Electroforming-Free Threshold Switching in Polycrystalline ErMnO₃ Films Based Two-Terminal Devices

Rare-earth hexagonal manganites h-<i>R</i>MnO₃ (<i>R</i>=Y, Er, Ho to Lu) have been extensively studied as multiferroic materials in the last 15 years. The study of non-volatile unipolar resistive switching behavior in polycrystalline YMnO₃ was reported, indicating that h-<i>R</i>MnO₃ may be promising candidates for memristive devices owing to their intriguing ferroelectric domain pattern - vortex lines where six-fold domains merge [1], [2].<br/>In this work, we report the first demonstration of volatile resistive switching behavior observed in polycrystalline ErMnO₃ thin films. The devices exhibit a repeatable unipolar threshold switching behavior. To fabricate the devices, 60 nm-thick ErMnO₃ thin films were deposited on Pt-coated Si wafers using RF sputtering. Subsequently, a post-deposition annealing process was carried out at 750 °C in N₂, resulting in the formation of polycrystalline ErMnO₃ thin films with mixed orthorhombic and hexagonal crystalline phases. Further engineering was realized through a subsequent thermal treatment at 400 °C in air. Top Pt electrodes were then patterned. The two-terminal Pt/ErMnO₃/Pt devices exhibit high endurance (&gt;10⁴ cycles) and low variability. To gain a clear understanding of the physical origin of the threshold switching and to open a path for potential applications, a detailed structural study (X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy) and extensive electrical characterization (current-voltage and voltage-current measurements including as a function of temperature, conductive atomic force microscopy) were performed. We propose a physical model to explain the observed threshold switching effect and current controlled negative-differential resistance behavior. Our model involves trap-assisted and thermally-activated Poole-Frenkel conduction, along with local Joule heating effects. We will discuss how the engineering of the microstructure, specifically the inclusions of minor orthorhombic phase within the hexagonal phase, plays a crucial role in achieving the threshold switching behavior. These devices hold great promise as candidates for building spiking neurons in leaky integrate-and-fire (LIF) neuron circuits [3].<br/><br/><br/><b>References</b><br/>[1] H. Schmidt, Appl. Phys. Lett. 118, 140502 (2021), doi: 10.1063/5.0032988.<br/>[2] V. R. Rayapati <i>et al.</i>, J. Appl. Phys. 126 (2019), doi:10.1063/1.5094748.<br/>[3] Y.Ding,<i> et al.,</i> Front. Neurosci. 1732 (2022), doi: 10.3389/fnins.2021.786694.