Epitaxial growth of Fe-Si compounds on the silicon (111) face

In a molecular-beam-epitaxy (MBE) chamber, epitaxial growth of all the silicides existing in the low-temperature portion of the iron-silicon phase diagram [e.g., bcc Fe (+Si), FeSi, and the semiconducting silicide β-${\mathrm{FeSi}}_2$] was achieved on the (111) face of silicon by deposition of pure iron onto a heated silicon substrate. The epitaxial growth has been characterized by means of in situ reflection high-energy electron diffraction (RHEED), ultraviolet photoemission spectroscopy, and x-ray photoemission spectroscopy. A strained phase, s-${\mathrm{FeSi}}_2$, has been clearly identified and shown to be metallic. This phase could be stabilized by the anisotropic elastic field induced by the epitaxy on the silicon (111) face which prevents a solid-state Jahn-Teller effect from distorting the cubic ${\mathrm{FeSi}}_2$ structure and opening the gap. Also during the growth, dynamical transitions between the different epitaxial phases have been observed at definite thicknesses (typically in the range 10–20 Å): (i) At the temperature of the silicon substrate T=350 °C, strained s-${\mathrm{FeSi}}_2$→FeSi; (ii) at T=400 °C, strained s-${\mathrm{FeSi}}_2$→β-${\mathrm{FeSi}}_2$. In our work such dynamical transitions are experimentally observed in situ during the growth. This shows that the use of a MBE chamber and in situ RHEED are very powerful techniques for studying and controlling metallurgical transformations on nanometer scale in real time. We interpret these dynamical transitions as being due to the combination of two effects: a change in the silicon atomic flux coming from the silicon substrate to the surface and the relaxation of the strained phase.