Bismuth volatility effects on the perfection of $SrBi2Nb2O9$ and $SrBi2Ta2O9$ films

The volatility of bismuth and bismuth oxide species complicates the growth of phase-pure films of $SrBi2Nb2O9$ and $SrBi2Ta2O9.$ Films that appear phase-pure by x-ray diffraction can have microstructural defects caused by transient bismuth nonstoichiometry which have a significant impact on properties. Such defects are resolved by transmission electron microscopy. Post-growth loss of bismuth from a slowly cooled $SrBi2Ta2O9$ film resulted in the generation of a high density of out-of-phase boundaries (OPBs), which are demonstrated to be ferroelectrically inactive. In another film, the difference in the rate of desorption of bismuth oxides from $SrTiO3$ versus that from $SrBi2Nb2O9$ led to bismuth enrichment at the film–substrate interface, and the formation of an epitaxial reaction layer in an otherwise stoichiometric $SrBi2Nb2O9$ film. This different-composition layer would be expected to alter the electrical properties of the film as a whole. These results help explain the scatter in electrical data reported for similarly oriented films.