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Abstract
This work investigated the effect of sodium (Na) and potassium (K) ions permeation on optical properties of vanadium dioxide (VO2) deposited on three different glass substrates: silica-soda-lime (SL), silica-potash-soda (PS) and fused quartz (FQ), respectively. The VO2 thin films were prepared by reactive magnetron sputtering. Microstructure and compositions were determined by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Morphology of the films was characterized by atomic force microscopy (AFM), and the optical properties of the films, including refractive index and extinction coefficient, were characterized by spectroscopic ellipsometry (SE). Results show that the optical properties of the VO2 films grown on different substrates exhibited different dispersion trends at room temperature, which could be attributed to the penetration of Na and K ions. In addition, the influence of temperature on the optical properties of VO2 thin films was also studied by varying temperature ellipsometry. Below the phase transition temperature, the peak of the refractive index showed a clear blue shift with increasing temperature; above the phase transition temperature, the peak of refractive index also showed a blue shift, which was different from the trend below the phase transition temperature. This phenomenon indicates that the VO2 film has undergone a phase change.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In 1959, F.J. Morin discovered the first-order metal-to-semiconductor phase transition in vanadium dioxide (VO2). This phase transition was reversible across a critical temperature, 𝜏𝑐, around 341 K [1]. Below 𝜏𝑐, VO2 has a monoclinic structure, and above 𝜏𝑐 it converts to a rutile tetragonal structure [2]. In addition, the conductivity, magnetic susceptibility, light absorption, refractive index, and the specific heat capacity of VO2 could all change with temperature [3–7]. For a VO2/glass system, the transmittance of the glass could be controlled by varying the temperature. Thus, VO2 thin films can be widely applied for military and civilian needs [8–10]. Especially in recent years, application of VO2 thin films in smart window has attracted increasingly more attention [11–14].
VO2 films were usually deposited on fused silica (high purity silica) in previous studies [15–19]. However, due to the need for smart window research, it is also meaningful to study the behavior of VO2 thin films on other substrates. In recent years, depositing VO2 on other substrates has become increasingly popular. The temperature-driven behaviors of VO2 thin films, such as the phase transition, and the optical properties rely heavily on the substrate and a great quantity of investigation has been carried out [20–23]. However, there is a lack of research on the effect of ion infiltration on VO2 optical properties.
In this paper, VO2 films were deposited on three different glass substrates by magnetron sputtering. The structure, composition, morphology and optical properties of the films were characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and spectroscopic ellipsometry (SE). The experimental results were used to investigate the influence of the ion penetration on the optical properties of the VO2 films. Moreover, the optical properties of the VO2 films under changing temperature conditions were also studied.
2. Experimental methods
2.1 Film growth
VO2 films were deposited by reactively sputtering a 99.9% pure vanadium target (50.8 mm diameter × 6.35 mm thickness) using an Orion 5 UHV magnetron sputtering system (AJA International, Inc.). Three different kinds of glass substrates were used: silica-soda-lime (SL), silica-potash-soda (PS) and fused quartz (FQ). The thicknesses of the SL, PS and FQ substrates were 1.0 mm, 0.5 mm and 1.5 mm, respectively. The films were deposited while maintaining a substrate temperature of 475 K. The deposition power was about 100 W, the absolute deposition pressure was 0.47 Pa, and the deposition time was 21 min. Following the deposition, samples were annealed in quiescent air at 740K for 30 mins using a Barnstead Thermolyne 47900 furnace [24,25].
2.2 Surface morphology, structure and composition
Film surface topographies were characterized by AFM (Naio AFM, Nanosurf). In addition, the lattice structure of VO2 thin films were measured by XRD (D8ADVANCE) with X-ray wavelength λ = 1.51 Å. And the composition analysis was conducted by XPS (ESCALAB 250) and EDX (S-4800).
2.3 SE measurements
SE is a non-destructive method that can determine the optical constants of materials. It is based on the change of the polarization states psi (Ψ) and delta (Δ) of the light reflected from the measurement surface and the interface.
In this study, SE measurements were performed with a step of 0.6 nm in the spectral range of 400-800 nm (3.1-1.55 eV) using a rotary compensator ellipsometry (SE-VE Spectroscopic Ellipsometry) equipped with a Linkam heating system. The ellipsometry was supplied with 65 degrees of continuous spectral radiation. Firstly, these samples were measured by SE at room temperature. Then, the samples were mounted on a stress-free high temperature stage, with the temperature controlled between 300 K and 675 K with the help of a thermocouple. Nine temperature points were measured for each sample from 300 K to 675 K and experimental spectra were recorded based on the monitored sample surface temperature.
3. Experimental results and analysis
3.1 Surface morphology
AFM is a powerful tool to analyze thin film surface topography. The 3D images of VO2 thin films that grown on different substrates measured by AFM are shown in Fig. 1. The roughness of the surface is calculated based on the arithmetic square root (Ra) of AFM and listed in Table 1.
Fig. 1 3D topographical images of the VO2 film surfaces on three different substrates by AFM measurement: (a) SL, (b) PS and (c) FQ.
Table 1. The roughness data measured by AFM (Ra)
3.2 Structure
The XRD spectra for the VO2 films on SL, PS and FQ substrates are displayed in Fig. 2. There is a NaxV2O5 peak in the SL-VO2 film. This indicates that the sodium (Na) ion has penetrated from the substrate to the VO2 film. The reason why the peak does not appear on the PS-VO2 may be due to the insufficient penetration of Na ions. The results of the XRD information can be used to describe the diffusion process of Na ions. Previous work postulated that the sodium silicate chains in the SL decompose as the synthesis temperature (deposition and annealing) exceeds 475 K, and the Na ion mobility increases significantly with temperature [19]. In addition, the VO2 structure of this film is relatively open and easily adapts to the diffusion of Na ions. The penetration of Na ions produces a NaxV2O5 structure on the film.
3.3 Composition
Figure 3(a) shows the XPS spectra for the VO2 films on the PS and the SL substrates. Besides the expected V and O elements, the Na (Fig. 3(b) is the fit spectrum of the Na1s) and potassium(K) (Fig. 3(c) is the fit spectrum of the K2p) elements are also present in the VO2 films. However, both spectra showed a Na1s peak [26], although much more prominent in the SL substrate, indicating Na ion penetration all the way to the film surface. Moreover, a notable K2p peak was detected for the sample on PS indicating K ion diffusion from the PS substrate [27]. XRD and XPS results show that Na and K ions have penetrated into the VO2 films, and more importantly, the PS contained K2O which led to a mixed-alkali effect with Na2O. It has been reported in literature that the mixed-alkali effect changes alkali diffusivity in a non-linear fashion [28].
Fig. 3 (a) XPS survey spectra for the VO2 films on PS and SL substrates, (b) the fit spectrum of the Na1s in the SL-VO2 film, (c) the fit spectrum of the K2p in the PS-VO2.
Table 2 is the EDX results of the VO2 films on FQ substrates. By analyzing the EDX data, in addition to the expected V and O elements, carbon impurities may be contaminated with air and the films has almost no other impurities of VO2 films on FQ substrate.
Table 2. EDX results of VO2 films on FQ substrate.
3.4 Spectroscopic ellipsometry
The optical constants of the three substrates were determined first. Considering that the substrate is transparent material, the Cauchy model was built. Through the Cauchy model, the optical constants of the three substrates were obtained. The expression is given by:
where n is refractive index, λ is wavelength, the parameter “A” controls the “deviation” of the curve and parameters “B” and “C” control the curvature. Our SE is based on the least square method analysis. The quality of fit of the optical model is defined by the lowest mean squared error (MSE) [29]. The MSE quantifies the difference between the model and the experiment based on the following expression:
where n is the number of (Ψ and Δ) pairs, m is the number of free parameters, and the subscripts “exp” and “mod” refer to experimentally measured data and theoretically modeled data, respectively. Table 3 shows the best fitting parameters in the Cauchy model for the three kinds of substrates. Figure 4 shows the refractive index for the three kinds of substrates as a function of photon energy. The refractive indices of these three kinds of substrates are different. Due to the transparency of the substrates, the extinction coefficient in the visible band is almost 0. The refractive indices obtained by experimental fitting are consistent with those from literature [30–33]. Previous work on these substrates indicated an increasing amount of Na content of 0at%, 4.2at%, and 9.5at%, respectively [19]. Interestingly, the substrates refractive index is proportional to the content of Na.
Table 3. The best fitting parameters in the Cauchy model for different substrates and the corresponding MSE
Figure 5 shows the experimental and simulated ellipsometry data (Ψ and Δ) for VO2 films on different substrates. According to the analysis of the AFM, the VO2 films have a rough surface. The ellipsometry data are very sensitive to surface condition, so a surface roughness layer should be added in the model. Therefore, a four-layer model (air/roughness/VO2/substrate) was applied to describe the VO2 system. The schematic diagram of the optical model is shown in Fig. 6.
Fig. 5 Ellipsometry experimental (dots) and simulated (lines) data of the VO2 films on the three substrates.
The surface roughness layer was modeled by Bruggeman effective-medium approximation (EMA) with a mixture of the VO2 (50%) and voids (50%). The combination of three Lorentzian oscillators was used for the description of the optical properties. The Lorentzian oscillators expression is given by:
where Amp represents amplitude, Br stands for center energy width, En represents center energy. Table 4 displays the best fitting parameters in the Lorentz oscillators for all the samples. The thickness, roughness and MSE of the VO2 films obtained through SE are listed in Table 5. In this table, we also listed the roughness obtained by AFM, and found that these roughness results are relatively close.
Table 4. The best fitting parameters in the Lorentz oscillators for samples
Table 5. VO2 film thickness, roughness and MSE by SE
Figure 7 shows the optical constants – refractive index (n) and extinction coefficient (k) – as a function of photon energy for VO2 thin films deposited on the three different substrates. The optical constants of the VO2 film on the FQ substrate are consistent with that reported by J.B. Kana and Kakiuchida et al. [34,35]. However, the optical constants of the VO2 films on SL and PS substrates are significantly different from that of the VO2 films (pure) on FQ substrates. In previous reports [36], there is a clear absorption of Na around 0.75 eV in sputtered ZnO film on SL, which indicates that Na ions infiltrate into ZnO thin films, causing changes in dielectric constants. Similarly, we also believe that the optical constants of VO2 films on SL and PS substrates changed due to the infiltration of Na and K ions.
Fig. 7 The photon energy dependent optical constants of VO2 thin films deposited on different substrates: (a)SL, (b)PS, (c)FQ. Left vertical axis is the Refractive index (n). Right vertical axis is the Extinction coefficient (k). Inset is the second derivative spectrogram of the imaginary part of dielectric constant.
VO2 is a rutile structure, with the O ions as the closest packing of the hexagon and the V ions in the octahedral space of similar rules [37]. In SL substrate, the Na ion penetrates into the VO2 films to form NaxV2O5, which led to changes in the optical properties. In PS substrate, K ions also penetrated into the VO2 film, so the mixed-alkali effect of the Na, K ions might have caused the change of the optical properties of the films.
Comparing the optical properties of VO2 films on PS substrate with that on SL substrate, the peak position of refractive index on the PS substrate has red-shifted. This phenomenon may be caused by the presence of impurity bands near the Fermi level [38], and the impurity bands may be caused by the mixed-alkali action of the ions.
The inset in Fig. 7(c) displays the second derivative spectrogram of the imaginary part of the dielectric constant. In this spectrum, optical transitions labeled as x1 and x2 were observed to be located at about 2.95 eV, and 3.08 eV, respectively. This bimodal structure is due to the p-d interaction [39].
VO2 is a phase change material [1], therefore it is meaningful to study its optical properties at variable temperature. In previous work [19], Miller and Wang studied the transmittance of VO2 thin films on different substrates at different temperatures and found that a temperature above 200°C is enough to activate Na diffusion. For VO2 films on SL, the film did not show the desired thermochromic functionality. By replacing half of Na with K and using a mixed-alkali effect between Na and K, the VO2 film grown on the PS substrates showed the desired thermochromic properties and its performance matched or exceeded that of the sample on FQ. The mixed alkali effect was deemed advantageous on producing high quality VO2 thin films on PS. Therefore, further ellipsometry studies were performed in this work on VO2 thin films on SL and PS substrates with temperature varying from 300 to 675 K.
The fitted parameters of the models at 300 K can be used as the initial values for the fitting procedure at elevated temperature. The thickness of the VO2 layers, surface roughness and the dielectric functions of VO2 were considered for the high-temperature SE fitting. The thickness of the film will not change too much as the test temperature rises. Therefore, the thickness of the film is set as fixed value based on the dispersion models at 300 K. The best-fit results can be obtained at elevated temperatures by adjusting these fitting parameters within reasonable borders. All fitting parameters and MSE are listed in the appendix in Table 6.
Figures 8 and 9 are the variation diagrams of VO2 film refractive index and extinction coefficient under varying temperature. Figure 8(a) shows the refractive index of the VO2 film on the SL substrate. A blue shift in the peak of the refractive index could be observed with increasing temperature. In the temperature range of 375 ~675K (the peak position shifts from 2.56 eV to 2.7 eV). The change of the refractive index is significant, and this trend is different from that between 300 K and 350 K. This phenomenon is attributable to the reversible phase change of VO2. At approximately 341 K, the optical band gap and band structure change, and the optical properties change. Figure 8(b) shows a blue shift in the peak of the refractive index of the VO2 film on the PS substrate. In the temperature range of 375 K ~675 K (peak from 2.5 eV to 2.76 eV), the refractive index changes greatly, and the change trend is slower than that from 300 K to 350 K. Figure 9(a) shows the extinction coefficient of the VO2 film on SL substrate. A red shift in the peak of the extinction coefficient could be observed with increasing temperature. But this phenomenon was not found in Fig. 9(b). We will continue to study its underlying mechanism.
Fig. 8 The variation diagram of VO2 film refractive index under variable temperature: (a) SL-VO2 and (b) PS-VO2.
Fig. 9 The variation diagram of VO2 film extinction coefficient under variable temperature: (a) SL-VO2 and (b) PS-VO2.
4. Conclusion
In summary, VO2 thin films were deposited on three different glass substrates by reactive magnetron sputtering. The AFM results demonstrate that the films have uniform surface morphology. The XRD and XPS analysis show that Na and K ions in the SL and PS substrates have penetrated into the VO2 film. SE studies indicate that the penetration of Na and K ions have a great influence on the optical properties of VO2 films, which changed the internal structure of the film by permeating ions. In the future, we can quantitatively dope Na and K on VO2 films to change the optical constant. Variable temperature ellipsometry experiments show that with the increase of experimental temperature, the peak of the refractive index demonstrates a blue shift. Across the phase transition temperature, the blue shift displays a different tendency, which might be attributed to a possible phase change of VO2. This phenomenon might show a new way to determine the VO2 phase transition in visible light.
Appendix
Table 6. Ellipsometry fitting parameters and MSE at varying temperature
Funding
National Key Basic Research Program of China (grant number 2015CB921003); Key Research and Development Project of Shandong Province (grant number 2017GGX201008); Fundamental Research Funds of Shandong University (NO.2016JC027); the University of Washington Royalty Research Fund (grant no. 65-1984); Clean Energy Institute Student Training and Exploration Grant.
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