Abstract
We explore the thickness dependence of the converse piezoelectric coefficient (${d}_{33}$) in epitaxial thin films of ${\mathrm{BaTiO}}_{3}$ (BTO) grown on (001) ${\mathrm{SrTiO}}_{3}$ substrates. Piezoresponse force microscope was performed using an atomic force microscope equipped with an interferometric displacement sensor allowing direct quantification of electromechanical coupling coefficients in BTO free from unwanted background contributions. We find that 80-nm-thick films exhibit a ${d}_{33}$ of $\ensuremath{\sim}\phantom{\rule{0.16em}{0ex}}20.5\phantom{\rule{0.16em}{0ex}}\mathrm{pm}/\mathrm{V}$, but as the thickness is reduced, the ${d}_{33}$ reduces to less than 2 pm/V for a 10 nm film. To explain the atomistic origin of the effect, we performed molecular dynamics simulations with a recently developed ab initio-derived reactive force field, constructed using the ReaxFF framework. Simulations predict that under applied electric fields thin films of ${\mathrm{BaTiO}}_{3}$ show an increasing thickness, with compressive strain, of the region screening the depolarization-field. This study confirms quantitatively the drop in piezoelectric performance in BTO ultrathin films and again highlights the importance of the screening mechanisms when films approach the ultrathin limits in dictating the functional behaviors.
Highlights
The properties of ferroelectric films as they are scaled to the ultrathin limit has been a topic of considerable attention for decades, due to the novel physics that can arise [1,2]
Typical step and terrace morphology is observed with a surface roughness of less than 1 nm corresponding to unit cell surface structures, providing an ideal platform for surface probe microscopy measurements
Since the piezoresponse force microscopy (PFM) signal contains multiple contributions including from electrostatics and piezoelectric modulus [12], we have employed interferometric displacement sensing (IDS) to quantitatively measure the piezoresponse of the sample and eliminate unwanted signals arising from electrostatic effects and cantilever motion [26]
Summary
The properties of ferroelectric films as they are scaled to the ultrathin limit has been a topic of considerable attention for decades, due to the novel physics that can arise [1,2]. It has been theoretically predicted, for example, that BaTiO3 (BTO) thin films (below ∼30 nm) can exhibit highly peculiar phase diagrams when the electrochemical boundary conditions are properly considered, given that the surface electrochemical processes associated with screening can, at this thickness, have energies comparable with that from the bulk free energy arising from the polarization of the lattice [3,4]. One of the challenges with BTO is that thin films often suffer from high
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have