Study On Resistive Switching Properties Of Bi-based Multiferroic Oxide Epitaxial Films

Recently,the demands of memory and processor performance have become increasingly higer with the rapid development of big data driven Artificial Intelligence Era.While Moore’s Law is slowing down,the challenges faced in the information technology industry are increasingly serious.Driven by the future market demand,we urgently need to find new materials to development memory,and comprehensively improve the characteristics of memory.And the emergence of brain-like neural network computing based on artificial memristor is also expected to break down the barriers of Moore’s Law,providing a feasible way for high-efficiency,low-power data processing.As environment-friendly lead-free materials,Bi-based multiferroic perovskite oxides not only have excellent properties in terms of ferroelectricity,ferromagnetism,etc.,but also exhibit rich physical characteristics and novel experimental phenomena under external fields such as stress and light.So it has a huge application prospect in the research of the future generations of nonvolatile memory and memristor.BiFeO₃（BFO）as one of the few multiferroic materials with stable ferroelectricity and magnetic properties at room temperature has attracted great interest from researchers.BiMnO₃（BMO）is the only ferromagnetic insulator in the transition metal perovskite oxide and the research on structural diversity and ferroelectricity has long been concerned by researchers.In this article,we have investigated the physical characteristics based on BFO and BMO,and discover its potential applications in nonvolatile memories and memristors.Our researches mainly include the following three parts:1.Study on preparation and physical properties of BMO epitaxial filmsThe existence of ferroelectricity in BMO still remains controversial over the past several years.One of important reasons is that the pure monoclinic BMO epitaxial thin films cannot be obtained and is only present with the pseudo-cubic phase in the thick samples.Therefore,it is impossible to precisely and systematically reveal the correlation between the crystal structure evolution and the ferroelectricity of strained BMO films.Here,we provide a controllable route to stabilize the different structure of BMO thin films and demonstrate the structure dependence of ferroelectricity in high quality BMO epitaxial films.In the current work,SrRuO₃（SRO）as buffer layer can effectively reduce the lattice mismatch between BMO and STO,and we observed a structural transition from pseudo-cubic to monoclinic structure with increasing the thickness of BMO films by high resolution TEM measurement.The PFM measurements in combination with DFT calculations show that the pseudo-cubic BMO thin film is indeed ferroelectric at room temperature,while the monoclinic BMO has no or very weak ferroelectricity.This study would well explain the controversial issue on the ferroelectricity in BMO thin films,providing a controllable route to stabilize the ferroelectricity in epitaxial BMO thin films,which is essential for magnetoelectric coupling in this compound.Moreover,the strong ferromagnetic properties of the BMO films were also investigated.In the study of resistive switching properties,the memristive properties of BMO films induced by oxygen vacancies were discovered,which provide a new direction for the transition metal perovskite oxides as brain-like artificial synapses.2.Mimicking synaptic functions based on BFO ferroelectric memristorAffected by material properties,most memristors cannot guarantee the synaptic devices function in extremely low or high temperatures,which limits their applications in harsh environments.Due to the robust ferroelectricity of BFO at high temperatures（>500℃）,the BFO ferroelectric memristors have excellent resistive switching characterization and successfully mimic the key features of biological synapses,with long-term potentiation,depression,consecutive potentiation and depression and spike-timing dependent plasticity learning rule in a wide range of temperature from-170 to 300°C.The correlation between electronic transport and ferroelectric properties is established by the coincidence of resistance and ferroelectricity switch and the direct visualization of local current and domain distributions.The resistive switching characterization is attributable to the interfacial barrier modulated by the ferroelectric reversal of the BFO epitaxial films.Our research provides a feasible solution for artificial memristors to simulate synaptic function under extreme temperature conditions.3.Study on resistive switching properties by light in BFO heterstructure deviceIn ITO/BFO/SRO devices,we observe nonvolatile resistive switching characteristics of the negative photoconductivity effect.In contrast to conventional devices where optical excitations typically enhance conduction,the prepared device exhibits a pronounced decrease in conductivity after laser illumination at wavelengths of 405,532,and 1064 nm.Also,the negative optoelectronic memory could be reset using optical stimuli and set using an electrical pulse.Based on annealing treatments in different oxygen pressure,the fits for the conduction mechanism and the capacitance measurements,we confirm that the resistance switching behavior originates from the electron detrapping/retrapping processes of oxygen vacancies in ITO/BFO interface,which effectively modulates the interfacial potential profile.This BFO optoelectronic memory has the unique advantages such as“optically-erase/electrically-write”reversible operation,broadband optical absorption,giant switching ratio（>10⁴）and non-volatile information storage,which shows good application prospects in multifunctional electronic devices,including nonvolatile memory and neuromorphic visual systems.