The Fabrication And Properties Of Resistive Switching Memory Devices Based On Perovskite BTO And BST Films

RRAM device performance is closely connected with its resistance transition layer,and now many materials have been found to have nonvolatile resistance characteristics.Among them,the perovskite structure materials with unique physical and chemical properties,such as ferromagnetism,ferroelectricity.Its has great potential in multi-functional device,and become one of the widely study materials in RRAM.Ba Ti O₃(BTO)and Ba_xSr_1-xTi O₃(BST)are typical perovskite semiconductor materials.At present,RRAM devices based on these two kinds of films have been reported,but the preparation of BTO and BST films mostly adopts magnetron sputtering,pulsed laser deposition,sol gel and other methods.The films prepared by these methods need to undergo high temperature annealing before they can be crystallized.In addition,the resistance switching characteristics of the same material films prepared by different preparation methods are often different.In comparison,the hydrothermal method can prepare crystal thin films with high purity,controllable stoichiometric ratio and good adhesion at low temperature.So in this thesis,we use hydrothermal method to prepare crystal BTO and BST films at a lower temperature(?200?),and a study was carried out to analyze the resistive switching characteristics and mechanisms of crystal BTO and BST films.The main results are as follows:Firstly,epitaxial single crystallinity BTO films are grown on Sr Ti O₃:Nb(NSTO)substrates by hydrothermal means.(1)At room temperature,the BTO film is ferroelectric with a polarization voltage of 5V.(2)Pt/BST/NSTO device exhibits typical rectification characteristics under low voltage(-1V?1V)and forming-free bipolar resistive switching under high voltage(-8V?3V).(3)Multilevel resistive switching is achieved under various reset voltages or different compliance currents.(4)The device can obtain stable negative differential resistance by prolonging the action time of voltage on the devices.Secondly,epitaxial single crystallinity BST films are grown on Sr Ti O₃:Nb(NSTO)substrates by hydrothermal means.(1)(Pt or Ag)/BST/NSTO devices were fabricated based on single crystallinity BST films.Pt/BST/NSTO device exhibits typical rectification characteristics under low voltage(-1V?1V)and forming-free bipolar resistive switching under high voltage(-6V?3V),with the resistance ratio between the high and low resistance states exceeding four orders of magnitude.Furthermore,stable negative differential resistance achives with set voltage of+5V,and multilevel resistive switching is achieved under various reset voltages.These behaviors are due to trapping/detrapping of electrons by oxygen vacancies at the Pt/BST interface.(2)In Ag/BST/NSTO device,the resistive switching characteristics are similar to Pt/BST/NSTO devices.Rectification characteristics are shown under lower voltage(-1V?1V),and the bipolar resistance characteristic are revealed under higher voltage(-6V?+3V).Under various reset voltages,multilevel resistive switching is also achieved.Thirdly,we prepared polycrystalline BST films by hydrothermal method,and studied the resistive switching characteristics and mechanism of Pt/BST/Ti N/Pt.(1)The resistive switching characteristics start with a clockwise resistance switching(CWRS),and transformed into a counterclockwise resistance(CCWRS)while cycle time increasing.The mechanism of Pt/BST/Ti N/Pt is oxygen vacancy conductive filament.(2)The set and reset processes of CWRS correspond to the connection and disconnection of the oxygen vacancy conductive filaments in BST film,while the CCWRS process correspond to the increasing and decreasing of effective cross section of conductive filament respectively.Fourthly,the Pt/BST/Ti N/Pt simulation model was established by using Dynamic Monte Carlo method,and the resistance switching characteristics and micro-mechanism were studied.(1)This model can well simulate the performance of the resistive switching characteristics based on polycrystalline BST film,and the simulation results further prove that the mechanism based on polycrystalline BST film is the oxygen vacancy conductive filament mechanism.(2)The forming voltage of simulation RRAM cell decreases with the increasing of initial oxygen vacancy concentration.The micro-mechanism is that the electric field intensity is concentrated at the initial oxygen vacancies position,and the higher the initial oxygen vacancy concentration is,the more concentrated the electric field will be,which is more beneficial to the formation of oxygen vacancy conductive filaments.So the forming voltage is lower when the initial oxygen vacancy concentration is higher.