Study Of The Resistive Switching And Photoresponse Characteristics Of Nb:SrTiO₃-based Heterostructures

At present,in the big data era,the demands for high-speed,high-density,low-power-consumption,non-volatile memories have become extremely urgent.RS memories based on resistive switching（RS）effects have attracted extensive research interests in both the academia and the industry due to their simple structure,fast operating speed,more operating times,high storage density,low power consumption,multi-level storage and three-dimensional storage potential and good compatibility with traditional CMOS technologies.Many oxides demonstrate excellent RS performances,and exhist various peculiar physical properties,such as magnetism,ferroelectricity,piezoelectricity,superconductivity and so on.In this dissertation,RS and photoresponse characteristics of the devices are studied to explore the RS mechanisms and their possible applications in multi-functional optoelectronic devices.The main contents and results are as follows:1.The RS,light response and C-V characteristics of Pt/Nb:SrTiO₃（NSTO）Schottky junctions are studied.The devices have good RS,endurance and retention performances.The HRS/LRS ratio is up to 1.1×10⁶,and the IRS/LRS ratio is 160.It is expected to be applied in multi-level data storage.When illuminating by purple（λ=405 nm）and red（λ=650 nm）lights,it was found that the device has obvious light response,and which is stronger under purple light illumination.At the same time,the two kinds of lights have obvious modulation on the HRS.Under the purple and red light illumination,the ON/OFF ratios of the HRS are 90 and 5,respectively.C-V characteristics imply that there exists an interfacial insulation layer at the interface of Pt/NSTO Schottky junctions.Because of the non-uniform spatial distribution of the interface barrier,the interface layer at the lower-barrier area locally trapping/de-trapping electrons under the applied voltage,which leads to the decrease of local interface barrier and the RS effects of the device.The photocurrents of the device do not change with the resistances,and the variation of the open-circuit voltage depends on the resistance states of the device.This voltage-and light-modulated RS characteristics are expected to be applied to multi-functional photoelectric devices.2.Pt/NSTO devices with different Nb doping concentrations were fabricated by DC magnetron sputtering.The I-V measurements show that the window of I-V loops increase gradually with the increase of Nb doping concentration.Only devices with0.1%and 0.7%doping concentration exhibit obvious RS effects.This is due to the decrease of the width of depletion layer in NSTO with the increase of Nb doping concentration which is confirmed by capacitance tests.At the same time,the decrease of depletion layer width will lead to the decrease of light response intensity.The photocurrents of the RS devices are the same in HRS and LRS,which indicates that the RS effects of the devices are caused by local barrier changes in the interface.3.High quality CeO₂ films on NSTO substrates were prepared by pulsed laser deposition（PLD）technique to form Pt/CeO₂/NSTO heterostructures.XPS measurements confirmed the existence of oxygen vacancies in as-prepared CeO₂films.It was found that the devices have excellent RS performances with HRS/LRS ratio up to 3.2×10⁴ and good retention properties.By applying different voltage pulses,two intermediate resistive states can be produced in the device,which demonstrates the potential application in multilevel memories.When illuminating by light with a wavelength of 405 nm,the device has obvious photoresponse which exhibits significant switching characteristics at HRS with a ON/OFF ratio of 7.Both light-controlled RS and voltage-controlled photoresponse are demonstrated in this device.Such RS and photoresponse characteristics are attributed to the variation of the Schottky barrier in Pt/CeO₂ interface and the trapping/detrapping of electrons by oxygen vacancies near the interface.These devices show potential applications in multilevel RS memories and multifunctional photoelectric devices.4.High quality CoFe₂O₄ films on NSTO substrates were prepared by PLD technique to form Pt/CFO/NSTO heterostructures.It was found that the devices have excellent RS performances with HRS/LRS ratio up to 3.8×10⁴ and good retention properties.By applying different voltage pulses,two intermediate resistive states can be produced in the device,which demonstrates the potential application in multilevel memories.When illuminating by light of 405 nm wavelength,the devices have obvious photoresponse which exhibits significant switching characteristics at HRS and IRS2 with ON/OFF ratios of 480 and 5,respectively.By applying different voltage pulses,the resistance,photoresponse and magnetic properties of the device can be simultaneously modulated.This can be attributed to the variation of the Schottky barrier in the CFO/NSTO interface and the trapping/detrapping of electrons by oxygen vacancies near the interface.These devices show potential applications in multilevel RS memories and multifunctional magneto-photoelectronic devices.