Surface morphological changes during growth and the subsequent decomposition of very thin oxide on an Si(001) surface was observed in real time by RHEED combined with AES and microscopically by STM. The RHEED intensity ratio between half-order spots revealed that etching of the surface took place in a manner of nucleation and lateral growth of dimer vacancy on the terrace during two-dimensional (2D) oxide island growth at 690 degrees C, whereas the resultant oxide layer was decomposed at 709 degrees C with consumption of Si atom in a step-flow mode. STM observation of the partially oxide decomposed surface, however, showed that a number of Si islands with 10-20 angstrom in diameter remained randomly over the rather atomically flat terraces within voids in spite of the step-flow etching. From these results, it is concluded that the oxide grown by the 2D oxide island growth mode contains excess Si atoms and the Si-rich oxide is thermally decomposed with the phase separation between Si clusters and a more stoichiometric oxide matrix, leading to precipitation of Si islands on the oxide-removed terraces. (c) 2005 Elsevier B.V. All rights reserved.