Irregular Resistive Switching Characteristics and Its Mechanism based on NiO Unipolar Switching Resistive Random Access Memory (RRAM) Kyung-Chang Ryoo, Jeong-Hoon Oh, Hongsik Jeong*, and Byung-Gook Park Inter-University Semiconductor Research Center (ISRC) and School of Electrical Engineering, Seoul National University, 599, Kwanak-Ro, Kwanak-Gu, Seoul, Korea, 151-742 *DRAM Process Architecture Team, Memory Division, Semiconductor Business, Samsung Electronics Co., Ltd., Nongseo-dong, Giheung-gu, Yongin-si, Gyeonggi-Do, Korea, 445-701 Phone: +82-2-880-7282, Fax: +82-2-882-4658, E-mail address:
[email protected] Abstract Resistive switching characteristics are investigated for NiO resistive switching random access memory (RRAM) by adapting cross-pointed structure. Uniform transition characteristics from high resistive state (HRS) to low resistive state (LRS) are very important to evaluate high reset/set ratio with low switching current. A cell which shows an irregular switching behavior in the initial transition has been discovered and characteristics associated with it have been discussed. In order to prevent these undesirable effects, optimal process conditions have been addressed. 1. Introduction In recent years, various studies on “new memories” have been conducted to meet fast changing environment of current electrical industry. Table. 1 shows the comparisons between new memories and other competitive memories [1]. Unlike other competitive memories, new memory has numerous advantages such as nonvolatility, high-density and low power application. In particular, RRAM is a memory using resistance difference which depends on the applied bias. And low cost and CMOS process compatibility make it advantageous for mass production, thus many researches are being carried out. Despite such advantages, it has crucial problems such as unknown switching mechanism and reset/set distribution [2]. Moreover, it is very difficult to accomplish low switching current with high reset/set ratio due to its irregular switching characteristics. In this paper, we investigate the irregular switching behavior in NiO based resistive switching memory (RRAM). Relationships between conducting filament (CF) in the cell, which are very important for stable resistance transition characteristics, and initial irregular switching behavior are also examined. In addition, the mechanism of these irregular transitions is also discussed by using advanced filament mechanism. 2. Motivation Fig. 1 shows typical I-V curves for unipolar switching (a) [1] [3] [7] and bipolar switching (b) [1] [4]. In unipolar switching, there is no need to change the polarity. The switching direction depends on the power of positive applied voltage. I-V curves enable us to predict both types of resistive cell material and direction of current flow. In general, unipolar type cell structure using diode has smaller cell size than that of bipolar type cell structure. Fig. 2 represents the various types of proposed models for resistive switching [1] [2] [7]. Figures 2 (a) and 2 (b) show the models of set and reset states of filamentary conducting paths [1] [2] [6], and Fig. 2 (c) and 2 (d) show the models of set and reset states of n- type interface type conducting path, respectively [1] [4]. In case of filamentary conduction path model, electroforming process similar to soft breakdown is needed for forming initial conducting filament path [5]. In this experiment, we adopted the NiO with filamentary conduction path model to introduce our modified conduction path model, so called filament branch effect (FBE), which is easy to explain irregular switching characteristics that show step-like transition fluctuation when resistive switching occurs.
3. Results and Discussion Fig. 3 shows schematic drawing of our fabricated RRAM structure. NiO is sandwiched between two Ir metal electrodes. Word line and bit line are cross-pointed for selecting a NiO cell and data writing/reading, respectively [7] [8]. Typical I-V curves of reset and set states of our fabricated cell are as shown in Fig. 4 (a) and (b), respectively. Fig. 5 shows the I-V characteristics of the conventional unipolar RRAM. Images (a) and (b) in Fig. 5 show that in many cases in which cells are at their initial state, irregular set (direction from 1 to 4 in Fig. 5 (a)) and reset switching can be seen, and such changes can give a reverse impact on the cell distribution. To explain these initial irregular switching behaviors in more detail, possible scenario is generated using conducting filament (CF) shape and movement at the cell interface as shown in Fig. 6. When a bias of positive/negative values is applied close to the negative differential resistance (NDR) region, electrons can be transported to the top electrode by the formation of the initial CF. Such a pristine CF formed in forming process creates tiny lateral conducting branches (TLCB) different from initially formed CF before breaks (reset) and connects (set) with respect to the applied voltage. Fig. 6 (a) and (b) show that TLCB repeat breaking and connecting at the cell interface, which in turn affects initial switching, results in the irregular switching behavior. We call this filament branch effect (FBE). If initial forming process is defective, such irregular switching fluctuation is observed more frequently. There is a correlation between irregular switching characteristics and initial resistive cell condition especially forming state. In case of multi filament states as shown in Fig. 7, degradation of reset/set distribution has been accelerated by unexpected additional tiny filament branch in the NiO/Ir interface. Switching behavior is degraded after about ten cycles past one month when irregular switching occurs in the initial state, as shown in Fig. 8. It is possible that this irregular switching is one of the reasons for time-dependent degradation of cell property. By controlling the amount of filament branch at NiO/Ir interface, irregular switching can be decreased significantly. Therefore, interface engineering is very important to avoid these problems. 5. Conclusion Irregular switching behavior in the initial transition has been discovered and associated characteristics have been discussed. To explain such undesirable effects, mechanism of irregular switching using filament branch effect (FBE) is introduced. By controlling the filament branch with interface engineering to avoid these irregular switching characteristics, it is expected to reduce reset current with high reset/set ratio. References [1] Ryoo, K. C, et al., Si Nanoelec. Workshop, (2009), 63 [2] Waser, R., and Aono, M., Nat. Mater. (2007) 6, 833 [3] Gibbons, J. F., and Beadle, W. E., Solid-State Electron. (1964) 7, 785
[4] Simmons, J. G., and Verderber, R. R., Proc. R. Soc. London, Ser. A (1967) 301, 77 [5] Baek, I. G., et al., Tech. Dig. IEDM (2004), 587 [6] Kinoshita, K., et al., Appl. Phys. Lett. (2006) 89, 103509 [7] Sawa, A., Materials today. Vol. 11, pp.28-36, 2008 [8] Baek, I. G., et al., Tech. Dig. IEDM (2005), 750
Table. 1. Comparisons between RRAM and other competing memories.
Acknowledgement This work was supported by Samsung Electronics with a project entitled as ‘The Research on Structure and Characteristics of the Nonvolatile Memory Devices’.
Fig. 3. Schematic drawing of our fabricated RRAM structure.
(b) Fig. 5. I-V curves showing irregular set (a) and reset (b) switching behavior.
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(a) (b) Fig. 1. Typical I-V curves for unipolar (nonpolar) switching (a) and bipolar switching (b).
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(b) Fig. 6. Schematic drawings of filament branch effect (FBE). Possible scenario of irregular reset (a) and set (b) transition behaviors is explained above.
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(b) Fig. 4. Typical I-V curves of reset (a) and set (b) transitions of our fabricated cell, respectively.
(b) Fig. 7. Filament branch effect (FBE) in multi-filament state.
(c) (d) Fig. 2. Sketches of set (a) and reset (b) states of filamentary path model, and set (c) and reset (d) states of n type interface conducting path model. (a) Fig. 8. Filament branch effect (FBE) of initial irregular switching behavior after 10 cycles.
