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| Home > Publications database > Nanocrystalline alkaline earth titanates and their electrical conductivity characteristics under changing oxygen ambients |
| Home > Publications database > Nanocrystalline alkaline earth titanates and their electrical conductivity characteristics under changing oxygen ambients |
| Dissertation / PhD Thesis/Book | PreJuSER-32043 |
2003
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/181
Report No.: Juel-4070
Abstract: Electroceramic materials, especially the perovskite-type titanates, have been subject of intense research in recent years aiming for their integration into state-of-the-art electronic devices like memories or sensors. Due to continuous miniaturization, typical feature dimensions range down to several nanometers. Today, the discussion of thin film electrical behavior (like leakage currents or degradation effects) is still based on the established model of bulk defect chemistry. However, the observed electrical properties of thin films are often inconsistent with this model. Against the background of single crystals, the present work represents the first systematic study of the conductivity characteristics of alkaline earth titanates in the form of polycrystalline and heteroepitaxial thin films as well as nanocrystalline ceramics as a function of temperature and the ambient oxygen partial pressure. This was made possible by using a specially designed measurement setup that allowed the reliable determination of resistances of up to several gigaohms at temperatures between 600 °C and 1000 °C under continuously adjustable oxygen partial pressures ranging from 10$^{-20}$ bar to 1 bar. In contrast to the well -known `v-shaped' log $\delta$ - log pO$_{2}$ profile of single crystals, due to n-type and p-type conduction, the conductivity behavior of CSD-prepared, polycrystalline SrTiO$_{3}$ thin films with a grain size of $\sim$ 50 nm differs radically. Besides markedly shifted absolute values, the most prominent characteristics are a sharp drop under reducing conditions followed by a broad plateau region. Furthermore, the p-type regime does not appear under oxidizing conditions. Tailored investigations on heteroepitaxial as well as polycrystalline thin films grown by PLD and especially by studies on nanocrystalline BaTiO$_{3}$-ceramics with a mean grain size of $\le$ 100 nm allowed an unambiguous assignment of these effects to morphological properties. Besides the surface-to-volume ratio in particular the high grain boundary density is a decisive measure for the different conductivity profiles of nanocrystalline titanates. This work shows for the first time that nanocrystalline titanates reveal special conductivity characteristics which cannot be explained by the classical model alone. Additionally, important aspects for a model description were developed taking into account the strongly inhomogeneous habit of nanocrystalline materials.
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