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Modeling ferroelectric interfaces using DFT methods.

N. Plugaru, National Institute for R&D in Microtechnologies (IMT-Bucharest)

Ultrathin (< 10 nm) ferroelectric layers are key ingredients for applications in high-density ultra-low power devices, e.g. multiple-states memories, ferroelectric tunnel junctions (FTJs), synaptic interconnects for neuromorphic computing or electro-optical sensors. During the last two decades, theory assisted computational modeling has become ubiquitous in materials research, showing their usefulness in speeding up the process of materials discovery and industrial-oriented research. Density functional theory (DFT) calculations are the most widely used method in materials modeling for two main reasons. First, this method may provide detailed information at atomic scale on the quantum physics and chemistry in practically any low dimensional system. Second, integrated into high throughput calculations platforms, such as AiiDA (https://www.aiida.net/), it is a convenient method to generate training data sets for machine learning technologies toward the prediction of material properties.
Here, recent results by first principles calculations on the structural, electronic and transport properties of several ferroelectric interfaces will be presented, and the main handles to control their functional properties will be highlighted.
First, results of DFT+NEGFs calculations performed on a magnetoelectric SrRuO₃/BaTiO₃/m-SrTiO₃/SrRuO₃ FTJ will be discussed [1]. We show that:
i) the sign of the spin polarized transmission can be changed between negative and positive values by varying the interlayer thickness, ii) Large values of the TER and TMR coefficients, characterising the tunnel electroresistance and tunnel magnetoresistance effects, respectively, can be achieved simultaneously, and iii) the interfacial magnetoelectric coupling is not strong enough to allow the polarization control of the tunnel magnetoresistance, in spite of robust Ru 4d band ferromagnetism.
Then, we focus on SrRuO₃/PbTiO₃/SrRuO₃ capacitors and discuss the effects of ferroelectric size, interface asymmetry and polarization on the ferroelectric instability and interface-specific properties, including local structure, potential and charge density distributions, interface dipole and band alignment [2].
Next, the effect of oxygen vacancies (VOs) located at the electrode-ferroelectric interfaces in La_0.7Sr_0.3MnO₃/BaTiO₃/BaSnO₃ (001) layers on polarization, microscopic dipole, band offsets and conductivity will be discussed [3]. Also, the VOs influence on the local magnetic moments at the interfacial layers, and emergent "weak polarity" of the otherwise formally neutral BaO and TiO₂ layers due to the mixed ionic and covalent bonds character will be examined.
Finally, following the idea that a built-in electric field in a photovoltaic device may be advantageous for the mechanism of charge separation and charge transfer [4], we will present very recent results on the effect of interfacial symmetry on interface dipole and band alignment in BaSnO₃/BaTiO₃/CH₃NH₃PbI₃ (001) interfaces [5]. References

[1] "First principles electron transport in magnetoelectric SrRuO₃/BaTiO₃/SrTiO₃/SrRuO₃ interfaces", N. Filipoiu, N. Plugaru, T. Sandu, R. Plugaru, C. Tibeica and G.A. Nemnes. Submitted to Nanotechnology.
[2] "Designing functional ferroelectric interfaces from first-principles: Dipoles and band bending at oxide heterojunctions", D. Rusu, L. Filip, L. Pintilie, K. T. Butler and N. Plugaru, New J. Phys. 21, 113005 (2019).
[3] "Structural and electronic effects of oxygen vacancies in La_0.7Sr_0.3MnO₃/BaTiO₃/BaSnO₃ interfaces: a DFT study", N. Plugaru, R. Plugaru, N. Filipoiu and T. Sandu, Work in progress.
[4] "Atomistic Simulations of Methylammonium Lead Halide Layers on PbTiO₃ (001) Surfaces", N. Plugaru, G. A. Nemnes, L. Filip, I. Pintilie, L. Pintilie, K. T. Butler, and A. Manolescu, J. Phys. Chem.C, 121, 9096-9109 (2017).
[5] "Effect of interfacial symmetry on interface dipole and band alignment in BaSnO₃/BaTiO₃/CH₃NH₃PbI₃ interfaces", N. Plugaru, K. T. Butler and R. Plugaru, Work in progress.