Showing papers in "Japanese Journal of Applied Physics in 2019"
109 citations
59 citations
TL;DR: In this paper, the status and challenges in the development of GaN power devices are reviewed, and a recessed-gate AlGaN/GaN heterojunction field effect transistor using AlON as a gate insulator is presented.
Abstract: The status and challenges in the development of GaN power devices are reviewed. At present, normally-off gate injection transistors (GITs) on Si are commercially available. The updated structure known as a hybrid-drain-embedded GIT provides superior reliability that contributes to the stable operation of compact power switching systems with high efficiency. The fabricated vertical GaN transistor on GaN as a future challenge demonstrates extremely low specific on-state resistance and high breakdown voltage. Metal-insulator-semiconductor-gate GaN transistor is also a technical challenge for faster switching, since it would give greater freedom of gate driving as a result of both high threshold voltage and widened gate voltage swing. Normally-off operation free from hysteresis in the current–voltage characteristics is confirmed in a recessed-gate AlGaN/GaN heterojunction field effect transistor using AlON as a gate insulator. Fast switching characteristics are experimentally confirmed for both of the newly developed GaN devices, indicating their great potential for practical use.
51 citations
TL;DR: In this article, the spontaneous orientation polarization (SOP) is inherent in evaporated films of many organic semiconducting molecules with a permanent dipole moment, and a significant electric field is formed in the film due to SOP.
Abstract: Spontaneous orientation polarization (SOP) is inherent in evaporated films of many organic semiconducting molecules with a permanent dipole moment. A significant electric field is formed in the film due to SOP. Consequently, the properties of organic light-emitting diodes (OLEDs) incorporating such films are influenced. The polarization charge appearing at heterointerfaces dominates the charge injection and accumulation properties. Moreover, SOP correlates to device degradation. In this article, we review SOP of organic semiconductor films and its influences on the device properties of OLEDs.
44 citations
TL;DR: The output matrix of the reservoir computing can be trained to perform AND, OR and XOR functions with an input delay of up to three steps because of the static magnetization directions of the nanomagnets as reservoir states.
Abstract: An idea to use a magnetic nano-dots array for a reservoir computing is introduced. The mechanism of how the nonlinear calculation is carried out in the magnetic system is explained by showing the simplest case with three nano-dots system. The first trial to prove calculation ability and fabrication ability of the system is demonstrated. Since the proposed reservoir computing system may utilize integration technology of the magnetic random access memory (MRAM), it possesses a possibility to realize a large-scale reservoir computing system.
40 citations
TL;DR: In this article, the effects of alternate current poling (ACP) for four different plates of [100]-oriented (100 − x)Pb(Mg1/3Nb2/3)O3-pbTiO3 (PMN-xPT, x = 28, 29, 30, 31) single crystals (SCs) obtained from one ingot.
Abstract: We have investigated the effects of alternate current poling (ACP) for four different plates of [100]-oriented (100 – x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 (PMN–xPT, x = 28, 29, 30, 31) single crystals (SCs) obtained from one ingot. A record high piezoelectric coefficient d33 of 3000 pC N–1 for PMN–30PT SCs, and a high free dielectric permittivity of 8900 for PMN–28PT SCs by ACP, 27% higher than those of conventional direct current poling (DCP), were confirmed. X-ray analysis revealed a higher (004) peak intensity and angle of ACP SCs compared to those of DCP. We assume that ACP creates a monoclinic multiphase (MB and MC) and eliminates some portion of the tetragonal phase in SCs. In addition, (004) peak shifts were different between the different PT composition SC samples. This work demonstrates an ACP process with excellent cost performance can offer promising prospects and practical value for PMN–xPT SC piezoelectric transducers.
40 citations
TL;DR: In this paper, ScxAl1-xN samples were grown by plasma-assisted MBE on GaN-on-sapphire templates under a variety of growth conditions and pulsed supply of Sc and Al.
Abstract: Due to its outstanding polarization properties and the possibility of lattice-matched growth on GaN, ScxAl1-xN is a promising material among group III nitrides providing a wide field of potential applications in modern semiconductor technology. However, epitaxial growth of ScxAl1-xN by MBE is still in an early stage of research. In this work, ScxAl1-xN samples were grown by plasma-assisted MBE on GaN-on-sapphire templates under a variety of growth conditions and pulsed supply of Sc and Al, resulting in compositions ranging from Sc0.02Al0.98N to Sc0.69Al0.31N. Samples grown in the highly metal-rich regime showed phase degradation and high surface roughness, whereas growth in the N-rich and intermediate regime led to phase purity and surface roughness as low as 0.7 nm. Electrical characterization revealed a 2DEG for Sc0.2Al0.8N with a sheet resistance of 215 Ω/, a Hall mobility of 553 cm2 V−1s−1, and a sheet carrier density of 5.26 × 1013 cm−2 at 77 K.
39 citations
TL;DR: In this article, the influence of aluminum mole fraction of Al x Ga1-x N/Al y Ga 1-y N multiple quantum wells (MQWs) on the optical polarization, light extraction efficiency (LEE) and external quantum efficiency (EQE) of deep ultra violet light emitting diodes in the wavelength range between 264 and 220 nm is investigated.
Abstract: The influence of aluminum mole fraction of Al x Ga1-x N/Al y Ga1-y N multiple quantum wells (MQWs) on the optical polarization, light extraction efficiency (LEE) and external quantum efficiency (EQE) of deep ultra violet light emitting diodes in the wavelength range between 264 and 220 nm is investigated. The on-wafer EQE decreases from 0.6% to 0.00013% in this wavelength range. Polarization resolved photoluminescence and electroluminescence measurements show a change from dominant transverse-electric to dominant transverse-magnetic polarized light emission with increasing aluminum mole fraction in the MQW. The quantitative agreement with kp calculations allow to ascribe this shift to a change of the characteristic of the Γ7+ valance band. Ray tracing simulations predict a reduction of the on-wafer LEE from 4% to 1.5%. Therefore the dramatic drop of the EQE in this wavelength range cannot be attributed to a drop in LEE and is most likely dominated by charge carrier injection and radiative recombination efficiency.
38 citations
38 citations
TL;DR: In this paper, the history of the Na-flux method for growing single GaN crystals is summarized from its discovery in 1994 until the present, including several important innovations without which it would have been impossible to achieve current technical levels.
Abstract: In this review, the history of research and development of the Na-flux method for growing single GaN crystals is summarized from its discovery in 1994 until the present. Underlying the development of the Na-flux method, which has become one of the more important technologies for growing high quality GaN crystals, there have been several important innovations without which it would have been impossible to achieve current technical levels. Here, we describe the development of the Na-flux method through these innovations, including a method for controlling nucleation by adding carbon, single- and multipoint seed techniques, and a hybrid of the flux-film coated and multipoint seed approaches.
37 citations
TL;DR: In this paper, a review of the physics and emerging physics and science of these new nitride materials is presented, as well as their potential applications in future generations of devices that take advantage of the photonic and electronic devices eco-system based on transistors, light-emitting diodes, and lasers that have already been created by the nitride semiconductors.
Abstract: The nitride semiconductor materials GaN, AlN, and InN, and their alloys and heterostructures have been investigated extensively in the last 3 decades, leading to several technologically successful photonic and electronic devices. Just over the past few years, a number of new nitride materials have emerged with exciting photonic, electronic, and magnetic properties. Some examples are 2D and layered hBN and the III-V diamond analog cBN, the transition metal nitrides ScN, YN, and their alloys (e.g. ferroelectric ScAlN), piezomagnetic GaMnN, ferrimagnetic Mn4N, and epitaxial superconductor/semiconductor NbN/GaN heterojunctions. This article reviews the fascinating and emerging physics and science of these new nitride materials. It also discusses their potential applications in future generations of devices that take advantage of the photonic and electronic devices eco-system based on transistors, light-emitting diodes, and lasers that have already been created by the nitride semiconductors.
TL;DR: In this paper, the p-GaN layer was reduced so that the punch-through breakdown occurred before the hard breakdown, and the diode showed a high voltage of 4.9 kV with high reverse avalanche capabilities against sudden increase of reverse current over 5 orders of magnitudes.
Abstract: In order to avoid sudden catastrophic hard breakdown in high breakdown voltage vertical GaN p–n junction diodes, punch-through induced breakdown structures have been newly considered. Mg acceptor concentration in the p-GaN layer was reduced so that the punch-through breakdown occurred before the hard breakdown. By using a wafer with triple drift layers and the p-GaN layer with lowered Mg concentration of 3 × 1017 cm−3 grown on a freestanding n-GaN substrate, the diode showed a high breakdown voltage of 4.9 kV with high reverse avalanche capabilities against sudden increase of reverse current over 5 orders of magnitudes. No degradation was observed after fifteen repetitive measurements.
TL;DR: In this article, the dislocations in β-Ga2O3 single crystals grown by edge-defined film-fed growth (EFG) and halide vapor-phase epitaxy (HVPE) using synchrotron X-ray topography were investigated.
Abstract: Here, we investigate the dislocations in β-Ga2O3 single crystals grown by edge-defined film-fed growth (EFG) and halide vapor-phase epitaxy (HVPE) using synchrotron X-ray topography. The (001)- and ()-oriented crystals grown in the [010] direction by EFG exhibited dislocations along the 〈010〉 direction with some dislocations oriented in a line; in addition, wandering dislocations were observed on the (001) surface. Based on the invisibility criterion, the Burgers vector of some dislocations was determined to be 〈010〉. On the other hand, in the (001) film grown by HVPE over the EFG substrate, threading dislocations propagating in the [001] direction were observed. Furthermore, it was found that the dislocations on the substrate grown by EFG were inherited by the film formed by HVPE: a dislocation was generated in the film grown by HVPE at both ends of the void defects in the substrate grown by EFG.
TL;DR: In this article, a large reverse leakage current was generated by nanopipes, which were formed from screw dislocations in the homoepitaxial layer and the other was newly generated in the epilayer.
Abstract: We fabricated p−n diodes under different growth pressures on free-standing GaN substrates of the same quality and observed a noteworthy difference in the reverse leakage current. A large reverse leakage current was generated by nanopipes, which were formed from screw dislocations in the homoepitaxial layer. There were two types of screw dislocation observed in this study. The first type already existed in the substrate and the other was newly generated in the epilayer by the coalescence of edge and mixed dislocations. An increase in the growth pressure suppressed the transformation of screw dislocations into nanopipes, which led to a reduction in the reverse leakage current. To reduce the leakage current further, it is necessary to apply growth conditions that do not transform screw dislocation into nanopipes and to use a free-standing substrate without threading dislocations, that become nanopipes.
TL;DR: In this paper, the authors investigate AlN/sapphire templates grown by metalorganic vapor phase epitaxy (MOVPE) subjected to an intermediate high temperature annealing (HTA) step at different AlN film thicknesses.
Abstract: In this work, we investigate AlN/sapphire templates grown by metalorganic vapor phase epitaxy (MOVPE) subjected to an intermediate high temperature annealing (HTA) step at different AlN film thicknesses. To avoid cracking of the MOVPE layers during HTA at 1700 °C these layers have to be grown under low incorporation of tensile strain. For each sample the growth is stopped at a certain thickness between 230 nm and 1.2 μm followed by HTA. X-ray rocking curve FWHM of 0002 and 10–12 reflections lead to an estimation of threading dislocation densities (TDD) as low as cm−2 for the thickest AlN layers of 880 nm and 1.2 μm after annealing. For all layer thicknesses MOVPE growth is subsequently continued to a total AlN layer thickness of 1.5 μm to reach for a smooth surface. The change of the AlN strain state after HTA leads to an increased wafer curvature at room temperature and compressive strain in the subsequently grown AlN layers increasing the wafer bow at room temperature. Hence, to serve as a base layer, e.g. for ultraviolet light emitting diodes, a trade-off between low TDD, surface smoothness and wafer bow has to be found.
TL;DR: In this article, a new inductor was proposed theoretically in terms of spiral magnets, and the inductance L in the present inductor is inversely proportional to the cross section A of the system, which enables to reduce the size of the device.
Abstract: A new principle for the inductor is proposed theoretically in terms of spiral magnets. In sharp contrast to the conventional inductor made by the solenoid with ferromagnetic core, the bulk sample itself acts as an inductor without any composite structure. Furthermore, the inductance L in the present inductor is inversely proportional to the cross section A of the system, which enables to reduce the size of the device.
TL;DR: In this article, the authors investigated the impact of annealing temperature and time on the crystalline structures and dielectric properties of 10 nm thick Hf0.5Zr0.2 thin films.
Abstract: Hf0.5Zr0.5O2 thin films are not always ferroelectric. This work investigates the impact of annealing temperature and time on the crystalline structures and dielectric properties of 10 nm thick Hf0.5Zr0.5O2 thin films. It is found that the tetragonal phase crystal is formed from the amorphous film firstly, then transforms to the orthorhombic and monoclinic phases, in accordance with the annealing temperature and time. The volume fraction of the orthorhombic phase in the film, which is known as the origin of ferroelectricity, becomes dominant in a certain range of the annealing condition. Thus, the annealing temperature and time are responsible for the anti-ferroelectric, ferroelectric, and paraelectric characteristics of Hf0.5Zr0.5O2 thin films. This phase transformation behavior is discussed from the viewpoint of formation energies of the respective crystal phases, which accompanies the changes of unit cell volumes. The competition of transformation rates between the tetragonal to orthorhombic and the orthorhombic to monoclinic is key for the formation of ferroelectric films.
TL;DR: In this paper, the process of forming the ferroelectric orthorhombic phase was investigated for epitaxial 7% Y-doped (YHO7) films using in situ high-temperature X-ray diffraction.
Abstract: The process of forming the ferroelectric orthorhombic phase was investigated for epitaxial 7% Y-doped (YHO7) films using in situ high-temperature X-ray diffraction. Epitaxial YHO7 films were grown on (111) ITO-coated (111)YSZ substrates by pulsed laser deposition at room temperature and a subsequent heat treatment process. Films deposited at room temperature were crystallized as paraelectric monoclinic phase. The monoclinic phase partially changes to tetragonal phase above 600 °C and perfectly transformed around 950 °C during heating. The change from tetragonal phase to orthorhombic phase was detected at 300 °C, corresponding to the Curie temperature under the cooling process. These results clearly suggest that the tetragonal phase was more stable at 1000 °C for YHO7 films on heating than the other phases, and the formation of this tetragonal phase—the high-temperature paraelectric phase of the ferroelectric orthorhombic phase—is key to the formation of the ferroelectric orthorhombic phase.
TL;DR: In this paper, the currentvoltage characteristics of Al/Ti/4H-SiC Schottky barrier diodes have been investigated in the 85-445 K temperature range by means of a combined numerical and analytical simulation study.
Abstract: The current–voltage characteristics of Al/Ti/4H-SiC Schottky barrier diodes have been investigated in the 85–445 K temperature range by means of a combined numerical and analytical simulation study. Simulation results showed a good agreement with measurements in the whole explored current range from 10 μA to 10 mA. The main device electrical parameters, namely the barrier height (BH) and ideality factor, were found to be strongly temperature-dependent. In particular, the ideality factor decreases while the BH increases with increasing temperature. The observed behaviours have been successfully interpreted by using the thermionic emission theory with a triple Gaussian distribution of the BH in three different temperature ranges, i.e. 85 ≤ ΔT 1 ≤ 135 K, 180 ≤ ΔT 2 ≤ 270 K, and 315 ≤ ΔT 3 ≤ 445 K. The corresponding Richardson constants are A 1* = 149.26 A cm−2 K−2, A 2* = 138.19 A cm−2 K−2, and A 3* = 173.21 A cm−2 K−2, respectively. These values are close to the theoretical result of 146 A cm−2 K−2 expected for n-type 4H-SiC. Finally, it has been highlighted that the current flowing through the Schottky junction is also determined by the thermionic-field emission mechanism.
TL;DR: In this paper, the authors achieved the coalescence of α-Ga2O3 on the mask with the 〈10-10〉 stripe on a-plane sapphire substrates by epitaxial lateral overgrowths, owing to the enhanced lateral overgrowth.
Abstract: Selective area growths of α-Ga2O3 were demonstrated on c-, m-, and a-plane sapphire substrates by the mist-CVD method using SiO2 as a mask material. We successfully achieved the coalescence of the α-Ga2O3 on the mask with the 〈10-10〉 stripe on a-plane sapphire substrates by epitaxial lateral overgrowths, owing to the enhanced lateral overgrowth, that is, the largest ratio of the lateral to the vertical growth rates of 0.87 among the substrate orientation and the stripe direction investigated. In the cross-sectional transmission electron microscopy images, dislocations were not observed in the α-Ga2O3 layer on the mask, while the dislocations on the window area were propagated without bending.
TL;DR: In this paper, the current-dependent and temperature-dependent efficiency droops were investigated in InGaN-based blue and AlGaInP-based red light-emitting diodes (LEDs).
Abstract: We investigate the current-dependent and temperature-dependent efficiency droops ("J-droop" and "T-droop", respectively) in InGaN-based blue and AlGaInP-based red light-emitting diodes (LEDs). It is found that the blue and red LEDs show different droop behaviors with increasing current density and temperature. The J-droop is significant in the blue LED while the T-droop is severe in the red LED. In case of the blue LED, the carrier accumulation caused by the saturation of the radiative recombination rate is thought to increase the quasi-Fermi level rapidly, thus causing the J-droop. On the other hand, the T-droop of the red LED is influenced by redistribution of carriers due to the increased thermal energy with a small barrier height in the AlGaInP material system. The comparison of different droop characteristics of blue and red LEDs helps understand the recombination mechanisms of both LEDs and provides useful insight for improving the device performance further.
TL;DR: In this article, the carrier transport properties in the inversion layer of Si-face 4H-SiC MOSFETs with nitrided oxide were investigated, and it was revealed that the intrinsic phonon-limited mobility (μ phonon) of the SiC-based MOS-FET is a quarter or less than the conventionally presumed values.
Abstract: We propose a method to evaluate the carrier transport properties in the inversion layer of 4H–SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) experimentally. Our approach differs from conventional methods, which have adjusted the parameters in conventional mobility models. Intrinsic phonon-limited mobility (μ phonon) in the SiC MOSFET was observed by suppressing the severe impact of Coulomb scattering on the SiC MOS inversion layer by lowering the acceptor concentration (N A) of the p-type well region to the order of 1014 cm−3. In this study, we investigated the carrier transport properties in the inversion layer of Si-face 4H–SiC MOSFETs with nitrided oxide. It is revealed that the μ phonon of the SiC MOSFET is a quarter or less than the conventionally presumed values. Additionally, surface roughness scattering is found not to be the most dominant mobility-limiting factor even at high effective normal field (E eff) for the SiC MOSFET. These results demonstrate that conventional understanding of carrier scattering in the SiC MOS inversion layer should be modified, especially in the high E eff region.
TL;DR: In this paper, a modulation-doped fisseld-effect transistor (MODFET) and a metal-semiconductor field effect transistor (MESFET), using β-(AlGa)2O3 (010), were demonstrated.
Abstract: This paper demonstrates a modulation-doped fisseld-effect transistor (MODFET) and a metal-semiconductor field-effect transistor (MESFET) using β-(AlGa)2O3 (010). Ohmic contacts on Sn-doped (Al0.15Ga0.85)2O3 exhibit a fairly linear behavior, which has a specific contact resistivity and sheet resistance of 9 × 10−5 Ω cm2 and 75 kΩ/, respectively. The MODFET with Sn-doped (Al0.08Ga0.92)2O3 barrier layer showed a breakdown voltage of 610 V for gate-drain spacing (L gd) of 8 μm, while the (Al0.16Ga0.85)2O3-channel MESFET exhibited a breakdown voltage of 940 V for L gd of 20 μm. These results show the great potential of (AlGa)2O3 transistors for high-power applications.
TL;DR: In this article, a crystal growth technique for an n-AlGaN buffer layer (BL) and current spreading layer (CSL) was developed for obtaining a high internal quantum efficiency (IQE) from UVB-emitting multi quantum wells (MQWs).
Abstract: Smart, low cost and environmentally safe AlGaN-based UVB LEDs are promising in many real world applications including medical as well as agricultural sciences. The main purpose of this work is to develop a crystal growth technique for an n-AlGaN buffer layer (BL) including an n-AlGaN current spreading layer (CSL) for obtaining a high internal quantum efficiency (IQE) from UVB-emitting multi quantum wells (MQWs). By the reduction of the edge type threading dislocation densities in the n-AlGaN CSL, as well as the optimization of the quantum well (QW) thickness, the IQE of about 42% was improved for UVB MQWs, with an emission wavelength of 294 nm. Subsequently, the external quantum efficiency improved from 2.7% to 3.3% at 20 mA under the continuous wave (CW) operation and the maximum output power also improved from 10.8 mW to 12.5 mW at 126 mA, respectively. 293 nm UVB LED light sources are very useful for the application of vitamin D3 production in the human body.
TL;DR: In this article, the authors employed UV-vis spectroscopy to monitor real-time changes in the oxygen tension and concentration of reactive oxygen and nitrogen species (RONS) in deionized (DI) water during treatments with helium (He) and argon (Ar) gas plasma jets.
Abstract: We employed UV-vis spectroscopy to monitor real-time changes in the oxygen tension and concentration of reactive oxygen and nitrogen species (RONS) in deionized (DI) water during treatments with helium (He) and argon (Ar) gas plasma jets. He and Ar gas jets are both shown to de-oxygenate DI water with He being more efficient than Ar, whilst the plasma jets deliver and regulate the concentrations of hydrogen peroxide (H 2 O 2 ), nitrite (NO 2 - ) and nitrate (NO 3 - ) in DI water. The H 2 O 2 and NO 3 - production efficiency varied between He and Ar plasma jets, but was similar for NO 2 - . Whilst DI water fully equilibrated with ambient air prior to treatment (de-oxygenated by both plasma jets) when DI water was first de-oxygenated by an inert gas jet treatment, both plasma jets were found to be capable of oxygenating DI water. These insights were then used to show how different combinations of plasma jet and inert gas jet treatments can be used to modulate O 2 tension and RONS chemistry. Finally, potential further improvements to improve control in the use of plasma jets in regulating O 2 and RONS are discussed. © 2018 The Japan Society of Applied Physics.