Current Conduction Mechanism for Non-volatile Memory Fabricated with Conductive Polymer Embedded Au Nanocrystals

Abstract

AbstractMany researchers have investigated organic nonvolatile memory devices as one of candidates device for next generation nonvolatile memory because of their low-cost, flexible and simple fabrication. The memory phenomenon in these devices is based on the electrical bistability of the material, which has two resistance states. We report memory effect in organic molecules based on electrical bistability of the materials and the bistable phenomenon was observed in poly(N-vinylcarbazole) (PVK) layer, containing a high density of Au nanocrystals and sandwiched between Al electrodes. The device was fabricated on cleaned SiO2. First, Al for the bottom electrode was deposited on SiO2 substrate by thermal evaporation in a vacuum chamber (pressure ∼10−6 torr). The PVK was dissolved with chloroform, spin-coated on the Al electrode, and baked at 120¡ÆC for 2 min to evaporate the solvent away. Subsequently, a 5-nm-thick Au film was deposited on the PVK. Additional PVK was then spin-coated on the Au film and baked. Next, the device was cured at 300¡É for 2 h in air to produce the Au nano-crystals. This device showed good nonvolatile memory characteristics. It was confirmed that it shows several region of current levels, (ION, IOFF, IINTER). When the voltage increased from zero in the OFF state (low conductivity state), the current increased rapidly at the threshold voltage (Vth), and presented a regime of negative differential resistance (NDR) after writing. Moreover ON and OFF states could be set at voltages at Vprogram (or Vp) and Verase (or Ve), respectively, and could be read at 1 V. After the device was programmed by sweeping the voltage from 0 to Vp, the current followed the high conductivity state and stayed in the ON state. And the device was programmed by sweeping the voltage from 0 to Ve, the current followed the low conductivity state and stayed in the OFF state. Furthermore, they exhibited seven different reversible current paths (intermediate states) capable for approving electron charge or discharge on surface of Au nanocrystals by sweeping the voltage from 0 to VNDR. Our results demonstrate that the fundamental parameters of the device were stable; the values of Vth, Vp, and Ve were ∼2.8, ∼4, and ∼8 V, respectively. In particular, this device exhibited excellent nonvolatile memory behavior, with bistability (ION/IOFF) of >1×102 and an intermediate state for multi-bit operation. We suggest that the current conduction mechanism clearly follow space-charge-limited(SCLC) for low conductivity state, thermionic field emission for electron charge(writing) or discharge(erasing), and F-N tunneling after erasing.