The dynamics of surface roughening and healing induced by Cl on Si͑100͒-͑2ϫ1͒ were studied with variable-temperature scanning tunneling microscopy. Clean samples were exposed to Cl 2 at room temperature, heated to as high as 700 K, and imaged for periods that exceeded 20 h. Chlorine caused surface roughening via a Cl recycling pathway that created pits and regrowth islands with minimal desorption of SiCl 2. This reaction pathway is accessible at ϳ675 K if the surface is not saturated with Cl. Images showed dimer vacancy creation and pit growth, together with vacancy capture by pits. They also showed dimer vacancy escape from pits and pit annihilation, which had not been reported previously. When the terrace coverage was 0.93 ML, the diffusivity of dimer vacancies along the dimer rows was ϳ0.7 Å 2 /sec, about three orders-of-magnitude lower than that on clean Si͑100͒. Single Si adatoms were created on terraces and bonded to Si dimers that were Cl-free. They could form new regrowth dimers or they could be accommodated at the ends of existing islands. Adatoms could also be released from regrowth islands. Pairs of atom vacancy lines separated by one or two Si dimers were observed and the transition between those lines and dimer vacancy lines was recorded. Local areas with ͑3ϫ2͒ symmetry were created from the latter. These dynamic changes all require Cl-free Si dimers, and the rate of change of the surface morphology reflects the surface concentration. The rate of change increases with time as the density of bare dimers increases due to the creation of new sites for Cl on an increasingly rough surface and limited desorption.
Spontaneous desorption of Cl, Br, and I from n-and p-type Si͑100͒-͑2 ϫ 1͒ was studied with scanning tunneling microscopy at temperatures of 620-800 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 34k B . They were 19k B , 13k B , and 8k 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.
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