Desorption of hydrogen from Si(100)2×1 at low coverages: The influence of π-bonded dimers on the kinetics

Abstract

The recombinative desorption of hydrogen from the Si(100)2\ifmmode\times\else\texttimes\fi{}1 monohydride state was investigated by means of isothermal measurements using optical second-harmonic generation to monitor the hydrogen coverage. In agreement with previous results, we observe approximately first-order kinetics for hydrogen coverages above 0.1 monolayer. For lower hydrogen coverages, the desorption rate is found to decrease more rapidly. The data are interpreted within the recently developed model of hydrogen pairing at the Si dimers. We relate the observed kinetics to the \ensuremath{\pi} bonding of the Si-Si units and deduce an effective \ensuremath{\pi}-bond strength of 0.25 eV. The desorption activation energy determined from the present data is 2.48\ifmmode\pm\else\textpm\fi{}0.1 eV, with a first-order prefactor of \ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$.