Investigation of the optical properties of a BiFeO$_3$/SrTiO$_3$ heterostructure grown on an Al$_2$O$_3$(0001) substrate by RF cathode sputtering

The phase composition, structure, and optical properties of the BiFeO$_3$/SrTiO$_3$/Al$_2$O$_3$ ($c$-cut) heterostructure have been studied using XRD analysis, spectrophotometry, and multi-angle ellipsometry. BFO/STO/Al$_2$O$_3$ heterostructures have been obtained by high-frequency cathode sputtering in an oxygen atmosphere using the intermittent deposition technology. It was found that the BiFeO$_3$ and SrTiO$_3$ layers grew with an orientation in the direction of the [111] crystallographic axis parallel to the normal to the Al$_2$O$_3$ substrate. It has been shown that the damaged layer on the surface of the heterostructure does not exceed 2–3 nm, and no signs of the presence of boundary layers at the Al$_2$O$_3$–SrTiO$_3$ and SrTiO$_3$–BiFeO$_3$ interfaces have been identified. The dispersion dependences of the refractive indices of BFO and STO layers are calculated. The reasons for the revealed regularities are discussed. Keywords: thin films, multiferroic, optical properties, ellipsometry, bismuth ferrite.