Novel Method to Reversibly and Uniformly Strain Epitaxial Oxide Thin Films Using a Piezoelectric Substrate

K. Dörr,¹ M.D. Biegalski,² D.H. Kim,^3-4 and H. M. Christen^2
1CNMS USER Institute for Metallic Materials, IFW Dresden, Dresden, Germany
²Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee
³Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee
⁴Department of Physics, Tulane University, New Orleans, Louisiana

Achievement
A method to vary reversibly the strain in a single epitaxial film, while keeping all other parameters constant, is crucial to understanding the properties of nanoscale thin film materials. We have shown that the strain exerted from a piezoelectric PMN-PT (0.72Pb(Mg_1/3,Nb_2/3)O₃-0.28 PbTiO₃) substrate is reversible, and is uniform within the plane and through the thickness of an epitaxial thin film stack grown by pulsed laser deposition. The uniformity of strain in this method makes it singularly useful for investigations of elastic properties in epitaxial thin films. This novel method gives a direct window into the effects of strain, eliminating the effects of changing defect structure and composition inherent in earlier methods based on comparing sets of films with different lattice-mismatch or thicknesses.

In this study, piezoelectric substrates of pseudocubic PMN-PT were utilized to exert uniform and reversible strain to single-crystal epitaxial perovskite films. An electric field applied across the crystal significantly changes the lattice parameter of the PMN-PT crystal, as measured using 4-circle x-ray diffraction. This work shows that the biaxial strain is fully transferred to epitaxial films, regardless of defects and buffer layers. Using this method, strains in excess of 0.15% are readily achieved, and were remarkably linear with the applied voltage. Additionally, the strain achieved for a given voltage is essentially temperature-independent (from 80K to room temperature), enabling temperature-dependent strain-effect measurements. In-plane and out-of-plane lattice parameters can be determined for different films, from which the Poisson ratio of these materials can be determined directly without the need for additional information. Results for SrTiO₃, BiFeO₃, LaScO₃ and MgO are shown in Table 1. In addition to the measurement of mechanical properties, the technique enables measurement of the effects of strain on ferroelectric behavior (e.g. BiFeO₃) with the results showing a remarkable agreement with predictions.

Significance
This straightforward method provides a general and valuable technique to study the effects of strain on a great variety of material properties. This technique is applicable not only for epitaxial oxide thin films, but for a wide range of materials including metastable materials and nanostructured films. The distinct advantage of this technique over others is that inherent trends in material properties can be extracted directly from single samples, eliminating sample-to-sample artifacts and variations.

Publications:

1. M. D. Biegalski, K. Dörr, D. H. Kim, and H. M. Christen, “Reversible Uniform Strain in Epitaxial Oxide Films,” manuscript in preparation.
2. C. Thiele, K. Dörr, O. Bilani, J. Rödel and L. Schultz, “Influence of Strain on the Magnetization and Magnetoelectric Effect in La_0.7A_0.3MnO₃/PMN-PT(001) (A=Sr,Ca),” Phys. Rev. B 75, 054408 (2007).
3. O. Bilani-Zeneli, A. D. Rata, A. Herklotz, O. Mieth, L. M. Eng, L. Schultz, M. D. Biegalski, H. M. Christen, and K. Dörr, “SrTiO₃ on Piezoelectric PMN-PT(001) for Application of Variable Strain,” J. Appl. Phys. 104, 054108 (2008).

Table 1: Table of measured Poisson ratios for several oxide thin films and their bulk values. The values of Poisson’s ratio are similar to bulk values for the SrTiO₃ films and similar to the predicted values for BiFeO₃ thin films. LaScO₃ has never been measured; these results show the flexibility of the technique. The MgO film shows a deviation from the bulk properties because of residual strain in film illustrating the importance of measuring the elastic properties of thin films.

Figure 1: Schematic of the induced strain in the film stack due to an applied voltage across the sample. The applied voltage, due to the piezoelectric effect, causes the substrate to elongate along the direction of the applied voltage and shrink perpendicular to applied field direction. This shrinking is transferred to the film being examined and causes a biaxial strain.

Figure 2: Lattice constant of (a) PMN-PT Substrate, (b) SrTiO₃ and (c) BiFeO₃ as a function of applied voltage to the substrate with at maximum field of ~15 kV/cm. The results show that the change in lattice parameter is linear as a function of the applied voltage. This indicated the changes in the unit cell of film as a function of strain.

Figure 3: Ferroelectric properties of a BiFeO₃ thin film grown on a PMN-PT substrate with a (La_0.80,Sr_0.20)MnO₃ bottom electrode. The polarization is aligned with the pseudocubic (111) direction in bulk BiFeO3 and is only weakly dependent on strain. However, the measured projection of the remanent polarization along the (001) direction changes slightly, as predicted theoretically, but the coercive field has a strong and previously unknown dependence on strain.