Showing papers in "Applied Physics Express in 2016"
TL;DR: In this article, a large increase in oscillation frequency was achieved in resonant-tunneling-diode (RTD) terahertz oscillators by reducing the conduction loss.
Abstract: A large increase in oscillation frequency was achieved in resonant-tunneling-diode (RTD) terahertz oscillators by reducing the conduction loss. An n+-InGaAs layer under the air-bridge electrode connected to the RTD was observed to cause a large conduction loss for high-frequency current due to the skin effect. By introducing a new fabrication process removing the InGaAs layer, we obtained 1.92-THz oscillation, which extended the highest frequency of room-temperature electronic single oscillators. Theoretical calculations reasonably agreed with the experiment, and an oscillation above 2 THz is further expected with an improved structure of the slot antenna used as a resonator and radiator.
252 citations
TL;DR: In this article, the annealing of an AlN buffer layer in a carbon-saturated N2-CO gas on a sapphire substrate was investigated, and a 2µm-thick AlN layer was fabricated by metalorganic vapor phase epitaxy (MOVPE).
Abstract: The annealing of an AlN buffer layer in a carbon-saturated N2–CO gas on a sapphire substrate was investigated. The crystal quality of the buffer layer was significantly improved by annealing at 1650–1700 °C. An AlN buffer layer with a thickness of 300 nm was grown by metalorganic vapor phase epitaxy (MOVPE), and was annealed at 1700 °C for 1 h. We fabricated a 2-µm-thick AlN layer on the annealed AlN buffer layer by MOVPE. The full widths at half maximum of the (0002)- and ($10\bar{1}2$)-plane X-ray rocking curves were 16 and 154 arcsec, respectively, and the threading dislocation density was 4.7 × 108 cm−2.
160 citations
TL;DR: In this paper, thin-film corundum-structured gallium oxide (α-Ga2O3) Schottky barrier diodes were fabricated by growing α-Ga 2O3 layers on sapphire substrates by the safe, low-cost, and energy-saving MIST EPITAXY® technique.
Abstract: Thin-film corundum-structured gallium oxide (α-Ga2O3) Schottky barrier diodes (SBDs) were fabricated by growing α-Ga2O3 layers on sapphire substrates by the safe, low-cost, and energy-saving MIST EPITAXY® technique, followed by lifting off the α-Ga2O3 layers from the substrates. The SBDs exhibited on-resistance and breakdown voltage of 0.1 mΩcm2 and 531 V (SBD1) or 0.4 mΩcm2 and 855 V (SBD2), respectively. These results will encourage the future evolution of low-cost and high-performance SBDs with α-Ga2O3.
154 citations
TL;DR: In this paper, an efficiency enhancement was achieved in Cu2O-based heterojunction solar cells fabricated with a zinc-germanium-oxide (Zn1− x Ge x -O) thin film as the n-type window layer and a p-type Na-doped Cu 2O (Cu2O:Na) sheet prepared by thermally oxidizing Cu sheets.
Abstract: Efficiency enhancement was achieved in Cu2O-based heterojunction solar cells fabricated with a zinc–germanium-oxide (Zn1− x Ge x -O) thin film as the n-type window layer and a p-type Na-doped Cu2O (Cu2O:Na) sheet prepared by thermally oxidizing Cu sheets. The Ge content (x) dependence of the obtained photovoltaic properties of the heterojunction solar cells is mainly explained by the conduction band discontinuity that results from the electron affinity difference between Zn1− x Ge x -O and Cu2O:Na. The optimal value of x in Zn1− x Ge x -O thin films prepared by pulsed laser deposition was observed to be 0.62. An efficiency of 8.1% was obtained in a MgF2/Al-doped ZnO/Zn0.38Ge0.62-O/Cu2O:Na heterojunction solar cell.
150 citations
TL;DR: In this paper, NiO/β-Ga2O3 alloxide p-n heterojunction diodes were fabricated for the first time using p-type NiO epitaxial layers grown on n-type β-Ga 2O3 substrates.
Abstract: NiO/β-Ga2O3 all-oxide p–n heterojunction diodes were fabricated for the first time using p-type NiO epitaxial layers grown on n-type β-Ga2O3 substrates. The fabricated diodes exhibited good rectifying current–voltage characteristics, with a rectifying ratio greater than 108 at ±3 V. The capacitance–voltage measurements showed that the built-in voltage was 1.4 V. These results were discussed in terms of the energy band diagram of a type-II heterojunction, where the conduction band and valence band discontinuities were estimated to be 2.2 and 3.4 eV, respectively.
133 citations
TL;DR: In this article, the authors demonstrate the improved efficiency of a Cu2Zn(Sn1− x Ge x )Se4 (CZTGSe) thin-film solar cell with a conversion efficiency of 12.3%.
Abstract: We demonstrate the improved efficiency of a Cu2Zn(Sn1− x Ge x )Se4 (CZTGSe) thin-film solar cell with a conversion efficiency of 12.3%; this cell exhibits a greatly improved open-circuit voltage (V OC) deficit of 0.583 V and a fill factor (FF) of 0.73 compared with previously reported CZTGSe cells. The V OC deficit was found to be improved through a reduced band tailing via the control of the Ge/(Sn + Se) ratio. In addition, the high FF was mainly induced by a reduced carrier recombination at the absorber/buffer interface and/or in the space charge region, whereas parasitic resistive effects on FF were very small.
133 citations
TL;DR: In this article, highly doped GaN p-n tunnel junction (TJ) contacts on III-nitride heterostructures where the active region of the device and the top p-GaN layers were grown by metal organic chemical vapor deposition (MOCVD) and highly Doped nGaN was grown by NH3 molecular beam epitaxy to form the TJ.
Abstract: In this work, we demonstrate highly doped GaN p–n tunnel junction (TJ) contacts on III–nitride heterostructures where the active region of the device and the top p-GaN layers were grown by metal organic chemical vapor deposition and highly doped n-GaN was grown by NH3 molecular beam epitaxy to form the TJ. The regrowth interface in these hybrid devices was found to have a high concentration of oxygen, which likely enhanced tunneling through the diode. For optimized regrowth, the best tunnel junction device had a total differential resistivity of 1.5 × 10−4 Ω cm2, including contact resistance. As a demonstration, a blue-light-emitting diode on a () GaN substrate with a hybrid tunnel junction and an n-GaN current spreading layer was fabricated and compared with a reference sample with a transparent conducting oxide (TCO) layer. The tunnel junction LED showed a lower forward operating voltage and a higher efficiency at a low current density than the TCO LED.
113 citations
TL;DR: In this article, the full phase diagram of (Ga1− x In x )2O3 is obtained theoretically, and the phases competing for the ground state are monoclinic β (low x), hexagonal (x ~ 0.5), and bixbyite (large x).
Abstract: The full phase diagram of (Ga1− x In x )2O3 is obtained theoretically. The phases competing for the ground state are monoclinic β (low x), hexagonal (x ~ 0.5), and bixbyite (large x). Three disconnected mixing regions interlace with two distinct phase-separation regions, and at x ~ 0.5, the coexistence of hexagonal and β alloys with phase-separated binary components is expected. We also explore the permanent polarization of the phases, but none of them are polar. On the other hand, we find that e-Ga2O3, which was stabilized in recent experiments, is pyroelectric with a large polarization and piezoelectric coupling, and could be used to produce high-density electron gases at interfaces.
101 citations
TL;DR: In this article, the authors demonstrate interfacial spin-to-charge current conversion by means of spin pumping from a ferromagnetic permalloy (Py: Ni80Fe20) to a Cu/Bi2O3 interface.
Abstract: Here, we demonstrate interfacial spin-to-charge current conversion by means of spin pumping from a ferromagnetic permalloy (Py: Ni80Fe20) to a Cu/Bi2O3 interface. A clear peak owing to the spin-to-charge current conversion was observed in the voltage spectrum of a Py/Cu/Bi2O3 trilayer film, whereas no peak was observed in Py/Cu and Py/Bi2O3 bilayer films. We also found that the conversion coefficient strongly depended on the Cu thickness, reflecting the thickness-dependent momentum relaxation time for the Cu layer.
94 citations
TL;DR: In this paper, an X-ray-diffraction-based composition estimation of β-(Al x Ga 1− x )2O3 coherently grown on (010) β-Ga2O 3 was presented.
Abstract: We demonstrate X-ray-diffraction-based composition estimation of β-(Al x Ga1− x )2O3 coherently grown on (010) β-Ga2O3. The relation between the strain along the [010] direction and the Al composition of the β-(Al x Ga1− x )2O3 layer was formulated using the stress–strain relationship in the monoclinic system. This formulation allows us to estimate the Al composition using the out-of-plane lattice spacing determined by conventional X-ray ω–2θ measurements. This method was applied to molecular-beam-epitaxy-grown coherent β-(Al x Ga1− x )2O3/Ga2O3 heterostructures, and the Al composition in β-(Al x Ga1− x )2O3 agrees closely with the composition determined directly by atom probe tomography.
88 citations
TL;DR: In this paper, the electron mobility of few-layer InSe has been investigated on the basis of the density functional theory coupled with the Boltzmann transport equation with relaxation time approximation.
Abstract: On the basis of the density functional theory coupled with the Boltzmann transport equation with relaxation time approximation, we investigate the electronic structure and predict the electron mobility of few-layer InSe. Few-layer InSe has a tunable band gap on thickness. At the same time, it possesses a high carrier mobility on the order of 103 cm2 V−1 s−1. As we demonstrated, few-layer InSe has potential applications for next-generation electronic materials owing to its considerable band gap and high carrier mobility.
TL;DR: In this paper, the authors investigated the write error rate (WER) for voltage-driven dynamic switching in magnetic tunnel junctions with perpendicular magnetization, and they observed a clear oscillatory behavior of the switching probability with respect to the duration of pulse voltage, which reveals the precessional motion of magnetization during voltage application.
Abstract: We investigated the write error rate (WER) for voltage-driven dynamic switching in magnetic tunnel junctions with perpendicular magnetization. We observed a clear oscillatory behavior of the switching probability with respect to the duration of pulse voltage, which reveals the precessional motion of magnetization during voltage application. We experimentally demonstrated WER as low as 4 × 10−3 at the pulse duration corresponding to a half precession period (~1 ns). The comparison between the results of the experiment and simulation based on a macrospin model shows a possibility of ultralow WER (<10−15) under optimum conditions. This study provides a guideline for developing practical voltage-driven spintronic devices.
TL;DR: In this paper, the effects of air nonthermal plasma irradiation of seeds of Arabidopsis thaliana (L.) on their growth from the beginning of cultivation to their harvest were studied.
Abstract: We have studied the effects of air nonthermal plasma irradiation of seeds of Arabidopsis thaliana (L.) on their growth from the beginning of cultivation to their harvest. Three minute plasma irradiation of dry seeds resulted in growth acceleration in all the growth stages. Compared with the control, the plasma irradiation led to an 11% shorter harvest period, a 56% increase in total seed weight, a 12% increase in each seed weight, and a 39% increase in seed number.
TL;DR: In this paper, the role of nitrogen doping in ferroelectricity was discussed from the viewpoint of charge balance and bond-constraining effects, and it was found that N doping can cause the transition from a monoclinic phase to a highly symmetric phase.
Abstract: We report that nitrogen (N) doping can drive the ferroelectricity of HfO2. It was found that N doping can cause the transition from a monoclinic phase to a highly symmetric phase. The role of N doping is discussed from the viewpoints of charge balance and bond-constraining effects. The former is responsible for the structural transformation from a paraelectric phase to a ferroelectric phase by forming an oxygen vacancy. In addition, Hf?N and N?O bonds with covalent characteristics have strong effects on HfO2 structural and electrical properties, and thus contribute to a marked HfO2 para-/ferroelectric transition.
TL;DR: In this paper, the authors presented an alternative route to supply excessive selenium (Se) for the deposition of Sb2Se3 thin films by the co-evaporation of Se and Sb 2Se3.
Abstract: In this work, we present an alternative route to supply excessive selenium (Se) for the deposition of Sb2Se3 thin films by the co-evaporation of Se and Sb2Se3. Scanning electron microscopy (SEM) images showed that additional Se modified the growth process and surface morphology of Sb2Se3 thin films. X-ray diffraction (XRD) patterns confirmed that this co-evaporation process enhanced the beneficiary preferred orientations, and capacitance–voltage (C–V) measurement showed that the carrier concentration of the Sb2Se3 absorber increased with the additional evaporation of Se. Accordingly, the efficiency of the devices employing co-evaporated Sb2Se3 absorber layers increased significantly from 2.1 to 3.47% with a open-circuit voltage (V OC) of 364 mV, a short-circuit current density (J SC) of 23.14 mA/cm2, and a fill factor (FF) of 41.26%.
TL;DR: In this paper, a quasi-graded α-(Al x Ga 1−x )2O3 buffer layer was applied to reduce the defect density in corundum-structured α-Ga 2O3 thin films on sapphire substrates.
Abstract: Efforts have been made to reduce the density of defects in corundum-structured α-Ga2O3 thin films on sapphire substrates by applying quasi-graded α-(Al x Ga1− x )2O3 buffer layers. Transmission electron microscopy images revealed that most strains were located in the α-(Al x Ga1− x )2O3 buffer layers, and that the total density of dislocations in the α-Ga2O3 thin films was successfully decreased by more than one order of magnitude compared with that without buffer layers, that is, the screw and edge dislocation densities were about 3 × 108 and 6 × 108 cm−2, respectively.
TL;DR: In this paper, high-output-power 255 and 280 nm light-emitting diodes (LEDs) using direct bonding were fabricated using sapphire lenses at room temperature using either atomic diffusion bonding or surface activation.
Abstract: We fabricated high-output-power 255 and 280 nm light-emitting diodes (LEDs) using direct bonding. The LED chips were bonded to sapphire lenses at room temperature using either atomic diffusion bonding or surface-activated bonding. The LEDs with lenses had a higher light extraction efficiency than conventionally structured LEDs. As a result, at a forward current of 350 mA, the output power of the 255 nm LED increased by a factor of 2.8, reaching 73.6 mW, while that of the 280 nm LED increased by a factor of 2.3, reaching 153 mW.
TL;DR: In this article, a 1.5λ-cavity GaN-based vertical-Cavity surface-emitting laser with an n-type conducting AlInN/GaN distributed Bragg reflector (DBR) achieved a peak reflectivity of over 99.9% and the threshold current was 2.6 mA.
Abstract: The room-temperature continuous-wave operation of a 1.5λ-cavity GaN-based vertical-cavity surface-emitting laser with an n-type conducting AlInN/GaN distributed Bragg reflector (DBR) was achieved. A peak reflectivity of over 99.9% was obtained in the n-type conducting AlInN/GaN DBR so that the current was injected through the DBR for the operation. The threshold current was 2.6 mA, corresponding to the threshold current density of 5.2 kA/cm2, and the operating voltage was 4.7 V. A lasing spectrum with a peak wavelength of 405.1 nm and a full-width at half maximum of 0.08 nm was also observed.
TL;DR: In this paper, the photoluminescence and scintillation properties of Cs2HfCl6 and cs2ZrCl6 crystals were investigated and the decay time constants were found to be about 2.2 and 8.4 µs.
Abstract: The photoluminescence and scintillation properties of Cs2HfCl6 and Cs2ZrCl6 crystals were investigated. Two emission bands in the photoluminescence spectra were observed at 375 and 435 nm for the Cs2HfCl6 crystal and at 440 and 479 nm for the Cs2ZrCl6 crystal. Similar spectra were observed for radioluminescence. The decay time constants were found to be about 2.2 and 8.4 µs for Cs2HfCl6 and 1.5 and 7.5 µs for Cs2ZrCl6. The scintillation light yields were estimated to be 27,500 and 25,100 photons/MeV for Cs2HfCl6 and Cs2ZrCl6, respectively.
TL;DR: In this paper, a 260 nm LED containing a transparent p-AlGaN layer and a reflective metal electrode was used to increase the light extraction efficiency of a p-GaN LED.
Abstract: AlGaN LEDs have been studied as efficient light sources in DUV. One of the central issues in DUV LEDs is their low light extraction efficiency owing to the absorption in a p-contacting layer and a metal electrode. We report the fabrication of a 260 nm LED containing a transparent p-AlGaN layer. The LED showed a relatively good current injection with an increase in forward voltage compared with a p-GaN LED. Its efficiency reached 2%, almost equivalent to that of the p-GaN LED. In addition, a nearly zero absorption in the p-contacting layer increased the light extraction efficiency by using a reflective metal electrode.
TL;DR: In this paper, a bandgap gradation was formed by changing the Ge/Sn ratio in the depth direction of the CTGS layers, and the performance of the cells was improved to over 6.7% in conversion efficiency.
Abstract: We fabricated Cu2Sn1− x Ge x S3 (CTGS) solar cells with a graded bandgap structure in order to improve their photovoltaic performance. Bandgap gradation was formed by changing the Ge/Sn ratio in the depth direction of the CTGS layers. The composition profile of each sample was measured by secondary ion mass spectrometry, and we confirmed that the Ge/Sn ratio near the buffer layer was lower than that near the back electrode. This means that the bandgap increases with depth from the surface. The performance of the cells was improved to over 6.7% in conversion efficiency.
TL;DR: In this paper, a metasurface composed of printed cross-dipole elements with different arm lengths is designed, fabricated, and experimentally demonstrated to generate OAM vortex waves of dual polarizations and dual modes in the radio frequency domain simultaneously.
Abstract: A metasurface, which is composed of printed cross-dipole elements with different arm lengths, is designed, fabricated, and experimentally demonstrated to generate orbital angular momentum (OAM) vortex waves of dual polarizations and dual modes in the radio frequency domain simultaneously. The prototype of a practical metasurface is fabricated and measured to validate the results of theoretical analysis and design at 5.8 GHz. Numerical and experimental results verify that vortex waves with dual OAM modes and dual polarizations can be flexibly generated by using a reflective metasurface. The proposed method paves a way to generate diverse OAM vortex waves for radio frequency and microwave wireless communication applications.
TL;DR: In this paper, high-angle annular dark field and bright-field scanning transmission electron microscopy observations of infinite NiO2 planes of c-axis-oriented LaNiO2 epitaxial thin films with a layer stacking sequence of NiO 2/La/NiO 2.
Abstract: Epitaxial thin films of LaNiO2, which is an oxygen-deficient perovskite with "infinite layers" of Ni1+O2, were prepared by a low-temperature reduction of LaNiO3 single-crystal films on NdGaO3 substrates. We report the high-angle annular dark-field and bright-field scanning transmission electron microscopy observations of infinite NiO2 planes of c-axis-oriented LaNiO2 epitaxial thin films with a layer stacking sequence of NiO2/La/NiO2. Resistivity measurements on the films show T 2 dependence between 400 and 150 K and a negative Hall coefficient.
TL;DR: In this article, a high-efficiency solar thermophotovoltaic (STPV) system has been demonstrated using spectrally selective planar absorber/emitter systems and a GaSb TPV cell.
Abstract: A high-efficiency solar thermophotovoltaic (STPV) system has been demonstrated using spectrally selective planar absorber/emitter systems and a GaSb TPV cell In this study, a novel approach for designing the STPV system based on the efficiency of unidirectional radiative heat transfer has been introduced To achieve high extraction and photovoltaic conversion efficiencies, the spectrally selective absorber/emitter based on a coherent perfect absorber composed of a thin molybdenum layer sandwiched between hafnium layers was applied The extraction efficiency was further investigated with respect to the absorber/emitter area ratio The experimental efficiency of STPV reached 51% with the area ratio of 23
TL;DR: In this paper, the spin-torque ferromagnetic resonance (ST-FMR) signals in both Ni80Fe20/Ta and Co40Fe40B20 /Ta bilayers were analyzed and the spin Hall angles of these signals were compared.
Abstract: We have performed spectral analyses of spin-torque ferromagnetic resonance (ST-FMR) signals in both Ni80Fe20/Ta and Co40Fe40B20/Ta bilayers and compared the spin Hall angles of these signals. We found that the contribution of the inverse spin Hall effect to the total signal in ST-FMR measurements is marked particularly in the case of Co40Fe40B20/Ta bilayers, because the anisotropic magnetoresistance effect in Co40Fe40B20, i.e., the origin of the ST-FMR signal, is much smaller than that in Ni80Fe20. When we take into account the contribution of the inverse spin Hall effect, the spin Hall angle of Co40Fe40B20/Ta decreases to less than half of that estimated by conventional ST-FMR spectral analysis.
TL;DR: In this paper, the authors demonstrate pabively Q-switched erbium-doped fiber laser (EDFL) operation using a saturable absorber (SA) based on Fe3O4 nanoparticles (FONPs).
Abstract: We experimentally demonstrate pabively Q-switched erbium-doped fiber laser (EDFL) operation using a saturable absorber (SA) based on Fe3O4 nanoparticles (FONPs). As a type of transition metal oxide, the FONPs have a large nonlinear optical response and fast response time. The FONPbased SA pobebes a modulation depth of 8.2% and nonsaturable absorption of 56.6%. Stable pabively Q-switched EDFL pulses with an output pulse energy of 23.76 nJ, a repetition rate of 33.3 kHz, and a pulse width of 3.2 μs were achieved when the input pump power was 110mW. The laser features a low threshold pump power of > 15mW.
TL;DR: In this paper, a nanoscale plasmonic filter based on a single-stub coupled metal-dielectric-metal waveguide system is investigated theoretically and numerically.
Abstract: A nanoscale plasmonic filter based on a single-stub coupled metal–dielectric–metal waveguide system is investigated theoretically and numerically. A tunable wide band-stop can be achieved by loading a metal bar into the stub. The band-stop originates from the direct coupling between the resonance modes. The bandwidth and the center wavelength of the band-stop can be tuned by changing the parameters of the metal bar. Compared with previously reported filters, the plasmonic system has the advantages of easy fabrication and compactness. Our results indicate that the proposed system has potential to be utilized in integrated optical circuits and tunable filters.
TL;DR: In this article, high-temperature in situ annealing led to a high degree of B2-order in the Co2FeAl layers and cation order of the MgAl2O4 barrier.
Abstract: Epitaxial magnetic tunnel junctions (MTJs) with a Co2FeAl/CoFe (0.5 nm)/MgAl2O4/Co2FeAl(001) structure were fabricated by magnetron sputtering. High-temperature in situ annealing led to a high degree of B2-order in the Co2FeAl layers and cation order of the MgAl2O4 barrier. Large tunnel magnetoresistance (TMR) of up to 342% was obtained at room temperature (616% at 4 K), in contrast to the TMR ratio (%) suppressed by the band-folding effect in Fe/cation-ordered MgAl2O4/Fe MTJs. The present study reveals that the high degree of B2-order and the resulting high spin polarization in the Co2FeAl electrodes enable us to bypass the band-folding problem in spinel barriers.
TL;DR: In this article, a TiS3 cathode was used for Mg batteries with a specific capacity of 80 mAh/g for the first 50 cycles at room temperature, which is comparable to the capacity of the prototype cathode for mg batteries.
Abstract: Rechargeable performance is realized in Mg batteries using a TiS3 cathode without the nanometer-scale downsizing of electrode particles. The specific capacity is about 80 mAh/g for the first 50 cycles at room temperature. This observed specific capacity is comparable to that of the prototype cathode for Mg batteries. First-principles calculation indicates that TiS3 is a semiconductor with d–p orbital hybridized electronic structures around the Fermi level. The reversible electrode performance is likely assisted by the delocalized electronic distribution over metal–ligand units through d–p orbital hybridization.
TL;DR: In this paper, the two-dimensional concentration distribution of reactive oxygen species (ROSs) transported through an agarose-film tissue phantom by atmospheric-pressure plasma-jet irradiation is visualized using a KI-starch gel reagent.
Abstract: The two-dimensional concentration distribution of reactive oxygen species (ROSs) transported through an agarose-film tissue phantom by atmospheric-pressure plasma-jet irradiation is visualized using a KI-starch gel reagent. Oxygen addition to helium enhances ROS transportation through the film. A radial ROS distribution pattern at the plasma-irradiated film surface changes into a doughnut-shaped pattern after passing through the film. The ROS transportation speed is 0.14–0.2 mm/min. We suggest that there are two types of ROS transportation pathways in the plasma-irradiated film: linear and circular. The majority of ROSs are transported through the circular pathway. ROS concentration distributions changed markedly with irradiation distance. Diffusive ROS transportation due to a concentration gradient is negligible in plasma-irradiated films.