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Journal of Physics: Conference Series
Related content
PAPER • OPEN ACCESS
Phase formation polycrystalline vanadium oxide
via thermal annealing process under controlled
nitrogen pressure
To cite this article: S Jessadaluk et al 2017 J. Phys.: Conf. Ser. 901 012162
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This content was downloaded from IP address 80.82.77.83 on 13/11/2017 at 08:23
Siam Physics Congress 2017 (SPC2017)
IOP Conf. Series: Journal of Physics: Conf. Series 1234567890
901 (2017) 012162
IOP Publishing
doi:10.1088/1742-6596/901/1/012162
Phase formation polycrystalline vanadium oxide via thermal
annealing process under controlled nitrogen pressure
S Jessadaluk1, 2, N Khemasiri1, 3, S Rahong1, 2, 3,*, A Rangkasikorn1, 2, 3,
N Kayunkid1, 2, 3, S Wirunchit1, 2, 3, M Horprathum4, C Chananonnawathron4,
A Klamchuen5 and J Nukeaw1, 2, 3
College of Nanotechnology, King Mongkut’s Institute of Technology Ladkrabang,
Chalongkrung Rd., Ladkrabang, Bangkok 10520, Thailand
2
Nanotec-KMITL Center of Excellence on Nanoelectronic Devices, Ladkrabang,
Bangkok 10520, Thailand
3
Thailand Center of Excellence in Physics, Commission on Higher Education,
Ministry of Education, Bangkok 10400, Thailand
4
National Electronic and Computer Technology Center (NECTEC), NSTDA, 111
Thailand Science Park, Paholyothin Rd., Klong Luang 12120, Thailand
5
National Nanotechnology Center (NANOTEC), NSTDA, 111 Thailand Science
Park, Paholyothin Rd., KlongLuang 12120, Thailand
1
*E-mail: [email protected]
Abstract. This article provides an approach to improve and control crystal phases of the
sputtering vanadium oxide (VxOy) thin films by post-thermal annealing process. Usually,
as-deposited VxOy thin films at room temperature are amorphous phase: post-thermal annealing
processes (400 °C, 2 hrs) under the various nitrogen (N2) pressures are applied to improve and
control the crystal phase of V xOy thin films. The crystallinity of VxOy thin films changes from
amorphous to α-V2O5 phase or V9O17 polycrystalline, which depend on the pressure of N2 carrier
during annealing process. Moreover, the electrical resistivity of the V xOy thin films decrease
from 105 Ω cm (amorphous) to 6×10-1 Ω cm (V9O17). Base on the results, our study show a simply
method to improve and control phase formation of V xOy thin films.
1. Introduction
Metal-insulator transitions (MIT) are reversible changes in the conductivity of materials when the
temperature above or below the transition point, which are the smart transitions in advanced material
[1]. Vanadium oxide (VxOy) demonstrated excellent MIT characteristics at the transition temperature
(Tc) due to the lattice distortion such as VO2 (68ºC), V4O7 (-23ºC), V6O11 (-103ºC), V9O17 (-194ºC) [2].
Several techniques have been applied to deposit vanadium oxide films such as sputtering, pulsed laser
deposition and thermal evaporation. The sputtering technique demonstrated an excellent uniformity with
high deposition rate, consequently the sputtering technique is promising to prepare V xOy thin film [3].
However, the sputtering VxOy thin films, which prepared at room temperature have amorphous phase.
To improve the MIT or the crystallinity of VxOy films, the post-annealing controlled ambient processes
were applied to recrystallize and control phase formation of the material.
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution
of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Published under licence by IOP Publishing Ltd
1
Siam Physics Congress 2017 (SPC2017)
IOP Conf. Series: Journal of Physics: Conf. Series 1234567890
901 (2017) 012162
IOP Publishing
doi:10.1088/1742-6596/901/1/012162
2. Experiment and methods
Vanadium oxide thin films were deposited on glass slides from 2-inch vanadium sputtering target (Kurt
J. Lesker) by pulsed DC magnetron sputtering (ATC 2000-F, AJA International, Inc.) at room
temperature. High purity (99.999%) of Argon (Ar) gas and Oxygen (O2) gas were used as the sputtering
gas and the reactive gas, respectively, both of them were controlled by mass flow controller (1179A,
MKS). VxOy thin films have been growth at base pressure 2×10-6 mbar. To find out the oxide mode in
reactive sputtering process, the flow rate of Ar was fixed at 45 sccm and various flow rates of O2 were
controlled at 0, 5, 10 and 15 sccm before introduced into the chamber [4]. Then, the operating pressure
was set at 5×10-3 mbar by automatic gate valve. The pulsed DC power was set at 300 W and the deposited
time was 60 minutes, respectively. To improve crystallinity of VxOy, as-deposited thin films with O2
flow rate 15 sccm were annealed at 400 ºC for 120 minutes by using a contact heater under the various
controlled nitrogen pressure in vacuum chamber. Annealing treatment was not performed higher than
400 ºC to avoid the deformation of thin films at the high annealing temperature [5]. The electrical
transport characteristics were evaluated by a four-point probe technique using precision DC source
(6621, Keithley) and nano-voltmeter (2182A, Keithley). The temperature was controlled between 30120 °C in the air by precision hot plate (1000-1, Electronic micro systems). After that, the crystalline
phases of VxOy thin films were investigated by X-ray diffraction (XRD) (Smartlab, Rigaku) using CuKα (λ=1.54 Å) radiation. All measurements were taken by generator voltage of 40 kV and a current of
30 mA. Then, the chemical bondings of VxOy were confirmed by confocal Raman spectroscopy using
532 nm Ar laser (NTEGRA Spectra, NT-MDT).
3. Results and Discussion
X-ray diffraction pattern by general θ-2θ scanning mode shows in figure 1(a) According to database
number (01-088-2322), a diffraction peak of a thin film without O2 (0 sccm) shown at 2θ = 42.14°,
which matched with the (110) plane of vanadium. The broad area between 20º-35º indicates the
diffraction from the glass substrate. When we increased the O2 flow rate (5, 10 and 15 sccm), the
diffraction peaks of thin films were disappeared. Therefore, all of them were amorphous phase.
a
b
Volatage Bias (Volt)
Intensity (a.u.)
500
15 sccm
10 sccm
5 sccm
480
460
440
0
420
5
10
15
400
0 sccm
380
20
25
30
35
40
45
2(degree)
50
55
0
60
2
4
6
8
10
12
14
16
Oxygen flow rate (sccm)
Figure 1. (a) X-ray diffraction pattern of VxOy thin films at the O2 flow rate 0-15 sccm. (b) Voltage
bias on vanadium target with the various O2 flow rate. (Inset: corresponding the image of thin films)
Figure 1(b) illustrated the increasing of voltage bias of target when the reactive O2 flow rate increased
during the sputtering process. The voltage bias abruptly changed from 388 V to 481 V in case of O2
flow rate 5 sccm and slightly increasing for 10 sccm and 15 sccm. The difference in voltage biases
revealed the oxide mode or the poisoning effect on target surface during the reactive sputtering [4]. The
as-deposited thin films by pulsed DC magnetron sputtering with various O2 flow rate shown as inset in
figure 1(b). The sputtered thin films at the O2 flow rate 5 sccm and 10 sccm were sub-oxide thin films,
while the thin films at the O2 flow rate 15 sccm has a yellow colour and transparent, which was possible
to be the VxOy. Consequently, the O2 flow rate 15 sccm was used to optimize for the post annealing
process, and were used to characterize the electrical transport property, respectively.
2
Siam Physics Congress 2017 (SPC2017)
IOP Conf. Series: Journal of Physics: Conf. Series 1234567890
901 (2017) 012162
a
IOP Publishing
doi:10.1088/1742-6596/901/1/012162
b
5
10
44
cm)
(
Resistivity
cm)
Resistivity (
Resistivity (cm)
30 C
55
10
10
Pristine
10
10
2
10
-6
2x10
mbar
33
10
10
5x10
-3
mbar
1
10
5x10
0
10
5x10
22
10
10
-2
mbar
11
10
10
-1
mbar
00
10
10
1 mbar
Pristine sample
30
40
50
60
70
80
90
0.0-6
100 110 120
10
Temperature (oC)
-5
10
-4
-3
10
-2
10
-1
10
0
10 0.1
10
Operating pressure
pressure (mbar)
Operating
(mbar)
Figure 2. (a) The resistivity of VxOy thin films as a function of temperature before and after annealed
in N2 ambient. (b) The resistivity of VxOy thin films at 30 ºC as a function of operating pressure.
After performed post annealing treatment (400ºC, 120 minutes), figure 2(a) shown the decreasing of
the VxOy thin films resistivity as a function of temperature measured by four-point probe technique with
the various controlled nitrogen pressure. The resistivity at 30 °C by post annealing process decreased
when the operating pressure increased from 2×10-6 to 1 mbar, whereas the as-deposited thin film has the
resistivity higher than 105 Ω cm, as shown in figure 2(b). According to above results, it can be implied
that the crystal structure of the VxOy thin films has changed from amorphous to polycrystalline phase.
a
b
*
* *
(002)V O /(101)V O
2.0
1 mbar
9
Peak intensity ratio
Intensity (a.u.)
*
*
V9O17
V6O11
α-V2O5
β-V2O5
17
2
5
1.5
-1
5x10 mbar
-2
5x10 mbar
-3
5x10 mbar
1.0
0.5
-6
2x10 mbar
Pristine
10
15
20
25
30
35
40
2(degree)
45
50
55
0.0
-6
60
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
Operating pressure (mbar)
Figure 3. (a) XRD diffraction pattern of VxOy thin films, inverse triangle (▼), blank diamond (◊),
asterisk (*) and dense circle (●) represent to α-V2O5, β-V2O5, V6O11 and V9O17 phase respectively. (b)
The intensity ratios between the (002) plane of V9O17 and the (101) plane of α-V2O5 as a function of
nitrogen operating pressure during annealing process.
After post annealing process under various nitrogen operating pressures, the crystal structures of the
VxOy thin films could observe as figure 3(a). At the operating pressure 2×10-6 mbar, the peaks located
at 15.44º, 26.16º, 31.16º, 34.38º, 47.52º and 51.22º represent for the orthorhombic structure of α-V2O5
from the crystal plane (200), (101), (310), (301), (020) and (501), respectively. However, we found a
small diffraction peak at 2θ = 12.8º which designated to be a beta-phase polymorph vanadium pentoxide
(β-V2O5) [6]. That means the thin film at the operating pressure 2×10-6 mbar has mixed phase between
alpha and beta phase. When the operating pressure increased from 2×10-6 to 1 mbar, the component
peaks of mixed-phase V2O5 decreased, while the component peaks of mix phase of V6O11 and V9O17
were appeared and increased dramatically. At the nitrogen operating pressure 1 mbar, the diffraction
pattern of the VxOy film preferred to be V9O17 dominant rather than V6O11. The intensity ratios between
(002) of V9O17 and (101) of α-V2O5 were calculated, which were plotted as a function of nitrogen
operating pressure, as presented in figure 3(b).
3
Siam Physics Congress 2017 (SPC2017)
IOP Conf. Series: Journal of Physics: Conf. Series 1234567890
901 (2017) 012162
IOP Publishing
doi:10.1088/1742-6596/901/1/012162
Raman intensity (a.u.)
Further analysis was performed to confirm the chemical bonding by using Raman spectroscopy.
Figure 4 illustrated nitrogen pressure-dependent Raman spectra of the VxOy thin films as a function of
the operating pressure. The intensity of Raman shifted spectra from the V2O5 thin film decreased; on the
other hand the intensity of Raman shifted spectra from the V9O17 slightly increased when the operating
pressure increased.
*
*
*
*
*
V9O17
V2O5
1 mbar
-1
5x10 mbar
-2
5x10 mbar
-3
5x10 mbar
Figure 4. Raman spectra of the post
annealing VxOy thin films.
-6
2x10 mbar
Pristine
200
400
600
800
1000
1200
Raman shift (cm-1)
According to above results, the applied thermal energy under the ambient of nitrogen contributed to
deform grains and recrystallization of VxOy thin films. Since, the oxygen atoms in thin films were carried
out by thermal annealing process under controlled nitrogen pressure a vacuum chamber. The residual
oxygen gas in annealing ambient decreased with increasing pressure of nitrogen gas, which suppressed
higher valence state of VxOy thin films [7]. Consequently, the crystallinity of the VxOy thin films has
changed from amorphous to polycrystalline structures, which leading to the reduced of resistivity when
the operating pressure increased. Based on our strategy, we could improve the crystallinity and control
VxOy phase by the post-annealing process from as-deposited thin film growth. This method is simple
and flexible for crystal phase controlling of the VxOy, which is an alternative method to prepared VxOy
materials for various applications, which depend on the transition temperature (Tc).
4. Conclusions
We offered a strategy to improve crystallinity and phase controlling of as-deposited sputtering VxOy thin
films via thermal annealing controlled nitrogen pressure. After annealing process, the crystallinity of
VxOy has changed from amorphous to polycrystalline phase. Consequently, we could measure the
decreasing of resistivity from VxOy thin film when the nitrogen operating pressure increased. Our
research work has shown the flexible and simply method to improve crystallinity and phase controlling
of VxOy thin films for further development in advanced functional device.
5. Acknowledgement
This work has partially been supported by the National Nanotechnology Center (NANOTEC), NSTDA,
Ministry of Science and Technology, Thailand, through its program of Center of Excellence Network
and Thailand Center of Excellence in physics, Ministry of Education, Thailand.
References
[1] Yang Z, Ko C and Ramanathan S 2011 Annu. Rev. Mater. Res. 41 337
[2] Nag J and Haglund Jr R F 2008 J. Phys.: Condens. Matter 20 264016
[3] Shigesato Y, Enomoto M, and Odaka H 2000 Jpn. J. Appl. Phys. 39 6016
[4] Saringer C, Franz R, Zorn K, and Mitterer C 2016 J. Vac. Sci. Technol., A 34 041517
[5] Liewhiran C and Phanichphant S 2007 Sensor 7 650
[6] Baddour-Hadjean R, Smirnov M B, Konstantin K S, Kazimirov V Y, Gallardo-Amores J M,
Amador U, Arroyo-de Dompablo M E, and Pereira-Ramos J P 2012 lnorg. Chem. 51 3194
[7] Xu H, Huang Y H, Liu S, Xu K, Ma F, and Chu P K 2016 RSC Adv. 6 79383
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