  Integrating epitaxial and ferromagnetic Europium Oxide (EuO) directly on
silicon is a perfect route to enrich silicon nanotechnology with spin filter
functionality.
  To date, the inherent chemical reactivity between EuO and Si has prevented a
heteroepitaxial integration without significant contaminations of the interface
with Eu silicides and Si oxides.
  We present a solution to this long-standing problem by applying two
complementary passivation techniques for the reactive EuO/Si interface:
  ($i$) an $in\:situ$ hydrogen-Si $(001)$ passivation and ($ii$) the
application of oxygen-protective Eu monolayers --- without using any additional
buffer layers.
  By careful chemical depth profiling of the oxide-semiconductor interface via
hard x-ray photoemission spectroscopy, we show how to systematically minimize
both Eu silicide and Si oxide formation to the sub-monolayer regime --- and how
to ultimately interface-engineer chemically clean, heteroepitaxial and
ferromagnetic EuO/Si $(001)$ in order to create a strong spin filter contact to
silicon.
