ISOTOPE SHIFT OF LOCAL VIBRATIONAL-MODES AT TRANSITION-METAL IMPURITIES IN SEMICONDUCTORS

The observation of isotope effects in the fine structure of optical tr ansitions at transition-metal impurities in II-VI compounds is mainly due to a Jahn-Teller effect. In case of the transitions between the mu ltiplets T-3(1)(F) <----> T-3(1)(P) of Ni2+ centers in cubic ZnS, the positive isotope shift of various lines with respect to the masses of Ni-58, Ni-60, and Ni-61 is due to the coupling to local vibrational mo des of T-2 symmetry. The isotope shift of the Raman active modes at 26 .2 meV and 38.9 meV are calculated to be -28.3 mu eV/nucleon and -25.2 mu eV/nucleon, respectively. These results were obtained by using the valence-force model of Keating and Kane together with the long-range Coulomb forces and the scaling-factor approximation. The fine-structur e transitions between the multiplets T-2(2) <----> E-2 of Cu2+ centers in hexagonal ZnO show negative isotope effects with respect to the ma sses of Cu-65 and Cu-63 and in addition isotope effects with respect t o the ligands of the impurity O-16 and O-18. We point out that these i sotope shifts are due to the Jahn-Teller coupling of a local vibration al mode (LVM) of T-2 = A(1) x E symmetry in the T-2(2) multiplet and t hat the anisotropic vibration of the four O ligands of this LVM explai ns the difference in the O-16/18 isotope shift where the c axis O-16 a nd one of the off axis O-16 is replaced by O-18 respectively.