First-principles-derived rate constants for H adatom surface diffusion on Si(100)-2 x 1.

Abstract

We present the results of first-principles-derived Monte Carlo simulations of H adatom diffusion on the Si(100)-2\ifmmode\times\else\texttimes\fi{}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\ifmmode\times\else\texttimes\fi{}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 ${\mathit{D}}_{0}^{\mathrm{\ensuremath{\parallel}}}$=4.0\ifmmode\times\else\texttimes\fi{}${10}^{\ifmmode\pm\else\textpm\fi{}0.5}$ ${\mathrm{cm}}^{2}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, ${\mathit{E}}_{\mathit{a}}^{\mathrm{\ensuremath{\parallel}}}$=38.1\ifmmode\pm\else\textpm\fi{}1.7 kcal/mol, and ${\mathit{D}}_{0}^{\mathrm{\ensuremath{\perp}}}$=4.8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}1.0\ifmmode\pm\else\textpm\fi{}1.8}$ ${\mathrm{cm}}^{2}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, ${\mathit{E}}_{\mathit{a}}^{\mathrm{\ensuremath{\perp}}}$=62.8\ifmmode\pm\else\textpm\fi{}6.4 kcal/mol, respectively. These results confirm our previous suggestion that anisotropic diffusion of H adatoms on the Si(100)-2\ifmmode\times\else\texttimes\fi{}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 ${\mathrm{H}}_{2}$ desorption from Si(100)-2\ifmmode\times\else\texttimes\fi{}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).