PHASE-TRANSITION FROM PSEUDOMORPHIC FESI2 TO BETA-FESI2 SI(111) STUDIED BY INSITU SCANNING-TUNNELING-MICROSCOPY/

We have used the scanning tunneling microscope to study the Jahn-Telle r-like structural transition from pseudomorphic FeSi2 on Si(111) to th e stable bulk phase beta-FeSi2. Thin films of the pseudomorphic phase have been grown by molecular-beam epitaxy at room temperature. The cry stal structure has been determined to be a CsCl-derived defect phase w ith 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 beta-FeSi2. Upon annealing the crystalline order deteriorates when approaching th e transition, which manifests itself in the appearance of a smooth sur face corrugation on a 100-angstrom scale in scanning-tunneling-microsc ope topographs. This can be explained by a precursor phase in which th e cubic silicide is locally distorted towards orthorhombic beta-FeSi2. The actual transition is very sensitive to kinetic factors, i.e., the stoichiometry of the initial deposit. Apart from the (101) orientatio n of beta-FeSi2 occurring also in films grown by solid-phase epitaxy b oth a (001) and a (100) orientation have been observed. The correspond ing films exhibit a high degree of structural perfection with grains s everal mum in size which might become relevant to electro-optical appl ications. We discuss the surface reconstructions observed on the vario us types of beta-FeSi2 grains.