THEORY OF SPIN-POLARIZED TRANSPORT IN PHOTOEXCITED SEMICONDUCTOR FERROMAGNET TUNNEL-JUNCTIONS/

We present a theory for spin-polarized transport in tunnel junctions c onsisting of a ferromagnet and a semiconductor, in which spin-polarize d carriers are created by optical orientation. The model includes, for both spin orientations, the current due to tunneling between the ferr omagnet and the semiconductor surface as well as the photoinduced and the thermionic emission currents through the semiconductor subsurface region. Tunneling is described in terms of a spin-dependent tunnel con ductance, taking account of the magnetic structure of the ferromagnet. We consider spin depolarization of photoexcited electrons in the semi conductor bulk material and in surface states that have a spin-depende nt occupation. The total tunnel current is evaluated as well as curren t modulations due to modulated spin polarization of photoelectrons (CP M signal) or modulated optical intensity. The calculations show that t he CPM signal is proportional to the tunnel conductance polarization a nd is relatively insensitive to spin depolarization of photoelectrons during their transport to the surface. A severe signal reduction can, however, result from spin relaxation in semiconductor surface states. In addition, it is demonstrated that a crucial role is played by the o perating regime of the junction, i.e., photoamperic or photovoltaic, w here the selection is determined mainly by the choice of applied bias voltage. We find that the photovoltaic mode is favored, as it yields t he highest contribution from spin-polarized tunneling, combined with t he smallest sensitivity for unwanted light intensity modulations. [S01 63-1829(98)03207-X].