Date of Award

2002

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Physics

Advisor

Robert J. Lad

Second Committee Member

Charles W. Smith

Third Committee Member

Brian G. Frederick

Abstract

The electronic and mechanical properties of tungsten trioxide (W03) thin films have been successfully utilized in both surface acoustic wave and chemiresistive microsensor devices for the detection of certain gas-phase species. A critical material aspect that can affect microsensor performance is the type and degree of crystallization within the sensing film. However, the inclusion of this added dimension into a thin film limits the number of viable substrates for which heteroepitaxy may be realized. For W03, this constraint has restricted its usage as a sensing material to being primarily coupled with sapphire substrates. Not only does sapphire promote structured film growth in this system, but it also provides the necessary electronic insulation for proper chemiresistive sensor operation. In this thesis study, we have demonstrated that multi-domained, heteroepitaxial tungsten trioxide films can alternatively be integrated onto silicon (100) oriented substrates by way of a high quality, (1 11) oriented barium fluoride insulator film buffer layer. Specifically, within an optimal deposition temperature range of 400°C - 500°C, monoclinic phase W03 films maintained coexisting (002), (020), and (200) orientations on BaF2(1 11)/Si(100) substrates. The W03 films were grown by electron cyclotron resonance oxygen plasma-assisted electron beam evaporation of a W03 source. A thorough investigation of the BaF2/Si substrate revealed that the BaF2 layer consisted of multiple crystalline domains oriented about the BaF2 [ I l l ] axis, which produced similarly oriented domains within subsequent film layers. Further analysis indicated that an interfacial barium tungstate (BaW04) reaction product formed at the W03-BaF2 interface. Post-deposition air-anneal experiments indicated that the heteroepitaxial W03 films maintained their lattice structure when air-annealed for 24 hours at 400°C.

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