Anharmonicity of lattice vibrations induced by charged nickel impurities in II–VI semiconductors

The lattice vibrations induced by nickel impurities with a negative charge relative to the lattice in ZnSe: Ni, ZnO: Ni, ZnS: Ni, and CdS: Ni semiconductors are investigated using very sensitive field-induced vibronic spectroscopy. This technique is based on the interaction of lattice vibrations with impurity excitons and the effect of an ac electric field on these excitons. The phonon replicas of the zero-phonon line (ZPL) of impurity excitons (including intense peaks of combination replicas up to the eighth order) in the experimental spectra of the system under investigation are observed for the first time. These spectra make it possible to analyze the interaction between different vibrations. The experimental results are interpreted in terms of model calculations of the vibrations in a lattice with a charged impurity center and vibrations in a monoatomic chain with a strong anharmonicity. It is demonstrated that charged impurity centers initiate new lattice vibrations, namely, extrinsic anharmonic modes with a considerable third-or fourth-order anharmonicity.