Mechanism of Forming-Free Cu2o Solid-Electrolyte Based Conductive-Bridge Random Access Memory
Authors: Jin Soo-Min , Kwon Ki-Hyun , Kim Dong-Won , Kim Hea-Jee , Yang Hun-Mo , Kim Ji-Yeon , Park Jea-Gun 
Published: February 2020

Resistive random access memory (ReRAM) is one of the most promising candidates for the next-generation memory due to its simple structure, high switching speed and low power consumption [1]. Despite of such advantages, its non-volatile characteristic strongly depends on memory cell size. To overcome the weakness, conductive bridging random access memory (CBRAM) cells have been researched as an alternative to oxygen-vacancy type ReRAM cells since CBRAM cells showed independence of non-volatile memory characteristics on the memory cell size [2]. However, CBRAM cells usually require a forming process to produce metal filaments in the solid-electrolyte before memory operation. It had a negative effect on the device since it reflects a high-power consumption. Thus, in this work, Cu2O-based CBRAM cells with forming-free characteristic were presented. In addition, we mainly focused on understanding the dependency of forming-free process on annealing temperature. To confirm the presence of forming process, the resistive switching device was fabricated on a Pt-bottom electrode (BE) with a 250nm via-hole pattern, and annealed at the temperature of as-sputtered, 150, 250 and 300 ˚C. As shown in Fig. 1(a) and 1(b), as-sputtered and 150 ˚C post annealing CBRAM cells demonstrated forming-free characteristic. However, reset process was not effectively work. For the case of 300 ˚C post annealing CBRAM cell, it required forming process as shown in Figure. 1(d). CBRAM cell with 250 ˚C post annealing demonstrated both bi-stable I-V characteristic and forming free characteristic. To understand the mechanism of the forming-free process at 250 ˚C post annealing, EDS line scanning/mapping were carried out on the CBRAM cells annealed at 150, 250, 300 and 400 ˚C repectively (Fig.2 (a)-(d)). According to the EDS analysis, the diffusion amount of Ag in the Cu2O layer decreases as the annealing temperature increases. This tendency can be attributed to the decrease in the number of metal vacancies sites. In the saturated case, as shown in Fig. 2(a) and 2(b), the reset process did not effectively work due to the excessive density of VCu
2- leaving not enough room for breaking the filament. In the optimal case, as shown in Fig. 2(c), forming-free characteristic and both set/reset process worked successfully due to the optimal gap between the concentrated area of Ag and the bottom electrode. In the deficient case, as shown in Fig. 2(d), the forming process was required due to the low density of VCu
2- in solid-electrolyte layer and the high gap between the concentrated area of Ag and bottom electrode. In our study, we report the annealing temperature (250 ˚C is the optimal condition in this study) is the significant key to achieve the forming-free and stable non-volatile characteristics via controlling the concentration of negatively charged Cu vacancies, where Ag ions can diffuse through.
[1] Daniele Ielmini, Semicond. Sci. Technol. 31 (2016) 063002
[2] Ilia Valov, J. Phys. D: Appl. Phys. 46 (2013) 074005

This material is based upon work supported by the Ministry of Trade, Industry & Energy(MOTIE, Korea) under Industrial Technology Innovation Program (10068055).

Figure 1