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Atomic View of Filament Growth in Electrochemical Memristive Elements.

Lv H, Xu X, Sun P, Liu H, Luo Q, Liu Q, Banerjee W, Sun H, Long S, Li L, Liu M - Sci Rep (2015)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Key Laboratory of Microelectronics Devices and Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences, Beijing, 100029, China.

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.

No MeSH data available.


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.
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f3: 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.

Mentions: In order to clarify the growth kinetics of filament, HRTEM analysis on the physical nature of CF was carried out. After programing the device with a relatively large compliance current (1 mA/VG@2.5 V), the device was cut with a focused ion beam and characterized by HRTEM. Figure 3a,b show the cross-section of the Cu/HfO2/Pt device and its magnification, respectively. High-resolution images of the cross-section from the left to right corner (regions 1, 2, 3 and 4) are shown in Fig. 3c–f. A conical region with a low contrast in the HfO2 layer was found in the left corner. An electron energy-dispersive spectroscopy (EDS) analysis of this area revealed that it contained a large amount of Cu. The feature peak of the Cu signal in this area was much higher than that in other region (as shown in Figure S5), indicating the CF was copper rich. A gradual profile of Cu concentration was observed on the edge of CF, suggesting more copper in the CF center and less copper at the edges. The Cu electrode was found to have a crystalline structure, as indexed by the lattice fringes in Fig. 3c and yellow framed fast Fourier transformation (FFT) patterns in Fig. 3d–f. The HfO2 layer was found to be amorphous, as can be seen in the blue-framed FFT patterns in Fig. 3c–f. Interestingly, no clear lattice fringe was observed in the CF region, indicating the CF was weakly crystallized or amorphous, which was quite different from the learned knowledge that the CF was with crystalline metallic phase161718. From the refined FFT patterns (shown in the upper inserts of Fig. 3c), the CF edges presented amorphous phases, whereas crystalline phase was detected in the CF center. The measured fringe space (0.2 nm) matched with the d-space of <111> plane of face-centered cubic copper. These findings suggest a possible scenario, i. e. the incorporated Cu element is merged with the amorphous HfO2. If the CF is composed by pure Cu, the original HfO2 material in the CF region should be pushed aside. In such a case, serious structure deformation should take place, however, from the TEM image, no serious deformation around the CF was detected. This result suggests the incorporated Cu element occupy some places in HfO2 lattice. Similar results were also found in Ag/SiO2 system1718, where the Ag cluster was detected in SiO2 material with Ag atoms accumulated in the void position. The movement of Ag cluster from one site to another contributed to the formation of conductive filament. One point should be mentioned that no crystalline Ag was found in the space between the clusters, suggesting the Ag actually be merged with the SiO2 lattice, consistent with the result in this work. According to the formation energy of a Cu atom in the HfO2 lattice, the Cu atom is more likely to occupy the interstitial sites of the HfO2 matrix, rather than the substitutional sites3536. The growth of filament relies on the transportation of Cu element to the next adjacent interstitial site.


Atomic View of Filament Growth in Electrochemical Memristive Elements.

Lv H, Xu X, Sun P, Liu H, Luo Q, Liu Q, Banerjee W, Sun H, Long S, Li L, Liu M - Sci Rep (2015)

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.
© Copyright Policy - open-access
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC4543950&req=5

f3: 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.
Mentions: In order to clarify the growth kinetics of filament, HRTEM analysis on the physical nature of CF was carried out. After programing the device with a relatively large compliance current (1 mA/VG@2.5 V), the device was cut with a focused ion beam and characterized by HRTEM. Figure 3a,b show the cross-section of the Cu/HfO2/Pt device and its magnification, respectively. High-resolution images of the cross-section from the left to right corner (regions 1, 2, 3 and 4) are shown in Fig. 3c–f. A conical region with a low contrast in the HfO2 layer was found in the left corner. An electron energy-dispersive spectroscopy (EDS) analysis of this area revealed that it contained a large amount of Cu. The feature peak of the Cu signal in this area was much higher than that in other region (as shown in Figure S5), indicating the CF was copper rich. A gradual profile of Cu concentration was observed on the edge of CF, suggesting more copper in the CF center and less copper at the edges. The Cu electrode was found to have a crystalline structure, as indexed by the lattice fringes in Fig. 3c and yellow framed fast Fourier transformation (FFT) patterns in Fig. 3d–f. The HfO2 layer was found to be amorphous, as can be seen in the blue-framed FFT patterns in Fig. 3c–f. Interestingly, no clear lattice fringe was observed in the CF region, indicating the CF was weakly crystallized or amorphous, which was quite different from the learned knowledge that the CF was with crystalline metallic phase161718. From the refined FFT patterns (shown in the upper inserts of Fig. 3c), the CF edges presented amorphous phases, whereas crystalline phase was detected in the CF center. The measured fringe space (0.2 nm) matched with the d-space of <111> plane of face-centered cubic copper. These findings suggest a possible scenario, i. e. the incorporated Cu element is merged with the amorphous HfO2. If the CF is composed by pure Cu, the original HfO2 material in the CF region should be pushed aside. In such a case, serious structure deformation should take place, however, from the TEM image, no serious deformation around the CF was detected. This result suggests the incorporated Cu element occupy some places in HfO2 lattice. Similar results were also found in Ag/SiO2 system1718, where the Ag cluster was detected in SiO2 material with Ag atoms accumulated in the void position. The movement of Ag cluster from one site to another contributed to the formation of conductive filament. One point should be mentioned that no crystalline Ag was found in the space between the clusters, suggesting the Ag actually be merged with the SiO2 lattice, consistent with the result in this work. According to the formation energy of a Cu atom in the HfO2 lattice, the Cu atom is more likely to occupy the interstitial sites of the HfO2 matrix, rather than the substitutional sites3536. The growth of filament relies on the transportation of Cu element to the next adjacent interstitial site.

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Key Laboratory of Microelectronics Devices and Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences, Beijing, 100029, China.

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.

No MeSH data available.