Advancing the Development of Amorphous Chalcogenide Films

Abstract

This thesis focuses on the development of advanced Seebeck characterisation techniques to aid the study of thermoelectronic materials. A reference phenological model of the Seebeck effect, applied to silicon under variable wavelength monochromatic excitation, is developed and validated experimentally. To enable this, the development of a reliable experimental system was undertaken delivering unique capability with unmatched sensitivity. The system is able to perform Seebeck measurements of samples under vacuum or inert (e.g. N2) atmosphere. To overcome issues of working at high temperatures such as crystallisation, the system incorporates a liquid nitrogen-based cooling system to increase the range of temperature gradients. The ability to expose the sample under study to monochromatic light in the 400 nm to 1800 nm wavelength region, enabling photo-Seebeck measurements to be performed, is provided. The system has been validated using n- and p-type silicon and GeTe, with the results agreeing with previously reported measurements of the Seebeck coefficient. For the first time, photo-Seebeck measurements have been performed on amorphous undoped and Bi-doped GeTe thin films. Subtle differences in their Seebeck voltage upon illumination with monochromatic light are observed due to the light-induced increase in minority carrier concentration. Analysis of the results indicates that that the photo-Seebeck technique may be used to estimate the optical gap of thin films. Sub-bandgap photo-Seebeck studies also confirm the presence of optically active defect states. Photo-Seebeck measurements of the Bi-doped GeTe samples confirm that carrier type reversal (from p- to n-type) has occurred following implantation. Upon illumination with light it is shown that it is possible to obtain ‘optically induced compensation doping’ (i.e. photo doping) based on a reduction in the Seebeck voltage. In addition to this key result, the measurements performed help to clarify the role of Bi in the creation of trap states in amorphous GeTe, and in valence alternative pair (VAPs) modification.

Date of Award	1 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Richard Curry (Supervisor) & Matthew Halsall (Supervisor)