Heteroepitaxial growth of tungsten oxide films on silicon(100) using a BaF 2 buffer layer
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F. Santiago and S.L. Moran Naval Surface Warfare Center, Dahlgren Division, Dahlgren, Virginia 22448
R.J. Lad Laboratory for Surface Science and Technology, University of Maine, Orono, Maine 04469 (Received 3 June 2003; accepted 22 September 2003)
Multidomained heteroepitaxial WO3 films were grown on Si(100) substrates using a (111)-oriented BaF2 buffer layer at the WO3–Si interface. The 30-nm-thick BaF2 layer, grown by very low rate molecular-beam epitaxy, consisted of four equivalent crystalline domains oriented about the BaF2[111] axis, which provided templates for heteroepitaxial WO3 film growth. The WO3 films were grown by electron cyclotron resonance oxygen plasma-assisted electron beam evaporation of a WO3 source, and the temperature range was varied between 25 °C and 600 °C. At an optimal deposition temperature of approximately 450 °C, monoclinic-phase WO3 films were produced, which consisted of coexisting (002), (020), and (200) in-plane orientations with respect to the BaF2(111)/Si(100) substrate. During growth, an interfacial barium tungstate (BaWO4) reaction product formed at the WO3–BaF2 interface. The {112} planes of this BaWO4 layer also have a multidomained heteroepitaxial orientation with respect to the BaF2(111) buffer layer. Postdeposition annealing experiments in air for 24 h at 400 °C indicated that the heteroepitaxial BaWO4 and WO3 layers remain stable. A thermodynamic argument is used to explain the BaWO4 interfacial reaction during initial growth stages, and kinetically limited diffusion processes through the BaWO4 layer coupled with lattice matching across the WO3–BaWO4 interface are proposed to be responsible for the formation of stable WO3 films at later growth stages.
I. INTRODUCTION
Tungsten trioxide (WO3) thin films have been used in a wide variety of applications that take advantage of their optical,1–3 tribological,4,5 and electronic6–13 behavior. As early as 1973, the optical phenomenon known as electrochromism was discovered in WO3 by Deb.14 Although electrochromic properties have since been found in several other transition metal oxide materials (e.g., MoO3, TiO2),3 WO3 remains one of the most viable material options for electrochromic devices to date. In addition, WO3 has generated interest in tribological applications as a solid lubricant for high-temperature, oxidizing environments. Specifically, WO3 coatings have been reported to maintain low friction and low wear at temperatures up to 800 °C.4
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 12, Dec 2003
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Over the past decade, the electronic properties of WO3 thin films have also been studied in the context of semiconducting metal oxide (SMO) chemiresistive gas-sensing devices.6–13 In SMO sensors, the primary gas-sensing mechanism arises from oxidation/ reduction surface reactions occurring between the metal oxide film and a reactive gas. Such gas/film interactions can induce a reversible change in film
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