Role of phonons in the optical properties of magnetron sputtered ZnO studied by resonance Raman and photoluminescence

The temperature dependence of phonons in ZnO has been studied using resonance Raman and photoluminescence (PL) emission measurements. Excitation with wavelength 363.8 nm (photon energy 3.409 eV) is used to establish incoming resonance near room temperature. Broad PL emission is seen at room temperature with peak position at 3.25 eV. This coincides with the overtone of the longitudinal optic (LO) band. Up to six LO phonon orders are observed. Temperature dependence of the LO phonon energy is described by a two-phonon decay mechanism with energies 100 and 496 cm-1. The temperature dependence of the PL shift is interpreted based on electron-phonon interactions. A two-phonon description is sufficient to describe the temperature shift in the band gap through occupation at average acoustic and optic phonon energies 125 and 500 cm-1, respectively. LO phonon sidebands (PSBs) are also observed at low temperature (23 to 100 K). The temperature shift in the PSB energies is interpreted based on the band gap shift combined with established theory for the PSBs.