Spintronic device physics in Si/Ge Heterostructures.

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.

This project on spin injection and transport in SiGe quantum structures is fundamental research and the major impact will be in generating new knowledge and techniques. The scientific results will primarily be disseminated through high quality academic conferences and journals, with patents filed where appropriate.

The long term impact will be an advance in computer power by introducing spin in memory and possibly as the logic function. Using spin rather than charge means that the power required for each operation could be substantially reduced and device speed could increase dramatically. For these reasons spin based devices have found a place on the industry guiding ITRS Roadmap. Moreover, there is the ultimate goal of a quantum computer with intrinsically parallel operation. To realise a spin logic device requires asymmetric spin injection and improvements in our understanding of spin manipulation by an electric field. These are exactly the areas that our proposal addresses, through bandstructure and quantum well engineering available in the Si-Ge system.

There is also substantial academic interest in the science behind spin transport and the research proposed here should lead both to a better understanding of this and the fabrication of novel structures to act as test beds.

The Investigators have established track records in outreach and 'promotion of science' to non-specialist audiences. The end application of this work, and the intriguing nature of electron spin, make it highly appropriate for these purposes. The research will be widely disseminated to general audiences (members of the public and young people) through talks and workshops for schools, science fairs etc., popular science articles and via group websites, including audio and video content. Support for the team to prepare suitable (specialist and non-specialist) news articles, flyers, material for the web-site etc. will be provided by central communications and media relations officers in each university. The Ogden Teaching Fellow at Warwick will in particular assist with outreach to schools.

In the longer term, spintronics research could have vast societal impact, both directly and indirectly from new technological developments. For example, faster and highly parallel computers will allow climate modelling simulations to advance far beyond their current state and a future quantum computer would enable reliable predictions on appropriate time and spatial scales to anticipate adverse conditions. Other applications will take advantage of increased logic power in a more subtle way to create ubiquitous computing devices that are at the moment unimagined. For instance, healthcare devices would benefit enormously from remote monitoring of patients, through implantable devices to monitor, say blood sugar levels, with a computer in the same room, or small lab-on-a-chip tests that relay their data to remote doctors.

Many companies are interested in the long term prospects of Si compatible spintronic devices, for the reasons given above. In this project, we will collaborate with Toshiba Research Europe Ltd. who are well aware of the potential impact of Si/Ge spintronic devices, having recently demonstrated (through their Japanese parent company) passive integration of a magnetic tunnel junction spin memory cell directly on top of a Si MOSFET. TREL will benefit from first sight of our research output and would be a preferred route to commercialisation.

This project will provide an extremely stimulating research environment for the researchers and students engaged. They will receive excellent training in synthesis, deposition and characterisation techniques in a clean room environment together with exposure to the culture and materials requirements of electronics applications. This training will make them very desirable employees for a range of high tech industries.