Oxygen ion migration induced polarity switchable SrFeOx memristor for high-precision handwriting recognition

This paper reports a repeatable polarity reversal behavior in Au/SrFeO_x (SFO)/SrRuO₃ (SRO) memristor. The set and reset voltage direction can be inverted by applying a suitable reverse bias voltage, and the SFO device always exhibits fine cycling stability and retention performance regardless of operation polarity. The cross-sectional transmission electron microscope (TEM) images of the SFO memristor on positive and negative set statuses display apparent perovskite SrFeO_3-δ (PV-SFO) conductive filaments and brownmillerite SrFeO_2.5 (BM-SFO) barrier layer, respectively. The surface oxygen atomic ratio characterized by X-ray photoelectron spectroscopy (XPS) demonstrates the oxygen ions migration behaviors during the polarity reversal processes. Moreover, the energy band structure evaluated by first-principles calculation suggests that insufficient oxygen injection into the BM-SFO barrier layer could reduce the band gap of BM-SFO and form a PV-SFO/BM-SFO₊ interface observed by TEM with the lower Schottky barrier height, facilitating Schottky emission and conduction. Furthermore, the SFO device on negative set status shows preferable handwritten digit recognition accuracy of 98.81 % and faster convergence speed than positive set status due to the better synaptic weight update linearity. Hence, the alternated polarity reversal features of the SFO memristor may provide a new reference for resistive memory and synaptic memristors.