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First-principles study of divacancy defects in ferroelectric lead titanate and their role in the up-down asymmetry of thin films
Thursday December 17 12:00, IFAE: OSCAR PAZ, Department of Physics and Astronomy Rutgers, The State University of New Jersey
The ability to produce thin ferroelectric perovskite films with a high degree of uniformity has allowed the design of ferroelectric based memory devices. These non-volatile memories offer an overall increase in speed, reliability, cycling lifetime and data density. Unfortunately, the presence of defects, even at low concentrations, modify some intrinsic properties of the material, such as the bulk electric polarization, and may significantly reduce the long-term performance. Particularly, a behavior known as "imprint", a history-dependent up-down asymmetry in the polarization that ultimately reduce the data lifetime, has been experimentally observed.
In lead titanate (PTO), Pb-O vacancy pairs are neutral defects, with a local electric dipole
moment that can couple to the bulk polarization. Here we analyze the presence of these local dipolar defects in bulk PTO as a possible mechanism for the up-down asymmetry in the polarization of thin films. By means of first-principles electronic-structure calculations, we investigate the energetic, structural and dipolar properties of 1st, 2nd and 3rd-neighbor vacancy pairs in the tetragonal ferroelectric phase. Our most important result is that the lowest energy defect is a 2nd-neighbor vacancy pair oriented so as to break the bulk symmetry between the up and down polarization states. Berry-phase polarization calculations reveal that the total polarization is only slightly modified for vacancy pairs whose dipoles are partially aligned with the host polarization, but is significantly suppressed by antialigned vacancy pairs. These results are consistent with models in which a population of predominantly up-oriented vacancy pair defects, generated during growth ("bias") or resulting from a thermal history of domain orientation ("imprint"), can lead to an up-down asymmetry and consequent lack of retention in thin lead titanate films.