Thin oxynitride gate dielectric films were prepared by thermal oxidation of Low-Pressure Chemical Vapor Deposition (LPCVD) silicon-rich silicon nitride (SRN) at temperature ranging 850 to 1050 degreesC. X-ray photoelectron spectroscopy (XPS) was used to study the bonding structure of resulting oxynitride at the interface. We found that the Si atoms at the interface exist in the form of random bonding (RB) to oxygen and nitrogen atoms for samples oxidized at 850 and 950 degreesC. However, with heavy oxidation, the interface nitrogen atoms exist in the form of SiN4 phase and the interface oxynitride layer is a random mixture (RM) Of SiO4 and SiN4 phases for samples oxidized at 1050 degreesC. These results have great impact on the interface defect density. At interface with random mixture of SiO4 and SiN4 phases, the constraint force of Si atom in SiN4 phases is large and would result in higher interface trap density. Whereas in random bonding oxynitride/Si interface, the interface trap density is low because of less constraint force of Si atoms. In addition, the RB structure will help to minimize the composition, permittivity and bandgap fluctuations at the interface and improve the reliability of MOS devices. (C) 2004 Elsevier B.V. All rights reserved.