Depolarizing-Field Effects in Epitaxial Capacitor Heterostructures

Abstract

We identify a transient enhancement of the depolarizing field, leading to an unexpected quench of net polarization, during the growth of a prototypical metal-ferroelectric-metal epitaxial system made of BaTiO 3 and SrRuO 3 . Reduced conductivity and, hence, charge screening efficiency in the early growth stage of the SrRuO 3 top electrode promotes a breakdown of ferroelectric BaTiO 3 into domains. We demonstrate how a thermal annealing procedure can recover the single-domain state. By tracking the polarization state in situ, using optical second harmonic generation, we bring new understanding to interface-related electrostatic effects in ferroelectric capacitors.

Domain structure and dielectric properties of metal-ferroelectric superlattices with asymmetric interfaces

“…The appearance of domains is caused by the imperfect screening of the spontaneous polarization due to the finite intrinsic screening length at the metal-ferroelectric interface, and it is expected even for structurally perfect interfaces [5,45,46]. However, experimentally, the presence of domains was usually inferred indirectly from studies of polarization dynamics [47], film tetragonality [48], macroscopic piezoresponse [49,50], or second-harmonic generation [51]. Recently, depolarization-induced polarization arrangements [52] and periodic flux-closure structures [27] have been observed directly in electroded PbTiO 3 films using transmission electron microscopy (TEM), but a systematic study of these domains, their scaling with ferroelectric thickness, and their effect on the macroscopic properties is still lacking.…”

In-situ monitoring of interface proximity effects in ultrathin ferroelectrics

“…We rule out the intrinsic chemistry of the PTO termination as a possible explanation because, according to ab initio calculations 22 , both the PbO and the TiO 2 top termination favor an upwards polarization. Our tests with other ferroelectric perovskites rather point to non-stoichiometry as the likely origin because the striking polarization evolution (ii–iii) was only observed for materials with A-site volatility, i.e., PTO, Pb[Zr x Ti 1− x ]O 3 (PZT), and BiFeO 3 (BFO), in contrast to BaTiO 3 (BTO) 4 (see Supplementary Note 3 ). A-site volatility is usually compensated by A-site-excess targets in pulsed laser deposition (PLD) to recover the stoichiometry in the bulk of the films.…”

“…Finally, we test the robustness of the competitive and the cooperative polarization state by capping the PTO films with 18 u. c. of STO. STO isolates PTO from the charge-screening oxygen-rich growth atmosphere and thus enhances the depolarizing field, promoting a multidomain breakdown 4,31 . For the suppressed polarization state, this breakdown is indeed observed, as confirmed macroscopically and microscopically by the SHG and STEM measurements in Fig.…”

“…1 b). The origin of the ISHG drop could be sample degradation, polarization suppression, or domain formation 4 , 20 . We therefore conduct a post-deposition STEM analysis by mapping the atomic displacements throughout the PTO film.…”

“…For example, in ferroelectric materials, a major obstacle to stabilizing the electric polarization in ultrathin layers is uncompensated bound charge at the interfaces 1 – 4 . Approaches to combat the resulting depolarizing field focus on acting on the bound charge at either the bottom interface by introducing metallic buffer electrodes 2 , 4 and atomic-scale interface engineering 5 , or at the top interface by introducing charge-screening environments such as gases 6 , 7 or liquids 8 – 10 . Acting on both interfaces in the same experiment is hampered by the lack of separate, yet simultaneous experimental access for observing how these interfaces set the resulting polarization state of the film.…”