THE DESORPTION OF MOLECULAR-HYDROGEN FROM SI(100)-2X1 AND SI(111)-7X7SURFACES AT LOW COVERAGES

The mechanisms leading to desorption of molecular hydrogen from Si(100 )-2 x 1 and Si(111)-7 x 7 surfaces have been elucidated and refined by detailed examination of the thermal desorption kinetics with particul ar emphasis on low and very low coverages. In the case of hydrogen des orption from Si(100)-2 x 1, a lattice-gas model incorporating the inte ractions that are responsible for pairing and clustering of adsorbed h ydrogen atoms has been employed to fit temperature programmed desorpti on (TPD) peaks resulting from initial coverages between 0.01 and 1.0 m onolayer (ML). From analysis of our low coverage data, we find that th e pairing and clustering energies are (3.2 +/- 0.3) kcal mol(-1) and ( 3.4 +/- 0.5) kcal mol(-1), respectively. A subtle shift of the TPD pea k maximum position as the initial coverage increases from 0.2 to 1.0 M L indicates that the pre-exponential factor and activation energy are weakly coverage dependent. me discuss how this is consistent with coup ling of a dihydridelike transition state to its neighbors. The rate of molecular hydrogen desorption from Si(111)-7 x 7 is found to be very nearly second order in total hydrogen coverage when the initial covera ge is low. This result is consistent with a two site model involving p referential adsorption of hydrogen atoms at rest atom sites rather tha n adatom sites. (C) 1998 American Institute of Physics.