Growth mode induced carrier localization in InGaN/GaN quantum wells
The growth of InGaN quantum wells ( QWs) by metal-organic vapour phase epitaxy is investigated by atomic force microscopy ( AFM). It is shown that for low growth temperatures the surface morphology of the thin InGaN well layer ( 1.5 nm) exhibits a meandering behaviour, i. e. the formation of valleys. Actually, threading dislocation terminations act as "bridge piles'' hindering the step flow growth and consequently resulting in deep surface depressions. Then they coalesce to form valleys along the < 1100 > direction leaving low dislocation density areas in between. Time-resolved cathodoluminescence ( TRCL) studies have been further carried out on such InGaN/GaN QWs. Monochromatic CL maps at the high energy side of the QW luminescence peak reproduces the same features as those observed by AFM. This means that the valleys are characterized by higher energy transitions compared to the energy of the maximum luminescence peak intensity. Thus, the main carrier radiative recombinations take place in between the valleys which are surrounded by large potential barriers. This phenomenon may explain the high efficiency of InGaN/GaN QWs despite the large density of dislocations.