  We theoretically investigate the spin injection in different FM/I/n-Si tunnel
contacts by using the lattice NEGF method. We find that the tunnel contacts
with low barrier materials such as TiO$_2$ and Ta$_{2}$O$_{5}$, have much lower
resistances than the conventional barrier materials, resulting in a wider and
attainable optimum parameters window for improving the spin injection
efficiency and MR ratio of a vertical spin MOSFET. Additionally, we find the
spin asymmetry coefficient of TiO$_2$ tunnel contact has a negative value,
while that of Ta$_{2}$O$_{5}$ contact can be tuned between positive and
negative values, by changing the parameters.
