Ultrathin silicon oxynitride films with thickness in the range of 1.8-3.5 nm have been produced on Si(100) by nitridation of an NO-oxidized surface with an electron-cyclotron resonance plasma source. The films were annealed in N 2O at 950°C for times up to 60 s and formed into Al-gated capacitors for capacitance-voltage (CV) and current-voltage analysis. The rapid annealing increases the oxygen content of the films but results in capacitors with excellent electrical properties. For a plasma oxynitride with equivalent oxide thickness, t eq = 1.8 nm, current reductions of ∼20 over that for SiO 2 films have been obtained for gate voltages in the range 1-1.5 V. For comparison, the thickness of the oxynitrides was obtained by X-ray photoelectron spectroscopy of the Si 2p, N 1s, and O 1s photoelectrons. By analyzing the yield from thick silicon dioxide and silicon nitride films, the electron escape depth in silicon nitride was estimated to be 1.7 nm for the Si 2p electrons. By correcting the measurements of the oxygen/nitrogen concentration ratio obtained from the O 1s and N 1s XPS peaks, and calculating the dielectric constant with a Bruggeman effective medium approximation, the equivalent oxide thickness was calculated. Agreement to ∼0.2 nm was obtained with t eq determined by the CV analysis. Information obtained from the XPS analysis can also give information about bonding configurations and possible errors due to nonuniform stoichiometry as a function of depth.