Growth of nanoscale BaTiO 3 /SrTiO 3 superlattices by molecular-beam epitaxy
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D.A. Tenne Department of Physics, Boise State University, Boise, Idaho 83725
H.P. Sun and X.Q. Pan Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109
K.J. Choi and C.B. Eom Department of Materials Science and Engineering, University of Wisconsin, Madison, Wisconsin 53706
Y.L. Li and Q.X. Jia Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
C. Constantin and R.M. Feenstra Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213
M. Bernhagen, P. Reiche, and R. Uecker Institute for Crystal Growth, Max-Born-Straße 2, D-12489 Berlin, Germany (Received 29 January 2008; accepted 12 February 2008)
Commensurate BaTiO3/SrTiO3 superlattices were grown by reactive molecular-beam epitaxy on four different substrates: TiO2-terminated (001) SrTiO3, (101) DyScO3, (101) GdScO3, and (101) SmScO3. With the aid of reflection high-energy electron diffraction (RHEED), precise single-monolayer doses of BaO, SrO, and TiO2 were deposited sequentially to create commensurate BaTiO3/SrTiO3 superlattices with a variety of periodicities. X-ray diffraction (XRD) measurements exhibit clear superlattice peaks at the expected positions. The rocking curve full width half-maximum of the superlattices was as narrow as 7 arc s (0.002°). High-resolution transmission electron microscopy reveals nearly atomically abrupt interfaces. Temperature-dependent ultraviolet Raman and XRD were used to reveal the paraelectric-to-ferroelectric transition temperature (TC). Our results demonstrate the importance of finite size and strain effects on the TC of BaTiO3/SrTiO3 superlattices. In addition to probing finite size and strain effects, these heterostructures may be relevant for novel phonon devices, including mirrors, filters, and cavities for coherent phonon generation and control.
I. INTRODUCTION
Well-ordered BaTiO3/SrTiO3 superlattices with BaTiO3 and SrTiO3 layer thicknesses in the nanometer range are of interest to probe fundamental issues in ferroelectricity as well as for potential devices. For example, recent a)
Address all correspondence to this author. e-mail: [email protected] b) Present address: Seagate Technology, Bloomington, MN 55437. c) Present address: USAID, Washington, DC 20523. DOI: 10.1557/JMR.2008.0181 J. Mater. Res., Vol. 23, No. 5, May 2008
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theoretical studies predict that (i) the unstrained SrTiO3 layers in BaTiO3/SrTiO3 superlattices grown commensurately on cubic (100) SrTiO3 substrates are themselves tetragonal and poled by internal electric fields, (ii) the polarization of such superlattices can be enhanced beyond that achievable in unstrained BaTiO3 because of the biaxial compressive strain state of the BaTiO3 layers within the superlattice, and (iii) that ferroelectricity will persist in such superlattices for BaTiO3 layers as thin as the thickness of a single BaTiO3 unit cell (0.4 nm).1,2 © 2008 Materials Research Society IP address: 130.126.162.1
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