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Multi-level operation in VO2-based resistive
switching devices
Cite as: AIP Advances 12, 015218 (2022); doi: 10.1063/5.0077160
Submitted: 6 December 2021 •Accepted: 18 December 2021 •
Published Online: 13 January 2022
Xing Gao, Carlos M. M. Rosário, and Hans Hilgenkampa)
AFFILIATIONS
Faculty of Science and Technology and MESA+Institute for Nanotechnology, University of Twente, Enschede,
The Netherlands
a)Author to whom correspondence should be addressed: h.hilgenkamp@utwente.nl
ABSTRACT
Vanadium dioxide (VO2) is widely studied for its prominent insulator–metal transition (IMT) near room temperature, with poten-
tial applications in novel memory devices and brain-inspired neuromorphic computing. We report on the fabrication of in-plane VO2
metal–insulator–metal structures and reproducible switching measurements in these two-terminal devices. Resistive switching can be
achieved by applying voltage or current bias, which creates Joule heating in the device and triggers the IMT. We analyze the current/voltage-
induced resistive switching characteristics, including a pronounced intermediate state in the reset from the low to the high resistance state.
Controllable switching behavior is demonstrated between multiple resistance levels over several orders of magnitude, allowing for multibit
operation. This multi-level operation of the VO2-bridge devices results from exploiting sub-hysteresis loops by Joule heating.
©2022 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license
(http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0077160
INTRODUCTION
Inspired by the functions of biological neurons and synapses
in the brain, the memristor,1a two-terminal switchable resistive
memory, becomes the main functional unit for energy-efficient neu-
romorphic computing circuitry. In this development, there is a
need for controllable and versatile resistive switching technologies,
which is pursued along various routes.2–4 While many methods rely
on thermal- or electric field-induced atomic rearrangements in the
devices, an attractive option is offered by systems that exhibit hys-
teretic resistance vs temperature characteristics concomitant with
electronic phase transitions. A particular case is the Mott insula-
tor VO2, which exhibits an insulator–metal transition (IMT) just
above room temperature, with resistivity changes of several orders
of magnitude.5–8 The IMT can be tuned by chemical doping, epi-
taxial strain, and external stimuli, such as temperature and electrical
current/voltage. This makes VO2a suitable material for memristive
devices and the realization of artificial neurons.9For example, Yi
et al. demonstrated 23 biological neuron spiking behaviors within
one single VO2-based device.10 In a previous work of our group,
Rana et al. observed multiple stable resistive states between the
insulating and metallic states in VO2films by tailored temperature
sweeps or external electrical stimuli,11 following an earlier work by
Driscoll et al.12 The existence of the intermediate resistive states
is unique and particularly attractive for reconfigurable electronic
circuitry.
In this work, we fabricated planar bridge-structure devices from
VO2thin films. By applying voltage sweeps, which create Joule
heating in the device and trigger the IMT, we observed repeatable
switching behavior with a correlation between the switching power
and device dimensions. It shows a combination of digital switching
and analog-like switching,13 and the reset happens gradually with
steps, resulting in stable intermediate resistive states between the
high resistive state (HRS) and the low resistive state (LRS). By tun-
ing the applied voltage bias, we realized multistate memory within
one VO2-based memory cell (in our demonstration 3 bits per cell)
and reliable multilevel operation. Additionally, we show multistate
operation with a VO2-based two-terminal parallel-bridge device as
an outlook, providing another versatile route to realize multistate
memory.
METHODS
Epitaxial VO2thin films were deposited on single crystal
TiO2(001) substrates using pulsed laser deposition (PLD) from a
AIP Advances 12, 015218 (2022); doi: 10.1063/5.0077160 12, 015218-1
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polycrystalline V2O3target8[Fig. S1(a)]. The distance between the
target and the sample is ∼45 mm. A KrF excimer laser (λ=248 nm,
20 ns pulse duration) was used with an energy density of ∼1.3 J/cm2
and a pulse repetition rate of 10 Hz. The growth temperature was
400○C, and the oxygen background pressure was 10−2mbar. After
deposition, the samples were cooled at 10 ○C/min at the same oxygen
pressure. To check the crystalline quality of the film, x-ray diffraction
(XRD) scans were performed as a function of temperature. Atomic
force microscopy (AFM) scans were conducted in the tapping mode
to study the surface topography.
The as-deposited VO2films were patterned into single or paral-
lel bridges with photolithography and wet or dry etching techniques.
For single bridges [Fig. S1(b)], a 35% mass fraction nitric acid
solution was used, while Ar+ion beam etching was used for the
parallel-bridge devices [Fig. S3(a)]; for detailed settings, see Table
S1 of the supplementary material. Two-terminal devices were fabri-
cated with Ti (4.5 nm)/Au (50 nm) contact pads via RF sputtering
and lift-off. The dimensions (W×L) of the VO2single bridges mea-
sured in this work are 5 ×10, 5 ×15, and 5 ×20 μm2. The VO2film
thickness after all processing steps was in all cases ∼10 nm, as verified
by AFM [Figs. S1(d) and S1(e)].
Temperature-dependent transport measurements were carried
out in a Quantum Design Physical Properties Measurement Sys-
tem (PPMS), and the resistivity was derived using a Van der Pauw
method. Electrical measurements were performed in a probe station
with a Keithley 4200A-SCS parameter analyzer applying voltage or
current sweeps at room temperature.
RESULTS AND DISCUSSION
The AFM image [Fig. 1(a)] of the as-deposited film shows that
the VO2film is homogenous with an rms roughness of 0.6 nm. For
the XRD measurement, a 2θ/ωscan [Fig. 1(b)] was performed to col-
lect the diffraction peaks related to crystallographic planes parallel
to the sample’s surface, therefore pointing to the out-of-plane direc-
tion. There are two peaks in the spectra: one from the (001)-oriented
TiO2substrate at ∼62.76○and the other from the rutile phase of VO2
at ∼65.70○. The clearly resolved thickness fringes indicate high sam-
ple quality and the calculated thickness is ∼11 nm. There are no other
peaks in Figs. 1(b) and S2, which confirms the epitaxy and phase
purity of the VO2film, and the film is fully strained, as expected for
a thickness below the critical thickness shown by Rodríguez et al.14
Normally, this critical thickness is around 15 nm, and thicker films
display cracking patterns due to the relaxation from the strain and
consequent appearance of the monoclinic phase.15
To study the IMT of the VO2film, the temperature evolu-
tion of both the first-order structural transition Fig. 1(c) and the
resistivity Fig. 1(d) has been measured. Temperature-dependent
XRD measurements were carried out under 25, 30, 40, 50, 60, 70,
80, and 90○C, focusing on the VO2(002)Rdiffraction peak. As
marked by the black arrow in Fig. 1(c), the (002)Rpeak shifts
from ∼65.70○to ∼65.55○when the temperature increases from 25
to 90○C. The slight shift to a lower angle suggests the expansion
of the out-of-plane lattice parameter, but there is no phase tran-
sition. The hysteresis loop in Fig. 1(d) shows that the VO2film
undergoes a sharp IMT with a 4-orders-of-magnitude decrease in
resistivity. The transition temperature TIMT is 318 K (∼44.9○C),
defined by the midpoint of the transition in the curve measured
during the warm-up process. The IMT is normally accompanied
by structural transformation from an insulating monoclinic (M1,
P21/c) phase to a metallic rutile (R, P42/mnm) phase as well as
the dimerization of neighboring vanadium atoms.16 However, the
driving force behind the IMT of VO2still requires further inves-
tigation. The transition has been explained by two main theories.
One considers a Mott–Hubbard transition in which strong Coulomb
interaction between electrons splits the near-Fermi-level electronic
states,17 while the other proposes a Peierls transition where the
monoclinicrutile structural transformation with dimerization opens
the insulating gap.18 These two theories are relevant, although the
order of occurrence remains under debate.19,20 However, in this
work, the VO2film is fully strained and already in the rutile
phase at room temperature. The IMT happens while the tempera-
ture increases without an obvious crystallographic phase transition
according to the temperature-dependent XRD results [Fig. 1(c)],
which suggests that the IMT can be a purely electronic phase tran-
sition. It is also reported that the IMT in strained VO2films can
result from electronic softening of Coulomb correlations within
V–V singlet dimers happening at a lower temperature compared to
the TIMT.16
The coexistence of both the metallic and insulating phases
within the hysteresis span of the IMT results in intermediate resis-
tive states.12,21 As shown in Fig. 1(e), during the heating process, we
increased the maximum temperature in steps to gradually heat the
film. For the cooling process, after the complete IMT, the film was
gradually cooled by reversing the temperature midway [Fig. 1(f)].
The intermediate resistive states can be stabilized by tuning the
temperature during the sweep before complete transition for both
heating and cooling process. It is to be noted that the resistivity
changes at a lower temperature when the film was heated up from
the intermediate temperatures [Fig. 1(f)], which indicates that the
film needs less heating to transform from the intermediate states
to the metallic phase than the first transition. These minor loops
within the hysteresis span are similar to the mathematical Preisach
model of hysteresis,22 which is normally used for ferromagnetic and
ferroelectric material systems.
The above characterizations were done on unstructured VO2
films. Subsequently, the VO2films were fabricated into two-terminal
devices, according to the above-described procedure, and the I–V
characteristics were measured at room temperature. As shown in
Fig. 2(a), while operating the devices at room temperature, they
show typical volatile switching behavior, as the VO2bridges spon-
taneously return from the LRS to the HRS after the removal of
the applied voltage. For the set process, the current of the device
increases abruptly when the applied voltage surpasses the threshold
value (Vset). For the reset process, the current first drops gradu-
ally with the decreasing applied voltage, followed by a sudden drop
when the applied voltage is smaller than the hold voltage (Vhold).
Hence, within one VO2bridge, there are both fast digital switching
and continuous analog-like switching, which provides possibilities
to control the resistive states. In addition, the device requires more
voltage and power to switch for the first cycle than all the subsequent
cycles. This forming process leads to a slight decrease of the HRS,
making the device easier to switch with lower power.23 The form-
ing is not permanent and the devices normally recover in one or two
days, which is consistent with previously reported data.23 All other
resistive switching characteristics reported were measured after the
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FIG. 1. (a) AFM image of an as-deposited VO2film. The image shows an area of 1 ×1μm2. (b) X-ray diffraction (XRD) 2θ–ωscan of a VO2film grown on a TiO2(001)
single crystal substrate in the range of 60○to 70○. (c) Temperature dependence of the VO2(002)Rdiffraction peak. XRD 2θ–ωscans were taken under 25, 30, 40, 50, 60,
70, 80, and 90○C, respectively. (d) Temperature dependence of the resistivity of a VO2thin film, exhibiting the insulator–metal transition (IMT). Multiple resistance states
can be achieved when temperature sweeps are reversed halfway during (e) the heating process and (f) the cooling process.
forming. Figure 2(b) shows the I–Vcurve of the DC voltage sweep
in both positive and negative bias. The symmetrical curve indi-
cates that the switching is independent of bias polarity, as expected
from a Joule heating-based mechanism. A current compliance (ICC)
should be set to protect the devices. It plays an important role in the
switching behavior and the control of resistive states since it maxi-
mizes the Joule heating generated inside the VO2bridges. Therefore,
the I–Vcharacteristics have been measured with different current
compliances [Fig. 2(c)]. Surprisingly, an intermediate step occurs
during the reset process when higher current compliances were used.
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FIG. 2. (a) Typical I(V) characteristics of the VO2-based single-bridge devices (W=5μm, L=20 μm). The curves of the first, second, and 500th cycles are plotted. ICC =0.1
mA. (b) I(V) characteristics of a VO2-based single-bridge device (W=5μm, L=20 μm) in positive and negative bias. ICC =1 mA. (c) I(V) characteristics of the VO2-based
single-bridge device (W=5μm, L=20 μm) with varying current compliances ranging from 200 μA up to 5 mA. (d) I(V) characteristics of a VO2-based single-bridge device
(W=5μm, L=20 μm) for 100 cycles. The curves of all cycles are shown. The inset figure shows the I(V) characteristics of 1 cycle in linear scale. ICC =1 mA.
In addition, it is shown in Fig. 2(d) that this behavior is repeatable
and stable for over 100 cycles. It indicates that there are interme-
diate states between the LRS and HRS and it is possible to stabilize
them by tuning the applied voltage and the compliance current his-
tory. The correlation between the switching parameters and device
length can be found in Fig. 3 of the supplementary material. The
set voltage and power both scale with device length, while the set
current does not. Shorter devices require a lower voltage and power
to switch since they have lower resistance and more Joule heat-
ing at a given voltage. In addition, for longer devices, the window
between set and hold is wider. Figure S2(d) shows the ON-state
resistance (recorded at Vhold) and OFF-state resistance (at 0.5 V) of
devices with different lengths. The ON/OFF ratio is 1 order of mag-
nitude for all the devices due to the moderate measurement settings
(ICC =1 mA).
To map out the hysteresis loops for different intermediate resis-
tive states, first, the DC voltage is swept from 0 V to the maximum
value (5 V) to set the device to the LRS, and then it is swept back to an
intermediate value such as 1.7 V. Afterward, it is swept from 1.7 to
5 V again and from there all the way back to 0 V to reset the device
to the HRS. The voltage sweeping process can be found in Fig. S4
of the supplementary material. The results are plotted in Fig. 3(a),
showing that multiple resistance states can be achieved when volt-
age sweeps are reversed at different intermediate voltages during the
reset process. Similar to the temperature-dependent measurement
in Fig. 1(f), it requires less voltage to switch from the intermedi-
ate states to the LRS at the second set event. In addition to voltage
sweep measurements, static voltages have also been applied during
the reset process to study the stability over time of these interme-
diate states. For this, the device is first set to the LRS, and then
the voltage is swept back and maintained steady at a certain inter-
mediate value [2, 2.5, or 2.8 V in Fig. 3(b)]. As it can be seen in
Fig. 3(b), varying intermediate resistive states are stabilized. More-
over, the current compliance is still important to affect the resistance
value of the intermediate state. With a lower ICC (like <0.5 mA), the
LRS is still fairly high and the controllable range of resistive states
is correspondingly small. However, with a higher ICC (like >5 mA),
the LRS is very low, which raises the risk for irreversible breakdown
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FIG. 3. (a) Device resistance as a function of the voltage for one of the VO2-based single-bridge devices. The DC voltage is swept from 0 V to the maximum value (5 V)
and then is swept back and kept at an intermediate value, e.g., 1.7 V (as indicated by the black arrows). The voltage is then swept from 1.7 to 5 V again, and finally, all the
way back to 0 V (as indicated by the red dashed arrows). ICC =5 mA. (b) Different resistance states obtained for three different applied voltage values (2, 2.5, and 2.8 V) in
a VO2-based single-bridge device (W=5μm, L=15 μm). The upper panel shows the biasing scheme, while the bottom panel shows the time evolution of the resistance
under that biasing scheme for three different values of the compliance current. (c) Time evolution of the resistance of the same device as in (b) under four different applied
voltage values after an initial IMT step. The voltage was held for 5 min. The upper panel again shows the biasing scheme, and the correspondent measured resistance is
shown below.
of the device. Therefore, it is better to pick a medium value, such
as 1 mA, for the following measurements. To check the stability of
the intermediate resistive states, the static intermediate voltages have
been held for over 5 min [Fig. 3(c)], without a noticeable change of
resistance after some initial stabilization.
Figures 4(a) and 4(b) show that within one VO2-based mem-
ory cell, straightforwardly over eight separable levels of states could
be achieved, corresponding to 3 bits per cell. The intermediate
levels spread in a 1-order-of-magnitude range, and they are repro-
ducible and repeatable, as marked by the color-coded dashed lines.
In Fig. 4(a), the states are accessed in consecutive ascending order,
from lower resistive states to the higher ones, while Fig. 4(b) shows
the reverse process from higher resistive states to the lower ones.
Moreover, as shown in Fig. 4(c), the individual intermediate states
FIG. 4. Time evolution of the resistance of a VO2-based single-bridge device (W=5μm, L=15 μm) under different voltage-biasing schemes. Eight distinguishable states
(color-coded dashed lines) can be reached within one device. States are accessible in (a) consecutive ascending order or (b) consecutive descending order by adjusting
the intermediate voltage value. (c) Each level can also be independently reached. Upper panels show the applied voltage and calculated power plotted as a function of time,
and lower panels are the resistance as a function of time.
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FIG. 5. Filament schematics of VO2-based single-bridge devices embedded in the
device resistance vs voltage/power curves. The schematics show the status of the
filament in the VO2bridge under different conditions, including the insulating state,
the nucleation phase, the metallic state, and two intermediate resistive states (I
and II).
can be reached independently and randomly. While accessing the
states in consecutive descending order, under voltage bias, the device
is first set to the LRS before the resistance is fixed on the return to
the high resistance state. This is because set processes to the low
resistance states are too abrupt to stabilize under voltage bias. With
current control this can straightforwardly be accomplished though,
without the need to first go completely to the LRS. Fortunately, the
set–reset processes can happen comparatively faster as the required
temperature swings of a few degrees associated with this electronic
phase transition are low as compared to the devices relying on crys-
talline phase changes. The power is also calculated and plotted in the
upper panels of Fig. 4. The required power to switch to the LRS is
2.5 mW, and the one to maintain the intermediate states is less than
1 mW. Obviously, these are high values for practical devices, but the
values will be decreased if the device dimensions are reduced from
the microscale to the nanoscale.
The switching mechanism has been widely studied. It is known
that the conducting filament formation triggered by Joule heating
in the VO2bridge leads to resistive switching.24,25 With various
FIG. 6. (a) Voltage-controlled I(V) characteristics of three VO2-based parallel-bridge devices (W=5μm, d=30 μm) with different bridge lengths. ICC =5 mA. (b) Current-
controlled V(I) characteristics of three VO2-based parallel-bridge devices (W=5μm, d=30 μm) with different bridge lengths. VCC =20 V. To facilitate the comparison with
Fig. 6(a), we have retained the voltage as the xaxis and the current as the yaxis. (c) Current-controlled R(I) characteristics of three VO2-based parallel-bridge devices (W
=5μm, d=30 μm) with different bridge lengths. VCC =20 V. (d) Schematics of VO2-based parallel-bridge devices to show the switching status of bridges corresponding
to different resistive states.
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in situ characterization techniques, the forming process of the fila-
ments has been visualized both statically21,26 and dynamically.27,28
As reported before,28 the current flows inhomogeneously due to
intrinsic defects in the bridge, leading to the nucleation of the metal-
lic domains. Once a small portion transits to the metallic state,
the current increases and the Joule heating triggers filament forma-
tion and expansion. The filament formation is avalanche-like on a
time scale of nanoseconds.28 However, the rupture of the filament
is a much slower process on a time scale of milliseconds.25 During
the reset process, the metallic domains inside the filament relax to
the insulating state at different speeds due to inhomogeneity.24,25
The filament becomes partially metallic and partially insulating, and
this intermediate state can be held with a small external stimu-
lus since the residual metallic domains can attract the current and
maintain the IMT. When the device is set to different intermediate
resistive states, the ratio between the metallic domains and insu-
lating domains is being tuned. Figure 5 schematically shows the
filament status for the whole process of nucleation, formation, par-
tial retention, and rupture. There are two intermediate states shown,
intermediate state I with more metallic domains and intermedi-
ate state II with fewer metallic residues. To switch from I to II,
the applied voltage can be decreased so that the metallic domains
that remained will further become insulating. However, switching
from II to I requires the device to be fully set again, and then, the
applied voltage can be decreased to the right value. If the applied
voltage simply increases, the device will undergo the complete
IMT with a lower power instead of going to a lower intermedi-
ate resistive state since the filament formation is unpredictable and
ultrafast.
Beyond the VO2-based single-bridge memristive devices, we
also fabricated devices with two identical parallel VO2bridges with
varying bridge-to-bridge distances [see the device structure in Fig.
S5(a) of the supplementary material]. It is to be noted that the ini-
tial resistance of parallel-bridge devices is lower, possibly because
we used Ar+etching for the structuring of these bridges. Figure 6(b)
shows the I–Vcharacteristics measured with a current-controlled
sweep. Different from the voltage-controlled curves in Fig. 6(a),
there are two snapbacks in the set process providing intermedi-
ate resistance states [Fig. 6(c)]. The distance between bridges plays
an important role in the occurrence of the intermediate state (see
Fig. S3 of the supplementary material). If the bridges are close to
each other, in this experiment <20 μm, the heat dissipation of one
bridge will affect the other and both bridges will switch to the ON
state almost at the same time so that there is no stable intermedi-
ate state during the set process. It is likely that the characteristic
length scale for the occurrence of this thermal crosstalk-induced
synchronization depends on the individual VO2bridge dimensions
and the current pulse duration, requiring further study. Figure 6(d)
schematically shows the switching status of two distant bridges cor-
responding to different resistive states. In the beginning, both of the
bridges are OFF and the overall device shows a HRS. Once the cur-
rent is applied, due to the intrinsic inhomogeneity, one bridge will
switch to the ON state first; therefore, the device shows the interme-
diate state. Finally, the other bridge will also switch to the ON state
with increased current and the device shows a LRS. Systematic inves-
tigation on the stability and repeatability of the intermediate states
is still required. However, the current results indicate the potential
for multilevel operation of such parallel bridge configurations within
a single two-terminal VO2-based device. The concept can readily be
extended to more parallel channels and complex network configura-
tions. Moreover, one may further use multilayering to stack bridges
in the vertical direction and provide a further dimension for multibit
operations.
CONCLUSION
In conclusion, we have investigated two-terminal memristive
devices based on VO2thin film bridges. By tuning the applied
voltage or current, we realized multistate memory within one
VO2-based memory cell and reliable multilevel operation at room
temperature. This multi-level operation of the VO2-bridge devices
results from exploiting sub-hysteresis loops by Joule heating, which
provides opportunities for novel (neuromorphic) electronics.
SUPPLEMENTARY MATERIAL
See the supplementary material for the device structures and
additional measurement data showing the correlation between
switching behavior and device dimensions.
ACKNOWLEDGMENTS
This work was supported by the China Scholarship Council
(Grant No. 201906170041).
AUTHOR DECLARATIONS
Conflict of Interest
The authors declare no conflict of interest.
DATA AVAILABILITY
The data that support the findings of this study are available
from the corresponding author upon reasonable request.
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