Desorption induced formation of low-density GaN quantum dots: nanoscale correlation of structural and optical properties

We report on the formation process of GaN/AlN quantum dots which arises after the deposition of 1 - 2 monolayers of GaN on an AlN/sapphire template followed by a distinct growth interruption. The influence of the duration of a growth interruption on structural and optical properties of the GaN layer has been systematically investigated. Quantum dots develop from initially bulky GaN islands, which nucleate in close vicinity to bundles of threading dislocations. For prolonged growth interruptions a decreasing island size is observed which is consistent with a systematic blue shift of the emission wavelength. In addition, a fragmentation of the bulky GaN islands into several smaller islands occurs, strongly depending on local strain fields caused by threading dislocations as well as on a different facet orientation of the islands. This morphological transition during growth interruption eventually leads to GaN quantum dot formation which assemble as clusters with a density of 108 cm-2. Desorption of GaN is identified as the major source for this morphological transition. The growth interruption time allows for tuning of the quantum dot emission wavelength in the UV spectral range.