Kinetics of atomic hydrogen+adsorbed Br reactions on Si(100) and Si(111) surfaces

Abstract

The kinetics of adsorbed Br removal on Si(100) and Si(111) using atomic hydrogen are presented for surface temperatures from 50-560 °C. Both the surface H and Br coverages are measured in real time as the reaction proceeds using time-of-flight scattering and recoiling spectroscopy (TOF-SARS). For both surfaces below TS=400 °C, the Br removal rate has a first-order dependence on Br coverage (θBr) and first-order dependence on atomic hydrogen flux (FH). On Si(111), the Br removal rate also has a first-order dependence on the surface hydrogen coverage (θH). The data for TS > 400 °C are explained by a thermal HBr desorption process, with a measured activation energy of 45 kcal/mol on Si( 100) and 43 kcal/mol on Si(111). At low TS (< 400 °C), Br is removed via an apparently activationless mechanism with activation energies of 0.7 kcal/mol on Si(100) and 1.4 kcal/mol on Si(111). Although the experimental observations on Si(100) are consistent with an Eley-Rideal (ER) reaction mechanism, other mechanisms should be considered. We suggest that Hat may be partially accommodated at the surface in a mobile precursor state before reaction with adsorbed Br. During accommodation of Hat, chemical potential energy is delivered to the surface and may "chemically activate" a local area causing HBr desorption to occur. The degree of "chemical activation" will depend on the rate and degree of H at accommodation, with the maximum delivered in direct Si-H bond formation. © 1993 American Institute of Physics.