Many-electron multiplet effects in the spectra of Ni and Co in II–VI semiconductors

Abstract

The optical spectra of transition metal (TM) impurities in semiconductor hosts are dominated by strong intra-atomic interactions between the d electrons, reduced because of their hybridization with ligands. We present here a new approach in which covalency effects result from the interaction between different configurations pertaining to the TM ion and its first neighbouring ligands. In contrast to the standard crystal field theory, free-ion Racah coefficients are used, the resulting multiplet structure depending only on the relative energy of these different configurations and their interaction strengths. Making use of this approach, a calculation of the multiplet structure of Ni and Co impurities in ZnO, ZnS, and CdS is performed and compared with other recent multiplet theories and experimental spectra. The very good agreement between our results and the experimentally determined transitions suggests that this theory can be considered as an extension of the conventional crystal field theory to strongly covalent hosts.