Optical and structural characterisation of barium zirconate-titanate thin films

Jonathan Rackham, Engineering And Physical Sciences Research Council, David Scanlon, Neil Alford
2019
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 =
more » ... tered 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.
doi:10.25560/70848 fatcat:cpwdjcxuarf6pd4gwpngg6r2de