Sol-gel Processing and Characterization of Alkaline Earth and Rare-earth Fluoride Thin Films
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Sol-gel processing and characterization of alkaline earth and rare-earth fluoride thin films Munehiro Tada, Shinobu Fujihara,a) and Toshio Kimura Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1, Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan (Received 19 February 1998; accepted 19 October 1998)
Alkaline earth and rare-earth fluoride thin films were prepared on silica glass substrates by a sol-gel process using trifluoroacetic acid (TFA) as a fluorine source. Homogeneous solutions were obtained by stirring a mixture of alkaline earth or rare-earth metal acetates, TFA and H2 O, dissolved in isopropanol. The solutions were spin-coated and heated at 300–800 ±C. The fluoride thin films were obtained by heat treatment around 400 ±C in air. The crystallization behavior, the surface morphology, and the optical properties of the films depended on the heating temperature as well as the chemical species of the metal ions.
I. INTRODUCTION
Alkaline earth and rare-earth fluoride thin films are attractive materials in the field of optics. For example, MgF2 films1 have been widely applied as antireflective coatings. As the fluorides promote weak ionic bonding, the phonon energy of the fluorides are lower than that of the oxides, leading to increased infrared transmittance. When they are used as a matrix, their low phonon energy enables the radiative transition of rare-earth ions or luminescent materials with high efficiency. CaF2 2 and LaF3 3,4 have been examined as a matrix for the luminescent ions such as Nd31 and Pr31 . In addition, the fluoride materials have low refractive indices and optical dispersions at the surface. Therefore, the fluorides serve integrated photonic applications as low phonon energy host materials.5 The deposition of fluoride thin films has been performed via evaporation techniques in UHV.2–4 Some chemical solution techniques have also been applied to the preparation of MgF2 thin films.6–9 We have attempted to improve the latter by using the sol-gel process,10 which is recognized as a method for producing highpurity materials such as silicates and other oxides.11 One of the advantages of the sol-gel method is that the experimental setup is relatively inexpensive because no high-vacuum system is necessary. The sol-gel method also offers the possibilities for an improved compositional control and homogeneity. Although the application of the sol-gel process to the fluoride films is very attractive, it has been difficult because postfluorination of the oxide precursors by HF or F2 gas is always needed.12 Furthermore, O–H and C–H groups remaining in the sol-gel-derived films tend to degrade the optical properties as compared to the fluorides prepared by other techniques.13
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J. Mater. Res., Vol. 14, No. 4, Apr 1999
The improved sol-gel process enabled us to prepare MgF2 and LaF3 thin films10,14 directly on silica glass substrates by using trifluoroacetic acid (TFA; CF3
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