Molecular-cluster model of the electronic structure of substitutional copper in zinc oxide

The LCAO molecular-cluster approach, within the local spin-density approximation, has been used to investigate the electronic structure of a substitutional Cu impurity in hexagonal ZnO (ZnO:Cu). The pure and doped semiconductors have been represented by the trigonally symmetric OZnZn3O1218-, and OCuZn3O1218- clusters, respectively. Additionally, both clusters have been embedded in the ZnO crystalline potential. The perturbations induced into the host electronic structure by the point defect have been discussed. Remarkably, the Cu 3d levels have been found deep in the valence band, at variance with previous theoretical investigations. On the other hand, this result is in tune with high-quality literature data on transition-metal impurities in other more covalent semiconductors. A new assignment of the near infrared (NIR) absorption transition, consisting of an e(DBH) --> t2DBH excitation, with both levels significantly localised on the Cu impurity and the nearest oxygen atoms, has been proposed on the basis of transition-state calculations. These calculations did not allow discrimination between two alternative assignments of the ZnO: Cu green emission (DELTAE = 2.86 eV), i.e. between a transition from the conduction-band minimum to the half-filled acceptor level placed at 250 meV above the valence band maximum (DELTAE = 3.32 eV) and the transfer of a hole from the inner t2CFR to the t2DBH ones (DELTAE = 3.45 eV). Nevertheless, a careful examination of literature experimental data coupled to a thorough charge density analysis of the levels involved in the two alternative transitions lead us to favour the latter assignment.