Study of ferroelectric PbTiO3 nanostructures deposited onto substrates and prepared by a novel microemulsion mediated synthesis

Abstract

[ES] Los materiales ferroeléctricos presentan una serie de propiedades que les hace apropiados para un gran número de aplicaciones. Estos materiales presentan dos estados de polarización de igual energía entre los que se puede transitar mediante la aplicación de un campo eléctrico externo, convirtiéndolos en materiales muy atractivos para su utilización en memorias RAM.

Hoy en día, la tendencia de los dispositivos de memoria es aumentar la densidad de almacenamiento, que se encuentra actualmente en los Tb/inch2, manteniendo o disminuyendo su coste. Para ello, es imprescindible poder fabricar unidades de almacenamiento cada vez más pequeñas, manteniendo sus propiedades y a un coste lo más bajo posible. En la presente tesis doctoral se ha desarrollado un método novedoso de procesado basado en la tecnología “bottom-up” para la obtención de nanoestructuras ferroeléctricas de PbTiO3 sobre sustratos para su uso en memorias no volátiles, NV-FeRAMs. Este procedimiento implica el depósito sobre substratos de las disoluciones micelares resultantes de la mezcla de soles y microemulsiones mediante la técnica de Depósito Químico de Disoluciones (CSD). Para comparar los resultados que se obtendrán en el transcurso de esta tesis, se han preparado nanoestructuras explotando el fenómeno de la inestabilidad estructural de láminas ultradelgadas. Según este fenómeno, cuando el espesor de éstas es menor de un valor crítico, la película se rompe, dando lugar a estructuras aisladas. Se prepararon diferentes muestras, analizando el fenómeno antes y después de este espesor crítico, obteniéndose nanoestructuras aisladas con un límite inferior de tamaño lateral ~50 nm. Al analizar los depósitos, no se aprecia orden sobre el substrato. Mediante difracción de rayos-X de radiación sincrotrón en ángulo rasante, se ha determinado la orientación de las nanoestructuras, siendo ésta de fibra y presentando un cono de distribución direcciones de ±15°. Mediante microscopía de fuerzas en modo piezorespuesta (PFM), se ha comprobado el carácter ferro-piezoeléctrico a escala local de estas partículas. En esta tesis, se ha desarrollado una novedosa tecnología de procesado para la obtención de nanoestructuras ferroeléctricas de PbTiO3, basada en el depósito de soluciones micelares resultantes de la mezcla de soles y microemulsiones, y en la funcionalización de la superficie de los substratos. Se hipotetiza que las micelas formarán una red organizada una vez depositadas sobre el sustrato, de forma que el depósito de la disolución micelar dará lugar a una disposición ordenada sobre la superficie del sustrato de las nanoestructuras y que, además, tendrán un tamaño y forma controladas. Las micelas proporcionan un entorno aislado a las partículas de sol que se encuentran en su interior, pudiendo producirse las reacciones químicas de síntesis de los componentes. Esta característica de las micelas, sumada a su capacidad de auto ordenación, hace de ellas las “building units” o elementos primarios para las nanoestructuras ferroeléctricas de PbTiO3. Utilizando este método, se prepararon nanoestructuras sobre sustratos policristalinos de Pt-(100)Si, compatibles con la tecnología actual del silicio, y sobre sustratos monocristalinos de SrTiO3. Sobre los sustratos policristalinos de Pt-(100)Si, se obtuvieron nanoestructuras con un tamaño promedio de ~70 nm y con una morfología semejante a partir de disoluciones con diferentes concentración. Estas nanoestructuras son el resultado de la coalescencia entre un número finito de nanoestructuras primarias. Al analizar las distribuciones de tamaños de las partículas obtenidas, se deduce que éstas crecen de forma independiente. Esto contrasta con las nanoestructuras obtenidas mediante inestabilidad microestructural, que siguen un mecanismo de nucleación y difusión entre las nanoestructuras vecinas. Esta diferencia confirma la hipótesis de que las micelas actúan de “building units” de estas nanoestructuras. Sin embargo, las nanoestructuras preparadas mediante este procedimiento sobre los sustratos policristalinos no se disponen ordenadamente sobre éste, debido fundamentalmente a los defectos de la superficie del substrato.

[EN] 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 units” for 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.