Book Description

Ionic transport in nanostructures at high eld strength has recently gained attention, because novel types of computer memory with potentially superior properties rely on such phenomena. The applied voltages are only moderate, but they drop over the distance of a few nanometers and lead to extreme eld strengths in the MV/cm region. Such strong elds contributes signi cantly to the activation energy for ionic jump processes. This leads to an exponential increase of transport speed with voltage. Conventional high-temperature ionic conduction, in contrast, only relies on thermal activation for such jumps. In this thesis, the transport of minute amounts of oxygen through a thin dielectric layer sandwiched between two thin conducting oxide electrodes was detected semiquantitatively by measuring the conductance change of the electrodes after applying a current through the dielectric layer. The relative conductance change
G=G as a function of current I and duration t follows over several orders of magnitude a simple, empirical law of the form
G=G = CIAtB with t parameters C, A and B; A;B 2 [0; 1]. This empirical law can be linked to a predicted exponential increase of the transport speed with voltage at high eld strength. The behavior in the time domain can be explained with a spectrum of relaxation processes, similar to the relaxation of dielectrics. The in uence of temperature on the transport is strong, but still much lower than expected. This contradicts a commonly used law for high- eld ionic transport. The di erent oxide layers are epitaxial with thicknesses between 5 and 70 nm. First large-scale test samples were fabricated using shadow masks. The general behavior of such devices was studied extensively. In an attempt to achieve quantitative results with defect-free, miniaturized devices, a lithographic manufacturing process that uses repeated steps of epitaxial deposition and structuring of the layers was developed. It employs newly developed and optimized wet chemical etching processes for the conducting electrodes. First high-quality devices could be manufactured with this process and con rmed that such devices su er less from parasitic e ects. The lithographically structured samples were made from di erent materials. The results from the rst test samples and the lithographically structured samples are therefore not directly comparable. They do exhibit however in principle the same behavior. Further investigation of such lithographically structured samples appears promising