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Bulk ZnO samples, epitaxially grown ZnO layers, and ZnO nanostructures frequently exhibit a characteristic emission band at 3.31-eV photon energy whose origin is controversially discussed in the literature. Partly, this omnipresent band is ascribed to ͑e , A 0 ͒ transitions of conduction band electrons to acceptors, which are abundant in relatively high concentrations but have not positively been identified. The band is, in particular, often reported after intentional p-type doping of ZnO, preferentially with group V species. In the present work, we study the 3.31-eV band by low-temperature cathodoluminescence ͑CL͒ with high spatial resolution, by scanning electron microscopy, and by transmission electron microscopy ͑TEM͒. Line shape analyses at different temperatures give clear evidence that the band originates from an ͑e , A 0 ͒ transition where the acceptor binding energy is ͑130Ϯ 3͒ meV. The 3.31-eV luminescence is exclusively emitted from distinct lines on sample surfaces and cross sections representing intersections with basal planes of the wurtzite hexagons. Correlating monochromatic CL images with TEM images, we conclude that the localized acceptor states causing the 3.31-eV luminescence are located in basal plane stacking faults.

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Photoluminescence spectra of high-quality bulk AlN crystals are reported. In addition to the expected linear luminescence features like free excitons and donor-bound excitons, nonlinear processes like biexcitons and the exciton-exciton scattering band are seen for higher excitation densities. For temperatures above Ϸ150 K the electron-hole plasma becomes clearly visible in the spectra. A detailed analysis of the data yields an exciton binding energy of 55 meV, a biexciton binding energy of 28.5 meV, a band gap of 6.089 eV at low temperature, and a band gap of 6.015 eV at room temperature.