Switching kinetics of memristive devices employing topotactic brownmillerite-perovskite phase transitions (MEMTOP)

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Redox-based memristive devices are highly attractive candidates for future non-volatile memories and for artificial synapses in neuromorphic circuits. The memristive behavior is usually induced by the field-driven movement of oxygen ions leading to a local nanoscale redox reaction. In thin films of certain perovskites, such as manganites, ferrites and cobaltites, this redox process results in a topotactic phase transition between a conducting perovskite (PV) phase with disordered oxygen vacancies and an insulating brownmillerite (BM) phase with ordered oxygen vacancies. In such cases, the oxygen-ion transport kinetics are expected to determine the memristive device’s performance, in terms of switching speed, retention and plasticity of artificial synapses, but at present these ion transport kinetics are poorly understood. It is only known that resistive switching in BM films strongly depends on the orientation of the BM thin film. In this project, we will combine a variety of experimental and computational techniques to elucidate the relationship between the orientation-dependent oxygen-ion transport in cobaltite and ferrite BM thin films and the switching kinetics of the corresponding memristive devices. Specifically, we will develop fabrication routes for different crystal orientations for two different deposition techniques, pulsed laser deposition (PLD) and metal-organic chemical vapour deposition (MOCVD). Such samples will be subjected, on the one hand, to in situ X-ray diffraction (XRD) and in situ Raman spectroscopy to monitor the dynamics of the topotatic phase transition, and on the other hand, to 18O isotope exchange anneals and subsequent secondary ion mass spectrometry (SIMS) or Raman spectroscopy analysis to obtain tracer diffusion coefficients of oxygen in both PV and BM thin films. Both branches of these studies will be supported by molecular dynamics (MD) simulations. In this way, we aim to link the kinetics of the topotactic phase transition to the oxygen-ion transport kinetics along different crystalline directions. The third stage consists of elucidating the switching mechanism and the kinetics of BM memristive devices with different crystalline orientations by means of different operando switching techniques. Putting all the gained knowledge on topotactic phase transitions together, we will derive design rules for memristive devices with improved switching kinetics.