EVOLUTION OF SURFACE-MORPHOLOGY OF SI(100)-(2X1) DURING OXYGEN-ADSORPTION AT ELEVATED-TEMPERATURES

We used scanning tunneling microscopy and low-energy electron microsco py to study the etching of the Si(100)-(2 X 1) surface by oxygen at lo w pressures and elevated temperatures. At 5 X 10(-8) torr partial pres sure of oxygen, the transition from random etching of terraces to step etching occurs at approximately 580-degrees-C, while at the higher pa rtial pressure of 5 X 10(-7) torr, the transition temperature is appro ximately 625-degrees-C. The diffusing species is a dimer vacancy, form ed by the desorption of two SiO molecules. Anisotropic diffusion of th e vacancies is observed, with a preferred direction along the dimer ro ws. The reaction probability, defined as the number of desorbed SiO mo lecules to the number of incident 02 molecules, is estimated to be 0.0 10+/-0.005.