In this thesis, we investigate, from first-principles, various means by which the energy gaps and the refractive index of BaTiO$_3$, SrTiO$_3$ and Ba$_{x}$Sr$_{1-x}$TiO$_3$ may be tuned, including epitaxial strain, alloying, and anionic doping. We start by discussing some limitations in the experimental spectroscopic ellipsometry technique that could affect the reliability of the measured optical spectra. Then, we investigate the effects of strain and alloying on the energy gaps of these materials. Interesting features in the gaps are observed as a function of strain, where it decreases and increases for small and high strains, respectively. These features are correlated with the spontaneous polarization of the system, whose behaviour is sensitive to the value of strain. The polarization also affects the behaviour of the bandgap of Ba$_{x}$Sr$_{1-x}$TiO$_3$ as a function of Sr content, which shows a non-monotonic trend. On the other hand, the refractive index of Ba$_{x}$Sr$_{1-x}$TiO$_3$ shows similar features with strain and alloying to those of the bandgaps. Strain is found to induce a decrease of up to $\sim 8\%$ in the refractive index and a birefringence of up to $\sim0.14$, whereas alloying has a smaller effect. Ferroelectric distortions are found to have a major role in decreasing the index, where much less effect is observed when they are inhibited. However, we show that the refractive index is insensitive to the ordering of Sr atoms in Ba$_{x}$Sr$_{1-x}$TiO$_3$. Finally, we investigate the effect of F and N codoping in BaTiO$_3$. The refractive index is sensitive to the atomic ordering of the dopant atoms, which prefer to be aligned in linear chains of F$-$Ti$-$N. The component of the index along the chain shows a dramatic increase with respect to pure BaTiO$_3$, while the other components are almost unaffected. Also, we show that the index can be further tuned by strain and doping concentration.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:676806 |
| Date | January 2015 |
| Creators | Alsaei, Jawad |
| Contributors | Mostofi, Arash ; Tangney, Paul ; Alford, Neil |
| Publisher | Imperial College London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/10044/1/28579 |
Page generated in 0.0024 seconds