We investigated exciton dynamics in ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ multiple quantum wells at 2 K and room temperature by means of time-resolved photoluminescence and pump-and-probe measurements. Under low excitation density, the temporal change of the spontaneous emission indicated slow dynamical features of the two-dimensional exciton localization, while, above a stimulation threshold density, the decay time of the emission was shortened to be less than $\sim 30\hspace{0.5em}\mathrm{ps}$ due to the stimulated emission process. Further, the time-resolved pump-and-probe measurement revealed a fast relaxation of photoexcited delocalized electron-hole $(e-h)$ pairs into localized states. Above stimulation threshold, localized states were saturated, and $e-h$ pairs at delocalized states were observed. Two-dimensional $e-h$ pairs at delocalized states relaxed into localized states and excitons were formed, from which the optical gain was formed in terms of the stimulated emission process.

• Received 26 May 1999

DOI:https://doi.org/10.1103/PhysRevB.60.16660