Tsang, Jone-him; (2023) Chemical Vapour Deposition of p-type First Row Transition Metal Oxides for Use in Sensing Volatile Organic Carbon Species. Doctoral thesis (Eng.D), UCL (University College London).

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Abstract

There has been an increased attention towards gas detection due to levels of toxic and hazardous gases to which humans are exposed to everyday. Semiconductor gas sensors, mainly metal oxide sensors have good sensitivity due to difference in conductivity even at low concentrations. Metal oxide are desirable materials for sensors due to their low cost, robust structure, relatively low energy consumption and good response to a variety of gases and vapours. Several metal oxides have been used for gas sensor devices, including but not limited to SnO2, WO3 and TiO2. Since the discovery for its use as gas sensors, SnO2, a n-type material is one of the most widely used gas sensor material. However, there are drawbacks such as poor baseline stability, interference from humid conditions and low selectivity.1,2 Recently Cr2-xTixO3, a n-type doped p-type material has improved properties com- pared to SnO2, a pure n-type MOS. It displays better stability towards relative humidity, a low baseline drift, higher selectivity and better stability.3 Since its discovery as a gas sensing material by Moseley and Williams, it has been widely used in research and com- mercially with the ability to detect various gases and vapours such as NH3, CO, H2S and different alcohols.4,5 Ceramic substrate are still commonly used as the based for MOS sensors due to its low cost, ease of fabrication, chemical stability and robustness of the substrate (sensor can be made via screen-printing). However, there are certain limitation, which is why Micro-electro-mechanical system (MEMS) technologys have been extensively researched upon over recent years. Some of the reasons for such drive is because MEMS miniaturised ii heating element (micro-hotplates) can operate at high temperatures quickly and preserve power consumption (mW ranges) due to its low thermal mass.6 However, due the MEMS substrate brittleness, certain synthetic techniques are not viable such as screen printing. Therefore, one aspect of the project is to directly and successfully deposit CTO sensing ma- terial onto the MEMS substrate via AACVD. Once deposited, its sensitivity and selectivity towards VOCs will be investigated and compared to ceramic screen printed sensors. This project is carried out in collaboration with Alphasense Ltd (http://www.alphasense.com/), a leading UK gas sensor manufacturer. The mechanism of gas sensing has long been debated and described. Here two models are described, band bending model and surface trap limited model. The latter was used to explain the mechanism and interaction of the sensors tested in this project. Another aspect of this project is to find optimum parameters to generate n-type doped CWO and p-type doped CNO thin films via AACVD. Detailed analysis (elemental compo- sition, oxidation state, structural information and morphology) of the deposited thin films were studied using XRD, EDX and XPS. These materials sensing abilities were tested towards various different VOCs. This project was carried out in collaboration with Alphasense Ltd, a leading UK gas sensor company. The project was divided into the following main sections: • Synthesis of CTO thin films onto glass, alumina and gas sensing substrates • Characterisation of CTO thin films • Synthesis of CTO on MEMS platforms • Gas sensor tests at Alphasense

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