Processing-microstructure-property relationships of tin oxide thin films for gas sensor applications

Tin dioxide (SnO₂) with rutile type structure is a wide band n-type semiconductor which exhibits unique electronic and optical properties. In application of this material as gas sensors, a film form of SnO₂ provides high surface area to volume ratio and leads to high sensitivity and fast responses. It has been found that the substrate material, the deposition conditions and the annealing procedure may directly influence the microstructure of thin films, hence control gas sensing properties. This thesis describes a concerted effort to study the microstructure-property relationship in SnO ₂ thin film sensors. Our studies help to elucidate the effects of microstructure on sensor performance and provide some fundamental understanding of sensor design principals.; Thin films with different microstructures were obtained by using two deposition techniques, namely pulsed laser deposition (PLD) and electron-beam evaporation, and a variety of substrates, such as Al₂O₃(1¯012), Al₂O₃(112¯0) and Al₂O₃(0001). The obtained SnO₂ thin films include single crystalline films, compact epitaxial films with different grain boundary density, and porous films with rough surface. Property measurements reveal that single crystal has low gas sensitivity and the performance of compact films depends on the grain boundary density. On the other hand, porous films exhibit high sensitivity. Based on the experimental results, a model is proposed to interpret the observed phenomena in terms of depletion and grain boundary.; We also investigated the effects of film thickness and additives, both bulk and surface, on gas sensitivity. Among the three films examined with thickness of 20nm, 60nm and 100nm, the thinnest film showed better sensitivity than the thicker ones. Dopants influence the sensitivity through the modification of depletion region. Trivalent additives (acceptor type) result in increased depletion layer thickness, hence improve sensor performance. On the contrary, films doped by pentavalent additives (donor type) only have very low gas sensitivity due to the thin depletion layer in the films. We believe that the volume ratio of the depletion region over the total volume is a key factor in determining sensitivity. Addition of surface Pt, even with a thickness of 1nm, is an effective way for improving gas sensitivity.