ZnO nanostructures have received great attention during the past decade due to numerous potential applications. In order to enable the fabrication of functional devices, reproducible preparation of such nanostructures is necessary. Therefore, a good understanding of the fundamental processes involved in the formation of ZnO nanostructures is indispensable for improving the controllability of nanostructure growth. This work elucidates various aspects of the essential nucleation and growth mechanisms at work during the growth of ZnO nanostructures by catalyst-assisted pulsed laser deposition (PLD).
ZnO nanowires and triangular nanosheets have been grown on sapphire substrates by Auassisted PLD. In a first study, the influence of thermal substrate pretreatments on the size and density of the ZnO nanostructures is investigated. It has been found that the presence of surface nucleation sites can compete with nucleation at the Au catalyst and lead to reduced nanostructure sizes and densities. Furthermore, it has been observed that the ZnO morphology switches from nanowires to triangular nanosheets upon increasing oxygen partial pressure in the growth chamber. Electron microscopy results indicates that the catalyst-nanowire growth interface plays an important role in this morphology change. Formation mechanisms of the two different nanostructure types are presented and possible links between oxygen pressure and morphology via growth kinetics and supersaturation considerations are discussed. Additionally, the epitaxial relationships between the two ZnO nanostructure types and sapphire substrates have been investigated in detail by combining x-ray pole figure measurements with both transmission and scanning electron microscope observations. ZnO nanowires growing tilted on c-plane sapphire showed an epitaxial alignment with a buried and inclined substrate plane. Two degenerate configurations have been identified for these tilted wires, promoted by equally low lattice mismatches. On a-plane sapphire, ZnO nanosheets and -wires show distinct differences in the epitaxial relationships with the substrate, indicating a direct correlation to the morphology.
The findings about ZnO nanostructures presented in this thesis help to improve control over catalyst-assisted nanostructure growth techniques and provide a further step towards reproducible nanostructure fabrication.
In a second part of the thesis, the electrical and optical properties of Al-doped ZnO (AZO) thin films grown on GaAs substrates by PLD have been investigated. AZO is a promising candidate for substituting indium tin oxide as transparent electrodes in optoelectronic applications. The aim of the study is to assess the suitability of AZO as a transparent electrode shell around GaAs nanowire solar cells for direct charge carrier pathways. Furthermore, the portability of previously reported results obtained on transparent substrates such as glass or sapphire to the opaque GaAs substrate is discussed.