High-resolution electron microscopy study of (Ca,Sr)F2/GaAs grown by molecular-beam epitaxy

GaAs/(Ca,Sr)F2/GaAs structures have been studied by means of high-resolution electron microscopy; two substrate orientations, (100) and (111)As, were considered. The (Ca,Sr)F2 layers were observed to be monocrystalline, while their crystalline orientation was found to be the same as that of the substrate (A type). The surface morphology of the (Ca,Sr)F2 layers appeared to be highly sensitive to the orientation: it was observed to be atomically flat for the (111) case and to consist of facets, {111} oriented for the (100) orientation. For both orientations, the transmission electron microscopy observations show that the interface seems to be composed of coherent and incoherent structures. These results are discussed and compared to those obtained on similar heteroepitaxial systems.