Effect of grain coalescence on dislocation and stress in GaN films grown on nanoscale patterned sapphire substrates

High-quality GaN films on nanoscale patterned sapphire substrates (NPSSs) are required for micro-light-emitting diode (micro-LED) display. In this study, two types of nucleation layers (NLs), including in situ low-temperature grown GaN (LT-GaN) and ex situ physical vapour deposition AlN (PVD-AlN), are applied on cone-shaped NPSS. The coalescence process of GaN grains on the NPSS is modulated by adjusting the three-dimensional (3D) growth temperatures. Results show that low 3D temperatures help to suppress the Ostwald ripening of GaN grains on the NPSS, facilitating the uniform distribution of 3D GaN grains. Higher 3D temperatures lead to a decrease in the edge dislocation density, accompanied by an increase in residual compressive stress. Compared with LT-GaN NLs, PVD-AlN NLs can effectively improve the growth uniformity, suppress the tilting and twisting of GaN grains grown on NPSSs, and promote the orientation consistency of crystal facets during coalescence. The GaN films grown on NPSSs with PVD-AlN NLs exhibit a decrease in the coalescence time from 2000 s to 500 s, a reduction in dislocation densities from 2.8 x 108 cm-2 to 1.4 x 108 cm-2, and an increase in the residual compressive stress from 0.98 GPa to 1.41 GPa compared to those grown on LT-GaN NLs. This study elucidates trends in dislocation and stress evolution in GaN films on NPSSs with analysis of grain coalescence. The effect of grain coalescence on the dislocation and stress in GaN films grown on nanoscale patterned sapphire substrates with low-temperature grown GaN and physical vapour deposition AlN nucleation layers is comparably investigated.