Unipolar nonvolatile memory devices with composites of poly(9-vinylcarbazole) and titanium dioxide nanoparticles
Introduction
Several types of organic electronics, including organic light emitting diodes, transistors, solar cells, and nonvolatile memory devices, have attracted considerable attention due to a variety of advantages such as printability, flexibility, low processing cost, and easy fabrication [1], [2], [3], [4]. Among these organic electronics, organic memory devices have been investigated as a promising alternative to the conventional semiconductor-based nonvolatile memory devices [5], [6], [7], [8], [9], [10]. In particular, organic memory devices employing nanoparticles in the active memory materials have been extensively studied due to easily controllable processing factors such as different kinds of particles, particle size, and concentration of particles [11], [12], [13]. Much research has been done to construct organic memory devices with high specifications including highly reproducible electrical characteristics, a large ON/OFF ratio, and a long retention time by optimizing these processing factors [11], [12], [13], [14]. Bozano et al. showed that different types of nanoparticles (NPs) in hybrid organic–inorganic switching devices could tune the memory performance parameters such as threshold voltages and the ON/OFF ratio [14]. As compared with the organic memory devices with metallic NPs, relatively few studies have been conducted on the organic memory devices containing semiconducting NPs [15], [16]. Memory devices with the hybrid organic–semiconducting NPs composites have been particularly attractive due to their unique advantages of low cost, inert chemical properties, and feasibility of various chemical compositions [15], [16]. Li et al. reported data for bistability and operating mechanisms of memory devices fabricated with core/shell-type CdSe/ZnS NPs and chemically self-assembled ZnO NPs [15], [16]. However, the detailed memory performances such as cell-to-cell uniformity, distribution of threshold voltages, and endurance cycles, which must be considered for realistic memory device application, have not been thoroughly investigated.
In this study, we report on the operation and performance of unipolar resistive switching devices involving titanium dioxide nanoparticles (TiO2 NPs) embedded in a poly(9-vinylcarbazole) (PVK) matrix layer in an 8 × 8 cross-bar array structure. The switching mechanism and the charge transport were studied by electrical measurements. The statistical distribution of the ON and OFF states in the 8 × 8 cell array was investigated to quantify cell-to-cell uniformity. Endurance cycle and retention tests were also performed to evaluate the performance of the memory devices.
Section snippets
Experimental
Organic memory devices using TiO2 NPs embedded in poly(9-vinylcarbazole) (PVK) in the 8 × 8 cross-bar array structure were fabricated on indium tin oxide (ITO) (sheet resistance of ∼8 Ω/□) on glass substrates. ITO-coated glass substrates were pre-cleaned with a typical ultrasonic cleaning process. ITO electrodes with 8 line patterns of 100 μm line-width were prepared as bottom electrodes (Fig. 1) by conventional photolithography and a subsequent etching process. PVK (molecular weight ∼1,100,000)
Switching characteristics
Fig. 2a shows current–voltage (I–V) characteristics of a memory cell in the 8 × 8 cross-bar array devices consisting of the ITO/PVK:TiO2 NPs (volume ratio 150:1)/Al structure. As the voltage was swept to a positive bias, the memory device exhibited an abrupt increase of current by three orders of magnitude near 3.4 V (set voltage), indicating an electrical resistance transition from a high resistance state (OFF state) to a low resistance state (ON state) (1st sweep). When the applied bias was
Conclusions
In summary, we demonstrated unipolar nonvolatile resistive switching devices with an 8 × 8 array structure based on composites of PVK and TiO2 NPs. The TiO2 NP induced the bistability and its concentration within PVK was a critical factor to control ON/OFF ratio. From electrical characterizations, the switching was mainly governed by the filamentary conduction mechanism and the transport accompanied the change from thermally activated transport (OFF state) into tunneling transport (ON state). The
Acknowledgements
This work was supported by the National Research Laboratory (NRL) Program of the Korea Science and Engineering Foundation (KOSEF), the Program for Integrated Molecular System at GIST, and System IC 2010 project of Korea Ministry of Knowledge Economy.
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2019, Materials Today: ProceedingsCitation Excerpt :Different conventional thin film forming techniques can be used to prepare such layers [7,33]. A number of molecules (both inorganic and organic as well as their mixture) have already been tested [14–33]. Inorganic materials have advantages over organic one with respect to switching stability, while organic molecules stand out in terms of high mechanical flexibility, ease of fabrication process and cost effectiveness.