Direct evidence for selective impurity incorporation at the crystal domain boundaries in epitaxial ZnO layers

A direct correlation of structural properties with the spatial distribution of bound exciton luminescence in ZnO epitaxial layers has been achieved on a microscopic scale using highly spatially and spectrally resolved cathodoluminescence. The morphology of the high quality ZnO layer is characterized by a distinct domain structure. While the laterally integrated cathodoluminescence spectrum shows narrow (full width at half maximum ≪3 meV) I₈ luminescence, a pronounced emission line at I₀/I₁ emerges in the local spectra taken at domain boundaries. In complete contrast, no I₀/I₁ emission is found inside the domains. Monochromatic images further evidence the selective incorporation of impurities at the grain boundaries of domains. Micro mappings of the I₈ peak wavelength directly visualize the strain relaxation across the domains toward their very center, where a drop in quantum efficiency indicates enhanced defect concentration.