Strain Engineering of SrTiO3

Due to their tendency to form ionic states, transition metal oxides and especially SrTiO3 exhibit extraordinary ferroelectric properties. However, they typically exhibit these extraordinary properties close to the ferroelectric phase transition temperature, which usually deviates significantly room temperature. The question arises as to whether, and how, these extraordinary properties can be utilized. It is therefore of major interest to engineer these materials to fully exploit and understand their potential, and to make them suitable or more suitable for various applications.One method of engineering the properties of these materials is to use mechanical strain. In particular, epitaxial strain, which is automatically generated in epitaxial films grown on lattice-mismatched substrates, allows crystalline oxides to be elastically strained up to percent levels. Under such strain, the properties of the transition metal oxides can be altered significantly.In this article, we review the use of epitaxial strain to modify the ferroelectric, electronic, and structural properties of SrTiO3. We discuss how the ferroelectric properties can be tuned systematically by strain. This includes the tuning of the ferroelectric transition temperature, permittivity, and the type of ferroelectricity. We demonstrate that these epitaxially strained films typically represent textbook-like relaxor ferroelectrics and are highly tunable. Furthermore, we show that even the conductance of the nominally insulating material can be modified by epitaxial strain. With adequate strain, SrTiO3 not only becomes semiconductor-like, it also exhibits an “electronic plasticity”, which is of interest for applications ranging from memristor to neuromorphic devices such as artificial synapses (e-synapses). The examples discussed demonstrate how elastic epitaxial strain represents an exciting option for engineering and fine-tuning the properties of SrTiO3 thin films