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Title: Quantum mechanical study of point and molecular defects in perovskite nano-systems
Author: Al-Hamadany, Raied Abass Saleh
ISNI:       0000 0004 5367 0948
Awarding Body: University of Newcastle Upon Tyne
Current Institution: University of Newcastle upon Tyne
Date of Award: 2014
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Strontium titanate is a perovskite dielectric material with a wide band-gap of 3.25 eV and a large relative dielectric permittivity of 300 at room temperature. The combi- nation of these properties makes SrTiO₃ a promising candidate for various industrial applications. However, there is growing evidence that oxygen vacancies have a sig- nificant impact upon its use, with the diffusion and deep donor level of the oxygen vacancy leading to electrical leakage. A qualitative understanding of the diffusion and electrical properties of oxygen vacancies can help to provide a clearer picture of many phenomena such as resistive switching and leakage current. Utilising SrTiO₃ thin films in various devices leads it to be in contact with other substances such as metal electrodes or other oxides. The lattice-mismatch between thin film SrTiO₃ and other material means that thin film SrTiO₃ is grown under bi-axial strain. The magnitude and the value of strain are driven by the lattice parameters of the material it is in contact with and the strain might be compressive or tensile. Here, the results of first principle density functional theory calculations performed using the AIMPRO code are presented. It is found that thin film SrTiO₃undergoes a transition from cubic to tetragonal structure with polarisation along the [001] and [110] directions under compressive and tensile (001) strain respectively. As a key parameter for tailoring the properties of SrTiO₃, the diffusivity of oxygen vacancies under bi-axial tensile or com- pressive strain has been investigated. The structural transition yields anisotropy in oxygen vacancy diffusion for diffusion within and between planes parallel to the plane of strain. Under (001) compressive strain it is found that, in the range of strains consistent with common substrate materials, diffusion energies in different directions are significantly affected, and for high values of strain may be altered by as much as a factor of two. The resulting diffusion anisotropy is expected to impact upon the rate at which oxygen vacancies are injected into the film under bias. By contrast, under (001) tensile strain, the diffusion of oxygen vacancies is predicted to increase in all directions (in-plane and inter-plane), albeit more so in the direction perpendicular to the plane of strain. Doping with a foreign element, namely a transition metal, is an alternative proce- dure for controlling the properties of SrTiO₃. The structural, thermodynamic, electri- cal and electronic properties of zinc-doped SrTiO₃ have been studied in the framework of density functional theory. The interaction of Zn with oxygen vacancies, which is a central theme in this thesis, has also been considered. In oxygen-lean conditions, however, the formation of oxygen vacancies is strongly favoured. It is found that VO may be bound to ZnTi with a binding energy of around 0.81±0.08 eV and no states in the gap. The role that ZnTi may have in the compensation for or capture of oxy- gen vacancy effects is discussed, along with the thermodynamics of Zn under various growth conditions.
Supervisor: Not available Sponsor: Iraqi Ministry of Higher Education and Scientific Research (MOHESR)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available