Cathodoluminescence spatially resolves optical transitions in thick group-III and N-polar InGaN films

Abstract

The growth of thick group-III polar InGaN films beyond the critical thickness remains a challenge due to the large miscibility gap and lattice mismatch between InN and GaN leading to phase separation and inhomogeneous distribution of indium that impacts the luminescence properties across the film. The growth of N-polar InGaN can circumvent these challenges due to the increased stability of InN, thus improving film quality. However, overall luminescence from N-polar InGaN films is consistently lower than that of group-III polar InGaN films grown under identical conditions. In this study, spatially resolved cathodoluminescence (CL) measurements are used to reveal the optical properties of group-III and N-polar thick InGaN films. In the case of group-III polar films, predominant CL luminescence arises from the periphery of V-pits which are regions with a high accumulation of indium, while negligible CL luminescence is observed at the apex of the V-pit, indicative of centers for non-radiative recombination. Large differences in the CL luminescence intensity between the group-III polar and N-polar InGaN are a result of significant differences in the carrier lifetimes of the respective polarities (∼50-200 ps: N-polar, ∼500-700 ps: group-III polar InGaN). Since the decay behavior of the transient in N-polar InGaN is bi-exponential, it is suspected that oxygen impurities play a dominant role in the overall luminescence quenching in N-polar InGaN films.