Homogeneous surface iron silicide formation on Si(111): Thec(8×4)phase

Abstract

The early stages of iron silicide formation on Si(111) were studied by scanning tunneling microscopy (STM), low-energy electron diffraction, and Auger electron spectroscopy. While the initial iron interaction with Si(111) in the submonolayer regime gives rise to inhomogeneous island nucleation, deposition of 1.5 monolayers (ML) iron at room temperature and subsequent annealing at 550--600$\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ leads to a flat and homogeneous film with $c(8\ifmmode\times\else\texttimes\fi{}4)$ surface periodicity. This $c(8\ifmmode\times\else\texttimes\fi{}4)$ surface reconstruction is linked to a definite film thickness and thus seems to be stabilized directly through the interface. The film is terminated by a layer of adatoms whose lateral positions form a $(2\ifmmode\times\else\texttimes\fi{}2)$ periodic lattice. At negative tip bias voltages, STM images show an alternating arrangement of darker and brighter adatoms corresponding to the $c(8\ifmmode\times\else\texttimes\fi{}4)$ supercell. While the $(2\ifmmode\times\else\texttimes\fi{}2)$-periodic adatom arrangement develops in a wide temperature regime (450--600$\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$ and also for thicker films, the long range ordered $c(8\ifmmode\times\else\texttimes\fi{}4)$ structure can be observed only for 1--2 ML Fe coverage and after high temperature annealing at about 600$\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ Then single $c(8\ifmmode\times\else\texttimes\fi{}4)$ domains can extend to diameters of several hundred nanometers. The atomic structure of the new phase can be derived from a CsCl $(B2)$ structure, and a number of structural details are elucidated on the course towards the development of a complete structural model.