Microstructural development of BaTiO 3 heteroepitaxial thin films by hydrothermal synthesis
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Microstructural development of BaTiO3 heteroepitaxial thin films by hydrothermal synthesis A. T. Chien, L. Zhao, M. Colic, J. S. Speck, and F. F. Lange Materials Research Laboratory and Materials Department, University of California, Santa Barbara, California 93106 (Received 30 April 1997; accepted 1 August 1997)
The hydrothermal growth of epitaxial BaTiO3 thin films on single-crystal SrTiO3 substrates occurs by the island growth mode. The aqueous solution chemistry is found to control interfacial characteristics and plays an important role in controlling film formation and faceting. Island faceting can be changed by the introduction of additional cations during synthesis. Electrophoretic data, confirmed by adsorption measurements, show that barium is a potential determining counterion and adsorbs on SrTiO3 surfaces. Initial electrical measurements show that the BaTiO3 films have a dielectric constant of 141 with a loss tangent of 0.9 that decreases with heat treatment.
I. INTRODUCTION
Barium titanate is an important material in the electronics industry due to its high dielectric constant and ferroelectric properties. Epitaxial BaTiO3 thin films and other perovskite materials have predominantly been prepared by vapor phase methods (e.g., pulsed laser deposition, metal-organic chemical vapor deposition, and sputtering).1 Our interest is in using direct solution synthesis routes to grow epitaxial thin films at low temperatures.2 Hydrothermal synthesis is a technique that involves the growth of oxides directly from aqueous solutions, generally at elevated temperatures (,100–400 ±C) and pressures (,0.1–15 MPa).3,4 The epitaxial growth of BaTiO3 on SrTiO3 single-crystal substrates has been successfully demonstrated in our previous work5 at 90 ±C and at ambient pressures and by Kajiyoshi et al.6 at high temperatures and pressures (T , 400–800 ±C, P 10 MPa) by using the SrTiO3 substrate to template the growth of the BaTiO3 thin film. This is a technique we call hydrothermal epitaxy. Recently, the epitaxial growth of PbTiO3 and Pb(Zrx Ti12x )O3 has also been demonstrated by the hydrothermal epitaxy technique.7,8 Several possible scenarios (listed below) have been proposed to describe the chemical reactions that synthesize BaTiO3 under hydrothermal conditions9–11 : Ba21 1 TisOHd622 saqd BaTiO3 1 3H2 O ,
(Ref. 9)
Ba21 1 2OH2 1 TiO2 ssd BaTiO3 1 H2 O , (Ref. 10)
Ba21 1 TisOHd4 saqd BaTiO3 1 H2 O 1 2H1 . (Ref. 11) J. Mater. Res., Vol. 13, No. 3, Mar 1998
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In all cases OH2 anions play a strong role in the reaction, inferred from the observation that the synthesis of BaTiO3 is inhibited at low pH.11 Thermodynamic calculations of the Ti–Ba–H2 O system have been used to assess the relative stability of BaTiO3 in aqueous solutions as a function of pH, reagent (Ba21 ) concentration, and temperature.10,12 The equilibrium phase diagram resulting from these calculations demonstrates that BaTiO3 is thermodynamically favored
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