Effects of the fluorine plasma treatment on the surface potential and Schottky barrier height of AlxGa1-xN/GaN heterostructures

The variations in surface potential and the Schottky barrier height phi(B) in fluorine-plasma-treated AlxGa1-xN/GaN heterostructures are systematically studied by x-ray photoelectron spectroscopy, providing insights to the mechanisms underlying the strong threshold voltage (V-th) modulation achieved by the F plasma treatment technology. It is found that a large amount of AlF3 appeared on the surface after the treatment, indicating a fluorinated surface. In addition, the surface potential of Al0.25Ga0.75N/GaN heterostructure was increased by similar to 0.38 eV during the first 60 s of the treatment and then rises slowly with additional treatment. Annealing at 400 degrees C in N-2 ambient for 10 min does not affect the surface potential, but results in quick reduction in AlF3, consistent with the relative poor thermal stability of AlF3 reported in literature. phi(B) between Ni and F-plasma-treated Al0.25Ga0.75N surface was extrapolated from the shift in Ga 2p(3/2) core-level and exhibits a small increase of 0.20 eV. The enhanced phi(B) is much smaller than the positive shift in V-th observed from high electron mobility transistors fabricated with the same treatment conditions, suggesting that the primary factor responsible for the conversion from depletion-mode to enhancement-mode AlxGa1-xN/GaN by F plasma treatment is not the surface modifications but rather the negative fixed charges carried by F ions in AlxGa1-xN/GaN heterostructures. (C) 2010 American Institute of Physics. [doi:10.1063/1.3446895