In inhomogeneity and emission characteristics of InGaN

Recombination dynamics of spontaneous and stimulated emissions have been as sessed in InGaN-based light emitting diodes (LEDs) and laser diodes (LDs), by employing time-resolved photoluminescence and pump and probe spectroscop y. As for an In0.02Ga0.98N ultraviolet LED, excitons are weakly localized b y 15 meV at low temperature, but they become almost free at room temperatur e (RT). It was found that addition of a small amount of In results in the r eduction of nonradiative recombination centres originating from point defec ts. The internal electric field does exist in InGaN active layers, and indu ces a large modification of excitonic transitions. However, it alone does n ot explain the feature of spontaneous emission observed in an In0.3Ga0.7N b lue LED such as an anomalous temperature dependence of peak energy, almost temperature independence of radiative lifetimes and mobility-edge type beha viour, indicating an important role of exciton localization. The lasing mec hanism was investigated for In0.1Ga0.9N near ultraviolet (390 nm), In0.2Ga0 .8N violet-blue (420 nm) and In0.3Ga0.7N blue (440 mn) LDs. The optical gai n was contributed from the nearly delocalized states (the lowest quantized levels (LQLs) within quantum wells) in the violet LD, while it was from hig hly localized levels with respect to the 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 I ps) transferred to the LQL, and then relaxed to the localized tail within the timescale of a few ps, giving rise to the optical gain. Such gain spectra were saturated and other bands appeared in the vicinity of the LQL under higher photo-excitation.