Observation of Ferroelectric Critical Thickness of BaTiO3 Thin Film by Cs-corrected STEM

Abstract

BaTiO3 is well known for its ferroelectricity. However, its ferroelectricity can be changed under various external conditions. Particularly, the critical thickness that retains ferroelectricity have attracted continuous attention. Thus, it is very important to reveal critical elements that determine the ferroelectricity of BaTiO3 in thin film structure to apply BaTiO3 for electronic devices. In this study, we investigate the possibility of reducing the critical thickness of BaTIO3 of 3.5 unit cell by introducing dielectric SrTiO3 layers, for which, 1unit cell and 10unit cell of SrTiO3 on 2.5unit cell BaTiO3 between SrRuO3 electrodes are compared. Using high-resolution HAADF-STEM imaging, we measured displacements of cations to examine the existence of ferroelectricity for each samples. We extracted atomic peak positions from HAADF-STEM images and calculated ionic displacements of B cations (Ti, Ru), δA-B. From this δA-B, we found that there is ferroelectricity in 1unit cell SrTiO3 sample, however, ferroelectricity disappears 10 unit cell SrTiO3 sample. Ferroelectricity in 1 unit cell SrTiO3 sample was also measured in piezoresponce force microscopy. This difference could be originated from theoretically expected, polarization penetration into electrodes. This result demonstrates that the thickness of the dielectric SrTiO3 layer on BaTiO3 can affect the critical thickness of BaTiO3 ferroelectricity.