Defect Induced Memristive Switching in Off-Stoichiometric SrTiO₃ and CaTiO₃ studied by In Situ Transmission Electron Mircroscopy

In the quest for neuromorphic computing systems that emulate the intricacies of the human brain, the transition of digital memory to an analog state is paramount. A central scientific question revolves around directing materials to incorporate synaptic plasticity. Among the most promising and technologically advanced strategies for achieving this are resistive random access memory (ReRAM) devices. Traditional ReRAMs operate through the stochastic formation and breakage of conductive filaments within an insulator storage medium, making control challenging. This paper presents a novel approach to memristive devices, focusing on single crystalline SrTiO3 and CaTiO3 thin films. By deliberately introducing an A cation deficiency of up to about 16% through metal organic vapor phase epitaxy (MOVPE), we have successfully realized resistive switching without the need for filament formation, achieving on/off ratios as high as 10³. Our investigation integrates various techniques, including electrical measurements, transmission electron microscopy (TEM), and in-situ X-ray studies at a synchrotron, to provide insights into the underlying mechanisms.<br/><br/>Our results suggest that the resistive switching phenomenon in off-stoichiometric films can be attributed to trap-assisted tunneling through Ti antisite defects, which induce a switchable polarization. Crucial parameters such as on/off ratio and retention time depend on the extent of off-stoichiometry. This study presents a comprehensive TEM analysis of these materials, including high-resolution scanning transmission electron microscopy (S-TEM), electron energy loss spectroscopy (EELS), and dynamic in-situ TEM measurements with electrical bias and heating. Our results show that approximately 50% of the V_sr sites are occupied by Ti and that these antisite defects are responsible for inducing ferroelectric polarization. Differential phase contrast measurements reveal the polarization of these domains. Preliminary in-situ TEM experiments confirm the resistive switching behavior observed in ex-situ electrical measurements. Furthermore, our in-situ studies suggest that these polar defects combine under bias to form nanopolar domains that are statistically distributed throughout the film. This is visually confirmed by contrast inversion in dark-field images using forbidden reflections, consistent with macroscopic observations in synchrotron experiments. We attribute this contrast inversion to the alignment of off-centered polar defects induced by the applied electric field. Reducing the voltage to 0 V results in a stable state, albeit with the polarization erased, returning the local film to a high-resistance state. These preliminary in-situ I-V measurements confirm our recent findings that resistive switching in Sr-deficient SrTiO3 thin films occurs at significantly lower voltages than those required for filament formation, offering promising prospects for future neuromorphic computing applications.