Tunneling in Ferroelectric Heterostructures: A Tale of Two 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 electrode/ferroelectric interface in an ll oxide heterostructure.<br/>The first part of the talk discusses our recent 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. The resonant current amplitude and voltage are tunable by the switchable ferroelectric polarization of the ultrathin BaTiO₃ layers with the NDR ratio modulated by ~3 orders of magnitude and an OFF/ON resistance ratio exceeding a factor of 20 000. An average tunnel lifetime of 0.47 fs on the resonant states is obtained, which is orders of magnitude smaller than those obtained in semiconductor QWs. This work appears in Ma et al , Adv. Mater. 2022, 34, 2205359.<br/>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/BaTiO₃interface in Pt/BaTiO₃/Nb:SrTiO₃ ferroelectric tunnel junctions (FTJs), benefitting from the double interface effect. That is, both the BaTiO₃/Nb:SrTiO₃ and La_0.8Ca_0.2MnO₃/BTO interfaces are synchronously metal or insulator phases in response to the ferroelectric polarization switching, thus amplifying the difference in resistance between the high resistance state (OFF state) and low resistance state (ON state). It is found that a thicker La_0.8Ca_0.2MnO₃ layer leads to the decrease of the TER. This is attributed to the decreased barrier height/width at the BaTiO₃/Nb:SrTiO₃ interface as revealed by analysis of the electron transport mechanism. We show it occurs by Fowler–Nordheim (FN) tunneling. Our reports provide a pathway, namely interface control of the oxide heterostructure, to achieve high- performance ferroelectric tunneling devices for future oxide electronics at the nanoscale.<br/>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.