Multi Level Operation of CuO Based Cbram with Cute Electrode

Conducting Bridge Random Access Memory (CBRAM) is a one of the most promising new memories due to its simple structure, low power consumption, high scaling potential, large on/off margin and high speed. It has been known that resistivity switching mechanism of CBRAM is induced by filament formation and rupture due to metal cation movement in the solid electrolyte. Typically CBRAM has capacitor like structure that a solid electrolyte is inserted between two metal electrodes. One electrode must be reactive metal and the other must be inert. Ag and Cu have been mainly used for a reactive electrode in CBRAM due to high field-induced-diffusivity of ion in the solid electrolyte. Such a reactive metal acts as an ion supplying source to form metal filament in solid electrolyte.

In some cases, since too strong filament was formed in the solid electrolyte due to high field-induced-diffusivity of ion causing a reset stuck, it is hard to control filament formation and rupture. Although many studies on CBRAM have been carried out, there are only a few studies on controlling filament. Therefore we investigated how the filament formation is controlled with CuTe electrode and demonstrated muiti level operation of CuO based CBRAM with CuTe electrodes. In particular, we compared CuTe with Cu electrodes to understand the role of Te in CuO based CBRAM with CuTe electrode. Fig.1 shows that CuO based CBRAM structure and I-V curves with Cu and CuTe electrode, respectively.  In case of Cu electrode, reset stuck occurs at operation condition with high compliance current (10^-3A) while switching behavior appears at operation condition with low compliance current, as shown in Fig.1 (b). On the other hand, CuO based CBRAM with CuTe electrode shows stable switching behavior at operation condition with low compliance current and even at the high compliance current, as shown in Fig.1 (e). In case of Cu electrode, it was expected that too strong and thick filament was formed in the CuO solid electrolyte at operation condition with high compliance current. On the other hand, partially localized thin-filament is formed in case of CuTe electrode so that, Te acts as diffusion barrier of Cu. In addition, On/Off ratio of CuO based CBRAM with CuTe electrode was bigger than that of Cu electrode so that, HRS (High Resistive State) of the CuO based CBRAM with CuTe electrode was lower than that of the CuO based CBRAM with Cu electrode. These results indicate that CuO based CBRAM with CuTe electrode showed superior properties in MLC.  CuO CBRAM with CuTe electrode shows retention time 5x10⁴sec, DC program/erase cycles of 10², and memory margin (I_on/I_off) higher than 10³ with four different state, as shown Fig. 2.

In the conference meeting, we present switching characteristics at high compliance current in CuO based CBRAM by using CuTe electrode and discuss Multi Level Cell (MLC) operation by varying compliance current.

* This work was financially supported by the Industrial Strategic Technology Development Program (10039191, The Next Generation MLC PRAM, 3D ReRAM, Device, Materials and Micro Fabrication Technology Development) funded by the Ministry of Trade, Industry and Energy (MOTIE), Republic of Korea and the Brain Korea 21 Plus, Republic of Korea.

Reference

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[2] L. Goux et al, Influence of the Cu-Te composition and microstructure on the resistive switching of CuTe/Al₂O₃/Si cells, Applied Physics Letters 99, 053502 (2011)