Optical Properties of Cd-O and Zn-O Complexes in GaP

We report sharp line spectra, due to Cd and O impurities in GaP, which show that the red luminescence previously attributed to recombination between distant donor-acceptor pairs arises instead from nearest-neighbor Cd-O complexes. The binding energy of isolated O donors is thus shown to be {~}1 eV. The energy levels and the optical selection rules for the complexes are found to be consistent with the C₃ᵥ symmetry of these centers. The spectra reveal three local phonon modes which we identify with the symmetric modes of vibration of the O atom and its four nearest neighbors. We show that the broad red emission band from Zn- and O-doped GaP is attributable to similar complexes having a strongly damped local vibrational mode.