A. Pignolet et al., Orientation dependence of ferroelectricity in pulsed-laser-deposited epitaxial bismuth-layered perovskite thin films, APPL PHYS A, 70(3), 2000, pp. 283-291
Thin films of bismuth-layered perovskites such as SrBi2Ta2O9, Bi4Ti3O12, an
d BaBi4Ti4O15.With preferred orientations were grown by pulsed laser deposi
tion 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 m
icroscopy, and scanning and transmission electron microscopy showed that th
e films consisted of both c-axis-oriented regions and mixed (110)-, (100)-,
and (001)-oriented regions. The regions with mixed orientation featured re
ctangular 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 w
ell as microscopic measurements of the ferroelectric properties of regions
with pure c-orientation and of regions with mixed orientations showed a cle
ar relationship between their ferroelectric properties and their morphology
and crystallographic orientation. In the regions with mixed orientation, t
he films exhibited saturated ferroelectric hysteresis loops with well-defin
ed remnant polarisation P-r and coercive field E-c. The regions having c-ax
is orientation with a smooth surface morphology in contrast exhibited a lin
ear P - E curve with no hysteretic behaviour for SrBi2Ta2O9 and BaBi4Ti4O15
and a weak ferreoelectric behaviour for Bi4Ti3O12 This clearly showed that
the ferroelectric properties of bismuth-layered ferroelectric oxides depen
ded on the crystalline orientation of the film and that the observed ferroe
lectric hysteresis loops in SrBi2Ta2O9 and BaBi4Ti4O15 films were solely du
e 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 h
aving 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-l
ayered ferroelectric oxides for the fabrication of non-volatile FeRAM memor
ies.