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Electrical control of glass-like dynamics in vanadium dioxide for data storage and processing

DOI:10.1038/s41928-022-00812-z
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Mohammad Samizadeh Nikoo at ETH Zurich
Mohammad Samizadeh Nikoo
Reza Soleimanzadeh at École Polytechnique Fédérale de Lausanne
Reza Soleimanzadeh
Krammer Anna at École Polytechnique Fédérale de Lausanne
Krammer Anna
Guilherme Migliato Marega at École Polytechnique Fédérale de Lausanne
Guilherme Migliato Marega
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Abstract and Figures

Metal–oxide–semiconductor junctions are the building blocks of modern electronics and can provide a variety of functionalities, from memory to computing. The technology, however, faces constraints in terms of further miniaturization and compatibility with post–von Neumann computing architectures. Manipulation of structural—rather than electronic—states could provide a path to ultrascaled low-power functional devices, but the electrical control of such states is challenging. Here we report electronically accessible long-lived structural states in vanadium dioxide that can provide a scheme for data storage and processing. The states can be arbitrarily manipulated on short timescales and tracked beyond 10,000 s after excitation, exhibiting features similar to glasses. In two-terminal devices with channel lengths down to 50 nm, sub-nanosecond electrical excitation can occur with an energy consumption as small as 100 fJ. These glass-like functional devices could outperform conventional metal–oxide–semiconductor electronics in terms of speed, energy consumption and miniaturization, as well as provide a route to neuromorphic computation and multilevel memories. Electronically accessible states in vanadium dioxide can be arbitrarily manipulated on short timescales and tracked beyond 10,000 s after excitation.
Tracing the state dynamics of VO2 switches with incubation time a, Schematic of the ultrafast time-domain experimental setup. The SEM image shows the VO2 switch. Scale bar, 5 μm. The devices investigated had lengths varying from 50 nm to 3 μm. b, Transient conductance of the VO2 channel corresponding to different relaxation times T, as well as the very first switching cycle. The incubation time (tinc) and conductance of the insulating state (Gins) were studied. c, Incubation time versus relaxation time. Here tinc is a logarithmic function of T. The error bars are smaller than the symbol dimension. d, Gins versus relaxation time. After ~1 s, variations in conductance are no longer detectable. e, Incubation time versus increased conductance ((Gins−Ḡ)/Gins\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$(G_{{{{\mathrm{ins}}}}} - \bar G)/G_{{{{\mathrm{ins}}}}}$$\end{document}, where Ḡ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar G$$\end{document} is the average over Gins for measurements corresponding to 10 s relaxation) for different relaxation times for a 100-nm-long device, showing the reproducibility of the results in nano-devices. The inset illustrates the fast relaxation of temperature and resistance, as well as the slow dynamics of tinc. f, Monitored incubation time for 5,000 measurements with two different relaxation times (10 and 100 ms), showing that the effect is reversible and consistent.
Tracing the state dynamics of VO2 switches with incubation time a, Schematic of the ultrafast time-domain experimental setup. The SEM image shows the VO2 switch. Scale bar, 5 μm. The devices investigated had lengths varying from 50 nm to 3 μm. b, Transient conductance of the VO2 channel corresponding to different relaxation times T, as well as the very first switching cycle. The incubation time (tinc) and conductance of the insulating state (Gins) were studied. c, Incubation time versus relaxation time. Here tinc is a logarithmic function of T. The error bars are smaller than the symbol dimension. d, Gins versus relaxation time. After ~1 s, variations in conductance are no longer detectable. e, Incubation time versus increased conductance ((Gins−Ḡ)/Gins\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$(G_{{{{\mathrm{ins}}}}} - \bar G)/G_{{{{\mathrm{ins}}}}}$$\end{document}, where Ḡ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar G$$\end{document} is the average over Gins for measurements corresponding to 10 s relaxation) for different relaxation times for a 100-nm-long device, showing the reproducibility of the results in nano-devices. The inset illustrates the fast relaxation of temperature and resistance, as well as the slow dynamics of tinc. f, Monitored incubation time for 5,000 measurements with two different relaxation times (10 and 100 ms), showing that the effect is reversible and consistent.
… 
Manipulation of post-firing state of VO2 switches a, Schematic showing the excitation of the VO2 switch with single or multiple pulses and monitoring tinc at the sensing pulse after a long relaxation time T. b, Measured tinc for different excitation pulse numbers (N) for ΔT = 1 ms and T = 1 s. c, After multiple pulses, the film undergoes different relaxation trajectories, and monitoring tinc can identify the different original excitations.
Manipulation of post-firing state of VO2 switches a, Schematic showing the excitation of the VO2 switch with single or multiple pulses and monitoring tinc at the sensing pulse after a long relaxation time T. b, Measured tinc for different excitation pulse numbers (N) for ΔT = 1 ms and T = 1 s. c, After multiple pulses, the film undergoes different relaxation trajectories, and monitoring tinc can identify the different original excitations.
… 
Exploration on possible mechanisms behind the long memory a, Schematic of an experimental setup for a synchronized measurement in which the memory is induced by a heat pulse. b, Illustration of waveforms in the synchronized measurement. Channel 1 triggers the IMT in isolated pulsed heaters, which ensures thermal-driven phase transition in the middle switch. The effect of the heat-induced IMT is evaluated in the incubation of the middle switch excited by channel 2. c, Measurements indicate that memory can be induced by thermal IMT, showing the generality of the effect. d, Schematic of an experimental setup for a synchronized measurement on a VO2 cross structure. The effect of IMT triggering in the horizontal path is measured by monitoring the incubation time in the vertical path. Scale bar (false-coloured SEM image), 2 μm. e, Output current at terminal 4 of the cross with a 1 s history of IMT triggered in the horizontal path (red) and without memory (blue). This indicates that the memory effect originates from the VO2 channel and rules out the possible effect from the metal–VO2 junction. f, Evaluation of memory effect on VO2 devices with different gap lengths ranging from 50 to 500 nm. Although devices with smaller channel lengths show a higher increase in conductance, the memory embedded in the incubation time is nearly constant (Extended Data Fig. 7).
Exploration on possible mechanisms behind the long memory a, Schematic of an experimental setup for a synchronized measurement in which the memory is induced by a heat pulse. b, Illustration of waveforms in the synchronized measurement. Channel 1 triggers the IMT in isolated pulsed heaters, which ensures thermal-driven phase transition in the middle switch. The effect of the heat-induced IMT is evaluated in the incubation of the middle switch excited by channel 2. c, Measurements indicate that memory can be induced by thermal IMT, showing the generality of the effect. d, Schematic of an experimental setup for a synchronized measurement on a VO2 cross structure. The effect of IMT triggering in the horizontal path is measured by monitoring the incubation time in the vertical path. Scale bar (false-coloured SEM image), 2 μm. e, Output current at terminal 4 of the cross with a 1 s history of IMT triggered in the horizontal path (red) and without memory (blue). This indicates that the memory effect originates from the VO2 channel and rules out the possible effect from the metal–VO2 junction. f, Evaluation of memory effect on VO2 devices with different gap lengths ranging from 50 to 500 nm. Although devices with smaller channel lengths show a higher increase in conductance, the memory embedded in the incubation time is nearly constant (Extended Data Fig. 7).
… 
Evidence of glass-like dynamics a, Illustration of the excitation signal of a VO2 switch for measurements under laser light. Every second, two identical pulses with time separation T are applied to the device and incubation times are monitored (tincref\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$t_{{{{\mathrm{inc}}}}}^{{{{\mathrm{ref}}}}}$$\end{document} and tinc). b, Relative change in the incubation time (tinc−tincref)/tincref\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left( {(t_{{{{\mathrm{inc}}}}} - t_{{{{\mathrm{inc}}}}}^{{{{\mathrm{ref}}}}})/t_{{{{\mathrm{inc}}}}}^{{{{\mathrm{ref}}}}}} \right)$$\end{document} versus increased Gins in a 200-nm-long channel VO2 switch. Measurements under a CW 532 nm laser light with a power density of ~100 W cm⁻² show a considerably higher conductivity; however, the memory effect is unchanged. The experiment shows that the memory effect is likely structural. c, Relative change in incubation time after T = 10 ms with respect to the reference pulses with relaxation time of T = 1 s at different temperatures. The more pronounced change in tinc at higher temperatures indicates faster relaxation (inset). d, Transient current density of the VO2 switch corresponding to two consecutive triggering events with Vset = 290 mV at a chuck temperature of 55 °C. The results show a notable change in tinc.
Evidence of glass-like dynamics a, Illustration of the excitation signal of a VO2 switch for measurements under laser light. Every second, two identical pulses with time separation T are applied to the device and incubation times are monitored (tincref\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$t_{{{{\mathrm{inc}}}}}^{{{{\mathrm{ref}}}}}$$\end{document} and tinc). b, Relative change in the incubation time (tinc−tincref)/tincref\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left( {(t_{{{{\mathrm{inc}}}}} - t_{{{{\mathrm{inc}}}}}^{{{{\mathrm{ref}}}}})/t_{{{{\mathrm{inc}}}}}^{{{{\mathrm{ref}}}}}} \right)$$\end{document} versus increased Gins in a 200-nm-long channel VO2 switch. Measurements under a CW 532 nm laser light with a power density of ~100 W cm⁻² show a considerably higher conductivity; however, the memory effect is unchanged. The experiment shows that the memory effect is likely structural. c, Relative change in incubation time after T = 10 ms with respect to the reference pulses with relaxation time of T = 1 s at different temperatures. The more pronounced change in tinc at higher temperatures indicates faster relaxation (inset). d, Transient current density of the VO2 switch corresponding to two consecutive triggering events with Vset = 290 mV at a chuck temperature of 55 °C. The results show a notable change in tinc.
… 
Electrical control of glass-like dynamics for computation and data storage a, Dynamic change in energy barrier (Ebar) of a 2-μm-long VO2 switch with the number of switching events for different pulse separations. b, Schematic showing that shorter pulse separations result in lower Ebar. c, Considered neural network. A VO2 switch is placed in the interconnection between each of the two nodes. ΔEA and ΔEB matrices represent the reduced energy barriers corresponding to the Ai − Xj and Xj − Bk interconnections. d, Labels used to train a network for a 3 × 3 pixel image classification. e, Post-training ΔEA × ΔEB matrix represents the effective reduced energy barrier corresponding to each output. f, Simulated output signals at three nodes: B1 (circles), B2 (triangles) and B3 (squares). Exposing the network to the inputs and monitoring the output currents successfully classifies the samples. g, Measured current of a VO2 nanodevice showing the potential of glass-like relaxation for ultrascaled memory devices. The inset shows the false-coloured SEM image of the VO2 switch. Scale bar, 100 nm. h, Measured current of the VO2 nanodevice (inset of g) excited with Vset = 2.5 V (red line) and Vset = 3.0 V (blue line), showing that electrically induced memory can take place in the sub-nanosecond timescales.
Electrical control of glass-like dynamics for computation and data storage a, Dynamic change in energy barrier (Ebar) of a 2-μm-long VO2 switch with the number of switching events for different pulse separations. b, Schematic showing that shorter pulse separations result in lower Ebar. c, Considered neural network. A VO2 switch is placed in the interconnection between each of the two nodes. ΔEA and ΔEB matrices represent the reduced energy barriers corresponding to the Ai − Xj and Xj − Bk interconnections. d, Labels used to train a network for a 3 × 3 pixel image classification. e, Post-training ΔEA × ΔEB matrix represents the effective reduced energy barrier corresponding to each output. f, Simulated output signals at three nodes: B1 (circles), B2 (triangles) and B3 (squares). Exposing the network to the inputs and monitoring the output currents successfully classifies the samples. g, Measured current of a VO2 nanodevice showing the potential of glass-like relaxation for ultrascaled memory devices. The inset shows the false-coloured SEM image of the VO2 switch. Scale bar, 100 nm. h, Measured current of the VO2 nanodevice (inset of g) excited with Vset = 2.5 V (red line) and Vset = 3.0 V (blue line), showing that electrically induced memory can take place in the sub-nanosecond timescales.
… 
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Articles
https://doi.org/10.1038/s41928-022-00812-z
1Power and Wide-band-gap Electronics Research Laboratory (POWERlab), Institute of Electrical and Micro Engineering, École Polytechnique Fédérale
de Lausanne (EPFL), Lausanne, Switzerland. 2Solar Energy and Building Physics Laboratory, Institute of Civil Engineering, École Polytechnique Fédérale
de Lausanne (EPFL), Lausanne, Switzerland. 3Laboratory of Nanoscale Electronics and Structures (LANES), Institute of Electrical and Micro Engineering
and Institute of Materials Science and Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland. 4Department of Materials
Science and Engineering (MSE), Pohang University of Science and Technology (POSTECH), Pohang, Republic of Korea. 5Laboratory of Quantum Materials
(QMAT), Institute of Materials (IMX), École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland. 6Max Planck Institute for the Structure
and Dynamics of Matter, Hamburg, Germany. ✉e-mail: mohammad.samizadeh@epfl.ch; elison.matioli@epfl.ch
Strongly correlated materials—in which several physical inter-
actions involving spin, charge, lattice and orbit are simultane-
ously active—can exhibit notable electrical properties1. Among
them, the first-order insulator–metal transition (IMT) in vanadium
dioxide (VO2), which happens close to room temperature, is of par-
ticular interest2–6. From a physical point of view, understanding the
underlying mechanism of phase switching in VO2 is still a challenge,
as several models ranging from Peierls to Mott–Hubbard types
were not successful in explaining the broad range of phenomena
occurring in the material7. Different types of excitation—including
temperature, electric field and doping—can induce an IMT, which
makes the understanding of phase switching more difficult2,8. From
a technological point of view, the bulk conductivity and abrupt
phase transition in VO2 can potentially overcome some of the
fundamental limitations in conventional metal–oxide–semicon-
ductor electronics, including the limited conductance imposed by
Thomas–Fermi screening9 and the thermionic subthreshold-slope
limit imposed by Boltzmann tyranny10.
In addition to the application of such phase-change materials in
traditional electronics, the rich variety of phenomena in VO2 (refs.
11–15) can provide novel functionalities for future electronic devices.
In this Article, we report electrically controllable glass-like states in
VO2, which could be used to create a platform for information pro-
cessing and storage. We show that two-terminal devices exhibit a
continuous spectrum of states that are revealed by the incubation
time of the IMT: the time at which the nucleation of phase transi-
tion percolates to form the first conductive filament between the
two terminals of the switch. The state can be imposed by a sequence
of binary switching events and can be tracked for hours after
excitation.
Electrical manipulation and probing of VO2 devices
Figure 1a shows an ultrafast time-domain experimental setup that
can precisely collect the temporal response of a two-terminal VO2
switch (inset). The device was integrated with radio-frequency
pads (ground–signal–ground configuration) that—together with
high-frequency probes—enable accurate measurements with time
resolutions down to ~5 ps (ref. 16). A square-pulse generator applies
repetitive 10-μs-long pulses with a fixed amplitude (set voltage
Vset = 2.1 V) to a two-port 3-μm-long VO2 switch. The waveform of
the current passing through the device is measured at the 50 Ω port
of a high-frequency oscilloscope, and the transient conductance of
the device is extracted. Following an applied pulse, the VO2 film ini-
tially exhibits an insulating behaviour; only after an incubation time
tinc, it undergoes an IMT (Fig. 1b). The measurements indicate that
the incubation time strongly depends on the history of the previous
phase transitions. The very first switching curve (Fig. 1b) shows an
incubation time of ~1.4 μs. Triggering an IMT and measuring the
incubation time after a 10-ms-long relaxation time (T) results in a
ten times shorter incubation time. Longer relaxation times after the
first phase transition cause longer incubation times; however, the
value of tinc is still lower than that of the very first switching, even
after T = 10,000 s.
Incubation time versus relaxation time (Fig. 1c) indicates a loga-
rithmic relation tinc = (78 ns)log[T/(160 μs)]. Although tinc has strong
dependence on the previous switching events, device conductance
Electrical control of glass-like dynamics in
vanadium dioxide for data storage and processing
Mohammad Samizadeh Nikoo 1 ✉ , Reza Soleimanzadeh1, Anna Krammer2,
Guilherme Migliato Marega 3, Yunkyu Park4, Junwoo Son 4, Andreas Schueler2, Andras Kis 3,
Philip J. W. Moll 5,6 and Elison Matioli 1 ✉
Metal–oxide–semiconductor junctions are the building blocks of modern electronics and can provide a variety of functionalities,
from memory to computing. The technology, however, faces constraints in terms of further miniaturization and compatibility
with post–von Neumann computing architectures. Manipulation of structural—rather than electronic—states could provide a
path to ultrascaled low-power functional devices, but the electrical control of such states is challenging. Here we report elec-
tronically accessible long-lived structural states in vanadium dioxide that can provide a scheme for data storage and process-
ing. The states can be arbitrarily manipulated on short timescales and tracked beyond 10,000 s after excitation, exhibiting
features similar to glasses. In two-terminal devices with channel lengths down to 50 nm, sub-nanosecond electrical excitation
can occur with an energy consumption as small as 100 fJ. These glass-like functional devices could outperform conventional
metal–oxide–semiconductor electronics in terms of speed, energy consumption and miniaturization, as well as provide a route
to neuromorphic computation and multilevel memories.
NATURE ELECTRONICS | VOL 5 | SEPTEMBER 2022 | 596–603 | www.nature.com/natureelectronics
596
Content courtesy of Springer Nature, terms of use apply. Rights reserved
... Neuromorphic computing represents a computational paradigm that aims to emulate characteristics of information processing in the brain. Mott semiconductors offer a powerful platform to realize neuromorphic hardware via electrically driven conductance transitions (1)(2)(3). For instance, a single vanadium dioxide (VO 2 ) device connected to a capacitor can serve as an oscillatory artificial neuron in a more compact manner (4-7) compared to traditional silicon (Si) complementary metal-oxide semiconductor layouts. ...
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Article
  • Oct 2002
The results of first principles electronic structure calculations for the metallic rutile and the insulating monoclinic phase of vanadium dioxide are presented. In addition, the insulating phase is investigated for the first time. The density functional calculations allow for a consistent understanding of all three phases. In the rutile phase metallic conductivity is carried by metal orbitals, which fall into the one‐dimensional band, and the isotropically dispersing bands. Hybridization of both types of bands is weak. In the phase splitting of the band due to metal‐metal dimerization and upshift of the bands due to increased p‐d overlap lead to an effective separation of both types of bands. Despite incomplete opening of the optical band gap due to the shortcomings of the local density approximation, the metal‐insulator transition can be understood as a Peierls‐like instability of the band in an embedding background of electrons. In the phase, the metal‐insulator transition arises as a combined embedded Peierls‐like and antiferromagnetic instability. The results for VO2 fit into the general scenario of an instability of the rutile‐type transition‐metal dioxides at the beginning of the d series towards dimerization or antiferromagnetic ordering within the characteristic metal chains. This scenario was successfully applied before to MoO2 and NbO2. In the compounds, the and bands can be completely separated, which leads to the observed metal‐insulator transitions.
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Generation of Tunable Stochastic Sequences Using the Insulator–Metal Transition
Article
  • Jan 2022
Probabilistic computing is a paradigm in which data are not represented by stable bits, but rather by the probability of a metastable bit to be in a particular state. The development of this technology has been hindered by the availability of hardware capable of generating stochastic and tunable sequences of "1s" and "0s". The options are currently limited to complex CMOS circuitry and, recently, magnetic tunnel junctions. Here, we demonstrate that metal-insulator transitions can also be used for this purpose. We use an electrical pump/probe protocol and take advantage of the stochastic relaxation dynamics in VO2 to induce random metallization events. A simple latch circuit converts the metallization sequence into a random stream of 1s and 0s. The resetting pulse in between probes decorrelates successive events, providing a true stochastic digital sequence.
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Spatiotemporal characterization of the field-induced insulator-to-metal transition
Article
  • Aug 2021
Watching a metal filament grow Resistive switching is a process in which the electrical resistance of a sample changes abruptly in response to a voltage pulse, often by orders of magnitude. This process is at the heart of many neuromorphic computing approaches but visualizing it in both space and time is tricky. del Valle et al . monitored the resistive switching in three different vanadium oxide compounds by measuring time- and space-resolved optical reflectivity (see the Perspective by Hilgenkamp and Gao). A characteristic conducting filament was quickly nucleated on the inhomogeneities in the sample and then propagated due to Joule heating. —JS
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Nanoscale Imaging and Control of Volatile and Non‐Volatile Resistive Switching in VO 2
Article
  • Nov 2020
Control of the metal‐insulator phase transition is vital for emerging neuromorphic and memristive technologies. The ability to alter the electrically driven transition between volatile and non‐volatile states is particularly important for quantum‐materials‐based emulation of neurons and synapses. The major challenge of this implementation is to understand and control the nanoscale mechanisms behind these two fundamental switching modalities. Here, in situ X‐ray nanoimaging is used to follow the evolution of the nanostructure and disorder in the archetypal Mott insulator VO2 during an electrically driven transition. Our findings demonstrate selective and reversible stabilization of either the insulating or metallic phases achieved by manipulating the defect concentration. This mechanism enables us to alter the local switching response between volatile and persistent regimes and demonstrates a new possibility for nanoscale control of the resistive switching in Mott materials.
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Nucleation and Growth Bottleneck in the Conductivity Recovery Dynamics of Nickelate Ultrathin Films
Article
We investigate THz conductivity dynamics in NdNiO3 and EuNiO3 ultrathin films (15 u.c., ∼ 5.7 nm thick) following a photoinduced thermal quench into the metallic state and reveal a clear contrast between first- and second-order dynamics. While in EuNiO3 the conductivity recovers exponentially, in NdNiO3 the recovery is non- exponential and slower than a simple thermal model. Crucially, it is consistent with first-order dynamics and well-described by a 2d Avrami model, with supercooling leading to metastable phase coexistence on the nano- to mesoscopic scale. This novel observation is a fundamentally dynamic manifestation of the first order character of the insulator-to-metal transition, which the nanoscale thickness of our films and their fast cooling rate uniquely enable us to detect. The large transients seen in our films are promising for fast electronic (and magnetic) switching applications.
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