Controlled Nucleation and Growth in the Hydrothermal-Electrochemical Formation of BaTiO 3 Films
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INTRODUCTION Thin films of the ferroelectric ABO 3 phase have been proposed for microelectronic devices, such as DRAM's and non volatile memories. The use of low temperature processes is an important improvement for these applications. A low temperature hydrothermal and hydrothermal-electrochemical route was developed by Yoshimura et al. [1]. Formation of BaTiO 3 layers was achieved by immersion of a titanium substrate in a Ba(OH) 2 solution at 100-200 °C, in open circuit or under anodic conditions. These films were about 100 nm thick and always resulted in the cubic non ferroelectric phase. Tetragonal BaTiO 3 films were produced at 450 'C by treating titanium during 4 hours in a 4.0 N solution [2]. Recently Cho et al. reported the hydrothermal preparation of ferroelectric, tetragonal BaTiO 3 films at 220 'C using 24 hour treatments in 2 N Ba(OH) 2 [3]. The mechanism of BaTiO 3 film formation has been characterized by using cyclic voltammetric and chronoamperometric techniques. These studies characterized the anodic and cathodic reactions involved in the hydrothermal process and suggested forms to electrochemically control the growth of BaTiO 3 films using the hydrothermal-electrochemical method. In the present work we report the application of controlled potential techniques to the growth BaTiO 3 films. This procedure enabled the inhibition of pure hydrothermal growth and the control of nucleation and grain size in films formed under anodic conditions. In addition, the growth of tetragonal BaTiO 3 films was achieved at 150 °C, the lowest temperature reported to this date.
101 Mat. Res. Soc. Symp. Proc. Vol. 403 0 1996 Materials Research Society
EXPERIMENTS The experiments were performed in a three-electrode Cortest Inc. high pressure electrochemical cell. The working electrode was a 1.5 cm 2 titanium flag. The counterelectrode was a 4 cm 2 platinum foil. The external reference electrode was saturated Ag/AgCI at 25 *C. All potentials in the text below are given with respect to this reference. The electrodes were connected to a potentiostat/galvanostat (EG&G model 273). The treatment temperature was 150 'C in all the cases. Hydrothermal-electrochemical experiments normally incorporate an initial pure hydrothermal step due to reactions during the heating period. This effect was avoided in the present experiments by immersing the titanium electrode in the electrolyte just when the working temperature was reached. The films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Scanning electron microscopy (SEM). Three types of experiments were carried out, dealing with cathodic protection, control of the microstructure, and control of the BaTiO 3 phase. RESULTS Inhibition of hydrothermal growth by cathodic protection In the first set of experiments, a 0.1 M Ba(OH)2 solution was used. Titanium electrodes were cathodically protected since immersion in the electrolyte applying potentials from -250 mV to -500 mV, during 30 s. Efficiency of BaTiO 3 growth inhibition was assessed by me
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