The cathodoluminescence (CL) of cubic (c-) GaN epitaxial layers is investigated at temperatures between 50 K and 300 K. The low temperature CL spectra show three well resolved emission lines (3.26 eV, 3.17 eV and 3.08 eV) which are due to excitonic, donor-acceptor and free to acceptor transitions. Spatially resolved measurements of the intensity of the excitonic emission demonstrate the homogeneity of the layers which are free of microcrystalline inclusions. The room temperature CL of the layers has a full width at half maximum of 56 meV and is due to excitonic recombination as is concluded from the zero-shift of the line position when the excitation intensity is varied over some orders of magnitude. The intensity of a broad emission band at 2.4 eV shows a strong nonlinear variation of the intensity at high excitation levels. Using a rate equation model for the near band edge and the deep 2.4 eV emission we are able to describe the intensity variation of these radiative transitions as a function of the excitation intensity. Depth resolved CL measurements reveal a homogeneous depth distribution of deep recombination centres responsible for the deep 2.4 eV luminescence band.

We have grown ZnSe layers by molecular beam epitaxy (MBE) on GaAs substrates which were deoxidized by atomic hydrogen at different temperatures. The surface roughness of the GaAs substrate was measured by atomic force microscopy (AFM); that of the growing ZnSe layers was determined from AFM measurement and the full width at half maximum of the reflection high-energy electron diffraction (RHEED) reflexes during the nucleation process and the layer growth. We find a strong influence of the substrate temperature during the deoxidization process on the surface roughness and the density of extended defects in the ZnSe layers. The density of extended defects was measured using high-resolution x-ray diffraction and spatially resolved cathodoluminescence (CL) images which were obtained at the defect-related Y 0 emission at about 474 nm. We find that the intensity ratio of the defect-related Y 0 emission and the near band edge excitonic emission is proportional to D 2.9 where D is the density of extended defects in the layer. We explain this relation by a model calculation which assumes radiative and non-radiative recombination of bound excitons as well as radiative recombination of electron-hole pairs via recombination centres which are involved in the Y 0 transition.