Forming-Free Cu-Doped Amorphous-Carbon-Oxide Based Resistive-Random-Access-Memory and Memory Operation Mechanism

Abstract

Recently, the demand for more data storage and fast processing has been dramatically increased for the big data markets such as artificial intelligence (AI), virtual reality (VR), autonomous car, and internet of things (IoT). Thus, a new memory such as a storage class memory (SCM) has been introduced since it can perform a reasonable latency compared to DRAM and a lower bit-cost than NAND flash memory [1]. Remind that, generally, a SCM has been fabricated with three-dimensional cross point memory cell array [2]. As a candidate memory cell for SCM, resistive-random-access-memory (ReRAM) has been proposed; i.e., called storage-type SCM. Santini, C. A. et al. demonstrated amorphous carbon oxide (a-COx) based ReRAM-cell having a reasonable memory window margin (I on/I off > 100), a fast switching speed of 20~50 ns at ~ 100-nm-diameter memory cell size. However, it showed extremely a high forming voltage (V forming) of ~ 5.0 V and a high reset voltage (V reset) of ~ – 4.0 V [3]. In particular, the a-COx based ReRAM-cell presented a different bi-stable memory characteristic for another typical ReRAM-cells; i.e., its bias directions of set and reset were opposite to a typical ReRAM, which mechanism was not evidently proved. In addition, the forming process for this ReRAM cell would be highly undesirable since it caused an extra burden for initializing memory-cells and degraded the write and erase endurance cycles [4].Here, for the first time, we designed a forming-free Cu-doped amorphous-carbon-oxide based ReRAM cell, which did not need a forming process and we reviewed the memory operation mechanism by understanding electrical and chemical properties of the Cu-doped a-COx based ReRAM cells. A typical a-COx based ReRAM-cell needed a forming process; i.e., a forming voltage of – 2.20 V and a set voltage of – 1.05 V, as shown in Fig. 1 (a). Otherwise, a Cu-doped a-COx based ReRAM-cell could achieve a forming-free process; i.e., a forming voltage (i.e. - 0.85 V) was the same as a set voltage (i.e. – 0.85 V), as shown in Fig. 1 (b). In addition, it demonstrated a write and erase endurance cycles of ~106 by sustaining a memory margin of ~1.3×102, being able to be utilized for a commercial nonvolatile memory-cell, as shown in Fig. 1(c). To clarify the forming-free mechanism, the depth profiles of C, Cu, and O atom in the Cu-doped a-COx memory cell were observed in detail under pristine, after set, and after reset process, which were obtained from intensity line-profiles of EELS/EDS elemental mapping images at C-K edges, O-K edges, Cu-Kα, Pt-La1, and W-La1. For the pristine state, C, Cu, and O atoms are uniformly distributed in the Cu-doped a-COx layer, as shown in Fig. 1(d). In addition, after a set process, since a negative voltage was applied to the top Pt electrode, Cu atoms evidently moved and segregated toward the top Pt electrode, as shown in a of Fig. 1(e), while O atoms evidently migrated and pile up toward the bottom W electrode, as shown in b of Fig. 1(e). This result means that the conductive C-C sp2 filaments in the Cu-doped a-COx layer were produced when oxygen atoms migrated and piled up toward bottom W electrode and the conductive Cu-atom filaments were formed in the Cu-doped a-COx layer since Cu atom moved and segregated toward the top Pt electrode. Hence, both conductive C-C sp2 filaments and Cu-atom filaments were generated simultaneously in the Cu-doped a-COx layer, achieving a set process without a forming process. On the other hand, after a reset process, since a positive voltage was applied to the top Pt electrode, Cu and O atoms were redistributed inside the Cu-doped a-COx, resulting in breaking both C-C sp2 filaments and Cu-atom filaments, as shown in a and b of Fig. 1(f). In our presentation, we will demonstrate and review the mechanisms between a set process without forming process and a reset process in detail by electrical and chemical composition depth profiles depending on the applied bias condition. In particular, we will demonstrate a different ReRAM behavior of the Cu-doped a-COx based ReRAM from a typical ReRAM or CBRAM. Acknowledgement This material is based upon work supported by the Ministry of Trade, Industry & Energy(MOTIE, Korea) under Industrial Technology Innovation Program (10068055). Reference [1] Matsui, C. et al. Integration 2019, 69, 62-74.[2] Hady, F. T. et al. Proceedings of the IEEE 2017, 105, (9), 1822-1833.[3] Santini, C. A. et al. Nature Communications 2015, 6, (1), 8600.[4] Skaja, K. et al. Scientific Reports 2018, 8. Figure 1