Chlorine adsorption on silicon

Chlorine is the only other gas whose interaction with silicon surfaces has been studied in a reasonably detailed and systematic manner. Florio and Robertson [246] reported on the kinetics of the reaction of Cl2 with Si 111 7x7 over a decade ago and, more recently, several groups [247—250] have used various types of photoelectron spectroscopy to evaluate the chemical binding sites and electronic states which occur when a saturation coverage of Cl2 is chemisorbed on Si lll] 2 x l,Si lll 7x7 and Si 100 2x1. These results, relating to Cl-induced surface energy bands, have been compared with self-consistent pseudo-potential [247, 250] and tight-binding [249] calculations. 

The major desorption product at all temperatures is SiCl4, but there is evidence for two distinct adsorption sites for the chlorine. At low temperatures ( 650 K), there is a rapid initial decrease in the surface chlorine 

This chemical picture of the silicon surface—chlorine interaction has been augmented by UPS [249] and angle-resolved photoemission measurements using synchrotron radiation [248, 250]. These experiments have provided rather detailed information on the atomic position of adsorbed chlorine and the chemical binding states involved, although no attempt has been made to relate the results to the kinetic data. 

Turning to the LDOS calculations, the four peaks labelled A, B, C and D in Fig. 27 can be identified in the following ways. For the one-fold site, peak A is due to Si s states, B arises from the o-bond between Si sp3 and Cl pz states, C results from non-bonding Cl px and py states (7r-bonding states), while D corresponds to p-like Si-Si bonds which have been perturbed by a Cl atom. In the three-fold site, where no o-bonds are involved, peak C is broadened and contains all three Cl p states (px, py and pz). The pz states are not completely degenerate with px and py, however, and the relative energy positions of Cl pz states with respect to those of Cl px and py is the main distinguishing feature between the two possible adsorption sites. 

SchlUter et al. [247] used polarization selection rule effects with s-and p-polarized photons, together with energy distribution measurements, to show a good level of agreement between experiment and calculation for the one-fold site. This result was confirmed by Pandey et al. [249] and extended by Larsen et al. [250] to include the derivation of the two-dimensional energy bands from measurements of the energy positions of 

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