(Invited) Study of Carrier Localization, Carrier Transportation and Carrier Recombination Processes in Blue-Emitting InGaN/GaN MQWs

InGaN/GaN MQWs grown on c-plane sapphire substrate show high internal quantum efficiency, regardless of their high threading dislocation density (10⁸ cm^-2~ 10¹⁰ cm^-2) that is introduced by lattice mismatch. This high-IQE phenomenon is attributed to bandgap energy fluctuation and carrier localization in the QW layer [1].

In the present work, temperature-dependent PL, confocal microscopy, and nanometer-scale time-resolved photoluminescence (TRPL) spectroscopy were employed to analyze the carrier localization, carrier transportation and carrier recombination processes in blue-emitting InGaN/GaN MQWs.

The temperature-dependent PL spectra were taken from 10 K to 300 K. The decreased spectrum-integrated PL intensity with increasing temperature was observed, which was fitted with second-order Arrhenius equation, as shown in Fig. 1. The fitting indicated two non-radiative channels: the one with high activation energy (dominated at high temperature) corresponded to thermal activation of carriers out of the strongly localized states, while the one with low activation energy (dominated at low temperature) corresponded to thermal activation of carriers out of the weakly localized states. The S-shaped shift of PL peak energy and the inverse S-shaped shift of PL FWHM were also observed, as shown in Fig. 2 and Fig. 3, which were explained with carrier localization and carrier dynamics.

The confocal microscope image collecting near bandedge emission showed inhomogeneous PL intensity distribution, as shown in Fig. 4. Previous analysis [2] has shown that the bright region had smaller bandgap energy than dark region did. Nanometer-scale TRPL spectra were taken at different places in the confocal microscope image. As shown in Fig. 5, TRPL spectra taken at the center of bright region and the center of dark region show single exponential decay, which are characterized by a single lifetime and corresponds to carrier localization in strongly localized state and weakly localized state, respectively. Comparing with bright region, dark region has shorter lifetime, indicating a higher probability of nonradiative recombination and carrier transportation from dark region to bright region. TRPL spectrum taken at the boundary of bright region, however, shows double exponential decay, as shown in Fig. 5, which is characterized by two lifetimes. The short lifetime corresponds to carrier transportation from weakly localized state to strongly localized states, while the long lifetime corresponds to carrier recombination at local strongly localized state.

References:

1. R. A. Oliver, et al., Journal of Physics D-Applied Physics, 43, 35 (2010)

2. C. Li, et al., ECS Journal of Solid State Science and Technology, 2, 11 (2013)