Showing papers in "Japanese Journal of Applied Physics in 2016"

High-quality β-Ga2O3 single crystals grown by edge-defined film-fed growth

Abstract: β-Ga2O3 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 103 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 × 1017 cm−3 and independent of the Si concentration.

Current status of Ga2O3 power devices

Abstract: Gallium oxide (Ga2O3) 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 Ga2O3 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 Ga2O3-based devices that have been fabricated to date demonstrate the performance potential for power electronics applications.

Evolution of corundum-structured III-oxide semiconductors: Growth, properties, and devices

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.

Magnetic anisotropy of the single-crystalline ferromagnetic insulator Cr2Ge2Te6

Abstract: Cr2Ge2Te6 (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.

Electron channel mobility in silicon-doped Ga2O3 MOSFETs with a resistive buffer layer

Abstract: The electron mobility in depletion-mode lateral β-Ga2O3(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 Ga2O3 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 cm2 V−1 s−1 at carrier concentrations of low- to mid-1017 cm−3, with small in-plane mobility anisotropy of 10–15% ascribable to anisotropic carrier scattering.

AlGaN/GaN high-electron-mobility transistor technology for high-voltage and low-on-resistance operation

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 presented by introducing a three-dimensional field plate (3DFP) in AlGaN/GaN HEMTs, and its possibility of realizing true collapse-free operation is described.

Conductivity control of Sn-doped α-Ga2O3 thin films grown on sapphire substrates

Abstract: We achieved the successful fabrication of Sn-doped α-Ga2O3 thin films with higher electron mobility and wider conductivity controls by improving the crystal quality. α-Ga2O3 films showed n-type conductivity with a maximum electron mobility of 24 cm2 V−1 s−1. The carrier concentration was successfully controlled in the range of 1017–1019 cm−3. Crystal defects such as dislocations severely compensate the free carriers in α-Ga2O3 films and restrict the mobility at low carrier concentrations. Therefore, to achieve further conductivity control and higher mobility, improving the crystallinity of α-Ga2O3 films is necessary.

Structural evaluation of defects in β-Ga2O3 single crystals grown by edge-defined film-fed growth process

Abstract: We have structurally evaluated β-Ga2O3 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.

Heteroepitaxial growth of ε-Ga2O3 thin films on cubic (111) MgO and (111) yttria-stablized zirconia substrates by mist chemical vapor deposition

Abstract: In this study, epitaxial e-Ga2O3 thin films are successfully grown on cubic (111) MgO and (111) yttria-stablized zirconia (YSZ) substrates by mist chemical vapor deposition. Pure-phase hexagonal e-Ga2O3 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 e-Ga2O3 and the (111) of the two substrates is e-Ga2O3 ∥ substrates . Using (111) MgO substrates with a 2.5% lattice mismatch, the epitaxial e-Ga2O3 films are successfully grown at a low temperature of 400 °C. The optical direct and indirect bandgaps of pure e-Ga2O3 thin films are estimated as 5.0 and 4.5 eV, respectively.

Ferroelectricity of nondoped thin HfO2 films in TiN/HfO2/TiN stacks

Abstract: We report on the impact of TiN interfaces on the ferroelectricity of nondoped HfO2. Ferroelectric properties of nondoped HfO2 in TiN/HfO2/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 HfO2 clearly appears by thinning HfO2 film down to ~35 nm. We directly revealed in thermal treatments that the ferroelectric HfO2 film on TiN was maintained by covering the top surface of HfO2 with TiN, while it was followed by a phase transition to the paraelectric phase in the case of the open surface of HfO2. Thus, it is concluded that the ferroelectricity in nondoped HfO2 in this study was mainly driven by both of top and bottom TiN interfaces.

Homoepitaxial growth of beta gallium oxide films by mist chemical vapor deposition

Abstract: Homoepitaxial single-crystal beta gallium oxide (β-Ga2O3) 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-Ga2O3 films grown on Sn-doped β-Ga2O3(010) substrates, the fabrication of Schottky barrier diodes was demonstrated. Furthermore, we fabricated electrically conductive Sn-doped β-Ga2O3 films on semi-insulating Fe-doped β-Ga2O3(010) substrates. The carrier concentrations were between 1 × 1018 and 5 × 1020 cm−3. The Hall mobility was 45 cm2 V−1 s−1 at the carrier concentration of 1 × 1018 cm−3.

Acceleration of stable interface structure searching using a kriging approach

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.

Conduction mechanism in highly doped β-Ga2O3 single crystals grown by edge-defined film-fed growth method and their Schottky barrier diodes

Abstract: Edge-defined fed-grown β-Ga2O3 single crystals with high electron concentration of 3.9 × 1018 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 cm2/(Vs) 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/cm2 at the forward voltage of 1.6 V was obtained.

Terahertz imaging system with resonant tunneling diodes

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.

Quenching effects for piezoelectric properties on lead-free (Bi1/2Na1/2)TiO3 ceramics

Abstract: Lead-free ferroelectric and piezoelectric ceramics, (Bi0.5Na0.5)TiO3 (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 piezoelectric properties.

Origins of etch pits in β-Ga2O3(010) single crystals

Abstract: Etch pits of various shapes were observed on etched ?-Ga2O3(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 ?-Ga2O3(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.

Relationship between crystal defects and leakage current in β-Ga2O3 Schottky barrier diodes

Abstract: We fabricated Schottky barrier diodes (SBDs) on the entire surface of a β-Ga2O3 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 × 103 to 6 × 104, and its average was 1.1 × 104 cm−2. The void etch pit density on the surface ranged from <5 × 102 to 7 × 103, and its average was 6 × 103 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.

Defect characterization of β-Ga2O3 single crystals grown by vertical Bridgman method

Abstract: The characteristics of structural defects observed on (100) wafers in β-Ga2O3 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-Ga2O3 single crystals with a low mean dislocation density of 2.3 × 103 cm−2. Line-shaped defects up to 150 µm long in the [010] direction were detected at a mean density of 0.5 × 102 cm−2, which decreased with decreasing growth rate. The line-shaped defect structure and formation mechanism were discussed.

Threshold-voltage bias-temperature instability in commercially-available SiC MOSFETs

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.

Thermoelectric properties of supersaturated Re solid solution of higher manganese silicides

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.

Review on thin-film transistor technology, its applications, and possible new applications to biological cells

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.

Crystallographic and optical properties and band structures of CuInSe2, CuIn3Se5, and CuIn5Se8 phases in Cu-poor Cu2Se–In2Se3 pseudo-binary system

Abstract: We prepared CuInSe2 and Cu-poor Cu–In–Se (CIS) phases such as CuIn3Se5 and CuIn5Se8 in the composition of (1 − x)Cu2Se–(x)In2Se3 with 0.5 ≤ x ≤ 1.0. The crystal structure of the sample changed from chalcopyrite-type CuInSe2 to hexagonal CuIn5Se8 through stannite-type CuIn3Se5 with increasing x (decreasing Cu/In ratio). The band-gap energies of Cu-poor CIS samples, i.e., CuIn3Se5 (1.17 eV) and CuIn5Se8 (1.22–1.24 eV), are larger than that of chalcopyrite-type CuInSe2 (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 CuIn3Se5 is 0.4 eV larger than that of chalcopyrite-type CuInSe2. The ionization energy of CuIn5Se8 is 0.1–0.3 eV larger than that of CuIn3Se5. These results show that the energy position of the VBM from the vacuum level of Cu-poor CIS phases, such as CuIn3Se5 and CuIn5Se8, is deeper than that of CuInSe2. To understand the electronic structure of Cu-poor CIS compounds, we performed first-principles band structure calculations on stannite-type CuIn5Se8 and a reference compound, tetragonal chalcopyrite-type CuInSe2, using the HSE06 nonlocal screened hybrid density functional. The calculated band-gap energy of tetragonal stannite-type CuIn5Se8 (1.19 eV) is larger than that of chalcopyrite-type CuInSe2 (0.94 eV).

Accurate energy bands calculated by the hybrid quasiparticle self-consistent GW method implemented in the ecalj package

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), SiO2, HfO2, ZrO2, SrTiO3, 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.

Observation of nanometer-sized crystalline grooves in as-grown β-Ga2O3 single crystals

Abstract: On the surface of as-grown β-Ga2O3 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.

Nanoarchitectonic atomic switch networks for unconventional computing

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 ~109 units/cm2 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 next-generation ASN devices using combined top-down and bottom-up processing and experimentally demonstrate their utility as reservoir computing hardware. Leveraging their intrinsic dynamics and transformative input/output (I/O) behavior enabled waveform regression of periodic signals in the absence of embedded algorithms, further supporting the potential utility of ASN technology as a platform for unconventional approaches to computing.

Humidity effects on the redox reactions and ionic transport in a Cu/Ta2O5/Pt atomic switch structure

Abstract: Redox reactions at the Cu/Ta2O5 interface and subsequent Cu ion transport in a Ta2O5 film have been investigated by means of cyclic voltammetry (CV) measurements. Under positive bias to the Cu electrode, Cu is preferentially oxidized to Cu2+ 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.

Effect of organic moieties on the scintillation properties of organic–inorganic layered perovskite-type compounds

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, (C4H9NH3)2PbBr4 (C4), (C6H5CH2NH3)2PbBr4 (Ben), and (C6H5C2H4NH3)2PbBr4 (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.

Liquid phase crystallized silicon on glass: Technology, material quality and back contacted heterojunction solar cells

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 mu m. 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 mu m or when the minority carrier lifetime is short. Finally, we discuss possible routes for device optimization and propose a hybride cell structure to circumvent current limitations in device design. (C) 2016 The Japan Society of Applied Physics

Thallium magnesium chloride: A high light yield, large effective atomic number, intrinsically activated crystalline scintillator for X-ray and gamma-ray detection

Abstract: We report the luminescence and the scintillation properties of a newly developed thallium magnesium chloride (TlMgCl3) 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.