  Two types of nucleation layers (NLs), including in-situ low-temperature grown
GaN (LT-GaN) and ex-situ sputtered physical vapor deposition AlN (PVD-AlN), are
applied on cone-shaped nanoscale patterned sapphire substrate (NPSS). The
initial growth process of GaN on these two NLs is comparably investigated by a
series of growth interruptions. The coalescence process of GaN grains is
modulated by adjusting the three-dimensional (3D) temperatures. The results
indicate that higher 3D temperatures reduce the edge dislocation density while
increasing the residual compressive stress in GaN films. Compared to the LT-GaN
NLs, the PVD-AlN NLs effectively resist Ostwald ripening and facilitate the
uniform growth of GaN grains on NPSS. Furthermore, GaN films grown on NPSS with
PVD-AlN NLs exhibit a reduction of over 50% in both screw and edge dislocation
densities compared to those grown on LT-GaN NLs. Additionally, PVD-AlN NLs
result in an increase of about 0.5 GPa in the residual compressive stress
observed in GaN films.

Materials Science