Asymmetry in the excitonic recombinations and impurity incorporation of the two polar faces of homoepitaxially grown ZnO films

Homoepitaxial ZnO layers were grown on O-polar and Zn-polar surfaces of ZnO single crystal substrates by chemical vapor deposition. While the structural properties (surface roughness and rocking curve half width) were identical within experimental error, the optical properties as monitored by photoluminescence (PL) were strikingly different. Four excitonic recombination lines are exclusively found on the O-polar surface. In order to understand the defects involved, secondary ion mass spectrometry was employed which clearly demonstrated that the impurity incorporation is substantially higher on the O-polar surface. Temperature and power dependent PL measurements provide further insight into the initial-final state recombinations. The observed recombinations are caused by excitons bound to a neutral defect complex. In order to account for the thermalization behavior found in the temperature dependent measurements, splittings in the excited as well as in the ground state must be present. Possible defect models are discussed.