| Metal oxide thin films are an important class of materials from both scientific and technological perspectives.Metal oxides have rich magnetic,optical,electrical and chemical properties.Thin films are particularly attractive owing to their relevance in devices and also for the ability to pursue structure-property relation studies using controlled microstructures.In oxide thin film materials,oxygen vacancy will change the valence state of metal ions and the energy band structure of the material,which has a significant effect on the functional properties of the oxide material.The use of electrolyte gating to electrically control electronic,magnetic and optical properties of materials has seen strong recent growth.Under an external electric field,an electric double layer(EDL)will form at the interface between the electrolyte and the film,which will modulate the properties of the material.EDL can not only modulate the carrier density in the oxide films,but also lead to a three dimensional chemical doping by voltagedriven infiltration,such as insertion or extraction of oxygen ions in oxide films.In other words,EDL can control the concentration and distribution of oxygen vacancies.Ion migration in the electrolyte is very similar to the neurotransmitter transport in synapses.So the electric double layer transistor can work as a synaptic device.The synaptic transistor can meet the function of memory computing and realize ultra-low power consumption.Compared with other synaptic devices,the synaptic transistor has advantages in linearity,symmetry and repeatability,and is becoming an important branch of neuromorphic devices.In this doctoral thesis,three research works related to oxygen vacancies in metal oxide films and EDL controlled synaptic transistors have been conducted:1.The relation between the direction of oxygen vacancy channels in SrFeO2.5 films and the orientation of SrTiO3 substrates:SrFeO2.5 thin film has a brownmillerite structure,which is a lattice structure formed by alternately stacking the oxygen tetrahedral layer and the oxygen octahedral layer.A one-dimensional ordered oxygen vacancy channels exists in this structure,which provides a pathway for ions inserting or extracting.Although the direction of oxygen vacancy channels in epitaxial SrFeO2.5 film on SrTiO3 substrate plays a key role on the ion migration rate,it has not been well studied due to the difference in symmetry between the substrate and the film.More research should be carried out to determine the direction of oxygen vacancy channels during epitaxial growth.Here,we propose a method for analyzing oxygen vacancy channels and epitaxial relation using Xray diffraction(XRD).Epitaxial SrFeO2 5 films were grown on SrTiO3(001),(111)and(011)substrates.It is found that the oxygen vacancy channels of the samples grown on SrTiO3(001)substrate have two directions,three directions on SrTiO3(111)substrate and one direction on SrTiO3(011)substrate.By analyzing XRD results of the characteristic crystal plane,we found that all the oxygen vacancy channels lie in the plane of the films.Our results provide a new idea for analyzing the epitaxy of crystals with different symmetries.2.Solid-state electrolyte gated synaptic transistor based on SrFeO2.5 film channel:Perovskite SrFeO3-δ,owing to the instability of formlal FeGLvalence in perovskites,combines very low enthalpy of formation of oxygen vacancies with unusually high diffusivity of those vacancies.Perovskite SrFeO3-δ has a high metallic conductivity,while brownmillerite SrFeO2.5 is a high resistive semiconductor.The topotactic phase transformation between SrFeO3-δ and SrFeO2.5 doesn’t change the pseudo-perovskite structure.Reversible transformation between two phases at room temperature can be achieved by electrochemical gating approaches.In addition,the EDL transistor can realize the memory response of channel conductance to the gate voltage pulse,which can be used to design the synaptic transistor.Herein,a novel three-terminal synaptic transistor is designed and constructed using a solid-state electrolyte gate and a brownmillerite SrFeO2.5 thin film channel.Inspired by biological synaptic structure,we constructed high-impedance relaxation mode pulses to achieve long-term synaptic plasticity with excellent symmetry.The essential synaptic learning functions including excitatory postsynaptic current,and short-term/long-term plasticity,are successfully mimicked in the three-terminal synaptic transistor.This synapse device achieves a wide adjusted range and multistable plasticity.demonstrating a new way to achieve key component of upcoming neuromorphic circuitry.3.Solid-state IGZO optoelectronic synapse transistor using a LaF3 gate dielectric:In recent years,inspired by emergence of optogenetics in neuroscience and visual sensing,light has been incorporated into synaptic devices.Optoelectronic synaptic devices enable the bidirectional conversion between electrical and optical signals,which are capable of combining visual sensing,signal processing,and memory.Compared with crystalline semiconductors,amorphous oxide semiconductors(AOS)are more suitable for optoelectronic synaptic devices,because persistent photoconductivity phenomenon enables these devices to mimic synaptic plasticity upon optical stimulation.Three-terminal optoelectronic synaptic devices have a unique feature of achieving linear conductance switching,high uniformity,long-term operation durability,and artificial sensory platform.Herein,an all-solid state optoelectronic synaptic transistor using indium gallium zinc oxide(IGZO)as the channel and lanthanum fluoride(LaF3)as the gate dielectric is fabricated and its photoelectric plasticity is investigated.LaF3 is a solid superionic conductor with plenty of mobile fluoride ions,which can be used to achieve a wide tunability of carrier density and synaptic plasticity of IGZO channel upon electrical stimulation.Synaptic plasticity by light pulse stimuli can also be accomplished with the assistance of the PPC effect in IGZO channel.Furthermore,synaptic plasticity induced by light stimuli can be modulated by the gate voltage.These results provide a new way to control of synaptic function and extend the function of optoelectronic synaptic devices. |
