Isotope shift in semiconductors with transition-metal impurities:: Experiment and theory applied to ZnO:Cu

Isotope shifts for various lines associated with excitations of transition-metal impurities in semiconductors are considered. Special attention is paid to ZnO:Cu, for which experimental results are presented. Isotope shifts are measured for the so-called photoluminescence alpha and beta zero-phonon lines associated with excitations of bound excitons, and of the zero-phonon line associated with the intracenter Cu2+(T-2(2)-E-2) transition. These shifts appear to be negative and nearly equal. A theoretical model explaining these results is proposed, which incorporates the mode softening mechanism and the covalent swelling of the impurity d electron wave functions. It is shown that, contrary to transitions in simple neutral impurities, this mechanism works both for the excited and ground states of all processes in transition-metal impurities considered here. Using reasonable values of the parameters of the system, we are able to explain both the sign and value of the isotope shifts. [S0163-1829(98)04115-0].