Direct Observation of Large Atomic Polar Displacements in Epitaxial Barium Titanate Thin Films

Abstract

The development of ferroelectric perovskite oxides having a controlled polarization direction is an ongoing and challenging topic of research. Here we report direct observation of large atomic polar displacements, which correspond to a polar density of ~0.9C m -2 pointing upwards, in an epitaxial BaTiO 3 (BTO) film grown by molecular beam epitaxy on a SrTiO 3 substrate. Aberration-corrected scanning transmission electron microscopy is used to map the polarization displacement with unit-cell resolution. Oxygen vacancies and other types of defects are examined and mapped using electron energy-loss near-edge structure analysis. The contributions from strain, strain gradient and defects are quantitatively modeled in order to explain the large polarization. Calculations show that strain (through a defect dipole-enhanced polarization) creates the large atomic polar displacements, and strain gradient (through inverse Vegard electrochemical strain effect) compensates the polarization. These two effects may explain the preferred polarization direction and the anomalous flexoelectric effect in ferroelectric thin films.

Full Control of Polarization in Ferroelectric Thin Films Using Growth Temperature to Modulate Defects

“…Further investigation is required to fully understand this phenomenon, which could be similar to the interface effects recently reported by other groups. [ 53,54 ] However, for the rest of the sample volume, these results are in line with a gradient of lead oxide dipolar vacancies inducing a gradient in polarization, which leads to a gradient in strain.…”

Ferroelectricity and resistive switching in BaTiO$_3$ thin films with liquid electrolyte top contact for bioelectronic devices

“…Fig. 4(b)] corresponds to P r = 0.6 mC/cm 2 of the BTO thin film -a higher value than typical polarization values of epitaxial BTO films, which are in the range below 0.1 mC/cm 2 [34]. We identified two possible causes for this observation.…”

BTO-Coupled CIGS Solar Cells with High Performances

“…Various ferroelectric materials, such as PVDF-TrFE [ 8 ], Pb(Zr,Ti)O 3 [ 11 ], BiFeO 3 , Bi 2 FeCrO 6 , BTO, LiNbO 3 [ 12 ], Be x Cd y Zn 1- x - y O, PbTiO 3 , and Pb (1- x ) La x ZrTiO 3 [ 13 ], have been applied to enhance the internal electric field of a solar cell by the combination of the ferroelectric depolarization field with the p - n junction field. Among these ferroelectric materials, BTO shows superior properties of a wide bandgap ( E g ~ 3.4 eV), room-temperature ferroelectricity ( T c ~ 120 °C), substantial remnant polarization ( P r = 0.5 C/m 2 ), environmental-friendly advantage, easy fabrication, excellent stability, and so on [ 14 , 15 , 16 , 17 , 18 , 19 , 20 ]. The internal electric field of a solar cell is expected to be significantly enhanced by the insertion of a BTO ferroelectric layer into the device.…”