Stimulated emission and excitonic bleaching in GaN epilayers under high-density excitation

Measurements of edge emission on a GaN epilayer under N-2 laser excitation demonstrate stimulated emission and gain. The mechanisms involved are inves tigated by using laser pulses of 250 fs duration to measure simultaneously the photoluminescence and the time-resolved reflectance near the band edge, over a range of excitation densities. The excitons are bleached at high de nsities due to free carriers. A broad luminescence band is seen, extending similar to 100 meV below the low-density emission peak. We show that the on set of the stimulated emission coincides with the bleaching of the excitons (the Mott transition).