Emission Mechanism in InGaN-Based Light Emitting Devices
Yoichi KAWAKAMI, Kunimichi OMAE, Yukio NARUKAWA, Shuji NAKAMURA and Shigeo FUJITA
Abstract: 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 InxGa1-xN-InyGa1-yN MQWs [(a): x=0.1, y=0.02, (b): x=0.2, y=0.05], whose stimulated emissions correspond to near ultraviolet (390 nm) and violet (420 nm), respectively. Almost no Stokes shift was observed for the sample (a) (x=10 %). However, Stokes shift in the sample (b) (x=20 %) was as large as 250 meV, 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 InxGa1-xN 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 250 meV 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. Key Words:InGaN active layers, Exciton localization, Pump & probe spectroscopy, Gain formation