Showing papers in "Applied Physics Express in 2015"

Homoepitaxial growth of β-Ga2O3 layers by halide vapor phase epitaxy

Abstract: Thick high-purity β-Ga2O3 layers of high crystalline quality were grown homoepitaxially by halide vapor phase epitaxy (HVPE) using gaseous GaCl and O2 on (001) β-Ga2O3 substrates prepared by edge-defined film-fed growth. The surface morphology and structural quality of the grown layer improved with increasing growth temperature. X-ray diffraction ω-rocking curves for the (002) and (400) reflections for the layer grown at 1000 °C had small full widths at half maximum. Secondary ion mass spectrometry and electrical characteristics revealed that the growth of high-purity β-Ga2O3 layers with low effective donor concentration (Nd − Na < 1013 cm−3) is possible by HVPE.

1.8 mΩ·cm2 vertical GaN-based trench metal–oxide–semiconductor field-effect transistors on a free-standing GaN substrate for 1.2-kV-class operation

Abstract: In this paper, we report on 1.2-kV-class vertical GaN-based trench metal–oxide–semiconductor field-effect transistors (MOSFETs) on a free-standing GaN substrate with a low specific on-resistance. A redesigned epitaxial layer structure following our previous work with a regular hexagonal trench gate layout enables us to reduce the specific on-resistance to as low as 1.8 mΩcm2 while obtaining a sufficient blocking voltage for 1.2-kV-class operation. Normally-off operation with a threshold voltage of 3.5 V is also demonstrated. To the best of our knowledge, this is the first report on vertical GaN-based MOSFETs with a specific on-resistance of less than 2 mΩcm2.

High-mobility β-Ga2O3() single crystals grown by edge-defined film-fed growth method and their Schottky barrier diodes with Ni contact

Abstract: High-Hall-electron-mobility and high-performance Schottky barrier diodes for edge-defined fed-grown () β-Ga2O3 single crystals have been demonstrated. A high electron mobility of 886 cm2/(Vs) at 85 K was obtained. By theoretical specific scattering mechanisms, it was found that the electron mobility for >200 K is limited by optical phonon scattering and that for <100 K by ionized impurity scattering. On Schottky barrier diodes with Ni contacts, the current density for the forward voltage was 70.3 A/cm2 at 2.0 V, and a nearly ideal ideality factor of 1.01 was obtained.

Heterojunction solar cell with 6% efficiency based on an n-type aluminum–gallium–oxide thin film and p-type sodium-doped Cu2O sheet

Abstract: In this paper, we describe efforts to enhance the efficiency of Cu2O-based heterojunction solar cells fabricated with an aluminum–gallium–oxide (Al–Ga–O) thin film as the n-type layer and a p-type sodium (Na)-doped Cu2O (Cu2O:Na) sheet prepared by thermally oxidizing copper sheets. The optimal Al content [X; Al/(Ga + Al) atomic ratio] of an AlX–Ga1−X–O thin-film n-type layer was found to be approximately 2.5 at. %. The optimized resistivity was approximately 15 Ω cm for n-type AlX–Ga1−X–O/p-type Cu2O:Na heterojunction solar cells. A MgF2/AZO/Al0.025–Ga0.975–O/Cu2O:Na heterojunction solar cell with 6.1% efficiency was fabricated using a 60-nm-thick n-type oxide thin-film layer and a 0.2-mm-thick Cu2O:Na sheet with the optimized resistivity.

Halide vapor phase epitaxy of twin-free α-Ga2O3on sapphire (0001) substrates

Abstract: The halide vapor phase epitaxy of α-Ga2O3 is demonstrated for the first time. The films are twin-free and heteroepitaxially grown on sapphire (0001) substrates using gallium chloride and oxygen as precursors. X-ray ω–2θ and pole figure measurements reveal that the film is single-crystalline (0001) α-Ga2O3 with no detectable formation of β-Ga2O3. The optical bandgap is determined to be 5.16 eV based on the transmittance spectrum. The growth rate monotonically increases with the partial pressures of the raw material gases, reaching approximately 150 µm/h, which is over two orders of magnitude larger than those of conventional vapor phase epitaxial growth techniques, such as mist CVD or MBE.

Cu2SnS3 thin-film solar cells fabricated by sulfurization from NaF/Cu/Sn stacked precursor

Abstract: Cu2SnS3 thin films were prepared by crystallization in a sulfur/tin mixing atmosphere from stacked NaF/Cu/Sn precursors deposited by the sequential evaporation of Sn, Cu elements, and NaF. The NaF mole ratio was changed at (x = 0 to 0.12). From X-ray diffraction patterns and Raman spectra, the Cu2SnS3 thin films were considered to have a monoclinic structure. The grain size of the Cu2SnS3 thin films decreased with increasing NaF/Cu mole ratio. The band-gap energies of the Cu2SnS3 thin films determined from quantum efficiency spectra were 0.93 and 1.02 eV. The solar cell with x = 0.075 demonstrated the best performance, namely, Voc = 283 mV, Isc = 37.3 mA/cm2, FF = 0.439, and ? = 4.63%.

Ultra-broadband dual-comb spectroscopy across 1.0–1.9 µm

Abstract: We have carried out dual-comb spectroscopy and observed in a simultaneous acquisition a 140-THz-wide spectrum from 1.0 to 1.9 µm using two fiber-based frequency combs phase-locked to each other. This ultrabroad-wavelength bandwidth is realized by setting the difference between the repetition rates of the two combs to 7.6 Hz using the sub-Hz-linewidth fiber combs. The recorded spectrum contains five vibration-rotation bands of C2H2, CH4, and H2O at different wavelengths across the whole spectrum. The determined transition frequencies of C2H2 agree with those from the previous sub-Doppler resolution measurement of individual lines using CW lasers within 2 MHz.

A 24.4% solar to hydrogen energy conversion efficiency by combining concentrator photovoltaic modules and electrochemical cells

Abstract: The highest efficiency of 24.4% for the solar-to-hydrogen (STH) energy conversion was obtained in an outdoor field test by combining concentrator photovoltaic (CPV) modules with InGaP/GaAs/Ge three-junction cells and polymer-electrolyte electrochemical (EC) cells. The high efficiency was obtained by using the high-efficiency CPV modules (~31% under the present operation conditions) and the direct connection between the CPV modules and the EC cells with an almost optimized number of elements in series. The STH efficiency bottleneck was clarified to be the efficiency of the CPV modules, the over-potential of the EC cells, and matching of the operation point to the maximal-power point of the CPV modules.

Manifestation of unexpected semiconducting properties in few-layer orthorhombic arsenene

Abstract: In this letter, we demonstrate that few-layer orthorhombic arsenene is an ideal semiconductor. Owing to the layer stacking, multilayer arsenenes always behave as intrinsic direct bandgap semiconductors with gap values of approximately 1 eV. In addition, these bandgaps can be further tuned in its nanoribbons. Based on the so-called acoustic phonon limited approach, the carrier mobilities are predicted to approach as high as several thousand square centimeters per volt–second and to simultaneously exhibit high directional anisotropy. All these characteristics make few-layer arsenene promising for device applications in the semiconducting industry.

Epitaxial stabilization of pseudomorphic α-Ga2O3on sapphire (0001)

Abstract: Heteroepitaxial Ga2O3 was grown on c-plane sapphire by molecular beam epitaxy, pulsed-laser deposition, and metalorganic chemical vapor deposition. Investigation by scanning transmission electron microscopy (STEM) revealed the presence of a three-monolayer-thick pseudomorphically grown layer of trigonal α-Ga2O3 at the interface between the c-plane sapphire substrate and the β-Ga2O3 independent of the growth method. On top of this pseudomorphically grown layer, plastically relaxed monoclinic β-Ga2O3 grew in the form of rotational domains. We rationalize the stable growth of the high-pressure trigonal α-phase of Ga2O3 in terms of the stabilization of the α-Ga2O3 phase by the lattice-mismatch-induced strain.

Conversion of sound radiation pattern via gradient acoustic metasurface with space-coiling structure

Abstract: We designed, fabricated, and experimentally demonstrated a gradient acoustic metasurface to manipulate sound radiation patterns. The gradient metasurface is constructed on the basis of a coiling-up space in a tunable interdigitated structure, which exhibits relative refractive index in a discretized classic hyperbolic secant profile. Capable of generating secondary sound sources with desired gradient phase shifts, the metasurface shows the ability of controlling sound radiation such as by cylindrical-to-plane-wave conversion, plane wave focusing, and effective tunable acoustic negative refraction. Owing to its deep-subwavelength thickness, the metasurface may reduce the size of acoustic devices and offer potential applications in imaging and scanning systems.

50 A vertical GaN Schottky barrier diode on a free-standing GaN substrate with blocking voltage of 790 V

Abstract: This paper reports on vertical GaN Schottky barrier diodes (SBDs) fabricated on a free-standing GaN substrate with different sizes of Schottky electrode. The fabricated SBDs with 3 ? 3 mm2 Schottky electrodes exhibited both a forward current of 50 A and a blocking voltage of 790 V. To our knowledge, the characteristics of operation with a simultaneous high forward current and high blocking voltage are reported for the first time for vertical GaN SBDs on free-standing GaN substrates. The dependence of these characteristics on the Schottky electrode size is also reported in detail.

Fano resonance and plasmon-induced transparency in waveguide-coupled surface plasmon resonance sensors

Abstract: A planar multilayer structure that allows coupling between surface plasmon polaritons and waveguide modes is proposed. Calculated reflectivity curves exhibit sharp resonances due to the Fano resonance and plasmon-induced transparency arising from the coupling. Electric field profiles obtained at the resonances demonstrate the hybrid nature of the modes excited. When the Fano resonance is used for sensing, the sensitivity with intensity modulation is enhanced by two orders of magnitude relative to that of conventional surface plasmon resonance sensors.

CuSbSe2 photovoltaic devices with 3% efficiency

Abstract: Recent technical and commercial successes of existing thin-film solar cell technologies encourage the exploration of next-generation photovoltaic (PV) absorber materials. Of particular scientific interest are compounds that do not exhibit conventional tetrahedral semiconductor bonding, such as CuSbSe2. CuSbSe2 has a 1.1 eV optical absorption onset, a 105 cm−1 absorption coefficient, and a hole concentration of 1017 cm−3. Here, we demonstrate CuSbSe2 PV prototypes with efficiencies >3%, prepared by a self-regulated sputtering process using a conventional substrate device architecture. Bulk recombination, device engineering issues, and a nonideal CuSbSe2/CdS band offset likely limit the promising initial result.

High-sensitivity plasmonic temperature sensor based on photonic crystal fiber coated with nanoscale gold film

Abstract: We demonstrate a photonic crystal fiber temperature sensor based on surface plasmon resonance and evaluate it using the finite element method. A temperature-sensitive material is injected into the central air hole of the photonic crystal fiber. The air hole is coated with nanoscale gold film. Six cores are formed by removing air holes in the second layer, which supports the core mode. The coupling between the core mode and the surface plasmon polariton mode occurs as the phase matching condition is satisfied. The average sensitivity and linearity become −2.15 nm/°C and 0.99991, respectively. The length of this fiber is only 1 mm. Our temperature sensor is competitive within the temperature sensor field.

Perpendicular magnetic anisotropy of Ir/CoFeB/MgO trilayer system tuned by electric fields

Abstract: The perpendicular magnetic anisotropy of an Ir/CoFeB/MgO trilayer was investigated after annealing at temperatures ranging from 200 to 350 °C. In the trilayer system annealed at 300 °C, we measured an interface anisotropy energy of 1.9 mJ/m2. Further annealing led to mixing of the buffer and ferromagnet, degrading the properties of the latter. In addition, we show the dependence of the magnetic anisotropy on the bias voltage. The presented system is important for the development of perpendicular magnetic tunnel junctions for storage applications.

Room-temperature continuous-wave operation of GaN-based vertical-cavity surface-emitting lasers fabricated using epitaxial lateral overgrowth

Abstract: We have successfully demonstrated the room-temperature continuous-wave operation of GaN-based vertical-cavity surface-emitting lasers (VCSELs) with all-dielectric reflectors, which were fabricated using epitaxial lateral overgrowth. The VCSELs exhibited a threshold current of 8 mA and a threshold voltage of 4.5 V at a lasing wavelength of 446 nm. The maximum output power was 0.9 mW for an 8-µm-diameter current aperture, which was made possible because of the high thermal conductivity of the GaN substrate.

Deep-red persistent luminescence in Cr3+-doped LaAlO3 perovskite phosphor for in vivo imaging

Abstract: We report deep-red long-lasting persistent luminescence in Cr3+-doped LaAlO3 perovskite phosphor synthesized by solid-state reaction. The LaAlO3:Cr3+ sample showed persistent luminescence peaking at a very long wavelength of 734 nm due to Cr3+:2E → 4A2 transition after ultraviolet excitation. The Sm3+ ion was found to be a good codopant for increasing the persistent luminescence intensity more than 35-fold. For the Cr3+–Sm3+-codoped LaAlO3 sample, the radiance of persistent luminescence in mW sr−1 m−2 unit was higher than or comparable to that of ZnGa2O4:Cr3+ phosphor, which is a candidate phosphor with a peak luminescence at 694 nm for in vivo imaging application.

Voltage induction of interfacial Dzyaloshinskii–Moriya interaction in Au/Fe/MgO artificial multilayer

Abstract: The Dzyaloshinskii–Moriya interaction (DMI) is an antisymmetric exchange interaction that plays a decisive role in the formation of chiral magnetic structures and in the determination of magnetoelectric properties. This study investigated the impact of an external voltage on the magneto-static surface waves in a Au/Fe/MgO multilayer. Spin waves were excited and detected using two coplanar waveguides and controlled by an external DC voltage. The DC bias voltage dependence of the resonant frequency in the spin waves revealed that the voltage effect has both directionally symmetric and asymmetric components, signifying voltage control of both interfacial magnetic anisotropy and interface-DMI.

Negative Poisson’s ratios in few-layer orthorhombic arsenic: First-principles calculations

Abstract: Using first-principles calculations we demonstrate for the first time that few-layer orthorhombic arsenic possesses a negative Poisson's ratio. For a single layer of arsenic, the negative Poisson's ratio is predicted to be ~−0.09. As the number of layers increases, the magnitude of the negative Poisson's ratio increases and finally approaches a limit at four layers, becoming very close to the bulk value of −0.13. To understand these layer-dependent negative Poisson's ratios, we propose a rigid mechanical model in which the intra-layer bond lengths and the normal Poisson's ratio of the in-layer plane play key roles.

Y3Al5−xGaxO12:Cr3+: A novel red persistent phosphor with high brightness

Abstract: We report on red persistent phosphors of Y3Al5−xGaxO12:Cr3+ garnet (YAGG:Cr3+, x from 0 to 5). In these materials, Cr ions act as both emission centers and electron traps. The trap depth, which is regarded as the energy gap between the bottom of the conduction band and the electron trap, can be optimized by modifying the Ga3+ substitution content (x). After ceasing UV illumination, the persistent luminescence of the YAGG:Cr3+ (x = 3) phosphor was nearly 5 times higher than that of the widely used ZnGa2O4:Cr3+ red persistent phosphor. Such novel red persistent phosphors have great potential for an improved in vivo bioimaging application.

Improvement of the open-circuit voltage of Cu2ZnSnS4 solar cells using a two-layer structure

Abstract: In Cu2ZnSnS4 (CZTS) photovoltaic cells, a low open-circuit voltage (VOC) principally causes low conversion efficiency. We investigated the deposition of a CZTS layer by a two-layer process to improve the VOC of the CZTS cells. In this process, the first CZTS layers near a Mo electrode have a high Cu content and the second layer near the surface has a low Cu content. The two-layer process improved the VOC of the CZTS cells from 0.66 to 0.78 V. Finally, the best CZTS cell showed a conversion efficiency of 8.8%.

Analysis of efficiency droop in 280-nm AlGaN multiple-quantum-well light-emitting diodes based on carrier rate equation

Abstract: The efficiency droop in 280-nm AlGaN multiple-quantum-well (MQW) ultraviolet (UV) light-emitting diodes (LEDs) is analyzed using the carrier rate equation. It is shown that the internal quantum efficiency (ηIQE), injection efficiency (ηinj), light-extraction efficiency (ηLEE), Shockley–Read–Hall recombination coefficient (A), and Auger coefficient (C) can be determined by the carrier rate equation using the theoretical radiative recombination coefficient (B), experimentally measured wavelength spectrum, and external quantum efficiency (ηEQE). The results show that the carrier spillover from the MQWs to the p-AlGaN layer is the main cause of the efficiency droop.

Robust micromagnet design for fast electrical manipulations of single spins in quantum dots

Abstract: Tailoring spin coupling to electric fields is central to spintronics and spin-based quantum information processing. We present an optimal micromagnet design that produces appropriate stray magnetic fields to mediate fast electrical spin manipulations in nanodevices. We quantify the practical requirements for spatial field inhomogeneity and tolerance for misalignment with spins, and propose a design scheme to improve the spin-rotation frequency (to exceed 50 MHz in GaAs nanostructures). We then validate our design by experiments in separate devices. Our results will open a route to rapidly control solid-state electron spins with limited lifetimes and to study coherent spin dynamics in solids.

Room-temperature operation of Si spin MOSFET with high on/off spin signal ratio

Abstract: We experimentally demonstrate a Si spin metal–oxide–semiconductor field-effect transistor (MOSFET) that exhibits a high on/off ratio of source–drain current and spin signals at room temperature. The spin channel is nondegenerate n-type Si, and an effective application of gate voltage in the back-gated structure allows the spin MOSFET operation. This achievement can pave the way for the practical use of the Si spin MOSFET.

Perpendicular magnetic tunnel junctions with strong antiferromagnetic interlayer exchange coupling at first oscillation peak

Abstract: We systematically developed a perpendicularly magnetized synthetic antiferromagnetically (p-SAF) coupled reference structure with strong interlayer exchange coupling (IEC) at the "first peak", where the thickness of the Ru layer between the Co/Pt superlattice layers was 0.4–0.5 nm. A high IEC energy density (2.2 erg/cm2) and a high magnetoresistance ratio (110–150%) with an ultralow resistance-area product (2–5 Ωµm2) were simultaneously achieved in the integrated magnetic tunnel junctions of the p-SAF structure. The use of p-SAF coupling with strong IEC is attractive because of its ability to produce highly stable and unidirectional reference layers for memory applications.

Optical antennas for tunable enhancement in tip-enhanced Raman spectroscopy imaging

Abstract: The use of optical antennas in tip-enhanced Raman spectroscopy (TERS) makes it a powerful optical analysis and imaging technique at the nanoscale. Optical antennas can work as nano-light sources in the visible region. The plasmonic resonance of an antenna depends on its length; thus, by varying the length, one can control the enhancement in TERS. In this study, we demonstrated a fabrication method based on focused ion beam milling to realize optical antennas with desired lengths. We then measured the resonances of these fabricated antennas and performed TERS imaging of carbon nanotubes to demonstrate the antenna length dependence on plasmonic resonance.

Phonon transport in perovskite SrTiO3 from first principles

Abstract: We investigate phonon transport in perovskite strontium titanate (SrTiO3), which is stable above its phase transition temperature (~105 K), by using first-principles molecular dynamics and anharmonic lattice dynamics. Unlike conventional ground-state-based perturbation methods that give imaginary phonon frequencies, the current calculation reproduces stable phonon dispersion relations observed in experiments. We find that the contribution of optical phonons to the overall lattice thermal conductivity is larger than 60%, which is markedly different from the usual picture with the dominant contribution from acoustic phonons. Mode-dependent and pseudopotential-dependent analyses suggest the strong attenuation of acoustic phonon transport originating from strong anharmonic coupling with the transversely polarized ferroelectric modes.

Highly enhanced piezoelectric property of co-doped AlN

Abstract: We propose a series of new compositions of AlN-based piezoelectric material based on the results of first-principles calculation. The composition is expressed by Al1−xN, where the elements α and β are selected to maintain the charge neutrality of the host AlN. We found that the selection of Mg2+ for α and Hf4+ and Zr4+ for β with y = 0.5 show good chemical stability with much better piezoelectric properties than pure AlN. Our results indicate broad compositional freedom for improving the piezoelectric properties of AlN by using the co-doping technique.

Fabrication of vertical Schottky barrier diodes on n-type freestanding AlN substrates grown by hydride vapor phase epitaxy

Abstract: Thick Si-doped AlN layers were homoepitaxially grown by hydride vapor phase epitaxy on AlN(0001) seed substrates. Following the removal of the seed substrate, an n-type AlN substrate with a carrier concentration of 2.4 × 1014 cm−3 was obtained. Vertical Schottky barrier diodes were fabricated by depositing Ni/Au Schottky contacts on the N-polar surface of the substrate. High rectification with a turn-on voltage of approximately 2.2 V was observed. The ideality factor of the diode at room temperature was estimated to be ~8. The reverse breakdown voltage, defined as the leakage current level of 10−3 A/cm2, ranged from 550 to 770 V.