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.

Improved Performance of GaN-Based Light-Emitting Diodes Grown on Si (111) Substrates with NH3 Growth Interruption

We demonstrate the highly efficient, GaN-based, multiple-quantum-well light-emitting diodes (LEDs) grown on Si (111) substrates embedded with the AlN buffer layer using NH3 growth interruption. Analysis of the materials by the X-ray diffraction omega scan and transmission electron microscopy revealed a remarkable improvement in the crystalline quality of the GaN layer with the AlN buffer layer using NH3 growth interruption. This improvement originated from the decreased dislocation densities and coalescence-related defects of the GaN layer that arose from the increased Al migration time. The photoluminescence peak positions and Raman spectra indicate that the internal tensile strain of the GaN layer is effectively relaxed without generating cracks. The LEDs embedded with an AlN buffer layer using NH3 growth interruption at 300 mA exhibited 40.9% higher light output power than that of the reference LED embedded with the AlN buffer layer without NH3 growth interruption. These high performances are attributed to an increased radiative recombination rate owing to the low defect density and strain relaxation in the GaN epilayer.