Relaxation of InGaN thin layers observed by x-ray and transmission electron microscopy studies

Double-crystal and triple-axis x-ray diffractometry and transmission electr on microscopy are used to characterize the microstructure, strain, and comp osition of InGaN layers grown on GaN by metalorganic chemical vapor deposit ion (MOCVD). Three different samples with increasing In concentration have been studied, all grown on GaN deposited on sapphire either with GaN or AIN buffer layers. It was found that InGaN layers with nominal 28% and 40% InN content consist of two sub-layers; the first sub-layer is pseudomorphic wi th the underlying GaN with lower In content than nominal. The top sub-layer is fully relaxed with a high density of planar defects and In content clos e to the nominal value. This is in contrast to a common assumption applied to InGaN quantum wells that 'quantum-dot like areas' are formed with differ ent In content. The sample with the nominal indium concentration of 45% doe s not exhibit any intermediate strained layer, is fully relaxed and the In concentration (43.8%) agrees well with the nominal value.