Yingqiang Xu

Blue emission and Raman scattering spectrum from AlN nanocrystalline powders

Photoluminescence and Raman scattering spectra from AIN nanocrystalline powders are studied. A blue emission band centered at 420 nm (2.95 eV) is observed. This band may be ascribed to the transition from the shallow level of V-N to the ground state of the deep level of the V-Al(3-)-3 x O-N(+) defect complexes. A phonon structure resulting fi om the transition from the shallow level to the excited states of the deep level is also observed. The broadening of peaks and the low-wave-number-shift of the phonon frequency observed on the Raman scattering spectra are the result of nano-sized effects

Growth of GaSb layers on GaAs (0 0 1) substrate by molecular beam epitaxy

GaSb 1 mu m-thick layers were grown by molecular beam epitaxy on GaAs (001). The effects of the growth conditions on the crystalline quality, surface morphology, electrical properties and optical properties were studied by double crystalline x-ray diffraction, atomic force microscopy, Hall measurement and photoluminescence spectroscopy, respectively. It was found that the surface roughness and hole mobility are highly dependent on the antimony-to-gallium flux ratios and growth temperatures. The crystalline quality, electrical properties and optical properties of GaSb layers were also studied as functions of growth rate, and it was found that a suitably low growth rate is beneficial for the crystalline quality and electrical and optical properties. Better crystal quality GaSb layers with a minimum root mean square surface roughness of 0.1 nm and good optical properties were obtained at a growth rate of 0.25 mu m h(-1).

Phase relations in the system Y2O3–CaO–B2O3

The subsolidus phase relations of the system Y2O3 -CaO-B2O3 are investigated by means of X-ray diffraction (XRD) analyses. There are six binary compounds and three ternary compounds in this system. The phase diagram comprises 13 three-phase regions. The ternary compound CaYBO4 is refined from powder XRD data by the Rietveld method. It has a warwickite-like structure with cell parameters a=10.4354(3) Angstrom, b=9.6126(2) Angstrom, c=3.5880(1) Angstrom and space group Nam. The rationality of the CaYBO4 structure is examined by Brown bond valence model and IR spectra

MBE growth of very short period InAs/GaSb type-II superlattices on (0 0 1)GaAs substrates

First, GaSb epilayers were grown on (0 0 1)GaAs substrates by molecular beam epitaxy. We determined that the GaSb layers had very smooth surfaces using atomic force microscopy. Then, very short period InAs/GaSb superlattices (SLs) were grown on the GaSb buffer layer. The optical and crystalline properties of the superlattices were studied by low-temperature photoluminescence spectra and high resolution transition electron microscopy. In order to determine the interface of SLs, the samples were tested by Raman-scattering spectra at room temperature. Results indicated that the peak wavelength of SLs with clear interfaces and integrated periods is between 2.0 and 2.6 µm. The SL interface between InAs and GaSb is InSb-like.

Structure and superconducting properties of chemically oxidized La2CuO4+y under hydrothermal conditions

Stoichiometric La2CuO4 was chemically oxidized under hydrothermal conditions in KMnO4 solutions above 100 degrees C. The structure and superconducting properties of the oxidized La2CuO4+y were studied by X-ray powder diffraction and magnetic susceptibility measurements. A large orthorhombic distortion was observed for the oxidized samples from X-ray diffraction. The oxidized La2CuO4+y shows a high superconducting transition up to 42 K. Chemical oxidation under hydrothermal conditions is effective in inserting oxygen into La2CuO4 because of equilibrium and of higher oxidation temperatures

Very high quantum efficiency in InAs/GaSb superlattice for very long wavelength detection with cutoff of 21 μm

The authors report the dependence of the quantum efficiency on beryllium concentration in the active region of type-II InAs/GaSb superlattice infrared detector with a cutoff wavelength around 21 μm. It is found that the quantum efficiency and responsivity show a clear delineation in comparison to the doping concentration. The quantum efficiency is further improved by gradually doping in the absorbing region. At 77 K, the 50% cutoff wavelength of the VLWIR detector is 18 μm, and the R0A is kept at a stable value of 6 Ω cm2. Different beryllium concentration leads to an increase of an average quantum efficiency in the 8–15 μm window from 35% to 55% with a π-region thickness of 3.0 μm, for Ubias = −0.3 V, and no anti-reflection coating. As for a further result, the quantum efficiency reaches at a maximum value of 66% by gradually doping in the absorbing region with the peak detectivity of 3.33 × 1010 cm Hz1/2/W at 15 μm.

Characteristic of rapid thermal annealing on GaIn"N…"Sb…As/GaAs quantum well grown by molecular-beam epitaxy

Effect of rapid thermal annealing on photoluminescence (PL) properties of InGaAs, InGaNAs, InGaAsSb, and InGaNAsSb quantum wells (QWs) grown by molecular-beam epitaxy was systematically investigated. Variations of PL intensity and full width at half maximum were recorded from the samples annealed at different conditions. The PL peak intensities of InGaAs and InGaNAs QWs initially increase and then decrease when the annealing temperature increased from 600to900°C, but the drawing lines of InGaAsSb and InGaNAsSb take on an “M” shape. The enhancement of the PL intensity and the decrease of the full width at half maximum in our samples are likely due to the removal of defects and dislocations as well as the composition’s homogenization. In the 800–900°C high-temperature region, interdiffusion is likely the main factor influencing the PL intensity. In–N is easily formed during annealing which will prevent In out diffusion, so the largest blueshift was observed in InGaAsSb in the high-temperature region.

A structural investigation of high-quality epitaxial thin films

The (PZT 53/47) and (PZT 20/80) films deposited on substrates by RF magnetron sputtering are single-phase epitaxial films. In order to prevent large thermal stresses being induced in the films during the preparation process, the PZT films were cooled slowly through the Curie temperature. Scanning electron microscopy (SEM) showed that the PZT 53/47 film had a pyramid structure on its surface, whereas the PZT 20/80 film had a smooth surface without any observable features. Transmission electron microscopy (TEM) observations showed similar features: the PZT 53/47 film had a coarse columnar structure, whereas the PZT 20/80 film had a fine crystalline structure; this is due to the better lattice matching in the latter.

The microstructure of and films

(PZT 53/47) is a polycrystalline film with coarse square-shaped grains. The surface morphology can be improved by reducing the cooling rate at the Curie temperature region; however this morphology still does not meet the requirement of device engineering. (PZT 20/80) film has been synthesized, to reduce the lattice mismatch between PZT and substrate. This film has a smooth surface without any evident crystalline features. A TEM of the cross-sectional (YBCO) integrated film shows there is no interaction between the PZT, YBCO and substrate. particles are detected in the PZT film.

Molecular beam epitaxy growth of high electron mobility InAs/AlSb deep quantum well structure

InAs/AlSb deep quantum well (QW) structures with high electron mobility were grown by molecular beam epitaxy (MBE) on semi-insulating GaAs substrates. AlSb and Al0.75Ga0.25Sb buffer layers were grown to accommodate the lattice mismatch (7%) between the InAs/AlSb QW active region and GaAs substrate. Transmission electron microscopy shows abrupt interface and atomic force microscopy measurements display smooth surface morphology. Growth conditions of AlSb and Al0.75Ga0.25Sb buffer were optimized. Al0.75Ga0.25Sb is better than AlSb as a buffer layer as indicated. The sample with optimal Al0.75Ga0.25Sb buffer layer shows a smooth surface morphology with root-mean-square roughness of 6.67 A. The electron mobility has reached as high as 27 000 cm2/Vs with a sheet density of 4.54 × 1011/cm2 at room temperature.