Visualização, caracterização e manipulação de paredes de domínios em filmes finos ferroelétricos
Gonçalves, André Marino
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In this work, conductivity in ferroelectric domain walls, in materials whose domains are insulating, was investigated at the nanoscale using a combination of techniques based on atomic force microscopy. Polycrystalline and epitaxial Pb(Zr,Ti)O3 tetragonal thin films were synthesized by different routes, with crystallization after deposition (ex-situ) and during deposition (in-situ). The manipulation of the domain structures and properties of the walls was explored, from the synthesis parameters, aiming the obtaining, optimization and manipulation of the conductivity in walls of ferroelectric domains. In polycrystalline films, complex domains composed of 180° and 90° walls were obtained. The reorientation of the polarization by the application of localized electric field, using the AFM tip, revealed substantially different responses in thin films with different thicknesses and grain sizes. Through the epitaxial growth path, films oriented in the  direction with monodomain structure and, with multidomain structure with walls of 90 °, were obtained by changing the epitaxial strain exerted on the film, using different substrates. With the application of localized electric field using the AFM tip, it was demonstrated the possibility of creation/destruction, with a certain level of control, of both 180° and 90° walls. The investigation of the local conductivity revealed that, in both the epitaxial and the polycrystalline films, the 90° walls are more conductive than the 180° walls, related to the greater deviation of the electroneutrality position, imposed by the mechanical restrictions on the 90° walls. It was also observed that, using oxides, the conductivity is stable, whereas with metal electrodes there is a decay of the current over time, possibly related to the trapping of carriers due to oxygen vacancies accumulated in the film/metal electrode interface. Finally, the processing route was also decisive in the electrical properties. Better electrical responses were obtained in films crystallized during the deposition than in films crystallized after the deposition. The observed differences are possibly related to the quality of the interface obtained by the different routes.