Ferroelectric fatigue in layered perovskites from self-energy corrected density functional theory
We employed self-energy corrected density functional theory (GGA-1/2) to investigate the band alignment between platinum and the layered perovskite Aurivillius ferroelectrics SrBi2Ta2O9 (SBT), Bi4Ti3O12 (BIT), and La-substituted BIT (BLT). The original GGA-1/2 method was found to not give satisfactory band gaps for these layered materials, despite yielding substantially better band gaps than GGA. We show that in such layered materials the cutoff radius for the self-energy potential in GGA-1/2 is strongly inhomogeneous across layers, therefore requiring different cutoff radii assigned to oxygen anions located in bismuth oxide layers and in pseudo-perovskite layers. After a 2D optimization of the oxygen cutoff radii, the calculated band gaps for these materials were found within 0.3 eV of experimental values. Next, we developed stoichiometric interface models for Pt/SBT and Pt/BIT, assuming platinum was connected to bismuth oxide layers as suggested by experiments. The calculated Schottky (hole) barriers for abrupt interfaces are 1.58 eV (2.50 eV) and 2.06 eV (1.41 eV) for SBT and BIT, respectively. For Pt(Bi) alloyed interfaces, where according to experiments some Bi diffuses inside the metal, we calculated a downshift of the Fermi level, lowering the barrier for holes. Because the barrier height for holes is lower in Pt/BIT than in Pt/SBT, a higher space-charge-limited-conduction hole current is expected to leak through metal alloyed Pt(Bi)/BIT/Pt(Bi) capacitors than to the corresponding SBT capacitors. Finally, replacing some Bi in the pseudo-perovskite layers with La significantly increased the barrier for holes. Based on the results above, we propose a phenomenological model for ferroelectric fatigue in Pt/BIT/Pt, whereby the formation of a non-ferroelectric Bi-deficient phase is the main reason for fatigue, which is accelerated by the lowered barrier for holes promoting the accumulation of Bi vacancies. The fatigue-free nature of Pt/SBT/Pt and Pt/BLT/Pt, on the other hand, stems from large barriers for both electrons and holes, which prevent the neutralization and further accumulation of charged defects. Our conclusions are consistent with the available experimental data.