Abstract
Ferroelectric films may lose polarization as their thicknesses decrease to a few nanometers because of the depolarizing field that opposes the polarization therein. The depolarizing field is minimized when electrons or ions in the electrodes or the surface/interface layers screen the polarization charge or when peculiar domain configuration is formed. Here, we demonstrate ferroelectric phase transitions using thermooptical studies in ∼5-nm-thick epitaxial Pb0.5Sr0.5TiO3 films grown on different insulating substrates. By comparing theoretical modeling and experimental observations, we show that ferroelectricity is stabilized through redistribution of charge carriers (electrons or holes) inside ultrathin films. The related high-density of screening carriers is confined within a few-nanometers-thick layer in the vicinity of the insulator, thus resembling a two-dimensional carrier gas.
Highlights
Growth technology, fundamental understanding, and applications of epitaxial perovskite oxide ferroelectric films has been progressing significantly in the last decades[1,2,3,4,5,6]
The films are pseudomorphic to the substrates, experience biaxial compressive in-plane strain, and exhibit out-of-plane elongation compared to bulk PSTO
Our modeling and experimental results strongly suggest that a single-domain ferroelectric state can be stabilized in ultrathin films by redistribution of the charge carriers therein
Summary
Fundamental understanding, and applications of epitaxial perovskite oxide ferroelectric films has been progressing significantly in the last decades[1,2,3,4,5,6]. If ferroelectric polarization in an ultrathin film on a crystal substrate is oriented perpendicular to the substrate surface (in the out-of-plane direction of the film), the associated bound polarization charge creates an internal electric field, opposing polarization. This depolarizing field may lead to instability of the polarization and disappearance of the ferroelectric phase below a critical thickness[7,8,9]. We investigate thermooptical behavior and demonstrate the ferroelectric phase transitions in electrodeless epitaxial PSTO films of ∼5 nm in thickness grown on insulating substrates. Our modeling and experimental results strongly suggest that ferroelectricity is stabilized owing to redistribution of the charge carriers in ultrathin films
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