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

In this paper, we reviewed our recent study on ultraviolet stimulated emission and lasing observed at room temperature from nano-structured ZnO thin films. The nano-structured ZnO thin films were grown on sapphire substrates using Laser-Molecular-Beam-Epitaxy (L-MBE). The thin film was consisted 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 largely enhanced, which is favored to the radiate recombination of exciton at room temperature. Excited using the frequency-tripled output of a YAG laser, the nano-structured ZnO thin film showed strong ultraviolet lasing at room temperature with a threshold as low as 24kW/cm2. At a moderate pumping intensity, the room temperature stimulated emission is associated with an exciton-exciton collision process. At higher pumping density, the excitons are dissociated, and the ultraviolet 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 reaches as high as 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 of commercial semiconductor lasers, the excitonic stimulated emission can be realized at relatively low external pumping density. The observation of excitonic lasing effect at room temperature might be valuable in realization of practical ultraviolet semiconductor laser devices.