Nanostructured Columnlike Tungsten Oxide Film by Anodizing Al/W/Ti Layers on Si

An Al/W/Ti trilayer sample prepared by a consecutive sputter deposition of titanium, tungsten, and aluminum layers onto a silicon wafer is first anodized in an oxalic-acid electrolyte at 27 V to convert the Al layer into nanoporous aluminum oxide. Further reanodizing the sample to 100 V results in the simultaneous growth of a 40 nm tungsten oxide layer beneath the alumina pores and discrete, columnlike regions of tungsten oxide (40 mn wide, 170 nm long, 30 nm apart) penetrating the alumina pores. The penetration of the alumina pores is assisted by the substantially increased transport number for tungsten species (0.48). The columnlike film derived after selective dissolution of the alumina matrix is mainly composed of amorphous WO3 (with cation vacancies in the surface layer) and suboxides W2O5 and WO2. Besides, Al2O3 and Si-, Ti-, and C-containing species are identified as minor components in the film. Annealing the film at 700−800 °C promotes the formation of polycrystalline phases of monoclinic WO3 with Pc symmetry, monoclinic WO2.72 with P2/m symmetry, and tetragonal WSi2 with the body-centered lattice. The impact of these experimental findings on the detailed understanding of ionic transport processes determining the growth of the tungsten oxide nanostructures is considered.