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Title: Fabrication, microstructure and properties of new chalcogenide thin films and characterization of functional compounds
Author: Mousavi, Tayebeh
ISNI:       0000 0004 6352 6499
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2015
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Fe-chalcogenides are of great interest because they have the simplest structure in the Fe-based superconductors, and show low anisotropies, high critical fields and high current densities all of which make these compounds useful for understanding Fe-based superconductors, a new class of superconductors with unusual properties, both for fundamental physics and potential high-field applications. The main part of this thesis concentrates on the fabrication and characterization of FeySe1-xTex (Fe-11) thin films in order to link superconductivity and microstructure in this system. Since sputtering has been rarely used for the deposition of Fe(Se,Te) films, we used sputtering to study the feasibility of this technique for this purpose and to optimize the processing conditions. Phase evolution and texture development as a function of processing conditions are studied, and the optimum conditions for the best quality film are found. Crystallization temperature of the 11-phase was found to be 250°C, and texture is shown to be dependent on substrate, substrate temperature and film thickness which strongly control in-plane alignment. The best texture is obtained at 315°C for films thinner than 50nm on the MgO. Texture of the films becomes poorer on the substrates in the order MgO > LAO > STO > CaF2 > LiF > LSAT, and the relationship between texture and substrate is shown to be more related to the growth mechanism rather than lattice misfit. 2D layer-by-layer growth is found for the LSAT, while the films on the LAO and MgO show 3D island growth, and a combination of both growths is found for the film on the STO. The films grown by in-situ sputtering are shown to have better quality than those grown by the ex-situ. A sharp TC of about 10.2K is obtained for a single crystal with a thickness of 58nm. Most of the deposited films are Fe-rich (y > 1) due to compositional variation in the target and the volatility of Se and Te. Extra Fe is the main detrimental parameter for the suppression of superconductivity in these films showing the necessity of using multiple sources in sputtering. Nano-scale inhomogeneities are also observed in the majority of the films possibly as a result of different Se/Te ratios and Fe3O4 impurity. Substitution of 2% Mn for Fe in the FeySe1-xTex structure leads to the stabilization of tetragonal 11 phase and improvement of crystallographic texture. Mn atoms are shown to occupy interstitial Fe sites in the tetragonal unit cell. I also applied TEM technique to functional materials from external collaborators including Bi2Te3 and CaFe2As2. Studies of Mn-doped Bi2Te3 single crystals reveal that at doping concentrations, Mn incorporates with the Bi2Te3 structure differently. At low doping concentrations (9%Mn), Mn atoms are randomly substituted onto the Bi sites resulting in the variation in both composition and lattice parameter. At high doping concentrations (15%Mn), supersaturation occurs and the extra Mn atoms occupy the interstitial sites in addition to forming nano-scale Mn-rich precipitates. CaFe2As2 single crystals grown by the FeAs-flux method are unstable FeAs-supersaturated crystals containing a high concentration of stacking faults. Annealing at high temperature (700°C) causes the extra FeAs to form needle-shaped precipitates perpendicular to the c-axis by Ca diffusion, and the matrix develops dislocations. Annealing at lower temperature (400°C) results in a number of stacking faults and similar precipitates with low Ca. As a result, the microstructure of the CaFe2As2 single crystals, especially the amount of strain, can be controlled by the annealing temperature resulting in similar effects to applying external pressure.
Supervisor: Grovenor, Chris ; Speller, Susannah Sponsor: Clarendon Fund
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available