A Transmission Electron Microscopy Study of Dislocation Substructures in PLD-grown Epitaxial Films of (Ba,Sr)TiO 3 on (0
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A Transmission Electron Microscopy Study of Dislocation Substructures in PLD-grown Epitaxial Films of (Ba,Sr)TiO3 on (001) LaAlO3 I. B. Misirlioglu, A. L. Vasiliev, M. Aindow R. Ramesh1 and S. P. Alpay Department of Metallurgy and Materials Engineering and Institute of Materials Science, University of Connecticut, Storrs, CT 06269 1 Department of Materials and Nuclear Engineering, University of Maryland, College Park MD 20742
ABSTRACT Epitaxial Ba0.6Sr0.4TiO3 films were grown onto (001) LaAlO3 by pulsed-laser deposition, and the dislocation structures of the films were investigated using transmission electron microscopy. Misfit dislocations with a periodicity of about 7 nm and Burgers vectors b = a were observed at the interface. High densities of threading dislocations was present in the films with Burgers vector b = a. The observations reveal that threading dislocations are not generated as the result of half-loop climb from the deposit surface as proposed previously, but are instead formed when misfit dislocations are forced away from the interface during island coalescence.
INTRODUCTION Over the last few years, significant effort has been devoted to understanding the effects of microstructural features such as grain boundaries, texture, and formation of structural (90o) domains on the physical and dynamic properties of ferroelectric films. There has, however, been very little attention paid to the possible effects of dislocations despite the fact that these constitute the vast majority of the defects in such films. In a recent study by Canedy et al. [1] it was shown that the dielectric response in epitaxial ferroelectric films is strongly dependent on the dislocation microstructure. Dislocations in heteroepitaxial thin films can be divided into two types: misfit and threading dislocations (MDs and TDs, respectively). MDs lie in the epitaxial interface and accommodate the lattice mismatch between the film and substrate. As a result of extensive research on metallic and semiconducting heteroepitaxial systems, the mechanisms by which MDs can be introduced are now well understood. For deposits that grow in the 3D Volmer-Weber mode, this usually occurs by the introduction of edge dislocations at island edges during growth [2]. For deposits growing in the 2D Frank-Van der Merwe mode, however, this cannot occur and misfit dislocations are often introduced by the introduction of glissile half-loops either from the deposit surface or from regenerative dislocation sources in the deposit [3]. TDs lie within the film and run from the interface to the film surface. The origins of these TDs are also well established for semiconductor thin films. These were originally explained on the basis of dislocation “copying” wherein dislocations in the substrate which emerged at the surface were duplicated into the deposit when they were overgrown [4, 5]. It has been shown subsequently that this mechanism cannot be used to explain most of the TDs observed in epitaxial deposits since the density of these features typically exce
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