S. F. Cui

X-RAY ANALYSIS OF STRAIN RELAXATION IN STRAINED-LAYER SUPERLATTICES

Theoretical simulations of x‐ray double‐crystal‐diffraction rocking curves for strain‐relaxed superlattices have been successfully carried out based on x‐ray dynamical diffraction theory. The strain relaxation, the misorientation between the superlattice layers and the substrate, and the effect of peak broadening due to the formation of misfit dislocations have been taken into account. The influence of possible strain relaxation mechanisms and relaxation ratios on the rocking curves have been investigated. It was found that both the mechanism and degree of the strain relaxation of the superlattice can be determined by fitting the angular positions and the relative intensities of the experimental superlattice satellites. By using this method, an InxGa1−xAs/GaAs superlattice sample and a GexSi1−x/Si superlattice sample were analyzed. The different strain‐relaxation mechanisms were found in these two samples.

Interfacial structure of molecular beam epitaxial grown cubic-GaN films on GaAs(001) probed by x-ray gazing-angle specular reflection

We report on a study of interfacial structure of GaN films grown on GaAs(001) substrates by plasma-assisted molecular beam epitaxy using x-ray grazing-angle specular reflection. We show that interfacial layers with electron densities differing from those of GaN and GaAs were formed upon deposition of GaN. It is also found that the interfacial structure of our systems depends strongly on the course of the initial layer deposition. The phase purity of the GaN films was examined by x-ray reciprocal space mapping. A simple kinetic growth model suggested by our results has been presented.

Thermal stability of cubic GaN film grown by molecular-beam epitaxy on GaAs(001)

The thermal stability of cubic-phase GaN (c-GaN) film grown by molecular-beam epitaxy was investigated by Raman scattering spectroscopy and X-ray scattering. The results of Raman scattering shows that, after annealing at 1000degreesC, the intensity of transverse (TO) and longitudinal (LO) optical peaks from cubic phase obviously decreases while the intensity of TOb peak from the boundary effect slightly decreases, but the transformation of the hexagonal phase (alpha-GaN) can not be detected due to a little of alpha-GaN inclusion. X-ray reflectivity measurements indicate that there is a high-electron-density layer between the substrate and the GaN film, and it becomes uniform and much thinner after high-temperature annealing, counting for the Raman results of the intensity change of the TOb peak. The results of high-angle X-ray diffraction and X-ray reciprocal space mapping revealed that the relative content of alpha-GaN obviously increases after annealing at 1000degreesC, and (10 (1) over bar1) is the most stable diffraction lattice of the alpha-GaN hexagonal phase

The interface roughness exponent in superlattices

The interface roughness and scaling exponent of strained-layer superlattices (SLSs) grown by molecular beam epitaxy (MBE) have been measured by x-ray reflectivity and diffuse-scattering methods. We have found that for samples grown under identical conditions the root mean square (rms) roughnesses , the roughness exponents (h), and the lateral correlation length are dependent upon the thickness and/or the substrate temperatures of the superlattice. The incorporation of a surfactant, such as antimony, can retard interface widening and smooth the interface.

Advances in design, growth and application of piezoelectric crystals with langasite structure

Crystals with langasite structure consist of more than one hundred compounds. Only about 20 compounds are grown and characterized. In this work, we first use first-principles calculation to simulate the crystal structure of 29 ordered langasite compounds and then forecast their piezoelectric related properties including dielectric constants, elastic constants, piezoelectric coefficients and electromechanical coefficients. Four known ordered crystals including SNGS, STGS, CTGS, CNGS, and several novel crystals including BTGS, CTAS, CNAS were grown by Czochralski method and characterized at room temperature and at high temperature from 400 to 900°C. The experimental results of piezoelectric properties verified the validity of theoretical forecast and that errors between experimental and theoretical results were in reasonable range. Gallium-free crystals shown good prospect in application of piezoelectric sensors, combining the advantages of low cost of raw materials and high performance. More efforts should be paid to the growth of these novel crystals.

Surface roughness exponent and non-designed cap layer in PbZr0.53Ti0.47O3/La1.85Sr0.15CuO4 bilayers

The surface and interface roughness and roughness exponent of PbZr0.53Ti0.47O3 (PZT)/La1.85Sr0.15CuO4 bilayers deposited on SrTiO3 (001) substrates by rf/dc magnetron sputtering have been measured by x-ray reflectivity and diffuse-scattering methods. We have found that the surface roughness increases and the roughness exponent decreases with the increase of the thickness of the PZT layers; and that there exist non-designed cap layers on the upper surfaces of the PZT layers. The growth character of the bilayer films is discussed.

A semi-kinematic approach to x-ray diffraction of real crystals with small defects

A semi-kinematic approach to x-ray diffraction of real crystals with small defects is proposed by incorporating an extinction function into the kinematic diffraction formula. The effect of multiple reflections is described by the extinction coefficient which can be calculated by Darwin’s recursion relations. In this approach the intensity of the coherent scattering is related to the static Debye–Waller factor E, while the intensity of diffuse scattering is determined by the correlation function of the distortion field. When E=1, i.e., for perfect crystals, it gives exactly the same result as Darwin’s theory does. The feasibility of this approach is demonstrated by investigating the reciprocal space mapping and the crystal truncation rod of a GaAs crystal.

X‐ray reflectivity studies of the effect of surfactant on the growth of GeSi superlattices

X‐ray reflectivity is applied to investigate the effect of a surfactant on the growth of Ge1−xSix/Si superlattices. It is demonstrated that the antimony layer deposited on the surface can effectively prevent the intermixing of silicon and germanium. The specular reflectivity curves show that the width of the interface is sufficiently reduced by the surfactant. The transverse scans show that the interface roughening exponent h for the sample with surfactant is larger than for the sample without surfactant, and the in‐plane correlation length for the former is much larger than for the latter. This indicates that the surfactant makes less jagged and smoother interfaces and induces a different surface growth mode.

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.

SURFACE SCATTERING OF X-RAYS FROM INP(001) WAFERS

We have studied the surface scattering of x rays from mechanical‐chemical polished InP (001) wafers with sulfur and/or iron doping. The scattering intensities in the scans transverse to the specular reflection rod were found to contain two components. A simple surface model was proposed to explain the experimental data. The results were also compared with those obtained from crystal truncation rod measurements.