Dynamics of spontaneous and stimulated emissions in GaN-based semiconductors

Recombination dynamics of spontaneous and stimulated emissions have been assessed in InGaN-based light emitting diodes (LEDs) and laser diodes (LDs), by employing time-resolved photoluminescence and pump and probe spectroscopy. As for an In_0.02Ga_0.98N-ultraviolet -LED, excitons are weakly localized by 15 meV at low temperature, but they become almost free at room temperature (RT). It was found that addition of small amount of In results in the reduction of nonradiative recombination centers originating from point defects. The internal electric field does exist in InGaN active layers, and induces a large modification of excitonic transitions. However, it alone does not explain the feature of spontaneous emission observed in an In_0.3Ga_0.7N-blue-LED such as an anomalous temperature dependence of peak energy, almost temperature independence of radiative lifetimes and mobility-edge type behavior, indicating an important role of exciton localization. The lasing mechanism was investigated for In_0.1Ga_0.9N-near ultraviolet (390 nm), In_0.2Ga_0.8N-violet blue (420 nm) and In_0.3Ga_0.7N-blue (440 nm) LDs. The optical gain was contributed from the nearly delocalized states [the lowest-quantized levels (LQL) within quantum wells] in the violet-LD, while it was from highly localized levels with respect to LQL by 250 meV for the violet blue-LD, and by 500 meV for the blue-LD. It was found that the photo-generated carriers rapidly (less than 1 ps) transferred to LQL, and then relaxed to the localized tail within the time-scale of few ps, giving rise to the optical gain. Such gain spectra were saturated and other bands appeared in the vicinity of LQL under higher photo-excitation.