First principles total energy calculations of the adsorption of single Cl2 and Br2 molecules on the Si(001)-c(2x4) surface

We have performed first principles total energy calculations to investigate the adsorption of Cl-2 and Br-2 molecules on the Si(001)-c(2 x 4) surface. First of all, our results indicate that the adsorption of Cl-2 and Br-2 as molecules is not energetically favorable. Instead, Cl-2 (Br-2) dissociates into two Cl (Br) subunits. The most stable geometry was achieved when each Cl (Br) subunit is bonded to a Si atom of the same Si dimer along the dangling bonds. Three other geometries were considered in our study: the first one corresponds to adsorption of the two subunits on Si dimer sites in adjacent silicon dimer rows. The second corresponds to adsorption of the two Cl (Br) subunits along dangling bonds of two consecutive dimers of the same Si dimer row, with the two Cl(Br) atoms in the same side of the Si dimer row. The third one is similar to the second, but the ad-atoms are now in a staggered arrangement, occupying sites in opposite sides of the Si dimer row. These are adsorption configurations that have been observed experimentally for Cl-2 on Si(001). Our results indicate that energetics and atomic geometries are similar for Cl-2 and Br-2. We have also calculated energy barriers for the ad-atom to move along and across the Si dimer rows. It is found that the barriers for Br are smaller than the barriers for Cl. This result can explain the differences observed in the adsorption Of Cl-2 and Br-2 molecules. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim