In this thesis, field effect gas sensors (Schottky diodes, MOS capacitors,and MOSFET transistors) based on wide band gap semiconductors like siliconcarbide (SiC) and gallium nitride (GaN), as well as resistive gas sensorsbased on indium oxide (In2O3), have been developed for the detection ofreducing gases (H2, D2) and oxidising gases (NOx, O2). The development ofthe sensors has been performed at the Institute for Micro- andNanoelectronic, Technical University Ilmenau in co-operation with (GE)General Electric Global Research (USA) and Umwelt-Sensor-Technik GmbH(Geschwenda). Chapter 1: serves as an introduction into the scientificfields related to this work. The theoretical fundamentals of solid-stategas sensors are provided and the relevant properties of wide band gapmaterials (SiC and GaN) are summarized. In chapter 2: The performanceof Pt/GaN Schottky diodes with different thickness of the catalytic metalwere investigated as hydrogen gas detectors. The area as well as thethickness of the Pt were varied between 250 × 250 µm2 and 1000 × 1000 µm2,8 and 40 nm, respectively. The response to hydrogen gas was investigated independence on the active area, the Pt thickness and the operatingtemperature for 1 vol.% hydrogen in synthetic air. We observed asignificant increase of the sensitivity and a decrease of the response andrecovery times by increasing the temperature of operation to about 350°Cand by decreasing the Pt thickness down to 8 nm. Electron microscopy of themicrostructure showed that the thinner platinum had a higher grain boundarydensity. The increase in sensitivity with decreasing Pt thickness points tothe dissociation of molecular hydrogen on the surface, the diffusion ofatomic hydrogen along the platinum grain boundaries and the adsorption ofhydrogen at the Pt/GaN interface as a possible mechanism of sensinghydrogen by Schottky diodes. The response to deuterium D2, NOx, and O2of metal-oxide-semiconductor (MOS) and metal-metaloxide-oxide-semiconductor (MMOOS) structures with rhodium (Rh) gate wereinvestigated in dependence on the operating temperature and gas partialpressures was investigated in chapter 3. The response of the sensor wasmeasured as a shift in the capacitance-voltge (C-V) curve along the voltageaxis. Positive and negative flat-band voltage shifts up to 1 V wereobserved for oxidizing and reducing gases, respectively. Depending on thetype of insulator that is chosen, differences in the sensitivity of thesensor were observed. In chapter 4: The performance of SiC-based fieldeffect transistors (FETs) with different gate materials (mixture of metaloxides: indium oxide and tin oxide (InxSnyOz), indium oxide and vanadiumoxide (InxVyOz), as well as mixtures of metal oxides with metal additives)were investigated as NOx, O2, and D2 gas detectors. The response to thesegases was investigated in dependence on the operating temperature and gaspartial pressures. The composition and microstructure of the sensing gateelectrode are the key parameters that influence the sensing mechanism, andhence key performance parameters: sensitivity, selectivity, and responsetime. By choosing the appropriate temperature and catalyst material (gatematerial), devices that are significantly sensitive to certain gases may berealized. In addition, the temperature of maximum response varies dependenton the gas species being measured. This information, along with a carefulchoice of catalyst (gate material) can be used to enhance deviceselectivity. In chapter 5: Polycrystalline and nano-structured In2O3thin films were investigated with the aim to obtain information about theirNOx and O2 gas sensing properties. The response to these gases wasinvestigated in dependence on the operating temperature and gas partialpressures. The analysis in the presence of different partial pressures ofNOx has shown that both thin films are able to detect nitrogen oxide, buttheir responses exhibit different characteristics. In particular,nano-structured In2O3 thin films were found to have the higher response toNOx. This is most probably due to the enlarged overall active surface areaof the sensing layer as a consequence of the small grain size (highersurface to volume ratio) so that the relative interactive surface area islarger, and the density of charged carriers per volume is higher. We havefound that reducing the grain size of the sensing material to the ~10 nmregime can have a substantial effect on performance. The optimum detectiontemperatures of the nano-structured In2O3 occur in the range of 100-175°Cfor NOx considering the sensitivity as well as the response time. In thisrange of temperatures the response to O2 is very low indicating that thesensor is very suitable for selective detection of NOx at low temperaturesIn addition, nano-structured In2O3 thin films were found to be moresuitable to be used in the field of application for detecting low partialpressures. Chapter 6: offers conclusions of the current work. In thischapter we compare also all studied gas sensors according to theirsensitivity, selectivity, and response time and then we compare them withthe related works by other authors available in the scientific literature.
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