Tunneling in Ferroelectric Heterostructures—Making it Better with Hybrid Interfaces

The various demonstrations of robust ferroelectricity down to a few unit cells at room temperature has triggered the quantum era of ferroelectrics. Nearly two decades ago the concept of a ferroelectric tunnel junction (FTJ) was brought to the fore leading to a flurry of activity in the field of polarization control of tunneling current and related tunneling electro-resistance (TER) effects studies. In this presentation we cover two types of new ferroelectric devices based on the tunneling effect, both founded on the control of interface design in an all oxide heterostructure.

The first part of the talk starts with our demonstration of a ferroelectric resonant tunneling diode (RTD) , which exploits the switchable electric polarization state of the quantum-well (QW) barrier to tune the device resistance at room temperature. We show robust room-temperature ferroelectric-modulated resonant tunneling and negative differential resistance (NDR) behaviors in all-perovskite-oxide BaTiO₃/SrRuO₃/BaTiO₃ RTDs. This work appears in Ma et al , Adv. Mater. 2022, 34, 2205359. We then present results extending the same concept to the popular multiferroic bismuth ferrite based QW heterostructures. Not only we observe clear signatures of NDR but there is an intriguing change of the resistance switching mechanism – from polarization controlled tunneling to Resonance effects. We hope to shed insight on these by correlating the device behavior to the polarization structure at the nanoscale under bias.

In the second study we explore how electronic phases at the interfaces can be deterministically harnessed to modulate the TER. We find a large tunneling electroresistance (TER) of 6 orders of magnitude when inserting a 0.5 nm-thick La_0.8Ca_0.2MnO₃ layer between the Pt/BaTiO₃ interface in Pt/BaTiO₃/Nb:SrTiO₃ ferroelectric tunnel junctions (FTJs), benefitting from the double interface effect. When compared to “regular” BTO/NSTO junctions we see a remarkable improvement in the TER values and show that this enhancement can be finely tuned based on the thickness of the inserted LCMO layer. Our reports thus provide a pathway, namely interface control of the oxide heterostructure, to achieve high- performance ferroelectric tunneling devices for future oxide electronics at the nanoscale. The speaker would like to acknowledge the support of an Australian Research Council (ARC) Discovery Project and Australian Research Council Centre of Excellence in Future Low-Energy Electronics Technologies (Project No. CE170100039) grant.