Depth dependence of optical property beyond the critical thickness of an InGaN film

We study the depth-dependent variation of optical property beyond the critical thickness in an InGaN thin film. In the sample, both free-carrier and localized-state recombination activities are observed. The emission peak corresponding to the localized states in photoluminescence (PL) measurement becomes more prominent with increasing sample depth, implying stronger clustering in the deeper layers. Although the PL spectral peak variation is weak, that of cathodoluminescence (CL), corresponding to the activities of the localized states, shows a clear red shift trend with depth. The red shift trend is attributed to the stronger clustering behavior and possibly stronger quantum-confined Stark effect in the nano-clusters, which is due to the residual strain beyond the critical thickness, in a deeper layer.