Room-temperature ferroelectricity in strained SrTiO3

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 (ε_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 ε_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.