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.