Optical characteristics of MOCVD-grown InGaN/GaN multiple quantum wells investigated by excitation energy dependent PL and PLE spectroscopy

摘要

We have systematiclaly studied the optical properties of InGaN/GaN multiple quantum wells (MQWs) at 10K under different excitation conditions using photoluminescence (PL) and PL excitation (PLE) spectroscopy. The MQWs were grown on 1.8-#mu#m-thick GaN on c-plane sapphire films by metal-organic chemical vapor deposition. Structures consist of 12 MQWs with 3-nm-thick In_(0.2)Ga_(0.8)N wells, 4.5-nm-thick GaN barriers, and a 100-nm-thick Al_(0.07)Ga_(0.93)N capping layers. We found that the PL emission consists of a strong main peak at 2.80 eV and a much weaker and broader secondary peak at approx 2.25 eV. We observed that both the peak position and the spectral width of the main peak vary with the excitation energy. The PL peak blueshifts and narrows when the excitation energies are varied from 3.81 eV (above the bandgap of the AlGaN capping layer) to 2.99 eV (below the bandgap of the GaN barrier layers). The intensity ratio of the main peak to the secondary peak also varied with excitation energy. The PLE studies, combined with the excitation dependent PL emission studies, reveal that the two observed emission peaks originate from different layers of the MQWs. In addition, we found that the InGaN-related spontaneous emission processes are strongly affected by different carrier generation conditions with further complications associated with compositional fluctuations and possible carrier localization. The differences in the observed emission processes under varying excitation conditions can be explained by considering the transfer of carriers between the different layers of the MQW structures.