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Hydrothermal epitaxy of functional perovskite thin films.

Velasco-Davalos, Ivan Alejandro (2016). Hydrothermal epitaxy of functional perovskite thin films. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 122 p.

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Functional oxides and their thin film epitaxy have become increasingly requested and further improvement in the existing technologies of information storage and energy harvesting heavily depends on the performance of such heterostructures at the nanoscale. Ferroelectric materials show a spontaneous electric polarization, which can be switched repeatedly by applying an external electric field. Since the discovery of ferroelectric BaTiO3 as the first oxide ferroelectric, the period prior to 1988 was mainly restricted to modelling ferroelectric phase transitions and discovering new ones. However, the focus has been significantly changed in the nineties, when thin films were developed and integrated into semiconductors at the nanoscale. This down scaling and bi-stable polarization of ferroelectrics were attractive in memory devices through ferroelectric random access memory, smart cards etc., and in tunable microwave devices through phase shifters, delay lines, resonators etc., apart from the conventional capacitor applications. Similarly, work on multiferroic can be traced back to pioneering research in the 1950s and 1960s, but there has been a recent resurgence of interest driven by long-term technological aspirations. For more than a decade, BiFeO3, being a magnetic and a strong ferroelectric material at room temperature, has been renowned as a multiferroic materials that addresses a range of possible applications that no other material class exhibits so far. Out of many possible options, photovoltaic applications are being extensively considered due to the relatively low band-gap (~2.5 eV). These two material systems, BaTiO3 and BiFeO3 are widely considered as the model systems for ferroelectric and multiferroic properties and hence epitaxial thin film growth on lattice matched SrTiO3 substrates by an inexpensive hydrothermal method are considered in this thesis. One of the main requirements of producing high quality epitaxial thin films on SrTiO3 substrates is the single termination of its surface. To this effect, a novel microwave-assisted hydrothermal etching was successfully applied to the surface preparation of pure and Nb-doped SrTiO3 single crystals with (100), (110) and (111) orientations. Without the possibility of fluorine contaminations from the Teflon liner and by avoiding the etching chemistry involved with HF widely used, the surface structure appears perfect within the limitations of the in-plane and out-of-plane miscut angles. These results indicate that the utilization of this method, without any corrosive chemicals during the preparation steps, to achieve atomically flat surface with single chemical termination of SrTiO3 substrates is feasible and compatible with batch processing. This technique is inexpensive, fast, safe, environmentally benign, compatible with batch processes, and showed remarkable reproducibility. Further, this method does not need an ultra-high vacuum environment and long annealing time at high temperatures. The possibility to reduce the etching time significantly avoids the formation of etch pits and holes on the substrate surface. The hydrothermal technique is shown to be a feasible way to obtain good crystalline quality thin films of BaTiO3 and BiFeO3. This method is an inexpensive alternative technique, which is defined as any chemical reaction in presence of aqueous solvents conducted at autogenous pressure, which corresponds to the vapor pressure above room temperature and below the critical point, generally lower than 370 °C for water, in a closed system. As for the synthesis of the thin films, two hydrothermal technique modes were employed; conventional hydrothermal for BaTiO3 and microwave assisted hydrothermal for BiFeO3. The conventional hydrothermal mode, where the heating process happens by convection at heating rates of 10 °C/min using a stainless steel reactor in a conventional oven; and, the microwave assisted hydrothermal mode, which consists in utilizing a relatively low budget high strength polymer reactor in a microwave oven for which the heating process happens by absorption through water molecules of the 2.45 GHz radiation which allows a more efficient heating process to synthesize films in less time compared to the conventional process. In the case of BaTiO3 films, TiO2 nanoparticles dispersed in the Ba(OH)2 alkaline solution are used as precursors. The incorporation of H2O2 into precursor solution served as a strong oxidant and catalyst for the uniform nucleation of BaTiO3 on the substrate surface. Polarization reversal in single phase epitaxial and polycrystalline BaTiO3 thin films were demonstrated on Nb:SrTiO3 and Pt/Al2O3/SiO2/Si substrates, respectively. Raman scattering studies revealed the necessity of cumulative depositions with 10 min of microwave radiation at a power of 120W for the single phase formation of BaTiO3 films. No traces of impurity phases were present according to the x-ray diffraction and Raman scattering results which is primarily due to the absence of mineralizers. Local phase hysteresis of BaTiO3 thin film on Nb:SrTiO3 substrate confirms ferroelectricity and 180o switching. Highly ordered BiFeO3 thin films were successfully produced by hydrothermal synthesis on SrTiO3:Nb (100) substrates. Surfactants were avoided and significantly reduced the concentration of potassium hydroxide (KOH) by a factor of 10 compared to reports in literature to reduce the leakage current. Due to this reduction of mineralizer, leading to less solubility of the precursors and decreasing the reaction rate, deposition time of ~18 h were required to grow a 40 nm thin film. As-grown BiFeO3 films were also annealed at 500 °C under nitrogen flow for better crystallization and to remove hydrogen. For the first time, polarization reversal is demonstrated by successful reduction of electronic leakage. A first demonstration of the bulk photovoltaic effect in hydrothermally grown BiFeO3 shows that this technique can be used to produce multiferroics for photovoltaic applications. Our experimental results confirm realization of single termination of SrTiO3 and epitaxial deposition of BaTiO3 and BiFeO3 by an inexpensive hydrothermal method with or without microwave radiation.

Type de document: Thèse
Directeur de mémoire/thèse: Ruediger, Andreas
Informations complémentaires: Résumé avec symboles
Mots-clés libres: functional oxides; perovskit; BaTiO3; BiFeO3; thin films; epitaxial growth; SrTiO3; single crystal; substrate surface preparation; etching; step-terrace structure; single termination; hydrothermal synthesis; atomic force microscopy; ferroelectric polarization switching; microwave-assisted hydrothermal method; conventional hydrothermal process; topography; bulk photovoltaic effect; photocurrent
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 20 oct. 2016 19:33
Dernière modification: 20 oct. 2016 19:33
URI: http://espace.inrs.ca/id/eprint/4800

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