Showing papers in "Applied Physics Express in 2018"
117 citations
TL;DR: In this article, a highly reflective photonic crystal (HR-PhC) was introduced into the surface of the p-AlGaN contact layer, thereby achieving a high external quantum efficiency (EQE).
Abstract: We increased the light-extraction efficiency (LEE) of AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) by introducing a highly reflective photonic crystal (HR-PhC) into the surface of the p-AlGaN contact layer, thereby achieving a high external quantum efficiency (EQE). A low-damage HR-PhC with a lattice period of approximately 250 nm was fabricated using nanoimprinting and dry etching. A reflective Ni/Mg p-type electrode was deposited on the HR-PhC layer using a tilted-evaporation method. The EQE of a conventional DUV LED with emission around 283 nm was increased from 4.8 to 10% by introducing the HR-PhC and the reflective Ni/Mg electrode. A simple estimation of the effective reflectance of the HR-PhC p-AlGaN contact layer with the Ni/Mg electrode indicated a value exceeding 90%.
109 citations
TL;DR: In this article, a bevel-field-plated mesa Schottky barrier diodes with good ideality and low reverse leakage were realized on the epitaxial material.
Abstract: We report (010)-oriented β-Ga2O3 bevel-field-plated mesa Schottky barrier diodes grown by low-pressure chemical vapor deposition (LPCVD) using a solid Ga precursor and O2 and SiCl4 sources. Schottky diodes with good ideality and low reverse leakage were realized on the epitaxial material. Edge termination using beveled field plates yielded a breakdown voltage of −190 V, and maximum vertical electric fields of 4.2 MV/cm in the center and 5.9 MV/cm at the edge were estimated, with extrinsic R ON of 3.9 mΩcm2 and extracted intrinsic R ON of 0.023 mΩcm2. The reported results demonstrate the high quality of homoepitaxial LPCVD-grown β-Ga2O3 thin films for vertical power electronics applications, and show that this growth method is promising for future β-Ga2O3 technology.
99 citations
TL;DR: In this paper, an epitaxial β-Ga2O3/GaN-based vertical metal-heterojunction-metal (MHM) broadband UV-A/UV-C photodetectors with high responsivity (3.7 A/W) at 256 and 365 nm, UVto-visible rejection >103, and a photo-to-dark current ratio of ~100.
Abstract: We demonstrate epitaxial β-Ga2O3/GaN-based vertical metal–heterojunction-metal (MHM) broadband UV-A/UV-C photodetectors with high responsivity (3.7 A/W) at 256 and 365 nm, UV-to-visible rejection >103, and a photo-to-dark current ratio of ~100. A small (large) conduction (valence) band offset at the heterojunction of pulsed laser deposition (PLD)-grown β-Ga2O3 on metal organic chemical vapor deposition (MOCVD)-grown GaN-on-silicon with epitaxial registry, as confirmed by X-ray diffraction (XRD) azimuthal scanning, is exploited to realize detectors with an asymmetric photoresponse and is explained with one-dimensional (1D) band diagram simulations. The demonstrated novel vertical MHM detectors on silicon are fully scalable and promising for enabling focal plane arrays for broadband ultraviolet sensing.
76 citations
TL;DR: In this article, an isotropic active analog of electromagnetic induced transparency through conductivity tuning of vanadium dioxide at terahertz frequencies is presented, where the unit cell consists of metallic split ring resonators and a metallic cross, which have identical resonance frequencies for the excitable lowest order modes but very different linewidths.
Abstract: We present an isotropic active analog of electromagnetically induced transparency through conductivity tuning of vanadium dioxide at terahertz frequencies. The unit cell of the designed metasurface consists of metallic split ring resonators and a metallic cross, which have identical resonance frequencies for the excitable lowest order modes but very different linewidths. By integrating vanadium dioxide into the bottom of the metasurface, an obvious tuning of the transparency window occurs under different conductivities. Calculated results show that resonant transmission frequency of the electromagnetically induced transparency remains stable with respect to the polarization and incident angle of electromagnetic waves.
74 citations
TL;DR: Magnetocardiography (MCG) and magnetoencephalography (MEG) signals were detected at room temperature using tunnel magneto-resistance (TMR) sensors.
Abstract: Magnetocardiography (MCG) and magnetoencephalography (MEG) signals were detected at room temperature using tunnel magneto-resistance (TMR) sensors. TMR sensors developed with low-noise amplifier circuits detected the MCG R wave without averaging, and the QRS complex was clearly observed with averaging at a high signal-to-noise ratio. Spatial mapping of the MCG was also achieved. Averaging of MEG signals triggered by electroencephalography (EEG) clearly observed the phase inversion of the alpha rhythm with a correlation coefficient as high as 0.7 between EEG and MEG.
71 citations
TL;DR: In this article, the three-dimensional imaging of threading dislocations in GaN films using two-photon excitation photoluminescence was demonstrated using dark lines.
Abstract: The three-dimensional imaging of threading dislocations in GaN films was demonstrated using two-photon excitation photoluminescence. The threading dislocations were shown as dark lines. The spatial resolutions near the surface were about 0.32 and 3.2 µm for the in-plane and depth directions, respectively. The threading dislocations with a density less than 108 cm−2 were resolved, although the aberration induced by the refractive index mismatch was observed. The decrease in threading dislocation density was clearly observed by increasing the GaN film thickness. This can be considered a novel method for characterizing threading dislocations in GaN films without any destructive preparations.
66 citations
TL;DR: In this paper, a vertical β-Ga2O3 metal-oxide-semiconductor field effect transistor featuring a planar-gate architecture was presented, which was fabricated by an all-ion-implanted process without requiring trench etching or epitaxial regrowth.
Abstract: A vertical β-Ga2O3 metal–oxide–semiconductor field-effect transistor featuring a planar-gate architecture is presented. The device was fabricated by an all-ion-implanted process without requiring trench etching or epitaxial regrowth. A Mg-ion-implanted current blocking layer (CBL) provided electrical isolation between the source and the drain except at an aperture opening through which drain current was conducted. Successful transistor action was realized by gating a Si-ion-implanted channel above the CBL. Thermal diffusion of Mg induced a large source–drain leakage current through the CBL, which resulted in compromised off-state device characteristics as well as a reduced peak extrinsic transconductance compared with the results of simulations.
66 citations
60 citations
TL;DR: In this article, a portion of this work was carried out in the UCSB nanofabrication facility, which is part of the NSF NNIN network (ECS-0335765) and was supported by the MRSEC Program of the National Science Foundation under Award No. DMR 1121053.
Abstract: This work was funded by the King Abdulaziz City for Science and Technology (KACST) Technology Innovations Center (TIC) program and the KACST-KAUST-UCSB Solid State Lighting Program. A portion of this work was carried out in the UCSB nanofabrication facility, which is part of the NSF NNIN network (ECS-0335765), as well as the UCSB MRL, which is supported by the MRSEC Program of the National Science Foundation under Award No. DMR 1121053. D.H. was supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1144085. The authors would like to acknowledge Dr. Tom Mates for his assistance with SIMS.
56 citations
TL;DR: In this paper, a transparent metamaterial absorber with simultaneously high optical transparency and broadband microwave absorption is presented, which consists of a two-layer soda-lime glass substrate and three-layer patch-shaped indium tin oxide (ITO) films.
Abstract: A transparent metamaterial absorber with simultaneously high optical transparency and broadband microwave absorption is presented in this paper. Consisting of a two-layer soda-lime glass substrate and three-layer patch-shaped indium tin oxide (ITO) films, the proposed absorber has advantages of broadband absorption with an absorptivity higher than 85% in the range from 6.1 to 22.1 GHz, good polarization insensitiveness, a high transparency, a low profile, and wide-incident-angle stability. A prototype of the proposed absorber is fabricated and experimentally measured to demonstrate its excellent performance. The measured results agree well with the theoretical design and numerical simulations.
TL;DR: In this article, a blue GaN-based vertical-cavity surface-emitting laser (VCSEL) under continuous-wave operation was presented, where a long-cave (10λ) structure was introduced.
Abstract: High output power values of 15.7 mW at 20 °C and 2.7 mW at 110 °C were obtained from a blue GaN-based vertical-cavity surface-emitting laser (VCSEL) under continuous-wave operation as a result of introducing a long-cavity (10λ) structure. The threshold current and voltage at 20 °C were 4.5 mA and 5.1 V, respectively. Owing to the reduced thermal resistance provided by the long-cavity structure and the adjusted reflectivity of the front cavity mirror, this VCSEL also exhibited a high slope efficiency of 0.87 W/A, a differential quantum efficiency of 31%, and a wall-plug efficiency of 8.9%.
TL;DR: In this article, an ultra-fast in-line graphene optical modulator on a silicon waveguide with a bandwidth exceeding 100 GHz, very small power consumption below 15 fJ/bit, and insertion loss of 1.5 dB is presented.
Abstract: We present a design of an ultra-fast in-line graphene optical modulator on a silicon waveguide with a bandwidth exceeding 100 GHz, very small power consumption below 15 fJ/bit, and insertion loss of 1.5 dB. This is achieved by utilizing the transverse-electric-mode silicon slot to tailor the overlap of graphene electrodes, thus significantly reducing the capacitance of the device while maintaining a low insertion loss and using conservative estimates of the graphene resistance. Our design is substantiated by comprehensive finite-element-method simulations and RC circuit characterization, as well as fabrication feasibility discussion.
TL;DR: In this paper, a metal-exchange catalytic effect was used to increase the growth domain and growth rate of the O plasma-assisted molecular beam epitaxy of 2O3 by adding the element In during growth.
Abstract: We demonstrate a marked increase in the possible growth domain and growth rate of the O plasma-assisted molecular beam epitaxy of ?-(Al x Ga1? x )2O3, by adding the element In during growth. We explain these enhancement results from a metal-exchange catalytic effect. This mechanism allows us to synthesize ?-(Al x Ga1? x )2O3/?-Ga2O3 heterostructures at growth conditions that are not accessible in the absence of In, stabilizing the monoclinic ?-phase. We demonstrate the growth of ?-(Al x Ga1? x )2O3 at growth temperatures up to 900 ?C. Moreover, we illustrate how additional In on the ?-(Al x Ga1? x )2O3 surface acts as a surface active agent, improving the crystal quality of the synthesized ?-(Al x Ga1? x )2O3/?-Ga2O3 heterostructures. These structures are shown to be of the highest crystal quality up to an Al concentration of x = 0.2. We predict the novel growth mode introduced for ternary III?O thin film synthesis ? shown by the example of ?-(Al x Ga1? x )2O3 ? to be applicable for a wide range of thin film materials, whose individual constituents possess material properties similar to those discussed for the constituents contributing to ?-(Al x Ga1? x )2O3.
TL;DR: In this paper, the effects of postmetallization annealing (PMA) on the interface properties of GaN metaloxide-semiconductor (MOS) structures using Al2O3 prepared by atomic layer deposition were investigated.
Abstract: In this study, we investigated the effects of postmetallization annealing (PMA) on the interface properties of GaN metal–oxide–semiconductor (MOS) structures using Al2O3 prepared by atomic layer deposition. Excellent capacitance–voltage (C–V) characteristics without frequency dispersion were observed in the MOS sample after PMA in N2 ambient at 300–400 °C. The PMA sample showed state densities of only at most 4 × 1010 cm−1 eV−1. A geometric phase analysis of transmission electron microscopy images after PMA revealed a uniform distribution of the lattice constant near the Al2O3/GaN interface, leading to the improved bond termination and bonding order configuration along the interface.
TL;DR: A review of spin-enabled neuromorphic computing, its status, and challenges and future prospects are outlined in this paper, where several spin-device structures have been proposed as the core building blocks of neuromorphic circuits and systems to implement brain-inspired computing.
Abstract: "Spintronics" refers to the understanding of the physics of electron spin-related phenomena. While most of the significant advancements in this field has been driven primarily by memory, recent research has demonstrated that various facets of the underlying physics of spin transport and manipulation can directly mimic the functionalities of the computational primitives in neuromorphic computation, i.e., the neurons and synapses. Given the potential of these spintronic devices to implement bio-mimetic computations at very low terminal voltages, several spin-device structures have been proposed as the core building blocks of neuromorphic circuits and systems to implement brain-inspired computing. Such an approach is expected to play a key role in circumventing the problems of ever-increasing power dissipation and hardware requirements for implementing neuro-inspired algorithms in conventional digital CMOS technology. Perspectives on spin-enabled neuromorphic computing, its status, and challenges and future prospects are outlined in this review article.
TL;DR: In this paper, the polarization values of κ- and e-Ga2O3 were analyzed to overcome the inconsistency between experimental and theoretical studies, and the bandgap values of 4.62 and 4.27 eV were estimated with the hybrid functional method.
Abstract: Physical properties of κ- and e-Ga2O3 are investigated using density functional theory. We utilized the supercell method considering the partial occupancies in e-Ga2O3. The polarization values of these materials were analyzed to overcome the inconsistency between experimental and theoretical studies. The polarization values of κ- and e-Ga2O3 were ~26.39 and 24.44 µC/cm2, respectively. The bandgap values of 4.62 and 4.27 eV were estimated with the hybrid functional method, which suggested an underestimation of the PBEsol functional values of 2.32 and 2.06 eV for κ- and e-Ga2O3, respectively.
TL;DR: In this article, the authors investigated the topology of the band structure of Lamb waves in a thin phononic crystal plate and showed that when inversion symmetry is broken, a valley pseudospin degree of freedom is formed around K and K' valleys for the A0 Lamb mode, which is decoupled from the S0 and SH0 modes in the low frequency regime.
Abstract: We investigate the nontrivial topology of the band structure of Lamb waves in a thin phononic crystal plate. When inversion symmetry is broken, a valley pseudospin degree of freedom is formed around K and K' valleys for the A0 Lamb mode, which is decoupled from the S0 and SH0 modes in the low-frequency regime. Chiral edge states are explicitly demonstrated, which are immune to defects and exhibit unidirectional transport behaviors when intervalley scattering is weak. The quantum valley Hall effect is thus simulated in a simple way in the context of Lamb waves.
TL;DR: In this paper, the first demonstration of III-nitride vertical-cavity surface-emitting lasers (VCSELs) with tunnel junction (TJ) intracavity contacts grown completely by metal-organic chemical vapor deposition (MOCVD) was reported.
Abstract: We report the first demonstration of III–nitride vertical-cavity surface-emitting lasers (VCSELs) with tunnel junction (TJ) intracavity contacts grown completely by metal–organic chemical vapor deposition (MOCVD). For the TJs, n++-GaN was grown on in-situ activated p++-GaN after buffered HF surface treatment. The electrical properties and epitaxial morphologies of the TJs were first investigated on TJ LED test samples. A VCSEL with a TJ intracavity contact showed a lasing wavelength of 408 nm, a threshold current of ~15 mA (10 kA/cm2), a threshold voltage of 7.8 V, a maximum output power of 319 µW, and a differential efficiency of 0.28%.
TL;DR: Optically pumped lasing from AlGaN/AlN multiple quantum wells grown on single-crystalline AlN substrates with lasing thresholds as low as 6 kW/cm2 is demonstrated via the reduction of unintentional point defects in the active region and waveguide, which reduces the nonradiative recombination by 2 orders of magnitude.
Abstract: Optically pumped lasing from AlGaN/AlN multiple quantum wells grown on single-crystalline AlN substrates with lasing thresholds as low as 6 kW/cm2 is demonstrated via the reduction of unintentional point defects in the active region and waveguide, which reduces the non-radiative recombination by 2 orders of magnitude. A higher lasing threshold of 11 kW/cm2 is observed for AlGaN barriers, owing to the reduced localization of electrons and holes in the wells. It is shown that for electrically injected UVC laser diodes, AlGaN barriers are essential.
TL;DR: In this article, a b-doped p-BaSi2 epitaxial layers with a hole concentration of 1.1 × 1018 cm−3 were grown on n-Si(001) using molecular beam epitaxy to fabricate BaSi2/n-Si solar cells.
Abstract: B-doped p-BaSi2 epitaxial layers with a hole concentration of 1.1 × 1018 cm−3 were grown on n-Si(001) using molecular beam epitaxy to fabricate p-BaSi2/n-Si solar cells. The thickness (d) of the p-BaSi2 layer was varied from 20 to 60 nm to investigate its effect on the solar cell performance. The conversion efficiency under an AM1.5 illumination increased with d reaching a maximum of 9.8% at d = 40 nm, which is nearly equal to the highest efficiency (9.9%) for p-BaSi2/n-Si solar cells on Si(111). This study indicated that Si(001) substrates are promising for use in BaSi2 solar cells.
TL;DR: In this paper, the source of carrier compensation in metalorganic vapor phase epitaxy (MOVPE)-grown n-type GaN was quantitatively investigated by Hall-effect measurement, deep-level transient spectroscopy, and secondary ion mass spectrometry.
Abstract: The source of carrier compensation in metalorganic vapor phase epitaxy (MOVPE)-grown n-type GaN was quantitatively investigated by Hall-effect measurement, deep-level transient spectroscopy, and secondary ion mass spectrometry. These analysis techniques revealed that there were at least three different compensation sources. The carrier compensation for samples with donor concentrations below 5 × 1016 cm−3 can be explained by residual carbon and electron trap E3 (E C − 0.6 eV). For samples with higher donor concentrations, we found a proportional relationship between donor concentration and compensating acceptor concentration, which resulted from a third source of compensation. This is possibly due to the self-compensation effect.
TL;DR: The thermal conductivity of AlN single crystals grown by physical vapor transport (PVT) and hydride vapor phase epitaxy (HVPE) was measured in the range of 30 to 325 K by the 3ω method as discussed by the authors.
Abstract: The thermal conductivity of AlN single crystals grown by physical vapor transport (PVT) and hydride vapor phase epitaxy (HVPE) was measured in the range of 30 to 325 K by the 3ω method. The measured room-temperature thermal conductivity ranged from 268 to 374 W m−1 K−1. Higher thermal conductivity correlated with higher transparency at 265 nm and lower total impurity levels.
TL;DR: A magnetically and electrically polarization-tunable terahertz emitter that integrates a ferromagnetic heterostructure and large-birefringence liquid crystals is demonstrated in this article.
Abstract: A magnetically and electrically polarization-tunable terahertz emitter that integrates a ferromagnetic heterostructure and large-birefringence liquid crystals is demonstrated. The heterostructure and the liquid crystal cell act as the broadband terahertz source and the phase retarder, respectively. The polarization state is switched between linear and circular by changing the direction of the external magnetic field. The phase retardation for frequencies higher than 1 THz is continuously adjustable over a range of π/2 by applying a low voltage. This compact, broadband, economical, and easy-to-regulate terahertz emitter can be widely used in polarization-sensitive research and engineering applications.
TL;DR: In this article, a two-step growth method was employed to fabricate a high-quality e-Ga2O3 thin film on a c-plane sapphire substrate, which was evaluated by atomic force microscope, and the density of screw-type threading dislocations determined by an X-ray diffraction rocking curve was as low as 1.8 × 108 cm−2.
Abstract: Layer-by-layer morphology is a crucial signature of the quality of epitaxial thin films. In this study, layer-by-layer growth of an e-phase gallium oxide (e-Ga2O3) thin film is demonstrated using metal–organic chemical vapor deposition. A two-step growth method, in which a nucleation layer is grown at 600 °C and an epilayer is grown at 640 °C, is employed to fabricate a high-quality e-Ga2O3 thin film on a c-plane sapphire substrate. The morphology of the e-Ga2O3 film is evaluated by atomic force microscope. The density of screw-type threading dislocations determined by an X-ray diffraction rocking curve is as low as 1.8 × 108 cm−2.
TL;DR: In this paper, the authors demonstrate true-blue 450 nm tunnel junction (TJ) laser diodes (LDs) grown by plasma-assisted molecular beam epitaxy (PAMBE).
Abstract: We demonstrate true-blue 450 nm tunnel junction (TJ) laser diodes (LDs) grown by plasma-assisted molecular beam epitaxy (PAMBE) The absence of hydrogen during PAMBE growth allows us to achieve TJs with low resistance We compare TJ LDs with LDs of standard construction with p-type metal contact For both types of LD, the threshold current density is around 3 kA/cm2 and the slope efficiency is 05 W/A We do not observe any significant changes in optical losses and differential gain in TJ LDs compared with standard LDs The differential resistivity of the TJs for current densities higher than 2 kA/cm2 is below 10−4 Ωcm2
TL;DR: In this article, the anomalous Hall effect and magnetic circular dichroism (MCD) spectroscopy indicate that the high-temperature ferromagnetism in (In,Fe)Sb thin films is intrinsic and originates from the zinc-blende (In-Fe)-Sb alloy semiconductor.
Abstract: Over the past two decades, intensive studies on various ferromagnetic semiconductor (FMS) materials have failed to realize reliable FMSs that have a high Curie temperature (T C > 300 K), good compatibility with semiconductor electronics, and characteristics superior to those of their nonmagnetic host semiconductors. Here, we demonstrate a new n-type Fe-doped narrow-gap III–V FMS, (In1− x ,Fe x )Sb. Its T C is unexpectedly high, reaching ~335 K at a modest Fe concentration (x) of 16%. The anomalous Hall effect and magnetic circular dichroism (MCD) spectroscopy indicate that the high-temperature ferromagnetism in (In,Fe)Sb thin films is intrinsic and originates from the zinc-blende (In,Fe)Sb alloy semiconductor.
TL;DR: In this paper, a search for high-piezoelectric wurtzite materials by identifying materials with smaller c/a values was conducted, and it was shown that the piezoclectricity of ZnO can be significantly enhanced by substitutions with Zn with Ca.
Abstract: Longitudinal piezoelectric constant (e 33) values of wurtzite materials, which are listed in a structure database, are calculated and analyzed by using first-principles and statistical learning methods. It is theoretically shown that wurtzite materials with high e 33 generally have small lattice constant ratios (c/a) almost independent of constituent elements, and approximately expressed as e 33 ∝ c/a − (c/a)0 with ideal lattice constant ratio (c/a)0. This relation also holds for highly-piezoelectric ternary materials such as Sc x Al1− x N. We conducted a search for high-piezoelectric wurtzite materials by identifying materials with smaller c/a values. It is proposed that the piezoelectricity of ZnO can be significantly enhanced by substitutions of Zn with Ca.
TL;DR: In this article, a 2 µm Tm-doped double-closeted double-clad fiber laser with a 10/90 fiber loop mirror was used as a nonlinear optical loop mirror for initiating mode locking.
Abstract: We report direct generation of a high-power, large-energy dissipative soliton resonance (DSR) in a 2 µm Tm-doped double-clad fiber laser. A compact σ-shaped cavity is formed by a fiber Bragg grating and a 10/90 fiber loop mirror (FLM). The 10/90 FLM is not only used as an output mirror, but also acts as a nonlinear optical loop mirror for initiating mode locking. The mode-locked laser can deliver high-power, nanosecond DSR pulses at 2005.9 nm. We further perform a comparison study of the effect of the FLM's loop length on the mode-locking threshold, peak power, pulse energy, and optical spectrum of the DSR pulses. We achieve a maximum average output power as high as 1.4 W, a maximum pulse energy of 353 nJ, and a maximum peak power of 84 W. This is, to the best of our knowledge, the highest power for 2 µm DSR pulses obtained in a mode-locked fiber laser.
TL;DR: In this paper, photo-electrochemical (PEC) etching was used to fabricate deep trench structures in a GaN-on-GaN epilayer grown on nGaN substrates.
Abstract: Photo-electrochemical (PEC) etching was used to fabricate deep trench structures in a GaN-on-GaN epilayer grown on n-GaN substrates. A 50-nm-thick layer of Ti used for an etching mask was not removed even after etching to a depth of >30 µm. The width of the side etching was less than 1 µm with high accuracy. The aspect ratio (depth/width) of a 3.3-µm-wide trench with a PEC etching depth of 24.3 µm was 7.3. These results demonstrate the excellent potential of PEC etching for fabricating deep trenches in vertical GaN devices.