Microstructure of columnar-grained SrTiO 3 and BaTiO 3 thin films prepared by chemical solution deposition
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Microstructure of columnar-grained SrTiO3 and BaTiO3 thin films prepared by chemical solution deposition C. L. Jia and K. Urban Institut f¨ur Festk¨orperforschung, Forschungszentrum, J¨ulich GmbH, D-52425 J¨ulich, Germany
S. Hoffmann and R. Waser Institut f¨ur Werkstoffe der Elektrotechnik, Rheinisch-Westf¨alische Technische Hochschule Aachen, D-52056 Aachen, Germany (Received 29 November 1996; accepted 17 October 1997)
The microstructure and lattice defects of SrTiO3 and BaTiO3 thin films with columnar grains prepared by a chemical solution deposition technique have been investigated by means of transmission electron microscopy. The columnar grains in the SrTiO3 films exhibit a preferential orientation with a crystallographic k111l direction parallel to the normal of film, which, in turn, follows the orientation texture of the substrate Pt layer. Cubic-to-cubic relationships have been found for the two materials. For the BaTiO3 films the columnar grains are oriented in a random way without any preferential relationship to the substrate Pt layer. Pores, lattice defects, and grain boundaries occur in either type of film, however in different configurations. This reflects the individual nature of the materials and the different formation and growth mechanisms of the films under the preparation conditions.
I. INTRODUCTION
Today, the integration of perovskite-structured ceramic thin films in semiconductor technology for microelectronic devices represents one of the most active areas in materials research (for an overview see, for example, Ref. 1). Specifically, SrTiO3 , BaTiO3 , and solid solutions Ba12x Srx TiO3 are considered as high-permittivity dielectrics in dynamic random access memory (DRAM) cells2–4 and integrated capacitors.5 Vapor phase deposition techniques, e.g., molecular beam epitaxy (MBE),6 pulsed laser deposition (PLD),7 rf sputtering,8 ion-beam sputter deposition,9 and metalorganic chemical vapor deposition (MOCVD),10 have frequently been employed to grow epitaxial alkaline earth titanate thin films on appropriate substrates. In contrast, chemical solution deposition (CSD) of BaTiO3 and SrTiO3 at temperatures of about 600 –800 ±C typically results in polycrystalline, granular films11–13 due to the trend toward homogeneous nucleation in the pyrolyzed precursor as shown by Mecartney and Gust.14 Epitaxial (Ba, Sr) (Ti, Zr)O3 films can be grown by CSD and SrTiO3 and LaAlO3 substrates applying temperatures of about 1000 ±C to promote sufficient long-range mass transport.15 This is in contrast to Pb(Ti, Zr)O3 films which can easily be grown by CSD into a columnar or epitaxial structure because of heterogeneous nucleation from a nanocrystalline pyrochlore/fluorite phase into the perovskite phase at the substrate.16 –21 The electrical properties of the titanate films show a significant dependence on their microstructure. For SrTiO3 thin films prepared by rf sputtering on Pt-coated 2206
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J. Mater. Res., Vol. 13, No. 8, Aug 1998
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