Impurity induced bond-softening and defect states in ZnO:Cu

Phonons and optical properties of Cu-doped ZnO have been investigated using micro-Raman and photoluminescence (PL) spectroscopy. Two new modes found in Raman spectra are assigned to Cu-O impurity vibrations analogous to polar A(1) and E-1 modes of ZnO on the basis of a Cu-O force constant lower than that of the Zn-O bond. The reduction in the frequencies of the nonpolar E-2 modes also appears to arise due to softening of the mixed crystal. In the PL spectrum at 80 K a prominent blue emission arises due to Cu-related intraband transitions. Temperature dependent PL spectrum confirms the existence of donor acceptor pair transition at 3.305 eV in ZnO:Cu system. (C) 2011 American Institute of Physics. [doi:10.1063/1.3624918]