Semiconductor Spintronics: electrical spin injection from ferromagnetic metals

Berry T. Jonker
Naval Research Laboratory, Washington DC

The use of spin angular momentum as a new state variable in semiconductor devices promises new functionality and performance. Electrical injection, transport, manipulation and detection of spin polarized carriers are essential requirements in a future semiconductor spintronics technology. Electrical injection has been a particularly vexing issue. We describe results using ferromagnetic (FM) metal contacts, and review the on-going effort to understand fundamental issues such as the role of band symmetries, conductivity mismatch and spin lifetimes. 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 quantum well and GaAs/InGaAs quantum dot based LEDs, with high injection efficiencies. We demonstrate via the Rowell criteria that tunneling is the dominant transport process, and confirm that majority spin electrons are responsible. We determine the atomic structure of the Fe/AlGaAs spin injecting interface using a combination of z-contrast transmission electron microscopy and image simulations based on density functional theory calculations of several structural candidates. These results show that the interface is atomically abrupt, and that spin injection efficiency is positively correlated with the coherence and abruptness of the interface.