Splitting of X-ray diffraction and photoluminescence peaks in InGaN/GaN layers

Abstract

The presence of two, or more, X-ray diffraction (XRD) peaks from an InGaN epilayer is sometimes regarded as an indicator of phase segregation. In this report, detailed characterisation of an InGaN/GaN bilayer by a combination of XRD and Rutherford backscattering spectrometry (RBS) showed that splitting of the XRD peak may occur in the absence of phase decomposition. An XRD reciprocal space map performed on the (105) plane showed that one component of the partially resolved InGaN double peak is almost aligned with that of the GaN buffer, indicating that part of the layer is pseudomorphic to the GaN template. From a consideration of the effect of strain on the c- and a- lattice constants, both the partially relaxed and the pseudomorphic components were shown to have the same indium content. The layer composition deduced from XRD measurements is confirmed by RBS. Depth-resolving RBS/channelling angular scans also shows that the region closer to the GaN/InGaN interface is nearly pseudomorphic to the GaN substrate, whereas the surface region is almost fully relaxed. Furthermore, photoluminescence (PL) spectroscopy shows a double peak corresponding to the strained and relaxed regions of the sample. The presence of the XRD and PL double peaks is shown to be unrelated to compositional grading or phase separation.