Understanding Memristive Switching in Off-Stoichiometric SrTiO₃ for Neuromorphic Applications by Advanced In Situ Transmission Electron Microscopy

For neuromorphic computing that mimics the human brain, digital memory has to turn analogue. A key scientific question is how to teach materials to adopt synaptic plasticity. One of the most promising and technologically advanced approaches for memristive devices is the resistive random-access memory (ReRAM) [1, 2]. Common ReRAMs operate by formation and breaking of conductive filaments through the insulator storage medium [3, 4]. The disadvantage of this type of component is that the filament formation process is stochastic, the formation of defects is difficult to control, and such devices suffer from non-linearity. Switching mechanisms based on formation of nanopolar domains or ferroelectric switching have the advantage to be non-destructive and add complex functionality to the device such as metaplasticity, e.g., a transition from short term plasticity (short time memory) to long term plasticity (long term memory), which is one of the key learning mechanisms in biological synapses.<br/>Recent experimental results of our group show growth by metal organic vapor phase epitaxy (MOVPE) allows to control the off-stoichiometry of SrTiO₃ keeping the structure single crystalline up to a Sr deficiency of ~16%. Samples with high off-stoichiometry show forming-less resistive switching with on/off ratios as high as 10³ without a forming step [5]. Combining electrical measurements, transmission electron microscopy (TEM) and in-situ X-ray at a synchrotron, we speculate that trap assisted tunnelling through the Ti antisite that induces a switchable polarization causes this switching. Important parameter like the on-off ratio and the retention time depend on the off-stoichiometry. In this study we present a detailed TEM study of this off-stoichiometry material combining high-resolution (S)TEM, electron energy-loss spectroscopy (EELS) and dynamic measurements using <i>in-situ</i> TEM based on electrical biasing and heating.<br/>By combining low-angle and high-angular dark field imaging STEM and EELS, and considering a variety of defect complexes (Ti_Srand V_Sr,-V_O), we conclude that roughly 50% of the V_sr is occupied by Ti forming antisite defects. It is well known that this causes in off-centre defect that induces a ferroelectric polarization. In preliminary <i>in-situ</i> TEM results applying a bias to FIB prepared thin lamella reproduce the resistive switching found in ex-situ electrical measurements of these samples. Our <i>in-situ</i> measurements further suggest these polar defects under bias couple to form nanopolar domains that are distributed statistically throughout the layer, visible by contrast inversion in dark-field images using forbidden reflections. This confirms macroscopic findings of <i>in-situ</i> measurements using synchrotron radiation. We assign this contrast inversions, to an alignment of the of centre polar defects by the applied electric field. When reducing the voltage to 0V, the system stays in a stable state, but the polarization can be erased, with the local film reversing back to a high-resistance state. These preliminary <i>in-situ</i> I-V measurements are in good agreement with <i>ex-situ</i> I-V curves and confirm our recent findings that resistive switching in Sr deficient SrTiO₃ thin films is observed at substantially lower voltages than those required to form filaments. We think that resistive switching based on trap assisted tunneling and ferroelectric switching potentially offers an added functionality that could mimic metaplasticity in the human brain. Properties of off-stoichiometric SrTiO₃ can be controlled by MOCVD growth and thus can be the properties.<br/>References:<br/>[1] Ren S <i>et al.</i> Journal of Physics: Condensed Matter <b>28</b>, 5 (2016).<br/>[2] Yang J J <i>et al.</i> Nature Nanotechnoly. <b>8</b>, 13–24 (2013).<br/>[3] Waser R <i>et al.</i> Nature Materials <b>6</b>, 833–40 (2007).<br/>[4] Terabe K <i>et al.</i> Nature <b>433</b>, 47–50 (2005).<br/>[5] Baki A <i>et al.</i> Scientific Reports <b>11</b> (2021), 7497.