Polarity inversion of GaN films by trimethyl-aluminum preflow in low-pressure metalorganic vapor phase epitaxy growth

The polarity of GaN films grown on nitrided (0001) sapphire substrates by low-pressure metalorganic vapor phase epitaxy was controlled by trymethyl-aluminum (TMAl) preflow prior to the growth of GaN buffer layer. The TMAl preflow served as forming a few monolayers of Al to modify the nitrided sapphire surface. The effects of the TMAl preflow on GaN epilayer polarities were investigated by coaxial impact collision ion scattering spectroscopy. It was shown that, by increasing the TMAl preflow time, the polarities of GaN epilayers were changed from a N polarity to a mixed polarity, and finally to a pure Ga polarity when the preflow time was over than 5 s. A schematic model of "two monolayers of Al" was proposed to understand the related mechanisms. The effects of the TMAl preflow on the epilayer quality were also evaluated by high-resolution x-ray diffraction. (C) 2002 American Institute of Physics.