In-plane anisotropic strain in a-ZnO films grown on r-sapphire substrates

We deposited (112¯0) nonpolar a-plane ZnO (a-ZnO) films on (011¯2) r-sapphire substrates using metalorganic chemical vapor deposition. Unit cell deformation due to interfacial strain in films was determined by triple-axis x-ray diffraction. Due to low symmetry of a-plane, anisotropic strain is observed along the [0001] (c-axis) and [11¯00] (m-axis) in-plane axes. Out-of-plane strain along [112¯0] (a-axis) is tensile and relaxes for film thickness ⩾2μm. The in-plane m-axis is under tensile strain and c-axis is under compressive strain. Increase in film thickness increases in-plane strain anisotropy due to faster relaxation along the m-axis. Thermal and lattice mismatch strains are separated by curve fitting.