Theoretical investigation of polarization-compensated II-IV/I-V perovskite superlattices

Recent work suggested that head-to-head and tail-to-tail domain walls could be induced to form in ferroelectric superlattices by introducing compensating “delta doping” layers via chemical substitution in specified atomic planes [Phys. Rev. B 73, 020103(R) (2006)]. Here we investigate a variation in this approach in which superlattices are formed of alternately stacked groups of II-IV and I-V perovskite layers, and the “polar discontinuity” at the II-IV/I-V interface effectively provides the delta-doping layer. Using first-principles calculations on ${\text{SrTiO}}_3/{\text{KNbO}}_3$ as a model system, we show that this strategy allows for the growth of a superlattice with stable polarized regions and large polarization discontinuities at the internal interfaces. We also generalize a Wannier-based definition of layer polarizations in perovskite superlattices [Phys. Rev. Lett. 97, 107602 (2006)] to the case in which some (e.g., KO or ${\text{NbO}}_2$) layers are non-neutral and apply this method to quantify the local variations in polarization in the proposed ${\text{SrTiO}}_3/{\text{KNbO}}_3$ superlattice system.

Figure 1

(Color online) Sketch of a possible II-IV $\left({\text{SrTiO}}_3\right)/\text{I-V}$ $\left({\text{KNbO}}_3\right)$ superlattice grown along [001].

Figure 2

(Color online) Two possible II-IV/I-V superlattice arrangements yielding compensating heterointerfaces and stabilized ferroelectric discontinuities. (a) Both materials are ferroelectric, with antiparallel polarizations along the growth direction. (b) One material is paraelectric and the other ferroelectric. Green (nonbold) and red (bold) interface charges denote polar-discontinuity and polarization-induced charges, respectively.

Figure 3

Calculated electronic density of states for a supercell with $n=2$ (two ${\text{SrTiO}}_3$ layers and two ${\text{KNbO}}_3$ layers). Inset shows the gradual reduction in band gap for $n{\text{SrTiO}}_3/n{\text{KNbO}}_3$ supercells as $n$ increases.

Figure 4

(Color online) Layer-by-layer local polarization profile of a supercell with $n=4$ (four ${\text{SrTiO}}_3$ units and four ${\text{KNbO}}_3$ units), with corresponding results for $n=2$ also indicated near each of the interfaces.