Selective reaction and chemical anisotropy in epitaxial bismuth layer-structured ferroelectric thin films

Bismuth layer-structured ferroelectric thin films consisting of a stacking of pseudoperovskite and bismuth oxide blocks along the c-axis were epitaxially grown on single-crystal substrates by metalorganic chemical vapor deposition (MOCVD), and a selective reaction of the bismuth oxide layer with HCl was demonstrated. Epitaxial films with contrasting crystal orientations were used for the acid treatment in order to probe chemical anisotropy. For a/b-axis-oriented SrBi2Ta2O9, Bi4Ti3O12, and SrBi4Ti4O15 films with sequential stacking of the two vertical blocks, notable structural selectivity of the reaction was observed only for SrBi2Ta2O9. For this film, the pseudoperovskite block remained and the bismuth oxide block was removed, while both blocks of Bi4Ti3O12 and SrBi4Ti4O15 dissolved into the acid. In addition to the bismuth, the other cations in the pseudoperovskite blocks, strontium and titanium, also decreased for Bi4Ti3O12 and SrBi4Ti4O15. The selective reaction observed for a/b-axis-oriented SrBi2Ta2O9, however, was not observed for c-axis-oriented SrBi2Ta2O9 films in which the pseudoperovskite and bismuth oxide blocks were stacked horizontally. The results clearly show that SrBi2Ta2O9 has sub-nano-order structural selectivity and chemical anisotropy in the unit cell in the reaction with HCl.