Abstract:
Dilute magnetic semiconductors (DMSs) are a group of promising materials
for spintronics applications, in which both the electric charge and intrinsic
magnetism (spin) of electrons are utilised to add novel functionality
to electronic devices. The proposed devices all rely on access to materials
that show strong coupling between magnetic and electrical properties, and
it is here that DMS systems offer potential. They consist of a conventional
semiconductor doped with a magnetic element in concentration sufficient to
promote ferromagnetism. However, magnetic ordering may also appear due
to the inclusion of undesirable metal-clusters or secondary phases and their
complexes, necessitating careful correlation between structural, electronic,
and magnetic properties of DMS systems.
ZnO is a promising DMS host material with the potential to show room
temperature ferromagnetism under appropriate doping. Transition metal
doped ZnO has received considerable attention, with mixed results regarding
the resulting magnetic state. Rare-earth (RE) elements have large magnetic
moments and can, in principle, be doped into ZnO to form DMSs. Thus far
there has been only limited investigation of such systems, and no consistent
picture is available of the magnetic interactions, the electrical transport, or
their dependence on the microstructure of the RE-doped ZnO.
This thesis presents an experimental study of rare-earth doped ZnO prepared
by ion implantation into ZnO single crystals and thin films, with
the investigation covering the structural, electronic, and magnetic properties.
These properties were investigated by systematically varying the rareearth
concentration implanted into the ZnO, and by studying the effect of
post-implantation annealing conditions. The primary focus was on ZnO:Gd,
which was supported by work on ZnO:Er and ZnO:Tb. Compositional and
structural characterisation was performed using a combination of Rutherford
backscattering spectrometry and channeling, Raman spectroscopy, XRD, and
TEM. The results show that in as-implanted samples the majority of RE
atoms occupy substitutional lattice sites. Annealing of the samples heals the
implantation-induced disorder in the ZnO lattice, but it prompts a significant
fraction of the RE atoms to be driven away from the lattice sites resulting in
an inhomogeneous RE distribution with evidence for RE-rich nanoclusters.
The electronic transport properties are consistent with heavily doped
semiconductor behaviour, although the temperature dependent resistivity
and carrier concentration show anomalous behaviour at low temperatures.
Ferromagnetic ordering is observed to occur in many of the samples, being
especially strong in samples annealed at temperatures around 650 °C. Surprisingly,
this ferromagnetic ordering also exists in unimplanted but annealed
ZnO samples, indicating that its origin is a magnetic defect other than the implanted
RE. However, the ferromagnetism is actually suppressed in ZnO:Gd
samples with a high concentration of Gd, indicating that the RE elements
do play a role in the magnetism. Possible mechanisms for the ferromagnetic
ordering are discussed. The transport data indicates a rather strong coupling
between the electrical conduction and the magnetism.