Spin-polarized electron transport processes at the ferromagnet/semiconductor interface

Circularly polarized light was used to excite electrons with a spin polarization perpendicular to the film plane in ferromagnet/semiconductor hybrid structures. The Schottky characteristics at the interface were varied by using NiFe, Co and Fe as the ferromagnet. The Schottky characteristics were clearly observed with NiFe and Co/GaAs, while almost ohmic I-V characteristics were seen with Fe/GaAs. At negative bias a helicity-dependent photocurrent, dependent upon the magnetization configuration of the film and the Schottky barrier height, was detected upon modulating the polarization from right to left circular, For the magnetization along or perpendicular to the surface normal, the helicity-dependent photocurrent Iⁿ or I ⁰, respectively, was measured. The asymmetry P=(Iⁿ-I⁰)/(Iⁿ+I⁰) of the helicity-dependent photocurrent decreases upon increasing the doping density of the GaAs substrates. P also decreases with photon energy hν as found for the polarization of photoexcited electrons in GaAs. In NiFe/GaAs samples for hν=1.59 eV, P=16% for n⁺=10²³ m^-3 and P=-23% for p^-=10²⁵ m^-3 doped substrates, i.e. P is comparable in magnitude to the theoretically predicted spin polarization of 50% for the optically pumped conduction band in GaAs. The results provide unambiguous evidence of spin-polarized electron transport through the ferromagnet/semiconductor interface and show that the Schottky barrier height controls the spin-polarized electron current passing from the semiconductor to the ferromagnet. The asymmetry data indicates that spin-polarized electrons are transmitted from the semiconductor to the ferromagnet with a high efficiency