Compositionally Asymmetric Tri-Color Superlattices Grown by Pulsed Laser Deposition
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Compositionally Asymmetric Tri-Color Superlattices Grown by Pulsed Laser Deposition H. N. Lee, H. M. Christen, C. M. Rouleau, S. Senz1, S. K. Lee1, D. Hesse1, and D. H. Lowndes Condensed Matter Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA 1 Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D-06120 Halle/Saale, Germany
ABSTRACT
Compositionally asymmetric tri-color superlattices (TCS) with a combination of BaTiO3/SrTiO3/CaTiO3 have been grown by pulsed laser deposition (PLD) on atomically-flat SrRuO3-covered (001) SrTiO3 substrates. Conducting SrRuO3 films with single-terrace steps that closely mimic those of the SrTiO3 substrate also were grown by PLD and serve as bottom electrodes. In order to achieve atomic control of each layer, we have calibrated precisely the number of laser pulses required to grow one unit-cell-thick layers (~200 for a laser spot of 0.4 mm2). These conditions allowed recording of pronounced oscillations of the reflection highenergy electron diffraction (RHEED) specular spot intensity over the entire growth run - even for TCS layers totaling more than 1000 nm in total thickness.
INTRODUCTION
Advances in modern epitaxial growth techniques have enabled the growth of nearly perfect thin films, superlattices, etc. by the atomic-scale control of surfaces and interfaces. For instance, oxide heterostructures with atomically-flat interfaces and single unit-cell steps on the surface can be grown on well-prepared single-stepped substrates. Moreover, artificial superlattices – i.e., materials that do not exist in bulk forms – can be AO AO grown with great control on such substrates and are expected to result in unprecedented physical BO2 B''O2 properties. Epitaxial thin films with arbitrary (but A''O AO precisely controlled) stacking of layers with an A/B/C/…A/B/C geometry - so called tri-color or BO2 B'O2 three-component superlattices (TCS), cf. Fig. 1 A'O AO can exhibit compositionally-broken inversion symmetry (CBIS), which results in strong selfBO2 BO2 poling due to the ionic size and/or electrostatic AO AO interaction effects by the isovalent and heterovalent substitutions, respectively [1]. Figure 1. Tri-color superlattices of A-site Consequently, TCSs with CBIS may result in [(A1/3A'1/3A''1/3)BO3] (left) and B-site anomalous piezoelectricity and dielectric [A(B1/3B'1/3B''1/3)O3] (right) substitutions. responses, and will allow a wide tunability of The inversion symmetry is broken by either ferroelectric properties [2]. The growth of isovalent or heterovalent substitution. atomically-flat layers, however, is not trivial due
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to lattice misfit, different growth modes, and lack of atomically-flat conducting layers for electric characterization. For example, growing BaTiO3 on SrTiO3 (misfit ~2%) involves the layer-thenisland growth mechanism which usually destroys the sharpness of subsequent interfaces if the BaTiO3 thickness exceeds two unit cells [3]. Thus, in order to design and fabricate functional materials and structures thereof, it is pa
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