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Title: Optical and structural characterisation of barium zirconate-titanate thin films
Author: Rackham, Jonathan
ISNI:       0000 0004 7963 7847
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2019
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Light emitting diodes (s LED s) operating in the C-spectrum ultraviolet region ( UV-C , wavelength 200 to 290nm ) are desirable for water sterilisation, but existing III-nitride materials are inefficient ( < 1% ) and difficult to manufacture. This thesis presents an investigation of perovskite oxides as an alternative wide band gap material for UV-C emission. Barium zirconate-titanate, ( BaZr x Ti 1-x O 3 , films have been grown by pulsed laser deposition from solid state sintered targets, x = 0, 0.25, 0.5, 0.75 and 1 . These were initially characterised by X-ray diffraction ( XRD ), UV-visible absorption spectroscopy ( UV-Vis ), atomic force microscopy ( AFM ) and X-ray photoelectron spectroscopy and found to be smooth ( ∼ 1nm RMS roughness) and of good crystallinity. The structural and electronic properties of ultra-thin films ( ∼ 20 to 2nm ) were also measured by XRD , UV-Vis and AFM as well as variable-angle spectroscopic ellipsometry. BZT was found to exhibit an indirect band gap for all compositions and film thicknesses. The relationship between primary band gap and composition shows a third-order dependence. The relationship between band gap and film thickness shows competing influences that are likely to prevent a change in primary band gap character similar to MoS 2 . It is concluded that BZT is unlikely to be useful for UV-C LED s. The results from chapter 5 investigate the disparity between local- and micro-structure in BZT with reciprocal space maps and Raman spectroscopy. Local tetragonal distortions are seen in BZT alloys x < 1, irrespective of their room-temperature bulk ferroelectric behaviour. With reducing film thickness, the ratio of I [ A 1 ( LO 3 )] to I [ A 1g ] increases for films of 5nm thickness, suggesting increased ferroelectric ordering. It is suggested that this is due to interaction between polar nanoregions and both surfaces of the film simultaneously.
Supervisor: Alford, Neil ; Scanlon, David Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral