Surface Roughness, Residual Stress, and Optical and Structural Properties of Evaporated VO₂ Thin Films Prepared with Different Tungsten Doping Amounts

Abstract

This study investigates the effects of different tungsten (W) doping contents on the optical transmittance, surface roughness, residual stress, and microstructure of evaporated vanadium dioxide (VO₂) thin films. W-doped VO₂ thin films with varying tungsten concentrations were fabricated using electron beam evaporation combined with ion-assisted deposition techniques, and deposited on silicon wafers and glass substrates. The optical transmittances of undoped and W-doped VO₂ thin films were measured by UV/VIS/NIR spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The root mean square surface roughness was measured using a Linnik microscopic interferometer. The residual stress in various W-doped VO₂ films was evaluated using a modified Twyman–Green interferometer. The surface morphological and structural characterization of the W-doped VO₂ thin films were performed by field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). Raman spectroscopy was used to analyze the structure and vibrational modes of different W-doped VO₂ thin films. These results show that the addition of tungsten significantly alters the structural, optical, and mechanical properties of VO₂ thin films.

1. Introduction

Vanadium dioxide (VO₂) has been widely used as a thermochromic smart window material. In 1959, Morin [1] discovered in the Bell Laboratory that vanadium dioxide has unique and reversible metal–insulator phase transition (MIT) properties. At the same time, VO₂ will undergo a structural phase transition, changing from a low-temperature monoclinic rutile structure to a high-temperature tetragonal rutile structure. As the crystal structure changes, the optical and electrical properties of VO₂ will undergo sudden changes. Therefore, many scholars have conducted research on VO₂ thin films. In the past few decades, the practical commercial applications of VO₂ thermochromic films are limited by their high phase transition temperature (Tc), low light transmittance, and poor solar modulation ability [2,3,4,5]. The VO₂ thermochromic thin film has metal–insulator phase transition characteristics near room temperature. At the VO₂ phase transition temperature, it has a monoclinic crystal structure and is transparent to the infrared band. After the VO₂ thin film reaches a temperature above the phase transition temperature, it has a rutile structure, and then becomes opaque to the infrared band, which has huge application potential in smart glass for energy-saving buildings. There are many methods for preparing vanadium dioxide films, including chemical vapor deposition (CVD) [6], atomic layer deposition (ALD) [7], the sol–gel method [8], reactive evaporation [9], sputtering [10,11], pulse laser deposition [12,13], electron beam deposition [14], etc. The early preparation method of VO₂ thin films was mainly based on sputtering technology, but only under controlled oxygen partial pressure can a single VO₂ (M) phase be synthesized.

Ping and Sakae [15] used dual-target magnetron sputtering to replace W⁶⁺ with VO₂ to form V_1−xW_xO₂. When the replacement amount x = 0~0.026, the change in its conversion temperature will be linear. When the doping ratio of W⁶⁺ is higher, the conversion temperature of V_1−xW_xO₂ is lower. The experimental results indicate that the use of the double-target magnetron sputtering method to prepare a W-doped VO₂ single-phase film is an efficient method. From their experiments, it can be proven that W⁶⁺ is the most suitable doping ion to reduce the VO₂ phase transition temperature. However, this research requires a substrate temperature of about 400 °C; so, reducing the process temperature is also an important issue. Lee and Kim [16] used electron beam evaporation to deposit medicinal materials of vanadium powder onto a glass substrate, followed by rapid thermal annealing (RTA). They observed that annealing times between 20 and 30 s produced the greatest thermal denaturation effects and reported that the optical properties degraded beyond 40 s at 400 °C in all vanadium dioxide films examined. All had strong thermochromic effects. However, the visible light transmittance of the prepared VO₂ thin film was relatively low. Tsai et al. [17] studied the relationship between residual stress and phase transition temperature in VO₂ film deposition. Quantitative evaluation of residual stress in polycrystalline VO₂ films was performed through X-ray diffraction (XRD). The results demonstrated that the inhibition of the phase transition in films with a (011) preferred orientation is attributed to residual tensile stress, which contributes to substrate contraction. Furthermore, the crystallinity of VO₂ films was found to play a crucial role in determining the residual stress in the as-deposited state. Analysis by electron spectroscopy for chemical analysis (ESCA) revealed that films exhibiting higher residual stress and reduced crystallinity possessed core-level binding energies approaching those of stoichiometric VO₂, a behavior likely associated with subtle shifts in the relative atomic arrangement of vanadium and oxygen.

In 2023, Wang et al. [18] examined how variations in film thickness, microstructure, grain size, and surface morphology influence residual stress in VO₂ thin films deposited by magnetron sputtering. Their findings revealed that the grain size grows with increasing film thickness. As the thickness increases, residual stress shifts from a compressive state to a tensile state. This transition is primarily linked to grain enlargement, a reduction in the grain boundary density, and the densification effect associated with the film growth process. In the same year, Lu et al. [19] used pulsed laser deposition (PLD) to prepare a series of VO₂ epitaxial films of different thicknesses on rutile TiO₂ (110) substrates. The strain relaxation phenomenon as the film thickness increases was also studied. As the film thickness increases from 14 nm to 88 nm and then to 300 nm, the average strain in the film decreases with increasing thickness. The strain distribution depends on the choice of substrate, the orientation of the substrate, and the thickness of the deposited VO₂ thin film.

In this study, we focus on the optical, surface, and residual stress properties of W-doped VO₂ films; some literature reports indicate that W-doping significantly changes the electrical behavior of VO₂ and may lead to partial metallization near the phase transition temperature or even at room temperature. Rajeswaran et al. [20] observed that when W doping is approximately 2.0 at.%, the phase transition temperature can be lowered from ~68 °C to approximately 25 °C, and a significant increase in the room-temperature carrier concentration was also recorded, indicating that W atoms have an electron donor effect and can improve room-temperature conductivity. Choi et al. [21] demonstrated that the resistivity response to W doping can vary significantly between VO₂ polymorphs, with some phases showing orders-of-magnitude decreases in resistance and others showing the opposite trend, underscoring the roles of the microstructure and phase composition. Ding et al. [22] reported that 1.62 at.% W-doped VO₂ films exhibited a sheet resistance of 64 kΩ/□ at 30 °C, compared to ~144 kΩ/□ for undoped films, along with a reduced resistance jump during the transition and a concurrent decrease in infrared transmittance—evidence that an increased carrier density directly impacts optical shielding. These results collectively imply that W-doped VO₂ films may exhibit a partially metallic character below the metal–insulator transition temperature, which could correlate with changes in infrared transmittance observed in this work.

In a previous work [23], the effect of higher W doping contents (3–5%) on the electrical, optical, structural, and thermomechanical properties of VO₂ thin films was reported. The experimental results demonstrate that when the tungsten doping concentration is increased from 3 wt.% to 5 wt.%, the residual stress in VO₂ thin films exhibits a progressive reduction, while the surface roughness correspondingly increases. Moreover, a slight enhancement in the refractive index is observed with higher tungsten incorporation, suggesting that tungsten not only modulates the stress state and surface morphology of the films but also exerts a subtle influence on their optical response. The residual stress and thermomechanical properties of vanadium dioxide thin films were studied [24]. The variation of residual stress in VO₂ thin films at different temperatures was measured using a modified Twyman–Green interferometer. This work found that the substrate has a significant impact on thermal stress, which is mainly caused by the mismatch in the coefficient of thermal expansion (CTE) between the thin film and the substrate. The thermal stress of VO₂ thin films can be measured in the range of room temperature to 120 °C using the double substrate method. Finally, the thermal expansion coefficient of the VO₂ thin film was measured to be 3.21 × 10⁻⁵ °C⁻¹, and the biaxial modulus was 517 GPa. The residual mechanical stress properties of VO₂ thin films are crucial for their reliability and durability in real-world applications.

Many studies published in the literature mainly focus on the phase transition temperature, structure, and electrical properties of VO₂ thin films [25,26,27,28]; however, the research on its surface roughness and residual stress properties is still very limited. This work expands on our previous research results by growing nanoscale vanadium dioxide (VO₂) thin films with different tungsten doping contents and investigates the effects of the tungsten doping content on the optical transmittance, surface roughness, residual stress, and structural properties of VO₂ thin films. This work distinguishes itself from our previous studies through several unique contributions. First, we present a systematic and feasible approach to thoroughly investigate the properties of evaporated W-doped VO₂ thin films with tungsten concentrations ranging from 1 to 5 at.% (atomic percent). This includes a detailed examination of optical transmittance, surface roughness, microstructure, and residual stress, with a particular focus on how tungsten doping influences the phase transition behavior. Second, in addition to conventional glass substrates, silicon wafers are employed to explore substrate-dependent variations in film properties. Third, the characterization techniques are significantly expanded, incorporating UV/VIS/NIR and FTIR spectroscopy for a comprehensive optical analysis; Linnik microscopic interferometry for precise surface roughness measurements; a modified Twyman–Green interferometer for residual stress evaluations; field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) for morphological and structural characterization; as well as Raman spectroscopy for vibrational mode and phase identification. Finally, rather than focusing solely on process optimization, this study systematically correlates tungsten doping levels with microstructural features, optical transmittance, surface morphology, and residual stress. This comprehensive analysis provides deeper insights into the underlying mechanisms governing the performance of evaporated W-doped VO₂ thin films, thereby offering valuable guidance for optimizing these materials in industrial applications.

2. Materials and Methods

In this study, electron beam evaporation combined with ion-assisted deposition were used to prepare tungsten-doped VO₂ thin films and compare related properties. The optical transmittance, residual stress, surface roughness, structural properties, and Raman spectra were measured for different vanadium dioxide films. These measurements are used to analyze the effect of varying W doping contents on the optical transmittance, residual stress, and surface roughness of electron beam-evaporated vanadium oxide. Exploring the generation mechanism of the metal–insulator phase transition (MIT) will be of great help in the application of thermochromic VO₂ thin films.

2.1. Synthesis of W-Doped VO₂ Thin Films with Different Doping Levels

The electron beam evaporation process represents a reliable thin-film deposition technique that enables uniform growth over large substrate areas. The evaporation chambers are generally designed to accommodate multiple substrates at once, thereby ensuring high production efficiency. This approach provides notable advantages, including shortened processing times and relatively simple maintenance, which makes it widely adopted in industrial applications such as optical coatings, transparent conductive films, and other functional layers. In the present study, an ion-assisted electron beam deposition system was employed to fabricate tungsten-doped vanadium oxide thin films. Six samples of VO₂ thin films with different tungsten (W) doping levels were prepared, including electron-beam evaporated W-doped VO₂ with 1 wt.% to 5 wt.% W (atomic percent), as well as undoped VO₂. Tungsten-doped VO₂ source materials were evaporated by directing an electron beam from the electron gun onto them, resulting in deposition onto the substrates. The chamber was evacuated using a high-vacuum system equipped with a mechanical booster pump and an oil rotary pump (RP), ensuring optimal vacuum conditions through the effective removal of residual water vapor. Prior to deposition, the chamber was evacuated to a base pressure below 2.6 × 10⁻⁴ Pa, which ensured a controlled environment conducive to high-quality thin film growth.

In the thin film coating experiment, VO₂ thin films were grown on 25 mm-diameter H-K9L and B270 glass substrates (The manufacturer is Youjun Optical Co., Ltd., Taichung, Taiwan), as well as on silicon wafers, to investigate the substrate-dependent properties of W-doped VO₂. All substrates were ultrasonically cleaned to eliminate surface contaminants. High-purity argon and oxygen gases (99.999%) were introduced during deposition. The electron gun operated at a peak power of 10 kW, with coating parameters set to 10 kV and 1 A. For ion-assisted deposition, the ion source operated with an anode voltage of 80–300 V, an anode current of 0.5–10 A, and ion energies between 50 and 200 eV. During VO₂ deposition, argon and oxygen flow rates were controlled at 16 and 20 sccm, respectively. Film thickness was monitored using a hybrid system combining quartz crystal oscillation (5 MHz) and optical spectroscopic monitoring in the range of 350–900 nm. Reflectance extrema detected by the optical system were used to regulate the deposition process, ensuring precision and accuracy. The final thickness of all VO₂ films was fixed at 90 nm. During deposition, the vacuum pressure was maintained below 6 × 10⁻⁴ Pa, with a working pressure of 3.0 × 10⁻² Pa. The evaporation beam power was stabilized at 7.5 kW, yielding a deposition rate of 0.1 nm/s. Substrate temperatures were controlled at 270 °C throughout the process.

In terms of the measurement and analysis of the W-doped VO₂ films’ characteristics, we measured the optical transmittance of thin film samples at different wavelength ranges. A UV−VIS−NIR spectrophotometer (Shimadzu UV-2600i, Kyoto, Japan) was used to measure the visible light transmittance. A Nicolet iS5 Fourier transform infrared (FTIR) spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) was used to measure the infrared transmission spectrum. A modified Twyman–Green interferometer was employed to measure the residual stress and thermal stress of the thin films. A temperature-controlled heating stage was used to ensure uniformity and stability during the heating temperature. The root-mean-square surface roughness was determined by a Linnik microscopic interferometer.

2.2. Structural, Optical, and Stress Characterization of W-Doped VO₂ Thin Films

To systematically evaluate the post-deposition properties of the W-doped VO₂ thin films, a combination of advanced characterization techniques was employed. Optical transmittance in the ultraviolet–visible–near-infrared (UV–VIS–NIR) range (200–1000 nm) was measured using a Shimadzu UV-2600i spectrophotometer, while the middle-infrared transmittance (3.0–5.5 µm) was assessed via Fourier transform infrared (FTIR) spectroscopy under normal incidence. Residual stress within the thin films was quantified using a custom-built Twyman–Green interferometer, enabling the precise determination of stress-induced optical path differences. Surface topography and roughness were characterized by a homemade Linnik microscopic interferometer specifically designed for high-resolution measurements of thin films deposited on well-polished glass substrates [29]. In addition, Raman spectroscopy was conducted to probe the crystallographic structure and phase composition of the deposited W-doped VO₂ thin films, providing insights into the effects of tungsten doping on lattice ordering.

3. Results

3.1. Effect of Tungsten Doping on the Optical Transmittance of VO₂ Thin Films

Optical transmittance refers to the amount of light that passes through a thin film, and in the case of thermochromic VO₂ thin films, it is highly sensitive to temperature changes. The metal–insulator transition (MIT) in VO₂ is triggered at around 68 °C, leading to a significant change in optical transmittance, particularly in the middle-infrared (MIR) region. In this study, the optical transmittance of the undoped VO₂ thin film (represented by the dotted line) and various W-doped thin films was measured with a Shimadzu UV-2600i spectrometer in the 350–850 nm wavelength range, as shown in Figure 1. The 1 wt.% W-doped VO₂ thin film (red line) has a transmittance of 52.9% at a wavelength of 550 nm. The transmittance of the 2 wt.% W-doped VO₂ thin film (green line) is significantly increased to 65%, while the 3 wt.% W-doped VO₂ thin film (blue line) slightly decreases to 63.3. The transmittance of the 4 wt.% W-doped VO₂ thin film (cyan line) is 56% at a wavelength of 550 nm, while the 5 wt.% W-doped VO₂ thin film (pink line) has a higher transmittance of 69.8%. There is no significant relationship between the transmittance in the visible light band and the tungsten (W) doping level of VO₂ thin films. It may be affected by the thickness of VO₂ thin film and visible light absorption These data show that different tungsten doping contents have an effect on the optical transmittance, and the transmittance of the 5 wt.% W-doped VO₂ thin film in the visible light band is the highest among all samples, followed by the 2 wt.% W-doped VO₂ thin film.

In the measurement of optical transmittance in the infrared band, the measurement results of the Fourier transform infrared (FTIR) spectrometer (Nicolet iS5, Thermo Fisher Scientific, Waltham, MA, USA) show that the average transmittance of undoped VO₂ and 1–5 wt.% W-doped VO₂ thin films in the MIR wavelength range of 3–5 μm are 46.57%, 41.19%, 37.08%, 33.37%, 30.03%, and 27.03%, respectively, as shown in Figure 2. FTIR measurement results show that the maximum transmittance is at the mid-infrared wavelength of 3.95 μm. These results also show that in the mid-infrared wavelength range, the transmittance of the undoped VO₂ thin film is the highest and the transmittance of the 5 wt.% W-doped VO₂ thin film is the lowest. They have good optical properties in the visible and mid-infrared wavelength ranges, making them suitable for use in smart windows and optical devices.

The incorporation of tungsten into VO₂ thin films leads to significant changes in their optical, mechanical, and thermal properties. Tungsten doping decreases the optical transmittance due to its high atomic mass and the resultant scattering effects. This could be beneficial in applications where controlled light transmission is required, such as in smart windows that need to block infrared light while allowing visible light to pass. The primary application for VO₂ thin films is in smart windows, where the material’s optical properties can be dynamically adjusted to control heat and light transmission. When VO₂ undergoes its MIT, the film transitions from a transparent insulator at low temperatures to a reflective metal at higher temperatures. This property allows the film to block infrared radiation (heat) in its metallic phase, while still allowing visible light to pass through in its insulating phase. As the temperature rises, the film should exhibit low optical transmittance in the infrared spectrum to prevent solar heat gain, while still allowing visible light to pass through.

Tungsten is a high atomic mass element that can introduce localized electronic effects and change the bonding and atomic arrangement in the VO₂ matrix. As the tungsten content increases, the refractive index typically increases with the increase in W doping contents, as shown in Figure 3. The refractive index of W-doped VO₂ films increases from 2.04 to 2.19. With increasing tungsten doping, the material may become more compact, which would lead to a higher density. A higher density can generally increase the refractive index because more atoms or molecules are present to interact with incident light. This work shows that for low doping concentrations (1–3%) of tungsten, the refractive index increases modestly compared to undoped VO₂ films. At higher doping levels (above 3%), the increase in the refractive index becomes more pronounced, especially in the infrared region. This is often attributed to changes in the film’s electronic structure, which makes the material behave more like a metal.

3.2. Residual Stress Measurement

Residual stress in thin films refers to the internal stress that remains after the deposition process and is a crucial factor affecting the mechanical properties and stability of thin films. It can be influenced by the deposition method, film composition, and thermal expansion mismatch between the thin film and substrate. The residual stress in the film was measured by using a Twyman–Green interferometer, and it was found that as the tungsten doping content increased from 1% to 5%, the residual compressive stress in the W-doped VO₂ thin films increased accordingly. In the case of an undoped VO₂ thin film, the residual compressive stress is about 0.236 GPa. When the tungsten doping content increases from 1 wt.% to 5 wt.%, the compressive stress of W-doped VO₂ thin films (thickness 90 nm) rises from 0.664 GPa to 1.89 GPa, as shown in Figure 4. In most studies, undoped VO₂ films exhibit relatively low residual stress (either tensile or compressive, depending on the deposition conditions). For tungsten-doped VO₂ films, the residual stress is generally observed to increase with higher tungsten contents. This indicates that a higher tungsten content will alter the VO₂ film structure, causing the lattice structure to shrink, thereby increasing compressive stress. In a previous study, we used the double substrate method to measure the thermal stress in undoped and W-doped VO₂ thin films [23,24]. The samples were heated from 30 °C to 100 °C, and the coefficient of thermal expansion (CTE) of the W-doped (1–5%) and undoped VO₂ thin films was measured by the Twyman–Green interferometer. The CTE values are 9.23 × 10⁻⁶ °C⁻¹, 9.29 × 10⁻⁶ °C⁻¹, 9.24 × 10⁻⁶ °C⁻¹, 9.23 × 10⁻⁶ °C⁻¹, 9.38 × 10⁻⁶ °C⁻¹, and 9.66 × 10⁻⁶ °C⁻¹ for undoped and W-doped VO₂ thin films, respectively. For undoped VO₂ thin films, the residual compressive stress is approximately 0.2 GPa. As the tungsten doping amount increases from 1% to 5%, the compressive stress gradually increases to 1.89 GPa. These results show that as the W doping content increases, the stress state of the nanoscale VO₂ film becomes more compact and compressive.

Residual stress is a crucial factor in the fundamental understanding and practical application of thin films. It significantly influences the mechanical stability, adhesion properties, and long-term reliability of the films. Excessive residual stress can induce cracking, delamination, or warping, which adversely affect the optical performance and operational lifespan of devices. The increase in residual compressive stress with higher tungsten contents suggests that the films undergo internal strain, likely due to differences in the lattice parameters of VO₂ and tungsten. This could affect the mechanical stability and durability of the films in practical applications. The results suggest that tungsten-doped VO₂ films, especially those with moderate doping levels (1–3%), may offer improved performance in thermochromic applications, smart windows, and other optoelectronic devices.

Residual stresses in W-doped VO₂ thin films can arise during deposition due to the mismatch in thermal expansion coefficients between the film and the substrate, as well as the specific deposition parameters, including temperature, pressure, and gas flow rate. Figure 5 presents the variation in residual stress as a function of substrate heating temperature for VO₂ thin films with different tungsten doping levels. Among them, the thermal stress of the 5% W-doped VO₂ film changes greatly at the temperature of 30~100 °C (the fitting slope is the largest), while the residual stress–temperature curve of the 3% W-doped VO₂ film increases relatively slowly (the fitting slope is the smallest), indicating that it has good thermal stability. For higher doping levels, tungsten may introduce local strain within the VO₂ film, either increasing or decreasing the overall stress, depending on how it modifies the structural properties and the extent of phase transition suppression. W atoms can induce strain because they have a different atomic radius compared to V, which affects how the film responds to temperature changes. This strain could either exacerbate or alleviate the residual stress, depending on the interaction between W and the VO₂ lattice. In summary, heating temperature affects the residual stress in W-doped VO₂ films by influencing thermal expansion, phase transition behavior, and the role of W atoms in changing the lattice structure. For the effect of the W content in VO₂ thin films on the phase transition temperature, Bleu et al. [30] reported that the phase transition temperature gradually decreased with increasing W content. For the highest doping concentration of 1.7 at.%, VO₂ showed a room-temperature transition (26 °C) with high luminous transmittance (62%), indicating great potential for optical applications. The results of this study provide a practical experimental basis for the development of new multilayer thermochromic materials, which may improve the performance of VO₂ thin films in applications where thermochromism is important.

3.3. Surface Roughness Measurement

Surface roughness affects the optical transmission and reflection properties of VO₂ thin films, which are critical for applications like smart windows and thermochromic coatings. Excessive roughness may impair the material’s ability to control light and heat transmission, making it less effective for energy-saving applications. The films’ surface roughness, expressed as the root-mean-square (RMS) value, was evaluated using a Linnik microscopic interferometer. The surface roughness measurement results of vanadium dioxide films show that the RMS roughness value of VO₂ thin films increases from 1.17 nm to 1.28 nm with an increase in the W doping content, as shown in Figure 6. The surface roughness of W-doped VO₂ thin films increases with increasing tungsten doping concentrations due to factors such as the formation of strain fields, changes in crystal growth mechanisms, and the tendency of tungsten to cluster in the film. Figure 7 reveals the surface topography of different W-doped vanadium dioxide thin films. Additionally, the surface roughness is affected by the deposition technique and post-deposition thermal treatments. Higher doping levels tend to introduce more inhomogeneity in the film’s surface morphology, leading to greater roughness, while lower doping concentrations usually result in smoother films. Therefore, careful control of the doping concentration, deposition conditions, and thermal treatments is essential to achieve the desired surface roughness for specific applications of W-doped VO₂ thin films.

3.4. Structural Properties of Different W-Doped VO₂ Thin Films

The W-doped VO₂ thin films were characterized using a Hitachi S-4800 field-emission scanning electron microscope (FE-SEM, Hitachi High-Tech, Tokyo, Japan). All images were acquired at identical magnifications to enable a direct comparison of the surface morphology and grain distribution. SEM observations provided comprehensive information on the films’ topography, grain structure, crystallinity, and possible defects. At low W doping levels (e.g., 1–2 at.%), the films display uniform and compact surfaces. At higher doping levels (e.g., 4–5 at.%), the films exhibit a more granular morphology with smaller and more uniformly distributed grains. The root-mean-square (RMS) roughness of the films increases with higher W doping concentrations. The incorporation of tungsten (W) into vanadium dioxide (VO₂) thin films deposited via electron beam evaporation induces significant modifications in the surface morphology, including variations in the grain size, grain shape, and grain distribution. These surface morphological changes are closely linked to the films’ thermochromic and electronic behaviors, as grain boundaries and surface uniformity can influence phase transition dynamics, optical modulation, and carrier transport. Consequently, controlling W doping provides a strategic approach to optimize VO₂ thin films for high-performance applications in smart windows and advanced optoelectronic devices. Figure 8 shows the FE-SEM images of different W doping VO₂ samples.

The structural properties of W-doped VO₂ thin films were investigated by X-ray diffraction (XRD) using a SIEMENS D-5000 diffractometer (Siemens, Munich, Germany), which allows for a precise analysis of the crystal phase and lattice structure. Glant incident XRD patterns were used for the thin film phase analysis in the range of 2θ = 10–80° at a scanning rate of 1°min using Cu-kα (λ = 0.15406 nm). The X-ray diffraction (XRD) analysis of W-doped vanadium dioxide (VO₂) thin films provides insights into their crystalline structure and the effects of tungsten incorporation. The primary diffraction peak typically corresponds to the (011) plane of the monoclinic VO₂ phase. At higher doping concentrations, such as 5 wt.% W, a phase transition from monoclinic to tetragonal (rutile) VO₂ can occur, leading to the emergence of new diffraction peaks. Figure 9 shows only one X-ray diffraction (XRD) peak for 5 wt.% tungsten-doped VO₂ thin films at approximately 2θ = 56.2°. It is indicative of the (220) reflection in certain crystalline materials. Other W-doped VO₂ thin films have no significant XRD diffraction peaks.

The Raman spectra of undoped and W-doped VO₂ thin films were measured with different W doping contents. All Raman spectra of the undoped and W-doped VO₂ thin films exhibit similar Raman spectral profiles. Figure 10 shows that the characteristic peaks for different doping contents are all located at 612 cm⁻¹. Among the W-doped VO₂ thin films, the sample with 5 wt.% tungsten exhibits the highest Raman peak intensity, which is indicative of enhanced crystallinity relative to the other compositions. The characteristic VO₂ vibrational mode is observed at 612 cm⁻¹, with no detectable signals corresponding to secondary vanadium oxide phases. As illustrated in Figure 8, the Raman peaks of both undoped and W-doped VO₂ thin films are in good agreement with previously reported data [31]. Notably, W-doped VO₂ films display significantly increased peak intensities and broader linewidths compared to the undoped VO₂ thin film, suggesting that tungsten incorporation stabilizes a local rutile structure within the lattice. This structural modification facilitates partial metallization of the original semiconducting VO₂ phase, consistent with previous reports on W-induced phase behavior [32].

4. Conclusions

This study investigates the influence of W doping (1% to 5%) on the properties of vanadium dioxide (VO₂) thin films. The effects of tungsten doping on optical transmittance, residual stress, surface roughness, and thermal stress behaviors are systematically analyzed. Our experimental results revealed that the addition of tungsten significantly alters the structural, optical, and mechanical properties of VO₂ thin films, making them potential candidates for applications in smart windows and thermochromic coatings. The tungsten doping content in VO₂ thin film causes the optical transmittance of the W-doped VO₂ thin films to decrease in the mid-infrared wavelength range, especially at higher wavelengths (above 5 μm), showing that the tungsten doping content affects the optical transparency of the undoped and W-doped VO₂ thin films. As the weight percentage of W doping increases, the residual compressive stress in the W-doped VO₂ thin films gradually increases. When the W doping content is 5%, the residual compressive stress reaches 1.89 GPa. This implies that the doping content of tungsten changes the internal lattice structure of the VO₂ thin film. During the heating process, as the tungsten doping content increases, the thermal stress in the VO₂ thin film gradually changes from a compressive stress to a tensile stress (Figure 5). Excessive residual stress can lead to mechanical failures, reducing the effectiveness of the material for thermochromic applications. Proper optimization of the deposition conditions and doping levels is essential to manage residual stress while maintaining the desired optical and thermochromic properties of VO₂ thin films. Therefore, an increase in the W doping content not only significantly changes the optical properties but also affects the stress behavior of VO₂ films. This shows that by adjusting the W doping content, the optical properties and residual stress of W-doped VO₂ films can be effectively controlled. In addition, this research potentially opens up new avenues for tailoring the thermochromic properties of VO₂ thin films, which could be beneficial in applications such as smart windows, sensors, and energy-efficient devices.