increasing Re concentration, whereas the electron transport properties were not greatly affected. Consequently, the ZT of p-type HMS increased to 1.04 at 6 at. % Re from 0.4 of the Re-free sample.

Abstract On the surface of as-grown β-Ga 2 O 3 single crystals that are cut and polished, we found nanometer-sized grooves elongated in the [001] direction. We confirmed that these grooves terminate within the crystals in the [010] direction. This proves that the grooves are different from micropipes penetrating crystals. Their typical length and width are 50–1200 nm in the [001] direction and ∼40 nm in the [100] direction, respectively. The grooves tend to form an array in the [001] direction. The type of nanometer-sized grooves should be essentially different from etch pits.

Abstract Developments in computing hardware are constrained by the operating principles of complementary metal oxide semiconductor (CMOS) technology, fabrication limits of nanometer scaled features, and difficulties in effective utilization of high density interconnects. This set of obstacles has promulgated a search for alternative, energy efficient approaches to computing inspired by natural systems including the mammalian brain. Atomic switch network (ASN) devices are a unique platform specifically developed to overcome these current barriers to realize adaptive neuromorphic technology. ASNs are composed of a massively interconnected network of atomic switches with a density of ∼10 9 units/cm 2 and are structurally reminiscent of the neocortex of the brain. ASNs possess both the intrinsic capabilities of individual memristive switches, such as memory capacity and multi-state switching, and the characteristics of large-scale complex systems, such as power-law dynamics and non-linear transformations of input signals. Here we describe the successful nanoarchitectonic fabrication of…

Abstract The construction of functional systems with nanosized parts would not possible by simple technology (nanotechnology). It can be handled by certain kinds of more sophisticated carpenter work or artistic architectonics (nanoarchitectonics). However, architecting materials in the nanoscale is not very simple because of various unexpected and uncontrollable thermal/statistical fluctuations and mutual interactions. The latter factors inevitably disturb the interactions between component building blocks. Therefore, several techniques and actions, including the regulation of atomic/molecular manipulation, molecular modification by organic chemistry, control of physicochemical interactions, self-assembly/organization, and application of external physical stimuli, must be well combined. This short review describes the historical backgrounds and essences of nanoarchitectonics, followed by a brief introduction of recent examples related to nanoarchitectonics. These examples are categorized in accordance with their physical usages: (i) atom/molecule control; (ii) devices and sensors;…

Abstract An evaluation method for the energy level of the valence band (VB) top from the vacuum level (VL) for metals, dielectrics, and semiconductors from the results of X-ray photoelectron spectroscopy (XPS) is presented for the accurate determination of the energy band diagram for materials of interest. In this method, the VB top can be determined by the energy difference between the onset of VB signals and the cut-off energy for secondary photoelectrons by considering the X-ray excitation energy ( h ν). The energy level of the VB top for three kinds of Si-based materials (H-terminated Si, wet-cleaned 4H-SiC, and thermally grown SiO 2 ) has been investigated by XPS under monochromatized Al Kα radiation ( h ν = 1486.6 eV). We have also demonstrated the determination of the electron affinity for the samples by this measurement technique in combination with the measured and reported energy bandgaps ( E g ).

Abstract We present the development of a low-frequency nonthermal plasma-jet system, where the surrounding-gas condition of the plasma jet is precisely controlled in open air. By restricting the mixing of the ambient air into the plasma jet, the plasma jet can be selectively changed from a N 2 main discharge to an O 2 main discharge even in open air. In the plasma-jet system with the controlled surrounding gas, the production of reactive oxygen and nitrogen species is successfully controlled in deionized water: the concentration ratio of NO 2 − to H 2 O 2 is tuned from 0 to 0.18, and a high NO 2 − concentration ratio is obtained at a N 2 gas ratio of 0.80 relative to the total N 2 /O 2 gas mixture in the main discharge gas. We also find that the NO 2 − concentration is much higher in the plasma-activated medium than in the plasma-activated deionized water, which is mainly explained by the contribution of amino acids to NO 2 − generation in the medium.

Anisotropic optical properties are investigated on β-Ga 2 O 3 films and a single crystal by spectroscopic ellipsometry measurements. The films grown on (0001) α-Al 2 O 3 contain threefold in-plane rotational domains, and the refractive index and absorption coefficient α obtained by assuming an isotropic material are found to be smaller than those in the single crystal. By measuring the off-normal transmission ellipsometry spectra of the (010) β-Ga 2 O 3 substrate, the optical anisotropy in a biaxial crystal as well as the gradual increase in α are recognized as origins of the scattering in optically determined bandgap energies.

Abstract Redox reactions at the Cu/Ta 2 O 5 interface and subsequent Cu ion transport in a Ta 2 O 5 film have been investigated by means of cyclic voltammetry (CV) measurements. Under positive bias to the Cu electrode, Cu is preferentially oxidized to Cu 2+ and then to Cu + . Subsequent negative bias causes a reduction of the oxidized Cu ions at the interface. It was found that CV curves change drastically with varied relative humidity levels from 5 to 85%. At higher humidity levels, the ion concentrations and diffusion coefficients, estimated from the CV curves, suggest increased redox reaction rates and a significant contribution of proton conduction to the ionic transport. The results indicate that the redox reactions of moisture are rate-limiting and highlight the importance of water uptake by the matrix oxide film in understanding and controlling the resistive switching behavior of oxide-based atomic switches.

Abstract The N 2 –CO gas annealing technique was demonstrated to improve the crystalline quality of the AlN layer on sapphire. 300-nm-thick AlN layers were fabricated on sapphire substrates by a metal–organic vapor phase epitaxy method. The AlN layers were annealed in N 2 and/or N 2 –CO gas atmosphere at 1923–1973 K for 0.5–4 h. Many pits and voids were observed on the AlN surface annealed in N 2 atmosphere at 1973 K for 2 h. The rough surface was, however, much improved for the AlN annealed in N 2 –CO gas atmosphere. The thermodynamic principle of the N 2 –CO gas annealing technique is explained in this paper on the basis of the phase stability diagram of the Al 2 O 3 –AlN–C–N 2 –CO system. Voids and γ-aluminum oxynitride (γ-AlON) at the AlN/sapphire interface formed during the annealing, which is also explained on the basis of the phase stability diagram. The in-plane epitaxial relationships among AlN, γ-AlON, and sapphire are presented, and misfits among them are discussed.

Abstract Al 2 O 3 films were deposited on β-Ga 2 O 3 (010) and β-Ga 2 O 3 substrates by atomic layer deposition at 250 °C, and their interface state densities ( D it ) at shallow energies were evaluated using a high–low capacitance–voltage ( C – V ) method. Al 2 O 3 /β-Ga 2 O 3 (010) showed lower D it values (5.9 × 10 10 to 9.3 × 10 11 cm −2 eV −1 ) than Al 2 O 3 /β-Ga 2 O 3 (2.0 × 10 11 to 2.0 × 10 12 cm −2 eV −1 ) in an energy range of −0.8 to −0.1 eV. Cross-sectional transmission electron microscopy analysis indicated the formation of a uniform amorphous Al 2 O 3 layer on the β-Ga 2 O 3 substrate. In contrast, a crystalline Al 2 O 3 interlayer with a thickness of 3.2 ± 0.7 nm with an amorphous Al 2 O 3 top layer was formed on the β-Ga 2 O 3 (010) substrate, which effectively decreased D it . Moreover, thicker interlayers showing lower D it values at deep state levels were formed at deposition temperatures higher than 100 °C, which were evaluated by shifts in the C – V curves.

Abstract To replace mercury lamps with AlGaN-based deep-ultraviolet (DUV) LEDs, a simple and low-cost package with increased light extraction efficiency (LEE) is indispensable. Therefore, resin encapsulation is considered to be a key technology. However, the photochemical reactions induced by DUV light cause serious problems, and conventional resins cannot be used. In the former part of this study, a comparison of a silicone resin and fluorine polymers was carried out in terms of their suitability for encapsulation, and we concluded that only one of the fluorine polymers can be used for encapsulation. In the latter part, the endurance of encapsulation using the selected fluorine polymer was investigated, and we confirmed that the selected fluorine polymer can guarantee a lifetime of over 6,000 h at a wavelength of 265 nm. Furthermore, a 3 × 4 array module of encapsulated dies on a simple AlN submount was fabricated, demonstrating the possibility of W/cm 2 -class lighting.

Abstract A new technique is described for manufacturing silicon wafers with the highest capability yet reported for gettering transition metallic, oxygen, and hydrogen impurities in CMOS image sensor fabrication. It is demonstrated that this technique can implant wafers simultaneously with carbon and hydrogen elements that form the projection range by using hydrocarbon compounds. Furthermore, these wafers can getter oxygen impurities out-diffused from the silicon substrate to the carbon cluster ion projection range during heat treatment. Therefore, they can reduce the formation of transition metals and oxygen-related defects in the device active regions and improve electrical performance characteristics, such as dark current and image lag characteristics. The new technique enables the formation of high-gettering-capability sinks for transition metals, oxygen, and hydrogen impurities under device active regions of CMOS image sensors. The wafers formed by this technique have the potential to significantly reduce dark current in advanced CMOS image sensors.

Abstract We report the luminescence and the scintillation properties of a newly developed thallium magnesium chloride (TlMgCl 3 ) crystal. The crystal sample can be easily fabricated from the melt using the Bridgman method. The photoluminescence band appeared near the wavelength of 405 nm under excitation at 230 nm. An X-ray-induced scintillation spectrum showed an intense emission band near the wavelength of 405 nm. The decay time constant was estimated to be approximately 60 ns (∼25%) and 350 ns (∼75%) using a bi-exponential fitting. The scintillation light yield reached 46,000 photons/MeV with an energy resolution of 5% at 662 keV.

Abstract In this paper, a biomimetic microstructure related to cilia, which are effective fluidic and conveying systems in nature, is described. Authors have already reported that a magnetic elastomer pillar actuated by a rotating magnetic field can work like a natural cilium. In the present work, we show examples of a cilia array with a metachronal wave as the next step. A metachronal wave is a sequential action of a number of cilia. It is theoretically known that a metachronal wave gives a higher fluidic efficiency; however, there has been no report on a metachronal wave by artificial cilia. We prepared magnetic elastomer pillars that contain chainlike clusters of magnetic particles. The orientation of chains was set to be different in each pillar so that each pillar will deform with a different phase.

Abstract We have demonstrated the high-peak-power operation of an AlGaN-based ultraviolet laser diode (UV-LD) with a lasing wavelength of 338.6 nm. The UV-LD structure was fabricated on a bulk GaN(0001) substrate. The broad-area and vertical conductive structure of the UV-LD, whose ridge width and cavity length were 50 and 600 µm, respectively, was employed. The threshold current density and differential external quantum efficiency were estimated to be 38.9 kA/cm 2 and 8.5%, respectively. The characteristic temperature of threshold current was estimated to be 119 K, and the temperature dependence of lasing wavelength was obtained to be 0.033 nm K −1 . A peak power of over 1 W has been achieved in 338.6 nm under pulsed operation at room temperature, which is the highest peak power ever obtained for AlGaN-based UV-LDs.

Abstract Diamond possesses a combination of exceptional physical properties and is expected to be used as a semiconductor material in high-efficiency and high-power electronic devices. In this study, hole doping was observed when using NO 2 molecules on a H-diamond surface. The activation energy of hole concentration in NO 2 /H-diamond was measured as 0.006 eV, and holes were fully activated at room temperature. A thermal stabilization of the hole channel was realized by passivation with an atomic-layer-deposited Al 2 O 3 layer. The passivation method enabled the realization of a thermally stable high-performance diamond field-effect transistor (FET), which exhibited high-performance DC and RF characteristics. NO 2 hole-doping and Al 2 O 3 -passivation technologies enabled reproducible measurements of MOS structure electric properties. Such technologies also facilitated observations of two-dimensional holes at the MOS interface and type-II band alignment of Al 2 O 3 /NO 2 /H-diamond. Additionally, the band diagram under various gate bias conditions was proposed on the basis of…

Abstract The effects of organic moieties on the scintillation properties of organic–inorganic layered perovskite-type compounds have been investigated. Three kinds of single crystals were fabricated, namely, (C 4 H 9 NH 3 ) 2 PbBr 4 (C4), (C 6 H 5 CH 2 NH 3 ) 2 PbBr 4 (Ben), and (C 6 H 5 C 2 H 4 NH 3 ) 2 PbBr 4 (Phe). Among the single crystals, the light output of Phe was found to have the greatest value when exposed to X-ray radiation (67.4 keV). The light output of Phe was 0.62 times that of YAP:Ce. The relative values of the light outputs among the fabricated single crystals under X-ray radiation correlated well with those of the quantum efficiencies and the luminescence intensity under ultraviolet radiation.

Abstract The rogress of on-surface molecular nanoarchitectonics over the last two decades has been reviewed. Over the early period in the 1990s–2000s, molecular self-assemblies were intensively studied, where van der Waals (vdW) interaction was predominant. After that, in the 2000s, selective intermolecular interactions based on hydrogen bonds and metal–molecule coordination enabled one to direct the assembling behaviors. Here, the concept of this directed assembly is opposite to that of the vdW-based self-assembly because the resulting architectures are purposefully tailored by programing intermolecular interaction. These efforts brought forth fruit in the on-surface syntheses of covalent bond networks. Particularly in the 2010s, on-surface covalent coupling was applied to graphene nanoribbons, where widths and edge structures can be precisely defined on the atomic scale. These works have the potential to bridge fundamental material nanoarchitectonics and functional device fabrication. In this paper, such a historical development of on-surface molecular nanoarchitectonics is…

Abstract Nanomachine and molecular machines are state-of-the-art objects in current physics and chemistry. The operation and manufacturing of nanosize machines are top-level technologies that we have desired to accomplish for a long time. There have been extensive attempts to design and synthesize nanomachines. In this paper, we review the these attempts using the concept of nanoarchitectonics toward the design, synthesis, and testing of molecular machinery, especially at interfacial media. In the first half of this review, various historical attempts to design and prepare nanomachines are introduced as well as their operation mechanisms from their basic principles. Furthermore, in order to emphasize the importance and possibilities of this research field, we also give examples of two new challenging topics in the second half of this review: (i) a world wide nanocar race and (ii) new modes of nanomachine operation on water. The nanocar race event involves actual use of nanomachines and will take place in the near future, and nanomachine operation of a dynamic fluidic interface will…

Abstract In this paper, we discuss the generation mechanisms of third-order nonlinearity in surface acoustic wave (SAW) devices on the basis of simulation results, which are obtained by a proposed method for this discussion. First, eight nonlinear terms are introduced to the piezoelectric constitutive equations, and nonlinear stress and electric flux fields are estimated using linear strain and electric fields calculated by a linear analysis, i.e., the coupling of mode simulation. Then, their contributions are embedded as voltage and current sources, respectively, in an equivalent circuit model, and nonlinear signals appearing at external ports are estimated. It is shown that eight coefficients of the nonlinear terms can be determined from a series of experiments carried out at various driving and resulting frequencies. This is because the effect of each nonlinear term on the nonlinear signal outputs changes markedly with the conditions. When the coefficients are determined properly, the simulations agree well with some measurement results under various conditions.

Abstract We demonstrated a room-temperature (RT) continuous-wave (CW) operation of a GaN-based vertical-cavity surface-emitting laser (VCSEL) using a thick GaInN quantum well (QW) active region and an AlInN/GaN distributed Bragg reflector. We first investigated the following two characteristics of a 6 nm GaInN 5 QWs active region in light-emitting diode (LED) structures. The light output power at a high current density (∼10 kA/cm 2 ) from the 6 nm GaInN 5 QWs was the same or even higher than that from standard 3 nm 5 QWs. In addition, we found that hole injection into the farthest QW from a p-layer was sufficient. We then demonstrated a GaN-based VCSEL with the 6 nm 5 QWs, resulting in the optical confinement factor of 3.5%. The threshold current density under CW operation at RT was 7.5 kA/cm 2 with a narrow (0.4 nm) emission spectrum of 413.5 nm peak wavelength.

Abstract The thin films of 5,10,15,20-tetraphenyl-21 H ,23 H -porphyrin zinc, ZnTPP, were successfully prepared by the thermal evaporation technique. The structure formation and optical properties of ZnTPP thin films were studied. The surface morphology and structural characteristics of ZnTPP thin films were analyzed by atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques, respectively. The optical constants of ZnTPP films were measured by spectrophotometer measurements of the transmittance and reflectance at a normal incidence of light in the spectral wavelength range from 200 to 2500 nm. The absorption spectrum of ZnTPP films showed four absorption bands, namely, the Q, B, N, and M bands. Anomalous dispersion was observed in the absorption region and normal dispersion occurred in the transparent region of the spectrum. We adopted the multi-oscillator and the single-oscillator models to interpret the results of anomalous and normal dispersion characteristics, respectively. The energy band gap of ZnTPP films was measured and the type of electron transition was…

Abstract Sputter-deposited indium–tin oxide (ITO) electrodes became ohmic contacts for unintentionally doped β-Ga 2 O 3 (010) substrates with a carrier concentration of 2 × 10 17 cm −3 after rapid thermal annealing in a wide range of annealing temperatures of 900–1150 °C. The formation of an ohmic contact is attributed to interdiffusion between ITO and β-Ga 2 O 3 , as evidenced by the results of transmission electron microscopy and energy-dispersive X-ray spectroscopy. The interdiffusion decreases the band gap and increases the donor concentration of β-Ga 2 O 3 at the interface, and forms an intermediate semiconductor layer desirable for carrier transport. The ITO ohmic contact is particularly useful for future β-Ga 2 O 3 devices operated at high temperatures.

Abstract Liquid phase crystallization has emerged as a novel approach to grow large grained polycrystalline silicon films on glass with high electronic quality. In recent years a lot of effort was conducted by different groups to determine and optimize suitable interlayer materials, enhance the crystallographic quality or to improve post crystallization treatments. In this paper, we give an overview on liquid phase crystallization and describe the necessary process steps and discuss their influence on the absorber properties. Available line sources are compared and different interlayer configurations are presented. Furthermore, we present one-dimensional numerical simulations of a rear junction device, considering silicon absorber thicknesses between 1 and 500 µm. We vary the front surface recombination velocity as well as doping density and minority carrier lifetime in the absorber. The simulations suggest that a higher absorber doping density is beneficial for layer thicknesses below 20 µm or when the minority carrier lifetime is short. Finally, we discuss possible routes for…

Abstract Terahertz (THz)-transmission modulation through the Si substrate coated with four π-conjugated materials under various laser light irradiation conditions was investigated using THz time-domain spectroscopy. Two of the π-conjugated low molecules, the [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), and two of the π-conjugated polymer materials, poly[5-(2-ethylhexyloxy)-2-methoxycyanoterephthalyliden] (MEH-CN-PPV) and poly(benzimidazobenzophenanthroline) (BBL), were investigated. Among these materials, PCBM and TIPS-pentacene showed higher modulation efficiencies and it was also shown that thermal annealing is quite effective in obtaining THz modulation efficiencies higher than those previously reported for copper phthalocyanine. Utilizing these solution-processable π-conjugated materials, various types of THz materials and devices could be fabricated by printing technologies. Our findings may open the way to fabricating various types of THz active devices.

Abstract A liquid crystal (LC) based tunable microstrip line (ML) phase shifter featuring high performance is presented. The experimental results show an electrically tunable differential phase up to 360° at 10 GHz with an overall insertion loss <8.5 dB. The device possesses a high figure-of-merit (FoM) of 64°/dB at 9.8 GHz and 60°/dB between 7–10 GHz. This is achieved by simultaneously considering both of the LC tuned phase and overall loss in the design. The proposed device utilizes the inverted meander ML technology to minimize its size. Taking into account the real fabrication procedure, a novel impedance matching structure is applied, and the measured return loss is considerably improved. The FoM and phase tuning property of the fabricated device as optimized are compared with the state-of-art results published recently and show better performance for both of them.

Abstract We investigated a V-based electrode for the realization of low ohmic-contact resistivity in n-type AlGaN with a high AlN molar fraction characterized by the circular transmission line model. The contact resistivity of n-type Al 0.62 Ga 0.38 N prepared using the V/Al/Ni/Au electrode reached 1.13 × 10 −6 Ω cm 2 . Using this electrode, we also demonstrated the fabrication of UV light-emitting diodes (LEDs) with an emission wavelength of approximately 300 nm. An operating voltage of LED prepared using a V/Al/Ni/Au electrode was 1.6 V lower at 100 mA current injection than that prepared using a Ti/Al/Ti/Au electrode, with a specific contact resistance of approximately 2.36 × 10 −4 Ω cm 2 for n-type Al 0.62 Ga 0.38 N.