First-principles investigation of defects and interfaces in rutile, perovskite, and fluorite-structured metal oxides
- Author(s): Behtash, Maziar
- Advisor(s): Yang, Kesong
- et al.
Defects and interfaces are ubiquitous in polycrystalline metal oxide materials, and their presence gives rise to a host of beneficial and harmful effects. In this thesis we focused on substitutional impurities, heterointerfaces, oxygen vacancies, and grain boundaries, with the goal of enhancing material properties, using first-principles density functional theory calculations. In the first project we studied the energetic and electronic properties of pentavalent-cation-doped SnO2 transparent conducting oxide (TCOs) materials. We found that P-doped SnO2 (PTO), a non-toxic low-cost material, compares favorably with well-known TCOs Sb-doped SnO2 (ATO) and Ta-doped SnO2 (TTO). In addition, we showed that the theoretical charge carrier density (ne) of PTO, ATO, and TTO is nearly identical, indicating that the order-of-magnitude experimental variations in n e for the latter two materials may instead arise from differences in experimental conditions during synthesis. In the second project we explored the possibility of enhancing the properties of the two-dimensional electron gas (2DEG) which forms at the interface of the LaAlO3/SrTiO3 (LAO/STO) heterostructure through n-type layer doping with transition metal cations. We found that Ta(Nb) doping at the interfacial Ti site is energetically favorable, and can significantly enhance the charge carrier density and magnetism of the interfacial 2DEG in LAO/STO. In addition, we found that Ta(Nb) doping at the interfacial Al site can improve 2DEG charge confinement to only two TiO2 atomic layers of the STO substrate.
In the third project we examined polarization and the effect of externally-applied strain on 2DEG properties in LAO/STO. We found that the LAO film polarization (P LAO ) decreases with film thickness, and that there is a critical P LAO value above which 2DEG cannot form. We resolved the long-standing discrepancy between the experimental and theoretical 2DEG charge carrier density through the use of an appropriate slab model. In addition, we showed that  uniaxial tensile strain applied on the STO substrate can reduce P LAO and thereby enhance the charge carrier density, electron mobility, and interfacial charge concentration of the LAO/STO 2DEG. In the fourth project we studied termination stability and oxygen vacancy formation in a Σ5  twist grain boundary (GB) structure of SrTiO3 (STO). We found that of the three possible GB terminations, only the SrO/SrO (S/S) and SrO/TiO2 (S/T) can form in the chemical potential range necessary to guarantee bulk STO stability, while the TiO2 /TiO2 (T/T) termination cannot form. We showed that oxygen vacancies tend to segregate adjacent to the GB layer in the S/S system, but at the GB layer in the S/T system. In addition, we demonstrated that oxygen vacancies form more easily in the S/T GB system than the S/S GB system, leaving the door open for possible GB engineering applications. In the final project we considered substitutional impurity segregation to Σ5 (310)/ tilt grain boundary structures of ZrO2 , HfO2 , and yttria-stabilized ZrO2 (YSZ). We discovered a fundamental difference in the characteristic segregation profiles of aliovalent and isovalent impurities which is explained by differences in the local electrostatic potential energy. We successfully generated and structurally-optimized a YSZ GB structure bearing high concentrations of yttrium dopants and oxygen vacancies. Finally, we found that Si, Mg, and Ca are strong segregants in YSZ while Al, Ti, and Y are not.