Phase transition from pseudomorphicFeSi2to β-FeSi2/Si(111) studied byinsituscanning tunneling microscopy

Abstract

We have used the scanning tunneling microscope to study the Jahn-Teller-like structural transition from pseudomorphic ${\mathrm{FeSi}}_2$ on Si(111) to the stable bulk phase β-${\mathrm{FeSi}}_2$. Thin films of the pseudomorphic phase have been grown by molecular-beam epitaxy at room temperature. The crystal structure has been determined to be a CsCl-derived defect phase with a statistical occupation of the metal sites. In scanning tunneling spectroscopy we have found direct evidence for the atomic disorder on the metal sublattice. The fluorite structure was only observed after prolonged annealing at temperatures near the transition to β-${\mathrm{FeSi}}_2$. Upon annealing the crystalline order deteriorates when approaching the transition, which manifests itself in the appearance of a smooth surface corrugation on a 100-Å scale in scanning-tunneling-microscope topographs. This can be explained by a precursor phase in which the cubic silicide is locally distorted towards orthorhombic β-${\mathrm{FeSi}}_2$. The actual transition is very sensitive to kinetic factors, i.e., the stoichiometry of the initial deposit. Apart from the (101) orientation of β-${\mathrm{FeSi}}_2$ occurring also in films grown by solid-phase epitaxy both a (001) and a (100) orientation have been observed. The corresponding films exhibit a high degree of structural perfection with grains several μm in size which might become relevant to electro-optical applications. We discuss the surface reconstructions observed on the various types of β-${\mathrm{FeSi}}_2$ grains.