- Email: [email protected]
Influence of lattice distortion on phase transition properties of polycrystalline VO2 thin film
Accepted Manuscript Title: Influence of lattice distortion on phase transition properties of polycrystalline VO2 thin film Author: Tiegui Lin Langping Wang Xiaofeng Wang Yufen Zhang Yonghao Yu PII: DOI: Reference:
S0169-4332(16)30747-4 http://dx.doi.org/doi:10.1016/j.apsusc.2016.04.007 APSUSC 33008
To appear in:
APSUSC
Received date: Revised date: Accepted date:
21-12-2015 2-4-2016 4-4-2016
Please cite this article as: Tiegui Lin, Langping Wang, Xiaofeng Wang, Yufen Zhang, Yonghao Yu, Influence of lattice distortion on phase transition properties of polycrystalline VO2 thin film, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.04.007 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.
Influence of lattice distortion on phase transition properties of polycrystalline VO2 thin film Tiegui Lina, Langping Wanga,*, Xiaofeng Wanga, Yufen Zhanga, Yonghao Yub,* a
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, P.R. China
b
Academy of Fundamental and Interdisciplinary Science, Harbin Institute of Technology, Harbin 150001
* Corresponding author Professor Langping Wang E-mail addresses: [email protected]. Tel: +86-451-86418728 Fax: +86-451-86416186 Dr. Yonghao Yu E-mail addresses: [email protected]
Submitted to: APPLIED SURFACE SCIENCE (With 5 figures)
1. Polycrystalline VO2 thin films were fabricated by high power impulse magnetron sputtering.
2. The reported lowest phase transition temperature for undoped polycrystalline VO2 thin film was reduced to 32 °C by this research.
3. XRD patterns at varied temperatures revealed that the main structual change was a gradual shift in interplanar spacing with temperature. 1
ABSTRACT In this work, high power impulse magnetron sputtering was used to control the lattice distortion in polycrystalline VO2 thin film. SEM images revealed that all the VO2 thin films had crystallite sizes of below 20 nm, and similar configurations. UV-visible-near IR transmittance spectra measured at different temperatures showed that most of the as-deposited films had a typical metal–insulator transition. Four-point probe resistivity results showed that the transition temperature of the films varied from 54.5 to 32 °C. The X-ray diffraction (XRD) patterns of the as-deposited films revealed that most were polycrystalline monoclinic VO2. The XRD results also confirmed that the lattice distortions in the as-deposited films were different, and the transition temperature decreased with the difference between the interplanar spacing of the as-deposited thin film and standard rutile VO2. Furthermore, a room temperature rutile VO2 thin film was successfully synthesized when this difference was small enough. Additionally, XRD patterns measured at varied temperatures revealed that the phase transition process of the polycrystalline VO2 thin film was a coordinative deformation between grains with different orientations. The main 2
structural change during the phase transition was a gradual shift in interplanar spacing with temperature. Keywords: sputtering; thin film; lattice distortion; phase transition; polycrystal
1. INTRODUCTION Vanadium dioxide (VO2) is a material that undergoes a first-order metal–insulator transition (MIT) at around 68 °C, coupled with a structural transition from a low temperature monoclinic (M-VO2) to a high-temperature rutile (R-VO2) structure [1]. This metal–insulator transition can occur at ultrafast timescales (100 fs) [2]. In pure bulk single crystals, the transition leads to a change in resistivity of up to five orders of magnitude and a strong modification of the optical transmission [3]. These unique characteristics can be exploited in applications such as optical switching, thermochromic smart windows, laser protection, and energy harvesting systems [4-6]. VO2 thin films have been synthesized by a variety of deposition techniques, such as pulsed laser deposition (PLD) [5, 7-10], molecular beam epitaxy (MBE) [11], reactive sputtering [12-15], sol-gel processing [16], thermal oxidation [17], and reactive electron beam evaporation [18]. Previous studies have shown that the change in resistivity of epitaxial VO2 thin film can reach about four orders of magnitude, the transmittance change at 2500 nm can exceed 60%, and the transition temperature can be reduced to 20 °C [2, 19]. However, the more practical polycrystalline thin film only shows a resistivity change of one to two orders of magnitude [20-23], and its transition temperature is higher than 42 °C [4, 21, 24, 25]. 3
Reduction of the phase transition temperature will benefit the applications of polycrystalline VO2 thin film, and thus has been of wide interest to researchers in this area [11, 24, 26]. A number of studies have found that the transition temperature of polycrystalline VO2 thin film can be changed by altering the preparation parameters. Jiang et al. [25] fabricated polycrystalline VO2 thin films with different transition temperature by changing the oxygen partial pressure of the synthesis, but did not explain the reason for the reduction in transition temperature. Ba et al. [21] found that the greater the oxygen flow, the lower the transition temperature. They believed that the reason for this was an increase in the grain size of the films. However, Miller et al. [24] reported that a lower transition temperature was obtained at smaller grain size. Consequently, it can be deduced that although the transition temperature of the polycrystalline VO2 thin film can be reduced, the origin of the reduction has been unclear until the present. The phase transition of polycrystalline VO2 thin film is a lattice distortion process [10]. Previous studies have proven that the phase transition temperature can be reduced by lattice distortion at room temperature [27, 28]. Therefore, a prior lattice distortion that brings the lattice parameters of monoclinic VO2 close to those of the rutile structure should help reduce the phase transition temperature. High power impulse magnetron sputtering (HiPIMS) can generate a dense plasma and intense ion bombardment during the deposition process [14, 29], in which the ion bombardment can induce distortion of the crystalline lattice [30]. As a consequence, VO2 thin films with different lattice distortions may be obtained using a HiPIMS approach. 4
In this study, VO2 thin films were prepared by HiPIMS under different deposition conditions. The lattice distortions in the different films were characterized, and the influences of the lattice distortion on the phase transition properties of the films are discussed.
2. EXPERIMENTAL VO2 films were synthesized in a high power impulse magnetron sputtering apparatus. A turbo-molecular pump based vacuum system was used to achieve a base vacuum of less than 1 × 10−3 Pa. A vanadium target with a diameter of 100 mm and purity of 99.95% was used for the deposition process. The target-substrate distance was approximately 100 mm. The quartz glass substrates (30 × 30 × 1 mm3) were heated by a conduction heater from the back side to a surface temperature of 420 °C. The argon flow rate was set to 100 sccm, and the oxygen flow rate was varied from 5 to 6.7 sccm. The total pressure was maintained at about 0.85 Pa during the deposition process. A modulated pulsed power supply was used for the HiPIMS process. The pulse repetition frequency and duration time were 50 Hz and 400 s, respectively, and the peak power during the sputtering process was about 40 kW. Table 1 lists the main processing conditions for the different samples. Table 1 Main processing conditions for different samples. Sample
Ar flow rate
O2 flow rate
Deposition time
Thickness
No.
(sccm)
(sccm)
(min)
(nm)
1
100
6.7
30
133
2
100
5.2
30
120
3
100
5
30
118
5
4
100
6.5
15
77
5
100
6
15
65
6
100
5.5
15
86
The surface morphology of the obtained films was examined using a scanning electron microscope (SEM; Quanta 200, FEI Co., USA). The phase composition of the films was identified by X-ray diffraction at a grazing angle of 1 (XRD; Empyrean, Panalytical, Holland) with a Cu-Kα radiation source operated at 40 kV and 40 mA. The optical properties of the film between 200 and 2500 nm were measured using a UV-visible-near IR spectrophotometer (Lambda 950, PerkinElmer Co., USA). The electrical resistance of the films and its variation with temperature were measured with a four-point probe resistivity measurement system.
3. RESULTS AND DISCUSSION 3.1 Electrical and optical properties Figure 1 shows the temperature dependences of the resistance and the transmittance spectra of the as-deposited VO2 thin films. It is clear from the results that the phase transition temperature of the films was successfully varied. The transmittance of the VO2 films decreased obviously when the temperature was increased from 24 to 75 °C, which proves that a metal–insulator transition occurred during the heating process. However, sample 6 possessed very a low resistance and transmittance at room temperature, close to those of R-VO2. To compare the transition characteristics of different samples, the relative resistance change, transition temperature, sharpness of the transition, hysteresis width, 6
and absolute transmittance change of the different VO2 thin films were defined according to previous studies [2]. The relative resistance change (R) is defined as
R=R 0⁄R , 1
(1)
where R0 is the maximum resistance before the phase transition, and R1 is the minimum resistance after the phase transition. The transition temperature (TMIT ) is given by
TMIT = (Tc, cooling+ Tc, heating )/2, where Tc,
cooling
and Tc,
heating
(2)
are the transition temperatures at the center of the
derivative curve (d[log(R)]/dT) during cooling and heating, respectively. The sharpness of the transition (T) is defined as the full width half maximum (FWHM) of the derivative curve during cooling. The hysteresis width (H) is written as
H= Tc, heating − Tc, cooling.
(3)
The absolute transmittance change (Tr) is given by
Tr = Tr0 − Tr1,
(4)
where Tr0 is the maximum transmittance at 2500 nm before the phase transition, and Tr1 is the minimum transmittance at 2500 nm after the phase transition. Table 2 shows the electrical and optical properties of the different VO2 thin films. Note that the transition temperature could be reduced to 32 °C (sample 3), which is close to the lowest reported transition temperature of epitaxial VO2 film on TiO2(001) substrate [19]. For sample 4, the resistance change, hysteresis width, sharpness of the transition, and absolute transmittance change were 4500, 6 °C, 5.8 °C and 69.8 %, 7
respectively, which indicate that the electrical and optical properties of this VO2 film were close to the best properties reported for epitaxial VO2 film [2].
Table 2 Electrical and optical properties of the different VO2 thin films
Sample No
TMIT (°C)
T
H
(°C)
(°C)
1 2 3 4 5
52.5 45.5 32 54.5 48.3
6.5 14 18.5 5.8 13.5
9 3 2 6 4.2
R
Tr
2.3×103 3.2×102 28 4.5×103 1.2×102
42.5% 23.1% 16.1% 69.8% 29%
3.2 Microstructural characteristics The SEM images of the VO2 thin films are shown in Figure 2. All the films exhibited a crystallite size of below 20 nm, and their configurations were similar. Therefore, the influence of crystallite morphology on the phase transition properties of the films can be neglected in this work. The XRD patterns of samples 1–3 are shown in Figure 3(a). The obvious (011), (211), (022), and (202) peaks in the patterns confirmed that the as-deposited VO2 thin films were polycrystalline. According to the standard diffraction card of M-VO2 (JCPDS 44-0252), the 2θ values of the (011), (211), (022), and (202) plane reflections should be 27.86, 55.54, 57.50, and 70.44, respectively. However, as shown in 8
Figure 3(a), the positions of the same diffraction planes in the as-deposited VO2 thin films were obviously different. For example, the (011) peak of samples 1–3 appeared at 27.877, 27.828, and 27.771, respectively, which indicates that different lattice distortions were formed in the as-deposited films. A previous study has shown that the variation of oxygen pressure during deposition can affect the stoichiometric ratio of V to O, which can produce oxygen vacancies in the as-deposited VO2 thin films [2]. Additionally, the ion bombardment would have induced large stress in the as-deposited film [31]. As a consequence, the lattice distortion in the as-deposited VO2 thin films should originate from the ion bombardment as well as the oxygen vacancies. To determine the influence of the lattice distortion on the phase transition temperature, we compared the interplanar spacing of the films before and after the phase transition process. Because the intensity of the (011) peak was the greatest, only the interplanar spacing of (011) plane is discussed here. For sample 3, the interplanar spacing was 0.32104 nm, which is 99.7% of that of R-VO2 (JCPDS 44-0253). The spacings of samples 2 and 1 were 0.32043 and 0.31991 nm, respectively, 99.5% and 99.3% that of R-VO2. The measured phase transition temperatures of samples 3, 2, and 1 were 32, 45.5, and 52.5 °C, respectively. These results reveal that the transition temperature of the polycrystalline VO2 film can be greatly reduced by a sufficient lattice distortion. This deduction was confirmed by the results obtained for an epitaxial VO2 thin film [32]. In this study, the interplanar spacing of the (002) plane of epitaxial VO2 thin film on TiO2(001) was measured to be 0.14235 nm, 99.71% of 9
that of R-VO2, and the corresponding phase transition temperature was only 27 °C. The other diffraction peaks of the different samples, such as the (012), (211), (022), and (202) peaks, also differed. To compare the lattice distortion in different planes, the respective diffraction angles of standard R-VO2 (JCPDS 44-0253) and the as-deposited VO2 thin films have been marked with red and blue lines in the XRD patterns shown in Fig. 3(a). It is obvious that the deviations between the red and blue varied across the different samples. Because the magnitudes of the 2 deviations are not clear in Fig. 3(a), magnifications of the XRD patterns are shown in Fig. 3(b) and 3(c). For (012), (211), (022) and (202), a small 2 difference was beneficial to reducing the phase transition temperature, similar to the results obtained for the (011) plane. Additionally, because the diffraction angles of the (200) and (210) planes of undistorted M-VO2 are 37.00 and 42.24, respectively, which are very close to those of R-VO2 (37.12 and 42.28), the shift of the corresponding peaks during the phase transition process should be negligible. Therefore, the deviations in the interplanar spacings of the (200) and (210) planes should be as small as possible, which was confirmed by the results shown in Fig. 3(b). Consequently, the phase transition of a polycrystalline VO2 film should be easier and the transition temperature should be lower if the change in the interplanar spacing during the phase transition process is smaller. This principle is consistent with the results of Chen et al. [33], who reported that a shift in the diffraction angle of the (011) plane of M-VO2 nanorods to that of R-VO2 decreased the phase transition temperature 10
to about 47 °C during the cooling process. Conversely, the larger the difference in interplanar spacing, the higher the phase transition temperature. This result is also in accordance with that obtained by Azhan et al. [34], who reported that deviation of the diffraction angle for the (020) plane of an epitaxial VO2 thin film away from that of R-VO2 increased the phase transition temperature. To determine whether this principle still applies when the thickness of the film is changed, the XRD patterns and 2 differences of samples 4–6 are shown in Figure 4. The results obtained for samples 4 and 5 are in accordance with those for samples 1–3, and prove that the principle is correct for films with different thickness. The diffraction peaks of sample 6 coincided with those of R-VO2. Fig. 1 shows that the resistance and the transmittance of this thin film were extremely low. These results indicate that metallic-VO2 was synthesized in the case of sample 6, which confirms Y.Y. Cui et al.’s inference based on first principles calculations [35]. Figure 5(a) compares the XRD patterns of sample 1 during heating and cooling processes. The red lines mark the diffraction peaks of the standard diffraction card of R-VO2, while blue lines mark the experimentally obtained diffraction peaks at different temperatures during the heating and cooling processes. Magnifications of the deviations in 2 from the peaks of standard R-VO2 during the heating process are shown in Fig. 5(b) and (c), while those for the cooling process are exhibited in Fig. 5(d) and (e). During the heating process, the 2 deviations of the main diffraction peaks decreased gradually with increased temperature (Fig. 5(b) and (c)), which hints that 11
the interplanar spacing of the as-deposited VO2 thin film was shifted step by step towards that of standard R-VO2. In other words, the crystal structure of polycrystalline M-VO2 thin film gradually became more similar to that of R-VO2 during the heating process. This result is different from that expected according to the domain boundary movement principle [17]. When the temperature was 75 °C, the diffraction peaks of the as-deposited VO2 thin film coincided with those of standard R-VO2, revealing that the phase transition was completed. This was confirmed by the electrical and optical properties of the film measured at 75 °C. During the cooling process, the diffraction peaks gradually moved backward with decreasing temperature, which means that the crystal structure gradually transformed from R-VO2 to M-VO2. Therefore, the VO2 film was restored to its original high resistance and transmittance after cooling to room temperature. The diffraction angles measured at 50 °C were also found to be different during heating and cooling. The positions of the peaks were closer to those of R-VO2 during the cooling process than during the heating process, which hints that the crystal structure of the film was more similar to that of R-VO2 during the cooling process. As a result, the resistance of the film was lower during the cooling process than during the heating process (Fig. 5(f)). Consequently, the hysteresis of the transition process can be attributed to variation in the lattice distortion at the same temperature during the heating and cooling processes. Unlike single crystal VO2, polycrystalline thin films possesses deformation compatibility between differently orientated grains. Therefore, some grains having 12
specific orientations may possess irregular lattice distortions. During the heating process, the diffraction angle of the (022) plane increased and the interplanar spacing is decreased, which is opposite to that for the (011) plane. During the cooling process, the diffraction angle of the (012) plane decreased instead of increased. Based on these results, we can infer that the intermediate states discovered by Kumar, S et al. [23] were likely generated in the present polycrystalline VO2 thin films owing to deformation compatibility. Meanwhile, because the (200) and (210) planes of standard M-VO2 and R-VO2 have similar interplanar spacing, the interplanar spacing of these planes should not change obviously during the phase transition process. As shown in Fig. 5(b) and (d), the change in the diffraction angle of the (200) and (210) of the present films were negligible during the cooling and heating processes, which confirms this deduction. Consequently, the phase transition process of the polycrystalline VO2 thin film was found to be a coordinative deformation between grains with different orientations. The main structural change in the phase transition process was a gradual shift in interplanar spacing with temperature. For planes with similar diffraction angles to those of standard M-VO2, and R-VO2, the interplanar spacing did not change obviously. Additionally, some planes were inversely deformed as a result of coordinative deformation. Based on the above results, it was found that the change in the transmittance and resistance of the as-deposited VO2 thin film deteriorated with decreasing TMIT. Because a low transition temperature and good resistance change can be achieved 13
with epitaxial VO2 thin film [32], and because the phase transition of the polycrystalline of the VO2 thin film involves coordinative deformation of different grains, we believe that the best way to reduce TMIT with only a slight deterioration in the film properties to fabricate VO2 thin film with a high ordered crystalline orientation close to that of R-VO2.
4. CONCLUSIONS Polycrystalline VO2 thin films with different phase transition properties were prepared successfully by HiPIMS. The transition temperature of the films could be varied from 54.5 to 32 °C. The phase transition temperature was found to be controlled by the difference between the interplanar spacing of the as-deposited thin film and standard R-VO2. The smaller the difference in the interplanar spacing, the lower the transition temperature. Additionally, a room temperature R-VO2 thin film was synthesized when the interplanar spacing difference was small enough. The phase transition of the polycrystalline VO2 thin film was a coordinative deformation process between grains with different orientations. The main structural change occurring during the phase transition process was found to be a gradual shift in the interplanar spacing with temperature.
ACKNOWLEDGMENTS This work was supported by the fund of National Key Laboratory of High Power Microwave Technology (Contract No. 2014-763.xy.k). 14
REFERENCES [1] Y.-K. Dou, J.-B. Li, M.-S. Cao, D.-Z. Su, F. Rehman, J.-S. Zhang, H.-B. Jin, Oxidizing annealing effects on VO2 films with different microstructures, Applied Surface Science, 345 (2015) 232-237. [2] H. Kim, N. Charipar, M. Osofsky, S.B. Qadri, A. Piqué, Optimization of the semiconductor-metal transition in VO2 epitaxial thin films as a function of oxygen growth pressure, Applied Physics Letters, 104 (2014) 081913. [3] A.P. Peter, K. Martens, G. Rampelberg, M. Toeller, J.M. Ablett, J. Meersschaut, D. Cuypers, A. Franquet, C. Detavernier, J.-P. Rueff, M. Schaekers, S. Van Elshocht, M. Jurczak, C. Adelmann, I.P. Radu, Metal-Insulator Transition in ALD VO2Ultrathin Films and Nanoparticles: Morphological Control, Advanced Functional Materials, 25 (2015) 679-686. [4] Z.L. Huang, S.H. Chen, C.H. Lv, Y. Huang, J.J. Lai, Infrared characteristics of VO2 thin films for smart window and laser protection applications, Applied Physics Letters, 101 (2012). [5] S. Fan, L. Fan, Q. Li, J. Liu, B. Ye, The identification of defect structures for oxygen pressure dependent VO2 crystal films, Applied Surface Science, 321 (2014) 464-468. [6] Y. Ji, Y. Zhang, M. Gao, Z. Yuan, Y. Xia, C. Jin, B. Tao, C. Chen, Q. Jia, Y. Lin, Role of microstructures on the M1-M2 phase transition in epitaxial VO2 thin films, Scientific reports, 4 (2014) 4854. [7] M.R. Bayati, R. Molaei, F. Wu, J.D. Budai, Y. Liu, R.J. Narayan, J. Narayan, Correlation between structure and semiconductor-to-metal transition characteristics of VO2/TiO2/sapphire thin film heterostructures, Acta Materialia, 61 (2013) 7805-7815. [8] B.D. Ngom, M. Chaker, A. Diallo, I.G. Madiba, S. Khamlich, N. Manyala, O. Nemraoui, R. Madjoe, 15
A.C. Beye, M. Maaza, Competitive growth texture of pulsed laser deposited vanadium dioxide nanostructures on a glass substrate, Acta Materialia, 65 (2014) 32-41. [9] T.-H. Yang, S. Nori, S. Mal, J. Narayan, Control of room-temperature defect-mediated ferromagnetism in VO2 films, Acta Materialia, 59 (2011) 6362-6368. [10] V.R. Morrison, R.P. Chatelain, K.L. Tiwari, A. Hendaoui, A. Bruhacs, M. Chaker, B.J. Siwick, A photoinduced metal-like phase of monoclinic VO(2) revealed by ultrafast electron diffraction, Science, 346 (2014) 445-448. [11] F.H. Chen, L.L. Fan, S. Chen, G.M. Liao, Y.L. Chen, P. Wu, L. Song, C.W. Zou, Z.Y. Wu, Control of the metal-insulator transition in VO(2) epitaxial film by modifying carrier density, ACS applied materials & interfaces, 7 (2015) 6875-6881. [12] X. Li, A. Gloter, H. Gu, J. Luo, X. Cao, P. Jin, C. Colliex, Discovery of nanoscale reduced surfaces and interfaces in VO2 thin films as a unique case of prewetting, Scripta Materialia, 78-79 (2014) 41-44. [13] B. Viswanath, C. Ko, S. Ramanathan, Thermoelastic switching with controlled actuation in VO2 thin films, Scripta Materialia, 64 (2011) 490-493. [14] J.P. Fortier, B. Baloukas, O. Zabeida, J.E. Klemberg-Sapieha, L. Martinu, Thermochromic VO2 thin films deposited by HiPIMS, Solar Energy Materials and Solar Cells, 125 (2014) 291-296. [15] H. Zhang, Z. Wu, Q. He, Y. Jiang, Preparation and investigation of sputtered vanadium dioxide films with large phase-transition hysteresis loops, Applied Surface Science, 277 (2013) 218-222. [16] Y. Guo, H. Xu, C. Zou, Z. Yang, B. Tong, J. Yu, Y. Zhang, L. Zhao, Y. Wang, Evolution of structure and electrical properties with annealing time in solution-based VO2 thin films, Journal of Alloys and Compounds, 622 (2015) 913-917. 16
[17] X. He, T. Xu, X. Xu, Y. Zeng, J. Xu, L. Sun, C. Wang, H. Xing, B. Wu, A. Lu, D. Liu, X. Chen, J. Chu, In situ atom scale visualization of domain wall dynamics in VO2 insulator-metal phase transition, Scientific reports, 4 (2014) 6544. [18] J. Leroy, A. Bessaudou, F. Cosset, A. Crunteanu, Structural, electrical and optical properties of thermochromic VO2 thin films obtained by reactive electron beam evaporation, Thin Solid Films, 520 (2012) 4823-4825. [19] J. Jeong, N. Aetukuri, T. Graf, T.D. Schladt, M.G. Samant, S.S. Parkin, Suppression of metal-insulator transition in VO2 by electric field-induced oxygen vacancy formation, Science, 339 (2013) 1402-1405. [20] D. Brassard, S. Fourmaux, M. Jean-Jacques, J.C. Kieffer, M.A. El Khakani, Grain size effect on the semiconductor-metal phase transition characteristics of magnetron-sputtered VO2 thin films, Applied Physics Letters, 87 (2005) 051910. [21] C. Ba, S.T. Bah, M. D'Auteuil, P.V. Ashrit, R. Vallee, Fabrication of high-quality VO2 thin films by ion-assisted dual ac magnetron sputtering, ACS applied materials & interfaces, 5 (2013) 12520-12525. [22] S. Chen, J. Lai, J. Dai, H. Ma, H. Wang, X. Yi, Characterization of nanostructured VO2 thin films grown by magnetron controlled sputtering deposition and post annealing method, OPTICS EXPRESS 17 (2009) 24153-24161. [23] S. Kumar, J.P. Strachan, M.D. Pickett, A. Bratkovsky, Y. Nishi, R.S. Williams, Sequential electronic and structural transitions in VO2 observed using X-ray absorption spectromicroscopy, Advanced materials, 26 (2014) 7505-7509. [24] M.J. Miller, J. Wang, Influence of grain size on transition temperature of thermochromic VO2, 17
Journal of Applied Physics, 117 (2015) 034307. [25] M. Jiang, X. Cao, S. Bao, H. Zhou, P. Jin, Regulation of the phase transition temperature of VO2 thin films deposited by reactive magnetron sputtering without doping, Thin Solid Films, 562 (2014) 314-318. [26] X. Liu, S.-W. Wang, F. Chen, L. Yu, X. Chen, Tuning phase transition temperature of VO2 thin films by annealing atmosphere, Journal of Physics D: Applied Physics, 48 (2015) 265104. [27] X. He, Y. Zeng, X. Xu, C. Gu, F. Chen, B. Wu, C. Wang, H. Xing, X. Chen, J. Chu, Orbital change manipulation metal-insulator transition temperature in W-doped VO2, Physical chemistry chemical physics : PCCP, 17 (2015) 11638-11646. [28] Y. Wu, L. Fan, Q. Liu, S. Chen, W. Huang, F. Chen, G. Liao, C. Zou, Z. Wu, Decoupling the Lattice Distortion and Charge Doping Effects on the Phase Transition Behavior of VO2 by Titanium (Ti(4+)) Doping, Scientific reports, 5 (2015) 9328. [29] B. Agnarsson, F. Magnus, T.K. Tryggvason, A.S. Ingason, K. Leosson, S. Olafsson, J.T. Gudmundsson, Rutile TiO2 thin films grown by reactive high power impulse magnetron sputtering, Thin Solid Films, 545 (2013) 445-450. [30] M. Aiempanakit, U. Helmersson, A. Aijaz, P. Larsson, R. Magnusson, J. Jensen, T. Kubart, Effect of peak power in reactive high power impulse magnetron sputtering of titanium dioxide, Surface and Coatings Technology, 205 (2011) 4828-4831. [31] K. Sarakinos, J. Alami, S. Konstantinidis, High power pulsed magnetron sputtering: A review on scientific and engineering state of the art, Surface and Coatings Technology, 204 (2010) 1661-1684. [32] Y. Muraoka, Z. Hiroi, Metal–insulator transition of VO2 thin films grown on TiO2 (001) and (110) substrates, Applied Physics Letters, 80 (2002) 583. 18
[33] R. Chen, L. Miao, C. Liu, J. Zhou, H. Cheng, T. Asaka, Y. Iwamoto, S. Tanemura, Shape-controlled synthesis and influence of W doping and oxygen nonstoichiometry on the phase transition of VO2, Scientific reports, 5 (2015) 14087. [34] N.H. Azhan, K. Su, K. Okimura, J. Sakai, Radio frequency substrate biasing effects on the insulator-metal transition behavior of reactively sputtered VO2 films on sapphire (001), Journal of Applied Physics, 117 (2015) 185307. [35] Y. Cui, S. Shi, L. Chen, H. Luo, Y. Gao, Hydrogen-doping induced reduction in the phase transition temperature of VO2: a first-principles study, Physical chemistry chemical physics : PCCP, 17 (2015) 20998-21004.
Figure captions: Figure 1 Temperature dependences of resistance: (a) samples 1–3, (b) samples 4–6; Transmittance spectra at various temperatures: (c) samples 1–3, (d) samples 4–6
Figure 2 SEM images of as-deposited VO2 thin films
Figure 3 (a) XRD patterns of samples 1–3, (b) and (c) Magnified 2 differences between the practical diffraction angle of the as-deposited film and that of the standard R-VO2
Figure 4 (a) XRD patterns of samples 4–6, (b) and (c) Magnified 2 differences between the practical diffraction angle of the as-deposited film and that of the 19
standard R-VO2
Figure 5 (a) XRD patterns of sample 1 during heating and cooling processes, (b) and (c) Magnified 2 differences between the practical diffraction angle of the as-deposited film and that of the standard R-VO2 during the heating process, (d) and (e) Magnified 2 differences between the practical diffraction angle of the as-deposited film and that of the standard R-VO2 during the cooling process, (f) The resistance change of sample S1 during the heating and cooling process.
Figures :
Fig. 1
20
Fig. 2
21
Fig. 3
22
Fig. 4
23
24
Fig. 5
25
S0169-4332(16)30747-4 http://dx.doi.org/doi:10.1016/j.apsusc.2016.04.007 APSUSC 33008
To appear in:
APSUSC
Received date: Revised date: Accepted date:
21-12-2015 2-4-2016 4-4-2016
Please cite this article as: Tiegui Lin, Langping Wang, Xiaofeng Wang, Yufen Zhang, Yonghao Yu, Influence of lattice distortion on phase transition properties of polycrystalline VO2 thin film, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.04.007 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.
Influence of lattice distortion on phase transition properties of polycrystalline VO2 thin film Tiegui Lina, Langping Wanga,*, Xiaofeng Wanga, Yufen Zhanga, Yonghao Yub,* a
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, P.R. China
b
Academy of Fundamental and Interdisciplinary Science, Harbin Institute of Technology, Harbin 150001
* Corresponding author Professor Langping Wang E-mail addresses: [email protected]. Tel: +86-451-86418728 Fax: +86-451-86416186 Dr. Yonghao Yu E-mail addresses: [email protected]
Submitted to: APPLIED SURFACE SCIENCE (With 5 figures)
1. Polycrystalline VO2 thin films were fabricated by high power impulse magnetron sputtering.
2. The reported lowest phase transition temperature for undoped polycrystalline VO2 thin film was reduced to 32 °C by this research.
3. XRD patterns at varied temperatures revealed that the main structual change was a gradual shift in interplanar spacing with temperature. 1
ABSTRACT In this work, high power impulse magnetron sputtering was used to control the lattice distortion in polycrystalline VO2 thin film. SEM images revealed that all the VO2 thin films had crystallite sizes of below 20 nm, and similar configurations. UV-visible-near IR transmittance spectra measured at different temperatures showed that most of the as-deposited films had a typical metal–insulator transition. Four-point probe resistivity results showed that the transition temperature of the films varied from 54.5 to 32 °C. The X-ray diffraction (XRD) patterns of the as-deposited films revealed that most were polycrystalline monoclinic VO2. The XRD results also confirmed that the lattice distortions in the as-deposited films were different, and the transition temperature decreased with the difference between the interplanar spacing of the as-deposited thin film and standard rutile VO2. Furthermore, a room temperature rutile VO2 thin film was successfully synthesized when this difference was small enough. Additionally, XRD patterns measured at varied temperatures revealed that the phase transition process of the polycrystalline VO2 thin film was a coordinative deformation between grains with different orientations. The main 2
structural change during the phase transition was a gradual shift in interplanar spacing with temperature. Keywords: sputtering; thin film; lattice distortion; phase transition; polycrystal
1. INTRODUCTION Vanadium dioxide (VO2) is a material that undergoes a first-order metal–insulator transition (MIT) at around 68 °C, coupled with a structural transition from a low temperature monoclinic (M-VO2) to a high-temperature rutile (R-VO2) structure [1]. This metal–insulator transition can occur at ultrafast timescales (100 fs) [2]. In pure bulk single crystals, the transition leads to a change in resistivity of up to five orders of magnitude and a strong modification of the optical transmission [3]. These unique characteristics can be exploited in applications such as optical switching, thermochromic smart windows, laser protection, and energy harvesting systems [4-6]. VO2 thin films have been synthesized by a variety of deposition techniques, such as pulsed laser deposition (PLD) [5, 7-10], molecular beam epitaxy (MBE) [11], reactive sputtering [12-15], sol-gel processing [16], thermal oxidation [17], and reactive electron beam evaporation [18]. Previous studies have shown that the change in resistivity of epitaxial VO2 thin film can reach about four orders of magnitude, the transmittance change at 2500 nm can exceed 60%, and the transition temperature can be reduced to 20 °C [2, 19]. However, the more practical polycrystalline thin film only shows a resistivity change of one to two orders of magnitude [20-23], and its transition temperature is higher than 42 °C [4, 21, 24, 25]. 3
Reduction of the phase transition temperature will benefit the applications of polycrystalline VO2 thin film, and thus has been of wide interest to researchers in this area [11, 24, 26]. A number of studies have found that the transition temperature of polycrystalline VO2 thin film can be changed by altering the preparation parameters. Jiang et al. [25] fabricated polycrystalline VO2 thin films with different transition temperature by changing the oxygen partial pressure of the synthesis, but did not explain the reason for the reduction in transition temperature. Ba et al. [21] found that the greater the oxygen flow, the lower the transition temperature. They believed that the reason for this was an increase in the grain size of the films. However, Miller et al. [24] reported that a lower transition temperature was obtained at smaller grain size. Consequently, it can be deduced that although the transition temperature of the polycrystalline VO2 thin film can be reduced, the origin of the reduction has been unclear until the present. The phase transition of polycrystalline VO2 thin film is a lattice distortion process [10]. Previous studies have proven that the phase transition temperature can be reduced by lattice distortion at room temperature [27, 28]. Therefore, a prior lattice distortion that brings the lattice parameters of monoclinic VO2 close to those of the rutile structure should help reduce the phase transition temperature. High power impulse magnetron sputtering (HiPIMS) can generate a dense plasma and intense ion bombardment during the deposition process [14, 29], in which the ion bombardment can induce distortion of the crystalline lattice [30]. As a consequence, VO2 thin films with different lattice distortions may be obtained using a HiPIMS approach. 4
In this study, VO2 thin films were prepared by HiPIMS under different deposition conditions. The lattice distortions in the different films were characterized, and the influences of the lattice distortion on the phase transition properties of the films are discussed.
2. EXPERIMENTAL VO2 films were synthesized in a high power impulse magnetron sputtering apparatus. A turbo-molecular pump based vacuum system was used to achieve a base vacuum of less than 1 × 10−3 Pa. A vanadium target with a diameter of 100 mm and purity of 99.95% was used for the deposition process. The target-substrate distance was approximately 100 mm. The quartz glass substrates (30 × 30 × 1 mm3) were heated by a conduction heater from the back side to a surface temperature of 420 °C. The argon flow rate was set to 100 sccm, and the oxygen flow rate was varied from 5 to 6.7 sccm. The total pressure was maintained at about 0.85 Pa during the deposition process. A modulated pulsed power supply was used for the HiPIMS process. The pulse repetition frequency and duration time were 50 Hz and 400 s, respectively, and the peak power during the sputtering process was about 40 kW. Table 1 lists the main processing conditions for the different samples. Table 1 Main processing conditions for different samples. Sample
Ar flow rate
O2 flow rate
Deposition time
Thickness
No.
(sccm)
(sccm)
(min)
(nm)
1
100
6.7
30
133
2
100
5.2
30
120
3
100
5
30
118
5
4
100
6.5
15
77
5
100
6
15
65
6
100
5.5
15
86
The surface morphology of the obtained films was examined using a scanning electron microscope (SEM; Quanta 200, FEI Co., USA). The phase composition of the films was identified by X-ray diffraction at a grazing angle of 1 (XRD; Empyrean, Panalytical, Holland) with a Cu-Kα radiation source operated at 40 kV and 40 mA. The optical properties of the film between 200 and 2500 nm were measured using a UV-visible-near IR spectrophotometer (Lambda 950, PerkinElmer Co., USA). The electrical resistance of the films and its variation with temperature were measured with a four-point probe resistivity measurement system.
3. RESULTS AND DISCUSSION 3.1 Electrical and optical properties Figure 1 shows the temperature dependences of the resistance and the transmittance spectra of the as-deposited VO2 thin films. It is clear from the results that the phase transition temperature of the films was successfully varied. The transmittance of the VO2 films decreased obviously when the temperature was increased from 24 to 75 °C, which proves that a metal–insulator transition occurred during the heating process. However, sample 6 possessed very a low resistance and transmittance at room temperature, close to those of R-VO2. To compare the transition characteristics of different samples, the relative resistance change, transition temperature, sharpness of the transition, hysteresis width, 6
and absolute transmittance change of the different VO2 thin films were defined according to previous studies [2]. The relative resistance change (R) is defined as
R=R 0⁄R , 1
(1)
where R0 is the maximum resistance before the phase transition, and R1 is the minimum resistance after the phase transition. The transition temperature (TMIT ) is given by
TMIT = (Tc, cooling+ Tc, heating )/2, where Tc,
cooling
and Tc,
heating
(2)
are the transition temperatures at the center of the
derivative curve (d[log(R)]/dT) during cooling and heating, respectively. The sharpness of the transition (T) is defined as the full width half maximum (FWHM) of the derivative curve during cooling. The hysteresis width (H) is written as
H= Tc, heating − Tc, cooling.
(3)
The absolute transmittance change (Tr) is given by
Tr = Tr0 − Tr1,
(4)
where Tr0 is the maximum transmittance at 2500 nm before the phase transition, and Tr1 is the minimum transmittance at 2500 nm after the phase transition. Table 2 shows the electrical and optical properties of the different VO2 thin films. Note that the transition temperature could be reduced to 32 °C (sample 3), which is close to the lowest reported transition temperature of epitaxial VO2 film on TiO2(001) substrate [19]. For sample 4, the resistance change, hysteresis width, sharpness of the transition, and absolute transmittance change were 4500, 6 °C, 5.8 °C and 69.8 %, 7
respectively, which indicate that the electrical and optical properties of this VO2 film were close to the best properties reported for epitaxial VO2 film [2].
Table 2 Electrical and optical properties of the different VO2 thin films
Sample No
TMIT (°C)
T
H
(°C)
(°C)
1 2 3 4 5
52.5 45.5 32 54.5 48.3
6.5 14 18.5 5.8 13.5
9 3 2 6 4.2
R
Tr
2.3×103 3.2×102 28 4.5×103 1.2×102
42.5% 23.1% 16.1% 69.8% 29%
3.2 Microstructural characteristics The SEM images of the VO2 thin films are shown in Figure 2. All the films exhibited a crystallite size of below 20 nm, and their configurations were similar. Therefore, the influence of crystallite morphology on the phase transition properties of the films can be neglected in this work. The XRD patterns of samples 1–3 are shown in Figure 3(a). The obvious (011), (211), (022), and (202) peaks in the patterns confirmed that the as-deposited VO2 thin films were polycrystalline. According to the standard diffraction card of M-VO2 (JCPDS 44-0252), the 2θ values of the (011), (211), (022), and (202) plane reflections should be 27.86, 55.54, 57.50, and 70.44, respectively. However, as shown in 8
Figure 3(a), the positions of the same diffraction planes in the as-deposited VO2 thin films were obviously different. For example, the (011) peak of samples 1–3 appeared at 27.877, 27.828, and 27.771, respectively, which indicates that different lattice distortions were formed in the as-deposited films. A previous study has shown that the variation of oxygen pressure during deposition can affect the stoichiometric ratio of V to O, which can produce oxygen vacancies in the as-deposited VO2 thin films [2]. Additionally, the ion bombardment would have induced large stress in the as-deposited film [31]. As a consequence, the lattice distortion in the as-deposited VO2 thin films should originate from the ion bombardment as well as the oxygen vacancies. To determine the influence of the lattice distortion on the phase transition temperature, we compared the interplanar spacing of the films before and after the phase transition process. Because the intensity of the (011) peak was the greatest, only the interplanar spacing of (011) plane is discussed here. For sample 3, the interplanar spacing was 0.32104 nm, which is 99.7% of that of R-VO2 (JCPDS 44-0253). The spacings of samples 2 and 1 were 0.32043 and 0.31991 nm, respectively, 99.5% and 99.3% that of R-VO2. The measured phase transition temperatures of samples 3, 2, and 1 were 32, 45.5, and 52.5 °C, respectively. These results reveal that the transition temperature of the polycrystalline VO2 film can be greatly reduced by a sufficient lattice distortion. This deduction was confirmed by the results obtained for an epitaxial VO2 thin film [32]. In this study, the interplanar spacing of the (002) plane of epitaxial VO2 thin film on TiO2(001) was measured to be 0.14235 nm, 99.71% of 9
that of R-VO2, and the corresponding phase transition temperature was only 27 °C. The other diffraction peaks of the different samples, such as the (012), (211), (022), and (202) peaks, also differed. To compare the lattice distortion in different planes, the respective diffraction angles of standard R-VO2 (JCPDS 44-0253) and the as-deposited VO2 thin films have been marked with red and blue lines in the XRD patterns shown in Fig. 3(a). It is obvious that the deviations between the red and blue varied across the different samples. Because the magnitudes of the 2 deviations are not clear in Fig. 3(a), magnifications of the XRD patterns are shown in Fig. 3(b) and 3(c). For (012), (211), (022) and (202), a small 2 difference was beneficial to reducing the phase transition temperature, similar to the results obtained for the (011) plane. Additionally, because the diffraction angles of the (200) and (210) planes of undistorted M-VO2 are 37.00 and 42.24, respectively, which are very close to those of R-VO2 (37.12 and 42.28), the shift of the corresponding peaks during the phase transition process should be negligible. Therefore, the deviations in the interplanar spacings of the (200) and (210) planes should be as small as possible, which was confirmed by the results shown in Fig. 3(b). Consequently, the phase transition of a polycrystalline VO2 film should be easier and the transition temperature should be lower if the change in the interplanar spacing during the phase transition process is smaller. This principle is consistent with the results of Chen et al. [33], who reported that a shift in the diffraction angle of the (011) plane of M-VO2 nanorods to that of R-VO2 decreased the phase transition temperature 10
to about 47 °C during the cooling process. Conversely, the larger the difference in interplanar spacing, the higher the phase transition temperature. This result is also in accordance with that obtained by Azhan et al. [34], who reported that deviation of the diffraction angle for the (020) plane of an epitaxial VO2 thin film away from that of R-VO2 increased the phase transition temperature. To determine whether this principle still applies when the thickness of the film is changed, the XRD patterns and 2 differences of samples 4–6 are shown in Figure 4. The results obtained for samples 4 and 5 are in accordance with those for samples 1–3, and prove that the principle is correct for films with different thickness. The diffraction peaks of sample 6 coincided with those of R-VO2. Fig. 1 shows that the resistance and the transmittance of this thin film were extremely low. These results indicate that metallic-VO2 was synthesized in the case of sample 6, which confirms Y.Y. Cui et al.’s inference based on first principles calculations [35]. Figure 5(a) compares the XRD patterns of sample 1 during heating and cooling processes. The red lines mark the diffraction peaks of the standard diffraction card of R-VO2, while blue lines mark the experimentally obtained diffraction peaks at different temperatures during the heating and cooling processes. Magnifications of the deviations in 2 from the peaks of standard R-VO2 during the heating process are shown in Fig. 5(b) and (c), while those for the cooling process are exhibited in Fig. 5(d) and (e). During the heating process, the 2 deviations of the main diffraction peaks decreased gradually with increased temperature (Fig. 5(b) and (c)), which hints that 11
the interplanar spacing of the as-deposited VO2 thin film was shifted step by step towards that of standard R-VO2. In other words, the crystal structure of polycrystalline M-VO2 thin film gradually became more similar to that of R-VO2 during the heating process. This result is different from that expected according to the domain boundary movement principle [17]. When the temperature was 75 °C, the diffraction peaks of the as-deposited VO2 thin film coincided with those of standard R-VO2, revealing that the phase transition was completed. This was confirmed by the electrical and optical properties of the film measured at 75 °C. During the cooling process, the diffraction peaks gradually moved backward with decreasing temperature, which means that the crystal structure gradually transformed from R-VO2 to M-VO2. Therefore, the VO2 film was restored to its original high resistance and transmittance after cooling to room temperature. The diffraction angles measured at 50 °C were also found to be different during heating and cooling. The positions of the peaks were closer to those of R-VO2 during the cooling process than during the heating process, which hints that the crystal structure of the film was more similar to that of R-VO2 during the cooling process. As a result, the resistance of the film was lower during the cooling process than during the heating process (Fig. 5(f)). Consequently, the hysteresis of the transition process can be attributed to variation in the lattice distortion at the same temperature during the heating and cooling processes. Unlike single crystal VO2, polycrystalline thin films possesses deformation compatibility between differently orientated grains. Therefore, some grains having 12
specific orientations may possess irregular lattice distortions. During the heating process, the diffraction angle of the (022) plane increased and the interplanar spacing is decreased, which is opposite to that for the (011) plane. During the cooling process, the diffraction angle of the (012) plane decreased instead of increased. Based on these results, we can infer that the intermediate states discovered by Kumar, S et al. [23] were likely generated in the present polycrystalline VO2 thin films owing to deformation compatibility. Meanwhile, because the (200) and (210) planes of standard M-VO2 and R-VO2 have similar interplanar spacing, the interplanar spacing of these planes should not change obviously during the phase transition process. As shown in Fig. 5(b) and (d), the change in the diffraction angle of the (200) and (210) of the present films were negligible during the cooling and heating processes, which confirms this deduction. Consequently, the phase transition process of the polycrystalline VO2 thin film was found to be a coordinative deformation between grains with different orientations. The main structural change in the phase transition process was a gradual shift in interplanar spacing with temperature. For planes with similar diffraction angles to those of standard M-VO2, and R-VO2, the interplanar spacing did not change obviously. Additionally, some planes were inversely deformed as a result of coordinative deformation. Based on the above results, it was found that the change in the transmittance and resistance of the as-deposited VO2 thin film deteriorated with decreasing TMIT. Because a low transition temperature and good resistance change can be achieved 13
with epitaxial VO2 thin film [32], and because the phase transition of the polycrystalline of the VO2 thin film involves coordinative deformation of different grains, we believe that the best way to reduce TMIT with only a slight deterioration in the film properties to fabricate VO2 thin film with a high ordered crystalline orientation close to that of R-VO2.
4. CONCLUSIONS Polycrystalline VO2 thin films with different phase transition properties were prepared successfully by HiPIMS. The transition temperature of the films could be varied from 54.5 to 32 °C. The phase transition temperature was found to be controlled by the difference between the interplanar spacing of the as-deposited thin film and standard R-VO2. The smaller the difference in the interplanar spacing, the lower the transition temperature. Additionally, a room temperature R-VO2 thin film was synthesized when the interplanar spacing difference was small enough. The phase transition of the polycrystalline VO2 thin film was a coordinative deformation process between grains with different orientations. The main structural change occurring during the phase transition process was found to be a gradual shift in the interplanar spacing with temperature.
ACKNOWLEDGMENTS This work was supported by the fund of National Key Laboratory of High Power Microwave Technology (Contract No. 2014-763.xy.k). 14
REFERENCES [1] Y.-K. Dou, J.-B. Li, M.-S. Cao, D.-Z. Su, F. Rehman, J.-S. Zhang, H.-B. Jin, Oxidizing annealing effects on VO2 films with different microstructures, Applied Surface Science, 345 (2015) 232-237. [2] H. Kim, N. Charipar, M. Osofsky, S.B. Qadri, A. Piqué, Optimization of the semiconductor-metal transition in VO2 epitaxial thin films as a function of oxygen growth pressure, Applied Physics Letters, 104 (2014) 081913. [3] A.P. Peter, K. Martens, G. Rampelberg, M. Toeller, J.M. Ablett, J. Meersschaut, D. Cuypers, A. Franquet, C. Detavernier, J.-P. Rueff, M. Schaekers, S. Van Elshocht, M. Jurczak, C. Adelmann, I.P. Radu, Metal-Insulator Transition in ALD VO2Ultrathin Films and Nanoparticles: Morphological Control, Advanced Functional Materials, 25 (2015) 679-686. [4] Z.L. Huang, S.H. Chen, C.H. Lv, Y. Huang, J.J. Lai, Infrared characteristics of VO2 thin films for smart window and laser protection applications, Applied Physics Letters, 101 (2012). [5] S. Fan, L. Fan, Q. Li, J. Liu, B. Ye, The identification of defect structures for oxygen pressure dependent VO2 crystal films, Applied Surface Science, 321 (2014) 464-468. [6] Y. Ji, Y. Zhang, M. Gao, Z. Yuan, Y. Xia, C. Jin, B. Tao, C. Chen, Q. Jia, Y. Lin, Role of microstructures on the M1-M2 phase transition in epitaxial VO2 thin films, Scientific reports, 4 (2014) 4854. [7] M.R. Bayati, R. Molaei, F. Wu, J.D. Budai, Y. Liu, R.J. Narayan, J. Narayan, Correlation between structure and semiconductor-to-metal transition characteristics of VO2/TiO2/sapphire thin film heterostructures, Acta Materialia, 61 (2013) 7805-7815. [8] B.D. Ngom, M. Chaker, A. Diallo, I.G. Madiba, S. Khamlich, N. Manyala, O. Nemraoui, R. Madjoe, 15
A.C. Beye, M. Maaza, Competitive growth texture of pulsed laser deposited vanadium dioxide nanostructures on a glass substrate, Acta Materialia, 65 (2014) 32-41. [9] T.-H. Yang, S. Nori, S. Mal, J. Narayan, Control of room-temperature defect-mediated ferromagnetism in VO2 films, Acta Materialia, 59 (2011) 6362-6368. [10] V.R. Morrison, R.P. Chatelain, K.L. Tiwari, A. Hendaoui, A. Bruhacs, M. Chaker, B.J. Siwick, A photoinduced metal-like phase of monoclinic VO(2) revealed by ultrafast electron diffraction, Science, 346 (2014) 445-448. [11] F.H. Chen, L.L. Fan, S. Chen, G.M. Liao, Y.L. Chen, P. Wu, L. Song, C.W. Zou, Z.Y. Wu, Control of the metal-insulator transition in VO(2) epitaxial film by modifying carrier density, ACS applied materials & interfaces, 7 (2015) 6875-6881. [12] X. Li, A. Gloter, H. Gu, J. Luo, X. Cao, P. Jin, C. Colliex, Discovery of nanoscale reduced surfaces and interfaces in VO2 thin films as a unique case of prewetting, Scripta Materialia, 78-79 (2014) 41-44. [13] B. Viswanath, C. Ko, S. Ramanathan, Thermoelastic switching with controlled actuation in VO2 thin films, Scripta Materialia, 64 (2011) 490-493. [14] J.P. Fortier, B. Baloukas, O. Zabeida, J.E. Klemberg-Sapieha, L. Martinu, Thermochromic VO2 thin films deposited by HiPIMS, Solar Energy Materials and Solar Cells, 125 (2014) 291-296. [15] H. Zhang, Z. Wu, Q. He, Y. Jiang, Preparation and investigation of sputtered vanadium dioxide films with large phase-transition hysteresis loops, Applied Surface Science, 277 (2013) 218-222. [16] Y. Guo, H. Xu, C. Zou, Z. Yang, B. Tong, J. Yu, Y. Zhang, L. Zhao, Y. Wang, Evolution of structure and electrical properties with annealing time in solution-based VO2 thin films, Journal of Alloys and Compounds, 622 (2015) 913-917. 16
[17] X. He, T. Xu, X. Xu, Y. Zeng, J. Xu, L. Sun, C. Wang, H. Xing, B. Wu, A. Lu, D. Liu, X. Chen, J. Chu, In situ atom scale visualization of domain wall dynamics in VO2 insulator-metal phase transition, Scientific reports, 4 (2014) 6544. [18] J. Leroy, A. Bessaudou, F. Cosset, A. Crunteanu, Structural, electrical and optical properties of thermochromic VO2 thin films obtained by reactive electron beam evaporation, Thin Solid Films, 520 (2012) 4823-4825. [19] J. Jeong, N. Aetukuri, T. Graf, T.D. Schladt, M.G. Samant, S.S. Parkin, Suppression of metal-insulator transition in VO2 by electric field-induced oxygen vacancy formation, Science, 339 (2013) 1402-1405. [20] D. Brassard, S. Fourmaux, M. Jean-Jacques, J.C. Kieffer, M.A. El Khakani, Grain size effect on the semiconductor-metal phase transition characteristics of magnetron-sputtered VO2 thin films, Applied Physics Letters, 87 (2005) 051910. [21] C. Ba, S.T. Bah, M. D'Auteuil, P.V. Ashrit, R. Vallee, Fabrication of high-quality VO2 thin films by ion-assisted dual ac magnetron sputtering, ACS applied materials & interfaces, 5 (2013) 12520-12525. [22] S. Chen, J. Lai, J. Dai, H. Ma, H. Wang, X. Yi, Characterization of nanostructured VO2 thin films grown by magnetron controlled sputtering deposition and post annealing method, OPTICS EXPRESS 17 (2009) 24153-24161. [23] S. Kumar, J.P. Strachan, M.D. Pickett, A. Bratkovsky, Y. Nishi, R.S. Williams, Sequential electronic and structural transitions in VO2 observed using X-ray absorption spectromicroscopy, Advanced materials, 26 (2014) 7505-7509. [24] M.J. Miller, J. Wang, Influence of grain size on transition temperature of thermochromic VO2, 17
Journal of Applied Physics, 117 (2015) 034307. [25] M. Jiang, X. Cao, S. Bao, H. Zhou, P. Jin, Regulation of the phase transition temperature of VO2 thin films deposited by reactive magnetron sputtering without doping, Thin Solid Films, 562 (2014) 314-318. [26] X. Liu, S.-W. Wang, F. Chen, L. Yu, X. Chen, Tuning phase transition temperature of VO2 thin films by annealing atmosphere, Journal of Physics D: Applied Physics, 48 (2015) 265104. [27] X. He, Y. Zeng, X. Xu, C. Gu, F. Chen, B. Wu, C. Wang, H. Xing, X. Chen, J. Chu, Orbital change manipulation metal-insulator transition temperature in W-doped VO2, Physical chemistry chemical physics : PCCP, 17 (2015) 11638-11646. [28] Y. Wu, L. Fan, Q. Liu, S. Chen, W. Huang, F. Chen, G. Liao, C. Zou, Z. Wu, Decoupling the Lattice Distortion and Charge Doping Effects on the Phase Transition Behavior of VO2 by Titanium (Ti(4+)) Doping, Scientific reports, 5 (2015) 9328. [29] B. Agnarsson, F. Magnus, T.K. Tryggvason, A.S. Ingason, K. Leosson, S. Olafsson, J.T. Gudmundsson, Rutile TiO2 thin films grown by reactive high power impulse magnetron sputtering, Thin Solid Films, 545 (2013) 445-450. [30] M. Aiempanakit, U. Helmersson, A. Aijaz, P. Larsson, R. Magnusson, J. Jensen, T. Kubart, Effect of peak power in reactive high power impulse magnetron sputtering of titanium dioxide, Surface and Coatings Technology, 205 (2011) 4828-4831. [31] K. Sarakinos, J. Alami, S. Konstantinidis, High power pulsed magnetron sputtering: A review on scientific and engineering state of the art, Surface and Coatings Technology, 204 (2010) 1661-1684. [32] Y. Muraoka, Z. Hiroi, Metal–insulator transition of VO2 thin films grown on TiO2 (001) and (110) substrates, Applied Physics Letters, 80 (2002) 583. 18
[33] R. Chen, L. Miao, C. Liu, J. Zhou, H. Cheng, T. Asaka, Y. Iwamoto, S. Tanemura, Shape-controlled synthesis and influence of W doping and oxygen nonstoichiometry on the phase transition of VO2, Scientific reports, 5 (2015) 14087. [34] N.H. Azhan, K. Su, K. Okimura, J. Sakai, Radio frequency substrate biasing effects on the insulator-metal transition behavior of reactively sputtered VO2 films on sapphire (001), Journal of Applied Physics, 117 (2015) 185307. [35] Y. Cui, S. Shi, L. Chen, H. Luo, Y. Gao, Hydrogen-doping induced reduction in the phase transition temperature of VO2: a first-principles study, Physical chemistry chemical physics : PCCP, 17 (2015) 20998-21004.
Figure captions: Figure 1 Temperature dependences of resistance: (a) samples 1–3, (b) samples 4–6; Transmittance spectra at various temperatures: (c) samples 1–3, (d) samples 4–6
Figure 2 SEM images of as-deposited VO2 thin films
Figure 3 (a) XRD patterns of samples 1–3, (b) and (c) Magnified 2 differences between the practical diffraction angle of the as-deposited film and that of the standard R-VO2
Figure 4 (a) XRD patterns of samples 4–6, (b) and (c) Magnified 2 differences between the practical diffraction angle of the as-deposited film and that of the 19
standard R-VO2
Figure 5 (a) XRD patterns of sample 1 during heating and cooling processes, (b) and (c) Magnified 2 differences between the practical diffraction angle of the as-deposited film and that of the standard R-VO2 during the heating process, (d) and (e) Magnified 2 differences between the practical diffraction angle of the as-deposited film and that of the standard R-VO2 during the cooling process, (f) The resistance change of sample S1 during the heating and cooling process.
Figures :
Fig. 1
20
Fig. 2
21
Fig. 3
22
Fig. 4
23
24
Fig. 5
25