Stoichiometry as key to ferroelectricity in compressively strained SrTiO₃ films

While strain is a powerful tuning parameter for inducing ferroelectricity in thin film oxides, the role of stoichiometry control is critical, but far less explored. A series of compressively strained SrTiO₃ films on (001) (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.35 substrates were grown by hybrid molecular beam epitaxy where the Ti cation was supplied using a metal-organic titanium tetraisopropoxide molecule that helps systematically and precisely control Sr:Ti stoichiometry in the resulting films. A stoichiometric growth window is located through X-ray diffraction and in-situ reflection high-energy electron diffraction measurements, which show a minimum out-of-plane lattice parameter as well as constant growth rate within the stoichiometric growth window range. Using temperature dependent optical second harmonic generation (SHG) characterization, a ferroelectric-to-paraelectric transition at T ∼ 180 K is observed for a stoichiometric SrTiO₃ film, as well as a higher temperature structural transition at T ∼ 385 K. Using SHG polarimetry modeling, the polar point group symmetry is determined to be tetragonal 4mm with the polarization pointing out-of-plane of the film. The SHG coefficients, d₃₁/d₁₅=3 and d₃₃/d₁₅=21, were determined at 298 K. The ferroelectric transition disappears in films grown outside the growth window, thus proving the critical role of stoichiometry control in realizing strain-induced ferroelectricity.