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Title: Silicon carbide based gas sensor arrays for deployment within hostile environments
Author: Furnival, Benjamin John Dylan
Awarding Body: University of Newcastle Upon Tyne
Current Institution: University of Newcastle upon Tyne
Date of Award: 2012
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The ability to determine the gaseous constituents of hazardous environments is of interest for a wide variety of applications including; emission control, nuclear waste storage and space exploration. Traditional Si-based technology has poor resilience and is unable to operate at temperatures much above 170°C. A viable alternative is SiC which has a wide band-gap and Si to C bonding, enabling it to operate at extreme temperatures, in high radiation flux and in corrosive ambients. SiC-based MIS capacitors have previously been shown to be sensitive to a range of gas species including; H2, O2 and H2S. However, these structures often reveal cross-sensitivity to multiple gas species and in order to enhance selectivity towards a target gas species or enable simultaneously monitoring of multiple gas species, a sensor array is required. The development of Ohmic contacts on 4H-SiC is crucial for a wide range of device structures. However, after high temperature post-processing, Ohmic behaviour has been found to deteriorate. This study reveals that careful replacement of the contact metallization can significantly recover the Ohmic behaviour, an enabling technology for the resilient sensor array. After optimization of a Ti02/Si02 high-x dielectric stack, a variety of SiC-based MIS capacitors were fabricated using combinations of Ni, Pd or Pt . gate contacts and Ti02 or Hf02 high-x dielectric layers. The C-V characteristics of these devices were investigated from 10KHz to 3 MHz and then at intervals during exposure to 400°C and 500 °C in air, showing reliable operation for at least 1000 hours and failure shortly after 143 hours respectively. The mechanisms responsible for this degradation were studied with SIMS and AFM analysis, which revealed that Ti and Cr have diffused into the Hf02 during annealing, possibly introducing additional defects. Using a combination of these capacitors a resilient SiC-based sensor array has been established. The C-V and I-V characteristics from this array have been examined in N2 at between 50°C and 400°C, prior to its C-V response being recorded at 300°C in a variety of H2, O2 and H2/02 ambients. The selectivity of a MIS capacitor between H2 and O2 has been found to be strongly dependant on contact material, whilst choice in high-x dielectric has a substantial influence on H2 sensitivity. The use of MLR analysis on the resulting data is found to enable the array to discriminate between H2/02 mixed gas ambients and make reasonable predictions of their concentrations, to generally within= 600 ppm for H2 and > 200 ppm for O2.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available