Electrical spin injection in hybrid ferromagnetic metal/semiconductor structures

An overview of the electrical spin injection from a ferromagnetic metal into a semiconductor is given. We present the fundamental theoretical obstacle known as the "impedance mismatch" problem and demonstrate from calculations based either on a diffuse or a "ballistic" approach that this obstacle can be overcome by the insertion of a tunnel barrier between the semiconductor and the ferromagnetic metal. We then review experimental results on the measurement of the electrons' spin polarization injected electrically in spin-light emitting diodes with tunnel barriers based on a Schottky barrier or a thin insulator layer such as alumina oxide or magnesium oxide.