| Zdroj: |
Jülich : Forschungszentrum Jülich, Zentralbibl., Verl. [u.a.], Berichte des Forschungszentrums Jülich 4266, X, 116 S. : graph. Darst. (2007). = Zugl.: Aachen, Techn. Hochsch., Diss., 2007 |
| Popis: |
The ever increasing memory densities of the so called Dynamic Random Access Memory (DRAM) lead to a drastic reduction of the area of the incorporated storage capacitors. As a consequence, the film thickness of the used dielectrics has to be decreased in a similar way. When considering state-of-the-art dielectrics showing a relative permittivity er of around 10-25, the necessary thicknesses are well below 100 nm, which leads to a drastic increase of the self-discharge in the capacitors. Alternative materials that display much higher permittivity values are the so called perovskites, which depending on their composition allow a theoretical increase of the memory density about a factor of 20-50, compared to standard materials. Investigations on BaSrTiO3 (BST) as the most intensively studied material in this context however revealed, that the permittivity is strongly suppressed when dealing with thin films (below 200 nm). This behavior is often tried to explain using the so called “dead-layer” model, which assumes a thin layer with reduced permittivity at the electrode/dielectric interface, resulting in an overall decrease of the measurable permittivity. The goal of this thesis is to identify and eliminate the extrinsic influences responsible for the observed decrease of the permittivity, as well as to examine additional intrinsic effects. For this purpose, capacitor samples are fabricated, which due to their epitaxial single-crystalline film growth show a crystal structure nearly free of any defects, thus allowing to gain further insight into the phenomenon of the reduced permittivity. Electrical analysis from temperature dependent measurements of capacitance and polarization give additional information about the thickness dependent ferroelectric phase transition as well as the values of “interface capacitance” and “bulk permittivity” derived from the dead-layer model. The experimentally achieved values are discussed theoretically using the approach of a modified Landau-Ginzburg-Devonshire theory of the ferroelectric phase transition. In addition to the influence of the misfit dislocation and its contribution to the strain in the films, also an impact of the depolarizing field will be considered, such that the shift of the phase transition temperature as well as the temperature dependence of the interface capacitance can explained more accurate. Investigations of samples with an SrRuO3 (SRO) upper electrode reveal excellent epitaxial interfaces between dielectric and electrodes. However, these samples show a strong dependence of the permittivity on the amplitude of the incorporated small signal excitation voltage, whereas this behavior cannot be found on samples wit Pt upper electrode. The reason for this peculiarity might be explained with a general co-existence of relaxor- and ferroelectric properties in the BST samples, which in case of Pt electrodes is suppressed by the considerably deteriorated interface between BST and Pt. |
