Local vibrational modes of the CuO4-cluster in ZnO

Zero phonon line isotope shifts of d-d transitions depend on the local vibrational properties of the transition metal. For copper in ZnO this isotope effect depend not only on the mass of the copper impurity but also on the mass of the oxygen ligands. To explain this behavior the local vibrational properties of copper in ZnO are investigated by means of resonant Raman spectroscopy and analyzed in a cluster calculation in the valence force model of Kane using the scaling factor approximation. The fit of the experimental data together with the calculations give insight into the local binding properties around the impurity, the localization and the symmetry of the local vibrational modes. The copper incorporation results in a local bond softening in the ZnO lattice. The copper and the oxygen mass dependences of the local vibrational modes of the CuO4-cluster are calculated. The mass dependence is mainly determined by the localization of the local vibrational modes. The anisotropic vibration of the CuO4-tetrahedron explains the ligand induced zero phonon line isotope shift which was observed for the intracenter Cu2+(T-2(2)-(2)E) transition.