Theoretical Approach to Electroresistance in Ferroelectric Tunnel Junctions

TL;DR

This paper presents a comprehensive theoretical framework combining Green's functions and polarization dynamics to model and explain tunneling electroresistance in ferroelectric junctions, addressing experimental controversies and device implications.

Contribution

It introduces a unified theoretical approach that accurately describes TER in various ferroelectric tunnel junctions and explains interface effects influencing experimental results.

Findings

Successfully models current-voltage characteristics of inorganic and organic ferroelectric junctions.

Addresses the sign controversy of TER by considering interface termination effects.

Explores the impact of CoOₓ buffer layers on memristor behavior and interface properties.

Abstract

In this paper, a theoretical approach, comprising the non-equilibrium Green's function method for electronic transport and Landau-Khalatnikov equation for electric polarization dynamics, is presented to describe polarization-dependent tunneling electroresistance (TER) in ferroelectric tunnel junctions. Using appropriate contact, interface, and ferroelectric parameters, measured current-voltage characteristic curves in both inorganic (Co/BaTiO$_{3}$/La$_{0.67}$Sr$_{0.33}$MnO$_{3}$) and organic (Au/PVDF/W) ferroelectric tunnel junctions can be well described by the proposed approach. Furthermore, under this theoretical framework, the controversy of opposite TER signs observed experimentally by different groups in Co/BaTiO$_{3}$/La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ systems is addressed by considering the interface termination effects using the effective contact ratio, defined through the effective screening length and dielectric response at the metal/ferroelectric interfaces. Finally, our approach is extended to investigate the role of a CoO$_{x}$ buffer layer at the Co/BaTiO$_{3}$ interface in a ferroelectric tunnel memristor. It is shown that, to have a significant memristor behavior, not only the interface oxygen vacancies but also the CoO$_{x}$ layer thickness may vary with the applied bias.