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.

Institute of Physics NAS of Ukraine 03028 Kyiv Ukraine

Institute of Physics of the Czech Academy of Sciences Na Slovance 2 18221 Prague Czech Republic

Institute of Problems for Material Sciences NAS of Ukraine 03028 Kyiv Ukraine

Microelectronics Research Unit Faculty of Information Technology and Electrical Engineering University of Oulu P O Box 4500 FI 90014 Oulu Finland

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