The effects of varying threading dislocation density on the optical properties of InGaN/GaN quantum wells

The effects of the threading dislocation (TD) density on the optical properties of a series of comparable In_xGa_1-xN/ GaN multiple QW structures were studied. The TD density ranged from 2 × 10⁷ cm^-2, for a structure grown on a free-standing GaN substrate, to 5 × 10⁹ cm^-2 grown on a sapphire substrate. Room temperature internal quantum efficiencies (IQEs) were determined by temperature dependent photoluminescence (PL); no systematic dependence of the IQE on the TD density was found. The excitation power density dependence of the efficiency was investigated, which also showed no systematic dependence on TD density. PL excitation spectroscopy was used to verify that equivalent carrier densities were generated within the QWs of each structure. The lack of systematic dependence of the optical properties on TD density is attributed to the strong carrier localisation in InGaN/GaN QWs. At the highest density of TDs studied, it is estimated that the average defect separation greatly exceeds the in-plane diffusion lengths of electrons and holes; consequently the majority of carriers in the system are isolated from the TDs.