The energy distribution of interface states in the Si band-gap present at ultra-thin silicon oxide/Si interfaces is obtained from XPS measurements under bias. The substrate Si 2p peak for the <Pt/silicon oxide/Si> MOS structure is shifted by the application of a bias voltage to the Si with respect to the Pt, due to the accumulation of charges in the interface states in the Si band-gap. The energy distribution of interface states can be obtained from measurements of the bias-induced shift of the Si 2p peak as a function of the bias voltage. All the observed interface state spectra have peaked-structure, and the number of' the peaks and their energies depend on the oxide formation method. From comparison with theoretical calculations, the observed interface state peaks are attributed to Si dangling bonds in various environments. The variation in the interface state spectra is attributed to different atomic densities of the oxide layers. The formation of Si-CN bonds at the interface using cyanide treatment is found to decrease the interface state density markedly. The cyanide treatment improves the electrical characteristics of the MOS tunneling diodes.