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

We deposited (11 (2) over bar0) nonpolar a-plane ZnO (a-ZnO) films on (01 (1) over bar2) 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 [1 (1) over bar 00] (m-axis) in-plane axes. Out-of-plane strain along [11 (2) over bar0] (a-axis) is tensile and relaxes for film thickness >= 2 mu m. 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. (C) 2008 American Institute of Physics.