Double Crystal X-Ray Diffraction Measurement of a Triclinicly Distorted and Tilted AlxGa1-xAs Unit Cell Produced by Growth on Offcut GaAs Substrates

Abstract

ABSTRACTCorrections are required for double crystal X-ray diffraction characterization of epitaxial AlxGa1-xAs layers grown on offcut GaAs (100) substrates. Double crystal X-ray diffraction measurements show that the cubic film unit cell defined by Vegard's law is triclinicly distorted and tilted with respect to the substrate unit cell. The distortion and tilt angles oppose each other defining a crystal geometry where the substrate and film <100= axes remain approximately coplanar with the surface normal. This film/substrate crystal geometry leads to formulation of a model describing heteroepitaxy on offcut (100) substrates. When film atoms are bonded to an offcut substrate, the already tetragonaly distorted film unit cell is subjected to additional cell distortions. The magnitude of this additional strain depends on where the film atoms are positioned on a substrate terrace. The first few layers of film atoms establish swain grades across individual substrate terraces. Constrained by the geometry of this interface region and driven by strain relaxation in the net growth direction, subsequent heteroepitaxy forms the measured film/substrate crystal geometry.