Chlorine chemisorption on silicon and germanium surfaces: Photoemission polarization effects with synchrotron radiation

The electronic states due to saturation Cl coverage on Si(111)2 × 1, Si(111)7 × 7, Si(100)2 × 1, and Ge(111)2 × 1 surfaces have been investigated with uv photoemission techniques employing synchrotron radiation. The energy position of the Cl-induced peaks in the photoemission spectra did not provide enough information to clearly identify the chemisorption geometry for each surface. Photon polarization effects have helped to solve this problem by determining the energy position of the $p_z$-like contributions to the density of filled states. The Cl adatoms have been found to occupy covalent onefold coordinated sites on Si(111)2 × 1 and threefold ionic sites on Ge(111)2 × 1. Both the above configurations coexist on Si(111)7 × 7, while for Si(100)2 × 1 the Cl-Si bonds are still covalent, but they are probably tilted with respect to the surface normal. Constant-initial-state curves reveal three Cl-induced structures in the density of empty states above the vacuum level. None of these empty state peaks seems to be related to the $\sigma$-antibonding states of the Si-Cl covalent bond on Si(111)2 × 1 which are therefore probably located below the vacuum level.