THE COPPER CENTER - A TRANSIENT SHALLOW ACCEPTOR IN ZNS AND CDS

Excitation measurements of the 2E(D)-2T2(D) Cu2+ luminescences in ZnS and CdS crystals are presented, revealing new excitation processes of the centres. In ZnS sharp resonances observed on the low energy onset of the charge transfer band at 1.17 eV enable us to identify a transient shallow acceptor state of the Cu2+ centre in ZnS. An energy transfer process by photoexcited holes between Cu2+ and Fe2+ centres yields an accurate value for the deep Cu2+-acceptor position in ZnS of (1.293 +/- 0.005) eV. In CdS the deep Cu2+-acceptor position is (1.20 +/- 0.02) eV above the valence band. Additional excitation bands just below the excitonic bandgap are observed and interpreted as transitions between the conduction band and the transient shallow acceptor state. The binding energies of these state are determined to be 119 and 94 meV for ZnS and CdS, respectively. The recombination energy of the transient shallow acceptor state is transferred efficiently to the excited 2E(D) state, leading to the internal Cu2+ luminescence.