ELECTROSTATICS OF THE SI(111) SURFACE - A SEMIEMPIRICAL AM1 STUDY

We applied the semiempirical AMI molecular orbital method to calculate the molecular electrostatic potential (MEP) near the Si(lll) surface which we represented by models where the bulk was replaced by hydrogen atoms. Rutting these on the Si(model)-Si(bulk) bonds at a distance of 190 pm, much larger than experimental SI-H bond lengths, the spurious charge transfer to hydrogens representing the bulk was minimized. We transformed the 7x7 unit cell into a hexagonal one as given by Tong et al. [10] and calculated the MEP for this structure by summing up nume rical values obtained for smaller models which extend to about one six th of the whole unit cell. The density of the MEP contour lines is pro portional to the magnitude of the molecular electrostatic field (MEF). We observed the maximum MEF values in the bisector of the adatom-rest atom distance which is compared to the experimental observation that this region is the site of attack for the dissociative adsorption of s mall molecules (H2O, NH3 and HCCH). The MEF is smaller near the centra l atom in the corner hole indicating its reduced reactivity.