The phase transition from Langmuir-type adsorption to two-dimensional (2D) oxide island growth during initial oxidation on the Si(001) surface was investigated by real-time Auger electron spectroscopy (AES) combined with reflection high-energy electron diffraction (RHEED). Curve-fitting analysis of the oxygen uptake curve obtained by O-KLL Auger electron intensity revealed that the phase transition occurs steeply at similar to630 degreesC and no oxidation occurs after completion of 2D growth of oxide islands, whereas oxides grows gradually at the interface following Langmuir-type adsorption. It was observed that the very thin oxide layer grown at 616 degreesC is more easily decomposed than that grown at 653 degreesC in spite of almost the same thickness. Furthermore, the RHEED intensity ratio between half-order spots indicated that etching of the surface starts suddenly just at the phase transition temperature of similar to630 degreesC. The steepness of the phase transition, the sudden start of SiO desorption and the difference in the interfacial oxidation and decomposition between two oxidation schemes are comprehensively interpreted using a surface reaction model in which O-2 adsorption on the Si(001) 2 x 1 surface changes drastically from barrier-less adsorption into dimer backbonds for Langmuir-type adsorption to formation of desorption precursor SiO* in pairs with dimer vacancies for 2D oxide island growth, and coalescence of SiO* leads to nucleation and 2D growth of oxide islands. (C) 2003 Elsevier Science B.V. All rights reserved.