EPITAXIAL PHASE-TRANSITIONS IN THE IRON SILICON SYSTEM

The properties of a newly discovered pseudomorphic FeSi/Si(111) phase with the CsCl structure are discussed. Upon annealing the new phase un dergoes phase transitions to the stable epsilon-FeSi phase, either in epitaxial or polycrystalline form, depending on film thickness. Instea d of transforming to epsilon-FeSi, very thin films (d < 15 angstrom) e xhibit an uptake of Si up to the stoichiometry of FeSi2. The symmetry of the resulting phase remains unchanged, however, except for prolonge d annealing close to the transition to beta-FeSi2, where the formation of fluorite gamma-FeSi2 grains is observed. We emphasize the far reac hing consequences of the epitaxial stability of the defect-CsCl phase over a large range of compositions.