Spatially resolved gas sensing

Sammanfattning: By using a Scanning Light Pulse Technique (SLPT) the gas response of catalytic Metal-Insulator semiconductor (MIS) devices can be spatially resolved.The application of SLPT to gas sensitive MIS devices can be divided into two areas. One is to produce response maps of large area MIS devices made with a spatial sensitivity and selectivity variation of its sensing layer. The response map can be regarded as a chemical fingerprint or artificial "olfactory" image. One such artificial olfactory image can contain several hundred "pixel" which corresponds to discrete sensors in sensor array.The other area of use for the SLPT is in fundamental investigations of the response mechanism of field-effect chemical sensors to which the MIS device belongs and development of new sensing layers.A major part of the work presented in this thesis belongs to this second area and several phenomena of interest for the understanding and use of gas sensitive field-effect devices have been documented. It has thus been possible to:Carefully monitor the way hydrogen consumption of along a palladium surface affects the response. The results are of great importance for the study of large area MIS devices with catalytic metal gates.Investigate the dependence of the response to ammonia, hydrogen, ethanol, ethylene and acetaldehyde on the morphology of thin layers of palladium, platinum and iridium produced in the form of metal stripes with a gradient in thickness.Obtain detailed information about the response of palladium and platinum to hydrogen, which has led to detailed models for these responses.Demonstrated the "locality" and "non-locality" of catalytic reactions on platinum in oxygen containing and inert atmospheres, respectively.Obtained an increased knowledge about the ammonia response mechanism of platinum field-effect devices.Constructed artificial "olfactory" images of complicated gas mixtures such as the emission from microorganisms in ageing meat.The SLPT is based on the fact that a current transient can be generated in a MIS structure by illuminating the semiconductor with light, which has a photon energy larger than the band gapof the semiconductor. The incoming light generates charge carriers that can be separated by the electric field in the depletion layer. Part of the separated carriers result in a current transient in the external circuit. The charge content of the current transient depends (among other things) on the work function of the metal gate. If the light is focused down to a small spot, it is the local value of the work function that determines the charge content. The response of the device is due to adsorbed and polarized (charged) species at the metal-insulator interface or on the bare insulator in the cracks, which affect the metal work function. By scanning a pulsed and well-focused light beam over the surface a map of the response of the device can be obtained.