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Influence of sputtering power on the phase transition performance of VO2 thin films grown by magnetron sputtering
Accepted Manuscript Influence of sputtering power on the phase transition performance of VO2 thin films grown by magnetron sputtering Y.Y. Luo, S.S. Pan, S.C. Xu, L. Zhong, H. Wang, G.H. Li PII:
S0925-8388(15)32013-2
DOI:
10.1016/j.jallcom.2015.12.222
Reference:
JALCOM 36311
To appear in:
Journal of Alloys and Compounds
Received Date: 11 September 2015 Revised Date:
8 December 2015
Accepted Date: 26 December 2015
Please cite this article as: Y.Y. Luo, S.S. Pan, S.C. Xu, L. Zhong, H. Wang, G.H. Li, Influence of sputtering power on the phase transition performance of VO2 thin films grown by magnetron sputtering, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2015.12.222. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical abstract
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Influence of sputtering power on the phase transition performance of VO2 thin films grown by magnetron sputtering
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Y. Y. Luo*, S. S. Pan, S. C. Xu, L. Zhong, H. Wang and G. H. Li* Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanostructures,
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Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China
Abstract
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The influence of sputtering power on the electrical and infrared properties of VO2 thin films was investigated. It was found that the controlling of sputtering power is very important in realizing the pure VO2 (M) thin film. The thin films grown at the sputtering powers of 350 W and above have a similar phase transition behavior to bulk VO2. The
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infrared transmittance and electrical resistance of the VO2 thin film also depend on the sputtering power, and the hysteresis width is controlled by the size effect. The film
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thickness and defect density affect the amplitudes of the phase transition and phase
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transition temperature.
Key words: VO2 thin film, sputtering power, phase transition, electrical resistance, infrared transmittance, switching performance
*
Authors to whom correspondence should be addressed. Electronic mail: [email protected], [email protected]
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1. Introduction Vanadium dioxide (VO2 (M)) has been widely studied because of its novel semiconductor-metal transition (SMT) from low-temperature semiconducting monoclinic
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phase to high-temperature metallic tetragonal phase at about 340 K for single crystalline VO2 (M) [1-4]. Bulk VO2 (M) has a narrow hysteresis width and a large amplitude of the
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transition. Thin film is widely used in practical energy saving applications [5]. It has been established that the optical transmittance of semiconducting phase VO2 thin films abide by
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the Beer-Lambert law, in which an increase in film thickness is always companying with a reduced optical transmittance. The SMT behaviors, such as the phase transition temperature, hysteresis width and the amplitude of the transition, depend strongly on the stoichiometry and microstructure of VO2 (M) thin films [6-8], and a slight change in film stoichiometry
optical/electrical
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and microstructure will bring about a considerable variation in the SMT behaviors and the properties
[9-13].
The
relationships
among the
stoichiometry,
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microstructures and SMT behaviors of VO2 (M) thin films are still not very clear [14], due to the complex stoichiometry of vanadium ions, the multiphase in VO2 [15], and the
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variations in film thickness, grain size distribution, grain boundaries, defects and strains [16-22], and fully understanding of the internal correlations among these factors is essential from basic research and for practical applications. Much effect has been devoted to grow high quality VO2 (M) thin films with excellent infrared and electrical switching performance. Different substrates like sapphire, Si, and glass have been used to grow VO2 thin films, in which the glass substrate with a low cost and small influence on the growth of the film has been commonly used. Nevertheless, there
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are only a few reports on optimizing the quality of VO2 (M) thin films on glass substrate. VO2 (M) thin film grown on glass substrates generally has a large amplitude of the transition but a very big hysteresis width of about 9 oC or higher during SMT, and big
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hysteresis width is not beneficial for practical application. Therefore, further study on the growth of VO2 (M) thin film on glass substrate is imperative. Various techniques have been
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used to grow the VO2 thin films, and the sputtering technique is an effective method to prepare high quality thin films. Much work has been carried out to modulate the SMT
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behaviors of VO2 (M) thin films by optimizing the sputtering parameters like sputtering gas pressure, O2/Ar ratio and sputtering temperature [23-25], though relatively few studies concern the sputtering power. In this paper, we report the growth and SMT behaviors of VO2 (M) thin films on glass substrates with different sputtering powers. It was found that
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the VO2 (M) thin film grown at 400 W has an excellent amplitude of the transition and a lower transition temperature.
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2. Experimental details
VO2 thin films were deposited on glass substrate at the sputtering power ranging from
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330 W to 400 W by radio frequency reactive magnetron sputtering. The details of the sputtering setup and substrate preparation can be found elsewhere [26]. In this study, the VO2 thin films were grown according to the followed conditions. The metal V target (purity: 99.99%) with a diameter of 60 mm was adopted as sputtering source material, and the distance between the target and substrate was 90 mm. The base pressure of the chamber is better than 5.0*10−5 Pa and the total sputtering gas pressure was kept at 0.3 Pa. Ar and O2 mixed gas were used as working and reacting gases, respectively, and the O2 ratio in mixed
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gas is kept 3.5 %. The sputtering powers are 330, 350 and 400 W. The morphology of the as-prepared thin films was examined by field-emission scanning electron microscopy (FESEM, Sirion 200). The phase structures were investigated by X-ray diffraction using
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Cu-Kα1 line (XRD, Philips X’Pert) and Raman scattering on a macroscopic confocal Raman spectrometer using a laser beam with an excitation wavelength of 514.5 nm. The
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electrical properties of the films in heating and cooling cycles were measured using a model 2612A semiconductor characterization system. All data were acquired at a temperature
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accuracy of 0.1 K with a 10 min waiting time at each temperature. Temperature-dependent optical transmittance spectrum was recorded by Nicolet Magna-IR750 Fourier transform infrared (FTIR) spectroscopy equipped with a heating system in the range of 400-4000
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cm-1.
3. Results and discussion
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3.1 Structure and morphology
Fig. 1 shows the XRD patterns of the thin films grown at different sputtering powers.
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One can see the thin film grown at sputtering power of 330 W is of a phase mixture of V6O13 (JCPDS card no. 00-025-1251) and VO2 (M) (JCPDS card no. 01-72-0514), as shown in curve (1) of Fig. 1. Simple quantitative analysis shows that the percentage of V6O13 phase is about 13%. When the sputtering power was increased to 350 W or higher, the V6O13 phase disappears and pure VO2 (M) phase was obtained, see curves (2) and (3) of Fig. 1 (For simplify, all the as-grown thin films are called VO2 films from here on). From Fig. 1 one also can see there is a strong diffraction peak at 2θ of about 27.9o from the (011)
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plane for all the thin films, indicating the grains in the films have a preferential orientation along (011) direction. The average grain size can be estimated from XRD pattern using the .λ
, where β is the full width at half maximum of a diffraction
β θ
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Scherrer’s formula: D =
peak at 2θ corrected for instrumental broadening, and the calculated average VO2 grain size based on the first five strong diffraction peaks is about 89.4, 119.7 and 129.8 nm at
size increases with increasing sputtering power [27].
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sputtering power of 330, 350, and 400 W, respectively. Obviously, the average VO2 grain
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Raman spectroscopy was used to further confirm the phase purity of the thin films grown at different sputtering powers. Fig. 2 shows room-temperature Raman spectra of the films. One can see that the Raman spectra of the films grown at 350 and 400 W have the same pattern, and the peaks at 143, 193, 223, 262, 308, 339, 389, 502 and 614 cm-1 can be
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assigned to the phonon vibration modes of VO2 phase, which is consistent with reports in literatures [28-30], and further proves that the films are pure VO2 phase. While the Raman
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peak at 131 cm-1 of the thin film grown at 330 W can be assigned to V6O13 phase [31], indicating coexistence of V6O13 and VO2 phases. Above results are in a good agreement
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with XRD results.
Fig. 3 shows typical FESEM images of the VO2 films grown at different sputtering powers. One can see the VO2 films are composed of random arrayed columnar particles due to self-shadowing of the incident atoms by those already incorporated into the film and low surface mobility. The particle size increases with increasing sputtering power, and the film thickness first increases with increasing sputtering power and then decreases at the sputtering powers of 400 W or over due to the target poisoning, see the cross-sectional
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FESEM images in Fig. 3. It was found the film grown at sputtering power over 400 W has a very large grain size with very thin thickness and some cracks due to the target poisoning,
3.2 Semiconductor-metal transition 3.2.1
Temperature-dependent resistance
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and thus the films grown at the sputtering powers higher than 400 W were discarded.
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To verify the electrical contact of the electrodes, in-plane I-V curves at different temperatures of the VO2 thin films grown at different sputtering powers were performed,
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and the results are shown in Fig. 4. A linear feature of all the I-V curves proves an ohmic contact of the metal-electrodes on top of VO2 film. The temperature-dependent electrical resistance of VO2 films at different sputtering powers is shown in Fig. 5, and the SMT behavior deduced form Fig. 5 is listed in Table 1, and a remarkable change in SMT can be
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seen. From Fig. 5 one can see all the films show an obvious thermal hysteresis behavior, and the film grown at 400 W has the lowest resistance at 80 oC. The SMT temperature
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during heating, Ttr, is determined from the intersection point of the back tangent and forward tangent during transition, and the resulted data are listed in Table 1. From this table,
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one can see the transition temperature is about 66.5, 56.0, and 65.5 oC for the VO2 films sputtered at 330, 350, and 400 W, respectively, and the film grown at 350 W has a narrower hysteresis width than that at 400 W. The amplitude of the SMT also can be obtained from the resistance ratio at the temperature above and below SMT, i.e. R80/R46 (R46 and R80 refer to the electrical resistance at 46 and 80 oC, respectively), and the results are listed in Table 1. From this table one can see the electrical resistance change is more than three orders after SMT.
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In the growth of VO2 films at the sputtering power of 350 W, we found that the higher the argon ionization degree, the higher the ion density and sputtering speed, and thus the higher the ion energy. High ion energy favors the transition of the V ions from a fully
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disordered state to partially ordered state, which will enhance the quality of the films [27]. On the other hand, high sputtering power will result in high energy ions, and the high
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energy ions will implant into VO2 surface, inducing a thermal effect in substrate, thereby leading to the increase in grain size and decrease in defect density. If the sputtering power
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is too high, such as 400 W in present study, the target poison will occur because of a notable chemisorption of oxygen on target surface, leading to a rapid drop in the deposition rate and the decrease in film thickness.
The grain size is main factor that affects the hysteresis width of the VO2 films, and the
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larger the grain size, the narrower the hysteresis width. As the grain size in the VO2 films increases with increasing sputtering power, and thus a narrowest hysteresis width is
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obtained for the film grown at 400 W. The amplitude of SMT and SMT temperature of the VO2 films are correlated with the microstructures like defects density and grain boundary,
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and in metallic state, high defect density will accelerate scattering of electrons and obviously increase the resistivity of the films, and thus the film grown at 400 W has the lowest resistance because of low defect density. The temperature dependence of resistance can be expressed by R(T)=R0exp(Ea/kBT) [21, 22], where R0 is the resistance of materials at infinite temperature, kB is Boltzmann constant, and Ea is activation energy. Fig. 6 shows the plot of ln[R(T)] vs (kBT)-1 of the VO2 films grown at different sputtering powers, from which the Ea can be derived from the
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slope of the linear fitting of ln[R(T)]~(1/kBT). The Ea is about 75.61/98.44, 209.84/118.15 and 218.44/140.05 meV in semiconductor/metal state of the VO2 films grown at sputtering power of 330, 350, and 400 W, respectively. One can see the Ea increases with increase
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sputtering power, and the higher the power, the higher the activation energy. It is worth noting that the Ea is higher in semiconductor state than in metal state for the films grown at
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350 and 400 W except for that at 330 W due to the presence of V6O13 phase. The increased Ea with increasing sputtering power indicates that the VO2 film grown at 400 W has the low
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defect density. [9-11]
3.2.2 Temperature-dependent Transmittance spectra
Fig. 7 shows the transmittance spectra at the wavelength of 2.5 µm of the VO2 films
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grown at different sputtering powers in heating and cooling cycles. The infrared switching parameters deduced from Fig. 7 are listed in Table 2. An obvious change in infrared
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transmittance can be observed across the SMT, and the change depends on the sputtering powers. The film grown at 350 W has a fairly high transmittance of 54.8 % at 30 oC in
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semiconductor state and nearly 0 % transmittance at 70 oC in metallic state, and the amplitude of SMT is about 54.8 %. It is worth noting that the transmittance in metallic phase is close to zero, corresponding to the value of bulk VO2, suggesting the film grown at a sputtering power of 350 W is pure VO2 (M) phase, which is in accordance with the XRD result. The SMT temperature of the film grown at 350 W is about 57 oC (lower than bulk VO2) with a hysteresis width of about 7.2 oC; while that of the film grown at 400 W is about 63 oC with a hysteresis width of 5 oC and a slightly increased amplitude of SMT.
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From Fig. 7 one also can see the maximum transmittance of the film grown at 350 W in semiconductor state almost has the same value as that of the film grown at 400 W. Compared Table 2 with Table 1, one can see the SMT parameters derived from
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transmittance data are slightly different from that deduced from resistance experiments, which is considered due to the different temperature errors in these two measurement
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systems. The poor infrared switching performance of the VO2 film grown at 330 W is due to the presence of V6O13 phase. It is worth noting that the hysteresis width of the VO2 film
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grown at 400 W is narrower than that at 350 W, which is attributed to the size effect, and is in agreement with the reported results [32]. Fig. 8 shows the transmittance Tλ maps of the VO2 film as a function of the sputtering power in the heating and cooling cycles. A hysteresis loop can be clearly seen, and the hysteresis width gradually decreases with
4 and 7.
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increasing sputtering power, which is in a good accordance with the results shown in Figs.
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According to Beer-Lambert law, the optical transmittance of the VO2 films in semiconductor state is exponentially dependent on the film thickness. From Table 2 one can
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see the film grown at 400 W has a smaller thickness than that grown at 350 W, and should have a high transmittance in semiconductor state, and in fact they almost have the same value. The results indicate that the infrared switching properties depend not only on the thickness but also on the factors like grain size, porosity, defects, substrate and stoichiometry of the films [14, 17-20]. Our XRD analysis confirmed that the films grown at 350 and 400 W are pure VO2 (M) phase, the different microstructures should be responsible for the slight difference in the infrared performance of these two films. It has been known
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that the amplitude of SMT and SMT temperature depend on the defect density, and a larger amplitude of SMT and a lower transition temperature can be realized with few defects.
amplitude of SMT and a lower SMT temperature.
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Being having a lower defect density, the VO2 film grown at 400 W has an excellent
Above results demonstrate that the pure VO2 (M) thin film with excellent switching
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performance can be obtained at the sputtering power of 400 W. It was found too low sputtering power will result in the formation of the second phase, and too high will poison
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the target, reducing the quality of the thin films. 4. Conclusions～
In summary, we have investigated the influence of sputtering power on the SMT and switching performance of the VO2 thin films on glass substrate. The controlling of
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sputtering power is very important in realizing the pure phase VO2 (M) thin film. The VO2 (M) films grown at sputtering power of 350 W and above exhibit an obvious transition
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below 68 °C, a large amplitude of SMT, and relatively small hysteresis width. It was found that the hysteresis width is controlled by the size effect, and film thickness and defect
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density simultaneously affect the amplitudes of the SMT and SMT temperature. The VO2 film grown at sputtering power of 400 W has a SMT temperature of about 63 oC and a hysteresis width of about 5 oC with transmittance close to zero in metallic state.
ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51471163 and 11104270).
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References [1] C. Z. Wu, F. Feng, Y. Xie, Chemical Society reviews, 42 (2013) 5157-5183 [2] S. Rathi, J. H. Park, I. y. Lee, J. M. Baik, K. S. Yi, G. H. Kim, Journal of Physics D:
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Applied Physics, 47 (2014) 295101
[3] H. Zhang, X. D. Xiao, X. M. Lu, G. Q. Chai, Y. M. Sun, Y. J. Zhan, G. Xu, Journal of
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Alloys and Compounds 636 (2015) 106-112
[4] Y. X. Guo, H. Y. Xu, C. W. Zou, Z. Y. Yang, B. Tong, J. Y. Yu, Y. J. Zhang, L. Zhao, Y. L.
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Wang, Journal of Alloys and Compounds 622 (2015) 913-917
[5] K. Nagashima, T. Yanagida, H. Tanaka, T. Kawai, Journal of Applied Physics 100 (2006) 063714.
[6] D. Y. Fu, K. Liu, T. Tao, K. Lo, C. Cheng, B. Liu, R. Zhang, H. A. Bechtel, and J. Q.
TE D
Wu, JOURNAL OF APPLIED PHYSICS 113 (2013) 043707
[7] S. Lafane, T. Kerdja, S. Abdelli-Messaci, Y. Khereddine, M. Kechouane, O. Nemraoui,
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Applied Physics A 112 (2012) 159-164
[8] J. Narayan, V. M. Bhosle, Journal of Applied Physics, 100 (2006) 103524
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[9] C. H. Chen, Y. Zhao, X. Pan, V. Kuryatkov, A. Bernussi, M. Holtz, Z. Y. Fan, Journal of Applied Physics, 110 (2011) 023707 [10] L. T. Kang, Y. F. Gao, Z. T. Zhang, J. Du, C. X. Cao, Z. Chen, and H. J. Luo, J. Phys. Chem. C, 114 (2010) 1901-1911 [11] H. W. Liu, L. M. Wong, S. J. Wang, S. H. Tang, X. H. Zhang, Applied Physics Letters, 103 (2013) 151908 [12] Ü. Akın, H. Şafak, Journal of Alloys and Compounds 647 (2015) 146-151.
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[13] Ö.D. Coşkun, S. Demirel, G. Atak, Journal of Alloys and Compounds 648 (2015) 994-1004.
of Physics D: Applied Physics, 46 (2013) 055310
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[14] Q. Yu, W. W. Li, J. Liang, Z. H. Duan, Z. G. Hu, J. Liu, H. D. Chen, J. H. Chu, Journal
[15] Y. D. Ji, T. S. Pan, Z. Bi, W. Z. Liang, Y. Zhang, H. Z. Zeng, Q. Y. Wen, H. W. Zhang,
SC
C. L. Chen, Q. X. Jia, Y. Lin, Applied Physics Letters, 101 (2012) 071902
[16] Y. X. Guo, Y. F. Liu, C. W. Zou, Z. M. Qi, Y. Y. Wang, Y. Q. Xu, X. L. Wang, F. Zhang,
M AN U
R. Zhou, Applied Physics A, 115 (2014) 1245-1250
[17] C. Clavero, J. L. Slack, A. Anders, Journal of Physics D: Applied Physics, 46 (2013) 362001
[18] R. Lopez, L. C. Feldman, R. F. Haglund, Jr Physical Review Letters, 93 (2004) 177403
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[19] T. H. Yang, R. Aggarwal, A. Gupta, H. H. Zhou, R. J. Narayan, J. Narayan, Journal of Applied Physics, 107 (2010) 053514
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[20] Y. J. Chang, J. S. Yang, Y. S. Kim, D. H. Kim, T. W. Noh, D. W. Kim, E. Oh, B. Kahng, J. S. Chung, Physical Review B, 76 (2007) 075118
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[21] J. W. Ma, G. Xu, L. Miao, M. Tazawa, S. Tanemura, Japanese Journal of Applied Physics, 50 (2011) 020215 [22] Z. Luo, X. Zhou, D. Yan, D. Wang, Z. Li, C. Yang, Y. Jiang, Thin Solid Films 550 (2014) 227-232. [23] P. Jin, G. Xu, M. Tazawa, K. Yoshimura, Applied Physics A: Materials Science & Processing, 77 (2003) 455-459 [24] X. B. Wei, Z. M. Wu, X. D. Xu, T. Wang, J. J. Tang, W. Z. Li, Y. D. Jiang, Journal of
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Physics D: Applied Physics, 41 (2008) 055303 [25] Y.Y. Luo, F.H. Su, S.S. Pan, S.C. Xu, C. Zhang, J. Pan, J.M. Dai, P. Li, G.H. Li, Journal of Alloys and Compounds 655 (2016) 442-447
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[26] Y.Y. Luo, L.Q. Zhu, Y.X. Zhang, S.S. Pan, S.C. Xu, M. Liu, G. H. Li, Journal of Applied Physics, 113 (2013) 183520.
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[27] K. Srinivas, M. Manivel Raja, D.V. Sridhara Rao, S.V. Kamat, Thin Solid Films 558 (2014) 349-355.
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[28] G. I. Petrov, V. V. Yakovlev, J. Squier, Applied Physics Letters, 81 (2002) 1023 [29] E. M. Heckman, L. P. Gonzalez, S. Guha, J. O. Barnes, A. Carpenter, Thin Solid Films, 518 (2009) 265-268
[30] H. T. Kim, B. G. Chae, D. H. Youn, G. Kim, K. Y. Kang, S. J. Lee, K. Kim, Y. S. Lim,
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Applied Physics Letters, 86 (2005) 242101
[31] T. D. Manning, I. P. Parkin, R. J. H. Clark, D. Sheel, M. E. Pemble, D. Vernadou,
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Journal of Materials Chemistry 12 (2002) 2936-2939. [32] R. A. Aliev, V. N. Andreev, V. M. Kapralova, V. A. Klimov, A. I. Sobolev, E. B.
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Shadrin, Physics of the Solid State, 48 (2006) 929-934
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Figure captions Fig. 1. XRD patterns of VO2 films at sputtering power of (1) 330, (2) 350 and (3) 400 W. The standard diffraction patterns from V6O13 (JCPDS card no. 00-25-1251) and VO2 (M)
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(JCPDS card no. 01-72-0514) are also given.
Fig. 2. Raman spectra of VO2 thin films grown at different sputtering powers.
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Fig. 3. FESEM images and the corresponding cross-sectional views of VO2 films grown at sputtering power of (a)-(b) 330, (c)-(d)350, and (e)-(f) 400 W.
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Fig. 4. In-plane I-V curves between two top electrodes at temperatures above ((a), (c) and (e)) and below ((b), (d) and (f)) SMT of VO2 thin films grown at different sputtering powers.
Fig. 5. Temperature-dependent resistance of the VO2 films grown at different sputtering
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powers.
Fig. 6. Plot of ln[R(T)]~1/kBT of VO2 films grown at different sputtering powers.
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Fig. 7. Temperature-dependent transmittance of VO2 films at sputtering power of curve (1) 330, (2) 350 and (3) 400 W.
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Fig. 8. Transmittance maps of VO2 films as a function of sputtering power in the cycle of (a) heating and (b) cooling.
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Table 1. Electrical switching parameters of VO2 films at different sputtering powers.
Ttr (oC)
width (oC)
R46/R80
amplitude
330
66.5
4.6
598.4/18.92
31.63
350
56
5.7
400
65.5
4.7
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power (W)
958.96
9178.23/5.68
1615.89
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7940.2/8.28
Table 2. Infrared switching parameters at 2.5 µm of VO2 films at different sputtering
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and 80 oC, respectively)
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powers (Ttr is SMT temperature in heating process, T30 and T80 are the transmittance at 30
Transmittance (%)
Thickness
Ttr
Hysteresis
(nm)
(oC)
width (oC)
T30
T80
SMT
330
540
67
4.5
18.8
0
18.8
350
700
57
7.2
54.8
0
54.8
400
370
63
5
56
0
56
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Power (W)
Amplitude of
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Fig. 1
Fig. 2
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Fig. 3
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Fig. 4
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Fig. 5
Fig. 6
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Fig. 7
Fig. 8
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Highlights 1) Optimal conditions are achieved to grow pure VO2 (M) thin film.
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2) Optical/electrical switching property of the VO2 film depend on sputtering power
3) Film thickness and defect density affect amplitude of transition
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4) Grain size effect controls the hysteresis width of phase transition
S0925-8388(15)32013-2
DOI:
10.1016/j.jallcom.2015.12.222
Reference:
JALCOM 36311
To appear in:
Journal of Alloys and Compounds
Received Date: 11 September 2015 Revised Date:
8 December 2015
Accepted Date: 26 December 2015
Please cite this article as: Y.Y. Luo, S.S. Pan, S.C. Xu, L. Zhong, H. Wang, G.H. Li, Influence of sputtering power on the phase transition performance of VO2 thin films grown by magnetron sputtering, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2015.12.222. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical abstract
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Influence of sputtering power on the phase transition performance of VO2 thin films grown by magnetron sputtering
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Y. Y. Luo*, S. S. Pan, S. C. Xu, L. Zhong, H. Wang and G. H. Li* Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanostructures,
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Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China
Abstract
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The influence of sputtering power on the electrical and infrared properties of VO2 thin films was investigated. It was found that the controlling of sputtering power is very important in realizing the pure VO2 (M) thin film. The thin films grown at the sputtering powers of 350 W and above have a similar phase transition behavior to bulk VO2. The
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infrared transmittance and electrical resistance of the VO2 thin film also depend on the sputtering power, and the hysteresis width is controlled by the size effect. The film
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thickness and defect density affect the amplitudes of the phase transition and phase
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transition temperature.
Key words: VO2 thin film, sputtering power, phase transition, electrical resistance, infrared transmittance, switching performance
*
Authors to whom correspondence should be addressed. Electronic mail: [email protected], [email protected]
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1. Introduction Vanadium dioxide (VO2 (M)) has been widely studied because of its novel semiconductor-metal transition (SMT) from low-temperature semiconducting monoclinic
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phase to high-temperature metallic tetragonal phase at about 340 K for single crystalline VO2 (M) [1-4]. Bulk VO2 (M) has a narrow hysteresis width and a large amplitude of the
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transition. Thin film is widely used in practical energy saving applications [5]. It has been established that the optical transmittance of semiconducting phase VO2 thin films abide by
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the Beer-Lambert law, in which an increase in film thickness is always companying with a reduced optical transmittance. The SMT behaviors, such as the phase transition temperature, hysteresis width and the amplitude of the transition, depend strongly on the stoichiometry and microstructure of VO2 (M) thin films [6-8], and a slight change in film stoichiometry
optical/electrical
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and microstructure will bring about a considerable variation in the SMT behaviors and the properties
[9-13].
The
relationships
among the
stoichiometry,
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microstructures and SMT behaviors of VO2 (M) thin films are still not very clear [14], due to the complex stoichiometry of vanadium ions, the multiphase in VO2 [15], and the
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variations in film thickness, grain size distribution, grain boundaries, defects and strains [16-22], and fully understanding of the internal correlations among these factors is essential from basic research and for practical applications. Much effect has been devoted to grow high quality VO2 (M) thin films with excellent infrared and electrical switching performance. Different substrates like sapphire, Si, and glass have been used to grow VO2 thin films, in which the glass substrate with a low cost and small influence on the growth of the film has been commonly used. Nevertheless, there
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are only a few reports on optimizing the quality of VO2 (M) thin films on glass substrate. VO2 (M) thin film grown on glass substrates generally has a large amplitude of the transition but a very big hysteresis width of about 9 oC or higher during SMT, and big
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hysteresis width is not beneficial for practical application. Therefore, further study on the growth of VO2 (M) thin film on glass substrate is imperative. Various techniques have been
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used to grow the VO2 thin films, and the sputtering technique is an effective method to prepare high quality thin films. Much work has been carried out to modulate the SMT
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behaviors of VO2 (M) thin films by optimizing the sputtering parameters like sputtering gas pressure, O2/Ar ratio and sputtering temperature [23-25], though relatively few studies concern the sputtering power. In this paper, we report the growth and SMT behaviors of VO2 (M) thin films on glass substrates with different sputtering powers. It was found that
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the VO2 (M) thin film grown at 400 W has an excellent amplitude of the transition and a lower transition temperature.
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2. Experimental details
VO2 thin films were deposited on glass substrate at the sputtering power ranging from
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330 W to 400 W by radio frequency reactive magnetron sputtering. The details of the sputtering setup and substrate preparation can be found elsewhere [26]. In this study, the VO2 thin films were grown according to the followed conditions. The metal V target (purity: 99.99%) with a diameter of 60 mm was adopted as sputtering source material, and the distance between the target and substrate was 90 mm. The base pressure of the chamber is better than 5.0*10−5 Pa and the total sputtering gas pressure was kept at 0.3 Pa. Ar and O2 mixed gas were used as working and reacting gases, respectively, and the O2 ratio in mixed
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gas is kept 3.5 %. The sputtering powers are 330, 350 and 400 W. The morphology of the as-prepared thin films was examined by field-emission scanning electron microscopy (FESEM, Sirion 200). The phase structures were investigated by X-ray diffraction using
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Cu-Kα1 line (XRD, Philips X’Pert) and Raman scattering on a macroscopic confocal Raman spectrometer using a laser beam with an excitation wavelength of 514.5 nm. The
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electrical properties of the films in heating and cooling cycles were measured using a model 2612A semiconductor characterization system. All data were acquired at a temperature
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accuracy of 0.1 K with a 10 min waiting time at each temperature. Temperature-dependent optical transmittance spectrum was recorded by Nicolet Magna-IR750 Fourier transform infrared (FTIR) spectroscopy equipped with a heating system in the range of 400-4000
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cm-1.
3. Results and discussion
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3.1 Structure and morphology
Fig. 1 shows the XRD patterns of the thin films grown at different sputtering powers.
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One can see the thin film grown at sputtering power of 330 W is of a phase mixture of V6O13 (JCPDS card no. 00-025-1251) and VO2 (M) (JCPDS card no. 01-72-0514), as shown in curve (1) of Fig. 1. Simple quantitative analysis shows that the percentage of V6O13 phase is about 13%. When the sputtering power was increased to 350 W or higher, the V6O13 phase disappears and pure VO2 (M) phase was obtained, see curves (2) and (3) of Fig. 1 (For simplify, all the as-grown thin films are called VO2 films from here on). From Fig. 1 one also can see there is a strong diffraction peak at 2θ of about 27.9o from the (011)
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plane for all the thin films, indicating the grains in the films have a preferential orientation along (011) direction. The average grain size can be estimated from XRD pattern using the .λ
, where β is the full width at half maximum of a diffraction
β θ
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Scherrer’s formula: D =
peak at 2θ corrected for instrumental broadening, and the calculated average VO2 grain size based on the first five strong diffraction peaks is about 89.4, 119.7 and 129.8 nm at
size increases with increasing sputtering power [27].
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sputtering power of 330, 350, and 400 W, respectively. Obviously, the average VO2 grain
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Raman spectroscopy was used to further confirm the phase purity of the thin films grown at different sputtering powers. Fig. 2 shows room-temperature Raman spectra of the films. One can see that the Raman spectra of the films grown at 350 and 400 W have the same pattern, and the peaks at 143, 193, 223, 262, 308, 339, 389, 502 and 614 cm-1 can be
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assigned to the phonon vibration modes of VO2 phase, which is consistent with reports in literatures [28-30], and further proves that the films are pure VO2 phase. While the Raman
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peak at 131 cm-1 of the thin film grown at 330 W can be assigned to V6O13 phase [31], indicating coexistence of V6O13 and VO2 phases. Above results are in a good agreement
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with XRD results.
Fig. 3 shows typical FESEM images of the VO2 films grown at different sputtering powers. One can see the VO2 films are composed of random arrayed columnar particles due to self-shadowing of the incident atoms by those already incorporated into the film and low surface mobility. The particle size increases with increasing sputtering power, and the film thickness first increases with increasing sputtering power and then decreases at the sputtering powers of 400 W or over due to the target poisoning, see the cross-sectional
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FESEM images in Fig. 3. It was found the film grown at sputtering power over 400 W has a very large grain size with very thin thickness and some cracks due to the target poisoning,
3.2 Semiconductor-metal transition 3.2.1
Temperature-dependent resistance
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and thus the films grown at the sputtering powers higher than 400 W were discarded.
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To verify the electrical contact of the electrodes, in-plane I-V curves at different temperatures of the VO2 thin films grown at different sputtering powers were performed,
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and the results are shown in Fig. 4. A linear feature of all the I-V curves proves an ohmic contact of the metal-electrodes on top of VO2 film. The temperature-dependent electrical resistance of VO2 films at different sputtering powers is shown in Fig. 5, and the SMT behavior deduced form Fig. 5 is listed in Table 1, and a remarkable change in SMT can be
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seen. From Fig. 5 one can see all the films show an obvious thermal hysteresis behavior, and the film grown at 400 W has the lowest resistance at 80 oC. The SMT temperature
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during heating, Ttr, is determined from the intersection point of the back tangent and forward tangent during transition, and the resulted data are listed in Table 1. From this table,
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one can see the transition temperature is about 66.5, 56.0, and 65.5 oC for the VO2 films sputtered at 330, 350, and 400 W, respectively, and the film grown at 350 W has a narrower hysteresis width than that at 400 W. The amplitude of the SMT also can be obtained from the resistance ratio at the temperature above and below SMT, i.e. R80/R46 (R46 and R80 refer to the electrical resistance at 46 and 80 oC, respectively), and the results are listed in Table 1. From this table one can see the electrical resistance change is more than three orders after SMT.
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In the growth of VO2 films at the sputtering power of 350 W, we found that the higher the argon ionization degree, the higher the ion density and sputtering speed, and thus the higher the ion energy. High ion energy favors the transition of the V ions from a fully
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disordered state to partially ordered state, which will enhance the quality of the films [27]. On the other hand, high sputtering power will result in high energy ions, and the high
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energy ions will implant into VO2 surface, inducing a thermal effect in substrate, thereby leading to the increase in grain size and decrease in defect density. If the sputtering power
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is too high, such as 400 W in present study, the target poison will occur because of a notable chemisorption of oxygen on target surface, leading to a rapid drop in the deposition rate and the decrease in film thickness.
The grain size is main factor that affects the hysteresis width of the VO2 films, and the
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larger the grain size, the narrower the hysteresis width. As the grain size in the VO2 films increases with increasing sputtering power, and thus a narrowest hysteresis width is
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obtained for the film grown at 400 W. The amplitude of SMT and SMT temperature of the VO2 films are correlated with the microstructures like defects density and grain boundary,
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and in metallic state, high defect density will accelerate scattering of electrons and obviously increase the resistivity of the films, and thus the film grown at 400 W has the lowest resistance because of low defect density. The temperature dependence of resistance can be expressed by R(T)=R0exp(Ea/kBT) [21, 22], where R0 is the resistance of materials at infinite temperature, kB is Boltzmann constant, and Ea is activation energy. Fig. 6 shows the plot of ln[R(T)] vs (kBT)-1 of the VO2 films grown at different sputtering powers, from which the Ea can be derived from the
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slope of the linear fitting of ln[R(T)]~(1/kBT). The Ea is about 75.61/98.44, 209.84/118.15 and 218.44/140.05 meV in semiconductor/metal state of the VO2 films grown at sputtering power of 330, 350, and 400 W, respectively. One can see the Ea increases with increase
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sputtering power, and the higher the power, the higher the activation energy. It is worth noting that the Ea is higher in semiconductor state than in metal state for the films grown at
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350 and 400 W except for that at 330 W due to the presence of V6O13 phase. The increased Ea with increasing sputtering power indicates that the VO2 film grown at 400 W has the low
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defect density. [9-11]
3.2.2 Temperature-dependent Transmittance spectra
Fig. 7 shows the transmittance spectra at the wavelength of 2.5 µm of the VO2 films
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grown at different sputtering powers in heating and cooling cycles. The infrared switching parameters deduced from Fig. 7 are listed in Table 2. An obvious change in infrared
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transmittance can be observed across the SMT, and the change depends on the sputtering powers. The film grown at 350 W has a fairly high transmittance of 54.8 % at 30 oC in
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semiconductor state and nearly 0 % transmittance at 70 oC in metallic state, and the amplitude of SMT is about 54.8 %. It is worth noting that the transmittance in metallic phase is close to zero, corresponding to the value of bulk VO2, suggesting the film grown at a sputtering power of 350 W is pure VO2 (M) phase, which is in accordance with the XRD result. The SMT temperature of the film grown at 350 W is about 57 oC (lower than bulk VO2) with a hysteresis width of about 7.2 oC; while that of the film grown at 400 W is about 63 oC with a hysteresis width of 5 oC and a slightly increased amplitude of SMT.
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From Fig. 7 one also can see the maximum transmittance of the film grown at 350 W in semiconductor state almost has the same value as that of the film grown at 400 W. Compared Table 2 with Table 1, one can see the SMT parameters derived from
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transmittance data are slightly different from that deduced from resistance experiments, which is considered due to the different temperature errors in these two measurement
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systems. The poor infrared switching performance of the VO2 film grown at 330 W is due to the presence of V6O13 phase. It is worth noting that the hysteresis width of the VO2 film
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grown at 400 W is narrower than that at 350 W, which is attributed to the size effect, and is in agreement with the reported results [32]. Fig. 8 shows the transmittance Tλ maps of the VO2 film as a function of the sputtering power in the heating and cooling cycles. A hysteresis loop can be clearly seen, and the hysteresis width gradually decreases with
4 and 7.
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increasing sputtering power, which is in a good accordance with the results shown in Figs.
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According to Beer-Lambert law, the optical transmittance of the VO2 films in semiconductor state is exponentially dependent on the film thickness. From Table 2 one can
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see the film grown at 400 W has a smaller thickness than that grown at 350 W, and should have a high transmittance in semiconductor state, and in fact they almost have the same value. The results indicate that the infrared switching properties depend not only on the thickness but also on the factors like grain size, porosity, defects, substrate and stoichiometry of the films [14, 17-20]. Our XRD analysis confirmed that the films grown at 350 and 400 W are pure VO2 (M) phase, the different microstructures should be responsible for the slight difference in the infrared performance of these two films. It has been known
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that the amplitude of SMT and SMT temperature depend on the defect density, and a larger amplitude of SMT and a lower transition temperature can be realized with few defects.
amplitude of SMT and a lower SMT temperature.
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Being having a lower defect density, the VO2 film grown at 400 W has an excellent
Above results demonstrate that the pure VO2 (M) thin film with excellent switching
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performance can be obtained at the sputtering power of 400 W. It was found too low sputtering power will result in the formation of the second phase, and too high will poison
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the target, reducing the quality of the thin films. 4. Conclusions～
In summary, we have investigated the influence of sputtering power on the SMT and switching performance of the VO2 thin films on glass substrate. The controlling of
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sputtering power is very important in realizing the pure phase VO2 (M) thin film. The VO2 (M) films grown at sputtering power of 350 W and above exhibit an obvious transition
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below 68 °C, a large amplitude of SMT, and relatively small hysteresis width. It was found that the hysteresis width is controlled by the size effect, and film thickness and defect
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density simultaneously affect the amplitudes of the SMT and SMT temperature. The VO2 film grown at sputtering power of 400 W has a SMT temperature of about 63 oC and a hysteresis width of about 5 oC with transmittance close to zero in metallic state.
ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51471163 and 11104270).
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References [1] C. Z. Wu, F. Feng, Y. Xie, Chemical Society reviews, 42 (2013) 5157-5183 [2] S. Rathi, J. H. Park, I. y. Lee, J. M. Baik, K. S. Yi, G. H. Kim, Journal of Physics D:
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Applied Physics, 47 (2014) 295101
[3] H. Zhang, X. D. Xiao, X. M. Lu, G. Q. Chai, Y. M. Sun, Y. J. Zhan, G. Xu, Journal of
SC
Alloys and Compounds 636 (2015) 106-112
[4] Y. X. Guo, H. Y. Xu, C. W. Zou, Z. Y. Yang, B. Tong, J. Y. Yu, Y. J. Zhang, L. Zhao, Y. L.
M AN U
Wang, Journal of Alloys and Compounds 622 (2015) 913-917
[5] K. Nagashima, T. Yanagida, H. Tanaka, T. Kawai, Journal of Applied Physics 100 (2006) 063714.
[6] D. Y. Fu, K. Liu, T. Tao, K. Lo, C. Cheng, B. Liu, R. Zhang, H. A. Bechtel, and J. Q.
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Wu, JOURNAL OF APPLIED PHYSICS 113 (2013) 043707
[7] S. Lafane, T. Kerdja, S. Abdelli-Messaci, Y. Khereddine, M. Kechouane, O. Nemraoui,
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Applied Physics A 112 (2012) 159-164
[8] J. Narayan, V. M. Bhosle, Journal of Applied Physics, 100 (2006) 103524
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[9] C. H. Chen, Y. Zhao, X. Pan, V. Kuryatkov, A. Bernussi, M. Holtz, Z. Y. Fan, Journal of Applied Physics, 110 (2011) 023707 [10] L. T. Kang, Y. F. Gao, Z. T. Zhang, J. Du, C. X. Cao, Z. Chen, and H. J. Luo, J. Phys. Chem. C, 114 (2010) 1901-1911 [11] H. W. Liu, L. M. Wong, S. J. Wang, S. H. Tang, X. H. Zhang, Applied Physics Letters, 103 (2013) 151908 [12] Ü. Akın, H. Şafak, Journal of Alloys and Compounds 647 (2015) 146-151.
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[13] Ö.D. Coşkun, S. Demirel, G. Atak, Journal of Alloys and Compounds 648 (2015) 994-1004.
of Physics D: Applied Physics, 46 (2013) 055310
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[14] Q. Yu, W. W. Li, J. Liang, Z. H. Duan, Z. G. Hu, J. Liu, H. D. Chen, J. H. Chu, Journal
[15] Y. D. Ji, T. S. Pan, Z. Bi, W. Z. Liang, Y. Zhang, H. Z. Zeng, Q. Y. Wen, H. W. Zhang,
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C. L. Chen, Q. X. Jia, Y. Lin, Applied Physics Letters, 101 (2012) 071902
[16] Y. X. Guo, Y. F. Liu, C. W. Zou, Z. M. Qi, Y. Y. Wang, Y. Q. Xu, X. L. Wang, F. Zhang,
M AN U
R. Zhou, Applied Physics A, 115 (2014) 1245-1250
[17] C. Clavero, J. L. Slack, A. Anders, Journal of Physics D: Applied Physics, 46 (2013) 362001
[18] R. Lopez, L. C. Feldman, R. F. Haglund, Jr Physical Review Letters, 93 (2004) 177403
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[19] T. H. Yang, R. Aggarwal, A. Gupta, H. H. Zhou, R. J. Narayan, J. Narayan, Journal of Applied Physics, 107 (2010) 053514
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[20] Y. J. Chang, J. S. Yang, Y. S. Kim, D. H. Kim, T. W. Noh, D. W. Kim, E. Oh, B. Kahng, J. S. Chung, Physical Review B, 76 (2007) 075118
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[21] J. W. Ma, G. Xu, L. Miao, M. Tazawa, S. Tanemura, Japanese Journal of Applied Physics, 50 (2011) 020215 [22] Z. Luo, X. Zhou, D. Yan, D. Wang, Z. Li, C. Yang, Y. Jiang, Thin Solid Films 550 (2014) 227-232. [23] P. Jin, G. Xu, M. Tazawa, K. Yoshimura, Applied Physics A: Materials Science & Processing, 77 (2003) 455-459 [24] X. B. Wei, Z. M. Wu, X. D. Xu, T. Wang, J. J. Tang, W. Z. Li, Y. D. Jiang, Journal of
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Physics D: Applied Physics, 41 (2008) 055303 [25] Y.Y. Luo, F.H. Su, S.S. Pan, S.C. Xu, C. Zhang, J. Pan, J.M. Dai, P. Li, G.H. Li, Journal of Alloys and Compounds 655 (2016) 442-447
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[26] Y.Y. Luo, L.Q. Zhu, Y.X. Zhang, S.S. Pan, S.C. Xu, M. Liu, G. H. Li, Journal of Applied Physics, 113 (2013) 183520.
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[27] K. Srinivas, M. Manivel Raja, D.V. Sridhara Rao, S.V. Kamat, Thin Solid Films 558 (2014) 349-355.
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[28] G. I. Petrov, V. V. Yakovlev, J. Squier, Applied Physics Letters, 81 (2002) 1023 [29] E. M. Heckman, L. P. Gonzalez, S. Guha, J. O. Barnes, A. Carpenter, Thin Solid Films, 518 (2009) 265-268
[30] H. T. Kim, B. G. Chae, D. H. Youn, G. Kim, K. Y. Kang, S. J. Lee, K. Kim, Y. S. Lim,
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Applied Physics Letters, 86 (2005) 242101
[31] T. D. Manning, I. P. Parkin, R. J. H. Clark, D. Sheel, M. E. Pemble, D. Vernadou,
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Journal of Materials Chemistry 12 (2002) 2936-2939. [32] R. A. Aliev, V. N. Andreev, V. M. Kapralova, V. A. Klimov, A. I. Sobolev, E. B.
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Shadrin, Physics of the Solid State, 48 (2006) 929-934
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Figure captions Fig. 1. XRD patterns of VO2 films at sputtering power of (1) 330, (2) 350 and (3) 400 W. The standard diffraction patterns from V6O13 (JCPDS card no. 00-25-1251) and VO2 (M)
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(JCPDS card no. 01-72-0514) are also given.
Fig. 2. Raman spectra of VO2 thin films grown at different sputtering powers.
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Fig. 3. FESEM images and the corresponding cross-sectional views of VO2 films grown at sputtering power of (a)-(b) 330, (c)-(d)350, and (e)-(f) 400 W.
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Fig. 4. In-plane I-V curves between two top electrodes at temperatures above ((a), (c) and (e)) and below ((b), (d) and (f)) SMT of VO2 thin films grown at different sputtering powers.
Fig. 5. Temperature-dependent resistance of the VO2 films grown at different sputtering
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powers.
Fig. 6. Plot of ln[R(T)]~1/kBT of VO2 films grown at different sputtering powers.
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Fig. 7. Temperature-dependent transmittance of VO2 films at sputtering power of curve (1) 330, (2) 350 and (3) 400 W.
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Fig. 8. Transmittance maps of VO2 films as a function of sputtering power in the cycle of (a) heating and (b) cooling.
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Table 1. Electrical switching parameters of VO2 films at different sputtering powers.
Ttr (oC)
width (oC)
R46/R80
amplitude
330
66.5
4.6
598.4/18.92
31.63
350
56
5.7
400
65.5
4.7
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power (W)
958.96
9178.23/5.68
1615.89
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7940.2/8.28
Table 2. Infrared switching parameters at 2.5 µm of VO2 films at different sputtering
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and 80 oC, respectively)
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powers (Ttr is SMT temperature in heating process, T30 and T80 are the transmittance at 30
Transmittance (%)
Thickness
Ttr
Hysteresis
(nm)
(oC)
width (oC)
T30
T80
SMT
330
540
67
4.5
18.8
0
18.8
350
700
57
7.2
54.8
0
54.8
400
370
63
5
56
0
56
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Power (W)
Amplitude of
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Fig. 1
Fig. 2
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Fig. 3
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Fig. 4
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Fig. 5
Fig. 6
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Fig. 7
Fig. 8
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Highlights 1) Optimal conditions are achieved to grow pure VO2 (M) thin film.
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2) Optical/electrical switching property of the VO2 film depend on sputtering power
3) Film thickness and defect density affect amplitude of transition
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4) Grain size effect controls the hysteresis width of phase transition