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Deterministic conversion between memory and threshold resistive switching via tuning the strong electron correlation.

Peng HY, Li YF, Lin WN, Wang YZ, Gao XY, Wu T - Sci Rep (2012)

Bottom Line: Intensive investigations have been launched worldwide on the resistive switching (RS) phenomena in transition metal oxides due to both fascinating science and potential applications in next generation nonvolatile resistive random access memory (RRAM) devices.It is noteworthy that most of these oxides are strongly correlated electron systems, and their electronic properties are critically affected by the electron-electron interactions.Moreover, from first-principles calculations and x-ray absorption spectroscopy studies, we found that the strong electron correlations and the exchange interactions between Ni and O orbitals play deterministic roles in the RS operations.

View Article: PubMed Central - PubMed

Affiliation: Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore.

ABSTRACT
Intensive investigations have been launched worldwide on the resistive switching (RS) phenomena in transition metal oxides due to both fascinating science and potential applications in next generation nonvolatile resistive random access memory (RRAM) devices. It is noteworthy that most of these oxides are strongly correlated electron systems, and their electronic properties are critically affected by the electron-electron interactions. Here, using NiO as an example, we show that rationally adjusting the stoichiometry and the associated defect characteristics enables controlled room temperature conversions between two distinct RS modes, i.e., nonvolatile memory switching and volatile threshold switching, within a single device. Moreover, from first-principles calculations and x-ray absorption spectroscopy studies, we found that the strong electron correlations and the exchange interactions between Ni and O orbitals play deterministic roles in the RS operations.

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Schematics illustrating the correlation between the band structure and the RS volatility.(a) Energy diagram of NiO as a charge-transfer insulator. Δ denotes the energy of the ligand-to-metal charge-transfer,  (: ligand hole). U represents the intra-atomic d-d Coulomb energy. Correspondingly, we observed (b) voltage-controlled memory switching. (c) Energy diagram of non-stoichiometric Ni1−xO which can be classified as a negative-charge-transfer insulator, where the band gap is determined by the split oxygen 2p bands. In devices made of such Ni-deficient NiO, we observed (d) voltage-controlled threshold switching.
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f1: Schematics illustrating the correlation between the band structure and the RS volatility.(a) Energy diagram of NiO as a charge-transfer insulator. Δ denotes the energy of the ligand-to-metal charge-transfer, (: ligand hole). U represents the intra-atomic d-d Coulomb energy. Correspondingly, we observed (b) voltage-controlled memory switching. (c) Energy diagram of non-stoichiometric Ni1−xO which can be classified as a negative-charge-transfer insulator, where the band gap is determined by the split oxygen 2p bands. In devices made of such Ni-deficient NiO, we observed (d) voltage-controlled threshold switching.

Mentions: We illustrate the key idea in Figure 1. As shown in Figure 1a, NiO is a charge-transfer insulator, whose band gap is between the oxygen 2p band and the nickel 3d upper Hubbard band31. The intra-atomic d-d Coulomb energy U is a direct result of the strong electron correlation. Since the first ionization state is of primarily oxygen 2p character rather than of nickel 3d character, the charge-fluctuation energy is determined by the change transfer gap (Δ) , where denotes a hole in the oxygen valence band. This charge transfer process is analogous to the redox reaction, involving electron transfer between oxygen and nickel, which is believed to play the pivotal role in memory switching27. As a charge-transfer insulator, NiO exhibits the voltage-controlled memory RS behavior (Fig. 1b). On the other hand, tuning the chemical stoichiometry in NiO can effectively modify the electron correlation and cause the band splitting. As shown in Figure 1c, the nickel deficiency decreases the charge transfer gap Δ and changes NiO toward the negative-charge-transfer insulator type31, in which the band gap is not of the p-to-d nor d-to-d but p-p type, , with a considerable mixture of d character into the p states. Our key hypothesis is that this alteration of the band structure will drastically affect the switching and give rise to the voltage-controlled threshold switching in RS devices made of Ni-deficient NiO (Fig. 1d).


Deterministic conversion between memory and threshold resistive switching via tuning the strong electron correlation.

Peng HY, Li YF, Lin WN, Wang YZ, Gao XY, Wu T - Sci Rep (2012)

Schematics illustrating the correlation between the band structure and the RS volatility.(a) Energy diagram of NiO as a charge-transfer insulator. Δ denotes the energy of the ligand-to-metal charge-transfer,  (: ligand hole). U represents the intra-atomic d-d Coulomb energy. Correspondingly, we observed (b) voltage-controlled memory switching. (c) Energy diagram of non-stoichiometric Ni1−xO which can be classified as a negative-charge-transfer insulator, where the band gap is determined by the split oxygen 2p bands. In devices made of such Ni-deficient NiO, we observed (d) voltage-controlled threshold switching.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3369197&req=5

f1: Schematics illustrating the correlation between the band structure and the RS volatility.(a) Energy diagram of NiO as a charge-transfer insulator. Δ denotes the energy of the ligand-to-metal charge-transfer, (: ligand hole). U represents the intra-atomic d-d Coulomb energy. Correspondingly, we observed (b) voltage-controlled memory switching. (c) Energy diagram of non-stoichiometric Ni1−xO which can be classified as a negative-charge-transfer insulator, where the band gap is determined by the split oxygen 2p bands. In devices made of such Ni-deficient NiO, we observed (d) voltage-controlled threshold switching.
Mentions: We illustrate the key idea in Figure 1. As shown in Figure 1a, NiO is a charge-transfer insulator, whose band gap is between the oxygen 2p band and the nickel 3d upper Hubbard band31. The intra-atomic d-d Coulomb energy U is a direct result of the strong electron correlation. Since the first ionization state is of primarily oxygen 2p character rather than of nickel 3d character, the charge-fluctuation energy is determined by the change transfer gap (Δ) , where denotes a hole in the oxygen valence band. This charge transfer process is analogous to the redox reaction, involving electron transfer between oxygen and nickel, which is believed to play the pivotal role in memory switching27. As a charge-transfer insulator, NiO exhibits the voltage-controlled memory RS behavior (Fig. 1b). On the other hand, tuning the chemical stoichiometry in NiO can effectively modify the electron correlation and cause the band splitting. As shown in Figure 1c, the nickel deficiency decreases the charge transfer gap Δ and changes NiO toward the negative-charge-transfer insulator type31, in which the band gap is not of the p-to-d nor d-to-d but p-p type, , with a considerable mixture of d character into the p states. Our key hypothesis is that this alteration of the band structure will drastically affect the switching and give rise to the voltage-controlled threshold switching in RS devices made of Ni-deficient NiO (Fig. 1d).

Bottom Line: Intensive investigations have been launched worldwide on the resistive switching (RS) phenomena in transition metal oxides due to both fascinating science and potential applications in next generation nonvolatile resistive random access memory (RRAM) devices.It is noteworthy that most of these oxides are strongly correlated electron systems, and their electronic properties are critically affected by the electron-electron interactions.Moreover, from first-principles calculations and x-ray absorption spectroscopy studies, we found that the strong electron correlations and the exchange interactions between Ni and O orbitals play deterministic roles in the RS operations.

View Article: PubMed Central - PubMed

Affiliation: Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore.

ABSTRACT
Intensive investigations have been launched worldwide on the resistive switching (RS) phenomena in transition metal oxides due to both fascinating science and potential applications in next generation nonvolatile resistive random access memory (RRAM) devices. It is noteworthy that most of these oxides are strongly correlated electron systems, and their electronic properties are critically affected by the electron-electron interactions. Here, using NiO as an example, we show that rationally adjusting the stoichiometry and the associated defect characteristics enables controlled room temperature conversions between two distinct RS modes, i.e., nonvolatile memory switching and volatile threshold switching, within a single device. Moreover, from first-principles calculations and x-ray absorption spectroscopy studies, we found that the strong electron correlations and the exchange interactions between Ni and O orbitals play deterministic roles in the RS operations.

Show MeSH