Epitaxial thin films of PbTiO 3 /SnO 2 heterostructures on sapphire
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Epitaxial films of single-layer SnO2 and PbTiO3/SnO2 heterostructures were obtained on (0001) sapphire (a-Al 2 O 3 ) substrates by metal-organic chemical vapor deposition. X-ray diffraction and transmission electron microscopy were used to characterize the structural properties of these films. The epitaxial relationship for the heterostructure PbTiO3/SnO2/sapphire was found to be (111) [0lT]PbTiO31| (100) [001]SnO2 || (0001) [lT00]o;-Al2O3. The fact that epitaxial ferroelectric films were obtainable in such a structurally highly heterogeneous system suggests that a wide range of material selection is possible in exploring the kind of applications that need to utilize epitaxial ferroelectric films in a multilayered heterostructure.
I. INTRODUCTION
In the past few years, there has been increasing interest in the area of epitaxial metal oxide thin films for various scientific and technological reasons. For example, epitaxial films are required or preferred for many potential applications utilizing ferroelectric oxide materials.1"3 In many cases, one needs not only to layer different materials together in an epitaxial, multilayered heterostructure, but also the crystallographic orientation relation between each layer has to be controlled. Very often, these kinds of applications need a bottom electrode underneath the ferroelectric films. The bottom electrode material has to be epitaxial and structurally and chemically compatible with the ferroelectric films in order for them to grow epitaxially. A number of electrically conductive oxides have been successfully prepared as bottom electrodes in epitaxial form by several research groups.4"7 Essentially all of them were perovskite materials with lattice constants closely matched to those of the ferroelectric materials of interest. The consideration of lattice matching limits the selection of materials that can be used. In our recent works,8'9 we have found that, for many ionic oxide heterostructures, high quality epitaxial films can be obtained despite the existence of fairly large lattice mismatch in these systems (typically 5 to 10%). This has encouraged us to search for more variety of ionic oxide materials as bottom electrodes for the above-mentioned applications. One system we have studied recently is a double-layer structure of PbTiO3/SnO2 grown on (0001) sapphire (a-Al 2 O 3 ) a)
Present address: Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208. b) Present address: Department of Materials Science, Fudan University, Shanghai 200433, China. 3108 http://journals.cambridge.org
substrates by metal-organic chemical vapor deposition (MOCVD). Sapphire, due to its various unique properties and low cost, is a widely used substrate for many thin film devices. Nonstoichiometric (oxygen-deficient) or impurity-doped SnO2 is electrically conductive and optically transparent in the visible light region; it has been extensively used as transparent electrodes for a variety of optoelectronic applications.10 In this work, we have suc
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