IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Thin Films Monday Sessions
       Session TF+MM-MoM

Paper TF+MM-MoM6
Charge Transport Mechanisms in Epitaxial Tungsten Oxide Films Used for Chemiresistive Sensors

Monday, October 29, 2001, 11:20 am, Room 123

Session: Thin Film Sensors
Presenter: S.C. Moulzolf, University of Maine
Authors: S.C. Moulzolf, University of Maine
R.J. Lad, University of Maine
Correspondent: Click to Email
Chemiresistive gas sensors fabricated from ultra-thin WO@sub 3@ films containing surface catalysts can be made highly sensitive towards a variety of target gases via manipulation of oxide surface chemistry. However, other important sensor characteristics including baseline stability, response time, and reproducibility are strongly dependent on the specific film microstructure and charge transport within the film. Using in situ Hall effect measurements coupled with structural analysis and gas testing experiments, we have determined a correlation between film deposition parameters, microstructure, and electrical response. WO@sub 3@ films were grown by rf magnetron sputtering on sapphire substrates to produce either epitaxial tetragonal or epitaxial monoclinic phases as deduced by RHEED and XRD. Exact film stoichiometries were controlled via post-deposition annealing treatments in vacuum and/or synthetic air environments. Four-point van der Pauw conductivity and Hall effect measurements as a function of temperature indicate that charge mobility is very small (<2cm@super2@V@super -1@s@super -1@) and that polaron hopping is the dominant conduction mechanism. The conductivity of the monoclinic phase is an order of magnitude larger than the tetragonal phase and exhibits temperature dependence similar to measurements from single crystal WO@sub 3@. The mobility of the tetragonal phase increases with temperature consistent with scattering from the increased number of grain boundaries and smaller grain size as observed by STM and XRD. Extended annealing in vacuum to reduce the oxide stoichiometry causes higher conductivity and temperature dependent mobility behavior that may be attributed to crystallographic shear plane defects in the WO@sub 3-x@ lattice. Upon gas exposure to H@sub 2@S or methanol, the tetragonal phase shows higher sensitivity compared to the monoclinic phase but a slower response which correlates with the lower Hall mobility.