Abstract
Four emission bands have been studied at low temperatures with differently doped ZnO crystals. In the Co2+(d7) ion, two transitions to the A24(F) ground state give rise to new luminescence bands, viz., starting from the T24(F) term (around 3600 cm1) and from the mixed T14(P),2 T1(G), )2 levels (around 15 100 cm1). Fine-structure and polarization properties of these transitions agree with conclusions on C3v and spin-orbit splittings drawn from previous absorption data. An emission of ZnO:Ni near 6000 cm1 is identified as the T24(F)4 A2(F) transition of the Ni3+(d7) ion in a trigonal environment. While the samples display the Ni2+(d8) transmission spectrum, a charge-transfer process leads to a donor-type conversion of the center entailing the transient Ni3+ state. A similar mechanism is proposed to explain the emission of ZnO:Cu in the 56006900-cm1 range. Its fine-structure and polarization behavior can be understood in terms of T2 3 T2(F) →A1 3 T1(F) transitions of a Cu3+(d8) ion. Its occurrence is established in a model of one-electron configurations also covering the other known Cu transitions. The excitation spectrum presented is explained by a Tanabe-Sugano type of reasoning for the d8 excited states which are reached in the d9d8 conversion.