Tesis doctoral de María Torres Sancho
Ferroelectric materials present some properties that make them suitable for a large number of applications. This materials present to states of polarization of equal energy, switchable by an external electric field. This makes of them a very attractive material for their use as random access memories (ram). Nowadays, the trend in memory devices is to increase the storage density, which is actually in the tb/inch2, maintaining or decreasing the fabrication cost. To achieve that, it is mandatory to be able to fabricate smaller storage units that keep their properties and at the lowest cost. in this thesis, a novel processing method based in the bottom-up technology is developed for the fabrication of ferroelectric pbtio3 nanostructures onto substrates for their use, for example, as non volatile ferroelectric ram, nv-ferams. This procedure implies the deposition onto substrates of micellar solutions, resulting of the mixture of sols and microemulsions by the chemical solution deposition technique (csd). in order to compare the results obtained in this thesis, nanostructures had been prepared using the phenomenon of the microstructural instability of ultrathin films. According to this phenomenon, when the thickness of an ultrathin film is below a critical one, it breaks, yielding isolated nanostructures. Different samples were prepared, studying the phenomenon before and after the critical thickness and obtaining isolated nanostructures of ~50 nm of lateral size. Self-assembly of the nanostructures onto the substrate was not observed. By synchrotron x-ray diffraction in grazing incidence, it was possible to determine the texture of the nanostructures: it is a fiber texture with an orientation distribution cone of ±15°. By piezoresponse force microscopy (pfm), the ferro-piezoelectric character of the nanostructures at a local scale was proved. in this thesis, it is proposed a novel processing technology for the fabrication of ferroelectric pbtio3 nanostructures, based in the deposition of micellar solutions resulting from the mixture of sols and microemulsions and in the functionalization of the substrates. Micelles are hypothesized to form a self-assembly network, once deposited onto the substrate, so that the deposit of the micellar solution will rise to a self-assembly onto the surface of the substrate with controlled size and shape. In addition, micelles provide an isolated environment for the sol particles in their inside, and the chemical reactions of synthesis of the components might occur. These properties of the micelles, makes of them the building unitsfor the ferroelectric pbtio3 nanostructures. By using this method, nanostructures were prepared onto polycrystalline pt-(100)si substrates, compatible with the actual si technology, and onto single crystal srtio3 substrates. nanostructures were obtained onto the polycrystalline pt-(100)si substrates with an average size of ~70 nm and with a similar morphology from solutions of different concentration. These nanostructures are the result of the coalescence of a finite number of primary nanostructures. From the analysis of the size of the obtained nanostructures it is deduced that they grow independently, which is contrast with the growing mechanism of the nanostructures obtained from the phenomenon of the ultrathin films instability, which mechanism is the nucleation and diffusion between neighbor nanostructures. This difference confirms the hypothesis of micelles acting as building units of the nanostructures. However, nanostructures prepared by this procedure onto polycrystalline substrates do not self-assemble onto the substrate, mainly due to the defects of the surface of the substrate. In order to improve the quality of this surface, it was functionalized by previously depositing a layer of microemulsion. When the micellar solution was deposited onto this functionalized surface, merged nanostructures were obtained after the crystallization process, as before. Primary nanostructures were also obtained with an average size of ~21 nm and a hexagonal short-range arrangement onto the substrate. The nanostructures have the crystalline structure of the pbtio3 perovskite with a fiber texture and two axes that present an orientation distribution cone of ±20°, as determined by synchrotron x-ray diffraction in grazing incidence. Pfm measurements confirmed the ferro-piezoelectric character of the nanostructures, measuring it in nanostructures of ~37 nm of lateral size and ~14 nm of height. To the best of the knowledge of this author, the size of this isolated nanostructure is below those reported in the literature where ferroelectric response had been measured. single crystal srtio3 substrates were used in order to utilize substrates with a surface closer to the ideal one than the polycrystalline ones used previously. Thus, once the validity of the microemulsion deposition procedure for the fabrication of primary nanostructures of controlled size and shape and with a self-assembly onto the surface of the substrate was proved, an improvement of the self-assembly onto the surface of the substrates was set as a target. However, the wetting of the substrate by the micellar solution was deficient, which yield a non-uniform coating of the single crystal substrate. This behavior was exploited to determine the type of growth and arrangement of the pbtio3 nanostructures on the srtio3 substrates, establishing that it is a frank-van der merwe growing type. The experiments of synchrotron x-ray diffraction in grazing incidence configuration confirmed the pbtio3 perovskite structure as well as the epitaxial growth onto the substrate. By pfm, the ferro-piezoelectric response of the nanostructures was measured. finally, in order to overcome the deficient wetting of the substrate by the micellar solution, the surface was functionalized by a chemical and thermal treatment, so that a uniform coating and a large-range arrangement of the pbtio3 nanostructures are observed in the whole substrate, when the micellar solution is deposited onto the srtio3 substrate after modifying the surface.
Datos académicos de la tesis doctoral «Study of ferroelectric pbtio3 nanostructures deposited onto substrates and prepared by a novel microemulsion mediated synthesis«
- Título de la tesis: Study of ferroelectric pbtio3 nanostructures deposited onto substrates and prepared by a novel microemulsion mediated synthesis
- Autor: María Torres Sancho
- Universidad: Carlos III de Madrid
- Fecha de lectura de la tesis: 06/11/2009
Dirección y tribunal
- Director de la tesis
- María Lourdes Calzada Coco
- Tribunal
- Presidente del tribunal: luís edmundo Fuentes cobas
- paula Vilarinho (vocal)
- marin Alexe (vocal)
- Jesús Ricote santamaria (vocal)