Materials design and characterization of the active regions of gas sensitive field-effect devices

University dissertation from Linköping : Linköpings universitet

Abstract: Metal-Insulator-Semiconductor (MIS) field-effect devices with catalytic metal layers are sensitive to several important gases, such as hydrogen, ammonia, hydrocarbons and alcohols. The gas sensitivity and selectivity depends on the properties of the active regions of the device. These active regions are primarily located at the metal surface and at the metal-insulator interface. For example, the hydrogen response occurs in three steps; (i) dissociation to form adsorbed hydrogen atoms on the catalytic metal surface, (ii) transport of the hydrogen atoms through the metal film and (iii) adsorption of hydrogen, forming a polarised layer at the interface giving rise to the response. In this thesis a characterisation of the active regions is presented together with studies on how modifications of the active layers affect the gas response.Pt films were grown, both by sputtering and evaporation, on Si02 at different conditions. A variation with a factor of ten of the maximum observed hydrogen responses for these devices is obtained. This large spread correlates with a surprisingly large variation of the metal morphology at the Pt-SiO2 interface. It is concluded that an increased coverage of Pt on the SiO2 surface reduces the hydrogen response. The corresponding increase in the Pt-SiO2 bond density, however, increases the effective adhesion of the metal layer in a sensor device.Another finding is that the metal layer can be divided into two layers with completely different properties. One layer, in contact with the insulator, can be used to optimise the metal-insulator interface. This layer can be catalytically inactive. The catalytic activity is provided by the second layer, forming the interface to the gas phase. This is illustrated with a Pt/TiN/SiO2/Si structure.The ammonia detection mechanism has been discussed for several years. In the thesis evidence for dissociation of ammonia molecules and detection of atomic hydrogen is presented. To be able to show this, it was necessary to produce Pt/SiO2/Si devices where the Pt films were completely free from pores down to the SiO2 surface. This was achieved by sputtering of Pt in Ar and in Ar + 02 and by co-deposition of Pt and SiO2. The co-deposited films resulted in an increased ammonia sensitivity compared to the pure Pt film, indicating a promoting effect by oxygen bound in the Pt film.The selectivity of MIS field-effect devices can be attributed to the catalytic activity on the metal surface and it is found that by modifying the surface, the selectivity will be influenced. The Pt surface was in this thesis modified by the deposition of different oxides (SiO2, SnO2 and AhO3), either on top or co-deposited with Pt. It was observed that the selectivity could be directed towards ammonia with proper choice of oxide material and thickness.

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