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Fluorine plasma treatment was used prior to the Schottky metal deposition on the undoped Al0.45Ga0.55N, which aimed at the solar-blind wavelength. After fluorine plasma treatment and before depositing the Ni/Au Schottky, the samples were thermal annealed in the N2 gas at 400 °C. The reverse leakage current density of Al0.45Ga0.55N Schottky diode was reduced by 2 orders of magnitude at −10 V. The reverse leakage current density was reduced by 3 orders of magnitude after thermal annealing. Further capacitance–frequency analysis revealed that the fluorine-based plasma treatment reduces the surface states of AlGaN by one order of magnitude at different surface state energies. The capacitance–frequency analysis also proved that the concentration of carriers in AlGaN top is reduced through fluorine plasma treatment. [ABSTRACT FROM AUTHOR]