Resistive random access memories (ReRAMs) have been researched to replace current NAND flash memory due to non-volatile memory characteristics, low power consumption, high operation speed, and minimum 4F² memory cell size[1]. However, to achieve high density of tera-bit level cells, it is needed to operate ReRAM cells with cross-point array structure which required selection device to limit the sneak current path in high-density ReRAM cells. Therefore, recently bi-directional selection devices with two-terminal structure such as ovonic threshold switching (OTS)[2], p-n-p type (PNP)[3], and mixed ionic electronic conduction (MIEC)[4] have been researched to suppress the sneak current path. In this work, we investigated the electrical features of the selection device with Cu-doped chalcogenide material to attain high non-linearity(>10⁵), high on-state current(I_on)(>10 MA/cm²) and low off-state current(I_off)(<0.01 kA/cm²) because Cu ions moved well by E-field and generated interstitials/vacancies acting as dopants. We fabricated the selection device of the structure of W / Cu-doped chalcogenide material / Pt by varying the pattern size of ranging 34 to 1,921 nm as shown in Fig.1. The device performance of I_on, I_off, threshold voltage(V_T), and non-linearity(I_{on @Von}/I_{off @1/2Von}) was characterized. The Cu-doped chalcogenide material layer was deposited by RF magnetron co-sputtering on W bottom electrode patterned by photo lithography process. Then, Pt top electrode was deposited by DC magnetron sputtering. The selection device with Cu-doped chalcogenide material demonstrated threshold voltage of ~1.0 V, low I_off of < 3.66 mA/cm², I_on of > 1.82 A/cm², and non-linearity of > 500 as shown in Fig.2. Finally, we present how the device performance is affected by varying the film thickness, annealing temperature, device size and stack-layer structure. In addition, the uniformity and the reliability of the selection device such as endurance cycling and retention time were investigated. In particular, the mechanism on operating behavior of the selection device will be presented by analyzing the composition and crystallinity of Cu-doped chalcogenide film by using Auger, energy-dispersive X-ray spectroscopy (EDS), cross sectional transmission electron microscopy (TEM) and X-ray diffraction (XRD).

*This work was financially supported by the Ministry of Trade, Industry & Energy(MOTIE, Korea) under Industrial Technology Innovation Program (10068055), the Brain Korea 21 Plus 2016, Republic of Korea and Hanyang University-Samsung Electronics selector research project.

Reference

[1] Choi, B. J et al., Advanced Materials 23, 3847-3852 (2011).

[2] S. Kim et al., VLSI, T240 (2013)

[3] S. Jo et al., IEDM, 6.7.1 (2014)

[4] G. Burr et al., VLSI, p41 (2012)