1. Department of Materials Science and Engineering, Penn State University, University Park, Pennsylvania 16802-5005, USA
2. Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
3. Naval Research Laboratory, 4555 Overlook Avenue S.W., Washington DC 20375, USA
4. Institute of Crystal Growth, Max-Born-Strae 2, D-12489 Berlin, Germany
5. Department of Materials Science & Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136, USA
6. Materials Science and Technology Division (MST-8), Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
7. Motorola Labs, 2100 East Elliot Road, Tempe, Arizona 85284, USA
8. Laboratoire de Céramique, Ecole Polytechnique Fédérale de Lausanne, Lausanne CH 1015, Switzerland
9. Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA

Correspondence to: D. G. Schlom¹ Correspondence and requests for materials should be addressed to D.G.S. (Email: schlom@ems.psu.edu).

Abstract

Systems with a ferroelectric to paraelectric transition in the vicinity of room temperature are useful for devices. Adjusting the ferroelectric transition temperature (T_c) is traditionally accomplished by chemical substitution—as in Ba_xSr_1-xTiO₃, the material widely investigated for microwave devices in which the dielectric constant (_r) at GHz frequencies is tuned by applying a quasi-static electric field^{1, 2}. Heterogeneity associated with chemical substitution in such films, however, can broaden this phase transition by hundreds of degrees³, which is detrimental to tunability and microwave device performance. An alternative way to adjust T_c in ferroelectric films is strain^{4, 5, 6, 7, 8}. Here we show that epitaxial strain from a newly developed substrate can be harnessed to increase T_c by hundreds of degrees and produce room-temperature ferroelectricity in strontium titanate, a material that is not normally ferroelectric at any temperature. This strain-induced enhancement in T_c is the largest ever reported. Spatially resolved images of the local polarization state reveal a uniformity that far exceeds films tailored by chemical substitution. The high _r at room temperature in these films (nearly 7,000 at 10 GHz) and its sharp dependence on electric field are promising for device applications^{1, 2}.

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