Microstructural instability in single-crystal thin films
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Microstructural instability in single-crystal thin films A. Seifert, A Vojta, J. S. Speck, and F. F. Lange Materials Department, College of Engineering, University of California, Santa Barbara, Santa Barbara, California 93106 (Received 23 August 1995; accepted 8 January 1996)
Epitaxial PbTiO3 thin films were produced from a mixed Pb–Ti double-alkoxide precursor by spin-coating onto single crystal (001) SrTiO3 substrates. Heat treatment at 800 ±C produces a dense and continuous, epitaxial lead titanate film through an intermediate Pb–Ti fluorite structure. A microstructural instability occurred when very thin single crystal films were fabricated; this instability caused the films to become discontinuous. Scanning electron microscopy and atomic force microscopy observations show that single crystal films with a thickness less than ,80 nm developed holes that expose the substrate; thinner films broke up into isolated, single crystal islands. The walls of the holes were found to be (111) perovskite planes. A free energy function, which considered the anisotropic surface energies of different planes, was developed to describe the microstructural changes in the film and to understand the instability phenomenon. The function predicted that pre-existing holes greater than a critical size are necessary to initiate hole growth, and it predicted the observed morphological changes in the current system. Morphological stability diagrams that explain the stability fields for different film configurations, i.e., either completely covered, with holes, or single crystal islands, can be calculated for any film/substrate system.
I. INTRODUCTION
It is generally assumed that thin films will remain stable during their use. However, it has been shown that continuous films are not always thermodynamically stable and that they can uncover the substrate during a high temperature heat treatment. This phenomenon has been observed for a wide range of polycrystalline metal films.1–7 Theoretical treatments for polycrystalline films have been reported by Srolovitz and Safran8,9 as well as Miller et al.10 for the case where the surface energy of the crystalline film is assumed to be isotropic. Miller et al. observed the breakup phenomenon for yttria-stabilized zirconia films on sapphire. They confirmed the hypothesis made by Srolovitz and Safran that grain boundary grooving can initiate the instability when the grain-size to film-thickness ratio exceeds a critical value. A similar phenomenon was observed in this work for the case of epitaxial PbTiO3 thin films grown on (001) SrTiO3 by the solution precursor method.11 As previously described,11 continuous and dense single crystal lead titanate films, ,120 thick, were prepared by depositing two layers of a Pb–Ti double-alkoxide precursor on SrTiO3 substrates with an intermittent drying period of approximately 2 min. Subsequent heat treatment in a PbO environment at 800 ±Cy1 h transformed the precursor film to a single-crystal PbTiO3 film with the same
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