Stimulated emission and time-resolved photoluminescence in rf-sputtered ZnO thin films

Stimulated emission (SE) was measured from ZnO thin films grown on $c$ -plane sapphire by rf sputtering. Free exciton transitions were clearly observed at 10 K in the photoluminescence (PL), transmission, and reflection spectra of the sample annealed at $\text{950\hspace{0.05em}°}\mathrm{C}.$ SE resulting from both exciton-exciton scattering and electron hole plasma formation was observed in the annealed samples at moderate excitation energy densities. The SE threshold energy density decreased with increasing annealing temperature up to $\text{∼950\hspace{0.05em}°}\mathrm{C}.$ The observation of low threshold exciton-exciton scattering-induced SE showed that excitonic laser action could be obtained in rf-sputtered ZnO thin films. At excitation densities below the SE threshold, time-resolved PL revealed very fast recombination times of $\text{∼74\hspace{0.17em}}\mathrm{ps}$ at room temperature, and no significant change at 85 K. The decay time for the SE-induced PL was below the system resolution of $\text{<45\hspace{0.17em}}\mathrm{ps}.$