STABILITY OF ORDERED MISSING-DIMER STRUCTURES AND THE ORDERING DYNAMICS ON SI(001)

The stability of (2 x n)-ordered missing-dimer structures on Si(001) a nd the ordering dynamics of missing dimers are investigated. The surfa ce free energy of (2 x n)-ordered missing-dimer structure was calculat ed under strains parallel or perpendicular to the dimer row using the Stillinger-Weber potential among Si atoms. The (2 x n)-ordered missing -dimer structure becomes more stable than the (2 x 1)-ordered surface without missing-dimers only if a compressive strain larger than 0.5% i s applied parallel to the dimer row. The relaxation process towards th e equilibrium configuration of excess missing dimers formed initially at random on the (001) surface was studied by a time-dependent Monte C arlo simulation based on the lattice gas model. The ordering of missin g dimers towards the (2 x n) structure evolves via a three-stage relax ation process at 300 K, while it evolves via a one-stage, fast relaxat ion process at 600 and 900 K. The degree of ordering at equilibrium is significantly improved as the annealing temperature decreases and the missing-dimer concentration increases. The effect of the missing-dime r hopping across the dimer row is also studied. (C) 1998 Elsevier Scie nce B.V.