Preparation And Performance Of Memory Device Based On Polyimide With Low Power Consumption And High Storage Density

With the advent of 5G era,the development process of high-tech fields such as cloud computing,big data,internet of things,driverless,smart cities and smart homes has accelerated.This will put forward higher and more stringent requirements for the development of storage technology.In the future,the design and manufacturing standards of memory devices will increase gradually.High density,large capacity,low power consumption,low cost,fast response and miniaturization of memory devices are important development goals.However,the traditional semiconductor storage technologies based on Si and Ge are facing a miniaturization bottleneck,which is difficult to break through.On the one hand,the reduced feature size of the storage device may cause problems such as dielectric breakdown and misreading.On the other hand,the longer switching time(microsecond level)and fewer erasable times(one hundred thousand)make it difficult to meet future memory requirements.Therefore,it is the key to the future development of storage devices to seek new storage materials,new switching mechanism,new device structures and device integration methods.In this context,developing new materials is an effective method.Polymer-based memory devices active layers get attention for the following advantages,such as good mechanical properties,processability,flexibility,3D stacking and multilevel storage ability.In recent years,aromatic polyimide(PI)has been widely studied for memory device application.PI is a special engineering material with excellent comprehensive performance with great thermal stability,chemical stability,and tunable molecular structure.PI-based memory materials with diverse molecular structure and different memory behaviors have been prepared.However,the relationship between PI structure and memory performance is still ambiguous due to the large difference in aspect of structure.This makes it difficult to form a unified mechanism of resistance change.Hence,the PIs with similar structure but different storage behaviors should be prepared by reasonable design to establish the relationship between structure and performance.This will be a great significance to the practical application of PI-based memory devices in the future.From the perspective of molecular design,the relationship between PIs’pendant group structure and memory performance will be explored by pendant group modification.It is divided into the following four parts for explanation.First,two diamine monomers withπ-πconjugated pendant groups are prepared.The pendant groups for TZPEDA and TZBPDA are phenyl and biphenyl,respectively.Two novel PIs,named TZPEDA-6FDA and TZBPDA-6FDA,are obtained by two-step condensation polymerization with 6FDA dianhydride monomer.The difference between the two PIs is the different pendant chainπ-πconjugate lengths.The memory devices based on the two polyimides both exhibit non-volatile WORM-type memory characteristics.The threshold voltage of the ITO/TZPEDA-6FDA/Al and ITO/TZBPDA-6FDA/Al are-2.6 V and-2.3 V,respectively.And the ON/OFF current ratio is up to 10~5for both devices.The difference in threshold voltage between the two devices is caused by different pendant chain conjugate lengths.The WORM-type memory behavior is governed by the combination of Ohmic,trap-limited space charge limited conduction,charge transfer and charge trapping.The biphenyl pendant group introduced into TZBPDA-6FDA can extend theπ-πconjugate structure of the donor unit and optimize the electrostatic surface potential distribution of the PIs,which are beneficial for improving the charge-carrier migration and reducing the threshold voltage.Then,two diamine monomers with p-πconjugated pendant groups are prepared,namely TZMPDA with methoxy side group and TZAPDA with diethylamine pendant group,respectively.Two PIs,TZMPDA-6FDA and TZAPDA-6FDA are prepared by condensation polymerization with 6FDA.The devices based on the two PIs both exhibit WORM-type memory behavior.With the enhancement of the p-πconjugation ability from methoxy group to diethylamine,the threshold voltage of the device decreases.The threshold voltage of ITO/TZAPDA-6FDA/Al is-1.8 V,which is much lower than that of ITO/TZMPDA-6FDA/Al-2.4 V.The WORM-type memory behavior is dominated by the combination of Ohmic,trap-limited space charge limited conduction and charge transfer.In comparison with TZMPDA-6FDA,the HOMO level of TZAPDA-6FDA is increased due to the stronger electron-donating ability of diethylamino group than methoxy group.The high-lying HOMO levels facilitate the intra-and inter-molecular charge transfer,which benefit to optimize the performance of the memory device.The results suggest that the HOMO level has a significant impact on the threshold voltage.Next,on the basis of TZAPDA-6FDA,the influence of the introduction of different contents of quaternary ammonium cations on memory behavior is explored.PIs with four kinds of quaternary ammonium salts in the pendant chain were prepared by adding different amounts of bromoethane,namely TZAPDA-6FDA-Br-10,TZAPDA-6FDA-Br-30,TZAPDA-6FDA-Br-50 and TZAPDA-6FDA-Br-70,respectively.The introduction of quaternary ammonium cations with different contents has obvious influence on the memory behavior.With the increasement of quaternary ammonium cations,the threshold voltages of ITO/TZAPDA-6FDA-Br-10/Al,ITO/TZAPDA-6FDA-Br-30/Al,ITO/TZAPDA-6FDA-Br-50/Al gradually decrease(-1.71,-1.61 and-1.06 V)and the ON/OFF current ratio also gradually reduce(10~3,10~2,10~2).For ITO/TZAPDA-6FDA-Br-10/Al,ITO/TZAPDA-6FDA-Br-30/Al and ITO/TZAPDA-6FDA-Br-50/Al,the memory mechanism of the devices mainly be controlled by charge transfer in TZAPDA-6FDA.At the same time,the quaternary ammonium cations have a synergistic effect on the storage behavior,including reducing threshold voltage and ON/OFF current ratio.With the increasement of quaternary ammonium salt,the high content of quaternary ammonium salt will provide continuous transport channel for charge carriers.This makes the device remain ON state without obvious memory characteristic.Finally,three diamine monomers with different carbazole pendant groups are prepared,namely TZCz EDA with ethyl alkyl chain,TZCz PDA with propyl chain alkyl,and TZCz BDA with butyl alkyl chain.Three novel PIs,named TZCz EDA-6FDA,TZCz PDA-6FDA and TZCz BDA-6FDA are obtained by two-step condensation polymerization with 6FDA dianhydride monomer.The devices based on the three PIs show ternary memory characteristic and WORM-type memory behavior.As the alkyl chain length increases,the first threshold voltage of the devices gradually decrease(-2.52,-2.27 and-2.07 V),and the second threshold voltage are similar(-3.18,-3.18 and-3.13 V).The memory behavior is controlled by two mechanism.The first threshold voltage of the devices is caused by the conformational transformation of the carbazole units to form a face-to-face structure under the applied electric field.The carriers can hop via carbazole groups in the direction of electrical field.With the increasement of the length of the alkyl chain,the energy barrier will reduce for forming conformation transition.Thus,the first threshold voltage of the three devices gradually decrease.The second threshold voltage is caused by the charge transfer that occurs on the main chain.Because the donor and acceptor structures of the three polymer backbones are the same,the threshold voltages are similar.