X-ray and Raman characterization of AlSb/GaSb strained layer superlattices and quasiperiodic Fibonacci lattices

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Abstract
Double-crystal rocking curves of samples grown on (001)-oriented GaSb substrates by molecular-beam epitaxy have been analyzed by fitting computer simulations to data for the symmetric (004) and (002) reflections and for asymmetric (115) reflections. Rocking curves revealed a multiplicity of superlattice diffraction peaks. Dynamical diffraction theory using Abeles matrix method [D. W. Berreman, Phys. Rev. B 14, 4313 (1976)] was applied for the symmetrical reflections. We compare our results to standard kinematical simulations, and we find that there are significant differences. For the asymmetric reflections a new dynamical computer simulation code [D. W. Berreman and A. T. Macrander, Phys. Rev. B 37, 6030 (1988)] involving an 8×8 matrix solution of Maxwell’s equations was used. Lattice incoherency was determined from measurements of the in-plane mismatch. Dramatic diffraction peak broadening was observed for incoherent superlattices, and this broadening was attributed to a mosaic structure formed by misfit dislocations. Peak broadening was used to infer both a growth direction as well as an in-plane coherence length. The in-plane coherence length was found to be somewhat less than the mean distance between misfit dislocation lines. Mosaic broadening of diffraction peaks of a quasiperiodic lattice was found to be qualitatively similar to that observed for the periodic superlattices. Raman measurements of zone-folded acoustic phonon spectra yielded superlattice periods that agreed with the x-ray measurements within a few percent. Analysis of Raman peak intensities to yield individual layer widths was not found to be quantitative within the confines of current analytic models.