Effects of strain on the dielectric properties of tunable dielectric SrTiO3 thin films

We fabricated an epitaxial ${\mathrm{Au/SrTiO}}_{\mathrm{3}}{\mathrm{(001)/SrRuO}}_{\mathrm{3}}$ $({\mathrm{or CaRuO}}_{\mathrm{3}}\text{)(001)/}{\mathrm{SrTiO}}_{\mathrm{3}}$ (or ${\mathrm{LaAlO}}_{\mathrm{3}}\text{)(001)}$ heterostructure to investigate the effect of the strain on the epitaxial tunable dielectric thin films. ${\mathrm{SrTiO}}_{\mathrm{3}}$ thin films showed very different dielectric properties depending on the bottom electrode with an opposite lattice mismatch. The ${\mathrm{SrTiO}}_{\mathrm{3}}$ thin films grown on the ${\mathrm{CaRuO}}_{\mathrm{3}}$ bottom electrodes showed nearly a two times larger tunability than that on ${\mathrm{SrRuO}}_{\mathrm{3}}.$ We think this is due to the different strain on the ${\mathrm{SrTiO}}_{\mathrm{3}}$ thin films. The tensile strain along the applied electric field in the parallel plate capacitor enhances the dielectric constant and the tunability, while the compressive strain decreases them. We believe this is consistent with the hardening of the soft mode phonon due to the compressive strain.