  We report on spin injection experiments at a Co/Al$_2$O$_3$/GaAs interface
with electrical detection. The application of a transverse magnetic field
induces a large voltage drop $\Delta V$ at the interface as high as 1.2mV for a
current density of 0.34 nA.$\mu m^{-2}$. This represents a dramatic increase of
the spin accumulation signal, well above the theoretical predictions for spin
injection through a ferromagnet/semiconductor interface. Such an enhancement is
consistent with a sequential tunneling process via localized states located in
the vicinity of the Al$_2$O$_3$/GaAs interface. For spin-polarized carriers
these states act as an accumulation layer where the spin lifetime is large. A
model taking into account the spin lifetime and the escape tunneling time for
carriers travelling back into the ferromagnetic contact reproduces accurately
the experimental results.
