Injection energy dependence of spin-polarized hot-electron transport through a ferromagnetic metal/oxide/semiconductor junction

Spin-polarized hot electron transport through a ferromagnetic metal/oxide/semiconductor junction is studied as a function of the electron injection energy in the range from a few eV up to 1 keV. The incident spin-polarized electrons are produced by a GaAs photocathode and are injected from vacuum into the thin metal layer. The current transmitted through the junction is measured in the semiconductor collector. A spin-dependent component of the transmitted current is detected when reversing either the spin polarization of the incident electrons or the magnetization of the metal layer. For injection energy in the hundreds of eV range, both the mean transmitted current and the spin-dependent transmitted current exhibit a spectacular increase, over several orders of magnitude. A transport regime is reached where electron transmission is larger than unity, providing a current gain, while the spin selectivity of the magnetic layer is still very high (close to 100%). This variation is analyzed in the framework of a transport model which accounts for the relaxation of the electron energy and velocity by secondary electron excitation. This model fits with the experimental data and evidences the crucial role of the metal/oxide/semiconductor barrier shape on the spin-dependent transport properties of the device.