Self-assembled ZnO nano-crystals and ultraviolet exciton lasing at room temperature

We review our recent studies on ultraviolet (UV) stimulated emission and lasing observed at room temperature from nano-structured ZnO thin films, which were grown on sapphire substrates using laser-molecular beam epitaxy. The thin film consists of regularly arrayed hexagonal nano-crystallite columns, whose facets form natural micro-cavities. These nano-crystallites confine the centre-of-mass motion of excitons. As a result of the quantum size effect, the oscillation strength of the excitons is greatly enhanced, which is favorable to the radiative recombination of excitons at room temperature. On excitation with the frequency-tripled output of a YAG laser, the nano-structured ZnO thin film shows strong UV lasing at room temperature with a threshold as low as 24kW/cm2. At a moderate pump intensity, the room temperature stimulated emission is associated with an 〖CM(44〗exciton-exciton collision process. At higher pump densities the excitons dissociate, and the UV stimulated emission is dominated by an electron-hole plasma recombination process. Because of the large enhancement of oscillator strength of the excitons, the optical gain of the stimulated emission measured at room temperature can reach 320 cm-1, which is an order higher than that observed in bulk ZnO crystals. In comparison with the electron-hole plasma stimulated emission in most commercial semiconductor lasers, the excitonic stimulated emission can be realized at relatively low external pumping density. The observation of excitonic lasing effects at room temperature might be valuable in the realization of practical UV semiconductor laser devices.