Structural, Electronic and Magnetic Properties of Rare-Earth Doped Zinc Oxide

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.