Electron-beam assisted adsorption on the Si(111) surface

Abstract

Observations by Auger emission spectroscopy on silicon surfaces have shown that considerable enhancement of the oxygen Auger peak occurs over the area irradiated continuously for many hours by a 2000 eV electron beam, while the silicon crystal is standing in a vacuum of 5 × 10 −10 torr at room temperature. On dirty silicon, i.e. on a surface partly covered with carbide, the oxygen saturation level inside the beam was at least an order of magnitude greater than that outside, while on clean silicon there was no accumulation of oxygen outside the beam. In both cases the rate of accumulation of carbon was the same inside as outside the beam, but on clean silicon the amount of carbon that built up was much less than that on the dirty silicon. The gas taking part in the beam-assisted adsorption was almost certainly carbon monoxide ; its initial sticking coefficient was calculated approximately, and found to be very similar to that observed experimentally by other workers for molecular oxygen. The minimum detectable surface concentrations of oxygen and of carbon, at the beam currents used, were estimated to be about 4 % and 1 %, respectively, of a monolayer. The results are interpreted in terms of electron dissociation of physisorbed carbon monoxide, followed by surface diffusion of carbon but not of oxygen. Chemisorption of carbon monoxide on silicon carbide particles is suggested to account for part of the greater accumulation of carbon on dirty silicon compared with that on clean silicon.