Wu Xiao-Shan

Effects of Yb-doping on flux pinning properties in YBa2Cu3O7-δ

Abstract The effects of Yb-doping on the critical current density (Jc) and the critical temperature (Tc) in YBa2Cu3O7-δ (YBCO) were investigated. Rietveld refinements of X-ray diffraction showed that Yb3+ ions had partly substituted Y sites in YBCO and the unit cell volume V decreased monotonically as x increasing. Although Tc almost had no change with increasing Yb-concentration, the increase of Jc occurred as a field was applied and displayed a maximum at x=0.08. We argued that the formation of nanometric inhomogeneity may lead to the enhancement of pinning force.

Crystal Tilts in Epitaxially Laterally Overgrown GaN Films Determined by Four-Circle X-Ray Diffraction

Crystal tilts in epitaxially laterally overgrown (ELO) GaN films via hydride vapour phase epitaxy (HVPE) on sapphire substrates have been investigated by using the four-circle x-ray diffraction method. Three diffraction peaks corresponding to the (0002) reflection of vertically epitaxial and tilted GaN domains are observable in the x-ray rocking curve. The angle separations Δω between the main peak and two lobes change with the azimuth angle . The dependence of Δω on and the crystal tilt angle θ has been calculated based on the standard kinetic x-ray diffraction model. The crystal tilt angle of a typical HVPE ELO GaN sample has been determined to be 2.379°.

Epitaxial growth of Ge1–xSnx films with x up to 0.14 grown on Ge (00l) at low temperature

We characterize the structures of Ge1-xSnx films with x up to 0.14 grown on Ge (00l) by molecular-beam epitaxy at low temperature. The results show that Ge1-xSnx films are fully strained even at high Sn composition. The in-plane lattice parameters remain exactly the same as that of the substrate. Depth sensitivity analysis of the lattice parameters indicates that the strains of the epitaxial films are all in homogeneity. The films are fully strained. Poisson ratios, the force constants for the bonds between Ge and Sn are estimated and discussed in the present paper. Raman results show Ge-Ge, Ge-Sn, Sn-Sn vibrational modes. The Sn-Sn bond aggregation may respond to the high quality of our films. The fully strained epitaxy films with high content of Sn may be useful in designing the high quality GeSn films.

Structural distortions and magnetisms in Fe-doped LaMn1−xFexO3 (0 < x ≤ 0.6)

X-ray absorption spectra (XAS) at Mn K-edge and Fe K-edge in LaMn1−xFexO3 show that with the increase of Fe substitution the chemical valence of Mn4+ decreases, while the chemical valence of Fe3+ remains unchanged. Structural distortions, such as the rotating and tilting for oxygen octahedron in the unit cell vary with iron content. A phase transition occurs at the Fe content values of 0.2~0.3. The evolutions of rotation and tilting angle of FeO6/MnO6 octahedral may be the vital factors to the structure and magnetism. We believe that the spin configuration of Fe3+ may vary from the intermediate spin t42ge1g (S = 3/2) to the higher spin t32geg2 (S = 5/2) near the phase transition.

An X-Ray Diffraction and Thermogravimetric Study of Layered Perovskite Y1−xBixBaCo4O7

Layer-structured oxides Y1−xBixBaCo4O7 (0.00 ≤ x ≤ 0.05) were successfully synthesized and their structural and oxygen absorption properties were investigated by x-ray diffraction and thermogravimetry. Though Bi solubility was limited to about 5%, corresponding to Y0.95Bi0.05BaCo4O7, it is found that the structure and oxygen absorption properties of Y1−xBixBaCo4O7 are affected significantly as the Bi content x increases. Rietveld refinement results show that Y1−xBixBaCo4O7 (x ≤ 0.05) is single phase with a hexagonal crystal structure (space group P63zmc). Unit cell parameters and volume are changed and CoO4 tetrahedra are distorted along the c-axis in Bi doped YBaCo4O7. The TG results show that Y1−xBixBaCo4O7 undergoes two oxygen absorption processes in oxygen from room temperature to 1000°C and the maximum mass increase of the doped samples is less than that of YBaCo4O7. Bi doping effects on the structure and oxygen absorption properties are discussed on the basis of average radius and disorder of the Y site, the valence of Bi and the oxygen activation energy.

Effect of Yb2O3 additives on structure and transport properties of YBa2Cu3O7-δ

Abstract The effects of Yb 2 O 3 particles on the microstructure and transport properties of YBa 2 Cu 3 O 7-δ (YBCO) were investigated through resistance-temperature (R-T), scanning electron microscope (SEM), X-ray diffraction (XRD) and the critical current density ( J c ) versus applied magnetic field ( J c -B) measurements. YBCO powder with Yb 2 O 3 additives was synthesized using standard solid state reaction technique. Rietveld refinements of X-ray diffraction showed that both Yb 2 O 3 and YBCO phases coexisted in the products. The critical current density ( J c ) exhibited a maximum at x =0.05 in YBCO+ x Yb 2 O 3 systems. Moreover, small additions of Yb 2 O 3 in YBCO could enhance J c by applying the magnetic field, although the superconducting temperature ( T c ) decreased monotonically with increasing additive content. The characteristic behavior of J c might come from the counterbalance of two effects simultaneously, which was caused by the inhomogeneity of grains distribution in nano-scale and degradation on superconducting properties.

Conversion Efficiency Enhancement of Multi-crystalline Si Solar Cells by Using a Micro-structured Junction*

A new approach for enhancing the conversion efficiency of solar cells is proposed. A surface with columnar structures is employed on the fabrication of multi-crystalline silicon solar cells, for increasing the collection probability of photon-generated minority carriers in the cells. For comparison, three types of surfaces (planar surface, surfaces with columns of 12 μm in radius and columns of 9 μm in radius, respectively) are used. It is demonstrated that the cells with columnar structured surfaces have better spectral response and higher efficiencies than the cells with planar surface, while the cells with columns of 9 μm in radius show better spectral response than the cells with columns of 12 μm in radius. However, the cells with columns of 12 μm exhibit higher efficiencies than the cells with columns of 9 μm in radius for their difference in fill factors. Moreover, the effect of the columnar structured surface on the minority carrier collection efficiency is verified with wafers of different minority carrier lifetimes (0.5 μs and 1.0 μs). This work may significantly consider its potential in the manufacturing of high-efficiency solar cells at low cost by using low quality materials.

Thermal Vibration and Twist Induced Semiconducting Behaviour in Short DNA Wires

We study the transport properties of electrons in a short homogeneous DNA molecule where thermal vibrations and twist fluctuations of the base molecules are considered. The nonlinear current–voltage curves can be derived by using the equivalent single-particle multichannel network. The voltage gap is sensitive to the strength of thermal vibrations and twist fluctuations of the base molecules. Our results are in good agreement with the recent finding of semiconducting behaviour in short poly(G)-poly(C) DNA oligomers. The present method can also be used to calculate the other molecular wires.

Microstructural stability of NiO-containing spin valves annealed at room temperature

Microstructure of NiO-containing Co/Cu/Co spin valves (CCC-SV) annealed at room temperature for nearly four years has been studied by synchrotron radiation X-ray diffraction. With the annealing time expanding, the thickness of each sub-layer remains nearly unchanged while the interface roughness varies obviously compared with that of samples without annealing. The roughness at the interface of NiO/Co decreases with the annealing time increasing for both of the samples with NiO layer on the top (TSV) and under the bottom (BSV) of CCC-SV. On the other hand, the roughness at Co/Cu interface increases with the annealing time expanding for BSV while it decreases for TSV. These results indicate that the structure of TSV is more stable than that of BSV.

Effect of Annealing on Thermal Expansion Behavior of Free-Standing GaN

Abstract During the rapid evolution progress of GaN based optoelectronic devices, GaN bulk material with high crystal quality is regarded as the hope of breakthrough for their efficiency and lifetime. In present work, free-standing GaN films with different thickness were obtained with laser lift-off (LLO) technique after the growth by hydride vapor phase epitaxy (HVPE). The samples were annealed at 1500 K. The temperature dependent lattice parameters of all samples were measured with high resolution X-ray diffractometer (HRXRD). The temperature dependent Raman scattering spectra were measured to analyze the stress in GaN films. The effect of annealing on the changing of thermal expansion coefficient of GaN was studied and the release of bending and residual strain of GaN was characterized.