Electrical transport in hybrid spintronic structures

Abstract

Injection and detection of spin polarised current in a metal/semiconductor deviceand the measurement of the degree of injected spin polarisation are two key issuesin the development of hybrid spintronics. This thesis touches on both themes asit details the development of planar Andreev spectroscopy as a tool to measureinjected spin and the electrical characterisation of MgO tunnel barriers for efficientspin injection and detection. Point contact Andreev reflection spectroscopy has been widely used tomeasuretransport spin polarisation in magnetic materials. Planar Andreev structures havean advantage over point contacts as they offer greater control over interface qualityand the possibility of spatially resolved information about the spin polarisationusing nanojunction arrays. We find that planar junctions compare favourably topoint contacts in that they can yield low interface barriers and minimal nonthermalsmearing. We show that a low interface barrier is critical for accurate detectionof spin polarisation, particularly in semiconductorswhere large Fermi velocitymismatchcontributes to the barrier. Furthermore, the fabrication method strongly affectsall parameter values. For Pb/InAs planar junctions we demonstrate that themost feasible way to obtain interfaces suitable for spin detection is an ?etch-back?processing strategy. The processing routes are shown to be scalable to nanoarrayfabrication to allow measurement of spin accumulation. We also examine the electrical properties of ultrathin MgO barriers grown onInAs epilayers and the dependence of barrier characteristics on InAs surface pretreatmentand growth conditions. Chemical pretreatment improves the yield oftunnel junctions and changes the roughness of the interface between the oxide andthe semiconductor. Electrical characterisation confirms that tunnel barriers withappropriate values of interface resistance for efficient spin injection/detection havebeen achieved. Using the Rowell criteria and various tunnelling models we showthat single step tunnelling occurs above 150 K and a thermal smearing model suggeststhat tunnelling is the dominant transport process down to 10 K.EThOS - Electronic Theses Online ServiceGBUnited Kingdo