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FTO structure
Infrared Physics & Technology 75 (2016) 82–86
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Electric field induced metal–insulator transition in VO2 thin film based on FTO/VO2/FTO structure Rulong Hao a, Yi Li a,b,⇑, Fei Liu a, Yao Sun b, Jiayin Tang a, Peizu Chen a, Wei Jiang a, Zhengyi Wu a, Tingting Xu a, Baoying Fang a a b
School of Optical-Electrical and Computer Engineering, University of Shanghai for Science and Technology, 516 Jungong Road, 200093 Shanghai, China Shanghai Key Laboratory of Modern Optical Systems, 516 Jungong Road, 200093 Shanghai, China
h i g h l i g h t s Electric field induced phase transition of VO2 thin film is proposed. Optical modulation has been studied based on FTO/VO2/FTO structure. Maximum transmission modulation value is 31.4% under different conditions.
a r t i c l e
i n f o
Article history: Received 14 December 2015 Available online 22 January 2016 Keywords: VO2/FTO DC magnetron sputtering Threshold voltage Electric induced phase transition
a b s t r a c t A VO2 thin film has been prepared using a DC magnetron sputtering method and annealing on an F-doped SnO2 (FTO) conductive glass substrate. The FTO/VO2/FTO structure was fabricated using photolithography and a chemical etching process. The temperature dependence of the I–V hysteresis loop for the FTO/VO2/ FTO structure has been analyzed. The threshold voltage decreases with increasing temperature, with a value of 9.2 V at 20 °C. The maximum transmission modulation value of the FTO/VO2/FTO structure is 31.4% under various temperatures and voltages. Optical modulation can be realized in the structure by applying an electric field. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction The phenomenon of metal–insulator reversible transition attracts much attention because of its scientific implications and the possibility of use in a wide variety of applications. Vanadium dioxide (VO2) has been shown to undergo a reversible transition between an insulating/semiconducting state and a conducting state, where the crystal structure of VO2 changes from the monoclinic structure to the rutile structure [1–3]. An abrupt change in resistivity and optical transmittance occurs during the phase transition. Recently, a VO2 thin film has been observed to exhibit this transition when applying an electric field to the film [4–7]. VO2 thin films have become attractive materials for optical switching, optical storage, optical modulators, smart windows, and microbolometers [8–12].
⇑ Corresponding author at: School of Optical-Electrical and Computer Engineering, University of Shanghai for Science and Technology, 516 Jungong Road, 200093 Shanghai, China. Tel.: +86 21 33621219; fax: +86 21 33621371. E-mail address: [email protected] (Y. Li). http://dx.doi.org/10.1016/j.infrared.2015.12.012 1350-4495/Ó 2016 Elsevier B.V. All rights reserved.
F-doped SnO2 (FTO) film [13–16] is a transparent conductive oxide film that has a regular tetrahedron rutile structure. FTO films have the advantages of high transmittance of visible light, good conductivity, chemical stability, the ability to withstand high temperature, and low cost. FTO is a broadly studied material and it is widely used in liquid crystal displays, light catalysis, thin film solar cells, and other fields. In the visible region, the optical constants of FTO films are between those of VO2 thin films and glass, the lattice constant of FTO is similar to VO2, and FTO has good flexibility. The introduction of an FTO film between glass and a VO2 thin film can effectively adjust the optical parameters of thin films, thereby improving the optical properties of the VO2 thin film [17]. In this paper, we investigate the growth and fabrication of a FTO/VO2/FTO structure by process control and study the optical characteristics by applying a voltage. A VO2 thin film with a preferential orientation is used in the structure. The structure is based on a three-layer thin film, where a VO2 thin film is deposited between two FTO films, which function as the top and bottom electrodes. The metal–insulator transition and the optical modulation characteristics of the VO2 thin film are analyzed through observation of the current response.
R. Hao et al. / Infrared Physics & Technology 75 (2016) 82–86
2. Experiments In this experiment, a sputtering system was used to deposit a pure vanadium thin film. The vanadium metal target was 120 mm in diameter and 5 mm in thickness, with a purity of 99.9%. FTO transparent conductive glass with a thickness of 2 mm was used as the substrate, where the conductive film thickness was 350 nm. The sputtering gas was argon, with a purity of 99.999%, and the flow of argon was controlled at 80 sccm by a gas mass flow meter. Before deposition, the sputtering chamber was evacuated to a vacuum of 4.2 10 3 Pa; then, the sputtering chamber was purged with argon for 10–15 min of pre-sputtering to clean the target surface. The operating pressure during sputtering was kept at 3.5 10 1 Pa, and the sputtering current and voltage were 2 A and 400 V, respectively. Throughout the process, the temperature of the chamber was kept at room temperature. A vanadium thin film with a thickness of 300 nm was obtained. The VO2/FTO composite films were prepared by annealing for 3 h with a flux of 60 sccm N2 and 40 sccm O2 at a temperature of 380 °C. The crystal structure of the VO2/FTO composite film was analyzed using a DX-2700 multifunction X-ray diffraction (XRD) apparatus. The surface morphology of the samples was studied using a Hitachi S4800 scanning electron microscope (SEM). A PHI 5000C ESCA X-ray photoelectron spectroscopy (XPS) system was used to study the relative content of the surface elements. The current– voltage characteristics were measured using a Keithley Model 4200-SCS semiconductor parameter analyzer. The optical properties of the FTO/VO2/FTO structure were determined using a Perkin Elmer Lambda 1050 UV/VIS/NIR spectrophotometer. Optical transmittance measurements were performed at wavelengths between 250 and 2500 nm. The optical modulation characteristics of the FTO/VO2/FTO structure were recorded using a Thorlabs PDA50B photodetector with a digital oscilloscope unit. A schematic representation of the FTO/VO2/FTO structure is shown in Fig. 1. FTO film was selected as the electrode material,
Fig. 1. Schematic of the FTO/VO2/FTO structure.
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which worked as the bottom electrode of the structure. VO2 thin film with a preferential orientation was deposited on the bottom electrode. Then, another FTO film was deposited on the VO2 thin film, using plasma-enhanced chemical vapor deposition, to function as the top electrode. The contact area between the FTO film and VO2 thin film was 8 mm 8 mm. Pt electrodes with dimensions of 50 lm 50 lm were patterned on the bottom and top of FTO electrodes using photolithography and chemical etching processes. The FTO/VO2/FTO structure was designed so that the optical signal was directed perpendicularly through the structure.
3. Results and discussion Fig. 2 shows the X-ray diffraction (XRD) data for the FTO film and the VO2/FTO composite films prepared under the above conditions. Fig. 2(a) shows the XRD spectrum of the FTO film. The FTO film has six major diffraction peaks, showing a film structure with (1 1 0), (1 0 1), (2 0 0), (2 1 1), (3 1 0), and (3 0 1) as the main crystal orientations. The crystal orientation of the FTO film shows no significant difference from the XRD spectra of the VO2/FTO composite films. The results indicate that there are no mixed oxides formed by V, Sn, and F in the DC magnetron sputtering and annealing process. The VO2 thin film does not react with FTO, but grows directly with a preferential orientation of (1 1 0) on the surface of the FTO. Because the crystal structure and lattice constant of the FTO film are similar to that of the VO2 thin film, the nucleation interface energy of VO2 thin film growth on FTO film can decrease. The FTO substrate can promote the growth of a VO2 thin film with a preferential orientation. To study the structural features of the VO2/FTO composite films, the surface morphologies of the FTO film and VO2/FTO composite films were observed by SEM at room temperature, as shown in Fig. 3. Fig. 3(a) shows that the surface colonies of the FTO film are cylindrical and tightly stacked. Fig. 3(b) presents an SEM image of the VO2/FTO composite film. VO2 colonies with square sheet structures are grown on the surface of the FTO grain group. The lateral dimension noticeably increases, and VO2 colonies have sizes of approximately 200–330 nm. VO2 and FTO have a similar crystal structure and lattice constant; VO2 is atomically transported and diffuses on the surface of the FTO film. It gradually forms small particles. As the crystallization process progresses, the small particles fuse into a sheet structure. The FTO film promotes the preferred orientation growth of the VO2 thin film and changes the surface morphology of the VO2 thin film. This is consistent with the results of XRD.
Fig. 2. X-ray diffraction spectra: (a) FTO thin film (b) VO2/FTO composite films.
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Fig. 3. Scanning electron micrographs: (a) FTO thin film (b) VO2/FTO composite films.
Fig. 4. X-ray photoelectron spectra of VO2/FTO composite films: (a) full spectrum scanning on the surface of VO2 film (b) narrow spectrum scanning on the surface of VO2 film about vanadium and oxygen (c) peak fitting V2p3/2 of the VO2 film surface (d) peak fitting O1s of the VO2 film surface.
Fig. 4 shows the X-ray photoelectron spectra of VO2/FTO composite films. The binding energy at 284.6 eV corresponds to the C1s peak. C1s is taken as a reference binding energy calibration. Fig. 4(a) presents the full spectrum scan on the surface of the VO2 thin film. Using a standard binding energy database, 530 eV and 516.3 eV correspond to the O1s peak and V2p3/2 peak, respectively. The surface of the VO2 thin film contains a large amount of vanadium oxide, and no other elements appear, which indicates
that no reaction occurs between vanadium metal and substrate during the high-temperature annealing. Fig. 4(b) presents the narrow spectrum scan on the surface of the VO2 thin film. There is a small peak at 524.8 eV that corresponds to V5+2p1/2 and originates from V2O5. Fig. 4(c) and (d) respectively present the peak fitting of V2p3/2 and O1s of the VO2 thin film surface. It can be seen from the XPS spectra that there is a small amount of V2O5 in the VO2 thin film.
R. Hao et al. / Infrared Physics & Technology 75 (2016) 82–86
Fig. 5. The temperature dependence of I–V hysteresis loops for the FTO/VO2/FTO structure.
The FTO/VO2/FTO structure is prepared using photolithography and chemical etching processes. With this structure, the voltage is applied on both of the transparent conductive films of the FTO/VO2/FTO structure. The temperature dependence of the I–V hysteresis loop for the FTO/VO2/FTO structure is shown in Fig. 5. The threshold voltage is 9.2 V at 20 °C. The threshold voltage decreases with increasing temperature. An abrupt change in the current can be seen when applying the voltage on the VO2 thin film at 20 °C, which is also an important characteristic of the VO2 thin
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film phase transition [18,19]. When the reverse voltage decreases, the current hysteresis effect is observed. The resistance of the structure also decreases with increasing temperature. The VO2 thin film contains a large number of grains, and each grain has a threshold voltage. Only when the voltage of the grain reaches the threshold voltage does the electrochromic phase transition occur. Another explanation is that the requirement of constant threshold current density implies that a certain bulk carrier density is needed for the phase transition. This phenomenon is similar to the Mott–Hubbard transition, where higher carrier density than the Mott criteria gives rise to an insulator to metal transition [20]. If the carrier density reaches a certain value, an applied voltage can lead to the formation of a stable conduction path. The optical transmittance spectra of the FTO/VO2/FTO structure are respectively presented in Fig. 6 when the VO2 thin films were subjected to voltages of 0 V, 4 V, 6 V, 8 V, and 10 V at temperatures of 20 °C, 40 °C, 60 °C, and 80 °C. The limiting factor for the optical transmittance at wavelengths above 2200 nm is the transmittance of the FTO films. Fig. 6(a) shows the transmittance of the FTO/VO2/ FTO structure with various applied voltages at 20 °C. The transmittance of the structure shows little change at visible wavelengths. However, the transmittance decreases with increasing applied voltage at infrared wavelengths. Fig. 6(b) is substantially similar to Fig. 6(a); the transmittance changes little when applying a voltage. Data from Fig. 6(c) indicate that the transmittance of the structure clearly decreases. Comparing Fig. 6(d) and (c), it can be seen that the transmittance of this structure changes little. When the applied voltage reaches the threshold voltage, the transmittance change is large. The change in optical transmittance is found to
Fig. 6. Optical transmittance spectra of the FTO/VO2/FTO structure.
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be 27.5% with temperatures ranging from 20 °C to 80 °C at the wavelength of 1200 nm when the VO2 thin films are not subjected to an applied voltage. The change in optical transmittance is 26.6% at 20 °C with voltages changing from 0 V to 10 V. The change in optical transmittance is 2.5% at 80 °C with voltages changing from 0 V to 10 V. The maximum modulation value of the FTO/VO2/FTO structure is 31.4% at the wavelength of 1200 nm under temperatures ranging from 20 °C to 80 °C and voltages between 0 V and 10 V. With applied voltage, the carrier concentration of the VO2 thin film increases and the migration rate increases. When light is incident on the film, the probability of photon collisions with carriers increases. It is concluded that the carrier concentration of the VO2 thin film promotes the phase transition. 4. Conclusion A VO2 thin film was prepared by a DC magnetron sputtering method on F-doped SnO2 conductive glass. Then, a FTO/VO2/FTO structure was fabricated using photolithography and a chemical etching process. An abrupt change in the current can be seen by applying a voltage on the VO2 thin film. The threshold voltage is 9.2 V at 20 °C and decreases with increasing temperature. Compared with the no-voltage situation, the maximum change in the optical transmittance of the FTO/VO2/FTO structure under the effect of voltage is up to 31.4% before and after transition. This structure shows stable performance for easy integration and miniaturization. The results indicate that the FTO/VO2/FTO structure can be used in a new generation of optoelectronic devices. Conflict of interest There is no conflict of interest. Acknowledgments This work is partly supported by the National High Technology Research and Development Program of China (Grant No. 2006AA03Z348), the Foundation for Key Program of Ministry of Education China (Grant No. 207033), the Key Science and Technology Research Project of Shanghai Committee, China (Grant No. 10ZZ94), the Shanghai Talent Leading Plan, China (Grant No. 2011-026). References [1] F.J. Morin, Oxides which show a metal-to-insulator transition at the Neel temperature, Phys. Rev. Lett. 3 (1959) 34–36.
[2] M.M. Qazilbash, K.S. Burch, D. Whisler, D. Shrekenhamer, B.G. Chae, H.T. Kim, D.N. Basov, Correlated metallic state of vanadium dioxide, Phys. Rev. B 74 (2006) 3840–3845. [3] S. Chen, H. Ma, X.J. Yi, H.C. Wang, X. Tao, M.X. Chen, X.W. Li, C.J. Ke, Optical switch based on vanadium dioxide thin films, Infrared Phys. Technol. 45 (2004) 239–242. [4] D. Ruzmetov, G. Gopalakrishnan, J. Deng, V. Narayanamurti, S. Ramanathan, Electrical triggering of metal-insulator transition in nanoscale vanadium oxide junctions, J. Appl. Phys. 106 (2009). pp. 083702–083702-5. [5] D. Matsunami, A. Fujita, Electrocaloric effect of metal-insulator transition in VO2, Appl. Phys. Lett. 106 (2015) 042901. [6] C.H. Chen, Z.Y. Fan, Changes in VO2 band structure induced by charge localization and surface segregation, Appl. Phys. Lett. 95 (2009). pp. 262106– 262106-3. [7] Y. Zhou, X. Chen, C. Ko, Z. Yang, C. Mouli, S. Ramanathan, Voltage-triggered ultrafast phase transition in vanadium dioxide switches, IEEE Electron Device Lett. 34 (2013) 220–222. [8] M. Dragoman, A. Cismaru, H. Hartnagel, R. Plana, Reversible metalsemiconductor transitions for microwave switching applications, Appl. Phys. Lett. 88 (2006). pp. 073503–073503-3. [9] J. Cao, Y. Gu, W. Fan, L.Q. Chen, D.F. Ogletree, K. Chen, N. Tamura, M. Kunz, C. Barrett, J. Seidel, J. Wu, Extended mapping and exploration of the vanadium dioxide stress-temperature phase diagram, Nano Lett. 10 (2010) 2667–2673. [10] B.Y. Fang, Y. Li, G.X. Tong, X.H. Wang, M. Yan, Q. Liang, F. Wang, Y. Qin, J. Ding, S.J. Chen, J.K. Chen, H.Z. Zheng, W.R. Yuan, Optical properties of vanadium dioxide thin film in nanoparticle structure, Opt. Mater. 47 (2015) 225–230. [11] G. Stefanovich, A. Pergament, D. Stefanovich, Electrical switching and Mott transition in VO2, J. Phys.: Condens. Matter 12 (2000) 8837–8845. [12] S. Kittiwatanakul, J. Lu, S.A. Wolf, Transport anisotropy of epitaxial VO2 films near the metal–semiconductor transition, Appl. Phys. Express 4 (2011) 544– 548. [13] G.X. Tong, Y. Li, F. Wang, Y.Z. Huang, B.Y. Fang, X.H. Wang, H.Q. Zhu, L. Li, Y.J. Shen, Q.X. Zheng, Q. Liang, M. Yan, Y. Qin, J. Ding, Thermal oxidation-grown vanadium dioxide thin films on FTO (Fluorine-doped tin oxide) substrates, Infrared Phys. Technol. 61 (2013) 37–41. [14] T.D. Cheng, N.J. Zhou, P. Li, Ferroelectric and photoelectricity properties of (Pb0. 52Zr0. 48) TiO3 thin films fabricated on FTO glass substrate, J. Mater. Sci.: Mater. Electron. 26 (2015) 7104–7108. [15] E.H. Jeon, S. Yang, Y. Kim, N. Kim, H.J. Shin, J. Baik, H.S. Kim, H. Lee, Comparative study of photocatalytic activities of hydrothermally grown ZnO nanorod on Si (0 0 1) wafer and FTO glass substrates, Nano. Res. Lett. 10 (2015) 1–8. [16] M. Guo, K. Zhang, X. Zhu, H. Li, Optical losses of CdS films on FTO, ITO, and AZO electrodes in CdTe–HgCdTe tandem solar cells, J. Mater. Sci.: Mater. Electron. 26 (2015) 7607–7613. [17] N. Shen, Y. Li, X.J. Yi, Preparation of VO2 films with nanostructure and improvement on its visible transmittance, Infrared Milli. Waves 25 (2006) 199–202. [18] J. Leroy, A. Crunteanu, A. Bessaudou, F. Cosset, C. Champeaux, J.C. Orlianges, High-speed metal-insulator transition in vanadium dioxide films induced by an electrical pulsed voltage over nano-gap electrodes, Appl. Phys. Lett. 100 (2012). pp. 213507–213507-4. [19] A. Crunteanu, J. Givernaud, J. Leroy, D. Mardivirin, C. Champeaux, J.C. Orlianges, A. Catherinot, P. Blondy, Voltage-and current-activated metal– insulator transition in VO2-based electrical switches: a lifetime operation analysis, Sci. Technol. Adv. Mater. 11 (2010) 065002. [20] C.R. Cho, S.I. Cho, S. Vadim, R. Jung, I. Yoo, Current-induced metal–insulator transition in VOx thin film prepared by rapid-thermal-annealing, Thin Solid Films 495 (2006) 375–379.
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Electric field induced metal–insulator transition in VO2 thin film based on FTO/VO2/FTO structure Rulong Hao a, Yi Li a,b,⇑, Fei Liu a, Yao Sun b, Jiayin Tang a, Peizu Chen a, Wei Jiang a, Zhengyi Wu a, Tingting Xu a, Baoying Fang a a b
School of Optical-Electrical and Computer Engineering, University of Shanghai for Science and Technology, 516 Jungong Road, 200093 Shanghai, China Shanghai Key Laboratory of Modern Optical Systems, 516 Jungong Road, 200093 Shanghai, China
h i g h l i g h t s Electric field induced phase transition of VO2 thin film is proposed. Optical modulation has been studied based on FTO/VO2/FTO structure. Maximum transmission modulation value is 31.4% under different conditions.
a r t i c l e
i n f o
Article history: Received 14 December 2015 Available online 22 January 2016 Keywords: VO2/FTO DC magnetron sputtering Threshold voltage Electric induced phase transition
a b s t r a c t A VO2 thin film has been prepared using a DC magnetron sputtering method and annealing on an F-doped SnO2 (FTO) conductive glass substrate. The FTO/VO2/FTO structure was fabricated using photolithography and a chemical etching process. The temperature dependence of the I–V hysteresis loop for the FTO/VO2/ FTO structure has been analyzed. The threshold voltage decreases with increasing temperature, with a value of 9.2 V at 20 °C. The maximum transmission modulation value of the FTO/VO2/FTO structure is 31.4% under various temperatures and voltages. Optical modulation can be realized in the structure by applying an electric field. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction The phenomenon of metal–insulator reversible transition attracts much attention because of its scientific implications and the possibility of use in a wide variety of applications. Vanadium dioxide (VO2) has been shown to undergo a reversible transition between an insulating/semiconducting state and a conducting state, where the crystal structure of VO2 changes from the monoclinic structure to the rutile structure [1–3]. An abrupt change in resistivity and optical transmittance occurs during the phase transition. Recently, a VO2 thin film has been observed to exhibit this transition when applying an electric field to the film [4–7]. VO2 thin films have become attractive materials for optical switching, optical storage, optical modulators, smart windows, and microbolometers [8–12].
⇑ Corresponding author at: School of Optical-Electrical and Computer Engineering, University of Shanghai for Science and Technology, 516 Jungong Road, 200093 Shanghai, China. Tel.: +86 21 33621219; fax: +86 21 33621371. E-mail address: [email protected] (Y. Li). http://dx.doi.org/10.1016/j.infrared.2015.12.012 1350-4495/Ó 2016 Elsevier B.V. All rights reserved.
F-doped SnO2 (FTO) film [13–16] is a transparent conductive oxide film that has a regular tetrahedron rutile structure. FTO films have the advantages of high transmittance of visible light, good conductivity, chemical stability, the ability to withstand high temperature, and low cost. FTO is a broadly studied material and it is widely used in liquid crystal displays, light catalysis, thin film solar cells, and other fields. In the visible region, the optical constants of FTO films are between those of VO2 thin films and glass, the lattice constant of FTO is similar to VO2, and FTO has good flexibility. The introduction of an FTO film between glass and a VO2 thin film can effectively adjust the optical parameters of thin films, thereby improving the optical properties of the VO2 thin film [17]. In this paper, we investigate the growth and fabrication of a FTO/VO2/FTO structure by process control and study the optical characteristics by applying a voltage. A VO2 thin film with a preferential orientation is used in the structure. The structure is based on a three-layer thin film, where a VO2 thin film is deposited between two FTO films, which function as the top and bottom electrodes. The metal–insulator transition and the optical modulation characteristics of the VO2 thin film are analyzed through observation of the current response.
R. Hao et al. / Infrared Physics & Technology 75 (2016) 82–86
2. Experiments In this experiment, a sputtering system was used to deposit a pure vanadium thin film. The vanadium metal target was 120 mm in diameter and 5 mm in thickness, with a purity of 99.9%. FTO transparent conductive glass with a thickness of 2 mm was used as the substrate, where the conductive film thickness was 350 nm. The sputtering gas was argon, with a purity of 99.999%, and the flow of argon was controlled at 80 sccm by a gas mass flow meter. Before deposition, the sputtering chamber was evacuated to a vacuum of 4.2 10 3 Pa; then, the sputtering chamber was purged with argon for 10–15 min of pre-sputtering to clean the target surface. The operating pressure during sputtering was kept at 3.5 10 1 Pa, and the sputtering current and voltage were 2 A and 400 V, respectively. Throughout the process, the temperature of the chamber was kept at room temperature. A vanadium thin film with a thickness of 300 nm was obtained. The VO2/FTO composite films were prepared by annealing for 3 h with a flux of 60 sccm N2 and 40 sccm O2 at a temperature of 380 °C. The crystal structure of the VO2/FTO composite film was analyzed using a DX-2700 multifunction X-ray diffraction (XRD) apparatus. The surface morphology of the samples was studied using a Hitachi S4800 scanning electron microscope (SEM). A PHI 5000C ESCA X-ray photoelectron spectroscopy (XPS) system was used to study the relative content of the surface elements. The current– voltage characteristics were measured using a Keithley Model 4200-SCS semiconductor parameter analyzer. The optical properties of the FTO/VO2/FTO structure were determined using a Perkin Elmer Lambda 1050 UV/VIS/NIR spectrophotometer. Optical transmittance measurements were performed at wavelengths between 250 and 2500 nm. The optical modulation characteristics of the FTO/VO2/FTO structure were recorded using a Thorlabs PDA50B photodetector with a digital oscilloscope unit. A schematic representation of the FTO/VO2/FTO structure is shown in Fig. 1. FTO film was selected as the electrode material,
Fig. 1. Schematic of the FTO/VO2/FTO structure.
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which worked as the bottom electrode of the structure. VO2 thin film with a preferential orientation was deposited on the bottom electrode. Then, another FTO film was deposited on the VO2 thin film, using plasma-enhanced chemical vapor deposition, to function as the top electrode. The contact area between the FTO film and VO2 thin film was 8 mm 8 mm. Pt electrodes with dimensions of 50 lm 50 lm were patterned on the bottom and top of FTO electrodes using photolithography and chemical etching processes. The FTO/VO2/FTO structure was designed so that the optical signal was directed perpendicularly through the structure.
3. Results and discussion Fig. 2 shows the X-ray diffraction (XRD) data for the FTO film and the VO2/FTO composite films prepared under the above conditions. Fig. 2(a) shows the XRD spectrum of the FTO film. The FTO film has six major diffraction peaks, showing a film structure with (1 1 0), (1 0 1), (2 0 0), (2 1 1), (3 1 0), and (3 0 1) as the main crystal orientations. The crystal orientation of the FTO film shows no significant difference from the XRD spectra of the VO2/FTO composite films. The results indicate that there are no mixed oxides formed by V, Sn, and F in the DC magnetron sputtering and annealing process. The VO2 thin film does not react with FTO, but grows directly with a preferential orientation of (1 1 0) on the surface of the FTO. Because the crystal structure and lattice constant of the FTO film are similar to that of the VO2 thin film, the nucleation interface energy of VO2 thin film growth on FTO film can decrease. The FTO substrate can promote the growth of a VO2 thin film with a preferential orientation. To study the structural features of the VO2/FTO composite films, the surface morphologies of the FTO film and VO2/FTO composite films were observed by SEM at room temperature, as shown in Fig. 3. Fig. 3(a) shows that the surface colonies of the FTO film are cylindrical and tightly stacked. Fig. 3(b) presents an SEM image of the VO2/FTO composite film. VO2 colonies with square sheet structures are grown on the surface of the FTO grain group. The lateral dimension noticeably increases, and VO2 colonies have sizes of approximately 200–330 nm. VO2 and FTO have a similar crystal structure and lattice constant; VO2 is atomically transported and diffuses on the surface of the FTO film. It gradually forms small particles. As the crystallization process progresses, the small particles fuse into a sheet structure. The FTO film promotes the preferred orientation growth of the VO2 thin film and changes the surface morphology of the VO2 thin film. This is consistent with the results of XRD.
Fig. 2. X-ray diffraction spectra: (a) FTO thin film (b) VO2/FTO composite films.
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Fig. 3. Scanning electron micrographs: (a) FTO thin film (b) VO2/FTO composite films.
Fig. 4. X-ray photoelectron spectra of VO2/FTO composite films: (a) full spectrum scanning on the surface of VO2 film (b) narrow spectrum scanning on the surface of VO2 film about vanadium and oxygen (c) peak fitting V2p3/2 of the VO2 film surface (d) peak fitting O1s of the VO2 film surface.
Fig. 4 shows the X-ray photoelectron spectra of VO2/FTO composite films. The binding energy at 284.6 eV corresponds to the C1s peak. C1s is taken as a reference binding energy calibration. Fig. 4(a) presents the full spectrum scan on the surface of the VO2 thin film. Using a standard binding energy database, 530 eV and 516.3 eV correspond to the O1s peak and V2p3/2 peak, respectively. The surface of the VO2 thin film contains a large amount of vanadium oxide, and no other elements appear, which indicates
that no reaction occurs between vanadium metal and substrate during the high-temperature annealing. Fig. 4(b) presents the narrow spectrum scan on the surface of the VO2 thin film. There is a small peak at 524.8 eV that corresponds to V5+2p1/2 and originates from V2O5. Fig. 4(c) and (d) respectively present the peak fitting of V2p3/2 and O1s of the VO2 thin film surface. It can be seen from the XPS spectra that there is a small amount of V2O5 in the VO2 thin film.
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Fig. 5. The temperature dependence of I–V hysteresis loops for the FTO/VO2/FTO structure.
The FTO/VO2/FTO structure is prepared using photolithography and chemical etching processes. With this structure, the voltage is applied on both of the transparent conductive films of the FTO/VO2/FTO structure. The temperature dependence of the I–V hysteresis loop for the FTO/VO2/FTO structure is shown in Fig. 5. The threshold voltage is 9.2 V at 20 °C. The threshold voltage decreases with increasing temperature. An abrupt change in the current can be seen when applying the voltage on the VO2 thin film at 20 °C, which is also an important characteristic of the VO2 thin
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film phase transition [18,19]. When the reverse voltage decreases, the current hysteresis effect is observed. The resistance of the structure also decreases with increasing temperature. The VO2 thin film contains a large number of grains, and each grain has a threshold voltage. Only when the voltage of the grain reaches the threshold voltage does the electrochromic phase transition occur. Another explanation is that the requirement of constant threshold current density implies that a certain bulk carrier density is needed for the phase transition. This phenomenon is similar to the Mott–Hubbard transition, where higher carrier density than the Mott criteria gives rise to an insulator to metal transition [20]. If the carrier density reaches a certain value, an applied voltage can lead to the formation of a stable conduction path. The optical transmittance spectra of the FTO/VO2/FTO structure are respectively presented in Fig. 6 when the VO2 thin films were subjected to voltages of 0 V, 4 V, 6 V, 8 V, and 10 V at temperatures of 20 °C, 40 °C, 60 °C, and 80 °C. The limiting factor for the optical transmittance at wavelengths above 2200 nm is the transmittance of the FTO films. Fig. 6(a) shows the transmittance of the FTO/VO2/ FTO structure with various applied voltages at 20 °C. The transmittance of the structure shows little change at visible wavelengths. However, the transmittance decreases with increasing applied voltage at infrared wavelengths. Fig. 6(b) is substantially similar to Fig. 6(a); the transmittance changes little when applying a voltage. Data from Fig. 6(c) indicate that the transmittance of the structure clearly decreases. Comparing Fig. 6(d) and (c), it can be seen that the transmittance of this structure changes little. When the applied voltage reaches the threshold voltage, the transmittance change is large. The change in optical transmittance is found to
Fig. 6. Optical transmittance spectra of the FTO/VO2/FTO structure.
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be 27.5% with temperatures ranging from 20 °C to 80 °C at the wavelength of 1200 nm when the VO2 thin films are not subjected to an applied voltage. The change in optical transmittance is 26.6% at 20 °C with voltages changing from 0 V to 10 V. The change in optical transmittance is 2.5% at 80 °C with voltages changing from 0 V to 10 V. The maximum modulation value of the FTO/VO2/FTO structure is 31.4% at the wavelength of 1200 nm under temperatures ranging from 20 °C to 80 °C and voltages between 0 V and 10 V. With applied voltage, the carrier concentration of the VO2 thin film increases and the migration rate increases. When light is incident on the film, the probability of photon collisions with carriers increases. It is concluded that the carrier concentration of the VO2 thin film promotes the phase transition. 4. Conclusion A VO2 thin film was prepared by a DC magnetron sputtering method on F-doped SnO2 conductive glass. Then, a FTO/VO2/FTO structure was fabricated using photolithography and a chemical etching process. An abrupt change in the current can be seen by applying a voltage on the VO2 thin film. The threshold voltage is 9.2 V at 20 °C and decreases with increasing temperature. Compared with the no-voltage situation, the maximum change in the optical transmittance of the FTO/VO2/FTO structure under the effect of voltage is up to 31.4% before and after transition. This structure shows stable performance for easy integration and miniaturization. The results indicate that the FTO/VO2/FTO structure can be used in a new generation of optoelectronic devices. Conflict of interest There is no conflict of interest. Acknowledgments This work is partly supported by the National High Technology Research and Development Program of China (Grant No. 2006AA03Z348), the Foundation for Key Program of Ministry of Education China (Grant No. 207033), the Key Science and Technology Research Project of Shanghai Committee, China (Grant No. 10ZZ94), the Shanghai Talent Leading Plan, China (Grant No. 2011-026). References [1] F.J. Morin, Oxides which show a metal-to-insulator transition at the Neel temperature, Phys. Rev. Lett. 3 (1959) 34–36.
[2] M.M. Qazilbash, K.S. Burch, D. Whisler, D. Shrekenhamer, B.G. Chae, H.T. Kim, D.N. Basov, Correlated metallic state of vanadium dioxide, Phys. Rev. B 74 (2006) 3840–3845. [3] S. Chen, H. Ma, X.J. Yi, H.C. Wang, X. Tao, M.X. Chen, X.W. Li, C.J. Ke, Optical switch based on vanadium dioxide thin films, Infrared Phys. Technol. 45 (2004) 239–242. [4] D. Ruzmetov, G. Gopalakrishnan, J. Deng, V. Narayanamurti, S. Ramanathan, Electrical triggering of metal-insulator transition in nanoscale vanadium oxide junctions, J. Appl. Phys. 106 (2009). pp. 083702–083702-5. [5] D. Matsunami, A. Fujita, Electrocaloric effect of metal-insulator transition in VO2, Appl. Phys. Lett. 106 (2015) 042901. [6] C.H. Chen, Z.Y. Fan, Changes in VO2 band structure induced by charge localization and surface segregation, Appl. Phys. Lett. 95 (2009). pp. 262106– 262106-3. [7] Y. Zhou, X. Chen, C. Ko, Z. Yang, C. Mouli, S. Ramanathan, Voltage-triggered ultrafast phase transition in vanadium dioxide switches, IEEE Electron Device Lett. 34 (2013) 220–222. [8] M. Dragoman, A. Cismaru, H. Hartnagel, R. Plana, Reversible metalsemiconductor transitions for microwave switching applications, Appl. Phys. Lett. 88 (2006). pp. 073503–073503-3. [9] J. Cao, Y. Gu, W. Fan, L.Q. Chen, D.F. Ogletree, K. Chen, N. Tamura, M. Kunz, C. Barrett, J. Seidel, J. Wu, Extended mapping and exploration of the vanadium dioxide stress-temperature phase diagram, Nano Lett. 10 (2010) 2667–2673. [10] B.Y. Fang, Y. Li, G.X. Tong, X.H. Wang, M. Yan, Q. Liang, F. Wang, Y. Qin, J. Ding, S.J. Chen, J.K. Chen, H.Z. Zheng, W.R. Yuan, Optical properties of vanadium dioxide thin film in nanoparticle structure, Opt. Mater. 47 (2015) 225–230. [11] G. Stefanovich, A. Pergament, D. Stefanovich, Electrical switching and Mott transition in VO2, J. Phys.: Condens. Matter 12 (2000) 8837–8845. [12] S. Kittiwatanakul, J. Lu, S.A. Wolf, Transport anisotropy of epitaxial VO2 films near the metal–semiconductor transition, Appl. Phys. Express 4 (2011) 544– 548. [13] G.X. Tong, Y. Li, F. Wang, Y.Z. Huang, B.Y. Fang, X.H. Wang, H.Q. Zhu, L. Li, Y.J. Shen, Q.X. Zheng, Q. Liang, M. Yan, Y. Qin, J. Ding, Thermal oxidation-grown vanadium dioxide thin films on FTO (Fluorine-doped tin oxide) substrates, Infrared Phys. Technol. 61 (2013) 37–41. [14] T.D. Cheng, N.J. Zhou, P. Li, Ferroelectric and photoelectricity properties of (Pb0. 52Zr0. 48) TiO3 thin films fabricated on FTO glass substrate, J. Mater. Sci.: Mater. Electron. 26 (2015) 7104–7108. [15] E.H. Jeon, S. Yang, Y. Kim, N. Kim, H.J. Shin, J. Baik, H.S. Kim, H. Lee, Comparative study of photocatalytic activities of hydrothermally grown ZnO nanorod on Si (0 0 1) wafer and FTO glass substrates, Nano. Res. Lett. 10 (2015) 1–8. [16] M. Guo, K. Zhang, X. Zhu, H. Li, Optical losses of CdS films on FTO, ITO, and AZO electrodes in CdTe–HgCdTe tandem solar cells, J. Mater. Sci.: Mater. Electron. 26 (2015) 7607–7613. [17] N. Shen, Y. Li, X.J. Yi, Preparation of VO2 films with nanostructure and improvement on its visible transmittance, Infrared Milli. Waves 25 (2006) 199–202. [18] J. Leroy, A. Crunteanu, A. Bessaudou, F. Cosset, C. Champeaux, J.C. Orlianges, High-speed metal-insulator transition in vanadium dioxide films induced by an electrical pulsed voltage over nano-gap electrodes, Appl. Phys. Lett. 100 (2012). pp. 213507–213507-4. [19] A. Crunteanu, J. Givernaud, J. Leroy, D. Mardivirin, C. Champeaux, J.C. Orlianges, A. Catherinot, P. Blondy, Voltage-and current-activated metal– insulator transition in VO2-based electrical switches: a lifetime operation analysis, Sci. Technol. Adv. Mater. 11 (2010) 065002. [20] C.R. Cho, S.I. Cho, S. Vadim, R. Jung, I. Yoo, Current-induced metal–insulator transition in VOx thin film prepared by rapid-thermal-annealing, Thin Solid Films 495 (2006) 375–379.