On indium segregation in InGaN/GaN quantum well structures

Indium segregation in InGaN is a crucial phenomenon controlling the optical characteristics of such a compound. Its formation is a natural process due to the large lattice mismatch between InN and GaN. In InGaN, indium compositional fluctuations and phase separated InN clusters can be observed, particularly when the nominal indium content is high. In this paper, we report the results of material characterization, including X-ray diffraction, high-resolution transmission electron microscopy, atomic force microscopy, etc., and optical studies, including photo-luminescence (PL) and stimulated emission (SE). In material characterization, we observed clear indium aggregation and phase separation. Such indium composition variations become more prominent as the nominal indium content increases. Disk-shape features with dimensions of a few nm by a few tens nm near the quantum well regions could be clearly seen. Regarding optical studies, the widely observed Sshape PL peak position variations with temperature were observed and from which the localization energy values of the localized states were calibrated. Also, the side-peak feature of PL spectrum was attributed to the phonon-replica transition. Such a transition could become quite prominent in a sample with high nominal indium content. In the stimulated emission study, the most prominent feature observed was the two-peak distribution of SE spectrum. These two peaks were supposed to originate from the recombination of localized and free carriers, respectively.