It was noted that an emission band at 3.31eV was frequently observed in low-temperature photoluminescence measurements of ZnO which was p-doped with group-V elements, and also in nominally undoped ZnO layers and nanostructures. It was also alternatively attributed to LO- or TO-phonon replicas of free excitons, to acceptor-bound excitons, to donor–acceptor pair transitions, to two-electron satellites of donor-bound excitons or to free-to-bound transitions. This band frequently dominated the photoluminescence of ZnO nanostructures and layers at room temperature. Annealing led to drastic changes in its intensity. Low-temperature cathodoluminescence measurements of very high spatial resolution and high-resolution transmission electron microscopic investigations were carried out on the same pieces of hetero-epitaxial ZnO sample with an unusual layer orientation. The data permitted an unambiguous correlation of this emission with c-plane stacking faults. The emission was found to be due to the recombination of a free electron with a hole bound to a relatively shallow acceptor state some 130meV above the valence band edge. Locally, these acceptor states occurred in concentrations of up to 1018/cm3, and thus led to strong two-dimensional perturbations of the free carrier concentration. They had severe implications for the conductivity of layers and nanostructures in general, and for the interpretation of Hall and luminescence data in particular.

The Role of Stacking Faults and Their Associated 0.13eV Acceptor State in Doped and Undoped ZnO Layers and Nanostructures. Thonke, K., Schirra, M., Schneider, R., Reiser, A., Prinz, G.M., Feneberg, M., Biskupek, J., Kaiser, U., Sauer, R.: Microelectronics Journal, 2009, 40[2], 210-4