  Due to the difficulty to grow high quality semiconductors on ferromagnetic
metals, the study of spin diffusion transport in Si was only limited to lateral
geometry devices. In this work, by using ultra-high vacuum wafer-bonding
technique, we have successfully fabricated metal semiconductor metal
CoFeB/MgO/Si/Pt vertical structures. We hereby demonstrate pure spin-current
injection and transport in the perpendicular current flow geometry over a
distance larger than 2\mu m in n-type Si at room temperature. In those
experiments, a pure propagating spin-current is generated via ferromagnetic
resonance spin-pumping and converted into a measurable voltage by using the
inverse spin-Hall effect occurring in the top Pt layer. A systematic study by
varying both Si and MgO thicknesses reveals the important role played by the
localized states at the MgO/Si interface for the spin-current generation.
Proximity effects involving indirect exchange interactions between the
ferromagnet and the MgO/Si interface states appears to be a prerequisite to
establish the necessary out-of-equilibrium spin-population in Si under the
spin-pumping action.
