Room-temperature ferroelectricity in strained SrTiO3

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Abstract

Systems with a ferroelectric to paraelectric transition in the vicinity of room temperature are useful for devices. Adjusting the ferroelectric transition temperature (Tc) is traditionally accomplished by chemical substitution—as in BaxSr1-xTiO3, the material widely investigated for microwave devices in which the dielectric constant (εr) at GHz frequencies is tuned by applying a quasi-static electric field1,2. 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 Tc in ferroelectric films is strain4,5,6,7,8. Here we show that epitaxial strain from a newly developed substrate can be harnessed to increase Tc 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 Tc 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 εr at room temperature in these films (nearly 7,000 at 10 GHz) and its sharp dependence on electric field are promising for device applications1,2.

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Figure 1: Expected shift in Tc of (100) SrTiO3 with biaxial in-plane strain, based on thermodynamic analysis.
Figure 2: In-plane dielectric constant (εr) and dielectric loss (tanδ) in strained epitaxial SrTiO3 films as a function of temperature and film thickness at a measurement frequency (f) of 10 GHz.
Figure 3: Dielectric tunability of the same 500-Å-thick SrTiO3 films grown on DyScO3 and LSAT as shown in Fig. 2.
Figure 4: Comparison of the morphology and microwave electro-optic response of tunable dielectric films at room temperature (just above Tc).

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Acknowledgements

We acknowledge discussions and interactions with M. D. Biegalski, J. Schubert, S. Trolier-McKinstry and J. Mannhart during the course of this work. In addition, the financial support of the National Science Foundation, the Office of Naval Research for the work performed at NRL, the Swiss National Science Foundation, and, for the work performed at ANL, the US Department of Energy, Basic Energy Sciences—Materials Sciences is gratefully acknowledged.

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Correspondence to D. G. Schlom.

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The authors declare that they have no competing financial interests.

Supplementary information

Supplementary Figure 1

Thickness dependence of the dielectric properties of strained SrTiO3 films. (PDF 481 kb)

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