Adsorption Dynamics on Si(111)-7 × 7 Surface Induced by Supersonic O2 Beam Studied Using Real-Time Photoelectron Spectroscopy

Abstract

The adsorption mechanisms of O2 on a Si(111)-7 × 7 surface were studied using real-time X-ray photoelectron spectroscopy with synchrotron radiation in conjunction with a supersonic molecular-beam technique. To study the dependence of the initial sticking probability, s0, saturation coverage of the O2 incident energy, En, and the oxygen uptake curves, which were evaluated from the time evolution of O 1s peak area, were measured over energy ranges from En = 0.03 eV (thermal gas) to 2.26 eV. It was found that a trapping-mediated adsorption mechanism is dominant at En < 0.06 eV. In contrast, an activated adsorption mechanism becomes prominent at En > 0.06 eV, along with the trapping-mediated adsorption mechanism which becomes less prevalent with increasing En. The saturation coverage at energies over 1.9 eV is approximately 1.6 times larger than that at 0.03 eV. Two different components of the O 1s spectrum and their energy dependence were verified. A marked increase in the tri oxygen component is clearly observed in the O 1s photoelectron spectrum for saturation at En > 0.39 eV, which is the evidence for the enhancement of dissociative adsorption of O2 into the subsurface layers; 0.39 eV was taken as the threshold energy for En dependence of both s0 and saturation coverage. The adsorption states induced via the novel reaction channel correlating with En were confirmed by the in situ spectroscopic surface analysis method.