Electrical spin injection into semiconductors

We present the results of a theoretical model describing electrical spin injection from a spin-polarized contact into a nonmagnetic semiconductor. The model includes the possibility of interface resistance due, for example, to a tunnel barrier at the contact/semiconductor heterojunction, and shows that such interface resistance can be critical in determining spin injection properties. With no interface resistance spin injection is very weak for contacts with typical metallic resistivities. For higher bulk resistivity contacts, such as doped semiconductors, or for completely spin-polarized contacts, strong spin injection is possible without significant interface resistance, However the spin polarization must be extremely close to complete for contacts with metallic resistivities. A tunnel barrier with spin-dependent interface resistance can greatly enhance spin injection. An insulating tunnel barrier with a spin-polarized contact, and a ferromagnetic insulator tunnel barrier, both have spin-dependent interface resistance, and provide two promising approaches to achieve significant electrical spin injection. The model is consistent with a variety of experimental observations, identifies the basic physics problems that must be addressed to achieve a high degree of spin injection, and suggests systematic strategies to achieve strong spin injection.