We investigated the nature of nitridated layers formed on sapphire (0001) substrates by heating at 1050°C in NH 3 gas and their influence on GaN layers grown by atmospheric pressure metalorganic vapor phase epitaxy. The intensity of the N ls nitrogen peak in X-ray photoelectron spectroscopy (XPS) rapidly increased with nitridation time, reached saturation after a few minutes, then continued to increase gradually. This behavior was found to correspond to a morphological change in the nitridated layer, from a flat layer to one with high-density (10 9–10 10 cm −2) protrusions, revealed by atomic force microscope (AFM) observations. We determined that the nitridated layers are amorphous AlO 1− x N x layers through transmission electron microscope (TEM) observations and energy dispersive X-ray spectrometry (EDX) measurements. A flat nitridated layer allows two-dimensional growth, leading to a thick GaN layer with a smooth surface and improved photoluminescence (PL) intensity due to recombination near the band-edge. However, nitridated surfaces with protrusions result in three-dimensional growth and a significant decrease in PL. Thus, to grow a thick high-quality GaN layer with a smooth surface, the surface of the nitridated layer must be controlled so that a crystal-amorphous-crystal (CAC) growth mechanism can occur.