Growth of N-Polar (0001) GaN in Metal–Organic Vapour Phase Epitaxy on Sapphire

We have systematically studied the growth of N-polar GaN on sapphire in metal–organic vapor phase epitaxy (MOVPE) on different misoriented (0001) sapphire substrates. The key parameter was the NH3 flow, which affects the roughness, growth rate, crystal quality, and impurities. Most parameters show a trend reversal at a V/III ratio around 500 and show either a maximum, such as the growth rate, the sizes of hexagonal hillocks on low misorientations, the yellow luminescence and the mobility, or show a minimum such as the FWHM in X-ray diffraction, the carrier concentration, the surface roughness of large misorientations, or the blue (430 nm) luminescence. This suggests that around a V/III ratio of 500, the surface changes from a Ga-terminated Ga-adlayer surface to a N-terminated 3N-H(2×2) surface. Using extremely low V/III ratios, a smooth N-polar GaN was obtained even on the standard 0.2° misorientation. However, good crystalline quality, low oxygen impurities and smooth surfaces together seem too challenging with low misorientation. The strain-dependent band edge shifted by 14 eV for strain along [0001], which is close to the values reported by Ga-polar GaN.