Micro-photoluminescence to investigate lateral diffusion of charge carriers in InGaN/GaN MQWs

Abstract

Micro-photoluminescence (μPL) pinhole scans of an InGaN multi-quantum well (MQW) and single-quantum well (SQW) LED structure show large PL spots compared to the relatively small excitation area. We observe a lateral diffusion over a wide range in the order of 10 µm in both samples. The excitation density is varied to identify a dependence of the diffusion length on the amount of injected charge carriers. Overall the MQW sample shows a smaller diffusion length compared to the SQW sample, which is attributed to the different sample quality. In the MQW sample no significant change in diffusion length with varying excitation power is observed, whereas in the SQW sample the charge carrier diffusion increases with smaller excitation densities. Our spectral resolved μPL setup allows to measure the spectra as a function of the distance, which gives the possibility to investigate the energetic shift. As a result the PL spectra of both samples show a pronounced blue-shift at the center of the PL spots which is assigned to the screening of the quantum confined Stark effect (QCSE) and the band-filling.