New Materials for Gas Sensitive Field-Effect Device Studies

Sammanfattning: Gas sensor control is potentially one of the most important techniques of tomorrow for the environment. All over the world cars are preferred for transportation, and accordingly the number of cars increases, unfortunately, together with pollutants. Boilers and powerplants are other sources of pollutants to the environment. Metal-Insulator-Silicon Carbide (MISiC) Field-effect sensors in car applications and boilers have the potential to reduce the amount of pollutants. These devices are sensitive to several gases in exhaust and flues gases, such as hydrogen, hydrocarbons, and ammonia (for the selective catalytic reduction (SCR) application). These applications require specific and long term stable sensors. The car industry for instance wants sensors that will stand at least 240 000 km.This thesis presents studies of the active layers in MIS Fieldeffect gas sensors. Fundamental studies of the sensor mechanism has been performed in ultra high vacuum, UHV, to understand the gas response mechanism in more detail, and to find out how the sensing mechanism is affected by the catalytic active gate material. The influence of four different insulating layers was studied at atmospheric pressure. The catalytic layer has also been altered to metal oxide nanoparticles with or without impregnation of catalytic metals.Nanoparticles are potential candidates to be used as the gate material for high temperature, long-term stable FET sensor devices. The combination of catalytic metals and metal oxides may prevent reconstruction of the metal. The use of nanoparticles will increase the number of triple points (catalytic material and insulator in contact with gas), which are crucial e.g. for the ammonia sensitivity. Another challenging aspect of nanoparticles is the possibility to get selectivity to different gases based on the particle size.The goal is to find new sensitive, selective and more long term stable materials, which meet the requirements above.From the UHV studies we learned that the two catalytic active metals Pt and Pd, do behave in a similar way, although there are some quantitative differences. Values for the heat of adsorption on both the Pd and Pt surfaces are estimated as well as the dipole moments for the adsorbates on the insulator surface.The insulators play an important role in the sensing mechanism, since the adsorption of hydrogen atoms (or protons) that are detected by the sensor occur on the insulator surface. By changing the insulator material the saturation response of the sensors is affected. It was shown that Al2O3 gave a higher saturated response to hydrogen in Pt-MIS capacitors at 140°C as compared to Ta2O5, SiO2, and Si3N4.We have tested wet synthesized ruthenium dioxide and ruthenium nanoparticles, which are electrically conducting and catalytically active sensing material. RuO2 is especially interesting as a high temperature material since it is already oxidized. Both materials show a sensitivity pattern comparable to porous platinum. The temperature dependence of the gas response indicates a higher catalytic activity of the RuO2 as compared to Ru nanoparticles.Nanoparticles synthesized by aerosol technology provide several advantages like a good adhesion of the particles to the substrate, many possible material combinations and efficient methods for particle separation according to size. The methods to use this technology for sensing materials in MISiC sensors are now under development and some preliminary results are obtained.