Monte Carlo simulations of a two-dimensional lattice gas in the grand canonical ensemble are performed to analyze the behavior of Si adatoms on Si(111)-1 × 1 surfaces. 2 × 2, c2 × 8 and c2 × 4 structures appear at 300 K under the condition that only Coulomb repulsion is introduced as a pair interaction between adatoms. The adatom density increases with the temperature, and √3 × √3 structures appear at 550 °C. These results agree with our recent STM observations.
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