Abstract
The objective of the study is to investigate effects of strain state, ion doping, and bottom electrode on BiFeO3 (BFO)/SrTiO3 (STO) superlattice structure. The BFO/STO and La-doped BFO/STO superlattice were deposited by radio frequency magnetron sputtering with optimized coating conditions. The epitaxial growth of superlattice films was confirmed by X-ray reflection and diffraction patterns. The BFO/STO superlattice with a well defined crystal structure, large strain state ~1.3% which approaches the critical value of 1.42% and large remnant polarization value at deposition temperature 600-700oC. The measurements of Pr values, piezoelectric coefficient d33, and the leakage current density of superlattice films demonstrate that ferroelectric properties of BFO/STO superlattice thin films could be enhanced with increasing in-plane strain. The BFO/STO superlattice with small BFO/STO sublayer thickness exhibits a pseudo-cubic structure without strain relaxation. Similar with tetragonal structure, superlattice films with pseudo-cubic structure exhibit a clear strain improvement on ferroelectric properties. The substitution of La (5 atomic %) in BFO sublayer can improve the leakage property of superlattice films. The La-doped BFO/STO superlattice films exhibited excellent and saturated hysteresis loops, larger piezoelectric coefficient (d33) on LaNiO3 (LNO) electrode than films on Nb-doped doped STO substrate. High leakage property of La-doped BFO sublayer and suitable resistivity of LNO bottom electrode are the major factor of enhancement of ferroelectric properties of superlattice films.