Chemisorption of fluorine onto the cluster modelled Si(111)7 × 7 surface: an ab initio Hartree-Fock/DFT study

Abstract

The chemisorption of fluorine on atomic clusters modelling an adatom and a rest-atom site of the Si(111)7 × 7 surface is studied using the first principles all-electron Hartree-Fock and Density Functional method embodied in the Gaussian92/DFT package. A number of equilibrium geometries corresponding to n fluorine atoms chemisorbing onto an adatom ( n ≤ 5) or rest-atom ( n ≤ 6) cluster are discussed. Our results show that, as progressively more fluorine is chemisorbed onto an adatom site, the fluorine bonded silicon adatom moves from its original 3-fold (T 4) site (one fluorine atom), to an adjacent bridge site (2 fluorine atoms) and then on top of a neighbouring first layer atom (3 fluorine atoms). For a fluorine bonded silicon rest-atom, a series of strained surface configurations are found as a result of the displacement of the rest-atom from its 3-fold equilibrium position. The lowest calculated desorption energy is 0.11 eV and corresponds to SiF 4 bonded to a rest-atom site. The smallest desorption energy for the adatom site is found to be 1.09 eV for the case of SiF 2. Extrapolating these results to the fluorinated Si(111)7 × 7 surface suggests that a low fluence of fluorine should produce a stable fluorosilyl layer which consists mainly of SiF and SiF 3 species, and corresponds to a fluorine coverage of 1.35 ML. The presented results may be of great importance for the understanding and modelling of the etching of silicon surfaces by fluorine.