The crystal geometry of Al xGa 1- xAs grown by MOCVD on offcut GaAs (100) substrates

Abstract

Double crystal X-ray diffraction measurements show that the cubic film unit cell defined by Vegard's law is triclinicly distorted and titled 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) vector normals remain approximately coplanar with the surface normal. Such film/substrate crystal geometry leads to formulation of a growth model. Buffer layer growth or homoepitaxy which conventionally precedes heteroepitaxial film deposition has been shown to form ordered substrate surfaces. During heteroepitaxy on offcut and ordered substrates, rapid lateral growth induces lateral strain relaxation which is interrupted periodicly by finite terrace dimensions. As it is the case for strained heteroepitaxy on substrates with zero offcut angle, strain relaxation occurs additionally in direction of the surface normal. The crystal lattice geometry produced across an average terrace length, defines the interface, which determined the observed film/substrate crystal geometry.