High performance CMOS-compatible perovskite oxide memristors: compositional control and nanoscale switching characteristics
Creators: name: Nili Ahmadabadi, Hussein
Publisher: name: RMIT University
Year: 2024

description: Nanoscale memristive devices have been dubbed as one of the main contenders for the next generation nonvolatile memories (NVM) and alternative logic architectures. Passive two-terminal metal-insulator-metal (MIM) memristive crossbar configurations based on functional transition metal-oxides (e.g. TiO2, SrTiO3) offer great potential for ultimate integration in contemporary electronic industry.<br><br>This thesis focuses on the realization and nanoscale characterization of high performance CMOS-compatible memristive devices utilizing functional perovskite oxides. A PVD based synthesis route for the realization of functional perovskite oxides with control over their composition and structure has been established. Utilizing the synthesis approach, first realization of memristive devices based on oxygen deficient amorphous SrTiO3 (a-STO) oxides has been demonstrated and their resistive switching performance has been studied in detail utilizing micro-scale crossbar MIM arrays and a sophisticated conductive nano-contact technique based on in situ electrical nanoindentation.<br><br>RF magnetron sputtering has been used in this work to synthesis perovskite oxide thin films on conventional silicon substrates. Firstly, a lead-free ferro/piezoelectric perovskite oxide (KxNa1‑xNbO3) was chosen to study the effects of sputtering parameters and post-deposition treatments on the composition and the structure of sputtered thin films. This study demonstrates that the crystal orientation, thickness and the elemental composition of the thin films sputtered from the same ceramic target can be effectively and reliably controlled via tuning the sputtering parameters (process gas, substrate temperature, etc.) and the oxide structure and secondary phases can be engineered through post-annealing treatments. The same procedure was employed for the synthesis of SrTiO3 thin films as a reliable resistive switching perovskite oxide. A low temperature synthesis of amorphous SrTiO3 (a-STO) thin films with precise control over the thickness, oxygen deficiency and A‑site/B-site dopants has been demonstrated for the first time.<br><br>The switching characteristics of a-STO cross-point devices suggest the possibility of fine tuning the memristive performance through tailoring the oxide composition and device structure. Outstanding switching performance (high switching ratios, excellent endurance and retention) is demonstrated in oxygen deficient a-STO devices. Also, it is shown that niobium doping through low temperature co-sputtering of Nb: a-STO result in significant improvements in device energy requirements.<br><br>Furthermore, nanoscale conduction and resistive switching mechanisms of these devices have been studied in detail utilizing a sophisticated in situ electrical nanoindentation technique, capable of forming nano‑contacts with controlled size and mechanical force. To this end, a unique empirical model has been developed that allows for a complete characterization of the electrical properties of the load controlled nano‑contact and therefore yields quantified insights into the conduction and switching mechanisms of a‑STO based memristive device at nanoscale. The results exhibit ultimately scalable and isolatedly controllable switching characteristics in these devices and also suggest the possibility of mechanically modulated nanoscale resistive switching in a‑STO based devices.<br><br>Overall, this thesis highlights a‑STO based devices as strong candidates for the ongoing development of the alternative memory technologies as well as applications in MEMS/NEMS devices.