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Cited by 303Advanced Memory and Neural ComputingFerroelectric and Negative Capacitance DevicesPhase-change materials and chalcogenides

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Evolution of the conductive filament system in HfO2-based memristors observed by direct atomic-scale imaging

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Zhang, Ying, Mao, Ge-Qi, Zhao, Xiaolong, Li, Yu, Zhang, Meiyun, Wu, Zuheng, Wu, Wei, Sun, Huajun, Guo, Yizhong, Wang, Lihua, Zhang, Xumeng, Liu, Qi, Lv, Hangbing, Xue, Kan-Hao, Xu, Guangwei, Miao, Xiangshui, Long, Shibing, Liu, Ming·2021·Nature Communications
DOI 10.1038/s41467-021-27575-zPMC PMC8668918
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Abstract

The resistive switching effect in memristors typically stems from the formation and rupture of localized conductive filament paths, and HfO 2 has been accepted as one of the most promising resistive switching materials. However, the dynamic changes in the resistive switching process, including the composition and structure of conductive filaments, and especially the evolution of conductive filament surroundings, remain controversial in HfO 2-based memristors. Here, the conductive filament system in the amorphous HfO 2-based memristors with various top electrodes is revealed to be with a quasi-core-shell structure consisting of metallic hexagonal-Hf 6 O and its crystalline surroundings (monoclinic or tetragonal HfO x). The phase of the HfO x shell varies with the oxygen reservation capability of the top electrode. According to extensive high-resolution transmission electron microscopy observations and ab initio calculations, the phase transition of the conductive filament shell between monoclinic and tetragonal HfO 2 is proposed to depend on the comprehensive effects of Joule heat from the conductive filament current and the concentration of oxygen vacancies. The quasi-core-shell conductive filament system with an intrinsic barrier, which prohibits conductive filament oxidation, ensures the extreme scalability of resistive switching memristors. This study renovates the understanding of the conductive filament evolution in HfO 2-based memristors and provides potential inspirations to improve oxide memristors for nonvolatile storage-class memory applications.

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Figures

Fig. 1 Structure and RS performance of the Pt/HfO2/Pt memristor. a SEM image and schematic illustration of the Pt/HfO2/Pt memristor. The effective line width of the memristor is 3 μm. b HRTEM image of the fresh Pt/HfO2/Pt stack and the typical FFT diffraction patterns of the marked region of the HfO2 RS layer, indicating the amorphous morphology of the as-fabricated HfO2 film. c Typical RS I–V characteristics of the device. d Retention characteristics of the HRS and LRS of the device for 104 s. e The resistance distribution of HRS and LRS of 100 switching cycles from the 10 randomly selected devices. f Typical V-t and I–t synchronous curves of the OFF and ON switching processes under the pulse mode, where both the LRS and the HRS can be reversed to their opposites within a 300 ns write or erase pulse.
Fig. 1 Structure and RS performance of the Pt/HfO2/Pt memristor. a SEM image and schematic illustration of the Pt/HfO2/Pt memristor. The effective line width of the memristor is 3 μm. b HRTEM image of the fresh Pt/HfO2/Pt stack and the typical FFT diffraction patterns of the marked region of the HfO2 RS layer, indicating the amorphous morphology of the as-fabricated HfO2 film. c Typical RS I–V characteristics of the device. d Retention characteristics of the HRS and LRS of the device for 104 s. e The resistance distribution of HRS and LRS of 100 switching cycles from the 10 randomly selected devices. f Typical V-t and I–t synchronous curves of the OFF and ON switching processes under the pulse mode, where both the LRS and the HRS can be reversed to their opposites within a 300 ns write or erase pulse.CC BY · PubMed Central
Fig. 2 Complete CFs and their m-HfO2 shells in the LRS of Pt/HfO2/Pt RS memristors. a HRTEM of a complete CF in the LRS device operated under 0.1 mA ICC with the typical polymorphous HfOx region, namely, h-Hf6O and m-HfO2 region, as confirmed by their FFT diffraction patterns in b and c. d HRTEM of the CF in the LRS memristor operated under 1 mA ICC, where the h-Hf6O CF is enclosed by the nonconductive m-HfO2 shell, as confirmed by their FFT diffraction patterns in e–g. The inset of d is the high-resolution close-up view of the h-Hf6O CF region marked with a green square.
Fig. 2 Complete CFs and their m-HfO2 shells in the LRS of Pt/HfO2/Pt RS memristors. a HRTEM of a complete CF in the LRS device operated under 0.1 mA ICC with the typical polymorphous HfOx region, namely, h-Hf6O and m-HfO2 region, as confirmed by their FFT diffraction patterns in b and c. d HRTEM of the CF in the LRS memristor operated under 1 mA ICC, where the h-Hf6O CF is enclosed by the nonconductive m-HfO2 shell, as confirmed by their FFT diffraction patterns in e–g. The inset of d is the high-resolution close-up view of the h-Hf6O CF region marked with a green square.CC BY · PubMed Central
Fig. 3 Emergence of t-HfO2 around the ruptured h-Hf6O CFs, which suffered a RESET process. a HRTEM image of the remaining ruptured h-Hf6O CF (in red) in the HRS Pt/HfO2/Pt RS memristor with both sides enclosed by t-HfO2 (in orange) and m-HfO2 (in blue), as confirmed by their typical FFT diffraction patterns in b–d. e HRTEM image of another ruptured CF region, where only t-HfO2 (in orange), a-HfO2 (in light blue), and t-HfO (in dark blue) are observed, while the CF is completely ruptured, as confirmed by their FFT diffraction patterns in f–h.
Fig. 3 Emergence of t-HfO2 around the ruptured h-Hf6O CFs, which suffered a RESET process. a HRTEM image of the remaining ruptured h-Hf6O CF (in red) in the HRS Pt/HfO2/Pt RS memristor with both sides enclosed by t-HfO2 (in orange) and m-HfO2 (in blue), as confirmed by their typical FFT diffraction patterns in b–d. e HRTEM image of another ruptured CF region, where only t-HfO2 (in orange), a-HfO2 (in light blue), and t-HfO (in dark blue) are observed, while the CF is completely ruptured, as confirmed by their FFT diffraction patterns in f–h.CC BY · PubMed Central
Fig. 4 Ab initio calculations of the HfOx system. a Relative free energy per HfOx formula unit of t-HfO2, t-HfO, t-Hf2O3, h-HfO, h-Hf2O, h-Hf3O, and h-Hf6O against the decomposed h.c.p Hf and m-HfO2. b The Gibbs free energy of t-HfOx with reference to m-HfOx at 300 K, evaluated at several VO concentrations by introducing various amounts of VO into a Hf32O64 supercell. c Atomistic structure of our Hf6O-in-m-HfO2 CF model, where the Hf6O metal core is shaded. d The local density of states on several marked Hf atoms in this model with the Fermi level aligned to zero.
Fig. 4 Ab initio calculations of the HfOx system. a Relative free energy per HfOx formula unit of t-HfO2, t-HfO, t-Hf2O3, h-HfO, h-Hf2O, h-Hf3O, and h-Hf6O against the decomposed h.c.p Hf and m-HfO2. b The Gibbs free energy of t-HfOx with reference to m-HfOx at 300 K, evaluated at several VO concentrations by introducing various amounts of VO into a Hf32O64 supercell. c Atomistic structure of our Hf6O-in-m-HfO2 CF model, where the Hf6O metal core is shaded. d The local density of states on several marked Hf atoms in this model with the Fermi level aligned to zero.CC BY · PubMed Central
Fig. 5 Schematic illustration of the CF system evolution in Pt/HfO2/Pt memristor. a Initial state of the device. b Under a positive electric field, dissociated O2− ions from HfO2 are generated near the BE and move from the BE to the TE, resulting in localized oxygen depletion. c Growth of Hf-rich or O-vacancy CFs in the RS layer. d The as-grown CF bridge and the annealing effect of Joule heat generated by the high current. e The as-formed quasi-core-shell CF system consists of conductive h-Hf6O and the surrounding nonconductive monoclinic HfO2 (m-HfO2) shell. f Longitudinal-section view of the core-shell CF system. g Transition of the surrounding m-HfO2 shell into t-HfO2 driven by the high current through the Hf6O CF in the RESET process. h Ruptured h-Hf6O CF and its t-HfO2 shell.
Fig. 5 Schematic illustration of the CF system evolution in Pt/HfO2/Pt memristor. a Initial state of the device. b Under a positive electric field, dissociated O2− ions from HfO2 are generated near the BE and move from the BE to the TE, resulting in localized oxygen depletion. c Growth of Hf-rich or O-vacancy CFs in the RS layer. d The as-grown CF bridge and the annealing effect of Joule heat generated by the high current. e The as-formed quasi-core-shell CF system consists of conductive h-Hf6O and the surrounding nonconductive monoclinic HfO2 (m-HfO2) shell. f Longitudinal-section view of the core-shell CF system. g Transition of the surrounding m-HfO2 shell into t-HfO2 driven by the high current through the Hf6O CF in the RESET process. h Ruptured h-Hf6O CF and its t-HfO2 shell.CC BY · PubMed Central

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Metadata

Authors
Zhang, Y., Mao, G.-Q., Zhao, X., Li, Y., Zhang, M., Wu, Z., Wu, W., Sun, H., Guo, Y., Wang, L., Zhang, X., Liu, Q., Lv, H., Xue, K.-H., Xu, G., Miao, X., Long, S., & Liu, M.
Affiliations
  • University of Science and Technology of China (CN)
  • Huazhong University of Science and Technology (CN)
  • Chinese Academy of Sciences (CN)
  • Beijing University of Technology (CN)
Corresponding author
  • Zhao, Xiaolong
  • Xue, Kan-Hao
  • Long, Shibing
  • Liu, Ming
ORCID
Venue
Nature Communications
Publisher
Springer Science and Business Media LLC
Published
2021-12-13
Volume
12
Issue
1
Pages
7232-7232
ISSN
2041-1723 (ISSN-L)
DOI
10.1038/s41467-021-27575-z
PMC
PMC8668918
OpenAlex
W4200157328
Cited by
300
References
75
Citations by year
2026: 47 · 2025: 72 · 2024: 70 · 2023: 85 · 2022: 26
FWCI
13.31
Type
journal article
Language
en
Open access
Licence
Locations
  • Nature Communications (publishedVersion, cc-by)
  • PubMed Central (publishedVersion, cc-by)
  • PubMed (publishedVersion)
  • RePEc: Research Papers in Economics (submittedVersion)
  • DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
Funders
National Natural Science Foundation of China (61925110, 61821091, 51961145110)
Topics
  • Physical Sciences › Engineering › Electrical and Electronic Engineering › Advanced Memory and Neural Computing
  • Physical Sciences › Engineering › Electrical and Electronic Engineering › Ferroelectric and Negative Capacitance Devices
  • Physical Sciences › Materials Science › Materials Chemistry › Phase-change materials and chalcogenides
Keywords
conductive filaments, resistive switching, amorphous HfO2, core-shell structure, tetragonal to monoclinic phase transformation, oxygen vacancies, Joule heating, non-volatile memory

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