Ferroelectric properties of compressively strained epitaxial SrTiO3 films on sapphire

Magnetron sputtered and laser deposited SrTiO3 thin films are deposited on CeO2 buffered sapphire substrates. Their structural and ferroelectric properties are analyzed and possible (mutual) correlations between these properties are investigated. It is shown, that the biaxial compressive strain imposed by the substrate on the ferroelectric films leads to a considerable increase of the Curie temperature, as well as the dielectric constant and the tenability of these films in technically relevant temperature regimes. Generally, the dielectric constant and the tuning decreases with increasing strain. However, the ferroelectric phase transition of the SrTiO3 films is shifted to higher temperatures compared to that of single crystalline SrTiO3. As a consequence, the dielectric constant of the films is larger than that of undistorted SrTiO3 single crystals for small strain (Δa/a < 0.005) and temperatures above the Curie temperature. Furthermore, a linear dependence of the loss tangent and the tunability on the dielectric constant is observed, which indicates, that all three properties are affected by the same mechanism that itself is affected by the lattice strain.