Investigation of VO₂ directly deposited on a glass substrate using RF sputtering for a smart window

Heat load management such as central control of heating and air conditioning in a building infrastructure is predicted to reduce approximately 30% of greenhouse gas emissions. ^1,2) This data has encouraged research on methods to save energy to maintain a comfortable indoor environment for a central system in buildings. One approach for heat management is to develop a functional material coating in which the thermochromic function is activated through temperature control of infrared (IR) radiation on glass windows in a building.

Thermochromic materials such as NbO₂, VO₂, and TiO₂ can be transformed from a semiconductor to a conductor and vice versa by controlling their temperature. ³⁾ Of these, many researchers have focused on VO₂ because its electrical conductance varies by several orders of magnitude at 68 °C from a semiconductor (with a high IR transmittance) to a metallic-like conductor (with a high IR reflectance). ⁴⁾ In our research group, a visible-light transparent solar cell and a sensor have been developed for a smart window with several functions for the Internet of Things. ^5,6) To add other functions to the smart window, we investigated VO₂ as a self-temperature control element by blocking IR radiation at high temperatures (over 68 °C). Furthermore, fabricating a large area of a VO₂ thin film on a glass substrate using a low-cost method and at an acceptable process, the temperature is necessary to realize a smart window for cost-effective optical applications.

Although several methods, such as chemical vapor deposition, ⁷⁾ pulsed laser deposition, ⁸⁾ metal-organic chemical vapor deposition, ⁹⁾ physical vapor deposition, ¹⁰⁾ and hydrothermal approach, ¹¹⁾ have been reported for VO₂ deposition, these methods have limitations such as the need for a small area and a high temperature of over 600 °C in fabricating a well-defined VO₂ film. To overcome these limitations, we used the sputtering method in this study, which can be employed to deposit a film on a large area using a relatively low temperature applied to the glass substrate. Zhao et al. conducted an experiment to achieve VO₂ deposition using sputtering at a deposition temperature in the range of 550 °C–700 °C. ¹²⁾ As the temperature increased, the ion state of V changed from V⁵⁺ → V⁴⁺ → V³⁺. A seed layer of ZnO, ¹³⁾ TiO₂, ^14,15) Si, ¹⁶⁾ muscovite, ¹⁷⁾ and SnO₂ ¹⁸⁾ is typically used to enhance the crystal quality and decrease the process temperature. However, in this work, adding a seed layer would have complicated the sputtering process and increased the fabrication cost. To achieve direct deposition of a VO₂ film on a glass substrate at an appropriate temperature, we investigated the dependence and variation of RF sputtering conditions while directly growing a VO₂ thin film on a soda-lime glass (SLG) substrate.

A 50 nm thick VO₂ thin film was deposited on cleaned SLG by RF sputtering. The SLG substrate was cleaned using acetone, followed by isopropyl alcohol in an ultrasonic cleaner for 30 min, respectively. During RF sputtering, the process temperature was fixed at 400 °C because the substrate is SLG, which has a strain temperature of approximately 500 °C. In reactive sputtering, the ambient directly affects the composition of the film, as reported in certain studies where VO₂ was deposited in an H₂ atmosphere to reduce oxygen from a V₂O₅ target. ¹⁹⁾ However, in the present experiment, a VO₂ target was used. We used a 4 in VO₂ target with a purity of 99.99% and the distance between the target and the substrate was 60 mm. During sputtering, oxygen from the VO₂ target easily turned volatile owing to the plasma; hence, forming VO₂ was difficult. Therefore, an Ar-based mixture of O₂ (5%) gas at a pressure of 2–3 Pa was used.

To characterize the crystallinity of the VO₂ thin film, we collected X-ray diffraction (XRD) data of the samples using a Rigaku Ultima IV X-ray diffractometer operated at 40 kV and 40 mA with Cu Kα radiation. The sample's XRD patterns were measured with a step size of 0.01° and a counting time of 4 s. X-ray photoelectron spectroscopy (XPS) measurement was performed using a JPS-9000MC (JEOL Ltd.) with a step size of 0.5 eV. To measure the depth profile, we etched the film with energy from an Ar ion beam of 300 eV using an XPS measurement system. The transmittance characteristics were measured using a UV–vis spectrophotometer (V-670, JASCO). The homemade heating system for VO₂ thin film deposited on the SLG glass was built in the V-670.

The crystallinity and orientation of VO₂ are strongly influenced by the surface state of the substrate, including the seed layer (ZnO, ²⁰⁾ TiO₂, ²¹⁾ SrTiO₃, ²²⁾ and MgO ²³⁾) and distortion. ²⁴⁾ In this experiment, ZnO and Al₂O₃ (c-sapphire substrate) were used as the seed layer to control the orientation and obtain better crystallinity of VO₂. A 50 nm thick ZnO seed layer was deposited on the glass by sputtering with an RF power of 100 W in Ar/O₂ (95/5) ambient. The sputtering method produced effects such as plasma damage on the substrate owing to the plasma. In this study, a relatively high RF power (180 W) was used; hence, the influence of RF power should be investigated first.

The representative XRD patterns of the VO₂ thin films deposited on different seed layers using an RF power of 180 W in Ar/O₂ (95/5) ambience are shown in Fig. 1. Diffraction peaks related to VO₂(100) and (200) orientations are shown on the amorphous glass substrate. In the case of an amorphous substrate, two types of patterns have been reported for VO₂(100) and (011), ^10,18) which will be discussed in the next section. The VO₂(020) peak at 39.9° appeared on the sapphire substrate as previously known, indicating that the orientation of VO₂ can be controlled and grown as described in a previous report ⁸⁾ even though the substrate is in a high plasma density. However, although a VO₂ thin film deposited on ZnO usually shows VO₂(020) orientation at 39.9° as reported in a previous study, ¹³⁾ a small peak related to the mixture of V₂O₅ and VO₂(002) can be observed on the 50 nm ZnO thin film in Fig. 1. The Zn atom can be easily diffused using a high substrate temperature and plasma energy; ²⁵⁾ consequently, VO₂ is not formed on the ZnO thin film according to the XPS results, contrary to the diffusion and oxidation states of V in VO₂ thin films deposited on Al₂O₃ and ZnO.

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Figure 2 shows the XPS spectra of the VO₂ thin film deposited on ZnO and sapphire after etching for 5 s and 30 s, respectively. The main peak of O 1 s for both films is at 531.1 eV; further, there is a V-OH contaminated peak at the VO₂ film surface (5 s etching) in Figs. 2(a) and 2(d) for VO₂ deposited on Al₂O₃ and ZnO, respectively. As shown in Fig. 2(f), the intensity of Zn 2p is detected even at the surface (after etching for 5 s). The calculated Zn atom ratios at the surface of VO₂ (5 s etching) and inside (30 s etching) VO₂ were 1.2% and 7.5%, respectively. The diffused Zn atoms interfere with the bond between vanadium and oxygen because Zn–O (<250 kJ mol⁻¹) has a lower enthalpy than V–O (637 kJ mol⁻¹). ²⁶⁾ Thus, the V⁵⁺ ion is preferred owing to the oxygen deficiency of vanadium, as shown in Fig. 2(e). In contrast, for the Al₂O₃ substrate, Al atoms do not diffuse into the VO₂ film, as shown in Fig. 2(c), and the film has more V⁴⁺ ions than V⁵⁺ ions, as shown in Fig. 2(b). Therefore, a seed layer that does not have an atom that can interfere with the composition of VO₂ should be selected.

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In fact, V is a transition metal that is difficult to synthesize as it exists in several oxidation states obtained by forming various binary oxides. ²⁷⁾ Of these oxidation states, VO₂(V⁴⁺) has a very narrow range in the V–O phase diagram; thus, VO₂ is sensitive to variations in the oxygen ratio. ²⁸⁾ A higher process temperature provides a wider acceptable O₂ density ratio range; ²⁹⁾ hence, previous works maintained a substrate temperature of over 500 °C. ^15,17,24) Moreover, the oxidation state of V varied from 5+ to 3+ as the temperature increased from 550 °C to 700 °C. ¹²⁾ In this experiment, the process temperature was fixed at 400 °C because the strain point (specific value at which the volume abruptly increases) of the SLG substrate is 490 °C. Therefore, the RF power was increased up to 180 W rather than increasing the substrate temperature to mitigate the sensitivity of oxygen.

Figure 3 shows the XRD patterns of the VO₂ thin film deposited on the SLG substrate at an applied temperature of 400 °C with different RF powers of 100, 150, and 180 W in Ar/O₂(95/5%). A small peak of VO₂(011) at 27.8° occurred at an RF power of 100 W. This agrees with the result presented in Ref. 30, and these samples show susceptibility to oxygen density. Interestingly, a VO₂ crystal orientation transition based on RF power is demonstrated for the first time. As the RF power increased, the orientation of VO₂ shifted from (011) to (100), which has a higher peak intensity. At an RF power of 150 W, the film shows the coexistence of the peak of VO₂(100) at 18.2° and the other intermediate compound (V₂O₅ and VO_x ). It shows well-defined VO₂(100) and VO₂(200) orientations at an RF power of 180 W. The oxidation process in plasma with oxygen is complex because of many excited metastable states of oxygen molecules; ³¹⁾ this requires further investigation in future works. In this analysis, two expectations are considered. First, a high plasma density can compensate for reaction energy because the enthalpies of vanadium oxides ³²⁾ are ∆ ${{H}}_{{{\rm{V}}}_{{\rm{2}}}{{\rm{O}}}_{{\rm{5}}}}$  = −1557 cal mol⁻¹ and ∆ ${{H}}_{{\rm{V}}{{\rm{O}}}_{{\rm{2}}}}$  = −713 cal mol⁻¹, which facilitates the formation of V₂O₅ rather than VO₂. A high plasma density provides the energy required to compensate for the difference in the enthalpy, similar to the effect of high temperature. Second, the strain state on the substrate changes because of high plasma density. The same pattern of changes in the crystal orientation reported in this work was also reported by Jung et al. ³³⁾ In the work by Jung et al., the trend of VO₂ orientation was investigated on different (a-, c-, m- and r-) plane sapphire substrates where it was reported that the out-of-plane direction of VO₂ occurred under compressive strain, whereas the in-plane VO₂ orientation occurred under tensile strain. Under high plasma density, an ion cloud with high kinetic energy could be formed to induce compressive strain on VO₂ film at low temperatures, producing an out-of-plane VO₂(100) orientation. ³⁴⁾

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The insulator–metal transition (IMT) temperature should be considered to practically utilize a smart window. The resistivities of the VO₂ thin film during heating and cooling were measured at temperatures ranging from 30 °C to 125 °C, as shown in Fig. 4(a). The VO₂ film exhibits clear IMT characteristics with two orders of resistance variation. The degree of resistance variation is relatively smaller than that observed in other studies ^20,23) because of the relatively thin (50 nm) VO₂(100) film. ²⁰⁾ The IMT temperature was obtained from the plot of dln R/dT versus T, as shown in Fig. 4(b). The IMT temperature is 72.5 °C, which was calculated as follows: (T_c + T_h)/2 (T_c: cooling branch, T_h: heating branch). This result is slightly higher than the ideal IMT temperature of VO₂ (68 °C). The VO₂(100) film also showed an IMT temperature of 68 °C. ⁷⁾ Hence, the increase in the IMT temperature was expected owing to the inhomogeneities of the VO₂ thin film, such as polycrystalline and Si impurity. ³⁵⁾

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To evaluate the thermochromic property of the VO₂ thin film for a smart window, we measured change in the transmittance. For smart window applications, the VO₂ film should always allow visible light and block IR radiation at high temperatures. The temperature-dependent transmittance of the VO₂ thin film deposited at a high RF power was measured at 25 °C and 80 °C, to estimate the modulation of transmission by the VO₂ thin film, as shown in Fig. 5. In this experiment, to test the IR transmittance modulation, the temperature was set as 80 °C because the maximum temperature of glass was assumed to be approximately 80 °C, as specified by Agrawal; ³⁶⁾ the set temperature is higher than the IMT temperature of the VO₂(100) thin film, obtained from Fig. 4(b). Thermochromic characteristics are clearly observed with drastic variations in the IR radiation at different temperatures. The VO₂(100) thin-film modulates the transmittance to a 35% drop at 80 °C. To quantitatively evaluate the IR modulation function, ³⁷⁾ we formulated the integral transmittance of IR as follows:

$$\begin{eqnarray*}&&{T}_{{\rm{I}}{\rm{R}}}=\displaystyle \int {{\rm{\Phi }}}_{{\rm{I}}{\rm{R}}}\left(\lambda \right)T\left(\lambda \right)d\lambda /\displaystyle \int {{\rm{\Phi }}}_{{\rm{I}}{\rm{R}}}\left(\lambda \right)d\lambda ,\end{eqnarray*}$$

where ${\rm{\Phi }}$ is the solar irradiance spectrum for an air mass of 1.5 corresponding to the Sun being positioned 37° above the horizon and λ ranges from 780 to 1700 nm for an IR function. The calculated difference in ${T}_{{\rm{I}}{\rm{R}}}$ at high temperatures and at room temperature is 17.6%. This integral IR transmittance modulation behavior was applicable to a smart window.

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Considerable work is still required to realize self-temperature control films capable of sensing temperature. This study investigated only the possibility of realizing such a device. These results of this study can serve as the first step toward realizing smart windows/greenhouse using high power density RF sputtered VO₂ thin films.

In this study, a VO₂ thin film was directly deposited on an SLG substrate by RF sputtering for a cost-effective optical implementation of self-temperature control, in which the RF sputter power was exclusively controlled. The study revealed that the crystalline of VO₂ was changed from VO₂(011) to VO₂(100) by RF power increasing. The obtained VO₂(100) at high RF power showed well-defined IMT property and IR transmittance modulation which are applicable for a smart window. Although further investigation is required to elucidate mechanisms such as the role of RF power and to enhance optical performance, the deposition of VO₂ thin film by sputtering using high RF power shows sufficient potential as a simple and low-cost method of creating smart windows.