Electrical spin injection from ferromagnetic metals and semiconductors*

Electrical injection, transport, manipulation and detection of spin polarized carriers in a semiconductor are essential requirements for utilizing the spin degree of freedom in a future semiconductor spintronics technology. Electrical injection has been a particularly vexing issue. We describe results using both ferromagnetic semiconductor (FMS) and FM metal contacts, and review the on-going effort to understand the fundamental issues. Spin-LED structures serve as test platforms, and the quantum selection rules provide a quantitative measure of the electron spin polarization, Pspin, produced in an AlGaAs/GaAs QW. The large values of Pspin=85{\%} obtained using semimagnetic ZnMnSe contacts enable detailed analysis of spin scattering by interface defects. True FM materials are preferable as contacts, and FMSs are promising candidates -- their exchange split band edges offer both spin injection and spin-selective transport. An $n$-type FMS is especially attractive. We describe measurements of electrical spin injection from such an n-type FMS, CdCr$_{2}$Se$_{4}$, into AlGaAs/GaAs LEDs, and discuss interface structure and band offsets. FM metals offer many desirable attributes as spin injecting contacts -- high Curie temperatures, low coercive fields and fast switching times -- and the fundamental issue of interface conductivity mismatch can be circumvented by utilizing a tunnel barrier. We have successfully injected polarized electrons from a reverse-biased Fe Schottky contact into AlGaAs/GaAs, with Pspin $>$ 32{\%}. We demonstrate via the Rowell criteria that tunneling is indeed the dominant transport process, and confirm that majority spin electrons are responsible. We compare these data with spin injection using Fe/Al$_{2}$O$_{3}$ contacts into identical structures.