Open Access
Add to BibSonomyAbstract
Vanadium dioxide undergoes a reversible metal-insulator phase change near 68°C. This transition is often recognized for its electronic properties; however, the concomitant change in optical properties is also attractive. One application is to exploit the optical properties within this transition region to control and tune the refractive index between its insulating and metallic states. To achieve this, we used atomic layer deposition to grow high-quality, low-temperature (≤150°C), ultrathin films of vanadium dioxide on sapphire substrates. Measurements of optical transmittance and reflectance as a function of temperature are then used to create wavelength and temperature dependent models of the complex optical refractive index. Our models create a foundation for developing vanadium dioxide as a tunable refractive index material.
© 2017 Optical Society of America
1. Introduction
Vanadium has a long history in colored glass, heat-absorbing glass, and recently as a dopant for its thermo-optic behavior, e.g., in fibers [1,2]. However, the discovery of the metal-to-insulator transition in the vanadium oxides [3] and their concomitant optical properties [4] generated interest in using the thermochromic properties of vanadium oxide for energy efficient windows [5] and later for passive thermal control in spacecraft [6].
In vanadium dioxide (VO2) a phase transition occurs near 68°C and coincides with a structural crystal change from monoclinic to tetragonal (specifically, rutile). The origin of this transition remains controversial (discussed by Park et al. [7]) as to whether the nature of the phase transition is due to a Mott transition [8], charge-transfer behavior [9], or a structural Peierls transition [10]. However, regardless of the mechanism, the phase transition causes the infrared optical properties of VO2 to change from a low-loss, semi-transparent material to a more lossy and reflective material above the transition temperature [11].
Previous research on the optical properties of VO2 have mostly used ellipsometry to calculate the complex refractive index of bulk materials or thick (>>100nm) sputtered films [6,11–16]. In addition, most prior work examined the infrared spectral range. These data prove difficult to interpret due to the challenges of using ellipsometry to study anisotropic materials as well as the nature of the (mostly) polycrystalline sputtered samples. For instance, Verleur et al. reported inconclusive data for their samples in the insulating phase within the ultraviolet and visible range, which was possibly due to the polycrystalline sample and surface traps [11]. The difficulty in comparing these different data sets led Konovalova et al. to comment that the “optical constants of VO2 films largely depend on their fabrication technology” [17].
More recently, groups are using thin films of VO2 and incorporating these thin films with optical substrates to create optical devices, such as few-micrometer-long absorption modulators [18], plasmonic modulators [19], hyperbolic and near-IR frequency-tunable metamaterials [20,21], tunable optical antennas [22], and perfect absorbers [23] to name a few. To aid this current research, we employ the new fabrication technique of atomic layer deposition (ALD) which enables high-quality, ultrathin films of VO2 [24–26]. This deposition technique produces oriented polycrystalline films as thin as a few nanometers with large area uniformity in composition and thickness at angstrom level control. Also, since it is a low temperature deposition method, films can be grown on almost any substrate which enables new areas of VO2 integration for a variety of applications. Finally, the ALD process offers conformality of the films on 3D structures or high surface area nanostructures, which is particularly useful for optical applications.
For our experiments ALD films were deposited on double-side polished sapphire to aid in optical measurements. We measured both the optical transmittance and reflectance at near-normal incidence in the visible and near-infrared region, and calculated the absorptance. Using these data, we created a model for the optical permittivity based on lossy Lorentz oscillators, and calculated the complex optical refractive index for our ALD VO2, to compare these values with previous results for films grown by other techniques. Finally, using our data we created a temperature and wavelength dependent model to use as a foundation for developing VO2 as a tunable refractive index material [27].
2. Fabrication
A thin (35nm), amorphous VO2 film was deposited on a c-plane sapphire substrate by ALD in an Ultratech Savannah 200 reactor at 150°C using tetrakis(ethylmethyl)amido vanadium and ozone precursors with optimized pulse and purge conditions that yield a growth rate of 0.9A/cycle. This particular vanadium precursor was chosen since it is already in the + 4 oxidation state and promotes the preferential formation of the VO2 phase. X-ray photoelectron spectroscopy revealed that beneath the ~1nm of atmospheric contamination, there was no residual carbon impurities and only a single VO2 peak was evident. To achieve MIT transitions, the as-deposited VO2 film was crystallized with an optimized ex-situ anneal at 560°C in 1.5x10−5 Torr of oxygen for 2 hours.
3. VO2 characterization
Atomic force microscopy shows, in Fig. 1, that the films consist of small polycrystalline grains on the order of 20-40nm in size. Still these films exhibited a roughness of only 3.2 nm which is similar to typical sputtered or PLD based films [28,29]. Raman spectroscopy was used to verify the presence of the monoclinic VO2 phase after the crystallization anneal, see Fig. 1. The narrow peaks in the Raman spectra indicate high crystalline quality. In addition, the hardening of the vanadium-vanadium low-frequency phonons (195 and 224 cm−1) and the 616cm−1 mode as well as the softening of the 390cm−1 mode signify these films have slight tensile strain [30,31]. X-ray diffraction measurements show a single VO2 peak at 39.0 degrees that independently verify the crystal quality and demonstrate the monoclinic (020) orientation is aligned with the underlying sapphire peak.
Fig. 1 (a) Atomic force microscopy of our VO2 shows crystal grain sizes on the order of 20-40nm and an RMS roughness of 3.2nm. (b) Raman spectroscopy verifies the quality of the crystals and that at room temperature they exhibit the monoclinic VO2 phase.
Next, optical measurements show, in Fig. 2, the broadband transmittance and reflectance as a function of temperature. The temperature was incrementally varied between 24 to 90 °C, and was stabilized for two minutes prior to measurement and maintained for the duration of the measurement. The measured temperature was from a thermistor mounted next to the sample on the heater stage, and was within 1 °C of the sample temperature. The light (from a tungsten lamp filtered by a scanning monochromator) was normally incident on the sample for transmittance, and at a 6 degree angle in reflectance. In these plots, changes occur in three general regions: visible (400-800nm), near-infrared (800-1100nm), and the infrared (1100-2200nm). Figure 2(a) shows that the temperature-dependent transmittance changes almost entirely in the infrared. This is more clearly illustrated in Fig. 2(c), which shows the change in transmittance as compared with the transmittance at the lowest (room) temperature. An interesting phenomenon is observed in the visible region, where the change in transmittance is of the opposite sign. While the magnitude of this change in the visible is small, this change demonstrates the same monotonic change with temperature as in the infrared region (except of the opposite sign).
Fig. 2 Spectral (a) transmittance and (b) reflectance of VO2 on c-plane Al2O3 changes monotonically as a function of temperature from 24 to 90°C (arrows indicate direction of increasing temperature). (c) The change in transmittance (T – T0)/ T0 and (d) the change in reflectance (R – R0)/ R0 highlight temperature dependent changes of VO2 by comparing to their value at room temperature (T0 and R0). The inset in (b) shows the change in transmittance near 2000 nm versus temperature to reveal a transition temperature of 61°C.
The reflectance of this sample is small, as shown in Fig. 2(b), but shows the complementary temperature-dependent changes to those in transmittance. Again, when plotting the change in reflectance, Fig. 2(d), the largest positive change occurs in the infrared. The change in reflectance exhibits similar (but opposite) behavior to the change in transmittance, with a sign reversal between properties in the infrared and visible regions. The exception is in the near-infrared region where the changing reflectance reverses sign near 1100 nm while the changing transmittance reverses sign near 800 nm, thus, the change in reflectance and transmittance both decrease in the near-infrared. Overall, the change in transmittance and reflectance (especially in the infrared region) as a function of temperature are shown by the representative trace in the inset to Fig. 2(b). From the inflection point of this curve a metal-insulator transition temperature of 61°C is determined.
Before advancing to our modeling section a few features can be noted by studying the absorptance of the entire optical structure, i.e., (1-Reflectance-Transmittance), since there is negligible absorption in the sapphire substrate as well as negligible attenuation by scattering in the high-quality film and substrate. In addition, this analysis is relevant to VO2 emittance structures, e.g., those used for passive thermal control [6]. Figure 3 displays the spectral absorptance of the sapphire-VO2 structure as a function of temperature. As the temperature is increased there is almost no change in absorptance in the visible region, shown in Fig. 3(b), except a small increase in absorptance from 440 to 520nm (2.4-2.8eV). This increased absorptance highlights the region of interband transitions between the oxygen 2p bands and the vanadium 3d bands with a Fermi level of approximately 2.5 eV [11]. In the infrared region, there is an increase in the absorptance, which is characteristic of free-carrier absorption in VO2 [11]. This increase also helps understand the seemingly odd behavior (noted earlier) in the spectral transmittance and reflectance plots in the near-infrared region. In other words, the spectral absorptance monotonically increases with wavelength, and that manifests itself differently in the spectral transmittance and reflectance.
Fig. 3 (a) The spectral absorbance of VO2 changes as a function of temperature (from 24 to 90 °C) with arrow indicating increasing temperature, as calculated from the transmittance and reflectance data. (b) The spectral absorptance normalized to the room temperature value shows the changing characteristics of VO2 with temperature and reveals subtle behavior near 500 nm.
4. Modeling the permittivity of VO2
We create a model for the optical permittivity of our ALD grown VO2 in both in the metallic and insulating phases. This phenomenological model is based on a collection of lossy Lorentz oscillators, and the complex optical refractive index is then computed from the permittivity. The formal definition of the model is:
where, ε is the complex dielectric permittivity as a function of photon energy, E. The permittivity is a function of the high frequency permittivity ε∞ plus a sum of n oscillators where An is the oscillator amplitude, En the oscillator energy, and Bn the oscillator damping.To find the transmittance and reflectance, the complex permittivity is related to the complex refractive index by: , where n and are the real and imaginary parts of the refractive index. Using this model, we calculate transmittance and reflectance data of our 35-nm VO2 on sapphire structure using a transfer matrix method. Several minimization techniques (simplex search methods like Nelder-Mead as well as differential evolution methods) were employed to achieve a good fit to the measured data.
Both our two- and three-term oscillator models provide good fits to our data (R2 from 0.96 to 0.99, except R2 = 0.91 for the reflectance of the 2-term metallic state), as shown in Fig. 4, with the model parameters listed in Table 1. A small inconsistency in the short-wavelength region of the two-term model is better fit in the three term model. The biggest challenge arises in the reflectance of the near-infrared metallic phase model and in the infrared insulating phase. Additional oscillator terms add model complexity while only marginally improving the fit in these regions.
Fig. 4 A series of complex oscillators are used to model the optical permittivity of VO2 in both the metallic (red lines) and insulating (black lines) phases. Both (a) two- and (b) three-oscillator models (solid lines) offer reasonable fits to the transmittance, reflectance, and absorptance data (open circles), however, the three-term oscillator model provides a better fit in the visible region.
Table 1. – Parameters for the two- and three-oscillator models of the permittivity of VO2 in its metallic and insulating phases
The free-carrier contribution to the permittivity primarily impacts the optical behavior in the infrared, and is addressed by adding a Drude term. In published bulk and sputtered VO2 work, the influence of the Drude term begins to appear near 800-900nm, with the impact of this term increasing with increasing wavelength into the infrared [11,16,32]. We included a Drude term in our model, however, these solutions produced poorer fits, and after error minimization techniques the Drude term was reduced to a negligible value. For this reason we tried a variety of minimization techniques from gradient-based to Nelder-Mead to differential evolution techniques along with a variety of initial conditions. These varied techniques and conditions converged upon similar solutions, and these solutions did not involve a Drude term.
The reason we do not observe a Drude contribution in our ALD VO2 could be due to a number of factors. Our recent measurements show the resistivity of similar ALD VO2 films is 820µΩ·cm, thus, more like a doped semiconductor than a metal. In addition, the metallic state of our VO2 could be such a poor metal that 1) the plasma frequency is shifted to the visible or near-IR, or 2) the collision rate (damping term) is sufficiently large, or 3) both. The Drude contribution is valid for frequencies much lower than the plasma frequency. Thus, in (1) if the plasma frequency is in the visible or near-IR the Drude contribution will be outside of our measured spectral region. For (2) if the damping rate is equal to or greater than the plasma frequency, then the Drude contribution to the dielectric function is negligible, especially in our measurement range. Even if the damping rate is a large fraction of the plasma frequency, the other oscillator terms will overpower its contribution in in our measurement range. Thus, the lack of a Drude term in our ALD VO2 permittivity is consistent with its poor metallic properties.
The complex permittivity and refractive index for our VO2 models are shown in Fig. 5, and compared with previous results for films grown by other techniques: sputtering [11,32–34] and pulsed laser deposition [18,20]. In the insulating phase (Figs. 5(a) and 5(b)), the complex permittivity and refractive index of our ALD VO2 agree well with the other fabrication methods. However, in the metallic state (Figs. 5(c) and 5(d)) our films are less lossy than those fabricated by other groups. While the absolute values of the optical constants vary between groups and fabrication methods, the spectral trends are similar in all the films. These films exhibit normal dispersion through most of the visible and near-infrared, but display anomalous dispersion in the blue (400-500nm) as well as in the infrared.
Fig. 5 Complex permittivity (a and c) and refractive index (b and d) from our model of VO2 in the insulating (a and b) and metallic phases (c and d). The plots compare our model (solid lines) with previous results for VO2 films grown by other techniques: sputtered [11,32,34]; and pulsed laser deposition [18,20].
5. Tuning the optical properties of VO2
VO2 has two phase states (insulating and metallic) each with distinct optical properties. However, the optical behavior is not binary, but instead displays a transition region between these states. This enables using VO2 as a tunable optical material [23]. To aid in the development of VO2 as a tunable refractive index material, we add temperature dependence to our spectral model to create a two-dimensional model (refractive index vs. temperature and wavelength). The premise of this new model assigns the gradual change in optical properties to a superposition of the insulating and metallic states as follows: , where is the refractive index of VO2 as a function of temperature, and are the dielectric permittivity of the insulating and metallic phases, and f(T) is a temperature-dependent function governing the distribution of insulating and metallic optical properties.
Since Fermi-Dirac statistics govern energy states in thermodynamic distributions, we choose a similar function f(T) to transition between the optical constants of the insulating and metallic phase states. In our Fermi-Dirac-like distribution we substitute: the transition temperature (Tt) for the chemical potential, and the actual temperature (T) for the state energy level. While Fermi-Dirac distributions describe behavior for energies within kBT, the transition temperature (Tt) is our fundamental energy quantity. Hence, instead of comparing to kBT we compare these to a variable W, which controls the width of the transition, times the transition temperature to create the temperature distribution: .
The model results are shown in Fig. 6 along with our experimental data for transmittance, reflectance, and absorptance. The transition temperature of our film is lower than the common 68°C (shown in Fig. 2 inset), so a model transition temperature of 60°C is used. The model shows good agreement with both the measured transmittance and reflectance. The challenging region for the model is in the low-temperature reflectance data at long wavelengths. However, the good quality of the fit produces a reliable predictor of the spectral performance of VO2 within the transition region between insulating and metallic phases. Using our models, the optical properties of VO2 can be predictably tuned by temperature, thickness, and wavelength to design optical systems that achieve static and dynamic goals.
Fig. 6 A temperature-tunable refractive index model was created for VO2. The open circles in the plot are the measured transmittance, reflectance, and absorptance at various temperatures, while the solid lines are the predicted values from our temperature- and wavelength-dependent model of VO2.
6. Conclusions
We fabricated VO2 on sapphire by atomic layer deposition, a new technique for VO2. The ALD process produced oriented polycrystalline films as thin as a few nanometers and allows us to conformally grow these films on almost any substrate at low temperatures (≤150°C). Optical transmittance and reflectance measurements in the visible through infrared characterized the spectral dependence of the optical properties of ALD VO2 as a function of temperature. These optical properties gradually change as VO2 undergoes a metal-insulator phase change, and compared with previous results for films grown by other techniques these films exhibit lower loss in the infrared region in the metallic state, suggesting our films are of high quality even at only 35-nm thickness.
In addition, we demonstrate our ability to model the spectral properties of VO2 through its metal-insulator transition by first modelling the spectral properties of both the metallic and insulating phases, and then creating a meta-model incorporating a temperature-dependent superposition of the individual metallic and insulating models.
These optical properties of VO2 make it desirable for perfect absorbers, optical modulation, antennas, and metamaterials. With two phase states (insulating and metallic) and their associated distinct optical states, VO2 has been used as bistable [35] or multistate [36] optical devices. However, the gradual transition of optical properties from one state to another establishes VO2 as a tunable refractive index material. Our new model permits prediction of the real and imaginary refractive index as a function of wavelength and temperature, and enables opportunities using VO2 as a tunable refractive index material. In addition, ultrathin VO2 films deposited by ALD on almost any substrate at low temperatures provide conformal and flexible optical coatings with a tunable refractive index that can significantly broaden the potential in optical design.
Funding
U.S. Naval Research Laboratory.
Acknowledgments
The authors gratefully acknowledge Colin Olson for his help with the minimization techniques, and especially his expertise in the differential evolution methods.
References and links
1. M. K. Davis and M. J. F. Digonnet, “Measurements of thermal effects in fibers doped with cobalt and vanadium,” J. Lightwave Technol. 18(2), 161–165 (2000). [CrossRef]
2. Z. Matjasec, S. Campelj, and D. Donlagic, “All-optical, thermo-optical path length modulation based on the vanadium-doped fibers,” Opt. Express 21(10), 11794–11807 (2013). [CrossRef] [PubMed]
3. F. J. Morin, “Oxides which show a metal-to-insulator transition at the neel temperature,” Phys. Rev. Lett. 3(1), 34–36 (1959). [CrossRef]
4. A. S. Barker, H. W. Verleur, and H. J. Guggenheim, “Infrared optical properties of vanadium dioxide above and below the transition temperature,” Phys. Rev. Lett. 17(26), 1286–1289 (1966). [CrossRef]
5. S. M. Babulanam, T. S. Eriksson, G. A. Niklasson, and C. G. Granqvist, “Thermochromic VO2 films for energy-efficient windows,” Sol. Energy Mater. 16(5), 347–363 (1987). [CrossRef]
6. M. Benkahoul, M. Chaker, J. Margot, E. Haddad, R. Kruzelecky, B. Wong, W. Jamroz, and P. Poinas, “Thermochromic VO2 film deposited on Al with tunable thermal emissivity for space applications,” Sol. Energy Mater. Sol. Cells 95(12), 3504–3508 (2011). [CrossRef]
7. J. H. Park, J. M. Coy, T. S. Kasirga, C. Huang, Z. Fei, S. Hunter, and D. H. Cobden, “Measurement of a solid-state triple point at the metal-insulator transition in VO2,” Nature 500(7463), 431–434 (2013). [CrossRef] [PubMed]
8. N. F. Mott, “The basis of the electron theory of metals, with special reference to the transition metals,” Proc. Phys. Soc. A 62(7), 416–422 (1949). [CrossRef]
9. J. Zaanen, G. A. Sawatzky, and J. W. Allen, “Band gaps and electronic structure of transition-metal compounds,” Phys. Rev. Lett. 55(4), 418–421 (1985). [CrossRef] [PubMed]
10. R. M. Wentzcovitch, W. W. Schulz, and P. B. Allen, “VO2: Peierls or Mott-Hubbard? A view from band theory,” Phys. Rev. Lett. 72(21), 3389–3392 (1994). [CrossRef] [PubMed]
11. H. W. Verleur, A. S. Barker, and C. N. Berglund, “Optical Properties of VO2 between 0.25 and 5 eV,” Phys. Rev. 172(3), 788–798 (1968). [CrossRef]
12. F. Guinneton, L. Sauques, J.-C. Valmalette, F. Cros, and J.-R. Gavarri, “Optimized infrared switching properties in thermochromic vanadium dioxide thin films: role of deposition process and microstructure,” Thin Solid Films 446(2), 287–295 (2004). [CrossRef]
13. A. Lafort, H. Kebaili, S. Goumri-Said, O. Deparis, R. Cloots, J. De Coninck, M. Voué, F. Mirabella, F. Maseri, and S. Lucas, “Optical properties of thermochromic VO2 thin films on stainless steel: Experimental and theoretical studies,” Thin Solid Films 519(10), 3283–3287 (2011). [CrossRef]
14. M. M. Qazilbash, M. Brehm, B.-G. Chae, P.-C. Ho, G. O. Andreev, B.-J. Kim, S. J. Yun, A. V. Balatsky, M. B. Maple, F. Keilmann, H.-T. Kim, and D. N. Basov, “Mott transition in VO2 revealed by infrared spectroscopy and nano-imaging,” Science 318(5857), 1750–1753 (2007). [CrossRef] [PubMed]
15. S. Cueff, D. Li, Y. Zhou, F. J. Wong, J. A. Kurvits, S. Ramanathan, and R. Zia, “Dynamic control of light emission faster than the lifetime limit using VO2 phase-change,” Nat. Commun. 6, 8636 (2015). [CrossRef] [PubMed]
16. H. Kakiuchida, P. Jin, S. Nakao, and M. Tazawa, “Optical properties of vanadium dioxide film during semiconductive–metallic phase transition,” Jpn. J. Appl. Phys. 46(5), L113–L116 (2007). [CrossRef]
17. O. P. Konovalova, A. I. Sidorov, and I. I. Shaganov, “Interference systems of controllable mirrors based on vanadium dioxide for the spectral range 06-106 micrometer,” J. Opt. Technol. 66(5), 391 (1999). [CrossRef]
18. R. M. Briggs, I. M. Pryce, and H. A. Atwater, “Compact silicon photonic waveguide modulator based on the vanadium dioxide metal-insulator phase transition,” Opt. Express 18(11), 11192–11201 (2010). [CrossRef] [PubMed]
19. L. A. Sweatlock and K. Diest, “Vanadium dioxide based plasmonic modulators,” Opt. Express 20(8), 8700–8709 (2012). [CrossRef] [PubMed]
20. M. J. Dicken, K. Aydin, I. M. Pryce, L. A. Sweatlock, E. M. Boyd, S. Walavalkar, J. Ma, and H. A. Atwater, “Frequency tunable near-infrared metamaterials based on VO2 phase transition,” Opt. Express 17(20), 18330–18339 (2009). [CrossRef] [PubMed]
21. H. N. S. Krishnamoorthy, Y. Zhou, S. Ramanathan, E. Narimanov, and V. M. Menon, “Tunable hyperbolic metamaterials utilizing phase change heterostructures,” Appl. Phys. Lett. 104(12), 121101 (2014). [CrossRef]
22. S. K. Earl, T. D. James, T. J. Davis, J. C. McCallum, R. E. Marvel, R. F. Haglund Jr, and A. Roberts, “Tunable optical antennas enabled by the phase transition in vanadium dioxide,” Opt. Express 21(22), 27503–27508 (2013). [CrossRef] [PubMed]
23. M. A. Kats, D. Sharma, J. Lin, P. Genevet, R. Blanchard, Z. Yang, M. M. Qazilbash, D. N. Basov, S. Ramanathan, and F. Capasso, “Ultra-thin perfect absorber employing a tunable phase change material,” Appl. Phys. Lett. 101(22), 221101 (2012). [CrossRef]
24. P. A. Premkumar, M. Toeller, I. P. Radu, C. Adelmann, M. Schaekers, J. Meersschaut, T. Conard, and S. V. Elshocht, “Process study and characterization of VO2 thin films synthesized by ALD using TEMAV and O3 precursors,” ECS J. Solid State Sci. Technol. 1(4), P169–P174 (2012). [CrossRef]
25. G. Rampelberg, D. Deduytsche, B. De Schutter, P. A. Premkumar, M. Toeller, M. Schaekers, K. Martens, I. Radu, and C. Detavernier, “Crystallization and semiconductor-metal switching behavior of thin VO2 layers grown by atomic layer deposition,” Thin Solid Films 550, 59–64 (2014). [CrossRef]
26. A. P. Peter, K. Martens, G. Rampelberg, M. Toeller, J. M. Ablett, J. Meersschaut, D. Cuypers, A. Franquet, C. Detavernier, J.-P. Rueff, M. Schaekers, S. Van Elshocht, M. Jurczak, C. Adelmann, and I. P. Radu, “Metal-insulator transition in ALD VO2 ultrathin films and nanoparticles: morphological control,” Adv. Funct. Mater. 25(5), 679–686 (2015). [CrossRef]
27. M. A. Kats, R. Blanchard, S. Zhang, P. Genevet, C. Ko, S. Ramanathan, and F. Capasso, “Vanadium dioxide as a natural disordered metamaterial: perfect thermal emission and large broadband negative differential thermal emittance,” Phys. Rev. X 3(4), 041004 (2013). [CrossRef]
28. D. Fu, K. Liu, T. Tao, K. Lo, C. Cheng, B. Liu, R. Zhang, H. A. Bechtel, and J. Wu, “Comprehensive study of the metal-insulator transition in pulsed laser deposited epitaxial VO2 thin films,” J. Appl. Phys. 113(4), 043707 (2013). [CrossRef]
29. P. Jin, K. Yoshimura, and S. Tanemura, “Dependence of microstructure and thermochromism on substrate temperature for sputter-deposited VO2 epitaxial films,” J. Vac. Sci. Technol. Vac. Surf. Films 15(3), 1113–1117 (1997). [CrossRef]
30. J. M. Atkin, S. Berweger, E. K. Chavez, M. B. Raschke, J. Cao, W. Fan, and J. Wu, “Strain and temperature dependence of the insulating phases of VO2 near the metal-insulator transition,” Phys. Rev. B 85(2), 020101 (2012). [CrossRef]
31. G. I. Petrov, V. V. Yakovlev, and J. Squier, “Raman microscopy analysis of phase transformation mechanisms in vanadium dioxide,” Appl. Phys. Lett. 81(6), 1023–1025 (2002). [CrossRef]
32. M. Tazawa, P. Jin, and S. Tanemura, “Optical constants of V1-xWxO2 Films,” Appl. Opt. 37(10), 1858–1861 (1998). [CrossRef] [PubMed]
33. E. N. Fuls, D. H. Hensler, and A. R. Ross, “Reactively sputtered vanadium dioxide thin films,” Appl. Phys. Lett. 10(7), 199–201 (1967). [CrossRef]
34. A. Joushaghani, “Micro- and Nano-scale Optoelectronic Devices Using Vanadium Dioxide,” Thesis (2014).
35. M. Nakano, K. Shibuya, D. Okuyama, T. Hatano, S. Ono, M. Kawasaki, Y. Iwasa, and Y. Tokura, “Collective bulk carrier delocalization driven by electrostatic surface charge accumulation,” Nature 487(7408), 459–462 (2012). [CrossRef] [PubMed]
36. N. Davila, R. Cabrera, and N. Sepulveda, “Programming and projection of near IR images using films,” IEEE Photonics Technol. Lett. 24(20), 1830–1833 (2012). [CrossRef]
