  Thickness-dependence of coercive field (EC) was investigated in ultrathin
BaTiO3 capacitors with thicknesses (d) between 30 and 5 nm. The EC appears
nearly independent of d below 15 nm, and decreases slowly as d increases above
15 nm. This behavior cannot be explained by extrinsic effects, such as
interfacial passive layers or strain relaxation, nor by homogeneous domain
models. Based on domain nuclei formation model, the observed EC behavior is
explainable via a quantitative level. A crossover of domain shape from a
half-prolate spheroid to a cylinder is also suggested at d~ 15 nm, exhibiting
good agreement with experimental results.
