Epitaxial growth and polarization reversal characteristics of hybrid improper ferroelectric Ca3Ti2O7 thin films

Layered perovskite oxides A3B2O7 with Ruddlesden–Popper structures have attracted great attention due to their intriguing hybrid improper ferroelectricity. In this work, we prepared high-quality epitaxial Ca3Ti2O7 (CTO) thin films on Nb-SrTiO3 (STO) and Pt–Si substrates by pulsed laser deposition. The epitaxial relationship between the CTO thin film and Nb-STO substrate is [010]CTO//[010]Nb-STO, while the CTO film on Pt–Si is the polycrystalline growth with the preferred orientation of (010). The piezoelectric force microscopy image is attested to the stable existence of ferroelectricity in CTO films at room temperature with different growth patterns. A resistive switching behavior with at least two orders of magnitude is observed in CTO films driven by their ferroelectric polarization reversal. Moreover, a downward self-poling phenomenon is observed in CTO films on different substrates, and the flexoelectric effect induced by the strain gradient is supposed to be the main origin.