Electrical properties and microstructure of tin dioxide thin films deposited by pulsed laser ablation for gas sensing applications

Despite the widespread use of solid state semiconductor oxide gas sensors in many applications, the basic principles of operation and fabrication are still not well understood. Thin film sensors hold many promises due to advances in thin film fabrication and integration to existing silicon microtechnology. This thesis focused on understanding structure-property relationships of thin SnO₂ films with customized microstructures and chemistries to improve fundamental sensor science.; We used surface templates and femtosecond pulsed laser ablation to grow SnO₂ with tailored microstructures and thickness. This system allowed the growth of high quality epitaxial single crystals, nanodomain epitaxial crystals and randomly oriented polycrystals. The electrical properties were very dependent on the film thickness due to interaction of charge carriers with the surface/interface and to a greater extent of film depleted of electrons from surface charge. Electron concentration was reduced by trapping at grain boundaries and planar defects. Grain boundary scattering and potential reduced the electron mobility in thin films. The gas sensing properties of thin films was improved as the film thickness decreased. Active grain boundaries in polycrystalline films resulted in a significant improvement of sensing properties over single crystal films.; We also investigated the properties of donor and acceptor doped single crystal and polycrystalline materials. Donor dopants were mainly electronically compensated and resulted in near degenerate electron conduction in the films. Acceptor dopants were ionically compensated and decreased the electron concentration in the films. A trend of decreasing electron concentration with increasing acceptor dopant radius was found. Acceptor dopants in nanocrystalline films tended to show non-uniform distribution near the grain boundaries. The amount of segregation was dependent on ionic radius, grain size and native ionic space charge in the films. The sensing properties of the films were improved by the introduction of acceptor dopants whereas for donor dopants the response to gases was decreased with respect to undoped film. The sensitivity was directly proportional to the resistance with different maxima at different film thickness. Some dopants affected not only the electronic but also the chemical properties of the material.; Epitaxial SnO₂ and TiO₂ heterolayers were studied as gas sensors. It was possible to grow high quality multilayers via pulsed laser deposition. The electronic properties of the resulting film could be tailored by changing the thickness and number of intercalating layers. The interfacial charge from misfit dislocations and bandgap misfit between the layers created n/N junctions that modulated the electron concentration. Non-linear I-V characteristics were measured across an epitaxial SnO₂/TiO ₂ bilayer. The heterostructures showed weak response to gases, although provide proof of principle of their potential usefulness.