Surface States in the Al_xGa_1-xN Barrier in Al_xGa_1-xN/GaN Heterostructures

Frequency-dependent capacitance-voltage (C-V) measurements have been performed on modulation-doped Al_0.22Ga_0.78N/GaN heterostructures to investigate the characteristics of the surface states in the Al_xGa_1-xN barrier. Numerical fittings based on the experimental data indicate that there are surface states with high density locating on the Al_xGa_1-xN barrier. The density of the surface states is about 10¹²cm^-2eV^-1, and the time constant is about 1 μs. It is found that an insulating layer (Si₃N₄) between the metal contact and the surface of Al_xGa_1-xN can passivate the surface states effectively.