GaN nucleation on patterned sapphire substrate with different shapes for improved GaN overgrowth

GaN was grown on cone- and dome-patterned sapphire (CPSS and DPSS) at different nucleation times of 40, 80 and 160 s. The GaN was also grown on flat sapphire substrate (FSS) for comparison. The results showed that the GaN growth improved by increasing the nucleation time, except for the case of FSS. Compared to CPSS, DPSS led to better GaN growth. This was due to the geometry of the dome patterns, which had more slanted curvature than cone patterns. Hence, more nucleation was promoted on the trenches and this was desirable to improve the GaN overgrowth. With 160 s of nucleation, the GaN coalescence was better for the growth on DPSS than on CPSS. This resulted in an improved surface of the overgrown GaN layer and lower full-width at half-maximum of XRD (FWHM-XRD) peaks. Strain study showed that the GaN layers grown on DPSS exhibited larger residual in-plane strain than the layers on CPSS due to the achievement of better overgrown GaN. Nonetheless, the strain reduced with nucleation time due to further dislocations inclination. Generally, the GaN layers on FSS showed better crystalline quality than on CPSS and DPSS. However, the layers suffered from larger strain.