In inhomogeneity and emission characteristics of InGaN

Abstract

Recombination dynamics of spontaneous and stimulated emissions have beenassessed in InGaN-based light emitting diodes (LEDs) and laser diodes (LDs),by employing time-resolved photoluminescence and pump and probe spectroscopy.As for an In0.02Ga0.98N ultraviolet LED, excitons are weaklylocalized by 15 meV at low temperature, but they become almost free at roomtemperature (RT). It was found that addition of a small amount of In resultsin the reduction of nonradiative recombination centres originating from pointdefects. The internal electric field does exist in InGaN active layers, andinduces a large modification of excitonic transitions. However, it alone doesnot explain the feature of spontaneous emission observed in anIn0.3Ga0.7N blue LED such as an anomalous temperature dependence ofpeak energy, almost temperature independence of radiative lifetimes andmobility-edge type behaviour, indicating an important role of excitonlocalization. The lasing mechanism was investigated for In0.1Ga0.9Nnear ultraviolet (390 nm), In0.2Ga0.8N violet-blue (420 nm) andIn0.3Ga0.7N blue (440 nm) LDs. The optical gain was contributed fromthe nearly delocalized states (the lowest quantized levels (LQLs) withinquantum wells) in the violet LD, while it was from highly localized levelswith respect to the LQL by 250 meV for the violet-blue LD, and by 500 meV forthe blue LD. It was found that the photo-generated carriers rapidly (less than1 ps) transferred to the LQL, and then relaxed to the localized tail withinthe timescale of a few ps, giving rise to the optical gain. Such gain spectrawere saturated and other bands appeared in the vicinity of the LQL underhigher photo-excitation.