The initial stage of the oxynitride on Si(1 0 0) 2 x 1 surface by the thermal annealing incorporated with the nitrogen ion was investigated using photoemission spectroscopy (PES). After growing about 20 Angstrom thermal SiO2 layer on Si(1 0 0) 2 x 1 surface, we implanted nitrogen ion with 200 eV ion energy on the surface at RT. Oxynitride and Si3N4 are found to co-exist during the thermal nitridation. PES results of Si 2p and N 1s were shown that N(-Si2O) was a major component at 800 degreesC annealing. The oxygen of this oxynitride was dissolved as a volatile SiO as increasing the nitridation temperature more than 1000 degreesC and Si3N4 became a majority components. Also, N 1s core-level spectra were shifted toward the low binding energy. (C) 2003 Elsevier Science B.V. All rights reserved.