High-resolution x-ray photoelectron spectroscopy (XPS) in conjunction with secondary-ion-mass spectrometry was used to study the chemical nature and distribution of N in oxynitride films formed by rapid thermal N2O processes (RTPs) or conventional furnace methods. The kinetics of furnace oxide growth in N2O are slower than that in O-2 During reoxidation the oxidation rate increased to that in pure O-2 and the N in the SiO2-Si interface region is displaced into the bulk of the oxide. High-resolution synchrotron Si 2p core-level photoemission spectroscopy (PES) was used to study the oxide-Si(100) interface suboxide structures produced by RTP with and without the presence of N. XPS N Is studies indicated that there are two types of N in the RTP oxynitride films. The chemical bond configuration of the first type of N is similar to that of N in Si3N4 and is mainly distributed within the first 1 nm from the interface. The second type of N is distributed mainly outside of the first 1 nm region, and the N is likely bonded to two Si and one oxygen atom. PES studies showed that Si formed suboxides with oxygen at the interface for all oxynitride films. It is found that there is no change in the Si+1 structure while there is a dramatic decrease in the Si+2 and Si+3 states with the inclusion of N in the oxide. Both the XPS and PES results are explained in terms of a strain reduction as N is incorporated in the film near the interface region, where Si3N4 functions as a buffer layer which reduces the stress caused by the large Si lattice mismatch between the bulk Si and the oxide overlayer. About 1/5 of the Si+2 and 1/3 of Si+3 atoms at the SiO2-Si interface have been replaced by the Si3N4, buffer layer at the oxynitride-Si interface.