Y.P. Xu

Low-temperature synthesis and photoluminescence of AlN

High-quality aluminum nitride powder was obtained through the reaction of aluminum metal with ammonia at 450 degrees C in an autoclave. The synthesized powder was characterized by X-ray powder diffraction, high-resolution transmission electron microscopy (HRTEM) and photoluminescence spectrum. X-ray diffraction and HRTEM indicated that the powder consisted of wurtzite AIN with an average size of about 32 nm. Photoluminescence spectrum confirmed the existence of a blue luminescence band in AIN due to nitrogen vacancies. A possible low-temperature synthesis mechanism was discussed

STRUCTURE AND DEBYE TEMPERATURE OF WURTZITE GaN

High Pure Wurtzite structure GaN has been synthesized by gas reaction method. Its structure was determined by powder X-ray diffraction using the Rietveld technique. The positions in the unit cell for Ga and N were refined to be (0, 0, 0) and (0, 0, 0.3814). The Debye temperature was determined as 586 K from the refined temperature factor by using the Debye approximation.

Phase relations in the Y2O3-BaO-B2O3 system

The subsolidus phase relations of the Y2O3-BaO-B2O3 system are investigated by means of X-ray diffraction (XRD) analyses. The system comprises eight three-phase regions. Nine binary compounds and four ternary compounds are found to exist. The ternary compounds of this system are BaYB9O16, Ba3YB3O9, Ba3Y2B4O12 and BaY3B3O10. The compound BaYB9O16 can be indexed by the DICVOL9I program based on a monoclinic unit cell with lattice parameters a = 15.568(2) Angstrom, b = 3.8934(8) Angstrom, c = 6.745(1) Angstrom and beta = 98.32(1)degrees. A new compound BaY3B3O10 is identified, which has a monoclinic structure with cell parameters a = 17.47(l) Angstrom, b = 4.684(5) Angstrom, c = 13.291(7) Angstrom and beta = 1 10.95(4)degrees. The compound Ba3YB3O9 has two modifications (alpha- and beta-phases), both of which crystallize in the hexagonal system but with different space groups

Thermal stability and crystal structure of β-Ba3YB3O9

A new compound, beta-Ba3YB3O9, has been attained through solid phase transition from alpha-Ba3YB3O9 at high temperatures. Differential thermal analysis (DTA) revealed the phase transition at about 1120degreesC, the melting temperature at about 1253degreesC. Its crystal structure has been determined from powder X-ray diffraction data. The refinement was carried out using the Rietveld method and the final refinement converged with R-p = 10.5% and R-wp = 13.7%. This compound belongs to the hexagonal space group R-3, with lattice parameters a = 13.0441(1) Angstrom and c = 9.5291(1) Angstrom. There are 6 formulas per unit cell and 7 atoms in the asymmetric unit. The structure of beta-Ba3YB3O9 is built up from Ba(Y)O-8, BaO6 and YB6O18 units formed by one YO6 octahedron and six BO3 triangles with shared O atoms

Nonlinear electrical characteristics and dielectric properties of Ca,Ta-doped TiO2 varistors

Abstract.Ca,Ta-doped TiO2 varistors with high nonlinear coefficients are obtained by a ceramic sintering. The nonlinear electrical and dielectric properties of the samples doped with 0.5mol% Ca and various concentrations of Ta (0.05∼2.0mol%) were investigated. The samples sintered at 1350xa0°C have nonlinear coefficients of α=5.1∼42.1 and high relative dielectric constants approach 105. The effects of Ta-doping on the nonlinear and dielectric properties of the Ca,Ta-doped TiO2 varistors are studied in greater detail. When the concentration of Ta is 0.5mol%, the sample possesses the highest nonlinear coefficient and a comparatively lower dielectric constant. The effects of Ta and the nonlinear electrical behavior of the TiO2 system are explained by analogy to a grain-boundary atomic defect model.

Regular arrays of GaN nanorods

Regular arrays of GaN nanorods have been synthesized by reacting gallium and ammonia using Au as a catalyst on a MgO single crystal substrate treated by chemical etching. They were characterized by field emission scanning electron microscopy, energy dispersive X-ray spectroscopy (EDX), selected area electron diffraction (SAED) and X-ray diffraction (XRD). EDX, XRD SAED indicated that the nanorods were wurtzite single crystal GaN. We suggest that regular tactic square shape with steps on the single crystal MgO and discontinuity of the Au film may play an important role during the formation of regular arrays of GaN nanorods.

Shock-induced brittle cracking in HVPE-GaN processed by laser lift-off techniques

A study on brittle cracking in GaN films processed by laser lift-off is presented. Two kinds of cracks were found in the N-polar face of GaN after the laser lift-off process, namely perpendicular cracks along the {1-1 0 0} planes and lateral cracks along the (0 0 0-1) plane, respectively. Single-shot laser damage is studied to understand the cracking mechanism. The damage morphology indicates that the GaN material on the edge of the laser ablation area experiences three loading modes: shear stress P-S, longitudinal compressive stress P-L and transverse tensile stress P-T. Under shock P-L, lateral cracks likely appear and extend from the illuminated region along the interface in mode I. Furthermore, two different kinds of perpendicular cracks were found, namely shear cracks (PC I) and deflection cracks (PC II). A strong P-S gives rise to PC I while a cooperative action of P-L and P-T results in PC II. In addition, there exist a critical effective spot size d(Pth) and a critical ratio of the laser spot size d(L) to the effective spot size d(P), when cracks occur over them.

High efficiency AlxGa1−xAsGaAs solar cells: Fabrication, irradiation and annealing effect

High efficiency AlxGa1-xAs/GaAs heteroface solar cells have been fabricated by an improved multi-wafer squeezing graphite boat liquid phase epitaxy (LPE) technique, which enables simultaneous growth of twenty 2.3 X 2.3cm(2) epilayers in one run. A total area conversion efficiency of 17.33% is exhibited (1sun, AM0, 2.0 x 2.0cm(2)). The shallow junction cell shows more resistance to 1 MeV electron radiation than the deep one. After isochronal or isothermal annealing the density and the number of deep level traps induced by irradiation are reduced effectively for the solar cells with deep junction and bombardment under high electron fluences.

A NOTE ON THE AlB2 TYPE STRUCTURE

In this paper, the crystal structures of a series of diborides are analyzed based on the hard ball model. We show that the packing efficiency has a close relation with the lattice parameter ratio c/a and the closest packing is achieved at c/a = 1.074. For the compounds with c/a 1.074 the packing is loose along the c-direction and the B–B bond is stretched. The structures will be unstable as the c/a value deviates from 1.074. MgB2 has the largest value of c/a among the examined diborides, except for lanthanide diborides and YB2.

RED EMISSION FROM GaN NANOCRYSTALLINE SOLIDS

The photoluminescence of the transparent GaN nanocrystallite solids is studied. Three new intensive isolated narrow red emission bands and one broad blue band are observed. A mechanism to explain the origin of these bands is proposed. The three red bands may originate from the transitions of deep donor levels to shallow acceptors, while the blue band may be from the transitions of shallow donors to deep levels with ground and excited states.