DOI:10.30919/esmm5f607

ES Materials & Manufacturing, 2019, 6, 62-67

Published online: 17 Dec 2019

Received 08 Dec 2019, Accepted 16 Dec 2019

Temperature-dependent Optical Characterization of VO2 Thin Film Prepared from Furnace Oxidation Method

Sydney Taylor, Jeremy Chao, Linshuang Long, Niko Vlastos and Liping Wang*

School for Engineering of Matter, Transport & Energy,

Arizona State University, Tempe, Arizona 85287, USA

* Corresponding author: [email protected] (L. Wang)

ABSTRACT

Thermochromic vanadium dioxide (VO2) thin films have been deposited on quartz substrates via a two-step furnace oxidation method. First pure metallic vanadium thin films are deposited on quartz substrates using electron beam evaporation. Then the vanadium films are oxidized in an atmospheric tube furnace to produce stoichiometric VO2. X-ray diffraction measurements confirm the composition of the prepared films. Temperature-dependent spectral transmittance measurements within the optical range reveal a 57% change upon transition at a wavelength of λ = 2.5 µm, indicating good potential for energy applications. The heating and cooling behaviors of the VO2 thin films on quartz are also investigated, and the VO2 transitions from an insulator to a metal at 345 K upon heating. There is approximately 20 K of hysteresis between the heating and cooling curves. The dielectric constants of the furnace oxidized VO2 for both the insulating and metallic phases are fitted to a dispersion model for the visible and near-infrared regime. Excellent thermal stability of the fabricated VO2 thin film from cryogenic to high temperatures is shown with in-situ optical measurements. 


Table of Content

Temperature-dependent optical properties of VO2 thin film prepared by furnace oxidation method are experimentally characterized and theoretically modelled.

 

 

 


Keywords: Vanadium dioxide; Thermochromic; Optical characterization; Property fitting


1. Introduction

Vanadium dioxide (VO2) is a thermochromic material that undergoes a dramatic change in optical properties near its insulator-to-metal phase transition temperature of 68 °C.1 At temperatures below its transition, VO2 is a monoclinic insulator, whereas at temperatures above its transition the VO2 is a rutile metal. Due to its considerable shift in optical properties upon phase transition, VO2 has been investigated for a variety of tunable devices in the near-infrared and mid-infrared wavelength regimes, including smart windows,2-7 absorbers/emitters,8,9 spacecraft thermal control coatings,10,11 and thermal diodes.12  In nearly every near-IR application, a large change in the VO2 transmittance is desired to facilitate the tunable behavior.  For example, smart window coatings based on VO2 seek to have high near-infrared transmittance when the temperature is low to allow sunlight to pass through the window. On the other hand, when the temperature is high, the near-infrared transmittance should be comparably lower to prevent heating of the room. The properties of the VO2 in the visible spectrum are also of interest for many tunable devices, including visible transmittance for smart windows and solar absorption for thermal control devices.2,13

Given the multitude of potential applications for VO2 in the visible and near-infrared ranges, it is critically important to develop a facile way to fabricate VO2 films with a high degree of thermochromism. VO2 is commonly fabricated via magnetron sputtering,6,14-16 which requires careful control of the chamber conditions, including substrate temperature, oxygen and argon partial pressures, and power. Another challenge is that the VO2 is typically reactively sputtered or post-annealed at high temperatures, typically greater than 450 °C, however significant progress has been made recently on the low temperature fabrication of VO2 films. Malarde et al. fabricated VO2 on floating glass substrates via APCVD deposition which showed a 58% change in transmittance at a wavelength of λ = 2.5 µm for smart window applications.17 Similarly, Taha et al. also fabricated VO2 on several substrates using DC magnetron sputtering and were able to achieve a 50% change in transmittance for their VO2 on quartz samples at λ = 2 µm.18 Chang et al. also fabricated Cr2O3/VO2 bilayers via a low-temperature DC sputtering process with a V2O3 ceramic target which achieved a ~55% change at λ = 2 µm.19 In addition to the development of new fabrication techniques, the optical properties of the fabricated thin film VO2 need to be extracted in each relevant wavelength to accurately predict the performance of designed devices. The behavior of VO2 at cryogenic and high-temperatures, is also of interest for a variety of space 2 applications, including spacecraft thermal control and optical propulsion.

In the present work, vanadium dioxide thin films are prepared on quartz substrates using a simple two-step thermal oxidation technique.20 The film composition is investigated using X-ray diffraction (XRD) and Raman spectroscopy, and the temperature-dependent visible and near-infrared transmittance is measured to provide a comparison to VO2 fabricated via other methods. Moreover, the heating and cooling curves of the VO2 thin film on quartz are characterized to illustrate the transition temperature, transition width, and hysteresis characteristics of the fabricated VO2 thin films. Models of the dielectric constants in the visible and near-IR regimes are presented for VO2 at both the insulating and metallic phases. Finally, the optical behavior of the VO2 at cryogenic and high temperatures is investigated to elucidate the thermal stability of the fabricated films. This furnace oxidization technique along with the optical property models will be useful for developing VO2 based tunable optical coatings.


2. Sample Preparation and Characterization

A 60-nm-thick VO2 thin film was fabricated on a 0.75” diameter, 0.50-mm-thick double side polished Z-cut quartz wafer (Precision Micro-Optics). First, 30-nm-thick pure vanadium was deposited onto the quartz substrate using electron beam evaporation (Lesker PVD75 Electron Beam Evaporator) from 99.99% pure 1/4” diameter 1/4” long vanadium pellets (Kurt. J. Lesker Co.). The deposition rate and chamber pressure were maintained at 0.7 Å/s and 3 × 10-6 Torr, respectively, during the deposition process. After the pure metal deposition, the precursor film was oxidized at 300 °C for 3 hours in a Thermco Minibrute ambient pressure tube furnace to produce a 60-nm-thick VO2 thin film. Throughout the oxidation process, a mixture of 0.5 SLPM of pure O2 and a 60 SLPM N2 was flowed through the quartz tube. After the oxidation, the sample was allowed to cool in a pure N2 purge flow for 10 minutes in the cold end of the furnace. The composition change of the thin film from metallic vanadium to insulating VO2 is accompanied by a visual color change from grey to yellow. Fig. 1(a) shows a photo of an unoxidized 30-nm-thick pure vanadium thin film on quartz substrate (left) before oxidation, which appears metallic grey with opaqueness, and that of vanadium dioxide film on quartz substrate (right) after oxidation, which looks yellow with good transparency to the text “VO2” underneath it.

The composition of the prepared thin film sample was characterized by XRD and Raman spectroscopy. Fig. 1(b) shows the XRD pattern for VO2 prepared on quartz substrate, where the peaks at the 2q  angles  of 28°, 37°, 43°, 56°, and 65° are typical of polycrystalline vanadium dioxide.21 There are no additional XRD peaks indicating the presence of other vanadium oxides, such as vanadium pentoxide (V2O5) or vanadium sesquioxide (V2O3), which do not contribute to the thermochromic switch of the fabricated thin film. Similarly, the Raman spectrum of the fabricated film as shown in Fig. 1(c) was also measured with a 488 nm laser source and 100× objective (Renishaw InVia spectroscopy system), which was also consistent with  polycrystalline VO2.20 The thin film thicknesses of both the pure V metal (30 nm) and the VO2 (60 nm) were determined using a contact profilometer (Bruker Dektak) to measure the height of a step prepared on the quartz substrate during the sample fabrication before and after the furnace oxidation process.

Fig. 1 (a) Photos of the pure vanadium metal film on a quartz substrate (left) and the yellow oxidized vanadium dioxide thin film on a quartz substrate (right). (b) XRD pattern and (c) Raman spectrum for the oxidized film with typical peaks for vanadium dioxide (VO2).

Temperature-dependent spectroscopic characterizations have been successfully used to study composition and optical properties of materials such as AlxGa1-xN alloy from 300 K to 823 K22 as well as to directly measure spectral emissivity for experimentally demonstrating high-temperature polaritonic radiation from ceramic nanotube antennas23 and coherent thermal emission from grating metamaterials due to magnetic polaritons.24 Here, temperature-dependent spectral transmittance of VO2 thin film at normal incidence was measured by a Fourier-transform spectrometer (Thermo Scientific, Nicolet iS50) with extended spectrum from mid-infrared to near-infrared and visible range enabled by a white light source and a quartz beam splitter. A silicon detector was used for the wavelengths from 400 nm to 1 mm, while a DTGS detector captured the spectra at longer wavelengths from 1 to 2.5 mm, with each spectral measurement averaged over 32 scans at a resolution of 16 cm-1. The sample was mounted on the cold finger inside a variable-temperature cryostat (Janis, VPF-800-FTIR) optically coupled through quartz windows with the spectrometer, and the sample temperature was accurately controlled in a wide range from 77 K to 800 K by a temperature controller (Lakeshore 335) along with liquid nitrogen and a built-in heater. Throughout the measurement the cryostat chamber was maintained at a vacuum pressure less than 1 × 10-4 Torr.

Fig. 2(a) shows the transmittance at room temperature when the VO2 is insulating, and also at 100 °C when the VO2 is metallic. Both the metallic and insulating VO2 have 40-50% transmittance in the visible wavelengths, with the transmittance of the metallic phase 7% greater than the insulating phase. In the longer wavelengths, the fabricated VO2 achieved a transmittance change of 57% at λ = 2.5 µm. This large change in near-infrared transmittance suggests that the furnace oxidized VO2 would have considerable potential as a switching material for near-IR device applications. To investigate the nature of the insulator to metal phase transition of the furnace oxidized VO2 sample, the heating and cooling curves were also measured. The in-situ temperature-dependent transmittance spectra were taken incrementally between room temperature and 370 K. After each set-point temperature was reached, 15 minutes were allotted for the sample temperature to stabilize at the set-point. Fig. 2(b) shows the heating and cooling curves at a wavelength of λ = 2.5 µm for the VO2 thin film fabricated on quartz. The transition temperature was determined as the temperature at which the transmittance reaches the midpoint of the transmittance curve, i.e., 45%. Therefore, the heating curve shows a typical VO2 transition temperature of 345 K, while the cooling curve shows a considerably lower transition temperature of 325 K, yielding a hysteresis of 20 K.

Fig. 2 (a) Temperature-dependent spectral transmittance of the fabricated VO2 thin film on quartz in the visible and near-infrared wavelength ranges. (b) Heating and cooling transmittance curves for the fabricated VO2 at the wavelength λ = 2.5 μm.


3. Dielectric Models of Fabricated VO2 Thin Films for both Phases

The dielectric constants for both the insulating and metallic VO2 are fitted to a dispersion model25 via a least-squares method by minimizing the objective function as:

                                                    (1)

where Texp is the experimentally measured transmittance data at each wavelength with index k and Ttheo is the theoretical transmittance calculated by considering the sample as a thin film on an incoherent slab with ray-tracing optics:26,27

                                                                       (2)

where τ is the internal transmittance of the slab, τa is the transmittance from the air through the thin film layer, τs is the transmittance between the substrate and air below, ρs is the air-substrate reflectance, and ρb is the reflectance for the slab-air interface. τa and ρs are determined from Fresnel’s equations, while τand ρb  are determined from thin-film optics.26 The optical properties of the quartz substrate are taken from Palik’s Handbook,28 while the VO2 optical properties are fitted to a Drude-Lorentz dispersion model:25

                                                                           (3)

where εis the high-frequency dielectric constant, ωn is the density parameter and ωc is the collision frequency for free carriers. Sj, ωj, and γj are respectively the high-frequency dielectric constant, phonon strength, phonon frequency, and damping coefficient for each oscillator. Four oscillators were used for the Lorentz term to achieve a satisfactory fit for both the metallic VO2 and insulating VO2. The least-squares parameter F was minimized according to ε, ωn, ωc, ωj, Sj, and γj using a genetic algorithm routine subject to bounds for each parameter. εwas constrained to be between -10 and 10, ωn  and ωc were limited between 500 and 30000, and Sj and γj were set to be between 0 and 10. The phonon frequency was bounded between 500 and 30000 cm-1.

The fitted parameters for both the insulating and metallic VO2 are summarized in Tables 1 and 2, respectively. Fig. 3(a) and 3(b) show the comparison of the experimental data from optical measurements and the calculation results from the fitted model the spectral transmittance for wavelengths from 0.4 to 2.5 mm for the insulating and metallic phases, respectively, where excellent agreement is clearly seen. The real and imaginary parts of the dielectric constants calculated from the dispersion model are shown in Fig. 3(c) and 3(d) for both insulating and metallic VO2. Note that the optical properties of VO2 strongly depend on its morphology from different fabrication methods,25 and hence the dielectric constants should be determined based on the optical spectroscopic measurements for accurate modelling to facilitate the design of novel optical devices based on VO2 films fabricated using this simple furnace oxidation process.

Fig. 3 (a) Comparison between the fitted model (solid black) and the experimental data for insulating VO2 (dashed red). (b) Comparison between the  fitted model (black) and the experimental data for metallic VO2 (dashed red). (c) Real part of the dielectric function for insulating and metallic VO2 . (d) Imaginary part of the dielectric function for insulating and metallic VO2.

Fig. 4 In-situ optical spectroscopic transmission measurements of 60-nm-thick VO2 thin film on quartz substrate from cryogenic to high temperatures in the wavelengths from 0.4 mm to 2.5 mm. Note that the VO2 is in the insulating phase from 77 K to 300 K, and in the metallic phase from 373 K to 750 K.

Table 1 Model Parameters Fitted for Insulating VO2.

Insulating VO2 Properties

ε

ωc

ωn

Oscillator

Index j

Sj

ωj

γj

5.16

27876

993

1

0.2711

25948

1.1436

2

0.2946

23277

0.0787

3

0.5720

16158

1.3605

4

0.7577

27127

0.7264

5

0.5052

29574

0.6017

6

0.0065

7733

0.9867

7

0.0189

24330

1.1995

8

0.6293

8652

0.7314

Table 2 Model Parameters Fitted for Metallic VO2.

Metallic VO2 Properties

ε

ωc

ωn

Oscillator

Index j

Sj

ωj

γj

5.08

 

 

 

 

 

 

 

7197

 

 

 

 

 

 

 

24410

 

 

 

 

 

 

 

1

0.06352

22043

0.6738

2

0.82578

4639

0.8577

3

0.75794

22149

0.2311

4

0.09844

21208

1.8399

5

0.85808

4715

0.4489

6

1.63554

1634

1.7125

7

0.20847

24103

0.3586

8

1.16358

4292

1.0351

 


4. Cryogenic and High-Temperature Stability of VO2 Thin Films

The thermal stability of VO2 films across a wide temperature range from cryogenic to high temperatures is of great interest for a number of applications, in particular for space applications like radiative thermal control and optical force modulation, while it has not been studied. The temperature-dependent spectral transmission provides a viable way to experimentally study the optical properties at different temperatures. First, the behavior of the insulating VO2 thin film on quartz sample at cryogenic temperatures was investigated with the customized cryostat coupled with the spectrometer using liquid nitrogen to cool the sample down to the lowest temperature of 77 K. With a built-in heater, the sample temperature was sequentially held at 77 K, 150 K, 250 K, and 300 K for 15 minutes at each temperature to ensure that the steady state was reached. Similarly, the VO2 on quartz sample was heated beyond the phase transition at 373 K, 450 K, 550 K, 650 K and 750 K, where VO2 is in the metallic phase, to inspect its high temperature behavior. Fig. 4 shows the temperature-dependent spectral transmittance in wavelengths from 0.4 mm to 2.5 mm for cryogenic to high temperatures. As it can be seen, the spectral transmission for the insulating VO2 exhibits barely changes with less than 3% variation from 77 K to 300 K within the visible and the near-infrared. The metallic phase behaves similarly with little changes in spectral transmission by no more than 10% at high temperatures from 373 K up to 750 K. The results clearly demonstrate the excellent thermal stability of VO2 thin films of both phases prepared from furnace oxidation method from cryogenic to high temperatures in vacuum conditions from visible to near-infrared regime. Note that there is a jump at the wavelength of 1 mm for the transmittance spectra at all temperatures. This is due to the fact that different detectors are used at wavelengths above and below 1 mm, at which both detectors have low responsivity with relatively larger error about 5%.    


5. Conclusion

In summary, the optical properties of VO2 thin film prepared by the furnace oxidation method were investigated in a wavelength range between 400 nm and 2.5 µm, which is of interest for applications such as smart windows, optical filters, and optical force coatings for spacecraft attitude control. The fabricated 60 nm VO2 on quartz films exhibited a 57% change in transmittance at a near-infrared wavelength of λ = 2.5 µm and a 7% change in transmittance in the visible wavelengths. The VO2 transition temperature was 345 K upon heating and 325 K upon cooling, yielding a hysteresis of 20 K. The dielectric constant of the insulating and metallic VO2 was fitted to a dispersion model to assist with device design with furnace fabricated VO2. Finally, the VO2 thin film was shown to be thermally stable in a wide temperature range from 77 K to 750 K. The VO2 thin film with large change in optical transmittance and excellent thermal stability achieved by the furnace oxidation process is promising as a switching material for tunable optical filters, smart windows, and thermal control applications.


Acknowledgments

This work is supported by a NASA Space Technology Research Fellowship NNX16AM63H (S.T.). We would like to thank support from the National Science Foundation (NSF) under Grant No. CBET-1454698 (L.L. and L.W.). The support from Arizona State University (ASU) Fulton Undergraduate Research Initiative Program is appreciated (J.C.). We would also like to thank the ASU NanoFab and Eyring Materials Center for access and use of their fabrication and characterization facilities supported in part by NSF under NNCI-ECCS-1542160.


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