Strain enhanced ferroelectric properties of multiferroic BiFeO3/SrTiO3 superlattice structure prepared by radio frequency magnetron sputtering

• BiFeO3/SrTiO3 (BFO/STO) superlattice structure was fabricated with rf sputtering.
• X-ray result shows the high quality of the BFO/STO superlattice structure.
• An epitaxial relation between the film and substrate was conformed.
• Strain of the BFO sublayer increase with decreasi the STO sublayer thickness.
• A strong effect of the strain state on the ferroelectric properties was observed.

Asymmetric superlattice structures consisting of SrTiO3 (STO) and multiferroic BiFeO3 (BFO) sublayers were deposited on a Nb-doped STO (001) substrate with a radio frequency magnetron sputtering system. For an investigation of the effect of strain on the microstructures, ferroelectric properties and piezoelectric properties resulting from a difference of lattice parameters of the BFO sublayer, varying the thickness of the STO sublayer in a range 1–2.5 nm introduced interfacial strain into the superlattice. The epitaxial relation and strain state of films were clearly observed with a synchrotron as a source of radiation. The results of X-ray diffraction measurements clearly showed that the decrease of strain was related to the thickness of the STO substrate. A separate diffraction signal of the superlattice structure with thickness ≧ 1.75 nm of the STO sublayer was found, indicating a large strain relaxation in these superlattice films. The remnant polarization values Pr, piezoelectric coefficient d33 and leakage properties of the BFO/STO superlattice thin films that improved with increasing in-plane strain provide strong evidence of a strain-enhancement effect. The BFO/STO superlattice with a well defined crystal structure and a large strain state showed a large Pr, greater than a BFO single layer of the same thickness.