Structure and Property of Electrical Resistive Memory Polymers

Abstract

Polymer materials exhibit easy processability, fiexibility, high mechanical strength, and good scalability. Moreover, polymer materials can be processed at low cost and can easily be fabricated to form multi-stack layer structures required for highly dense memory devices. A series of soluble poly(amic acid) precursors were prepared from a carbzole-containing monomer, 3,3’-bis[9-carbazole(ethyloxy)biphenyl]-4,4’-diamine (HAB-CBZ) with four different aromatic dianhydrides: pyromellitic dianhydride (PMDA), 3,3’,4,4’-biphenyltetracarboxylic dianhydride (BPDA), 3,3’,4,4’-diphenylethertetracarboxylic dianhydride (ODPA), and 3,3’,4,4’-diphenylsulfonyltetracarboxylic dianhydride (DSDA). From the precursors, nanoscale thin films of polyimides (PIs) were prepared by spin-coating and subsequent thermal imidization. All the PIs exhibited excellent thermal and dimensional stability. In particular, the PIs based on the PMDA and BPDA units revealed excellent chemical resistance to organic solvents, in addition to the high thermal and dimensional stability, which are required for the fabrication of high performance memory devices in 3D multi-stack structure. Devices fabricated with nanoscale thin PI films exhibited excellent unipolar write-once-read-many-times (WORM) memory behavior with a high ON/OFF current ratio of up to 1010. A high temperature polyimide bearing anthracene moieties, poly(3,3’-di(9-anthracenemethoxy)-4,4’-biphenylene hexafluoroisopropylidenediph-thalimide) (6F-HAB-AM PI) was studied. The polymer exhibited excellent thermal stability up to around 410 °C. This polymer was amorphous but oriented preferentially in the plane of nanoscale thin films. In device fabrications of its nanoscale thin films with metal top and bottom electrodes, no diffusion of the metal atoms or ions between the polymer and electrodes was found

however, the aluminum bottom electrode had undergone oxide layer (ca. 1.2 nm) formation at the surface during the post polymer layer formation process, which had no significant influence on the device performance. The polymer thin film exhibited excellent unipolar and bipolar switching behaviors over a voltage range of less than ±2 V. Further, the PI films showed repeatable writing-reading-erasing ability with long reliability and high ON/OFF current ratio (up to 107) in atmospheric conditions as well as at temperatures up to 200 °C. Three functional PIs bearing conjugated bis(triphenylamine) (2TPA) derivatives with electron-donating and accepting groups were examined. The PIs exhibited high thermal and dimensional stabilities and furthermore produced high-quality nanoscale thin films via conventional solution coating process. All of the PIs in the films were found to be amorphous, but oriented somewhat preferentially in the film plane, rather than randomly. Their film densities and interchain distances were measured, and the optical and electrochemical properties were determined. All of the PIs in the devices with aluminum top and bottom electrodes showed volatile or nonvolatile digital memory behavior, depending on the substituents of the 2TPA unit. The 2TPA-based PI, as well as the PI bearing 2TPA with electron-donating methoxy substituents, showed unipolar WORM memory behavior, whereas the 2TPA-based PI containing electron-accepting cyano groups exhibited unipolar dynamic random access memory (DRAM) behavior. All of the PI films revealed excellent retention abilities in both the OFF- and ON-state, even under ambient air conditions. Moreover, they all revealed high ON/OFF current ratios (106–1010). All of the memory behaviors were found to be governed by a mechanism involving trap-limited space-charge limited conduction and local filament formation. Such memory behaviors were further investigated in detail with taking into consideration the PI components’ chemical nature and molecular orbital levels, possible trapping sites, substituents’ effect, and the metal electrodes’ work function. A series of brush copolymers bearing N-phenylcarbazole (PK) and 2-biphenyl-5-(4-ethoxyphenyl)-1,3,4-oxadiazole (BEOXD) moieties in various compositions were studied in detail, in particular their electrical memory characteristics, optical and electrical properties, morphological structures, and interfaces. Nanoscale thin films of the brush copolymers in devices were found to exhibit excellent unipolar electrical memory versatility, which can easily be tuned by tailoring the chemical composition and by changing the film thickness. Moreover, the molecular orbitals and band gap can be tuned by changing the chemical composition. The novel memory characteristics of these copolymers originate primarily from the cooperative roles of the ambipolar PK and BEOXD moieties, which have different charge trapping and stabilization properties. The electrical memory behaviors were found to occur via a favorable hole injection from the electrode and to be governed by trap-limited space-charge limited conduction combined with Ohmic conduction and local filament formation. Linear-brush diblock copolymers bearing carbazole moieties in the brush block were studied. Various phase-separated nanostructures were found to develop in nanoscale thin films of the copolymers, depending on the fabrication conditions including selective solvent-annealing. This variety of morphologies and orientations means that these block copolymers exhibit digital memory versatility in their devices. Overall, the relationship between the morphology and digital memory performance of these copolymers has several important features. In particular, the carbazole moieties in the vertical cylinder phase with a radius of 8 nm or less can trap charges and also form local hopping paths for charge transport, which opens the mass production of advanced digital memory devices with ultra-high memory density. Charges can be transported through the layer when the dielectric linear block phase has a thickness of 10.6 nm

however, charge transport is not possible for a dielectric phase with a thickness of 15.9 nm. All the observed memory behaviors are governed by the trap-limited space-charge limited conduction mechanism and local hopping path (i.e., filament) formation.