The formation mechanism of barium titanate thin film under hydrothermal conditions
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Barium titanate thin films were produced under the hydrothermal conditions, and their formation mechanism was investigated. The phase structure and surface morphology of the resultant films were directly dependent on the reaction conditions. The films that have a reaction time shorter than 8 h below the processing temperature of 150 °C in the 0.8 N Ba(OH)2 solution were composed of compounds with the component ratio m(Ba)/ m(Ti) < 1, and a flower-like surface morphology. With extended reaction time under the higher temperature, the films showed a multiphase structure and a mosaic or island-like surface morphology. In the case of 1.0 N, 1.5 N Ba(OH)2 solution, well-crystallized, monoperovskite BaTiO3 thin films having uniform, mirror-like, and visible-defectless surfaces were produced at a processing temperature of 180 °C after 24 h. The experimental results were explained by hypothesizing that the formation mechanism consists of a "dissolutionscrystallization process".
I. INTRODUCTION
II. EXPERIMENTAL
Ferroelectric barium titanate (BaTiO3) thin films have gained importance in the electronics industry because of their application as capacitors and thermistors. Recently, hydrothermal methods, including the ordinary and so-called hydrofhermal-electrochemical methods where a direct current is passed between the anode and the cathode, have been used to prepare BaTiO3 thin films.1"6 The hydrothermal process does not require conductive substrates, and therefore films can be formed on any kind of substrate deposited with titanium metal. The electrochemical process has the advantage of enhanced purity and lower reaction temperatures, but the films are found to be less crystalline. In the hydrofhermal-electrochemical method,7 high thickness can be achieved in a short reaction time, but a more optimum condition is needed to improve the electrical properties and to minimize the surface inhomogeneities and cracks. With a view to understanding and improving hydrothermal techniques for preparation of BaTiO3 thin film, it is desirable to investigate in detail the formation mechanism of BaTiC>3 thin films on silicon substrates under hydrothermal conditions. In this paper, we present a systematic survey of the relationship between the hydrothermal reaction conditions, the phase structure, and surface morphology of the resultant films.
The hydrothermal experiments were performed in a tube-type autoclave with a large ratio of the height to internal diameter (approximately 18). The Ti (5000 A)deposited Si(100) plates used as substrates were cut to 20 X 10 mm size and suspended from a frame which was placed in the autoclave. Three heaters surrounded the autoclave and were used to establish an optimum temperature difference between the top and the bottom of the autoclave, which decided the vertical thermal convection of the reaction medium during hydrothermal reaction. The phase and crystallinity of the resultant films were characterized by x-ray diffraction techniques. (Rigaku, the fixed measurement conditions: CuK a , 30 kV, and 20
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