Hydrothermal Growth Kinetics of BaTiO 3 on TiO 2 Single Crystal Surfaces
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Hydrothermal Growth Kinetics of BaTiO3 on TiO2 Single Crystal Surfaces Víctor M. Fuenzalida and Judit G. Lisoni1 Universidad de Chile, Facultad de Ciencias Físicas y Matemáticas Santiago 6511226, CHILE 1 Present address: IMEC, SPT/ITS Division, Leuven, Belgium ABSTRACT TiO2 (rutile) single crystal plates with (001) and (110) orientation were immersed in an aqueous solution of Ba(OH)2 0.5 M at temperatures of 100 and 150 ºC for 1 and 4 hours in order to grow BaTiO3 on them. SEM micrographs of the samples fabricated on the (001) surface of rutile displayed isolated grains with an average height ranging from 200 nm at 100 ºC to 700 nm at 150 ºC. On the other hand, samples with the (001) orientation exhibited no growth at 100 ºC and only a few grains along lines attributed to the polishing process of the substrate at 150 ºC. The image of backscattered electrons indicated that barium is concentrated on the grains in all cases. Only the (001) samples exhibited reflections of cubic BaTiO3, as indicated by x-ray diffraction, as well as distinct Ba signals under x-ray photoelectron spectrometry. These results agree with the hypothesis of a dissolution-precipitation growth mechanism, in which dissolution is possible for the (001) face, but not for the (110) one, which is the most stable of the low-index faces of this material. Similar treatments were applied to ZrO2:Y2O3 crystals, leading to no film growth. INTRODUCTION Films of high-K materials are of increasing technical interest. Besides microelectronic applications, some of them exhibit piezoelectric or even ferroelectric properties, with a wide range of potential applications as sensors and actuators. Their conventional synthesis requires thermal treatments around 600 ºC in order to obtain polycrystalline films, which are amorphous if deposited below this temperature. Hydrothermal synthesis allows for the preparation of crystallized thin films of alkali earth titanates and zirconates [1], as well as molybdates and tungstates [2], and other complex oxides at lower temperatures. The basic process consists in the immersion of a titanium [1], zirconium, or a metallized substrate [3] in an aqueous alkali earth solution of hydroxides or other salts [4], followed by a thermal treatment at temperatures ranging from 55ºC to above that of hypercritical water (374 ºC). Several aspects of the growth kinetics of (Ba,Sr)(Ti,Zr)O3 films have been reported. Increasing Ba(OH)2 concentration leads to decreasing grain size [5], which can be explained by the higher nucleation rate at higher supersaturation on the surface, with a more regular grain size with increasing strontium content in the solution [6]. Even the smallest impurity concentration in a Ba(OH)2 solution can change dramatically the growth habit from single crystalline grains to flower-like aggregates [7]. It is accepted that a titanium oxide layer is a necessary precursor for the titanate film growth [8, 9]. The film dissolves in water as titanate ions [Ti(IV)Ox]4-2x leading to nucleation of BaTiO3 crystallites on the substrat
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