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Abstract

Growth, thermal reaction, and crystalline structure of ultrathin iron silicide films on Si(111) are studied by low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The structural development of silicide layers is monitored in dependence on iron coverage and annealing temperature. Below approximately 10 monolayers (ML) of iron, two film structures appear, that are not stable in bulk material, while above that limit a switch to the bulk structures is observed. The morphology of the films is strongly dependent on the growth conditions. Their homogeneity can be considerably improved by simultaneous deposition (coevaporation) of Fe and Si in the desired stoichiometry compared to annealing predeposited Fe films. This improvement is accompanied by the suppression of pinholes in the film. The Fe:Si stoichiometry of the (1x1) and (2x2) phase can be assigned 1:1 and 1:2, respectively. The crystal structure of the former was previously determined to be CsCl, so called c-FeSi. For codeposition in 1:2 stoichiometry an initially disordered (1x1) phase transforms to a well ordered (2x2) phase after annealing. For these phases, gamma-FeSi2 in CaF2 structure, the tetragonal alpha-FeSi2 or an iron depleted variant of the CsCl structure are compatible with LEED and angle resolved AES results. In case of 1:2 stoichiometric films, the stability range of the (2x2) periodic phase can be extended to more than 60 Angstrom (equivalent to more than 20 ML Fe) by coevaporation.