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C: Physical Processes in Nanomaterials and Nanostructures
Conductance Quantization in Nonvolatile Resistive Switching Memory Based on the Polymer Composite of Zinc Oxide Nanoparticles Yanmei Sun, and Dianzhong Wen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01120 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 26, 2018
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The Journal of Physical Chemistry
Conductance Quantization in Nonvolatile Resistive Switching Memory Based on The Polymer Composite of Zinc Oxide Nanoparticles Yanmei Sun, Dianzhong Wen* HLJ Province Key Laboratories of Senior-education for Electronic Engineering, Heilongjiang University, Harbin, 150080, China ABSTRACT: The composite of zinc oxide (ZnO) and polyurethane (PU) was used as active layer. The conductance quantization was demonstrated in nonvolatile resistive switching memory devices of indium tin oxide/PU+ZnO/Al sandwich-structure. The observed bipolar resistive switching behavior are estimated to be arised from the formation and dissolution of Zn filaments. And furthermore, conductance quantization with both integer and half integer multiples of G0 (2e2/h, where e is electron charge, and h is Planck’s constant) has been observed in which multilevel conductance steps are separated by half integer multiples of G0. The observed quantized conductance phenomena in multiple well circumscribed resistance states were appropriate for application in multilevel data memory cells. INTRODUCTION For the past few years, resistive random access memory (RRAM) has attracted great attention on account of its non-volatility, long endurance, and high data storage density, serving as one of the most potential candidates for next generation memory device and logic circuit applications.1-8 Up to now, the resistive switching memory behavior has been reported in many organic and inorganic materials.2,3,5- 9 The advantage of organic memory devices include excellent scalability, low manufacture cost and availability of chemical structure varieties, but its resistive switching parameter consistency is poor. And inorganic memory devices have generally high reproducibility and good reliability, but its limitations on the flexible devices application should not be overlooked. One of significant research interest in RRAM is multilevel data storage, many groups have studied the 1
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multilevel data storage properties of organic and inorganic materials in RRAM for memory applications. 10 - 13 These researches are conducive to searching the multilevel data storage manipulated by multiple channel. Broadly speaking, a metallic filament-based RRAM usually be made up of an active layer sandwiched between two electrode. Under applied bias, the memory device could be repeatedly converted between two resistance states, according to the formation and dissolution of conductive filaments consist of the active metal atoms.14-18 When the diameter of the metallic filament drops to the atomic scale, it was the natural thing to expect that quantum effects, for instance conductance quantization, may go into effect. The distance of the carrier travels less than the mean free path. In this case the carrier travels elastically, which come from interactions with charged impurities, lattice defects, phonons, and with other charged carriers.19 For conductance quantization, the conductance (G) in the low resistance state could be represented as G=I/V = nG0, n =1, 2, 3… where I is a representative of the current across the device and G0= 2e2/h (where e is the electron charge and h is Planck’s constant) signifies a single atomic point contact. 20-21 Such quantization of conductance is a well-studied phenomenon, having been found in a number of materials and systems of RRAM devices.22-26 Furthermore, quantum conductance in such systems attracted great attention not only on account of the opportunity it offers to study the physics nature of electron transport and resistive switching, but also due to it has promising applications in multilevel semiconductor memories, quantum information processing. Up to now, many types of polymer host matrix materials for inorganic nano-powder and small organic molecule, such as polyvinylpyrrolidone, 27 [6,6]-phenyl-C61-butyric acid methyl ester, 28 poly(3,4-ethylene dioxythiophene): poly(styrenesulfonate),
29
poly(methylmethacrylate),
30 - 31
polyvinyl alcohol, 32 and poly (3-hexylthiophene) 33 have been used extensively in resistive switching memory devices. Polymer as the host matrix have the advantages such as outstanding film-forming property, low cost, easily processed, and adequate adhesiveness for applications in 2
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The Journal of Physical Chemistry
electronic devices. In this work, an polymer, polyurethane (PU) has been used for preparation composite layer of PU and ZnO for the memory device. Compared to other polymers, PU have been selected as the host matrix on account of its water solubility and friendly to environment without environmental pollution in solution process. For this reason, we mixed ZnO nanoparticles with PU and researched the resistive switching behaviors of ITO/PU+ZnO/Al device. It is indicated that quantum conductance in composite of ZnO and PU-based RRAM devices in which multilevel conductance steps are segregated by integer and half integer multiples of G0. EXPERIMENTAL SECTION Commercial glasses covered with tin indium oxide (ITO) layer (~100 nm) purchased from Huananxiangcheng Co. Ltd (Shenzhen,China) were used as substrates. PU (Mw= 62000, MDL: MFCD01775440) was provided by Sigma-Aldrich. Monodispersed ZnO nanoparticles (MDL: MFCD00011300) of mean size 5 nm were provided by J&K Scientific Ltd. (Beijing, China). The ITO substrates were dipped into acetone, methanol and deionized water, in turn, and clean with ultrasonic washer for 15 minutes, respectively. Afterwards, the substrates were dried in vaccum oven. 25 mg ZnO with a diameter of approximately 5 nm was dissolved in 5 ml ethanol solution and then the dispersion was ultrasonicated for 1 h. Subsequently, the 0.6 ml ZnO dispersion was blended into 2 ml the previously filtered (PTFE- membrane micro filters with a pore size of 0.22 µm) PU aqueous solution (ZnO concentration in PU was 4.5%) and ultrasonicated for 1 h. After that, 250 µl blend solution of PU+ZnO was spin-coated to the ITO substrate at a rotation rate of 600 rpm for 18 s and then 4000 rpm for 45 s, followed by vacuum-dried at 70 ℃ overnight to remove the existing solvents. As a typical electrode material, Al is widely used in the manufacture of the resistive switching memory device. Al is an active metal with low cost and good flexibility. An oxide layer may naturally be formed during spin-coating and drying of the active layer. In many previous reports, the resistive switching effect of memory devices were concerned with top Al electrode.34-39 Taking 3
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this into account, the metal Al was selected as the top electrode, was thermally evaporated and patterned into many circular pads with a diameter of 500µm using a shadow mask. The resistive switching characteristics of ITO/PU+ZnO/Al device were characterized by a Keithley 4200-SCS semiconductor parameter analyzer. A two-terminal (between bottom and top electrode) I-V test was performed. The probe tip used 10 µm diameter tungsten wire attached to a nickel shaft with a point radius
