Atomic structure of cluster-ordered array on the Si(001) surface induced by aluminum

We studied the atomic structure of the Si(001)-c(4 x 12)-Al surface, which appears in scanning tunneling microscopy (STM) images as an array of cluste rs in the four-fold direction. Transmission electron diffraction (TED) patt erns showed (h/4, k/12) fractional-order spots after depositing about four monolayers of aluminum on clean Si(001) surfaces at a substrate temperature of 600 degreesC. A model, which accords with the TED intensity and the obs erved STM images, consists of a periodical terrace-and-trench geometry in t he 12-fold direction. In the four-fold direction, an aluminum ad-dimer is a dsorbed every 4a distance (a=0.384 nm) on the terraces terminated by double -height steps, making up the cluster array corresponding to bright protrusi ons in STM images. In addition to ad-dimers, rebonded atoms at the double-h eight step edge and zigzag isolated dimers in the trench are found to be th e building blocks of the c(4 x 12) structure. (C) 2000 Elsevier Science B.V . All rights reserved.