In this thesis the diffusion of anions and cations in HfO2 was investigated in detail.
Furthermore, the conductivity of HfO2 was probed and approaches to interpret
the results were presented. The diffusion of oxygen in dense ceramics of
monoclinic HfO2 (m-HfO2) was studied by means of (18O/16O) isotope exchange
annealing and Secondary Ion Mass Spectrometry (SIMS). All measured isotope
profiles showed complicated behaviour in exhibiting two features: the first feature,
closer to the surface, was attributed mainly to slow oxygen diffusion in an
impurity silicate phase; the second feature, deeper in the sample, was attributed
to oxygen diffusion in bulk m-HfO2. The activation enthalpy of oxygen tracer diffusion
in bulk m-HfO2 was found to be ΔHD∗ ≈ 0.5 eV. The diffusion of cations
in m-HfO2 was studied with samples prepared by cooperation partners, utilising
a low-temperature preparation method, atomic layer deposition, in order to produce
non-equilibrium samples. These were then used in diffusion annealing experiments
and investigated with SIMS. The measured isotope profiles displayed
two features, attributed to bulk diffusion and grain-boundary diffusion. A numerical
analysis produced a bulk diffusion activation enthalpy of ΔHb ≈ 2.1 eV
and a grain-boundary diffusion activation enthalpy of ΔHgb ≈ 2.1 eV. These
values are small compared to other AO2 systems and the difference is attributed
to the structural perturbations in the monoclinic system. A computational investigation
of cation diffusion in m-HfO2 using Density-Functional-Theory (DFT)
yielded migration enthalpies for individual cation jumps. Two jumps were found
with values comparable to the experiments (≈ 2 eV), allowing long-range diffusion
through the bulk. Molecular dynamics simulations in c-HfO2 with an applied
field were able to reproduce the activation enthalpy of bulk diffusion determined
experimentally and with DFT. However, molecular static simulations
instead produce results much closer to those of other AO2 systems. A cooperative
migration mechanism of oxygen and hafnium vacancies is proposed. The
conductivity of m-HfO2 was studied in dependence of the oxygen partial pressure
by means of high temperature equilibrium conductance measurements. In
reducing conditions the total conductivity was found to increase with oxygen
partial pressure. Numerical defect-chemical calculations showed that singly positively
charged oxygen vacancies are likely responsible for this behaviour. In the
intermediate oxygen partial pressure regime ionic conductivity dominated. In oxidising
conditions the total conductivity increased with oxygen partial pressure
due to electron holes.weiterlesen