Polar Dependence of Threading Dislocation Density in GaN Films Grown by Metal-Organic Chemical Vapor Deposition

We investigate the threading dislocation (TD) density in N-polar and Ga-polar GaN films grown on sapphire substrates by metal-organic chemical vapor deposition. X-ray diffraction results reveal that the proportion of screw type TDs in N-polar GaN is much larger and the proportion of edge type TDs is much smaller than that in Ga-polar. Transmission electron microscope results show that the interface between the AlN nucleation layer and the GaN layer in N-polar films is smoother than that in Ga-polar films, which suggests different growth modes of GaN. This observation explains the encountered difference in screw and edge TD density. A model is proposed to explain this phenomenon.