Enhancement of the Spin Accumulation at the Interface between a Spin-Polarized Tunnel Junction and a Semiconductor

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 1.2 mV for a current density of 0.34 nA center dot 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 traveling back into the ferromagnetic contact reproduces accurately the experimental results.