Phonon-activated electron-stimulated desorption of halogens fromSi(100)−(2×1)

Abstract

Spontaneous desorption of Cl, Br, and I from $n$- and $p$-type $\mathrm{Si}(100)\text{\ensuremath{-}}(2\ifmmode\times\else\texttimes\fi{}1)$ was studied with scanning tunneling microscopy at temperatures of $620--800\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ where conventional thermal bond breaking should be negligible. The activation energies and prefactors determined from Arrhenius plots indicate a novel reaction pathway that is initiated by the capture of electrons which have been excited by phonon processes into Si-halogen antibonding states. This configuration is on a repulsive potential energy surface, and it is sufficiently long lived that desorption can occur, constituting phonon-activated electron-stimulated desorption. Surprisingly, the Arrhenius plots for differently doped samples crossed and, above a critical temperature, the reaction with the largest activation energy had the highest rate. This is explained by large entropy changes associated with the multiphonon nature of the electronic excitation. For Cl desorption from $p$-type Si, these entropy changes amounted to $34{k}_{B}$. They were $19{k}_{B}$, $13{k}_{B}$, and $8{k}_{B}$ for Br desorption from $p$-type, lightly doped $n$-type, and heavily doped $n$-type Si, respectively. The desorption rates for I were nearly three orders of magnitude larger than the rates observed for Cl and Br. Here, the Si-I antibonding states overlap the conduction-band minimum, so that conduction-band electrons with this energy can be captured by the Si-I antibonding states. Together, these results reveal that a complex relationship exists between phonons and electronic excitations during chemical reactions at surfaces.