Spintronic device physics in Si/Ge Heterostructures.

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Spin injection and transport in semiconductors is under intense investigation by physicists around the world, motivated by fascinating new insights into condensed matter, aware of considerable potential for novel devices and ensuing technologies. However, spin injection and its detection pose exceptional challenges. Much focus has been on technologically important materials: GaAs, where optical properties aid spin detection, and more recently Si for its long spin lifetimes. Here, we propose a new approach based on germanium. Ge is compatible with Si technology, has a longer spin life time than GaAs, a higher room temperature hole mobility than GaAs or Si, and better modulation properties than Si due to its higher spin-orbit coupling. SiGe heterostructure technology also has the potential to increase spin diffusion lengths by virtue of dramatic enhancements in carrier mobility.We recently carried out optical experiments that demonstrated RT spin transport and extraction through Ge for the first time, based on a structure consisting of Ge grown epitaxially on GaAs and an electrodeposited Ni/Ge Schottky contact [C. Shen et al., Appl. Phys. Lett. 97, 162104 (2010)]. Here, we propose to build upon that work and use the Si-Ge system to its full extent, through delta doping and bandstructure-engineering to maximize spin transparency of the electrical contacts and using strain and low dimensionality to enhance coherent transport in the channel. The culmination of this project should be the exciting prospect of the elusive two-terminal semiconductor spin valve operating at room temperature and an early demonstration of spin modulation by a gate electrode in such a device.The programme will combine the complementary expertise of the partners: Warwick in SiGe epitaxy and in carrier transport, Southampton in Schottky barrier research, and Cambridge in semiconductor spin transport by optical and electrical means, together with the facilities of the Southampton Nanofabrication Centre and industrial support from Toshiba Europe Research Ltd.