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We have used reactive rf magnetron sputtering of a tungsten target in Ar/O2 mixtures and direct electron-beam evaporation of WO3 pellets with and without the presence of an electron cyclotron resonance (ECR) oxygen plasma to grow WO3 films on α-Al2O3(101̄2) single-crystal substrates (r-cut sapphire). The WO3 films exhibit a range of microstructures depending on deposition conditions. Using any of the deposition methods, the films are amorphous when grown at room temperature. Postdeposition annealing in O2 induces the formation of a random polycrystalline microstructure and an increased surface roughness. Growth of crystalline WO3 films can be achieved at deposition temperatures above 200 °C. During electron-beam evaporation of WO3 at 600 °C, reflection high-energy electron diffraction observations indicate that a tetragonal phase of WO3 grows epitaxially on r-cut sapphire with the (100) tetragonal plane coincident with the rectangular mesh of the r-cut sapphire substrate. Deposition of WO3 using plasma species generated by either a rf magnetron or ECR source promotes the formation of some nuclei of an orthorhombic and/or monoclinic phase coexisting with the tetragonal phase; the nearly identical lattice parameters make the orthorhombic and monoclinic phases difficult to distinguish. Compared to the polycrystalline films, the epitaxial films exhibit both a higher base-line conductivity and a larger conductivity change upon exposure to H2S gas.

Chemiresistive metal oxide gas sensors based on materials including SnO 2 , ZnO, TiO 2 , and WO 3 have been investigated extensively for a wide range of applications. The band bending model, based on the surface chemistry of highly reactive ionosorbed species (O 2 − or O − ) and the semiconducting material properties of SnO 2 , TiO 2 , and ZnO, adequately predicts the dependence of steady state response on target gas pressure and temperature for these materials. However, the assumptions associated with the band bending model are not valid for sensors based on reducible oxides such as WO 3 , MoO 3 , and V 2 O 5 , in which lattice oxygen reacts with adsorbed target gases creating oxygen vacancies which diffuse rapidly into the bulk and modulate the conductivity. Here, we develop a model that includes surface reactions and vacancy diffusion for the bulk conduction mechanism and describe characteristics of the sensor behavior that allow bulk conduction to be distinguished from the band bending transduction mechanism. We illustrate the predictions of the model regarding how the change in conductivity, Δσ, and response time, τ, depend on target gas pressure, temperature, and film thickness using well-characterized WO 3 sensors, including epitaxially oriented polycrystalline thin films and nanorod structured glancing angle deposition (GLAD) films. The physical/ chemical parameters of the model are determined in independent measurements. Expressions for the limiting cases in which τ is determined either by surface reactions or by bulk diffusion provide design criteria to predict the theoretical performance limits of sensors which operate via this transduction mechanism.

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