Anisotropic optical properties of free and bound excitons in highly strained A-plane ZnO investigated with polarized photoreflectance and photoluminescence spectroscopy

We have investigated the polarization dependence of the near-band-edge photoluminescence and photoreflectance spectra in nonpolar (A-plane) ZnO films under strong biaxial compressive strain. We show that anisotropic strain and the orientation of the nonpolar plane play an important role in determining the polarization selectivity and properties of excitonic transitions. We identified four distinct band-edge transitions at 3.449, 3.420, 3.386, and 3.326 eV. They were identified as E-2 and E-1 free excitons, E-1 excitons bound to a donor, and free-electron-to-bound-hole transition, respectively. Unlike previously reported results on relatively thick nonpolar films, the E-1 exciton (lowest energy) was mainly polarized to E perpendicular to c and weakly polarized to E parallel to c under strong biaxial compressive strain in the 100 nm thick film. The E-2 exciton (next higher energy) was exclusively polarized to E parallel to c. The localization energy of DX is 34 meV, which is much larger than that in polar ZnO, and the DX was not thermally delocalized even at room temperature. (c) 2008 American Institute of Physics.