Effects of annealing on structural and optical properties of InGaN/GaN multiple quantum wells at emission wavelength of 490 nm

We report on structural and optical properties of Si-doped InGaN/GaN multiple quantum wells (MQWs) investigated by employing high-resolution x-ray diffraction (HRXRD), reciprocal space mapping (RSM), continuous-wave photoluminescence (PL) spectroscopy, and time-resolved photoluminescence (TRPL) spectroscopy. The MQWs were grown on c-plane sapphire substrate by metal-organic chemical vapor deposition (MOCVD). HRXRD and RSM revealed that the MQWs are coherently strained on the GaN base layer and the coherence remains intact after annealing at temperatures up to 1100 °C. They also revealed an occurrence of In/Ga atoms interdiffusion across the QW interfaces at elevated temperatures. The shift in PL emissions towards their lower-energy sides with increasing annealing temperatures provides evidence for the enhanced formation of In-rich regions (quantum dots, QDs) within the QWs. An anomalous photon-energy dependence of PL lifetimes is observed at 5 K; however, the dependence reverts to normal when the temperature is increased to 200 K. This optical transition behavior is discussed based on the self-forming of QDs due to the enhanced In-clustering at elevated temperatures and the anneal-induced generation/activation of defects/impurities-related recombination centers.