Diffusion-promoted-desorption mechanism for D_2 desorption from Si(100) surfaces

摘要

Temperature-programmed-desorption (TPD) spectra and isothermal desorption rates of D_2 molecules from a Si(100) surface have been calculated to reproduce experimental β_(1,A)-TPD spectra and isothermal desorption rate curves. In the diffusion-promoted-desorption (DPD) mechanism, hydrogen desorption from the Si(100) (2×1) surfaces takes place via D atom diffusion from doubly-occupied Si dimers (DODs) to their adjacent unoccupied Si dimers (UODs). Taking a clustering interaction among DODs into consideration, coverages θ_(DU) of desorption sites consisting of a pair of a DOD and UOD are evaluated by a Monte Carlo (MC) method. The TPD spectra for the β_(1,A) peak are obtained by numerically integrating the desorption rate equation R = v_A exp (-E_(d,A)/k_BT)θ_(DU), where v_A is the pre-exponential factor and E_(d,A) is the desorption barrier. The TPD spectra calculated for E_(d,A) = 1.6 eV and v_A = 2.7 × 10~9/s are found to be in good agreement with the experimental TPD data for a wide coverage range from 0.01 to 0.74 ML. Namely, the deviation from first-order kinetics observed in the coverage dependent TPD spectra as well as in the isothermal desorption rate curves can be reproduced by the model simulations. This success in reproducing both the experimental TPD data and the very low desorption barrier validates the proposed DPD mechanism.