Effects of the fluorine plasma treatment on the surface potential and Schottky barrier height of Al_x Ga_1-x N/GaN heterostructures

The variations in surface potential and the Schottky barrier height φ_B in fluorine-plasma-treated Al_x Ga_1-x N/GaN heterostructures are systematically studied by x-ray photoelectron spectroscopy, providing insights to the mechanisms underlying the strong threshold voltage (Vth) 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 Al_0.25 Ga_0.75 N/GaN heterostructure was increased by ∼0.38 eV during the first 60 s of the treatment and then rises slowly with additional treatment. Annealing at 400 °C in N₂ 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. φ_B between Ni and F-plasma-treated Al_0.25 Ga_0.75 N surface was extrapolated from the shift in Ga 2 p_3/2 core-level and exhibits a small increase of 0.20 eV. The enhanced φ_B is much smaller than the positive shift in Vth 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 Al_x Ga_1-x N/GaN by F plasma treatment is not the surface modifications but rather the negative fixed charges carried by F ions in Al _x Ga_1-x N/GaN heterostructures.