Excitonic gain and stimulated ultraviolet emission in nanocrystalline zinc-oxide powder

We have studied ultraviolet photoluminescence from high-purity zinc-oxide powder over a wide temperature range (2-293 K). At low temperatures, the spontaneous emission is due to radiative recombination of excitons bound to donors and acceptors. At high temperature (>90 K), it mainly consists of recombination of free excitons, with exciton - exciton collision-induced recombination dominating the spectrum at higher pumping intensities. Emission from the exciton - exciton collision process shows clear stimulated-emission behavior. At sufficiently high pumping intensity, the stimulated emission band shifts abruptly to a longer wavelength due to the formation of an electron -hole plasma. Sharp emission peaks are observed in the stimulated emission bands. The possible origins of these sharp peaks are discussed.