Ferroelectricity in Strain-FreeSrTiO3Thin Films

Abstract

Biaxial strain is known to induce ferroelectricity in thin films of nominally nonferroelectric materials such as ${\mathrm{SrTiO}}_3$. By a direct comparison of the strained and strain-free ${\mathrm{SrTiO}}_3$ films using dielectric, ferroelectric, Raman, nonlinear optical and nanoscale piezoelectric property measurements, we conclude that all ${\mathrm{SrTiO}}_3$ films and bulk crystals are relaxor ferroelectrics, and the role of strain is to stabilize longer-range correlation of preexisting nanopolar regions, likely originating from minute amounts of unintentional Sr deficiency in nominally stoichiometric samples. These findings highlight the sensitive role of stoichiometry when exploring strain and epitaxy-induced electronic phenomena in oxide films, heterostructures, and interfaces.