Concepts for spin injection into semiconductors—a review

Spintronics is a topic that has raised a lot of interest during the past years. The transport and manipulation of spin polarized electrons or holes in semiconductors offers a huge potential for novel devices that combine non-volatile information storage with high processing speed at low power, and which may even be useful for quantum computation. However, one major ingredient that has been missing for a long time is the transfer of spin polarized carriers from a magnetic contact into a non-magnetic semiconductor. Highly efficient electrical spin injection was realized for the first time in 1999 (Fiederling R et al Nature 402 787). Since then, several experiments have successfully demonstrated spin injection into semiconductors and additional concepts for spin filters and spin aligners have been proposed. Some of the experiments also yielded high spin injection efficiencies; however, in other experiments no unambiguous results could be obtained, and for many of the proposed concepts even a proof of principle is still missing. In this review, the suitability of different contact types for spin injection will be discussed together with a review of possible detection mechanisms that can be used in the experiment. By using a simple model based on load-lines we will show that for most spin aligners a first estimate of the injection efficiency can easily be obtained without complicated calculations.