메탈 나노크리스탈이 삽입된 저분자 비휘발성 메모리의 메모리 효과 및 전류 전도 메커니즘에 대한 연구

Abstract

Organic nonvolatile memory was recently proposed as a next generation technology with the possibility of tera-bit integration and a variety of applications due to advantages such as a minimum feature size of 4F2, simple structure, low production cost, the combination of organic based devices (organic light emitting diode (OLED), organic photovoltaic cell etc.), and flexibility. The present study investigates the memory effects and current conduction mechanisms of organic nonvolatile memory-cells. Organic nonvolatile memory-cells that were embedded with uniform, well distributed metal nanocrystals were fabricated by O2 plasma oxidation. These memory-cells were reproducible, stable, and reliable [1, 2]. The detailed physical and chemical structure of the nanocrystal layer, which plays an important role in nonvolatile memory operations, was explored by physical and chemical analyses including transmission electron microscopy (TEM), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). The mechanisms of operation were correlated with the physical and chemical structures of memory-cells. Based on these results, we fabricated organic nonvolatile memories adjacent to commercial level, and obtained memory margins of over 103, stable multi-level operation, and favorable retention and endurance characteristics. We developed vertically stacked organic nonvolatile memory-cells with nonvolatile memory characteristics for both the top and bottom memory-cells, and verified the potentiality of the resulting high density memory devices [1]. We confirmed the temperature dependency of the small-molecule nonvolatile memory-cells through low temperature measurements. We then explored the exact operation mechanisms of the fabricated organic nonvolatile memory-cells. We applied a current conduction mechanism that was correlated with the physical structure of the memory-cell to influence the current characteristics of each voltage region (low current state, high current state, intermediate current state, Vth ~ Vp, NDR region, Ve ~) during memory operation procedures, and analyzed the current conduction mechanisms of each voltage region. The results of these analyses allowed us to identify the exact operation mechanism of the fabricated organic nonvolatile memory-cells. This operation mechanism was applied to assess the effects of small-molecule layer thickness on small-molecule nonvolatile memory characteristics, and the electrical characteristics elucidated through these experiments were correlated to calculated results using an equation that expressed the mechanism of operation [3]. We analyzed the performance of small-molecule nonvolatile memory-cells with representative electron transport and hole transport bilayers. The nonvolatile memory characteristics and reliability (expressed as retention time and endurance cycles) of a memory-cell with small-molecule bilayers embedded with Ni nanocrystals worsened, and the current level decreased, as the thickness of the hole transport layer increased. These results indicate that electrons, not holes, are the dominant carriers in small-molecule bilayer memory-cells irrespective of the carrier transport characteristics of small-molecule materials [4]. Finally, we investigated a double-stacked small-molecule nonvolatile memory-cell that was embedded with uniformly distributed nanocrystals using O2 plasma oxidation method. This memory-cell showed multi-level operation over four levels, a memory margin over 103, retention time of 105 sec at 85 ℃, and program/erase cycles of 103. These results are significant because they demonstrate the feasibility of producing high density, commercially attractive organic nonvolatile memory devices. Furthermore, our investigation of the exact and detailed mechanism of operation for small-molecule nonvolatile memory-cells provides a baseline for future research and development of organic nonvolatile memory.