Supplementary Material
Correlation between Thermal Annealing Temperature and JouleHeating Based Insulator-Metal Transition in VO2 Nanobeams
Servin Rathi1, Jinhyung Park1, Inyeal Lee1, Min Jin Kim2, Jeong Min Baik2, and Gil-Ho Kim1a)
1
School of Electronic and Electrical Engineering and Sungkyunkwan Advanced Institute of
Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Korea
2
School of Mechanical and Advanced Materials Engineering, Ulsan National Institute of Science
and Technology (UNIST), Ulsan 689-805, Korea
a)
Electronic mail: ghkim@skku.edu
1
A. Growth of vanadium dioxide nanobeams ( VO2 NBs)
Single-crystal VO2 NBs were grown by physical vapor deposition (PVD) at atmospheric
pressure. In a 10 cm long quartz boat, 0.1 g of fine mesh VO2 (99.9%, Aldrich) powder was
placed with a precleaned SiO2/Si substrate located approximately 5 mm from the VO2 powder.
The boat was then transferred to a quartz tube furnace where a high-purity He carrier gas
(99.999%) was flowed at 300 sccm. The furnace temperature was then increased to
approximately 700 °C and the sample growth proceeded for 2 h, following which the samples
were allowed to cool down to room temperature in flowing He; thereafter the nanobeamscovered substrate was removed from the furnace. From Raman analysis (Fig. S3), we confirmed
that the NBs were single-crystal monoclinic VO2 and grew along a preferential [100] direction.
The lengths of most of the nanobeams fall in the range 20-150 μm. The nanobeams width and
height, for a representative number of nanowires, as determined by Scanning Electron
Microscope (SEM), was found to be varying within 100 ~ 400 nm and 50 ~ 200 nm,
respectively.
B. Device Fabrication Procedure and Yield-Rate of VO2 NB Devices
For device fabrication, the as-grown VO2 NBs were first detached in isopropyl alcohol (IPA) by
ultra-sonication and then dispersed onto a clean p-type Si substrate covered with 300 nm of SiO2
by dropping a suspension of VO2 NBs. After a cleaning process with acetone/IPA and de-ionized
water and drying with N2 gas, electrode patterns were defined using standard optical lithography.
This was followed by a conventional e-beam metal vapour deposition of Ti/Au (10/200 nm)
electrodes and lift-off processes. In this study, the electrode pattern has a separation of 5 µm and
2
a single chip contains 16 such electrodes. Fig. S1 shows a schematic of the fabrication steps in
detail.
FIG. S1. Procedure for fabricating the selective VO2 NB device. Schematic of the (a) as-grown
VO2 NBs, which were first detached in isopropyl alcohol (IPA) by ultra-sonication, (b) dispersed
onto a clean p-type Si substrate covered with 300 nm of SiO2 (c) photoresist spin coating and (d)
electrode patterns defined using standard optical lithography, (e) e-beam metal vapour deposition
of Ti/Au electrodes and (f) lift-off processes.
The yield rate (device showing good transitions (thermal/voltage)) statistics of VO2 NBs
grown by PVD method varies from 20 to 45 %. Also, only those devices which have approx.
similar aspect-ratio (compared by SEM images) were selected for this study so as to rule out any
I_V variations due to change in the aspect-ratio. For the present experiment, about 60 devices
were fabricated out of which only less than half showed good transitions behavior. Even after
post-fabrication annealing, some of the devices showed huge variation in their conducting
3
behavior and such devices were treated as exceptions and hence were not included in the present
study (Fig. S2). Therefore, only those devices which demonstrated identical I_V and transition
characteristics were finally considered. The reason for these variations in the device resistance
from sample to sample can be attributed to the random variations in the density of defects,
interstitials and vacancies in VO2 material1 which cannot be precisely controlled in PVD growth
method.
8
10
7
Resistance ()
10
1-2 devices
6
10
5
10
4
10
6-7 devices
3
10
Pre-Annealed 200
300
400
500
o
Annealing Temperature ( C)
FIG. S2. Resistance variation with different annealing temperature. Red colour symbols indicate
exceptionally high resistance devices while black error bars show close variation in resistance as
shown by 6-7 devices for each annealing temperature.
C. Raman Spectra of a single VO2 NB
Raman spectroscopy is a non-destructive technique to assess the phase and orientation of
crystalline materials at ambient conditions.2 The peaks in Raman spectra of VO2 NB are
4
Intensity (a.u.)
VO2 NB
SiO2/Si Substrate
200
400
600
800
-1
1000
1200
Raman Shift (cm )
FIG. S3. Raman spectra of a VO2 NB (red) and the recorded spectra of SiO2/Si substrate (blue)
for reference.
identified at 197(Ag), 227(Ag), 264(Bg), 311(Bg), 343(Bg), 396(Ag), 443(Bg), 614(Ag) cm-1. The
low-frequency phonons (197(Ag) and 227(Ag)) correspond to V-V lattice motion and other
distinguishable peaks relate to V-O bonding.3-5 These Raman-active modes are the clear
5
signature of the monoclinic (M1) phase predicted by its space group 𝐶2ℎ
and the measured M1
Raman spectra was also found to be consistent with previously reported measurement on VO2
5
NBs.3-7 Fig. S3 clearly indicates the NB to be a single crystal monoclinic VO2 at room
temperature.
D. Modeling of experimentally observed VIMT and VMIT
In order to calculate VIMT and VMIT, using Eq. (2) and (3) of the main text, mean values of the
parameter k and k’ were obtained using the known parameters. Table SI lists the important
resistance and voltage parameters, as indicated in Fig. S4, for D200, D300 and D400 devices.
The TC for D300 and D400 devices were taken to be 64 oC and 57 oC, respectively.
TABLE SI. Observed resistances, VIMT and VMIT values of various devices at the indicated point,
i.e., RA, RB and Rc as illustrated in Figure S4.
Device
RA
(kΩ)
RB
(kΩ)
RC
(kΩ)
VIMT
(V)
VMIT
(V)
VM
𝑽𝟐𝑴
(V)
(V2)
D200
174
0.75
143
4.85
3.25
1.60
12.96
D300
62
0.99
59
2.20
1.35
0.85
3.02
D400
49
0.91
45
0.90
0.50
0.40
0.56
It may be interesting to note that although the thermal conductivity of the VO2 nanomaterial has
shown variations across the transition point8-10, there is no common consensus on the variation
trends. This is because the thermal conductivity in the metallic or monoclinic phase is a sum of
the independent contribution from the lattice and carrier conductivity. Although the carrier
dependent electrical conductivity (kC) increases by an order of magnitude across the transition,
but the metallic phase of VO2 exhibits a lower lattice thermal conductivity (kr), possibly caused
by a soft phonon mode in the high temperature phase.11,12 Therefore, depending upon the relative
6
contribution from the lattice and carrier dependent electrical conductivities, some analysis has
pointed towards positive temperature dependence,9 while others have reported a negative
temperature dependence of the total thermal conductivity of VO2 across the transition point.10,13
However, this variation in thermal conductivity of VO2 and hence the collective thermal
parameters k and k’ in this study, which include the thermal coefficients of the VO2 NBs, the
underlying substrate, etc., doesn’t really affect our analysis in eqn. 4 of the main text, as the
product k’(TMIT-To)RB can be readily neglected as compared to much higher k( TIMT-To)RA.
1.2
Upward Voltage Ramping
Downward Voltage Ramping
RB
0.8
0.6
0.4
Current (mA)
Current (mA)
1.0
0.03
0.02
0.01
0.4
0.5
0.6
Voltage (V)
0.2
RC
0.0
0.2
0.4
RA

0.6

0.8
1.0
Voltage (V)
FIG. S4. A typical I-V curve for the D400 device,  indicates VIMT and  indicates VMIT (the
inset shows the detail of the enclosed junction).
The VIMT and VMIT for D200, D300 and D400 devices were calculated (Table SII), using Eq. (2)
and (3) of the main text, and by replacing TIMT and TMIT with observed TC for simplicity. The
7
thermal coefficient values were calculated with known parameters, in case of heating and cooling
conditions and mean value of k’ was found to be increased by an order of magnitude as
compared to k, though exact values vary from device to device. The VIMT values were found to be
in agreement with the observed values; however VMIT of D200 was found to be out of range but
the values for D300 and D400 agreed approximately. The origin of these errors can be related to
the approx. values of the thermal parameters used and the complex stress and strain mechanism
at the interface of VO2 NB and the SiO2/Si substrate which affect metal/insulator domain
dynamics and has been widely studied in the literature. 6,7,12,14
TABLE SII. Predicted VIMT and VMIT of D200, D300 and D400 devices using k = 4.6 x 10-6 WK-1 and k’ = 8.2 x 10-5 W K-1.
Device
TC
(oC)
Observed
VIMT (V)
Calculated
VIMT (V)
Observed
VMIT (V)
Calculated
VMIT (V)
D200
67
4.85
4.00
3.25
1.11
D300
64
2.20
2.20
1.35
1.17
D400
57
0.90
1.50
0.50
0.86
References for supplementary material
1
C. N. Berglund, H. J. Guggenheim, Phys. Rev. B 185, 1022 (1969).
2
J. Chou, J. Lensch-Falk, E. Hemesath, L. Lauhon, J. Appl. Phys. 105, 034310 (2009).
3
J. Atkin, S. Berweger, E. Chavez, M. Raschke, Phys. Rev. B 85, 020101(R) (2012).
8
4
E. Donev, R. Lopez, L. Feldman, R. Haglund, Nano Lett. 9, 702 (2009).
5
C. Marini, E. Arcangeletti, D. D. Castro, L. Baldassare, A. Perucchi, S. Lupi, L. Malavasi, L.
Boeri, E. Pomjakushina, K. Conder, P. Postorino, Physical Review B 77, 235111 (2008).
6
S. Zhang, J. Chou, L. Lauhon, Nano Lett. 9, 4527 (2009).
7
A. Jones, S. Berweger, J. Wei, D. Cobden, M. Raschke, Nano Lett. 10, 1574 (2010).
8
C. Leroux, G. Nihoul, G. V. Tendeloo, Phys. Rev. B 57, 5111 (1998).
9
D. W. Oh, C. Ko, S. Ramanathan, D. G. Cahill, Appl. Phys. Lett. 96, 151906 (2010).
10
J. K. Chen, X. L. Liu, X. Yaun, Y. L. Zhang, Y. F. Gao, Y. F. Zhou, R. H. Liu, L. D. Chen, N.
F. Chen, Chin. Sci. Bull. 57, 3393 (2012).
11
H. Terauchi, J. B. Cohen, Phys. Rev. B 17, 2494 (1978).
12
D. Maurer, A. Leue, R. Heichele, V. Muller, Phys. Rev. B 60, 13249 (1999).
13
V. N. Andreev, F. A. Chudnovskii, A.V. Petrov, E.I. Terukov, Phys. Status Solidi a. 48, K153
(1978).
14
J. Sohn, H. Joo, D. Ahn, H. Lee, A. Porter, K. Kim, D. Kang, M. Welland, Nano Lett. 9, 3392
(2009).
9