Interface structure and anisotropic strain relaxation of nonpolar wurtzite (112¯) and (101¯) orientations: ZnO epilayers grown on sapphire

Abstract

The interface properties between nonpolar ZnO and sapphire have been studied using high resolution transmission electron microscopy. Two nonpolar orientations are investigated: a- and m-orientations corresponding to [112¯0] and [101¯0] crystallographic directions. After the definition of the epitaxial relationships and the resulting initial lattice mismatch, we show that nonpolar ZnO can be grown on sapphire with perfectly flat interfaces. Geometrical misfit dislocations are observed at the interface ZnO/sapphire and their density gives the residual strain in the layer. A strong anisotropy in the strain relaxation is found along the two perpendicular in-plane directions. This anisotropy may be explained in terms of initial anisotropic mismatch yielding different relaxation processes. A domain matching epitaxy is observed in m- and a-oriented layers for mismatches larger than 9% while a lattice matching epitaxy, in which the relaxation is driven by nucleation and glide of dislocations, is observed in a-oriented ZnO along the [0001] in-plane direction. In order to explain the observed relaxation the activated slip systems are calculated for both nonpolar orientations as a function of the in-plane stress due to the anisotropic mismatch. There is a major difference from the polar orientations. Low energy prismatic slip systems can be effective for plastic relaxation in the nonpolar orientations because they are no longer parallel to the growth direction, which is the case of c-oriented layers, nor to the applied stress. Our results can be directly extended to other nonpolar wurtzite structures such as III-nitrides.