Crystal symmetry breaking of wurtzite to orthorhombic in nonpolar a-ZnO epifilms

Crystal symmetry breaking of wurtzite C-6V to orthorhombic C-2V due to in-plane anisotropic strain was investigated for nonpolar (11 (2) over bar0) ZnO epifilms grown on the R-sapphire. X-ray diffraction results reveal the epilayer is subjected to a compressive strain along the polar c-axis and tensile strains along both a-[11 (2) over bar0] surface normal and in-plane p-[1 (1) over bar 00] axis. The polarized Raman spectra of E-2 modes reveal violation of the C-6V selection rules. Oppositely, the C-2V configuration satisfies the selection rules for the Raman modes. The observed E-1 and E-2 bands in polarized optical reflection and photoluminescence spectra confirm the anisotropic strain causes the structure change to the orthorhombic one. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3159470]