Z.H. Mai

Fabrication and characterization of heteroepitaxial bilayers of La-Ca-Mn-O/Y-Ba-Cu-O

We have fabricated La0.67Ca0.33MnO3/YBa2Cu3Oy heteroepitaxial bilayers on SrTiO3 (1 0 0) substrates by rf magnetron sputtering. The La0.67Ca0.33MnO3/YBa2Cu3Oy bilayers were characterized by X-ray diffraction, rocking curve measurements, scanning electron microscope and dc four-probe measurements. Both La0.67Ca0.33MnO3 and YBa2Cu3Oy layers show excellent crystallinity indicated. by their sharp and narrow rocking curves. The La0.67Ca0.33MnO3/YBa2Cu3Oy bildyers exhibit a very smooth surface. The relation between the superconducting critical current density of YBa(2)Cu(3)O(y)and temperature was alsoinvestigated

Thickness dependence of microstructures in La0.8Ca0.2MnO3 thin films

The thickness dependence of microstructures of La0.8Ca0.2MnO3 (LCMO)∕SrTiO3 (STO) thin films was investigated by high-resolution x-ray diffraction, small angle x-ray reflection, grazing incidence x-ray diffraction, scanning electron microscopy, and atomic force microscopy. The results show that all the LCMO films are well oriented in (00l) direction perpendicular to the substrate surface. Self-organized crystalline grains with a tetragonal shape are uniformly distributed on the film surface, indicating the deposition condition being of benefit to the formation of the crystalline grains. With increasing the film thickness, the crystalline quality of the LCMO film is improved, while the surface becomes rougher. There exists a nondesigned cap layer on the upper surface of the LCMO layer for all the samples. The mechanism is discussed briefly.

Microstructural characterization of YBa2Cu3O7-x thin films grown on Y-ZrO2

The effect of chemical plating with Ni-Co-P alloy on the properties of MH electrodes is investigated. The results show that the efficiency of storage alloy and the activation of MH electrode have been improved by introducing 1.74% cobalt in the Ni-Co-P alloy coating. The initial discharge capacity is 208 mAh/g, The maximum discharge capacity gets to 298.5 mAh/g. At the same time the cycle life of MH electrodes is improved. The discharge capacity of MH electrodes coated with Ni-Co-P is 88% of the maximum discharge capacity after 300 cycles. Whereas the discharge capacity of bare alloy electrodes retains 62% of the maximum capacity after 300 cycles. An increment of discharge capacity is mainly due to the superposition of the oxidation current of Co as well as improved efficiency of microcurrent collection. The effect of Ni-Co-P alloy coating by electroless plating on the kinetic properties of hydride electrode has been systematically investigated by electrochemical techniques. The results indicate that the kinetic properties of MH electrodes, including exchange current density, limiting current density, have been improved markedly. This improvement of kinetic properties leads to the decrease of the overpotential of anodic and cathodic polarization.

Microstructures of epitaxial thin films of YBa2Cu3O7?? deposited on silicon wafers

The microstructures of YBa2Cu3O7-delta (YBCO) thin films deposited on Si(001) wafers with an yttrium-stabilized ZrO2 (YSZ) single buffer or/and an Eu2CuO4(ECO)/YSZ double buffer layer were investigated by using high-resolution x-ray diffraction, small angle reflection and scanning electron microscopy (SEM), respectively. The results showed that the YBCO films were well oriented in the [001] direction perpendicular to the substrate surface. A thin intermediate layer of BaZrO3 was observed at the interface of YBCO/YSZ for samples with a YSZ single buffer. The composition of the YBCO film was also diverse from the 1-2-3 compound. In contrast, a well-defined interface was formed for samples with an ECO/YSZ double buffer. The film surface was more smooth and stable. The composition of the YBCO films was also found to approach that of the 1-2-3 compound. The results obtained indicate that highly epitaxial YBCO thin films could be successfully grown on Si wafers with a double buffer of ECO/YSZ. demonstrating advantages of such a double buffer structure.

Excellent buffer layer for growing high-quality Y-Ba-Cu-O thin films

Eu2CuO4 (ECO) has been used as a buffer layer for growing of YBa2Cu3O7-delta (YBCO) thin films on SrTiO3(100) and Y-stabilized ZrO2(100) substrates. The epitaxy, crystallinity, and surface of YBCO thin films have been significantly improved by using ECO buffer layer as investigated by x-ray diffraction, rocking curves, scanning electron microscope, surface step profiler, and x-ray small-angle reflection. The best value of the full width at half-maximum of the YBCO(005) peak can be greatly reduced down to less than 0.1 degrees. The scanning-electron-microscope photos indicate a very smooth surface for the YBCO thin films. The average roughness is less than 5 nm over a wide scanning region of 2000 mum. The results of x-ray small-angle reflection indicate a very clear and flat interface between YBCO and ECO layers. Meanwhile, the resistivity of ECO is about 20 times higher than that of PrBa2Cu3Oy at the boiling point of liquid nitrogen. Our results suggest that ECO should be a good barrier candidate for fabricating high-T-c, superconductor junctions.

Effect of strain on the transport and magnetoresistance properties of La0.8Ca0.2MnO3 epitaxial thin films

The true residual stress in La0.8Ca0.2MnO3 (LCMO) thin films of various thicknesses deposited on STO substrates under the same deposition conditions was measured quantitatively by x-ray diffraction sin2ψ method. The truly strain-induced effect on the transport and magnetoresistance (MR) properties of LCMO films was investigated. The in-plane residual stress (σ11) in the LCMO film is tensile, while the out-of-plane one (σ33) is compressive. Moreover, the value of σ33 is larger than that of σ11. With increasing film thickness, the crystalline unit cell of the LCMO film reduces; also both the in- and out-of-plane components of the residual stress in the LCMO film decrease. It was found that the resistivity, TMI and MR strongly depend on the in-plane tensile stress σ11 (or/and the out-of-plane stress σ33). With the increase in the in-plane stress σ11 (or/and the out-of-plane stress σ33), the values of resistivity and MR increase, while TMI decreases. The truly strain-induced effect on the transport and magnetoresistance properties of LCMO film is discussed briefly.

Studies of the microstructures of YBa2Cu3O7-x/La0.67Ca0.33MnO3 heterostructural films

Abstract The structures of YBa 2 Cu 3 O 7− x (YBCO) thin films grown on SrTiO 3 substrates with and without La 0.67 Ca 0.33 MnO 3 (LCMO) buffer layer were investigated by high resolution X-ray scattering techniques. All the films are demonstrated to be c -axis oriented. X-ray analysis shows that the introduction of the LCMO buffer does not improve the quality of the YBCO film. It was found that the LCMO and the YBCO monolayers are in different strain states for different samples. In the YBCO/LCMO heterostructure, there exists misorientation between the film and the substrate. The interface roughness of YBCO/LCMO, as well as the surface roughness of all layers was obtained. There exists a non-designed cap layer on the upper surface of the YBCO layer in the YBCO/LCMO heterostructure.

Micro-structural analysis of YBa2Cu3O7-x thin films grown on different substrates by X-ray techniques

Abstract YBa 2 Cu 3 O x (YBCO) thin films grown on different substrates with and/or without Eu 2 CuO 4 (ECO) buffer layer were investigated by X-ray wide angle diffraction, reflection, diffuse scattering and topography. The results show that for the yttria stabilized ZrO 2 (YSZ) substrate, the presence of an ECO buffer layer improves the crystalline quality of the YBCO film, while a negative effect is observed for the SrTiO 3 (STO) substrate. The lateral correlation length for a sample grown on a YSZ substrate with ECO buffer layer is much greater than that grown on an STO substrate. The STO substrate used has mosaic structure.

Microstructural changes of stearic acid films by immersion in salt solution

X-Ray reflectivity has been used to investigate the microstructural changes of solution-cast stearic acid films before and after immersion in CoCl, solutions. Before immersion, the films possess a well-defined layered structure with an interlayer spacing of 4.01 +/- 0.05 nm. After the films were immersed in the CoCl, solutions, a new set of equidistant diffraction peaks emerge, the corresponding interlayer spacing of which is 5.13 +/- 0.05 nm. The X-ray photoelectron spectra of the films indicate the existence of cobalt ions inside the films after immersion. It is concluded that the permeation of the cobalt ions into the hydrophilic interlayer causes the stearic acid molecules to reorient perpendicular to the films, resulting in the increase of interlayer spacing and the roughening of the interfaces

X-RAY-SCATTERING FROM A ROUGH-SURFACE AND DAMAGED LAYER OF POLISHED WAFERS

Theoretical and experimental investigations were performed to show the application of x-ray crystal truncation rod scattering combined with x-ray reflectivity in the measurements of surface roughness and near-surface damage of mechanochemically polished wafers. By fitting the measured crystal truncation rod curves it has been shown that polished wafers are divided into three parts -irregular steps on the surface, a damaged thin layer beneath the surface and a perfect bulk. The results show that the root mean square of the surface roughness of mechanochemically polished Fe-doped and/or S-doped InP wafers is one to two atom layers, and that the lateral correlation length of the surface roughness is about 3000-7500 Angstrom. The thickness of the damaged region is found to be about 1000 atom layers.