  We consider the spin injection from Fe into ZnSe and GaAs in the ballistic
limit. By means of the ab initio SKKR method we calculate the ground state
properties of epitaxial Fe|ZnSe(001) and Fe|GaAs(001) heterostructures. Three
injection processes are considered: injection of hot electrons and injection of
"thermal" electrons with and without an interface barrier. The calculation of
the conductance by the Landauer formula shows, that these interfaces act like a
nearly ideal spin filter, with spin polarization as high as 99%. This can be
traced back to the symmetry of the band structure of Fe for normal incidence.
