Emission Mechanism in InGaN-Based Light Emitting Devices

摘要

Dynamics of spontaneous and stimulated emissions in InGaN-based laser-diode (LD) structures have been assessed by employing time-resolved photoluminescence (TRPL) and pump & probe (P & P) spectroscopy at room temperature. The LDs are composed of In_xGa_(1-x)N-In_yGa_(1-y)N MQWs ((a) : x = 0.1, y = 0.02, ('3): x = 0.2, y = 0.05), whose stimulated emissions correspond to near ultraviolet (390nm) and violet (420nm), respectively. Almost no Stokes shift was observed for the sample (a) (x = 10 percent). However, Stokes shift in the sample (b) (x = 20 percent) was as large as 250meV, and it is probable that the origin of such deep localization is In-rich quantum dots self-formed during the growth. It is likely that large internal quantum efficiency in In-rich In_xGa_(1-x)N active layers is as a result of zero-dimensionality because capture-cross-section to non-radiative recombination centers are greatly reduced once excitons are trapped at deep localization centers. P & P spectroscopy has revealed that the optical gain was contributed from the nearly delocalized states (the lowest-quantized MQW levels (LQL)) in the sample (a), while it was from highly localized levels with respect to LQL by 250meV for the sample (b). It was found that the photo-generated carriers rapidly (within a few hundred fs) transferred to LQL, and then relaxed to the localized tail within the time-scale of a 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.