Temperature programmed desorption from the Si(100):Br and Si(100):D/Br surfaces: theory and experiment

Abstract

New experimental temperature programmed desorption (TPD) data have been obtained under carefully controlled UHV conditions following the adsorption of bromine and the co-adsorption of bromine and atomic deuterium on the single crystal Si(100): 2 × 1 surface. Coverages ranged from maxima of θ = 1.5 ML (atomic deuterium alone) and 1.0 ML (atomic bromine alone) and included a wide combination of the two co-adsorbed species down to a lower limit of 0.05 ML atomic deuterium and 0.2 ML atomic bromine. When bromine alone was absorbed at surface temperatures of 380 ± 10 K, the only observed TPD product was silicon dibromide. When the total coverage of bromine and deuterium atoms was high (≳ 1 ML), the TPD data were much more complex and as well as showing the usual ß 1 and ß 2 desorption peaks of deuterium, analogous peaks for DBr were also observed together with the above-mentioned SiBr 2 peak. The data have been interpreted using a kinetic lattice-gas model which describes the atomic bromine and deuterium co-adsorbed on the surface in terms of nine basic units: dideuteride (SiD 2), doubly occupied dimers (DSiSiD), singly occupied dimers (SiSiD), unoccupied dimers (SiSi) together with the analogous bromine substituted and mixed deuterium-bromine species. Using a quasi-equilibrium approximation and including up to five competing desorption pathways corresponding to the product peaks observed, the TPD spectra for deuterium, deuterium bromide and silicon dibromide are determined and compared with the observed experimental data. By fitting the simulated data to the experimental curves it was shown that both ß 1 desorption processes and desorption of SiBr 2 were consistent with first order kinetic behaviour. On the other hand both ß 2 processes were found to obey second order kinetics. The frequency factors for the first-order processes were determined as 1 × 10 15s −1 (D 2 and DBr) and 2 × 10 15s −1 (SiBr 2) with the corresponding activation energies being 57.5 kcal mol −1 (D 2), 61 kcal mol −1 (DBr) and 61 kcal mol −1 (SiBr 2). For the second-order processes, the frequency factors (expressed in first-order units) were found to be 2 × 10 15s −1 (for both D 2 and DBr) with the activation energies being 45 kcal mol −1 (D 2) and 49 kcal mol −1 (DBr).