Investigation Of Multilevel Data Storage On Silicon Based Ferroelectric Memory

Ferroelectric random access memories(FRAM)based on two stable spontaneous polarization states can work in a wide temperature range,have good retention and high speed,and consume little energy.Now they have been widely used in satellite,and other spacecraft for deep space exploration.However,there are two big problems which limit their application in civil field.The first problem is that the ferroelectric memory is not compatible with silicon technology,and the second problem is that the storage density is too small.In order to promote the application of ferroelectric memory,this paper explored the preparation process of polycrystalline ferroelectric thin films on Pt and Si substrate,investigated the ferroelectric properties of ferroelectric thin films and their application in multistate storage,and investigated the multistate storage mechanism.This study provides a new way to solve the problem of compatibility between FRAM and Si technology,and the problem of low storage density,it can promote the commercial application of FRAM.The main contents and results are summarized as following:1.The ferroelectric properties of different PbZr0.52Ti0.48O3(PZT)films prepared with different preparation parameters were investigated.PZT ultrathin films with good ferroelectric property were prepared,and the switching characteristics of Pt/PZT/Pt diode based on these PZT films were studied.PZT thin films with good ferroelectric property were prepared by controlling the preparation parameters.When the PZT film thickness is below 50 nm,the residual polarization value of PZT thin film decreases with the decrease of film thickness.And the ferroelectric properties are still obvious when the film thickness is thin to 29 nm.In addition,the switch ratio of Pt/PZT/Pt diode based on the polycrystalline film decreases with the decreasing of PZT film thickness.When the thickness of the ferroelectric thin film is as thin as 13 nm,the switching characteristic of the diode is still obvious,which indicates that the ultrathin polycrystalline ferroelectric film can still be used in the memory device.2.A ferroelectric memristor is designed,the oxygen vacancy concentration in ferroelectric thin films can be controlled with the use of active electrodes,and the oxygen vacancy migration and ferroelectric polarization switch can be controlled to realize memristor behavior with high switch ratio.PZT films with different initial oxygen vacancy concentration were prepared by adjusting the oxygen pressure during the film preparation,and the Ag/PZT/Pt memory cells were prepared.The correlation between the concentration of initial oxygen vacancies in PZT film and the memory behavior was studied.The device with PZT thin films prepared at 10 Pa oxygen pressure can achieve six logic states.And the device with PZT thin films prepared at 20 Pa oxygen pressure can achieve four logic states,the ratio between the highest and lowest resistance states of the device can reach 107~108%.These devices have high switching ratio and good retention performance,the resistance states can keep about 105 s when it is used as a two logic state device,indicating that it has a great potential for application in nonvolatile ferroelectric memory devices.3.Multistate storage unit based on the ZnO:Mn/PZT composite films was designed,through controlling the resistance of film layer by layer,a way to achieve four logic states storage was developed by controlling Ag ion migration,ferroelectric polarization reversal and oxygen vacancy migration.Firstly,the behavior of Ag/PZT(8 nm)/Pt and Ag/ZMO(20 nm)/Pt memory cells is investigated,and then the storage behavior of the multistate of the Ag/ZMO/PZT/Pt memory cell was studied.Ag/ZMO/PZT/Pt memory cell integrated the resistance switch mechanisms of Ag/PZT(8 nm)/Pt and Ag/ZMO(20 nm)/Pt units.It can realize four logic states storage by controlling Ag ion migration,ferroelectric polarization reversal and oxygen vacancy migration in the effect of different voltage The Ag/ZMO/PZT/Pt memory cell has good repeat write performance,and can keep the logic states in a short time.The research shows that it is feasible to construct a new type of multi logic state memory by combine different principles.4.In order to investigate the compatibility of Si technology with polycrystalline films applied to multi state storage.Pt/PZT/SiOx/Si ferroelectric tunnel junction unit was designed,and multistate storage was achieved by controlling ferroelectric polarization reversal and oxygen vacancy migration in it.PZT films with different thicknesses were grown on Si wafer,and the ferroelectric properties of PZT films were investigated.When Pt electrode is deposited on the PZT film,the switch ratio of the device changes as the thickness of PZT film.The device with 2.5 nm PZT film can achieve 8 resistance states,and keep stable in 100 s.It is proved the multistate storage of Pt/PZT/SiOx/Si ferroelectric tunnel junction is caused by the polarization and oxygen vacancy migration.The results provide a new idea for the development of ferroelectric memory compatible with Si process.5.Different thick BFO films were directly deposited on Si substrate to prepare multistate ferroelectric tunnel junctions,the storage properties of Pt/BFO/SiOx/Si unit were investigated in detail,then the numerical calculation method was used to verify the storage model.The results show that the storage mechanism of the memory cell and the number of the stored logic states are related to the thickness of the BFO film.When the thicknesses of the BFO films are about 2 nm or below 2 nm,the films have no ferroelectricity.When the thicknesses are about 3.5 nm or more over than 3.5 nm,these films have good ferroelectricity.The Pt/BFO/SiOx/Si device with 2 nm BFO film can achieve 2 logic states,the device with 3.5 nm BFO film can achieve 4 logic states,and the device with 5 nm BFO film can achieve 5 logic states.And numerical computing methods are used to prove the contribution of polarization and oxygen vacancy migration.The investigation provides guidance for the investigation of improving the performance of the Si compatible ferroelectric memory.