High temperature excitonic stimulated emission from ZnO epitaxial layers

The emission spectrum of high quality ZnO epilayers is studied from room temperature up to 550 K. At room temperature and low excitation power a single emission peak is observed which may be identified with the free exciton from its peak energy and dependence on temperature. However, when excitation intensities exceed 400 kWcm^-2 a sharp peak emerges at lower energy which we attribute to exciton-exciton scattering. At higher excitation intensities (>800 kWcm^-2) a second stimulated emission peak emerges at even lower energies: we attribute this peak to be stimulated emission of an electron hole plasma. Similar features are observed for all temperatures up to 550 K.