Shufan Cui

Relation between microstructures and magnetic properties upon annealing in Fe50Mn50/Ni80Fe20 films

The annealing-temperature-dependent change in the exchange bias and coercivity is investigated in Fe50Mn50/Ni80Fe20 films. It is interesting to note that, as the annealing temperature increases, the exchange bias first decreases, and then increases, and finally decreases for the case of annealing at the higher temperature. The coercivity will increase upon annealing at moderate temperature, but decrease upon higher-temperature annealing. We can qualitatively interpret the change of the magnetic properties with annealing temperature in connection to the microstructures by x-ray scattering technologies. The results show that both the large exchange bias field and low coercivity of Fe50Mn50/Ni80Fe20 films are dependent of not only the interfacial roughness but also the antiferromagnetic structure.

STUDY OF PERFECTION AND EVOLUTIVE MECHANISM OF MAGNETIC CZOCHRALSKI SI

The growth striation, resistance homogeneity, and oxygen concentration of silicon single crystal grown by both magnetic Czochralski and Czochralski methods were investigated. Theoretical analysis of the experimental results, including effect of magnetic field on the temperature fluctuation, effective segregation coefficient (keff), and oxygen concentration are discussed.

Determination of stress in GaAs/Si material

The stresses in GaAs/Si samples which were prepared under different growing conditions were determined by the x‐ray diffraction method. The results show that the stress in GaAs/Si samples is reduced by improving the homogeneity of the thermal field. As the thickness of the epitaxial film increases, the curvature of the sample increases, but the stress remains in the same order.

A new scheme for x‐ray grazing incidence diffraction

A new scheme for x‐ray grazing incidence diffraction (GID) under total external reflection conditions is presented. This simple scheme allows the grazing angles and the scattering angles to be adjusted independently so that the structural and/or the chemical profiles of materials can be determined at controlled depths. As an example, Si1−xGex/Si superlattice materials were studied. Both commensurate and incommensurate growths between the two quantum wells in the strained‐layer superlattice were observed by the GID technique.

Determination of surface roughness of InP (001) wafers by x‐ray scattering

The surface roughness of polished InP (001) wafers were examined by x‐ray reflectivity and crystal truncation rod (CTR) measurements. The root‐mean‐square roughness and the lateral correlation scale were obtained by both methods. The scattering intensities in the scans transverse to the specular reflection rod were found to contain two components. A simple surface model of surface faceting is proposed to explain the experimental data. The sensitivities of the two methods to the surface structure and the role of the resolution functions in the CTR measurements are discussed.

Investigation of GaAs/Si material by x‐ray double‐crystal diffraction

GaAs epilayer films on Si substrates grown by molecular‐beam epitaxy were investigated by the x‐ray double‐crystal diffraction method. The rocking curves were recorded for different diffraction vectors of samples. The results show that the unit‐cell volumes of GaAs epilayers are smaller than that of the GaAs bulk material. The strained‐layer superlattice buffer layer can improve the quality of the film, especially in the surface lamella. The parameter W’ = Wexpt/(√‖γh‖/γ0/sin 2θB) is introduced to describe the quality of different depths of epilayers. As the x‐ray incident angle is increased, W’ also increases, that is, the quality of the film deteriorates with increasing penetration distance of the x‐ray beam. Therefore, W’ can be considered as a parameter that describes the degree of perfection of the epilayer along the depth below the surface. The cross‐section transmission electron microscopy observations agree with the results of x‐ray double‐crystal diffraction.

New modulation structures observed in Bi2.13Sr1.87CuO6+delta superconductors

A new two-dimensional structure modulation along c- and b-axes has been discovered in superconducting single crystals of Bi2.13Sr1.87CuO6+delta (Bi2201) by x-ray scattering. Such modulation structure does not exist in non-superconducting Bi2201 single crystals, but instead lattice distortions are observed in the a-b-plane. This phenomenon may indicate that both strain relaxation and charge modulation in the a-b-plane are important to the occurrence of superconductivity in the copper oxides.

Study of the correlations between the critical current density and the structures of YBa2Cu3O7- delta thin films

The authors examine YBa2Cu3O7- delta thin films grown by DC magnetron sputtering on SrTiO3 substrates by using X-ray double crystal diffraction, X-ray topography and SEM, etc. Various microstructural and compositional parameters of the specimens with higher and lower critical current density (Jc) are compared. It is found that the Jc is closely correlated to the structural perfectness, the compositional uniformity and the grain orientation of the thin films. There is some evidence to show that the intrinsic and surface quality of the substrates influences the quality of the thin films. An optimum procedure for the surface polishing of SrTiO3 substrates is proposed. In addition, a remarkable influence of the growth procedure on the quality of the thin films is also reported.

STRUCTURAL-ANALYSIS OF HIGH-TC EPITAXIAL GDBA2CU3O7-X SUPERCONDUCTING THIN-FILMS

High-Tc superconducting epitaxial thin films of GdBa2Cu3O7-x, (GBCO), grown in situ by DC magnetron sputtering onto substrates of single-crystal (001) oriented LaAlO3, SrTiO3 and yttrium-stabilized ZrO2, have been studied by high resolution double crystal X-ray diffractometry, X-ray white beam Laue topography and scanning electron microscopy. The lattice mismatch between film and substrate in the (001) direction was determined to be 3.0% for the film on the LaAlO3 substrate, while the mismatch in the (110) direction was estimated to be 2.9% from extremely asymmetric X-ray rocking curves exploiting the tunability of synchrotron radiation. The data show that the unit cell of the GBCO film is greater than that of bulk GBCO materials, implying that the film is totally relaxed. It is proven that when grown on an yttrium-stabilized ZrO2 substrate the a and b axes of the unit cell of the film are rotated by 45 degrees with respect to the a axis of the substrate. Scanning electron microscopy observations reveal that there are many micrograins less than one micron in size on the surface of the films, confirming the results of double-crystal topography.

In Situ Epitaxial Growth of the Gd-Ba-Cu-O Superconducting Thin Films on (100) LaAlO3 by Dc-Magnetron Sputtering Using Single Planar Target

We report in situ epitaxial growth of the GdBa2Cu3O7 superconducting thin films on (100) LaA103 single crystal substrates by dc-magnetron sputtering using single planar target. The best films had Tco of 92.5K, transition width of 0.57K and critical current density of 3.6x106A/cm2 at 77K with very good reproducibility. The film structure was studied by X-ray spectra in different diffraction geometries. In the Bragg-Brentano (BB) focusing geometry merely (00L) lines of the film are observed indicating c-axis normal to the substrate surface. The rocking curve of the film is very narrow (0.43 degree, full width at half maximum). In the SeemannBohlin (SB) focusing geometry the structureless spectrum indicates single crystalline growth of the film.