- Email: [email protected]
Influence of bias voltage on microstructure and phase transition properties of VO2 thin film synthesized by HiPIMS
Surface & Coatings Technology 305 (2016) 110–115
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Influence of bias voltage on microstructure and phase transition properties of VO2 thin film synthesized by HiPIMS Tiegui Lin a, Langping Wang a,⁎, Xiaofeng Wang a, Yufen Zhang a, Yonghao Yu b,⁎ a b
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, PR China Academy of Fundamental and Interdisciplinary Science, Harbin Institute of Technology, Harbin 150001, PR China
a r t i c l e
i n f o
Article history: Received 27 April 2016 Revised 26 July 2016 Accepted in revised form 7 August 2016 Available online 08 August 2016 Keywords: Thin films High power impulse magnetron sputtering Pulsed bias voltage Crystalline size Phase transition
a b s t r a c t VO2 thin films were prepared on quartz glass substrates by using high power impulse magnetron sputtering with different pulsed bias voltages. X-ray diffraction patterns revealed that all as-deposited films were polycrystalline monoclinic VO2 and the preferred crystalline orientation of the as-deposited films varied with the magnitude of the bias voltage. Scanning electron microscopy images exhibited that the crystalline size of the VO2 thin films reduced with the magnitude of the bias voltage. X-ray photoelectron spectroscopy results revealed that oxygen vacancies were formed in all films and their quantities were similar. UV–visible-near IR spectra of the VO2 thin films at different temperatures confirmed that all films possessed typical metal–insulator transition properties. Fourpoint probes resistivity results exhibited that the phase transition temperature was reduced from 54 to 31.5°C when the magnitude of the bias voltage was increased from −50 to −250 V. © 2016 Published by Elsevier B.V.
1. Introduction Vanadium dioxide (VO2) is an archetypal correlated oxide showing a first order reversible metal–insulator transition (MIT) from a low temperature monoclinic (M1, P21/c) to a high-temperature rutile structure (R, P42/mnm) at around 68 °C [1]. The MIT leads to a significant change in physical properties, such as the resistivity and the infrared transmittance [2,3]. Thereby VO2 is an extremely interesting material suitable for many technological applications [4]. VO2 thin films have been fabricated by a variety of deposition techniques, such as DC magnetron sputtering [5,6], RF reactive sputtering [7–9], pulsed laser deposition (PLD) [10], reactive electron beam evaporation [11] and chemical vapor deposition (CVD) [12,13]. In some special applications, such as smart windows, which can help control the room temperature intelligently, the MIT temperature of 68 ° C is too high [14,15]. In recent years, much work has been done with the purpose of decreasing the transition temperature to room temperature [14–17]. For epitaxial VO2 thin films, the MIT temperature has been reduced to room temperature on some specific substrates, such as TiO2 (001) [18]. However, the applications of epitaxial VO2 thin film are limited greatly because of these specific substrates. Since polycrystalline VO2 thin film can be fabricated on a glass substrate, it has shown great potential in such applications. Up to present, the reported MIT temperature of undoped polycrystalline VO2 thin film is higher than ⁎ Corresponding authors. E-mail addresses: [email protected] (L. Wang), [email protected] (Y. Yu).
http://dx.doi.org/10.1016/j.surfcoat.2016.08.020 0257-8972/© 2016 Published by Elsevier B.V.
42 °C [3,15,17,19,20]. Although doping can decrease the phase transition temperature effectively, the change in resistivity and transmittance of the doped VO2 thin film was reduced significantly, which limited the applications of the VO2 thin film [21,22]. Therefore, synthesis of undoped polycrystalline VO2 thin film with low phase transition temperature is still a great challenge. Previous studies proved that polycrystalline VO2 thin film with a smaller crystalline size possessed lower phase transition temperature [17,23,24]. Compared with conventional magnetron sputtering, high power impulse magnetron sputtering (HiPIMS) can obtain high ionized plasma [25–28]. The energy of the ions impinged toward the substrate during a HiPIMS process can be controlled by varying the substrate bias voltage, and the grain size of the as-deposited thin films can be reduced greatly by this ion bombardment [29]. As a consequence, using the HiPIMS method with a bias voltage, fine crystalline VO2 thin film may be obtained. In this study, VO2 thin films were prepared by using HiPIMS with a pulsed bias voltage on quartz glass substrates. In addition, microstructure and phase transition characteristics of the as-deposited thin 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
T. Lin et al. / Surface & Coatings Technology 305 (2016) 110–115
deposition process. The target-substrate distance was approximately 100 mm. Argon and oxygen with respective mass flow rates of 100 and 6.5 sccm were introduced into the chamber simultaneously, and the total pressure was maintained at about 0.85 Pa during the deposition process. The whole deposition time was 30 min. A modulated pulsed power supply was used for the HiPIMS process. The discharge voltage and current waveforms of the vanadium target during the HiPIMS deposition process are shown in Fig. 1(a). An auxiliary pulse with a small voltage was applied before the main HiPIMS pulse, where the duration time for auxiliary and main pulses was 100 and 400 μs, respectively. The pulse repetition frequency was 50 Hz, and the peak power during the sputtering process was about 40 kW. 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. Since the quartz glass substrate is not conductive, a negative bias voltage applied on the substrate during the deposition process cannot accelerate the ions to the substrate. In order to apply an effective bias voltage on the substrate, the deposition process of the VO2 film was carried out in two steps. Firstly, in order to make the substrate be conductive, a pre-deposited VO2 thin film was deposited on quartz glass substrate by using above HiPIMs method but without bias voltage for 8 min, and the thickness of the pre-deposited thin film was about 33 nm. Secondly, a pulsed bias voltage was applied on the pre-deposited VO2 thin film, and VO2 thin films were synthesized on the pre-deposited thin film by using the same HiPIMS method for 22 min. In order to study the influences of the bias voltage on microstructure and phase transition properties, the magnitude of the bias voltage in the second step was varied from − 50 to − 250 V for different samples, and the bias voltages captured during the HiPIMS process are shown in Fig. 1(b). The pulse width and delay time of the bias voltage relative to the main HiPIMS voltage pulse were 300 and 0 μs, respectively. The total thicknesses of all as-deposited thin films deposited with −50, −100, −150 and −250 V bias voltage were about 92, 97, 93 and 86 nm, respectively. Table 1 lists the main processing conditions for VO2 thin films. 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 0.5° (XRD; Empyrean, Panalytical, Holland) with a Cu-Kα radiation source operated at 40 kV and 40 mA. The compositions of the as-deposited thin film were detected by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, USA). 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.
111
Table 1 Main processing conditions for VO2 thin films. Ar flow rate (sccm) O2 flow rate (sccm) Working pressure (pa) Substrate temperature (°C) Pulse frequency (Hz) Auxiliary pulse duration time (μs) Main pulse duration time (μs) Peak power (kW) Bias magnitude (V) Pre-deposition time (min) Whole deposition time (min)
100 6.5 0.85 420 50 100 400 40 −50, −100, −150, −250 8 30
3. Results and discussion The XRD pattern of the pre-deposited VO2 thin film is shown in Fig. 2(a), three weaker diffraction peaks of (001), (002) and (003) planes can be found. According to the standard diffraction card of VO2(B) (JCPDS 31-1438), the pre-deposited VO2 thin film should be metastable monoclinic phase VO2(B). The SEM image of the pre-deposited thin film is given in Fig. 2(b). It is noticeable that nano-crystalline thin film has been obtained, and its crystalline size is about 10 nm. Fig. 2(c) shows the temperature dependence of the resistance of the pre-deposited thin film. Different from VO2(M) thin film, the resistance-temperature curve of this VO2(B) thin film is basically linear and the temperature coefficient of the resistance is about 7%/K, which is in accordance with that obtained by Chen et al. [30]. The resistance-temperature curves obtained during the heating and cooling processes are almost identical, which hints that a hysteresis of the phase transition doesn't exist. Additionally, note that the pre-deposited VO2(B) thin film was conductive, thus the pulsed bias voltage was effectively applied for the subsequent HiPIMS process. XRD patterns of as-deposited VO2 thin films are exhibited in Fig. 3. The (011), (200), (20-2), (21-3), (211) and (202) peaks in the patterns confirmed that the as-deposited VO2 thin films were polycrystalline VO2(M) (JCPDS 44-0252). Obviously, the diffraction peaks of pre-deposited metastable VO2(B) disappeared in these thin films. Because metastable B-VO2 can be transformed to VO2(M) by an annealing process [30], and a heating to 420 °C for 22 min was conducted during the subsequent deposition process, pre-deposited VO2(B) was transformed to VO2(M) and the diffraction peaks of VO2(B) vanished in these XRD patterns. The sample fabricated with a bias magnitude of −50 V possessed the largest quantity of diffraction peaks, such as (011), (200), (210), (211) and (202) ones. When the bias magnitude was increased to − 100 and − 150 V, only (011), (211) and (202) peaks were obvious in the XRD patterns. Meanwhile, the intensity of (011) and (202) peaks decreased with the bias magnitude. When the bias magnitude
Fig. 1. (a) Discharge current and voltage and (b) bias voltage waveforms captured during the HiPIMS deposition process.
112
T. Lin et al. / Surface & Coatings Technology 305 (2016) 110–115
Fig. 2. (a) XRD pattern of pre-deposited VO2 thin film, (b) SEM image of the pre-deposited VO2 thin film, (c) Temperature dependence of resistance of the pre-deposited VO2 thin film.
was further increased to −250 V, (011) and (202) peaks disappeared, and the main diffraction peaks transformed to (20-2) and (21-3) ones, which should be a result of the preferential sputtering effect caused by the high energy ion impingement [26,29,31]. In addition, when the magnitude of the bias voltage was larger than −100 V, the crystallinity of the as-deposited thin films decreased with the bias magnitude, which should be derived from the lattice defects caused by the high energy ion bombardment. Because we applied an effective bias voltage on the substrate, the above results were different from other sputtering methods
Fig. 3. XRD patterns of as-deposited VO2 thin films fabricated with different bias voltages.
[20,32], in which the crystalline orientation and crystallinity did not change obviously. The average grain size of the as-deposited VO2 thin films can be estimated through applying the full-width at half maximum (FWHM) of the main XRD peak to the Scherrer equation [7]. The calculated average grain sizes of the VO2 thin films deposited with bias magnitudes of −50, − 100, − 150, and − 250 V are 22.68, 18.08, 15.52, and 11.89 nm, respectively. Obviously, the calculated average grain size reduces with the magnitude of the bias voltage. The SEM images of the as-deposited VO2 thin films are given in Fig. 4. It is obvious that all as-deposited films possess nano-crystalline structure. In addition, the relationship between the grain size and the magnitude of the bias voltage obtained in this figure is in consistent with the result calculated by the Scherrer equation. Particularly, for thin film fabricated with −250 V bias voltage, the crystalline size of polycrystalline VO2 thin film is smaller than other methods [10,17,19,33]. Therefore, a large magnitude of the bias voltage benefits the reduction of the crystalline size of the VO2 thin film. In order to study the chemical states of vanadium, XPS spectra of vanadium for different samples are shown in Fig. 5(a)–(d), where the spectra were calibrated by the C(1s) peak (284.6 eV). All V(2p) peaks were resolved into three main components at 515.2–515.7, 515.7– 516.2 and 516.9–517.2 eV, corresponding to V3+, V4+ and V5+, respectively [34]. According to the XPS spectra, the relative concentration of V3+, V4+ and V5+ in the as-deposited VO2 thin films were calculated and listed in Table 2. It can be found that the contents of V5+ are similar in all samples, and other researchers have pointed out that the existence of V5+ can be attributed to the surface oxidization during storage in air [34]. The percentages of V3+ were 25.6%, 23.1%, 25.2%, and 21.1% for thin films deposited in −50, −100, −150, and −250 V bias voltages, and the corresponding ratio of oxygen to vanadium were 1.75, 1.8,
T. Lin et al. / Surface & Coatings Technology 305 (2016) 110–115
113
Fig. 4. SEM images of as-deposited VO2 thin films fabricated with different bias magnitudes: (a) −50 V, (b) −100 V, (c) −150 V, (d) −250 V.
1.78, and 1.82, respectively. Because the XRD results have revealed that the crystal structure of the as-deposited thin films were polycrystalline
VO2(M), in order to equilibrium the charge of V3+, some oxygen vacancies should be formed in the as-deposited films [34–36]. In addition,
Fig. 5. XPS profiles of V(2p) for as-deposited VO2 thin films fabricated with different bias magnitudes: (a) −50 V, (b) −100 V, (c) −150 V, (d) −250 V.
114
T. Lin et al. / Surface & Coatings Technology 305 (2016) 110–115
Table 2 Content of V with different valence states.
Table 3 Electrical properties of different bias voltages.
Bias magnitude (V)
V3+
V4+
V5+
Bias magnitude (V)
TMIT (°C)
ΔH (°C)
ΔR
−50 −100 −150 −250
25.6% 23.1% 25.2% 21.1%
40.2% 41.8% 38.6% 46.7%
34.2% 35.1% 36.2% 32.2%
−50 −100 −150 −250
54 51 43.5 31.5
8 5 2 0.2
6 × 102 3.5 × 103 3 × 102 9.5
since the ratios of oxygen to vanadium are similar for all thin films, the influence of the bias magnitude on stoichiometry can be neglected in this study. Fig. 6 shows the temperature dependences of the resistance and the transmittance spectra at different temperatures of the as-deposited VO2 thin films. Note that the phase transition properties of the films were 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 has been occurred during the heating process. In addition, a shifting of the optical transmission edge toward higher energies for VO2 thin film deposited at −250 V was obvious, which should be derived from the poor crystallinity and unique orientations in this film [34,37]. To compare the transition characteristics of different samples, the relative resistance change, the transition temperature and the hysteresis width of the different VO2 thin films were defined according to previous studies [38]. The relative resistance change (ΔR) is defined as ΔR ¼ R0 =R1 ;
ð1Þ
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 T MIT ¼ T c;cooling þ T c;heating =2
ð2Þ
where Tc,cooling and Tc,heating are the transition temperatures at the center of the derivative curve (d[log(R)]/dT) during cooling and heating, respectively. The hysteresis width (ΔH) is written as ΔH ¼ T c;heating −T c;cooling
ð3Þ
Table 3 exhibits electrical properties of different VO2 thin films. It can be found that the phase transition temperature and the hysteresis width decrease significantly with the magnitude of the bias voltage. For thin film fabricated with −100 V bias voltage, the change in resistance and hysteresis width are 3500 times and 5 °C, respectively, which indicates
that the electrical properties of this VO2 film are close to the reported best one of epitaxial VO2 films [38]. The phase transition temperature of the polycrystalline VO2 thin film fabricated at a bias magnitude of −250 V can be reduced to 31.5 °C, which is far below the reported lowest phase transition temperature (42 °C) of undoped polycrystalline VO2 films. Simultaneously, the hysteresis width is almost completely suppressed to 0.2 °C, which is similar to that of doped epitaxial VO2 thin film [39]. In this study, a large variation of the MIT temperature has been obtained. However, the mechanism for the reduction of the phase transition temperature is not clear yet. According to our previous study [40], the MIT temperature of VO2 thin film can be influenced by a lattice distortion, and the transition temperature decreased with the difference between the interplanar spacing of the as-deposited thin film and standard rutile VO2. To compare the lattice distortion in different planes before and after the phase transition process, interplanar spacings of standard VO2(R) (JCPDS 44-0253) and the as-deposited VO2 thin films have been compared. Because the crystalline orientations of − 250 V thin film changed significantly, we only discussed the lattice distortions of thin films deposited at −50, −100 and −150 V. Additionally, since the intensity of the (011) peak was the greatest in these samples, only the interplanar spacing of (011) plane was discussed here. The spacings of thin films deposited at − 50, − 100 and − 150 V were 0.31863, 0.31898 and 0.31889 nm, respectively, 98.95%, 99.06%, and 99.03% that of VO2(R). According to our previous results, the lattice distortions in these samples can be considered as similar, thus the influence of the lattice distortion on the phase transition temperature can be neglected. In addition, because the ratios of O to V for different samples were similar, the variation of transition temperature with the bias voltage should not be correlated to film stoichiometry (oxygen to vanadium). In this study, the obvious influence of the bias magnitude is the reduction of the crystalline size of the as-deposited thin films, and other studies have proved that a fine crystalline size benefits the reduction of phase transition temperature [17,23,24]. Consequently, the main reason for the reduction of phase transition temperature caused by a large bias magnitude should be attributed to the decreased size of VO 2 nano-crystalline grains in the as-deposited thin films.
Fig. 6. (a) Temperature dependence of resistance of VO2 thin films, (b) Transmittance spectra of VO2 films at various temperatures during heating.
T. Lin et al. / Surface & Coatings Technology 305 (2016) 110–115
4. Conclusions Polycrystalline VO2 thin films were prepared on the quartz glass substrates using HiPIMS with different bias voltages. The preferred crystalline orientations of the as-deposited films varied with the magnitude of the bias voltage. Nano-crystalline structure was obtained in all VO2 films, and the crystalline size reduced with the magnitude of the bias voltage. Oxygen vacancies were formed in all films, and the ratio of O to V kept stable under different bias voltages. All films showed typical metal–insulator transition properties, and the phase transition temperature was reduced from 54 to 31.5 °C when the magnitude of the pulsed bias voltage was increased from −50 to −250 V. Acknowledgments This work was supported by the fund of Shanghai Association for Science and Technology (SAST2015059). References [1] M. Nakano, D. Okuyama, K. Shibuya, M. Mizumaki, H. Ohsumi, M. Yoshida, M. Takata, M. Kawasaki, Y. Tokura, T. Arima, Y. Iwasa, Distinct substrate effect on the reversibility of the metal-insulator transitions in electrolyte-gated VO2 thin films, Adv. Electron. Mater. 1 (2015), 1500093. [2] L.H. Yeo, A. Srivastava, M.A. Majidi, R. Sutarto, F. He, S.M. Poh, C. Diao, X. Yu, M. Motapothula, S. Saha, S. Ojha, D. Kanjilal, P.E. Trevisanutto, M.B.H. Breese, T. Venkatesan, A. Rusydi, Anomalous spectral-weight transfers unraveling oxygen screening and electronic correlations in the insulator-metal transition of VO2, Phys. Rev. B 91 (2015), 081112. [3] 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, Appl. Phys. Lett. 101 (2012), 191905. [4] 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 (Ti4+) doping, Sci. Rep. 5 (2015) 9328. [5] S.-Y. Li, K. Namura, M. Suzuki, G.A. Niklasson, C.G. Granqvist, Thermochromic VO2 nanorods made by sputter deposition: growth conditions and optical modeling, J. Appl. Phys. 114 (2013), 033516. [6] N.R. Mlyuka, G.A. Niklasson, C.G. Granqvist, Thermochromic multilayer films of VO2 and TiO2 with enhanced transmittance, Sol. Energy Mater. Sol. Cells 93 (2009) 1685–1687. [7] 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 magnetronsputtered VO2 thin films, Appl. Phys. Lett. 87 (2005), 051910. [8] Y.Y. Luo, L.Q. Zhu, Y.X. Zhang, S.S. Pan, S.C. Xu, M. Liu, G.H. Li, Optimization of microstructure and optical properties of VO2 thin film prepared by reactive sputtering, J. Appl. Phys. 113 (2013), 183520. [9] M. Jiang, Y. Li, S. Li, H. Zhou, X. Cao, S. Bao, Y. Gao, H. Luo, P. Jin, Room temperature optical constants and band gap evolution of phase pure M1-VO2 thin films deposited at different oxygen partial pressures by reactive magnetron sputtering, J. Nanomater. 2014 (2014) 1–6. [10] B.D. Ngom, M. Chaker, A. Diallo, I.G. Madiba, S. Khamlich, N. Manyala, O. Nemraoui, R. Madjoe, A.C. Beye, M. Maaza, Competitive growth texture of pulsed laser deposited vanadium dioxide nanostructures on a glass substrate, Acta Mater. 65 (2014) 32–41. [11] 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. [12] D. Vernardou, D. Louloudakis, E. Spanakis, N. Katsarakis, E. Koudoumas, Thermochromic amorphous VO2 coatings grown by APCVD using a single-precursor, Sol. Energy Mater. Sol. Cells 128 (2014) 36–40. [13] L. Kritikos, L. Zambelis, G. Papadimitropoulos, D. Davazoglou, Structure and electrical properties of selectively chemically vapor deposited vanadium oxide films from vanadium tri-i-propoxy oxide vapors, Surf. Coat. Technol. 201 (2007) 9334–9339. [14] 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), J. Appl. Phys. 117 (2015), 185307. [15] X. Liu, S.-W. Wang, F. Chen, L. Yu, X. Chen, Tuning phase transition temperature of VO2 thin films by annealing atmosphere, J. Phys. D. Appl. Phys. 48 (2015), 265104.
115
[16] 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 VO2 epitaxial film by modifying carrier density, ACS Appl. Mater. Interfaces 7 (2015) 6875–6881. [17] M.J. Miller, J. Wang, Influence of grain size on transition temperature of thermochromic VO2, J. Appl. Phys. 117 (2015), 034307. [18] 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. [19] 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 Appl. Mater. Interfaces 5 (2013) 12520–12525. [20] 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. [21] X. Wu, Z. Wu, H. Zhang, R. Niu, Q. He, C. Ji, J. Wang, Y. Jiang, Enhancement of VO2 thermochromic properties by Si doping, Surf. Coat. Technol. 276 (2015) 248–253. [22] N.R. Mlyuka, G.A. Niklasson, C.G. Granqvist, Mg doping of thermochromic VO2 films enhances the optical transmittance and decreases the metal-insulator transition temperature, Appl. Phys. Lett. 95 (2009), 171909. [23] J. Jian, W. Zhang, C. Jacob, A. Chen, H. Wang, J. Huang, H. Wang, Roles of grain boundaries on the semiconductor to metal phase transition of VO2 thin films, Appl. Phys. Lett. 107 (2015), 102105. [24] E. Radue, E. Crisman, L. Wang, S. Kittiwatanakul, J. Lu, S.A. Wolf, R. Wincheski, R.A. Lukaszew, I. Novikova, Effect of a substrate-induced microstructure on the optical properties of the insulator-metal transition temperature in VO2 thin films, J. Appl. Phys. 113 (2013), 233104. [25] 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. [26] K. Sarakinos, J. Alami, S. Konstantinidis, High power pulsed magnetron sputtering: a review on scientific and engineering state of the art, Surf. Coat. Technol. 204 (2010) 1661–1684. [27] F.J. Jing, K. Yukimura, H. Kato, Y.F. Lei, T.X. You, Y.X. Leng, N. Huang, Film characterization of titanium oxide films prepared by high-power impulse magnetron sputtering, Surf. Coat. Technol. 206 (2011) 967–971. [28] J.P. Fortier, B. Baloukas, O. Zabeida, J.E. Klemberg-Sapieha, L. Martinu, Thermochromic VO2 thin films deposited by HiPIMS, Sol. Energy Mater. Sol. Cells 125 (2014) 291–296. [29] Z. Wang, D. Zhang, P. Ke, X. Liu, A. Wang, Influence of substrate negative bias on structure and properties of TiN coatings prepared by hybrid HIPIMS method, J. Mater. Sci. Technol. 31 (2015) 37–42. [30] 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, Opt. Express 17 (2009) 24153–24161. [31] V. Stranak, A.-P. Herrendorf, H. Wulff, S. Drache, M. Cada, Z. Hubicka, M. Tichy, R. Hippler, Deposition of rutile (TiO2) with preferred orientation by assisted high power impulse magnetron sputtering, Surf. Coat. Technol. 222 (2013) 112–117. [32] 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, J. Alloys Compd. 664 (2016) 626–631. [33] Y.-X. Ji, G.A. Niklasson, C.G. Granqvist, M. Boman, Thermochromic VO2 films by thermal oxidation of vanadium in SO2, Sol. Energy Mater. Sol. Cells 144 (2016) 713–716. [34] Z. Zhang, Y. Gao, Z. Chen, J. Du, C. Cao, L. Kang, H. Luo, Thermochromic VO2 thin films: solution-based processing, improved optical properties, and lowered phase transformation temperature, Langmuir 26 (2010) 10738–10744. [35] T.-H. Yang, S. Nori, S. Mal, J. Narayan, Control of room-temperature defect-mediated ferromagnetism in VO2 films, Acta Mater. 59 (2011) 6362–6368. [36] 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, Appl. Surf. Sci. 345 (2015) 232–237. [37] G. Sun, H. Zhou, X. Cao, R. Li, M. Tazawa, M. Okada, P. Jin, Self-assembled multilayer structure and enhanced thermochromic performance of spinodally decomposed TiO2-VO2 thin film, ACS Appl. Mater. Interfaces 8 (2016) 7054–7059. [38] 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, Appl. Phys. Lett. 104 (2014), 081913. [39] M. Soltani, M. Chaker, E. Haddad, R.V. Kruzelecky, J. Margot, Effects of Ti–W codoping on the optical and electrical switching of vanadium dioxide thin films grown by a reactive pulsed laser deposition, Appl. Phys. Lett. 85 (2004) 1958. [40] T. Lin, L. Wang, X. Wang, Y. Zhang, Y. Yu, Influence of lattice distortion on phase transition properties of polycrystalline VO2 thin film, Appl. Surf. Sci. 379 (2016) 179–185.
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Influence of bias voltage on microstructure and phase transition properties of VO2 thin film synthesized by HiPIMS Tiegui Lin a, Langping Wang a,⁎, Xiaofeng Wang a, Yufen Zhang a, Yonghao Yu b,⁎ a b
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, PR China Academy of Fundamental and Interdisciplinary Science, Harbin Institute of Technology, Harbin 150001, PR China
a r t i c l e
i n f o
Article history: Received 27 April 2016 Revised 26 July 2016 Accepted in revised form 7 August 2016 Available online 08 August 2016 Keywords: Thin films High power impulse magnetron sputtering Pulsed bias voltage Crystalline size Phase transition
a b s t r a c t VO2 thin films were prepared on quartz glass substrates by using high power impulse magnetron sputtering with different pulsed bias voltages. X-ray diffraction patterns revealed that all as-deposited films were polycrystalline monoclinic VO2 and the preferred crystalline orientation of the as-deposited films varied with the magnitude of the bias voltage. Scanning electron microscopy images exhibited that the crystalline size of the VO2 thin films reduced with the magnitude of the bias voltage. X-ray photoelectron spectroscopy results revealed that oxygen vacancies were formed in all films and their quantities were similar. UV–visible-near IR spectra of the VO2 thin films at different temperatures confirmed that all films possessed typical metal–insulator transition properties. Fourpoint probes resistivity results exhibited that the phase transition temperature was reduced from 54 to 31.5°C when the magnitude of the bias voltage was increased from −50 to −250 V. © 2016 Published by Elsevier B.V.
1. Introduction Vanadium dioxide (VO2) is an archetypal correlated oxide showing a first order reversible metal–insulator transition (MIT) from a low temperature monoclinic (M1, P21/c) to a high-temperature rutile structure (R, P42/mnm) at around 68 °C [1]. The MIT leads to a significant change in physical properties, such as the resistivity and the infrared transmittance [2,3]. Thereby VO2 is an extremely interesting material suitable for many technological applications [4]. VO2 thin films have been fabricated by a variety of deposition techniques, such as DC magnetron sputtering [5,6], RF reactive sputtering [7–9], pulsed laser deposition (PLD) [10], reactive electron beam evaporation [11] and chemical vapor deposition (CVD) [12,13]. In some special applications, such as smart windows, which can help control the room temperature intelligently, the MIT temperature of 68 ° C is too high [14,15]. In recent years, much work has been done with the purpose of decreasing the transition temperature to room temperature [14–17]. For epitaxial VO2 thin films, the MIT temperature has been reduced to room temperature on some specific substrates, such as TiO2 (001) [18]. However, the applications of epitaxial VO2 thin film are limited greatly because of these specific substrates. Since polycrystalline VO2 thin film can be fabricated on a glass substrate, it has shown great potential in such applications. Up to present, the reported MIT temperature of undoped polycrystalline VO2 thin film is higher than ⁎ Corresponding authors. E-mail addresses: [email protected] (L. Wang), [email protected] (Y. Yu).
http://dx.doi.org/10.1016/j.surfcoat.2016.08.020 0257-8972/© 2016 Published by Elsevier B.V.
42 °C [3,15,17,19,20]. Although doping can decrease the phase transition temperature effectively, the change in resistivity and transmittance of the doped VO2 thin film was reduced significantly, which limited the applications of the VO2 thin film [21,22]. Therefore, synthesis of undoped polycrystalline VO2 thin film with low phase transition temperature is still a great challenge. Previous studies proved that polycrystalline VO2 thin film with a smaller crystalline size possessed lower phase transition temperature [17,23,24]. Compared with conventional magnetron sputtering, high power impulse magnetron sputtering (HiPIMS) can obtain high ionized plasma [25–28]. The energy of the ions impinged toward the substrate during a HiPIMS process can be controlled by varying the substrate bias voltage, and the grain size of the as-deposited thin films can be reduced greatly by this ion bombardment [29]. As a consequence, using the HiPIMS method with a bias voltage, fine crystalline VO2 thin film may be obtained. In this study, VO2 thin films were prepared by using HiPIMS with a pulsed bias voltage on quartz glass substrates. In addition, microstructure and phase transition characteristics of the as-deposited thin 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
T. Lin et al. / Surface & Coatings Technology 305 (2016) 110–115
deposition process. The target-substrate distance was approximately 100 mm. Argon and oxygen with respective mass flow rates of 100 and 6.5 sccm were introduced into the chamber simultaneously, and the total pressure was maintained at about 0.85 Pa during the deposition process. The whole deposition time was 30 min. A modulated pulsed power supply was used for the HiPIMS process. The discharge voltage and current waveforms of the vanadium target during the HiPIMS deposition process are shown in Fig. 1(a). An auxiliary pulse with a small voltage was applied before the main HiPIMS pulse, where the duration time for auxiliary and main pulses was 100 and 400 μs, respectively. The pulse repetition frequency was 50 Hz, and the peak power during the sputtering process was about 40 kW. 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. Since the quartz glass substrate is not conductive, a negative bias voltage applied on the substrate during the deposition process cannot accelerate the ions to the substrate. In order to apply an effective bias voltage on the substrate, the deposition process of the VO2 film was carried out in two steps. Firstly, in order to make the substrate be conductive, a pre-deposited VO2 thin film was deposited on quartz glass substrate by using above HiPIMs method but without bias voltage for 8 min, and the thickness of the pre-deposited thin film was about 33 nm. Secondly, a pulsed bias voltage was applied on the pre-deposited VO2 thin film, and VO2 thin films were synthesized on the pre-deposited thin film by using the same HiPIMS method for 22 min. In order to study the influences of the bias voltage on microstructure and phase transition properties, the magnitude of the bias voltage in the second step was varied from − 50 to − 250 V for different samples, and the bias voltages captured during the HiPIMS process are shown in Fig. 1(b). The pulse width and delay time of the bias voltage relative to the main HiPIMS voltage pulse were 300 and 0 μs, respectively. The total thicknesses of all as-deposited thin films deposited with −50, −100, −150 and −250 V bias voltage were about 92, 97, 93 and 86 nm, respectively. Table 1 lists the main processing conditions for VO2 thin films. 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 0.5° (XRD; Empyrean, Panalytical, Holland) with a Cu-Kα radiation source operated at 40 kV and 40 mA. The compositions of the as-deposited thin film were detected by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, USA). 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.
111
Table 1 Main processing conditions for VO2 thin films. Ar flow rate (sccm) O2 flow rate (sccm) Working pressure (pa) Substrate temperature (°C) Pulse frequency (Hz) Auxiliary pulse duration time (μs) Main pulse duration time (μs) Peak power (kW) Bias magnitude (V) Pre-deposition time (min) Whole deposition time (min)
100 6.5 0.85 420 50 100 400 40 −50, −100, −150, −250 8 30
3. Results and discussion The XRD pattern of the pre-deposited VO2 thin film is shown in Fig. 2(a), three weaker diffraction peaks of (001), (002) and (003) planes can be found. According to the standard diffraction card of VO2(B) (JCPDS 31-1438), the pre-deposited VO2 thin film should be metastable monoclinic phase VO2(B). The SEM image of the pre-deposited thin film is given in Fig. 2(b). It is noticeable that nano-crystalline thin film has been obtained, and its crystalline size is about 10 nm. Fig. 2(c) shows the temperature dependence of the resistance of the pre-deposited thin film. Different from VO2(M) thin film, the resistance-temperature curve of this VO2(B) thin film is basically linear and the temperature coefficient of the resistance is about 7%/K, which is in accordance with that obtained by Chen et al. [30]. The resistance-temperature curves obtained during the heating and cooling processes are almost identical, which hints that a hysteresis of the phase transition doesn't exist. Additionally, note that the pre-deposited VO2(B) thin film was conductive, thus the pulsed bias voltage was effectively applied for the subsequent HiPIMS process. XRD patterns of as-deposited VO2 thin films are exhibited in Fig. 3. The (011), (200), (20-2), (21-3), (211) and (202) peaks in the patterns confirmed that the as-deposited VO2 thin films were polycrystalline VO2(M) (JCPDS 44-0252). Obviously, the diffraction peaks of pre-deposited metastable VO2(B) disappeared in these thin films. Because metastable B-VO2 can be transformed to VO2(M) by an annealing process [30], and a heating to 420 °C for 22 min was conducted during the subsequent deposition process, pre-deposited VO2(B) was transformed to VO2(M) and the diffraction peaks of VO2(B) vanished in these XRD patterns. The sample fabricated with a bias magnitude of −50 V possessed the largest quantity of diffraction peaks, such as (011), (200), (210), (211) and (202) ones. When the bias magnitude was increased to − 100 and − 150 V, only (011), (211) and (202) peaks were obvious in the XRD patterns. Meanwhile, the intensity of (011) and (202) peaks decreased with the bias magnitude. When the bias magnitude
Fig. 1. (a) Discharge current and voltage and (b) bias voltage waveforms captured during the HiPIMS deposition process.
112
T. Lin et al. / Surface & Coatings Technology 305 (2016) 110–115
Fig. 2. (a) XRD pattern of pre-deposited VO2 thin film, (b) SEM image of the pre-deposited VO2 thin film, (c) Temperature dependence of resistance of the pre-deposited VO2 thin film.
was further increased to −250 V, (011) and (202) peaks disappeared, and the main diffraction peaks transformed to (20-2) and (21-3) ones, which should be a result of the preferential sputtering effect caused by the high energy ion impingement [26,29,31]. In addition, when the magnitude of the bias voltage was larger than −100 V, the crystallinity of the as-deposited thin films decreased with the bias magnitude, which should be derived from the lattice defects caused by the high energy ion bombardment. Because we applied an effective bias voltage on the substrate, the above results were different from other sputtering methods
Fig. 3. XRD patterns of as-deposited VO2 thin films fabricated with different bias voltages.
[20,32], in which the crystalline orientation and crystallinity did not change obviously. The average grain size of the as-deposited VO2 thin films can be estimated through applying the full-width at half maximum (FWHM) of the main XRD peak to the Scherrer equation [7]. The calculated average grain sizes of the VO2 thin films deposited with bias magnitudes of −50, − 100, − 150, and − 250 V are 22.68, 18.08, 15.52, and 11.89 nm, respectively. Obviously, the calculated average grain size reduces with the magnitude of the bias voltage. The SEM images of the as-deposited VO2 thin films are given in Fig. 4. It is obvious that all as-deposited films possess nano-crystalline structure. In addition, the relationship between the grain size and the magnitude of the bias voltage obtained in this figure is in consistent with the result calculated by the Scherrer equation. Particularly, for thin film fabricated with −250 V bias voltage, the crystalline size of polycrystalline VO2 thin film is smaller than other methods [10,17,19,33]. Therefore, a large magnitude of the bias voltage benefits the reduction of the crystalline size of the VO2 thin film. In order to study the chemical states of vanadium, XPS spectra of vanadium for different samples are shown in Fig. 5(a)–(d), where the spectra were calibrated by the C(1s) peak (284.6 eV). All V(2p) peaks were resolved into three main components at 515.2–515.7, 515.7– 516.2 and 516.9–517.2 eV, corresponding to V3+, V4+ and V5+, respectively [34]. According to the XPS spectra, the relative concentration of V3+, V4+ and V5+ in the as-deposited VO2 thin films were calculated and listed in Table 2. It can be found that the contents of V5+ are similar in all samples, and other researchers have pointed out that the existence of V5+ can be attributed to the surface oxidization during storage in air [34]. The percentages of V3+ were 25.6%, 23.1%, 25.2%, and 21.1% for thin films deposited in −50, −100, −150, and −250 V bias voltages, and the corresponding ratio of oxygen to vanadium were 1.75, 1.8,
T. Lin et al. / Surface & Coatings Technology 305 (2016) 110–115
113
Fig. 4. SEM images of as-deposited VO2 thin films fabricated with different bias magnitudes: (a) −50 V, (b) −100 V, (c) −150 V, (d) −250 V.
1.78, and 1.82, respectively. Because the XRD results have revealed that the crystal structure of the as-deposited thin films were polycrystalline
VO2(M), in order to equilibrium the charge of V3+, some oxygen vacancies should be formed in the as-deposited films [34–36]. In addition,
Fig. 5. XPS profiles of V(2p) for as-deposited VO2 thin films fabricated with different bias magnitudes: (a) −50 V, (b) −100 V, (c) −150 V, (d) −250 V.
114
T. Lin et al. / Surface & Coatings Technology 305 (2016) 110–115
Table 2 Content of V with different valence states.
Table 3 Electrical properties of different bias voltages.
Bias magnitude (V)
V3+
V4+
V5+
Bias magnitude (V)
TMIT (°C)
ΔH (°C)
ΔR
−50 −100 −150 −250
25.6% 23.1% 25.2% 21.1%
40.2% 41.8% 38.6% 46.7%
34.2% 35.1% 36.2% 32.2%
−50 −100 −150 −250
54 51 43.5 31.5
8 5 2 0.2
6 × 102 3.5 × 103 3 × 102 9.5
since the ratios of oxygen to vanadium are similar for all thin films, the influence of the bias magnitude on stoichiometry can be neglected in this study. Fig. 6 shows the temperature dependences of the resistance and the transmittance spectra at different temperatures of the as-deposited VO2 thin films. Note that the phase transition properties of the films were 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 has been occurred during the heating process. In addition, a shifting of the optical transmission edge toward higher energies for VO2 thin film deposited at −250 V was obvious, which should be derived from the poor crystallinity and unique orientations in this film [34,37]. To compare the transition characteristics of different samples, the relative resistance change, the transition temperature and the hysteresis width of the different VO2 thin films were defined according to previous studies [38]. The relative resistance change (ΔR) is defined as ΔR ¼ R0 =R1 ;
ð1Þ
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 T MIT ¼ T c;cooling þ T c;heating =2
ð2Þ
where Tc,cooling and Tc,heating are the transition temperatures at the center of the derivative curve (d[log(R)]/dT) during cooling and heating, respectively. The hysteresis width (ΔH) is written as ΔH ¼ T c;heating −T c;cooling
ð3Þ
Table 3 exhibits electrical properties of different VO2 thin films. It can be found that the phase transition temperature and the hysteresis width decrease significantly with the magnitude of the bias voltage. For thin film fabricated with −100 V bias voltage, the change in resistance and hysteresis width are 3500 times and 5 °C, respectively, which indicates
that the electrical properties of this VO2 film are close to the reported best one of epitaxial VO2 films [38]. The phase transition temperature of the polycrystalline VO2 thin film fabricated at a bias magnitude of −250 V can be reduced to 31.5 °C, which is far below the reported lowest phase transition temperature (42 °C) of undoped polycrystalline VO2 films. Simultaneously, the hysteresis width is almost completely suppressed to 0.2 °C, which is similar to that of doped epitaxial VO2 thin film [39]. In this study, a large variation of the MIT temperature has been obtained. However, the mechanism for the reduction of the phase transition temperature is not clear yet. According to our previous study [40], the MIT temperature of VO2 thin film can be influenced by a lattice distortion, and the transition temperature decreased with the difference between the interplanar spacing of the as-deposited thin film and standard rutile VO2. To compare the lattice distortion in different planes before and after the phase transition process, interplanar spacings of standard VO2(R) (JCPDS 44-0253) and the as-deposited VO2 thin films have been compared. Because the crystalline orientations of − 250 V thin film changed significantly, we only discussed the lattice distortions of thin films deposited at −50, −100 and −150 V. Additionally, since the intensity of the (011) peak was the greatest in these samples, only the interplanar spacing of (011) plane was discussed here. The spacings of thin films deposited at − 50, − 100 and − 150 V were 0.31863, 0.31898 and 0.31889 nm, respectively, 98.95%, 99.06%, and 99.03% that of VO2(R). According to our previous results, the lattice distortions in these samples can be considered as similar, thus the influence of the lattice distortion on the phase transition temperature can be neglected. In addition, because the ratios of O to V for different samples were similar, the variation of transition temperature with the bias voltage should not be correlated to film stoichiometry (oxygen to vanadium). In this study, the obvious influence of the bias magnitude is the reduction of the crystalline size of the as-deposited thin films, and other studies have proved that a fine crystalline size benefits the reduction of phase transition temperature [17,23,24]. Consequently, the main reason for the reduction of phase transition temperature caused by a large bias magnitude should be attributed to the decreased size of VO 2 nano-crystalline grains in the as-deposited thin films.
Fig. 6. (a) Temperature dependence of resistance of VO2 thin films, (b) Transmittance spectra of VO2 films at various temperatures during heating.
T. Lin et al. / Surface & Coatings Technology 305 (2016) 110–115
4. Conclusions Polycrystalline VO2 thin films were prepared on the quartz glass substrates using HiPIMS with different bias voltages. The preferred crystalline orientations of the as-deposited films varied with the magnitude of the bias voltage. Nano-crystalline structure was obtained in all VO2 films, and the crystalline size reduced with the magnitude of the bias voltage. Oxygen vacancies were formed in all films, and the ratio of O to V kept stable under different bias voltages. All films showed typical metal–insulator transition properties, and the phase transition temperature was reduced from 54 to 31.5 °C when the magnitude of the pulsed bias voltage was increased from −50 to −250 V. Acknowledgments This work was supported by the fund of Shanghai Association for Science and Technology (SAST2015059). References [1] M. Nakano, D. Okuyama, K. Shibuya, M. Mizumaki, H. Ohsumi, M. Yoshida, M. Takata, M. Kawasaki, Y. Tokura, T. Arima, Y. Iwasa, Distinct substrate effect on the reversibility of the metal-insulator transitions in electrolyte-gated VO2 thin films, Adv. Electron. Mater. 1 (2015), 1500093. [2] L.H. Yeo, A. Srivastava, M.A. Majidi, R. Sutarto, F. He, S.M. Poh, C. Diao, X. Yu, M. Motapothula, S. Saha, S. Ojha, D. Kanjilal, P.E. Trevisanutto, M.B.H. Breese, T. Venkatesan, A. Rusydi, Anomalous spectral-weight transfers unraveling oxygen screening and electronic correlations in the insulator-metal transition of VO2, Phys. Rev. B 91 (2015), 081112. [3] 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, Appl. Phys. Lett. 101 (2012), 191905. [4] 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 (Ti4+) doping, Sci. Rep. 5 (2015) 9328. [5] S.-Y. Li, K. Namura, M. Suzuki, G.A. Niklasson, C.G. Granqvist, Thermochromic VO2 nanorods made by sputter deposition: growth conditions and optical modeling, J. Appl. Phys. 114 (2013), 033516. [6] N.R. Mlyuka, G.A. Niklasson, C.G. Granqvist, Thermochromic multilayer films of VO2 and TiO2 with enhanced transmittance, Sol. Energy Mater. Sol. Cells 93 (2009) 1685–1687. [7] 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 magnetronsputtered VO2 thin films, Appl. Phys. Lett. 87 (2005), 051910. [8] Y.Y. Luo, L.Q. Zhu, Y.X. Zhang, S.S. Pan, S.C. Xu, M. Liu, G.H. Li, Optimization of microstructure and optical properties of VO2 thin film prepared by reactive sputtering, J. Appl. Phys. 113 (2013), 183520. [9] M. Jiang, Y. Li, S. Li, H. Zhou, X. Cao, S. Bao, Y. Gao, H. Luo, P. Jin, Room temperature optical constants and band gap evolution of phase pure M1-VO2 thin films deposited at different oxygen partial pressures by reactive magnetron sputtering, J. Nanomater. 2014 (2014) 1–6. [10] B.D. Ngom, M. Chaker, A. Diallo, I.G. Madiba, S. Khamlich, N. Manyala, O. Nemraoui, R. Madjoe, A.C. Beye, M. Maaza, Competitive growth texture of pulsed laser deposited vanadium dioxide nanostructures on a glass substrate, Acta Mater. 65 (2014) 32–41. [11] 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. [12] D. Vernardou, D. Louloudakis, E. Spanakis, N. Katsarakis, E. Koudoumas, Thermochromic amorphous VO2 coatings grown by APCVD using a single-precursor, Sol. Energy Mater. Sol. Cells 128 (2014) 36–40. [13] L. Kritikos, L. Zambelis, G. Papadimitropoulos, D. Davazoglou, Structure and electrical properties of selectively chemically vapor deposited vanadium oxide films from vanadium tri-i-propoxy oxide vapors, Surf. Coat. Technol. 201 (2007) 9334–9339. [14] 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), J. Appl. Phys. 117 (2015), 185307. [15] X. Liu, S.-W. Wang, F. Chen, L. Yu, X. Chen, Tuning phase transition temperature of VO2 thin films by annealing atmosphere, J. Phys. D. Appl. Phys. 48 (2015), 265104.
115
[16] 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 VO2 epitaxial film by modifying carrier density, ACS Appl. Mater. Interfaces 7 (2015) 6875–6881. [17] M.J. Miller, J. Wang, Influence of grain size on transition temperature of thermochromic VO2, J. Appl. Phys. 117 (2015), 034307. [18] 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. [19] 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 Appl. Mater. Interfaces 5 (2013) 12520–12525. [20] 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. [21] X. Wu, Z. Wu, H. Zhang, R. Niu, Q. He, C. Ji, J. Wang, Y. Jiang, Enhancement of VO2 thermochromic properties by Si doping, Surf. Coat. Technol. 276 (2015) 248–253. [22] N.R. Mlyuka, G.A. Niklasson, C.G. Granqvist, Mg doping of thermochromic VO2 films enhances the optical transmittance and decreases the metal-insulator transition temperature, Appl. Phys. Lett. 95 (2009), 171909. [23] J. Jian, W. Zhang, C. Jacob, A. Chen, H. Wang, J. Huang, H. Wang, Roles of grain boundaries on the semiconductor to metal phase transition of VO2 thin films, Appl. Phys. Lett. 107 (2015), 102105. [24] E. Radue, E. Crisman, L. Wang, S. Kittiwatanakul, J. Lu, S.A. Wolf, R. Wincheski, R.A. Lukaszew, I. Novikova, Effect of a substrate-induced microstructure on the optical properties of the insulator-metal transition temperature in VO2 thin films, J. Appl. Phys. 113 (2013), 233104. [25] 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. [26] K. Sarakinos, J. Alami, S. Konstantinidis, High power pulsed magnetron sputtering: a review on scientific and engineering state of the art, Surf. Coat. Technol. 204 (2010) 1661–1684. [27] F.J. Jing, K. Yukimura, H. Kato, Y.F. Lei, T.X. You, Y.X. Leng, N. Huang, Film characterization of titanium oxide films prepared by high-power impulse magnetron sputtering, Surf. Coat. Technol. 206 (2011) 967–971. [28] J.P. Fortier, B. Baloukas, O. Zabeida, J.E. Klemberg-Sapieha, L. Martinu, Thermochromic VO2 thin films deposited by HiPIMS, Sol. Energy Mater. Sol. Cells 125 (2014) 291–296. [29] Z. Wang, D. Zhang, P. Ke, X. Liu, A. Wang, Influence of substrate negative bias on structure and properties of TiN coatings prepared by hybrid HIPIMS method, J. Mater. Sci. Technol. 31 (2015) 37–42. [30] 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, Opt. Express 17 (2009) 24153–24161. [31] V. Stranak, A.-P. Herrendorf, H. Wulff, S. Drache, M. Cada, Z. Hubicka, M. Tichy, R. Hippler, Deposition of rutile (TiO2) with preferred orientation by assisted high power impulse magnetron sputtering, Surf. Coat. Technol. 222 (2013) 112–117. [32] 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, J. Alloys Compd. 664 (2016) 626–631. [33] Y.-X. Ji, G.A. Niklasson, C.G. Granqvist, M. Boman, Thermochromic VO2 films by thermal oxidation of vanadium in SO2, Sol. Energy Mater. Sol. Cells 144 (2016) 713–716. [34] Z. Zhang, Y. Gao, Z. Chen, J. Du, C. Cao, L. Kang, H. Luo, Thermochromic VO2 thin films: solution-based processing, improved optical properties, and lowered phase transformation temperature, Langmuir 26 (2010) 10738–10744. [35] T.-H. Yang, S. Nori, S. Mal, J. Narayan, Control of room-temperature defect-mediated ferromagnetism in VO2 films, Acta Mater. 59 (2011) 6362–6368. [36] 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, Appl. Surf. Sci. 345 (2015) 232–237. [37] G. Sun, H. Zhou, X. Cao, R. Li, M. Tazawa, M. Okada, P. Jin, Self-assembled multilayer structure and enhanced thermochromic performance of spinodally decomposed TiO2-VO2 thin film, ACS Appl. Mater. Interfaces 8 (2016) 7054–7059. [38] 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, Appl. Phys. Lett. 104 (2014), 081913. [39] M. Soltani, M. Chaker, E. Haddad, R.V. Kruzelecky, J. Margot, Effects of Ti–W codoping on the optical and electrical switching of vanadium dioxide thin films grown by a reactive pulsed laser deposition, Appl. Phys. Lett. 85 (2004) 1958. [40] T. Lin, L. Wang, X. Wang, Y. Zhang, Y. Yu, Influence of lattice distortion on phase transition properties of polycrystalline VO2 thin film, Appl. Surf. Sci. 379 (2016) 179–185.