posted on 2017-06-27, 11:23authored byAidan Wilkinson
The physics of two-phase systems is explored here, particularly magneto-transport and low temperature
d.c. conductivity in thin films. The extraordinary magnetoresistance (EMR) effect was
analysed in the context of previous experimental and theoretical considerations. The magnetoresistance
(MR) may be enhanced by up to two orders of magnitude by changing the geometry.
This was investigated using finite element analysis. Thin film samples consisting of a layered
structure of Germanium-Tin-Germanium (Ge-Sn-Ge) were created in collaboration with Shandong
University in China. Ge layers were kept at a constant thickness across all samples, with
variable Sn thickness. Regions of Sn form island-like shapes ten times larger than the average
film thickness, as is seen in scanning electron microscope (SEM) images. Raman spectroscopy
was conducted on these samples, from which it is concluded that the Ge layers are amorphous
in nature. It was seen that there is a relationship between the electrical resistance and the film
thickness which is indicative of a metal-insulator transition (MIT). The temperature dependence
of resistivity was subsequently investigated. The temperature coefficient of resistivity (TCR)
of the samples is seen to become negative as the thickness of the Sn layer is reduced below a
certain critical thickness. Depending on their thickness, samples were designated as metallic or
insulator, and various models associated with metals and insulators fitted to the data. While it
is impossible to be absolutely certain of the validity of each of the models, some are a better
fit than others. The same temperature dependence of resistivity was measured with an applied
magnetic field. This is compared with the previous EMR investigation, however the MR of the
samples is only of the order of a few percent which corresponds to ordinary MR, seen in most
metals. The magnetic field measurements suppress a resistivity down-turn at very low temperatures
(T<10K) which suggests the presence of superconductivity. Analysis of dr=dT shows
that the onset of superconductivity is lower for samples with a lower Sn thickness. Additionally,
the deposition rate of the Sn layer affects the resistivity significantly; a higher deposition rate
causes a decrease in resistivity. It is supposed that this is due to a change in the microstructure
of the film. Finally, piezo-resistivity was considered by applying mechanical compression to the
samples. The added pressure causes a drop in resistivity.
This work is made available according to the conditions of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) licence. Full details of this licence are available at: https://creativecommons.org/licenses/by-nc-nd/4.0/
Publication date
2017
Notes
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.