Investigation of inhomogeneities in (Al, Ga, In)N heterostructures by STEM and cathodoluminescence

Wurtzite (Al, Ga, In)N heterostructures grown by metal organic vaper phase epitaxy (MOVPE) were studied using quantitative analytical scanning transmission electron microscopy (STEM) and cathodoluminescence (CL). The STEM results can be summarized as follows. The interfaces in e.g. In0.12Ga0.88N/GaN single quantum wells (SQWs) appear to be asymmetric. The lower interface in the growth direction is more abrupt than the upper one, where a grading in the In concentration is significant. We found composition fluctuations in the nanometer range within the InGaN QWs, which are supposed to cause the localized exitonic behavior of the observed photoluminescence (PL) emission. In the sample with 17 nm thick InGaN QWs we observed relaxation effects, which are not present in the thin QWs of 2 nm thickness. CL results on both InGaN/GaN and AlGaN/GaN SQW structures show generally inhomogeneous emission intensity in panchromatic CL micrographs on a 1 μm scale, which is related to local variations of the interface quality. CL spectra recorded from defect sites in AlGaN/GaN SQWs are dominated by the so-called ‘yellow-emission’. In the samples containing InGaN layers, the grown-in hexagonal pyramids and mesa-like structures as well as micropipes were commonly observed.