The surface morphological change at an initial stage of thermal oxidation on Si(001) surface with O-2 Was investigated as a function of oxide coverage by a real-time monitoring method of Auger electron spectroscopy (AES) combined with reflection high energy electron diffraction (RHEED). At 653 degreesC where oxide islands grow laterally, protrusions were observed to develop under the oxide islands as a consequence of concurrent etching of the surface. The rate of etching was measured from a periodic oscillation of RHEED half-order spot intensity I((1/2,0)) and I((0,1/2)). At 549 degreesC where Langmuir-type adsorption proceeds, it was observed that both I((1/2,0)) and I((0,1/2)) decrease more rapidly in comparison with an increase of oxide coverage and the intensity ratio between them decreases gradually with 02 exposure time. These suggest that Langmuir-type adsorption occurs at sites where O-2 adsorbs randomly, leading to subdivision of the 2 x I and I x 2 domains by oxidized regions, and that Si atoms are ejected due to volume expansion in oxidation to chance the ratio between 2 x 1 and 1 x 2 domains. (C) 2002 Elsevier Science B.V. All rights reserved.