NON-MAGNETIC SEMICONDUCTOR SPINTRONICS: INNOVATIONS IN NANOSCALE, HIGHLY SPIN-ORBIT COUPLED QUANTUM WELL SYSTEMS

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The aim of this proposal is to provide new methods of injecting and detecting spin polarised currents in all non-magnetic semiconductor systems. This area, the control of spins using spin-orbit coupling effects, is the subject of intense theoretical activity around the world, though so far relatively little experimental work has been published. One of the primary reasons for the interest is that the challenges in either producing room-temperature magnetic semiconductors or in injecting from magnetic metals are both by-passed. One important reason for the paucity of experimental activity is the difficultly in controlling spin-orbit coupling in the semiconductor materials of primary importance for (opto-)electronics, where the effects are small. The methods presented here utilise the high spin-orbit coupling in narrow gap semiconductor (NGS) quantum well structures, using the latest theories of spin dependent ballistic transport. These materials have recently been shown by UK workers to be capable of delivering the ultimate in speed and power consumption for transistors, and the technology may now be considered as having reached maturity. This proposal will exploit and strengthen the UK's lead in this area, and turn it to new advantage. Devices for spin filtering, emission and detection are proposed which will provide a crucially important set of tools for the spintronic community.Successful development of novel spin-based electronic devices requires establishing new experimental methods of creating, measuring and manipulating spin-polarised currents. According to the International Semiconductor Roadmap in order to realise an all electrical semiconductor spintronic device one of two challenges must be met. Either successful engineering of the ferromagnetic metal/semiconductor interface has to be achieved to incorporate spin preserving tunnel barriers or the fabrication of room temperature dilute magnetic semiconductors compatible with the standard electronic industry's materials (Si, GaAs and GaN). The current proposal describes an alternative solution that utilises existing materials and technologies in a new way. This project will investigate spin kinetics and related phenomena in nanostructures, and apply the physical understanding gained to the creation of a spin polarisation within the semiconductor without reliance on the presence of an external ferromagnetic source. Simply put, I shall exploit the non-degeneracy of moving electron spin states in materials and structures which lack inversion symmetry. For most purposes and in most materials these spin splittings are negligibly small. However, it is possible to utilise the dichromatic nature of electron spin, for example in the ballistic regime at the interface between two materials of different spin-orbit coupling strength. In this emerging field, dubbed 'spin optics', spin dependent reflection has been demonstrated and a similar refraction (and negative refraction) -like effect has been proposed. The ballistic regime also presents the opportunity to use the spin dependent nature of cyclotron motion. Similarly, the spin splitting may be utilised in resonant tunnel diode structures for spin injection and detection, where a voltage applied along growth direction can preferentially allow transmission of either spin up or down electrons. These mechanisms have great potential for spin filters for semiconductor spintronic applications, which avoid the need for spin injection from ferromagnetic metals or dilute magnetic semiconductors.