The copper centre: a transient shallow acceptor in ZnS and CdS

Abstract

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 one 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+or-0.005) eV. In CdS the deep Cu²⁺-acceptor position is (1.20+or-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 states 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.