Stability of anisotropic missing dimer clusters on Si(001) surfaces

Two kinds of anisotropic structures of missing dimer clusters have been observed on (2 x 1) Si(001) surfaces: one is elongated along the dimer direction (missing dimer line) and the other along the dimer row direction (missing dimer row). The cohesive energy of missing dimer clusters was calculated as a function of the number, m, of missing dimers in a cluster, using Stillinger-Weber inter-atomic potential. The cohesive energy became positive in the range of nt 2 2 for missing dimer line (MDL) clusters, while that of missing dimer row (MDR) clusters became positive in the range of m greater than or equal to 3 but exceeded the cohesive energy of MDL clusters. MDL clusters are stabilized by alignment along the dimer direction of rebonded bonds, and the cohesive energy increases linearly with m. MDR clusters are stabilized by formation of alternate rebonded bonds along the dimer row direction, relaxing the strain energy caused by rebonded bonds. The cohesive energy of MDR clusters increases oscillating with In and the cohesive energy of a cluster in a size of 2n became smaller than that of (2n - 1). The growth of odd-numbered MDR clusters is endothermic but the growth of even-numbered clusters is exothermic. (C) 1999 Elsevier Science B.V. All rights reserved.