ISOTOPE SHIFT IN SEMICONDUCTORS WITH TRANSITION-METAL IMPURITIES - EXPERIMENT AND THEORY APPLIED TO ZNO-CU

Isotope shifts for various lines associated with excitations of transi tion-metal impurities in semiconductors are considered. Special attent ion is paid to ZnO:Cu, for which experimental results are presented. I sotope shifts are measured for the so-called photoluminescence alpha a nd beta zero-phonon lines associated with excitations of bound exciton s, 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 eq ual. A theoretical model explaining these results is proposed, which i ncorporates 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 fo r the excited and ground states of all processes in transition-metal i mpurities considered here. Using reasonable values of the parameters o f the system, we are able to explain both the sign and value of the is otope shifts. [S0163-1829(98)04115-0].