Interface properties of BCN/GaN metal-insulator-semiconductor (MIS) structures are investigated by X-ray photoelectron spectroscopy (XPS) and capacitance versus voltage (C-V) characteristics measurements. The BCN/GaN samples are fabricated by in situ process consisting of plasma treatment and deposition of BCN film in the plasma-assisted chemical vapor deposition (PACVD) apparatus. XPS measurement shows that the oxide formation at the BCN/GaN interface is suppressed by nitrogen (N-2) and hydrogen (H-2) plasma treatment. The interface state density is estimated from C-V characteristics measured at I MHz using Terman method. The minimum interface state density appears from 0.2 to 0.7 eV below the conduction band edge of GaN. The minimum value of the interface state density is estimated to be 3.0 x 10(10) eV(-1) cm(-2) for the BCN/GaN structure with mixed N-2 and H-2 plasma treatment for 25 min. Even after annealing at 430 degrees C for 10 min, the interface state density as low as 6.0 x 10(10) eV(-1) cm(-2) is maintained. (c) 2006 Elsevier B.V. All rights reserved.