Non-equilibrium Fe-Si thin films as potential spin injection materials

Fe-Si thin films are potential spintronics materials for its tunable structural, magnetic, and electric properties [1]. Our goal is to inject spin polarized electrons from iron-silicides into Si through a Schottky barrier, which is formed by choosing the proper doping level for Si, and a suitable Iron-silicide composition. We prepared Fe$_{1-x}$Si$_{x}$ (x = 0.25 - 0.5) films by electron beam co-evaporation from Fe and Si sources onto Si substrates under ultra-high vacuum conditions. Growth at 300 \r{ }C leads to a homogeneous Fe$_{1-x}$Si$_{x}$ magnetic alloy with both Tc and room temperature magnetization monotonically decrease with an increasing x. X-ray diffraction patterns show that a thin seed layer of FeSi at interface reduces the lattice mismatch between Si substrate and the bcc Fe$_{1-x}$Si$_{x}$ film, so that epitaxial growth can be realized. The seed layer also plays the role of reducing interdiffusion. A clear interface at Iron-silicide and silicon is obtained, resulting in a good Schottky barrier with height around 0.7 eV. By adjusting the Fe$_{1-x}$Si$_{x}$ composition, the resistivity of iron-silicide can be tailored. Ferromagnetic Fe$_{55}$Si$_{45}$ shows resistivity of 10$^{-3}$ ohm-cm, and magnetization 100 emu/cc at 300 K. By increasing the iron-silicide resistivity, one of the main obstacles for spin-injection from metal to semiconductor -- the resistivity mismatch -- can be overcome. Experiments based on spin-valve-type magneto-resistance for spin injection detection will be discussed. [1]. Ionescu et al. Physical Review B 71, 94401 (2005).