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. 2015 Aug 21:5:13311.
doi: 10.1038/srep13311.

Atomic View of Filament Growth in Electrochemical Memristive Elements

Hangbing Lv  1   2 , Xiaoxin Xu  1   2 , Pengxiao Sun  1   2 , Hongtao Liu  1   2 , Qing Luo  1   2 , Qi Liu  1   2 , Writam Banerjee  2 , Haitao Sun  1   2 , Shibing Long  1   2 , Ling Li  1   2 , Ming Liu  1   2
Affiliations

Atomic View of Filament Growth in Electrochemical Memristive Elements

Hangbing Lv et al. Sci Rep. .

Abstract

Memristive devices, with a fusion of memory and logic functions, provide good opportunities for configuring new concepts computing. However, progress towards paradigm evolution has been delayed due to the limited understanding of the underlying operating mechanism. The stochastic nature and fast growth of localized conductive filament bring difficulties to capture the detailed information on its growth kinetics. In this work, refined programming scheme with real-time current regulation was proposed to study the detailed information on the filament growth. By such, discrete tunneling and quantized conduction were observed. The filament was found to grow with a unit length, matching with the hopping conduction of Cu ions between interstitial sites of HfO2 lattice. The physical nature of the formed filament was characterized by high resolution transmission electron microscopy. Copper rich conical filament with decreasing concentration from center to edge was identified. Based on these results, a clear picture of filament growth from atomic view could be drawn to account for the resistance modulation of oxide electrolyte based electrochemical memristive elements.

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Figures

Figure 1
Figure 1. Real-time regulation of the current flow.
(a) The I-V curve of the common SET operation with VS swept from 0 ~ 1.3 V and VG constantly biased at 1.5 V. (b) The I-V curve of the RESET process with VD sweeping. (c) The SET curve for the varied VG programing scheme. VS is constantly biased with a voltage of 2 V and VG varies from 0 V to 1.5 V with an increasing rate of 0.005 V per step. (d) The IDS dependence of VG in the RESET process, with VD kept at 2 V and VS at ground.
Figure 2
Figure 2. The refined history of the resistance change during programming.
(a) The resistance of cell as a function of VG in the Fig. 1(c) test. A discrete change in resistance is observed. The insert is the magnification of the red solid dots region. (b) Histogram of the resistance states measured from 13360 points in 100 SET curves for VG < 1.1 V. The peaks are fitted with Gaussian distributions as a guide to the eye. (c) The fitting results of the red dots region in (a) after taking into account the series resistance (RS). (d) Histogram of the conductance from 20 SET curves of 10 cells after reducing the serial resistance to 700 Ω. The distribution peaks are positioned at integer or half-integer multiples of G0, which are fitted with Gaussian distributions as a guide to the eye.
Figure 3
Figure 3. Morphology and structure characterizations of the filament by HRTEM.
(a) The cross-section of the Cu/HfO2/Pt device. (b) The magnification of (a). (c) High resolution image of region ① in (b). The upper insertions are the FFT patterns of CF left edge, core edge and right edge. The lower left insertion is the FFT pattern of the HfO2 with no CF region. (d) A high resolution image of region ② in (b). (e) A high resolution image of region ③ in (b). (f) A high resolution image of region ④ in (b). The scale bars in (a) and (b) are 100 nm and in (c–f) are 2 nm. The yellow and blue framed insertions in (d,e) are the FFT patterns of the Cu electrode and HfO2 layer with no CF, respectively.
Figure 4
Figure 4. Description of the discrete resistance change using direct tunneling.
(a) The gap length vs. the VG curve fitted by the low voltage tunnel equation. The barrier height is 2.0 eV and the size of the filament tip is 2.5 nm in calculation. (b) Histogram of the tunnel gap lengths obtained from the resistance data in Fig. 2(b). (c–e) Schematic diagrams of filament growth.
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References

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