RF plasma synthesis and characterization of thin films for transparent conductors

Oxide-based transparent conductors constitute a novel class of materials, which finds applications in many technological fields such as photovoltaics and organic light emitting devices. They can be employed in the new generation solar cells as transparent charge collectors. The transparent and conductive oxide mostly used nowadays is indium tin oxide (ITO), however due to the high cost and scarcity of indium, other materials are under research and development as potential substitutes. Many candidates are currently under study, mainly doped-ZnO, doped-CdO, doped-SnO2, doped-TiO2. The work undertaken in this thesis is a study of the doping processes of thin films of TiO2 and ZnO, two cheap, chemically stable and non-toxic materials. Two main objectives were pursued in this work: (i) the optimization of the film deposition and doping conditions for a potential replacement of ITO and (ii) the understanding of the factors dominating the doping process as well as its limitations. The approach was to explore three doping methods of the films: intrinsic doping, extrinsic doping and, with the aim to combine the benefits of both, intrinsic-extrinsic co-doping. Since the structural defects (such as oxygen vacancies) are at the basis of the intrinsic doping, a control of their formation was searched through the variation of the growth process conditions of the ZnO and TiO2 films. Niobium was selected for the extrinsic doping of the TiO2 films. The films were grown by RF plasma sputtering in different atmospheres (Argon, Ar-O2 and Ar-H2 gas mixtures) and under different plasma power conditions and substrate temperature, onto silicon and quartz substrate. The Nb-containing films were obtained by co-sputtering of either a single composite TiO2 -Nb target or two distinct niobium and TiO2 targets. Many characterization techniques were applied to define the film structural, electronic, electrical and optical properties obtained upon doping. For chemical analysis, X-ray Photoelectron Spectroscopy (XPS) was used. The structure and morphology of the films were analyzed by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The chemical species present in various plasmas used in deposition process were investigated by Optical Emission Spectroscopy (OES). Further, the defect structure and properties of the obtained films were studied by Positron Annihilation Spectroscopy. Analysis by this technique shed more light on the nature of the vacancies/open volume and on the effect of the latter on the electrical and structural properties of the films. A study based on a joint use of XPS and optical measurements allowed to define the electronic properties of the films (valence band edge, Fermi level position, work function, ionization potential and electron affinity). Structural analysis results revealed the formation of both anatase and rutile nanocrystalline phases for intrinsic and extrinsic doping of TiO2, while with the co-doping method only anatase phase was obtained, a phase known to be favorable for Nb incorporation in TiO2 lattice. The intrinsic doping of TiO2 films showed high transparency in the visible range, but resulted in still high resistivity values (101-103 ï —xcm). The latter could be lowered by using Ar-H2 gas mixtures during film deposition. The same trend was observed in the case of intrinsically-doped ZnO films, an increase in the electrical conductivity was observed when the concentration of defects was increased. The lowest resistivity was achieved with niobium doping of TiO2, 5x10-3 ï —xcm, with an optical absorption coefficient in the visible range of ~1x104 cm-1, however the combination of the internal defects and Nb, in co-doping, did not improve the conductivity. Nonetheless, it was found that co-doping method strongly modified the electronic properties of the TiO2 films, allowing a control of the work function, an important parameter for transparent electrodes. Low cost transparent conductive oxides were obtained when niobium was successfully incorporated in TiO2 lattice. By optimization of the deposition process of the films (dopant concentration, RF power, atmosphere, and annealing temperature) the electronic, electrical and optical properties of doped- TiO2 films can be improved. The obtained results can contribute to the development of transparent electrodes and charge collectors by RF sputtering, a suitable technique for coating on large area substrates.