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Browsing by Author "Matei, Ion"

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    Electrochemical studies of MBE-grown CaF2/BaF2 heterolayers
    (2007) Matei, Ion; Maier, Joachim (Prof. Dr.)
    Ionic conductors, materials in which specific ions can migrate preferentially with high mobility, are of prime importance for electrochemical measurements, and for devices such as high-temperature batteries and fuel cells, chemical filters and sensors. This research study is focused on the dynamics of ion-conducting superlattices synthesized by molecular beam epitaxy (MBE) in which the interfaces are artificially tuned, with the aim of designing superior ionic conductors by controlling their interfaces. The interface is also expected to introduce lattice strain due to lattice mismatches and/or to change space charge distribution at the interfaces when superlattices of different ionic conductors are fabricated with a period of a few to a few hundred nanometres. Since the superlattice structure enables to tune the crystal structure to some extent, the ionic conductivity dependence on the structural parameters will also be investigated in this study. A qualitatively different conductivity behaviour is expected when the interface spacing is comparable to or smaller than the width of the space charge regions in comparatively large crystals: single layers lose their individuality and an artificial ionically conducting material with anomalous transport properties is generated. These results demonstrate mesoscopic ion conductivity effect in nanosystems (extremely thin films, nanocrystalline materials). In order to obtain more fundamental insight into the conductivity effects, some points still need to be clarified and are addressed in this study: (1) the detailed understanding of the defect chemical situation and the conductivity effects in parallel and in perpendicular direction to the interfaces; (2) the annealing effects; (3) theoretical model and numerical evaluation in periods of the mesoscopic situation (thinner than 50nm). To understand these effects in depth, electrical measurements on parallel (along the interfaces) and perpendicular (to the interfaces) configuration of the heterostructures as well as thermodynamic modelling are performed. Multilayers of CaF2/BaF2 have been prepared by molecular beam epitaxy on different substrates (Al2O3, Si, Nb-doped SrTiO3), with highly defined geometry, periodicity, interfacial spacings and layer sequence. The measured effective parallel conductivity (i.e. derived from the measurement of parallel conductance via the total thickness ~400nm) progressively increases with interfacial density. The purpose of the annealing experiment is to determine the anomalous decrease of the parallel conductivity of the sample as the annealing temperature increases. This can be associated with the existence of unstable dislocations not only at the interface, but also inside the layers that can be annealed out by thermal treatment. This results in a clear picture: in annealed samples there is a fluorite ( -ions) transfer from one phase to the other. In a non-annealed samples this is superimposed by charging of dislocations leaving vacancies in the vicinity. The heterostructures on conductive substrates were also prepared and allow us to carry out the conductivity measurement in the perpendicular direction to the interfaces. Mesoscopic size effects predict a decrease in the difference between parallel and perpendicular conductivity with the increase in the number of interfaces. This is very satisfactory as a parallel conductivity pronounces the highly conductive regions, while the perpendicular one emphasizes the less conductive regions. In this study, the thickness dependence of the layer conductivities is numerically calculated using both the Gouy-Chapman and the Mott-Schottky modes. The calculated concentration profile turns only out to be consistent with the charge density of the Mott-Schottky model if the frozen-in impurity profile is assumed to be moderately increased. In summary: 1. Heterolayers of CaF2/BaF2 have been prepared by molecular beam epitaxy (MBE) on different substrates (Al2O3, Nb-doped SrTiO3), with highly defined geometry, periodicity, interfacial spacings and layer sequence. 2. XRD and AFM measurements demonstrate that defined highly oriented heterostructures of CaF2/BaF2… can be prepared on different substrates. 3. The conductivity effects can be understood in terms of ionic space charge effects occurring as a consequence of a thermodynamic redistribution at equilibrium. 4. The influence of annealing effects on the resistance of the sample has been studied and analysed. Unstable dislocations appear to be charged by adsorption. 5. In this study, the thickness dependence of the layer conductivities is numerically calculated using both the Gouy-Chapman, and the Mott-Schottky models. In direct comparison to the experimental data, the modified Mott-Schottky mode (impurity profile with a gradient close to the interface) can reproduce the features of the experiments even in the mesoscopic range.
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