Structure, transport and magnetic properties of ultrathin and thin FeSi films on Si(111)

Using solid-phase and molecular-beam epitaxy methods at 350°C, polycrystalline and epitaxial films of iron monosilicide (FeSi) with a thickness of 3.2 to 20.35 nm were grown on a Si(111) substrate, which was confirmed by X-ray diffraction data. Morphological studies have shown that the films are continuous and smooth with a root-mean-square roughness of 0.4–1.1 nm when grown by solid-phase epitaxy, and in the case of molecular beam epitaxy, they have an increased roughness and consist of coalesced grains with sizes up to 1 μm and a puncture density up to 1 × 10⁷ cm^–2. In solid-phase epitaxy, an increase in thickness leads to incomplete silicide formation and the appearance of a layer of disordered iron monosilicide with a thickness of 10 to 20 nm. This is confirmed by a change in the temperature dependence of resistivity ρ from semiconductor to semi-metallic and a decrease in resistivity by one and a half to two times. The nonmonotonic nature of the temperature dependence of the resistivity ρ ultrathin FeSi film with a thickness of 3.2 nm has been established, in which a maximum at 230–240 K, a region of growth from 160 to 65 K with E_g = 14.8 meV and further growth without saturation to a temperature of 1.5 K are observed. With increasing thickness of FeSi films grown by molecular-beam epitaxy, the minimum and maximum are not observed, but the tendency of nonmonotonic growth of ρ(T) with decreasing temperature and the opening of the band gap E_g = 23 meV remains. The probable reasons for the occurrence of effects in the dependences ρ(T) are considered. In ultrathin and thin FeSi films grown by solid-phase and molecular-beam epitaxy, respectively, an anomalous Hall effect was found, which was confirmed by the weak ferromagnetic properties of the films. The results obtained proved the possibility of growing and controlling the properties of ultrathin and thin FeSi films on silicon obtained by solid-phase and molecular-beam epitaxy, which ensured the appearance of their unique transport and magnetic properties that are absent in single crystals.