OPTICAL ORIENTATION OF SPINS IN SEMICONDUCTORS USING THE FELIX AND FELBE FREE-ELECTRON LASER FACILITIES

Lead Research Organisation: University of Surrey
Department Name: ATI Physics

Abstract

Optical orientation of spins in semiconductors is a well established field that takes advantage of the selection rules for circularly polarised photon absorption. Using short pulsed lasers information can be gained on the spin dynamics and control. The proposal focusses mainly on narrow gap III-V semiconductors (NGSs) because they offer advantages for spintronic devices (i.e. devices that depend on the manipulation of spin rather than charge), due to their high mobility and strong relativistic effects that couple the spin and orbital motion. The fact that the materials of choice have small bandgap immediately indicates that far-infrared sources are required for excitation of spins from the valence band to the conduction band. In addition, in this wavelength range one can gain valuable information about dynamics using intraband excitation (e.g. of conduction electrons in doped samples) in both narrow and wide gap materials. Intraand excitation is important if e.g. photo-excited holes influence electron spin dynamics in interband experiments, or if the material under investigation has an indirect gap. This proposal is based on collaboration between groups at Surrey and Heriot-Watt Universities with combined experience in the use of Free Electron Laser (FEL) facilities in infrared and optical characterisation of ultrafast processes. We wish to hire a project student, based at Surrey, to study spin dynamics in bulk and quantum well (QW) structures of InSb and InAs at the FEL facilities, FELIX (at Utrecht) and FELBE (at Dresden). A key aspect of this activity will be the training and maintenance of expertise of UK personnel in the use of FEL facilities in preparation for the forthcoming Fourth Generation Light Source (4GLS) at Daresbury. The overall scientific objective is to resolve key issues related to spin manipulation (spin lifetime and spin relaxation). The programme will work closely with two partner institutions, Imperial College and QinetiQ (Malvern), who will provide a focused systematic set of NGS QW structures from parallel funded activities.

Publications

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Description Optical orientation of spins in semiconductors is a well established field that takes advantage of the selection rules for circularly polarised photon absorption. Using short pulsed lasers information can be gained on the spin dynamics and control. We focussed mainly on narrow gap semiconductors (NGSs) because they offer advantages for spintronic devices (i.e. devices that depend on the manipulation of spin rather than charge), due to their high mobility and strong relativistic effects that couple the spin and orbital motion. The fact that the materials of choice have small bandgap immediately indicates that far-infrared sources are required for excitation of spins from the valence band to the conduction band. This project concentrates on NGSs InSb (a III-V material), and PbTe (a IV-VI material), which are suitable candidates for high speed electronic and spintronic devices due to their strong (spin orbit) and relativistic effects. We used a combination of solid state laser systems and the Free-Electron Laser facility FELIX in the Netherlands. Semiconductors generally show that a high contrast electrical control of spins, as characterised by the Rashba effect, goes hand-in-hand with a short spin relaxation lifetime (or vice-versa). One reason for this is that the vast bulk of research has focussed on the non-centro-symmetric family of III-V crystals, although engineering of GaAs heterostructures with novel growth directions has produced some good results (but see our new results on the lead salts, below, where we combine a high Rashba coefficient with a long zero field spin lifetime).



Outcomes of this project are demonstrations that the spin lifetime can be controlled by:

a) the magnitude and orientation of an applied magnetic field. This work [published in Appl. Phys. Lett] paper has significant implications for semiconductor spintronics. It shows that spin coherence may be either enhanced or reduced by a given magnetic field depending on its orientation. This means that for hybrid ferromagnet/semiconductor devices, strategies must be incorporated to mitigate the effects of stray edge fields that could destroy injected spin polarisation. At the same time there are opportunities for new devices such as a spintronic compass.

b) asymmetry in their structures (this is the "Rashba effect"). In this work we showed sensitive control over the spin dynamics by heterostructure design [published in J Phys C].

c) the crystal symmetry so that the spin lifetime in IV-VIs in zero field is two orders of magnitude longer than in III-Vs. Here we show that lead-chalcogenide semiconductor heterostructures can exhibit long spin lifetimes by virtue of their centro-symmetric crystal structure, and a tuning of the lifetime of over one order of magnitude after appropriate structure control corresponding to a large Rashba coefficient. The results imply that this system can be the material of choice for certain semiconductor spintronic applications requiring control of spins [in draft].



We investigated the dynamics both with standard time-resolved spectroscopy (the experiments described above) and also with transient photoconductivity experiments in electrical device structures [in draft] and with two-photon absorption [in draft].



All the above lead to the conclusion that NGSs - and in particular IV-VIs - provide exciting opportunities for spin injection, detection and manipulation, and certainly must be considered as suitable materials for future spintronic devices.
Exploitation Route The spintronics programme, which is under-pinned by the present proposal, will result in future generations of very low power, low noise, devices for memory, logic and sensors. In the longer term these devices will have the potential for quantum computing and cryptography. The latter is particularly important for NGS where polarised sources in the mid-IR are required for free-space secure communications. There is an enormous academic community interested in spintronics generally and semiconductor spin physics who will benefit from better understanding of strongly spin-orbit coupled materials. The 4GLS user community (and indeed the spintronics, semiconductor and ultrafast optics communities) will benefit from output of trained staff. Magnetic metals have been successfully used for the development of a variety of spintronic field sensor and memory and devices (disk-drive read heads and MRAM); adding semiconductors to the spintronics toolkit will allow new active functions because of the tremendous control over transport properties.
Sectors Digital/Communication/Information Technologies (including Software),Electronics

URL http://www.surreyspintronics.com/