Observation of the early stages of heteroepitactic growth of BaTiO 3 thin-films
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The formation of thin-films of ferroelectric materials is of importance in a number of applications—for example, in infrared sensors and thin-film capacitors. For these applications it is essential that the films are epitactic and have the required stoichiometry and crystalline phase. The growth of ferroelectric thin-films is an area of active interest.1"6 In the present study, the formation and growth of thin epitactic films of the perovskite BaTiO3 by the pulsed-laser ablation technique have been investigated. The properties of thin films depend on their microstructure which, in turn, is controlled by the nucleation and early stages of heteroepitactic growth. The substrate exerts an important influence during this time which can be studied by deposition of very thin films directly onto electron transparent thin-foil substrates and examining them using transmission electron microscopy (TEM). Targets of BaTiO3 were prepared from high-purity powder which was cold-pressed and then sintered in flowing oxygen at 1100 °C for 2 h. The sintered pellets were held in a multitarget holder inside a stainless-steel vacuum chamber. The chamber was evacuated to better than 5 ^uTorr, then equilibrated to 400 mTorr of oxygen. The beam from a KrF (248 nm) excimer laser was focused onto the pellet and then rastered over the pellet surface to avoid localized preferential ablation. In order to study the early stages of growth, films were deposited using 400 laser pulses at a pulse repetition rate of 50 Hz. The laser energy was maintained at 85 mJ/pulse. The substrate was placed at a distance of 4 cm from the target. The substrates used were specially prepared electron-transparent thin-foils of (OOl)-oriented MgO which were mounted on copper foil on the substrate stage during laser ablation. A single-crystal MgO holder, which had a small hole drilled in it, was used to cover the substrate. The hole permitted passage of the ablated material onto the substrate. The substrate could be heated at temperatures up to 850 °C by means of a lamp inside the holder assembly. All reference to the temperature of the sample actually refers to the temper2762
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 12, Dec 1990
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ature of the substrate stage which was measured using a thermocouple embedded into the stage in close proximity to the substrate surface. Deposition was performed with the substrate stage at either 670 °C or 750 °C; the temperatures were chosen because, in a previous study3 epitactic growth was not observed below 600 °C on MgO. After deposition the chamber was filled with oxygen and then slowly cooled to room temperature under atmospheric pressure of oxygen. The preparation of the MgO thin-foil substrates has been described in detail elsewhere.7 Briefly, 3 nundiameter disks were cored from a bulk single-crystal substrate and were then mechanically polished and dimpled to a thickness of 10-20 ^m. The disks were then ion-milled to perforation using 5 kV Ar + ions. The perforated foils were chemically c
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