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.