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

We have performed long time scale tight-binding molecular dynamics (MD) simulations to study dimer flippings on perfect and defective Si(0 0 1) surfaces. The constant temperature MD simulations have been performed for temperatures ranging from 800 to 1300 K. From them we measured the average residence time between flips and assuming an Arrhenius equation we obtain a dynamical energy barrier for flipping. Our results extrapolate to a dimer flipping frequency of 3 x 10(6) Hz at 300 K in agreement with the symmetric appearance of surface dimers using STM. We have found that the correlated flippings of two neighbouring dimers are as important as single dimer flippings. The ratio of correlated flips with respect to the total number of flips is of 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 vacancy have three stable positions instead of two as in perfect surfaces. The dimers 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 temperatures even below the order-disorder transition temperature. (C) 2001 Elsevier Science B.V. All rights reserved.