Molecular dynamics study of dimer flipping on perfect and defective Si(001) surfaces

We have performed long time scale tight-binding molecular dynamics (MD) sim ulations to study dimer flippings on perfect and defective Si(0 0 1) surfac es. The constant temperature MD simulations have been performed for tempera tures ranging from 800 to 1300 K. From them we measured the average residen ce time between flips and assuming an Arrhenius equation we obtain a dynami cal energy barrier for flipping. Our results extrapolate to a dimer flippin g frequency of 3 x 10(6) Hz at 300 K in agreement with the symmetric appear ance of surface dimers using STM. We have found that the correlated flippin gs of two neighbouring dimers are as important as single dimer flippings. T he ratio of correlated flips with respect to the total number of flips is o f the order of 0.3 at 800 K and decreases to 0.2 at 1200 K. In the case of surfaces with a single dimer vacancy, the neighbouring dimers to the vacanc y have three stable positions instead of two as in perfect surfaces. The di mers on the same row as the defect flip faster than those of the other row, leading to their appearance as symmetric dimers in STM measurements at tem peratures even below the order-disorder transition temperature. (C) 2001 El sevier Science B.V. All rights reserved.