  Implementing spin functionalities in Si, and understanding the fundamental
processes of spin injection and detection, are the main challenges in
spintronics. Here we demonstrate large spin polarizations at room temperature,
34% in n-type and 10% in p-type degenerate Si bands, using a narrow Schottky
and a SiO2 tunnel barrier in a direct tunneling regime. Furthermore, by
increasing the width of the Schottky barrier in non-degenerate p-type Si, we
observed a systematic sign reversal of the Hanle signal in the low bias regime.
This dramatic change in the spin injection and detection processes with
increased Schottky barrier resistance may be due to a decoupling of the spins
in the interface states from the bulk band of Si, yielding a transition from a
direct to a localized state assisted tunneling. Our study provides a deeper
insight into the spin transport phenomenon, which should be considered for
electrical spin injection into any semiconductor.
