Synthesis and characterization of TiO 2 thin films on organic self-assembled monolayers: Part I. Film formation from aqu
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R. J. Collins Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106
M. R. De Guire and A. H. Heuer Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106
C. N. Sukenik Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106 (Received 1 August 1994; accepted 17 November 1994)
Self-assembled monolayers (SAMs) bearing sulfonate (-SO3H) surface functional groups, on single-crystal Si wafers, were used as substrates for the deposition of TiO2 thin films from aqueous solutions. Polycrystalline TiO2 thin films over 50 nm thick formed in 2 h by hydrolysis of TiCU in aqueous HC1 solutions at 80 °C. The films were pore-free, showed excellent adherence and uniformity, and consisted of anatase crystallites 2 - 4 nm in diameter. Annealing at temperatures up to 600 °C caused coarsening of the anatase grains, but no loss of adherence or structural integrity.
I. INTRODUCTION Thin films of titanium dioxide (TiO2) have many useful electrical and optical properties. Rutile is a highpermittivity but anisotropic material, the relative dielectric constant sr being 170 parallel to the optic axis (i.e., parallel to [001] and 89 perpendicular to the optic axis (parallel to [100]).1 These values are relatively independent of frequency up to the gigahertz range.2^* Also, TiO2 shows a high refractive index (average indices of refraction at 550 nm are 2.57 for natural crystals of anatase and 2.74 for synthetic crystals of rutile) and affords excellent transmittance of visible light.5 These properties make it useful in antireflective coatings,6 as dielectric layers (with values of er = 82-86 having been achieved7), and for optical filters.8 It is important to note that for thin film applications, the film properties depend on phase composition, microstructure, crystallinity, and chemical composition. For example, in unoriented polycrystalline TiO2 thin films,8 the refractive index increases from 2.0 to 2.6 and the optical absorption decreases during the phase transition from anatase to rutile. Other electronic applications exist for TiO2 thin films, for example in oxygen sensors9 and electrochromic windows.10 TiO2 films have been proposed for use as dielectric layers11 within metalinsulator-semiconductor (MIS) electronic devices because of its high permittivity. 692
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J. Mater. Res., Vol. 10, No. 3, Mar 1995
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TiO2 films have been deposited on various substrates, including Si, by CVD,12"16 MOCVD, 1718 and sol-gel techniques.19 Generally, the form of TiO2 in the films deposited by CVD varies with substrate temperature: films are amphorous below 300 °C, have the anatase structure between 350 °C and 600 °C, and form rutile only at substrate temperatures above 600 °C. Thus, rather high annealing temperatures are required to form stable rutile films if sol-gel techniques are used for film formation. Because interfacial reactions between semiconductors and oxide films at high temperatures may adv
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