Showing papers in "Applied Physics Express in 2012"

Structure of Silicene Grown on Ag(111)

Abstract: The structure of silicene, the two-dimensional honeycomb sheet of Si, grown on Ag(111) was investigated by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) combined with density functional theory (DFT) calculation. Two atomic arrangements of honeycomb configuration were found by STM, which are confirmed by LEED and DFT calculations; one is 4×4 and the other is √13×√13 R13.9°. In the 4×4 structure, the honeycomb lattice remains with six atoms displaced vertically, whereas the √13×√13 R13.9° takes the regularly buckled honeycomb geometry.

Device-Quality β-Ga2O3 Epitaxial Films Fabricated by Ozone Molecular Beam Epitaxy

Abstract: N-type Ga2O3 homoepitaxial thick films were grown on β-Ga2O3(010) substrates by ozone molecular beam epitaxy. The epitaxial growth rate was increased by more than ten times by changing from the (100) plane to the (010) plane. The carrier concentration of the epitaxial layers could be varied within the range of 1016–1019 cm-3 by changing the Sn doping concentration. Platinum Schottky barrier diodes (SBDs) on 1.4-µm-thick β-Ga2O3 homoepitaxial layers were demonstrated for the first time. The SBDs exhibited a reverse breakdown voltage of 100 V, an on-resistance of 2 mΩ cm2, and a forward voltage of 1.7 V (at 200 A/cm2).

AlGaN Deep-Ultraviolet Light-Emitting Diodes with External Quantum Efficiency above 10%

Abstract: Improvements of the internal quantum efficiency by reduction of the threading dislocation density and of the light extraction by using UV transparent p-type cladding and contact layers, UV reflecting ohmic contact, and chip encapsulation with optimized shape and refractive index allowed us to obtain the external quantum efficiency of 104% at 20 mA CW current with the output power up to 93 mW at 278 nm for AlGaN-based deep-ultraviolet light-emitting diodes grown on sapphire substrates

High Mobility in a Stable Transparent Perovskite Oxide

Abstract: We discovered that La-doped BaSnO3 with the perovskite structure has an unprecedentedly high mobility at room temperature while retaining its optical transparency. In single crystals, the mobility reached 320 cm2 V-1 s-1 at a doping level of 8×1019 cm-3, constituting the highest value among wide-band-gap semiconductors. In epitaxial films, the maximum mobility was 70 cm2 V-1 s-1 at a doping level of 4.4×1020 cm-3. We also show that resistance of (Ba,La)SnO3 changes little even after a thermal cycle to 530 °C in air, pointing to an unusual stability of oxygen atoms and great potential for realizing transparent high-frequency, high-power functional devices.

Thermoelectric Properties of Mineral Tetrahedrites Cu$_{10}$ Tr $_{2}$Sb$_{4}$S$_{13}$ with Low Thermal Conductivity

Abstract: We have investigated thermoelectric properties of synthesized mineral Cu10Tr2Sb4S13 (Tr = Mn, Fe, Co, Ni, Cu, and Zn) tetrahedrites, which have a cubic and complex crystal structure. The mother phase Tr = Cu shows metal–semiconductor transition and anomalous hysteresis. Through various Tr substitutions, the thermopower was increased and thermal conductivity was decreased. Results show that Tr = Ni had the largest dimensionless figure of merit Z T of 0.15 at 340 K. The main advantage for the large Z T is the quite low lattice thermal conductivity. Because of the large Z T and the environmentally friendly components, tetrahedrites are anticipated as a good thermoelectric material.

Efficient Mode-Locking of Sub-70-fs Ti:Sapphire Laser by Graphene Saturable Absorber

Abstract: The efficient passive mode-locking of a Ti:sapphire laser with a monolayer graphene saturable absorber is demonstrated for the first time. High-quality and large-area (1 in.) monolayer graphene, synthesized by chemical vapor deposition, exhibits ultrafast recovery times and excellent nonlinear absorption behavior for bulk solid-state laser mode-locking near 800 nm. The continuous-wave mode-locked Ti:sapphire laser generates 63-fs pulses with output powers up to 480 mW under stable operation at 99.4 MHz.

Evaluation of spin Hall angle and spin diffusion length by using spin current-induced ferromagnetic resonance

Abstract: We experimentally demonstrate the thickness dependence of the spin current-induced ferromagnetic resonance in Ni80Fe20/nonmagnetic (Pt,Pd) bilayer thin films The spectrum shape depends on the Ni80Fe20 layer thickness, due to extrinsic excitation in addition to the spin Hall effect Detailed analysis of the thickness dependence of the spectrum, both for Ni80Fe20 and nonmagnetic layers, provides the spin Hall angle (Pt: 0022±0004, Pd: 0008±0002) and spin diffusion length (Pt: 12±006 nm, Pd: 20±009 nm)

Demonstration of Nonpolar GaN-Based Vertical-Cavity Surface-Emitting Lasers

Abstract: Electrically injected, single-longitudinal-mode nonpolar m-plane vertical-cavity surface-emitting lasers (VCSELs) are reported with a room-temperature lasing wavelength of 411.9 nm, a full width at half maximum (FWHM) of 0.25 nm, and a peak output power of 19.5 µW under pulsed conditions. The polarization direction of the VCSELs is consistently oriented along the [1210] a-direction. The degree of polarization increases from 0.13 below threshold to 0.72 above threshold. A process is demonstrated that utilizes bandgap-selective photoelectrochemical (PEC) undercut etching of an intracavity-embedded In0.12Ga0.88N sacrificial layer to achieve precise cavity length control and allow for top and bottom dielectric distributed Bragg reflector (DBR) mirrors.

Deep-Ultraviolet Light-Emitting Diodes Fabricated on AlN Substrates Prepared by Hydride Vapor Phase Epitaxy

Abstract: AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) were fabricated on AlN substrates. The AlN substrates were prepared by growing thick hydride vapor phase epitaxy (HVPE)-AlN layers on bulk AlN substrates prepared by physical vapor transport (PVT). After growing an LED structure, the PVT-AlN substrates were removed by mechanical polishing. This process allowed the fabrication of DUV-LEDs on HVPE-AlN substrates with high crystalline quality and DUV optical transparency. The DUV-LEDs exhibited a single emission peaking at 268 nm through the HVPE-AlN substrates. The output power as high as 28 mW was obtained at an injection current of 250 mA.

High-Power (over 100 mW) Green Laser Diodes on Semipolar {2021} GaN Substrates Operating at Wavelengths beyond 530 nm

Abstract: Continuous-wave operation of InGaN green laser diodes (LDs) on semipolar {2021} GaN substrates with output powers of over 100 mW in the spectral region beyond 530 nm is demonstrated Wall plug efficiencies (WPEs) as high as 70–89% are realized in the wavelength range of 525–532 nm, which exceed those reported for c-plane LDs The longest lasing wavelength has reached 5366 nm under cw operation These results suggest that the InGaN green LDs on the {2021} plane are better suited as light sources for applications requiring wavelengths over 525 nm

Current Induced Tilting of Domain Walls in High Velocity Motion along Perpendicularly Magnetized Micron-Sized Co/Ni/Co Racetracks

Abstract: Kerr microscopy is used to investigate domain wall motion in response to nanosecond-long current pulses in perpendicularly magnetized micron-sized Co/Ni/Co racetracks. Domain wall velocities greater than 300 m/s are observed. The velocity is independent of the pulse length for a wide range of current densities. However, the domain wall dynamics depends on the pulse length just above the threshold current for motion, where slow creep motion occurs, and at very high current densities, where domain nucleation takes place. We also observe a tilting of the domain wall that cannot be accounted for by the Oersted field from the driving current.

Preparation of a Freestanding AlN Substrate from a Thick AlN Layer Grown by Hydride Vapor Phase Epitaxy on a Bulk AlN Substrate Prepared by Physical Vapor Transport

Abstract: The structural and optical quality of a freestanding AlN substrate prepared from a thick AlN layer grown by hydride vapor phase epitaxy (HVPE) on a bulk (0001)AlN substrate prepared by physical vapor transport (PVT) were investigated. The prepared HVPE-AlN substrate was crack- and stress-free. High-resolution X-ray diffraction ω-rocking curves of symmetric (0002) and skew-symmetric (1011) reflections had small full widths at half maximum (FWHMs) of 31 and 32 arcsec, respectively. Deep-ultraviolet optical transparency of the HVPE-AlN substrate was higher than that of the PVT-AlN substrate, which was related to lower concentrations of C, O impurities, and Al vacancy.

High-power, low-efficiency-droop semipolar (202̄1̄) single-quantum-well blue light-emitting diodes

Abstract: We demonstrate a small-area (0.1 mm2) semipolar (2021) blue (447 nm) light-emitting diode (LED) with high light output power (LOP) and external quantum efficiency (EQE) by utilizing a single 12-nm-thick InGaN quantum well. The LED had pulsed LOPs of 140, 253, 361, and 460 mW, and EQEs of 50.1, 45.3, 43.0, and 41.2%, at current densities of 100, 200, 300, and 400 A/cm2, respectively. The device showed little blue shift and had a narrow full width at half maximum (FWHM). Micro-electroluminescence (µ-EL) and scanning transmission electron microscope (STEM) images indicate a high-quality InGaN quantum well (QW) layer.

Fundamental Oscillation up to 1.31 THz in Resonant Tunneling Diodes with Thin Well and Barriers

Abstract: We report the dependence of oscillation frequency on the well and barrier thicknesses in a resonant tunneling diode (RTD) terahertz oscillator integrated with a planar slot antenna. The oscillation frequency increased with decreasing well and barrier thicknesses because of the reduction in dwell time in the resonance region. Room-temperature fundamental oscillation of up to 1.31 THz with an output power of about 10 µW was achieved in the RTD with a 3.9-nm-thick well and 1.0-nm-thick barriers.

AlGaN/GaN/AlGaN Double Heterostructures Grown on 200 mm Silicon (111) Substrates with High Electron Mobility

Abstract: In this work, we demonstrate, for the first time, Al0.35GaN/GaN/Al0.25GaN double heterostructure field effect transistors on 200 mm Si(111) substrates. Thick crack-free Al0.25GaN buffer layers are achieved by optimizing Al0.75GaN/Al0.5GaN intermediate layers and AlN nucleation layers. The highest buffer breakdown voltage reaches 1380 V on a sample with a total buffer thickness of 4.6 µm. According to Van der Pauw Hall measurements, the electron mobility is 1766 cm2 V-1 s-1 and the electron density is 1.16×1013 cm-2, which results in a very low sheet resistance of 306±8 Ω/square.

Anomalous nernst effect in an L1 0-ordered epitaxial FePt thin film

Abstract: The anomalous Nernst effect in a perpendicularly magnetized L10-ordered epitaxial FePt(001) thin film has been investigated, and the anomalous Nernst coefficient and the anomalous Nernst angle of the FePt thin film were experimentally evaluated. Furthermore, the voltage due to the anomalous Nernst effect in the spin-Hall device was simulated by the finite element method. A good agreement between the experiment and the simulation was found. It was revealed that the anomalous Nernst effect could be quantitatively discussed even in nanoscale devices.

High-Electron-Mobility InN Layers Grown by Boundary-Temperature-Controlled Epitaxy

Abstract: A boundary-temperature-controlled epitaxy, where the growth temperature of InN is controlled at its maximum, is used to obtain high-electron-mobility InN layers on sapphire substrates by molecular beam epitaxy. The Hall-effect measurement shows a recorded electron mobility of 3280 cm2 V-1 s-1 and a residual electron concentration of 1.47×1017 cm-3 at room temperature. The enhanced electron mobility and reduced residual electron concentration are mainly due to the reduction of threading dislocation density. The obtained Hall mobilities are in good agreement with the theoretical modelling by the ensemble Monte Carlo simulation.

High Spin Torque Efficiency of Magnetic Tunnel Junctions with MgO/CoFeB/MgO Free Layer

Abstract: We present the results of a perpendicular magnetic tunnel junction (MTJ) that displays simultaneously low critical switching current and voltage, as well as high thermal stability factor. These results were achieved using a free layer of the MgO/CoFeB/MgO structure by increasing the spin torque efficiency to an average of 3.0 kBT/µA for 37-nm-diameter junctions, about three times that of a MgO/CoFeB/Ta free layer, which makes it the highest value reported to date. By comparing two films with different RA, hence different switching voltage and power, we explore the contributions of heating and voltage-modulated anisotropy change to the switching properties.

Stress-Induced Perpendicular Magnetization in Epitaxial Iron Garnet Thin Films

Abstract: We have grown pseudomorphic Tm3Fe5O12 films (46–350 nm in thickness) with perpendicular magnetization on (111) Gd3Ga5O12 substrates. Among various garnets, Tm3Fe5O12 is selected because of a negatively large magnetostriction constant to overcome strong shape anisotropy in very thin films. A stress-induced anisotropy field as large as +25 kOe is estimated by calculation under a moderate in-plane tensile strain of +0.49%. The magnetization hysteresis loop and magnetic domain structure indicate the perpendicular easy axis. The domain size (W) in its maze pattern varies from 500 to 960 nm with increasing thickness (t) and agrees well with a scaling law of W∝√t.

Optical Thermometry through Green Upconversion Emissions in Er 3+ /Yb 3+ -Codoped CaWO 4 Phosphor

Abstract: Under 980 nm excitation, the temperature-dependent green upconversion emissions from Er3+/Yb3+-codoped CaWO4 phosphor were studied at temperatures from 294 to 923 K. By using the fluorescence intensity ratio technique, the maximum sensitivity for temperature measurement achieved here is approximately 0.0092 K-1, which is much higher than previously reported temperature sensors based on the fluorescence of Er3+ ions in other host materials. With the efficient upconversion fluorescence, the Er3+/Yb3+-codoped CaWO4 is a very promising candidate for optical high-temperature sensors with high sensitivity and good accuracy.

Thermal Stabilization of Hole Channel on H-Terminated Diamond Surface by Using Atomic-Layer-Deposited Al2O3 Overlayer and its Electric Properties

Abstract: We have established an atomic-layer-deposited Al2O3 overlayer deposition method, which makes the H-surface-terminated p-type channel diamond surface thermally stable and completely keeps the concentration and mobility high even at 150 °C. In a range from 230 to 500 K, the mobility is proportional to the inverse of temperature showing a property characteristic for degenerate hole gas. The ionization energy is estimated to be 6.1 meV, indicating that holes are not generated mainly by thermal activation. This thermal stabilization technology enables us to measure hole properties up to 230 °C and to realize H-terminated diamond field-effect transistors with a reproducible high drain current.

High-Mobility Thin-Film Transistors with Polycrystalline In–Ga–O Channel Fabricated by DC Magnetron Sputtering

Abstract: Oxide thin-film transistors (TFTs) were fabricated using a polycrystalline In–Ga–O (IGO) thin film as the n-channel active layer by direct current magnetron sputtering. The 50-nm-thick IGO TFT showed a field-effect mobility of 39.1 cm2 V-1 s-1, a threshold voltage of 1.4 V, and a subthreshold gate voltage swing of 0.12 V/decade. The polycrystalline IGO thin film showed the cubic bixbyite structure of In2O3 without an obvious preferred orientation. The average grain size of polycrystalline IGO was approximately 10 µm. The high mobility of IGO TFT is related to the In2O3 crystalline phase and large grain size of the IGO film.

Highly Efficient Color Filter Incorporating a Thin Metal–Dielectric Resonant Structure

Abstract: A highly efficient color filter that takes advantage of an ultrathin metal (Al)-dielectric (TiO2) resonant structure, where a subwavelength metallic grating is deposited as cladding in a planar dielectric waveguide, is demonstrated. A selective spectral response was obtained by virtue of the guided mode resonance between the diffracted mode and waveguide mode. The center wavelengths for the blue, green, and red filters were found to be 430, 520, and 630 nm, and the corresponding 3 dB bandwidths were 67, 84, and 80 nm, respectively. All three filters provided an overall transmission exceeding 70% and a satisfactory color image.

21.7 kV 4H-SiC PiN Diode with a Space-Modulated Junction Termination Extension

Abstract: Ultrahigh-voltage 4H-SiC mesa PiN diodes are fabricated and characterized. An original space-modulated two-zone junction termination extension (SM-two-zone JTE) has realized a laterally tapered profile of the JTE dose, which enlarged the tolerance to the deviation of effective JTE dose compared with a conventional two-zone JTE. We demonstrate a SiC PiN diode with a breakdown voltage of 21.7 kV (81% of the ideal breakdown voltage calculated from the epilayer structure), which is the highest breakdown voltage among any semiconductor devices ever reported.

Improved absolute frequency measurement of the 171 Yb optical lattice clock towards a candidate for the redefinition of the second

Abstract: We demonstrate an improved absolute frequency measurement of the 1S0–3P0 clock transition at 578 nm in 171Yb atoms in a one-dimensional optical lattice. The clock laser linewidth is reduced to ≈2 Hz by phase-locking the laser to an ultrastable neodymium-doped yttrium aluminum garnet (Nd:YAG) laser at 1064 nm through an optical frequency comb with an intracavity electrooptic modulator to achieve a high servo bandwidth. The absolute frequency is determined as 518 295 836 590 863.1(2.0) Hz relative to the SI second, and will be reported to the International Committee for Weights and Measures.

A Mode-Locked 1.91 µm Fiber Laser Based on Interaction between Graphene Oxide and Evanescent Field

Abstract: The use of a graphene oxide (GO)-deposited D-shaped fiber as a saturable absorber for mode locking of a thulium-doped fiber laser is experimentally demonstrated. By using the evanescent field interaction of an oscillating beam with GO, a passive mode locking operation at a wavelength of ~1.91 µm is shown to be achievable from a ring cavity. Stable picosecond pulses are readily obtained at a repetition rate of 15.9 MHz. This demonstration confirms that GO is a cost-effective saturable absorber applicable for ~2 µm ultrafast pulse generation.

High-Efficiency Conversion of Threading Screw Dislocations in 4H-SiC by Solution Growth

Abstract: The high-efficiency conversion of threading screw dislocations (TSDs) in 4H-SiC by solution growth provides an efficient method of obtaining ultra high-quality SiC crystals. The behavior of TSDs on on-axis and off-axis 4H-SiC0001 seed crystals was investigated by synchrotron X-ray topography. Almost all TSDs in the off-axis Si-face seed crystal were converted to Frank-type stacking faults on the basal planes. The conversion ratio of TSDs was highly influenced by the surface polarity of the seed crystal. The stacking faults laterally propagate toward the outside of the crystal.

Near-Infrared InGaN Nanocolumn Light-Emitting Diodes Operated at 1.46 µm

Abstract: We grew a heterojunction combination of Mg-doped Ga-rich InGaN cladding and In-rich InGaN active layers on top of Si-doped GaN nanocolumns with a diameter of ~300 nm; the uniformly arranged and dislocation-free GaN nanocolumns contributed to the high crystalline quality of In-rich InGaN, which led to the longest-wavelength (146 µm) operation of InGaN light emitting diodes The In content of the active layer estimated from the emission peak wavelength was 086 The selected-area diffraction analysis in the transmission electron microscope for the InGaN layers evinced that the In contents of the Ga-rich and In-rich InGaN layers were approximately 03 and 085, respectively

Single-Wire Light-Emitting Diodes Based on GaN Wires Containing Both Polar and Nonpolar InGaN/GaN Quantum Wells

Abstract: Single-wire light-emitting diodes based on radial p–i–n multi quantum well (QW) junctions have been realized from GaN wires grown by catalyst-free metal organic vapor phase epitaxy. The InxGa1-xN/GaN undoped QW system is coated over both the nonpolar lateral sidewalls and on the polar upper surface. Cathodo- and electroluminescence (EL) experiments provide evidence that the polar QWs emit in the visible spectral range at systematically lower energy than the nonpolar QWs. The EL of the polar or nonpolar QWs can be selectively activated by varying the sample temperature and current injection level.

Diamond Junction Field-Effect Transistors with Selectively Grown n+-Side Gates

Abstract: Diamond junction field-effect transistors (JFETs) were fabricated by the selective growth of phosphorus-doped n+-type diamond. The n+ diamonds were grown at the sidewalls of a boron-doped p-type channel, and lateral pn junctions were formed under optimized conditions of microwave plasma chemical vapor deposition. We confirmed that the drain current could be well modulated by controlling the depletion layers in the p-channel, and the devices turned into the off-current state when the channel was closed by the depletion layers. JFETs showed a very low leakage current in the 10-15 A regime, high on/off ratios of 107–108, and steep subthreshold swings of 95–120 mV/decade.