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 (TMA1) preflow prior to the growth of GaN buffer layer. The TMA1 preflow served as forming a few monolayers of A1 to modify the nitrided sapphire surface. The effects of the TMA1 preflow on GaN epilayer polarities were investigated by coaxial impact collision ion scattering spectroscopy. It was shown that, by increasing the TMA1 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 AI" was proposed to understand the related mechanisms. The effects of the TMA1 preflow on the epilayer quality were also evaluated by high-resolution x-ray diffraction. [ABSTRACT FROM AUTHOR]