Nanostructured materials for gas sensing applications

Abstract: In this Thesis I have investigated the use of nanostructured films as sensing, and contact layers, on field effect gas sensors in order to achieve higher sensitivity, selectivity, and long term stability of the devices in corrosive environments at elevated temperatures. Electrochemically synthesized Pd and Au nanoparticles deposited as sensing layers on capacitive field effect devices were found to give a significant response to NOx with small, or no responses towards H2, NH3, and C3H6. Pt nanoparticles incorporated in a TiC matrix are catalytically active, but the agglomoration and migration of the Pt particles towards the substrate surface reduces the activity of the sensing layer. Magnetron sputtered epitaxial films from the Ti-Si-C and the Ti-Ge-C were grown on 4H-SiC substrates in order to explore them as high temperature stable ohmic contact materials to SiC based field effect gas sensors. Ti3SiC2 thin films deposited on 4H-SiC substrates were found to yield ohmic contacts to n-type SiC after a high temperature rapid thermal anneal at 950 ºC. Investigations on the growth mode of Ti3SiC2 thin films with varying Si-content deposited on 4H-SiC substrates, showed the growth to be lateral step-flow with the propagation of steps with a height as small as half a unit cell. The amount of Si present during deposition leads to differences in surface faceting of the films and Si-supersaturation conditions gives growth of Ti3SiC2 films with the presence of TiSi2 crystallites. Current-Voltage measurements of the as-deposited Ti3GeC2 films indicate that this material is also a promising candidate for achieving long term stable contact layers to 4H-SiC for operation at elevated temperatures in corrosive environments. Further investigations into the Ti-Ge-C system showed that the previously unreported solid solutions of (Ti,V)2GeC, (Ti,V)3GeC2 and (Ti,V)4GeC3 can be synthesized, and it was found that the growth of these films is affected by the nature of the substrate.