Effects of the Sapphire Nitridation on the Polarity and Structural Properties of GaN Layers Grown by Plasma-Assisted MBE

The nitridation of the (0001) sapphire surface by a nitrogen rf-plasma source used in GaN molecular beam epitaxy has been investigated. Auger electron spectroscopy measurements were used to estimate the extent of the nitridation effect. Significant nitridation occurred during 100 min at a high temperature (HT) of 700 °C and the nitridated layers usually exhibited three-dimensional islands, while weak nitridation occurred at a low temperature (LT) of 165 °C and resulted to atomically flat surfaces. Linear time dependence has been found for the amount of HT nitridation. High nitridation temperature resulted in Ga-face and low temperature in N-face polarities of overgrown GaN films. Electron microscopy observations revealed that the N-face material exhibited a superior crystalline quality although presented higher surface roughness.