ETHYL GROUP DECOMPOSITION KINETICS FOLLOWING ADSORPTION OF DIETHYLSILANE, DIETHYLGERMANE, AND ETHYLSILANE ON SI(111)7X7

The kinetics of ethyl group decomposition on Si(111)7 x 7 were measured using laser-induced thermal desorption techniques following the adsorption of diethylsilane (DES), diethylgermane (DEG), and ethylsilane (ES). Earlier infrared spectroscopic studies have shown that ethyl species are present after the dissociative chemisorption of DES, DEG, and ES on silicon surfaces. As the ethyl groups decompose via a beta-hydride elimination mechanism, the hydrogen coverage increases and ethylene desorbs into the gas phase, i.e. Si-CH2CH3(ad) --> Si-H(ad) + CH2=CH2(g). LITD measurements determined that the ethyl group decomposition kinetics consisted of a fast initial step, followed by a slower second step. Decomposition kinetics for DES, DEG, and ES could be described by a model consisting of two concurrent first-order decomposition processes. The similar ethyl group decomposition rate constants for DES, DEG, and ES suggested that both hydrogen and ethyl groups transfer upon adsorption to free dangling bonds on Si(111)7 x 7. Measured activation barriers for the beta-hydride elimination reaction were slightly larger than the thermodynamic barrier, but much smaller than the calculated gas-phase barrier. The dependence of the beta-hydride elimination mechanism on the availability of free dangling bonds was studied by adsorption of additional atomic hydrogen following ES and DES exposures. The presence of additional hydrogen adsorption did not significantly affect the ethyl group decomposition kinetics. These results suggested that beta-hydride transfer occurs through a four-center transition state and is not dependent on adjacent free dangling bond sites.