Transmission electron microscopy study of (103)-oriented epitaxial SrBi 2 Nb 2 O 9 films grown on (111) SrTiO 3 and (111
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S.K. Streiffer Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439
M.E. Hawley Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received 28 August; accepted 3 November 2000)
Portions of the same epitaxial (103)-oriented SrBi2Nb2O9 film grown on (111) SrTiO3 for which we recently reported the highest remanent polarization (Pr) ever achieved in SrBi2Nb2O9 (or SrBi2Ta2O9) films, i.e., Pr ⳱ 15.7 C/cm2, have been characterized microstructurally by plan-view and cross-sectional transmission electron microscopy (TEM) along three orthogonal viewing directions. SrBi2Nb2O9 grows with its c axis tilted 57° from the substrate surface normal in a three-fold twin structure about the substrate [111], with the growth twins’ c axes nominally aligned with the three 〈100〉 SrTiO3 directions. (103) SrBi2Nb2O9 films with and without an underlying epitaxial SrRuO3 bottom electrode have been studied. Dark-field TEM imaging over a 12 m2 area shows no evidence of second phases (crystalline or amorphous). A high density of out-of-phase boundaries exists in the films.
I. INTRODUCTION 1
Due to their high fatigue resistance, the ferroelectric materials SrBi2Nb2O9 and SrBi2Ta2O9 have become important for ferroelectric device applications including nonvolatile random access memories2 and ferroelectric field-effect transistors.3 To make devices with higher integration densities, uniform film properties are needed on an increasingly finer length scale. Epitaxy is one approach to achieving such uniform film properties. However, epitaxial SrBi2Nb2O9 and SrBi2Ta2O9 films have had remanent polarization (Pr) values much lower than randomly-oriented polycrystalline films because of the low (often zero) component of their spontaneous polarization (Ps) parallel to the applied electric field for the particular epitaxial orientations and electrode geometries studied. Epitaxy also allows the anisotropy in the dielectric and ferroelectric properties to be studied, and this is the focus of our research. We recently reported4 the growth and electrical characterization of an oriented epitaxial SrBi2Nb2O9 film
a)
On leave at the Institute of Physics, Augsburg University, D-86159 Augsburg, Germany. b) On leave from Qingdao University, People’s Republic of China. J. Mater. Res., Vol. 16, No. 2, Feb 2001
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grown on (111) SrTiO3 with an epitaxial SrRuO3 bottom electrode. The film had a remanent polarization of 15.7 C/cm2. This is the highest Pr reported to date for SrBi2Nb2O9 (or SrBi2Ta2O9) films of any kind, polycrystalline or epitaxial. In this article, we present transmission electron microscopy (TEM) characterization of the same (103) SrBi2Nb2O9 /(111) SrRuO3/(111) SrTiO3 film that was used for the reported electrical characterization,4 as well as a (103) SrBi2Nb2O9 /(111) SrTiO3 film without an underlying SrRuO3 electrode layer. After introducing the crystal structure of SrBi2Nb2O9, we describe the epitaxial relatio
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