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Theses

Charge and spin injection from GaAs into metallic and magnetic layers.

Abstract : This thesis describes experimental and theoretical work concerning photo-assisted tunnelling between optically pumped GaAs and metallic surfaces. In particular, the first evidence for the spin dependence of the tunnel photocurrent into a magnetic surface is presented. The thesis is made up of two separate parts: Part 1: Reports studies of charge and spin injection of photoelectrons from an optically pumped, tipless GaAs microcantilever into both (nonmagnetic) Gold and (magnetic) Cobalt surfaces. A new model, which is used to analyse the bias, tunnel distance and spin dependence of the tunnel photocurrent, correctly predicts the behaviour observed on the Gold surfaces. With the addition of a Pockels' cell, the same experiment is used to modulate the spin polarisation of tunnelling electrons and to the spin dependence of photoelectron injection into Cobalt. A reversal of the relative spin polarisation of the photoelectrons to the magnetisation direction of the Cobalt results in a 6 % variation in the tunnel photocurrent. This compares with a value of 0.1 % observed on nonmagnetic Gold surfaces. A reduction in this variation with increasing applied bias is attributed to a reduction in the surface recombination velocity. An extension to the model developed for charge injection which accounts for the spin polarisation of the photoelectrons describes the experimental results well. Part 2: Reports studies on charge and spin transport in GaAs using an original polarised photoluminescence microscopy technique. This technique is firstly applied to photoluminescence imaging in GaAs tips to be used in imaging studies of nanomagnetism, in order to estimate the expected electronic polarisations at the tip apex. In combination with studies on equivalent planar geometry samples and by numerically solving the charge and spin diffusion equations, polarisations approaching 40 % are predicted. Independently, polarised luminescence microscopy is used to investigate charge and spin transport in planar oxidized and passivated thin films. Surface recombination is shown to play an important role in determining the effective charge and spin diffusion lengths. The effect of an increase in the surface recombination velocity between the passivated and oxidized sample from 103 cm/s to 107 cm/s reduces the charge and spin diffusion lengths from 21 micron and 1.3 micron, to 1.2 micron and 0.8 micron respectively.
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Submitted on : Friday, July 30, 2010 - 7:52:43 PM
Last modification on : Wednesday, March 27, 2019 - 4:18:02 PM
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  • HAL Id : pastel-00506565, version 1

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Vu Duong. Charge and spin injection from GaAs into metallic and magnetic layers.. Other [cond-mat.other]. Ecole Polytechnique X, 2010. English. ⟨pastel-00506565⟩

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