Evolution and control of dislocations in GaN grown on cone-patterned sapphire substrate by Metal Organic Vapor PhaseEpitaxy

Abstract

In this paper, three GaN epilayers grown on commercially available cone-patterned sapphire substrates (PSS) by Metal Organic Vapor Phase Epitaxy (MOVPE) were investigated. Voltage dependence of monochromatic cathodoluminescence (CL) was applied to investigate the threading dislocation (TD) behaviors. Fewer TDs were observed in CL images when the penetration depth of electron was made shorter by altering the acceleration voltage from 30 to 5 kV. It was shown that many TDs were generated on the peaks of the cones, in-between cones, and in the coalescence regions, while there was almost no TD above the slopes of the cones. When the growth pressure was changed from 200 to 100 Torr during the final stage, fewer TDs were observed in CL images, and the full widths at half maximum (FWHMs) for the (0 0 2) and (1 0 2) reflections’ X-ray diffraction (XRD) rocking curves decreased. Furthermore, when the growth temperature was decreased from 1040 to 1000 °C during the middle stage, a clear hexagonal distribution of TDs was observed and the FWHMs for the (0 0 2) and (1 0 2) reflections’ XRD rocking curves further decreased. Atomic force microscopy (AFM) was used to investigate the surface morphology of the samples. In situ optical reflectivity spectra were measured to monitor the growth mode during the different growth stages. Based on the CL and reflectivity spectra results, the process of TDs’ evolution and distribution was deduced and sketched.