  We have experimentally and theoretically investigated the spin
injection/detection polarization in a Si-based ferromagnetic tunnel junction
with an amorphous MgO layer, and demonstrated that the experimental features of
the spin polarization in a wide bias range can be well explained using our
theoretical model based on the band diagram of the junction and the direct
tunneling mechanism. It is shown that the spin polarization originates from the
band diagrams of the ferromagnetic Fe layer and n+-Si channel in the junction,
while the spin selectivity of the MgO tunnel barrier is not necessary. Besides,
we clarified the mechanism of the reduction in spin polarization when the bias
is high and nonlinear properties are prominent, where the widely-used spin
injection/detection model proposed by Valet and Fert is not applicable. The
dominant mechanism of such reduction is found to be spin accumulation
saturation (SAS) at the n+-Si interface in contact with the MgO layer as the
bias is increased in the spin extraction geometry, which is inevitable in
semiconductor-based ferromagnetic tunnel junctions. We performed numerical
calculations on a two-terminal spin transport device with a n+-Si channel using
the junction properties extracted from the experiments, and revealed that the
magnetoresistance (MR) ratio is suppressed mainly by SAS in a higher bias
range. Furthermore, we proposed methods for improving the MR ratio in
two-terminal spin transport devices. Our experiments and theoretical model
provide a deep understanding of the spin injection/detection phenomena in
semiconductor-based spin transport devices, toward the realization of high
performance under reasonably high bias conditions for practical use.
