  For electrical spin injection and detection of spin-polarized electrons in
silicon, we explore highly epitaxial growth of ferromagnetic full-Heusler-alloy
Co2FeSi thin films on silicon substrates using low-temperature molecular beam
epitaxy (LTMBE). Although in-situ reflection high energy electron diffraction
images clearly show two-dimensional epitaxial growth for growth temperatures
T_G of 60, 130, and 200 C, cross-sectional transmission electron microscopy
experiments reveal that there are single-crystal phases other than Heusler
alloys near the interface between Co_2FeSi and Si for T_G = 130 and 200 C. On
the other hand, almost perfect heterointerfaces are achieved for T_G = 60 C.
These results and magnetic measurements indicate that highly epitaxial growth
of Co_2FeSi thin films on Si is demonstrated only for T_G = 60 C.
