Film thickness dependence of in-plane ferroelastic domain structure in constrained tetragonal PbTiO3 films induced by isotropic tensile strain.

In this study, the ferroelectric phase of PbTiO₃ (PTO) thin films was grown on cubic single-crystal KTaO₃ (KTO) substrates using metal–organic chemical vapor deposition. X-ray diffraction (XRD) was used to reveal an a₁/a₂ domain structure, which remained unchanged down to a film thickness of 2 nm. The a₁ and a₂ polydomains do not have a simple tetragonal symmetry because a_a∥ and a_a⊥ do not have the same values. The crystallographic tilt angle, α, is defined based on the rotation angle of the PTO lattice with respect to the cubic phase of KTO substrates. The in-plane tetragonal distortion (c_a∥/a_a∥) and α decreased with the decrease in the film thickness, following the in-plane tetragonal geometric equation: α = tan − 1 ( c a ∥ a a ∥ ) − 45 ° . The isotropic tensile strains induced in-plane polarization directions along the [100] and [010] axes of the substrates. These axes are formed via the a₁/a₂ polydomain of the tetragonal-like phase. Moreover, synchrotron in-plane grazing incidence XRD and piezoelectric force microscopy were used to reveal the thickness dependency of the periodic domain width of the ferroelastic a₁/a₂ domain. The periodic domain width in the PTO films decreased, following Kittel's law, with the reduction in the film thickness. [ABSTRACT FROM AUTHOR]