Oxygen transport in thin oxide films at high field strength

Forschungszentrum Jülich, 2014 - 115 Seiten
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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 


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