Atomic force microscopy analysis of Ga-face and N-face GaN grown on Al2O3 (0001) by plasma-assisted molecular beam epitaxy

GaN thin films were grown on Al₂O₃ (0001) substrates by RF-plasma assisted molecular beam epitaxy with either N-face or the Ga-face polarity. Atomic force microscopy was used to investigate the surface morphology and the polarity of the GaN films. Etching in a KOH solution always resulted in a significant increase in the surface roughness of the N-face material, with characteristic pyramid-shaped features. On the contrary, the KOH solution did not modify the surface morphology of the Ga-face material. For both polarities, significant variations of the surface morphology were observed depending on the V/III flux ratio during growth. A range of potential surface morphologies was determined for each polarity which may be used for direct assessment of the film´s polarity