(B0.5-xSix)N0.5 films (0≤x≤0.5) were prepared by dual ion beam deposition. Buffer layers were added to improve the film adhesion. The film structure was characterized by x-ray photoelectron spectroscopy, infrared absorption, and x-ray diffraction. The hardness and elastic modulus were measured by a nanoindenter. The I–V curves of the Ti/(B0.5-xSix)N0.5/buffer/p-Si/Ti diodes were investigated. The films are composites of cubic-boron nitride (c-BN), h-BN, and Si–N. When x=0, the film contains 70–75 vol % c-BN and has a hardness ≈38 GPa, but peels off quickly from the substrate after exposure to air. When x increases to 0.013, a small amount of Si–N phase is formed, which serves to release part of the internal stress without affecting the volume fraction of c-BN or the mechanical strength, and good adhesion is achieved. For higher Si content (0.013≪x≤0.067), the c-BN phase is disrupted with simultaneous replacement by h-BN. Rapid drops in the hardness and elastic modulus follow. When the Si content continues to increase (0.067 to 0.51), the fraction of the h-BN phase decreases progressively with simultaneous replacement by Si–N. Both the hardness and elastic modulus rise and approach those of silicon nitride. I–V curves of Ti/(B0.5-xSix)N0.5/buffer/p-Si/Ti diodes show a strong rectifying effect, which becomes less pronounced when x is larger.© 1997 American Vacuum Society.