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ACKNOWLEDGMENTS
We thank the students and field assistants who collected behavioral
data over 27 years. We also thank S. Rotics for assistance with
survival analysis and Y. Ram, N. Pinter-Wollman, J. Firth, and
two anonymous reviewers for comments on earlier drafts.Funding:A.I. was supported by Israel Science Foundation grants 244/19
and 245/19. K.E.H. was supported by NSF Grants OISE1853934 and
IOS 1755089.Author contributions:A.I. and E.A. designed the
study. K.E.H. collected the data. A.I. analyzed the data and wrote the
manuscript, with input from all authors.Competing interests:
The authors declare no competing interests.Data and materials
availability:Data and code used in this study are available
at Zenodo ( 50 ).
SUPPLEMENTARY MATERIALS
science.sciencemag.org/content/373/6552/348/suppl/DC1
Materials and Methods
Figs. S1 to S13
Tables S1 to S7
References ( 51 – 55 )
10 April 2020; accepted 19 May 2021
10.1126/science.abc1966OXIDE ELECTRONICS
Universal phase dynamics in VO 2 switches revealed
by ultrafast operando diffraction
Aditya Sood1,2, Xiaozhe Shen^3 , Yin Shi^4 , Suhas Kumar^5 †, Su Ji Park^3 ‡, Marc Zajac^2 §, Yifei Sun^6 ¶,
Long-Qing Chen^4 , Shriram Ramanathan^6 , Xijie Wang^3 , William C. Chueh1,2, Aaron M. Lindenberg1,2,3
Understanding the pathways and time scales underlying electrically driven insulator-metal transitions
is crucial for uncovering the fundamental limits of device operation. Using stroboscopic electron
diffraction, we perform synchronized time-resolved measurements of atomic motions and electronic
transport in operating vanadium dioxide (VO 2 ) switches. We discover an electrically triggered,
isostructural state that forms transiently on microsecond time scales, which is shown by phase-field
simulations to be stabilized by local heterogeneities and interfacial interactions between the equilibrium
phases. This metastable phase is similar to that formed under photoexcitation within picoseconds,
suggesting a universal transformation pathway. Our results establish electrical excitation as a route for
uncovering nonequilibrium and metastable phases in correlated materials, opening avenues for
engineering dynamical behavior in nanoelectronics.
T
he insulator-metal transition (IMT) in
correlated oxide semiconductors is a
notable example of an emergent phe-
nomenon arising from the complex in-
terplay between lattice and electronic
degrees of freedom. Electronic and optical
properties change drastically across the IMT,
motivating applications in computing and
photonics ( 1 ). In vanadium dioxide (VO 2 ), an
archetypal correlated material, the IMT can be
driven primarily in three ways—through ther-
mal ( 2 – 4 ), optical ( 5 – 11 ), and electrical ( 12 – 14 )
excitation. Among these, the electrically trig-
gered IMT (E-IMT) is arguably the most useful
for future solid-state devices. It has been used
in applications including steep sub-Boltzmann
switching transistors ( 15 ), neuromorphic cir-
cuits ( 16 , 17 ), and reconfigurable photonics
( 18 – 20 ). In fact, almost all envisioned (and,
to date, demonstrated) applications of VO 2
involve two-terminal devices that are driven
electrically. However, despite its importance,
very little is understood about the mechanisms
underlying E-IMT. Notably, the transforma-
tion pathway from the insulating monoclinic
(M1) to metallic rutile (R) phase under an
electric field remains unknown. In general,
understanding the structural processes me-
diating electric field–driven phase transitions
remains a challenge in condensed-matter
physics. A major roadblock in addressing
these issues has been the lack of a direct
structural probe of the electrically triggered
transient state.
By contrast, there is a rich history of fun-
damental studies probing the ultrafast photo-
induced IMT (P-IMT). Several spectroscopicand structural techniques have shown that
femtosecond optical pulses trigger the trans-
formation ofM1toRon a picosecond time
scale ( 5 – 7 ). In some cases, the structural and
electronic transitions have been observed to
be decoupled, which points toward a photo-
induced isostructural, metallic monoclinic
(mM) phase ( 7 , 8 , 21 ). Given that E-IMT and
P-IMT occur on very different time scales
and are often studied under different experi-
mental conditions, it is still unclear whether
there is a connection between the pathways
followed by the two types of transformations.
In particular, although the existence of an elec-
trically drivenmMphase has been hypothe-
sized previously ( 13 , 22 ), a direct structural
observation of this transient state in operat-
ing devices has remained challenging.
To further our understanding of electric-
field effects and engineer the next generation
of electronic switches based on correlated oxide
semiconductors, it is essential to visualize
atomic motions within the electrically triggered
transition state on fast time scales. Here, we
introduce a stroboscopic mega–electron volt
ultrafast electron diffraction (MeV-UED) tech-
nique and report time-resolved measurements
of atomic structure in electrically excited VO 2
switches. By simultaneously probing changes
in both structure and electronic transport
under a pulsed electrical bias, we directly probe
the mechanisms underlying E-IMT. Figure 1A
shows a schematic of the operando experiment
built at the MeV-UED facility ( 23 , 24 ) at the
SLAC National Accelerator Laboratory [see
fig. S1 and ( 25 ) for details]. Two-terminal
devices were fabricated using 60-nm-thick
polycrystalline VO 2 films deposited on 50-nm-
thick free-standing silicon nitride membranes.
The device was pumped by a periodic train of
voltage pulses synchronized to the 180-Hz clock
of the UED system (Fig. 1B, top). After exci-
tation by a voltage pulse, the time-dependent
structural response was probed through dif-
fraction of a delayed ~100-fs electron pulse.
The resistance of the device was simultane-
ously monitored using a high-speed oscilloscope.352 16 JULY 2021•VOL 373 ISSUE 6552 sciencemag.org SCIENCE
(^1) Stanford Institute for Materials and Energy Sciences, SLAC
National Accelerator Laboratory, Menlo Park, CA 94025,
USA.^2 Department of Materials Science and Engineering,
Stanford University, Stanford, CA 94305, USA.^3 SLAC
National Accelerator Laboratory, Menlo Park, CA 94025,
USA.^4 Department of Materials Science and Engineering, The
Pennsylvania State University, University Park, PA 16802,
USA.^5 Hewlett Packard Labs, Palo Alto, CA 94304, USA.
(^6) School of Materials Engineering, Purdue University, West
Lafayette, IN 47907, USA.
*Corresponding author. Email: [email protected] (A.S.);
[email protected] (A.M.L.)†Present address: Sandia National
Laboratories, Livermore, CA 94550, USA.‡Present address: Center
for Functional Nanomaterials, Brookhaven National Laboratory,
Upton, NY 11973, USA. §Present address: Argonne National
Laboratory, Argonne, IL 60439, USA. ¶Present address: College of
Energy, Xiamen University, Xiamen 361102, China.
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