Orientation dependence of ferroelectricity in pulsed-laser-deposited epitaxial bismuth-layered perovskite thin films

Thin films of bismuth-layered perovskites such as SrBi₂Ta₂O₉, Bi₄Ti₃O₁₂, and BaBi₄Ti₄O₁₅ with preferred orientations were grown by pulsed laser deposition on epitaxial conducting LaNiO₃ electrodes on single-crystalline (100) SrTiO₃ or on top of epitaxial buffer layers on (100) silicon. A morphology and structure investigation by X-ray diffraction analysis, scanning probe microscopy, and scanning and transmission electron microscopy showed that the films consisted of both c-axis-oriented regions and mixed (110)-, (100)-, and (001)-oriented regions. The regions with mixed orientation featured rectangular as well as equiaxed crystalline grains protruding out of a smooth c-oriented background. A closer examination revealed that the regions with mixed orientation actually consisted of a c-axis-oriented sublayer growing directly on the epitaxial LaNiO₃ electrode, on top of which the growth of either (110)-, (100)-, or (001)-oriented grains took place. Macroscopic as well as microscopic measurements of the ferroelectric properties of regions with pure c-orientation and of regions with mixed orientations showed a clear relationship between their ferroelectric properties and their morphology and crystallographic orientation. In the regions with mixed orientation, the films exhibited saturated ferroelectric hysteresis loops with well-defined remnant polarisation P_r and coercive field E_c. The regions having c-axis orientation with a smooth surface morphology in contrast exhibited a linear P-E curve with no hysteretic behaviour for SrBi₂Ta₂O₉ and BaBi₄Ti₄O₁₅ and a weak ferroelectric behaviour for Bi₄Ti₃O₁₂. This clearly showed that the ferroelectric properties of bismuth-layered ferroelectric oxides depended on the crystalline orientation of the film and that the observed ferroelectric hysteresis loops in SrBi₂Ta₂O₉ and BaBi₄Ti₄O₁₅ films were solely due to the (100)- and (110)-oriented grains. The size of the (110)- and (100)-oriented grains being of the order of 100 nm and spontaneous polarisation having been observed and switched in a controlled manner is a demonstration that ferroelectricity can exist in structures of submicrometer size. These results might have a technological impact due to the relevance of bismuth-layered ferroelectric oxides for the fabrication of non-volatile FeRAM memories.