1ST-PRINCIPLES-DERIVED RATE CONSTANTS FOR H-ADATOM SURFACE-DIFFUSION ON SI(100)-2X1

We present the results of first-principles-derived Monte Carlo simulations of H adatom diffusion on the Si(100)-2 X 1 surface. We developed an analytical Si/H potential which was fit to the results of first-principles electronic-structure calculations of H adatom adsorption and diffusion on embedded silicon clusters designed to model Si(100)-2 X 1. With this interaction potential, we calculated the rate constants for a H adatom hopping from one site to another, both parallel and perpendicular to the silicon dimer rows, using Monte Carlo simulations to extract exact classical transition-state-theory rate constants. The diffusion constants for H adatoms moving parallel and perpendicular to the surface dimer rows both were found to obey an Arrhenius temperature dependence (over the temperature range T = 700-900 K) with preexponential factors and activation energies of D0parallel-to = 4.0 X 10(+/-0.5) cm2s-1, E(a)parallel-to = 38.1 +/- 1.7 kcal/mol, and D0perpendicular-to = 4.8 X 10(-10 +/- 1.8) cm2 s-1, E(a)perpendicular-to = 62.8 +/- 6.4 kcal/mol, respectively. These results confirm our previous suggestion that anisotropic diffusion of H adatoms on the Si(100)-2 X 1 surface will occur preferentially along the edges of silicon dimer rows. However, these predicted H adatom diffusion rates are orders of magnitude faster (along the dimer rows) or slower (across the dimer rows) than measured values for the rates of H-2 desorption from Si(100)-2 X 1-H. Thus these results suggest that diffusion of hydrogen atoms may not be involved in the rate-limiting step for hydrogen desorption from Si(100).