Structure of Lattice-Matched CaxSr1-xF2 Epilayers on GaAs(111)B Surface Analyzed by the X-Ray Standing-Wave Method

Abstract
The atomic structure of a single-crystal Ca x Sr1-x F2(x=0.43) film grown on GaAs(111)B surface has been determined by the X-ray standing-wave technique. A new method has been applied which allows complete analysis of mismatched structures using a single specimen. (Ca, Sr) atoms at the interface are found to be seven-coordinated, supporting the previously proposed growth model of epitaxial cubic fluoride films. The atomic layer spacing across the interface is contracted by 0.37 Å, compared with that calculated from the bulk bond lengths. The film is strained in such a way that the (111) interplanar distance is contracted by 0.28%, although the bulk-mixed fluoride exactly lattice matches GaAs at room temperature. The study is based on a theoretical reinvestigation of the standing-wave position with respect to atomic planes in absorbing noncentrosymmetric crystals.