Vacancy-Assisted Halogen Reactions on Si(100)- (2 × 1)

Scanning tunneling microscopy studies of etching of Si(100)-(2×1) show that the rate of terrace pit formation goes through a maximum for surface coverages of θ(Cl)  =  0.77±0.05 monolayer, in contrast to predictions of conventional models. Using recently calculated energies for different possible surface configurations, we show that a key component in desorption is the formation of a single-atom vacancy adjacent to a volatile SiCl₂ unit. The demonstration of vacancy-assisted reaction establishes a self-limited reaction and the sequence of events leading to desorption.