Precise tuning of the built-in bias in PbTiO$_{3}$ based superlattice thin films.

The artificial layering in ferroelectric superlattices is a potential source of polarization asymmetry that results in a built-in bias which shifts polarization hysteresis loops. Here, we studied the origin of polarization asymmetry for ferroelectric PbTiO$_{3}$ based superlattice films. Using off-axis RF sputtering, we prepared several compositions of PbTiO$_{3}$/SrTiO$_{3}$ (PTO/STO) superlattice thin films; and for comparison PbTiO$_{3}$/SrRuO$_{3}$ (PTO/SRO) superlattices, which have an additional intrinsic compositional asymmetry at the interface. From theoretical modeling and experiments such as heat treatment and repetitive electrical cycling on the samples, we were able to identify and study the impact of Pb-O vacancy defect dipoles on the built-in bias. In addition, we were able to detect the presence of vacancies at the interfaces in the superlattice with STEM-EELS. Finally, we show the ability to tune this built-in bias by depositing a hybrid superlattice that combines PTO/STO and PTO/SRO superlattices. By tuning the composition of the PTO/STO/PTO/SRO$_{\, }$superlattice, the built-in bias in this combo-superlattice can be reduced to zero.