Thermally induced structural changes in epitaxial SrZrO 3 films on SrTiO 3
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Thermally induced structural changes in epitaxial SrZrO3 films on SrTiO3 P.A. Langjahra), T. Wagner, and M. Ru¨hle Max-Planck-Institut fu¨r Metallforschung, Seestr. 92, 70174 Stuttgart, Germany
F.F. Lange Materials Department, College of Engineering, University of California, Santa Barbara, California 93106 (Received 20 August 1998; accepted 19 April 1999)
Epitaxial, continuous, approximately 40-nm-thick films of SrZrO3 on SrTiO3 substrates prepared by a chemical solution deposition method including a postdeposition heat treatment at 900–1000 °C were subjected to further heat treatments at higher temperatures (approximately 1200–1300 °C) to investigate their high temperature stability. Experimental investigations included scanning electron microscopy, atomic force microscopy, and conventional transmission electron microscopy. The investigations have demonstrated a morphological instability of the films. Concentration profiles of the cations determined by energy dispersive x-ray spectroscopy, as well as investigations by x-ray diffraction, revealed that the film islands consisted of a solid solution. As shown by high-resolution electron microscopy, the reaction between film and substrate also led to an increase in the separation distance of the misfit dislocations that were introduced during the lower temperature heat treatment to relax the lattice mismatch strain. The morphological and structural changes of the films are reported and discussed in this paper.
I. INTRODUCTION
It is well known that structures and morphologies of thin films influence the properties in device applications. One method of producing epitaxial films on singlecrystal substrates is the chemical solution deposition route with a post-deposition heat treatment at relatively high temperatures.1 Previous studies have investigated the mechanisms of epitaxial growth.2,3 However, little is known about the stability of the structure and the morphology of the epitaxial films and the atomic structure of the interface during a high temperature heat treatment. For simplicity, we considered the case of film and substrate possessing the same crystal structure. If film and substrate form a solid solution, interdiffusion between film and substrate may be caused by a driving potential due to chemical miscibility and reduction of the lattice mismatch strain. As shown in experimental and theoretical studies by various authors,4 – 9 compositional gradients, as they occur within diffusion zones in bicrystals or at graded heterojunctions, can lead to a redistribution of misfit dislocations at the interface between film and sub-
a)
Present address: Universita¨t Karlsruhe, Institut fu¨r Keramik im Maschinenbau (IKM), Zentrallaboratorium, Haid-und-Neu-Str. 7, 76131 Karlsruhe, Germany. J. Mater. Res., Vol. 14, No. 7, Jul 1999
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strate. Previous studies, however, have not (i) dealt with thin films produced by the chemical solution deposition method and (ii) encompas
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