Effect of Substrate Temperature on the Structural, Optical and Electrical Properties of DC Magnetron Sputtered VO2 Thin Films

This study focuses on the effect of the substrate temperature (TS) on the quality of VO2 thin films prepared by DC magnetron sputtering. TS was varied from 350 to 600 °C and the effects on the surface morphology, microstructure, optical and electrical properties of the films were investigated. The results show that TS below 500 °C favors the growth of V2O5 phase, whereas higher TS (≥500 °C) facilitates the formation of the VO2 phase. Optical characterization of the as-prepared VO2 films displayed a reduced optical transmittance (T˜) across the near-infrared region (NIR), reduced phase transition temperature (Tt), and broadened hysteresis width (ΔH) through the phase transition region. In addition, a decline of the luminous modulation (ΔT˜lum) and solar modulation (ΔT˜sol) efficiencies of the as-prepared films have been determined. Furthermore, compared with the high-quality films reported previously, the electrical conductivity (σ) as a function of temperature (T) reveals reduced conductivity contrast (Δσ) between the insulating and metallic phases of the VO2 films, which was of the order of 2. These outcomes indicated the presence of defects and unrelaxed lattice strain in the films. Further, the comparison of present results with those in the literature from similar works show that the preparation of high-quality films at TS lower than 650 °C presents significant challenges.


Introduction
Vanadium dioxide (VO 2 ) has been widely studied for applications such as optical switches, sensors, smart windows, and optical memory devices [1][2][3][4]. Currently, potential applications can also be found in neuromorphic computing materials and metamaterials [5][6][7][8]. The primary reason for its importance in electronics, photonics and optical switching applications is the fast metal-to-insulator phase transition (MIT) between the semiconductor and metallic phases. During the MIT, the VO 2 crystal undergoes an abrupt phase transformation from high temperature tetragonal phase to a low temperature monoclinic phase at around 68 • C [9]. Up to now, a variety of deposition methods (i.e., magnetron sputtering, pulsed laser deposition, chemical vapor deposition, electron beam evaporation etc.) have been employed to prepare VO 2 thin films [10][11][12][13][14][15]. Regardless of the method, high growth temperatures (450-800 • C) are employed to assure its stoichiometry [16][17][18][19], which is crucial in the synthesis of VO 2 thin films that display a sharp phase transition. However, instrumentation that delivers high substrate temperatures (T S ) is costly and not industry-friendly [20]. For this reason, there have been numerous efforts to bring the T S down to more practical temperatures [21][22][23][24][25][26][27]. A few studies have already reported the fabrication of the VO 2 thin films at low T S (down to around 300 • C) by applying a buffer layer such as ZnO, AZO, TiO 2 , SnO 2 or VO [21][22][23]. Other pronounced efforts typically include substrate biasing,

Materials and Methods
Single crystal p-Si (100) and r-cut sapphire were used as substrates for the thin film preparation. Prior to the deposition, the substrates were ultrasonically cleaned in ethanol and dried in air. After the cleaning process, the substrates were placed in a sputtering chamber. DC reactive magnetron sputtering was utilized to prepare VO 2 thin films. For this purpose, a high-purity vanadium target (99.95%, Plasmionique, Inc., Varennes, QC, Canada) was used. During every deposition, stable argon (Ar) and oxygen (O 2 ) flow with constant rates of 100 sccm and 1.3 sccm were maintained, respectively. The selected Ar and O 2 flow rates facilitate the growth of pure VO 2 films using the above-mentioned deposition process which has been also reported in Ref. [17]. All depositions were performed under a constant pressure of 1.3 Pa for a duration of 2 h. The thin films were prepared at six different T S values. The deposition conditions for each T S are summarized in Table 1. After deposition, the structural properties of the as-prepared thin films were examined at room temperature using Raman spectroscopy (Renishaw Invia Reflex Raman Microscope, Wharton-under-Edge, UK), scanning electron microscopy (Hitachi HT-7700 SEM, Tokyo, Japan), and X-ray diffractometry (Rigaku Ultima IV XRD, Tokyo, Japan). The average crystalline grain size was estimated using ImageJ (a software program developed by NIH, Bethesda, MD, USA [34]) on SEM images. Surface profilometry was used to examine the surface roughness of the films using a profilometer (Zygo OMP-0592, Middlefield, CT, USA). The optical transmittance spectrum was measured using a Lambda 900 UV-Vis-NIR Perkin-Elmer spectrophotometer (Waltham, MA, USA) in the wavelength (λ) range of 300-2500 nm. During the optical measurements, VO 2 -film/substrate sample was mounted on a purpose-built sample heating platform connected to a DC power supply (similar to the one in Ref. [32]). An Omega ® CN-16Pt-330 PID temperature controller (La Chauxde-Fonds, Switzerland) was used to control the sample holder temperature and monitor the sample temperature during the heating and cooling processes. The temperature (T) dependence of the optical transmittance ( T) measurements were conducted at a heating rate of~0.5 • C/min. The T-dependence of the conductivity (σ) of the as-prepared films on sapphire substrates were carried out by depositing gold (Au) contacts at room temperature with the size of (1 mm × 2 mm) using a radio frequency (RF) sputtering system. These measurements were carried out in a cryostat where the samples were placed on a copper (Cu) block which was connected to a DC power supply. The supply was used to heat the block. The temperature of the samples was measured by the use of a thermocouple which was mounted in close proximity to the sample. During these measurements, the heating of the sample was specifically kept at a low rate (~0.5 • C/min) to prevent a lag between the temperature reading and the actual temperature of the sample.

Microstructural Characterization by Raman Spectroscopy and XRD
The microstructural characterization of the vanadium oxide thin films prepared at different T S was carried out using Raman spectroscopy and x-ray diffraction (XRD) for which the results can be found in Figures 1 and 2, respectively. These figures show that the pure monoclinic VO 2 phase is achieved only for samples deposited at T S = 500 • C and higher temperatures. In Figure 1, the Raman peaks at 144, 193, 223, 260, 308, 334, 389, 497 and 613 cm −1 represent the pure monoclinic phase of VO 2 [15]. In Figure 2, the diffraction peaks at 2θ = 28 • , 56 • , and 58 • correspond to the (011), (220), and (022) of pure monoclinic phase of VO 2 [35]. For T S = 450 • C, the Raman peaks of pure VO 2 are not observed in Figure 1 and a small diffraction peak at 2θ = 20 • appears in Figure 2, which corresponds to the (001) peak of V 2 O 5 . As the T S is further decreased down to 350 • C, the (001) peak of V 2 O 5 dominates the XRD spectrum as can be seen in Figure 2a Figure 1a,b, signify the presence of V 2 O 5 phase to be the dominant in the thin film [15] below a T S of 450 • C. It can be seen that both Raman and XRD peaks of V 2 O 5 become the strongest when the T S reaches 350 • C. It is worth mentioning that the sharp Raman peak around 520 cm −1 for the film deposited T S = 350 • C on a Si substrate arises from the Si substrate itself and is absent in films deposited on sapphire substrates.     Figure 3 presents the SEM surface images of the as-prepared VO2 thin films. It can be observed that the TS affects the surface morphology of the thin films. As observed from (a) to (b) to (c) the films show a misoriented crystalline structure with grain sizes ranging between around 100 and 150 nm. The substrate temperatures of 450 and 400 °C produce V2O5 nanostructured films with smooth surfaces. When the TS is lowered to 350 °C the V2O5 grains become clearly discernable in the films. Surface profilometry shows that TS also affects the surface roughness of VO2 films. Figure 4 shows the surface profile images of the VO2 films deposited at various TS-values. These images are taken at a surface area of approximately 0.6 × 0.6 mm 2 of each film. Roughness measurements show that the rootmean-square (Rq) roughness of films on both Si and sapphire substrates increases as TS is lowered. The maximum Rq measured was obtained for VO2 films deposited at 500 °C, with 10 nm for films on Si and 8 nm for films on sapphire substrates. The minimum Rq is measured for films deposited at 600 °C as 4 nm on Si and 3 nm on sapphire substrates. Yet, these films show increased roughness compared to the films prepared at 650 °C [17]. The Rq is measured as 3 nm on Si and 2 nm on sapphire substrates for films prepared at 650 °C. These, in fact, manifest properties associated with high-quality VO2 films as investigated previously [36]. Nevertheless, the average roughness (Ra) does not exhibit a strong  Figure 3 presents the SEM surface images of the as-prepared VO 2 thin films. It can be observed that the T S affects the surface morphology of the thin films. As observed from (a) to (b) to (c) the films show a misoriented crystalline structure with grain sizes ranging between around 100 and 150 nm. The substrate temperatures of 450 and 400 • C produce V 2 O 5 nanostructured films with smooth surfaces. When the T S is lowered to 350 • C the V 2 O 5 grains become clearly discernable in the films. Surface profilometry shows that T S also affects the surface roughness of VO 2 films. Figure 4 shows the surface profile images of the VO 2 films deposited at various T S -values. These images are taken at a surface area of approximately 0.6 × 0.6 mm 2 of each film. Roughness measurements show that the root-mean-square (R q ) roughness of films on both Si and sapphire substrates increases as T S is lowered. The maximum R q measured was obtained for VO 2 films deposited at 500 • C, with 10 nm for films on Si and 8 nm for films on sapphire substrates. The minimum R q is measured for films deposited at 600 • C as 4 nm on Si and 3 nm on sapphire substrates. Yet, these films show increased roughness compared to the films prepared at 650 • C [17]. The R q is measured as 3 nm on Si and 2 nm on sapphire substrates for films prepared at 650 • C. These, in fact, manifest properties associated with high-quality VO 2 films as investigated previously [36]. Nevertheless, the average roughness (R a ) does not exhibit a strong variation with respect to T S , in contrast to the clear trend exhibited by R q . Table 2 and Figure 5 present the average grain size (GS avg ), R a and R q of the VO 2 thin films prepared on Si and sapphire substrates. The grain size in the films on sapphire substrates decreases from 153 nm to 108 nm as T S is lowered from 600 to 500 • C. Indeed, near-epitaxial growth on sapphire films has been reported in the literature [37]. On the other hand, the grain size of the films prepared on Si substrates, within experimental uncertainties, does not exhibit any compelling dependence on T S , even though there is seemingly a maximum average grain size at T S = 600 • C. Films on Si substrates prepared at 600 • C show grains with largely varying size (large standard deviation) and orientation. Even though the grain size is reduced for films prepared at T S = 550 and 500 • C the misorientation of grains is still evident. The thickness of the VO 2 films prepared at T S = 600, 550 and 500 • C are measured by using cross-section SEM images as 210 ± 8, 230 ± 14, and 289 ± 15 nm, respectively.      In (b) the dashed vertical line is an asymptote that indicates the near-epitaxial almost flat surface of the film prepared at 650 °C [17]. (c,d) present the average (Ra) and root-mean-square (Rq) roughness of films prepared on Si and sapphire substrates, respectively. The dashed lines between the data points are guides to the eye. The data points shown in hollow symbols belong to the films prepared at 650 °C (prepared as described in Ref. [17] but not reported therein).  . (c,d) present the average (R a ) and root-mean-square (R q ) roughness of films prepared on Si and sapphire substrates, respectively. The dashed lines between the data points are guides to the eye. The data points shown in hollow symbols belong to the films prepared at 650 • C (prepared as described in Ref. [17] but not reported therein).

Optical Characterization by Spectrophotometry
To understand the effect of T S on the structural VO 2 phase transition properties, the optical transmission ( T) spectrum of VO 2 thin films was measured at 300 K (27 • C) (in the insulating phase) and at 368 K (95 • C) (in the metallic phase) in a wavelength range between 300 and 2500 nm. Figure 6 presents the T spectra of these VO 2 thin films. In practical terms, no notable difference can be found in the trend of the T spectra for samples prepared at T S = 600 and 550 • C. In Figure 6a, for the film prepared at T S = 500 • C, T 2500nm is observed to be the lowest among other examined films, which is around 34% in the insulating phase. In Figure 6b,c the T of films deposited at T S = 550 and 600 • C on sapphire are observed to be very close. The drop of the T for films prepared at 500 • C can be related to the increased defects and the inferior crystal quality of VO 2 thin films, which can be clearly observed in Figure 3d. At 368 K, all the VO 2 films on both Si and sapphire substrates transition into a near-zero transmittance in the metallic phase. Table 3 presents the details of T values at 300 K and 368 K along with the optical transmittance contrast (∆ T) of the as-prepared VO 2 films obtained at NIR wavelengths of 1500, 2000 and 2500 nm. ∆ T is defined as the difference in the optical transmittance of the insulating and metallic phases. Figure 7a gives the temperature dependence of T 2500nm of the VO 2 thin films prepared at various T S -values. the IM and MI phase transitions. It is important to emphasize that the narrow and sharp hysteresis of IM and MI transitions in VO 2 films deposited at 650 • C on sapphire substrate in Ref. [17] is not observed in the thin films deposited at T S = 600 • C and below. The hysteresis width (∆H) of the films deposited on both Si and sapphire substrates become broader as the T S is lowered. The broadening appears to be about 14-16 K for films on Si and 13-21 K for films on sapphire.    Figure 8 presents the comparison of phase transition properties obtained at λ = 2500 nm. In Figure 8a the insulator-to-metal (T IMT ), metal-to-insulator (T MIT ) and the phase transition (T t ) temperatures are presented. The T t is calculated as the mean of the T IMT and T MIT and is given in Equation (1) below: By definition, T t depends on the hysteresis width (∆H). It is observed that the critical temperatures on silicon substrates are relatively the same as T S is lowered from 650 [17] to 550 • C, but eventually decrease sharply for films prepared at 500 • C. For films on Si, as the T t stays approximately constant for T S = 650, 600 and 550 • C at about 340 K but falls to 320 K for T S = 500 • C. On the other hand, the T t of the films on sapphire appear to be lower compared to that for the film prepared at 650 • C in Ref. [17], displaying the lowest T t around 322 K at 500 • C. There is no clear dependence of T t on T S for films prepared on sapphire substrates. This change in T t indicates increased defects in the VO 2 films. In addition, the change may be attributed to the spatial variation of local T IMT and T MIT due to increased number of coexisting metallic and insulating phases within the volume of the films as pointed out elsewhere [38]. Figure 8b presents the ∆H and the full width half maximum (FWHM) of the IM and MI transitions of the as-prepared VO 2 thin films. The increasing FWHM can be interpreted as the degrading sharpness of the phase transitions. Figure 8c introduces the comparison of transmittance contrast (∆ T) of the as-prepared films at λ = 1500, 2000, and 2500 nm which are commonly considered in NIR applications [39][40][41][42][43]. At lower T S , the ∆H becomes broader. The luminous (visible) and solar transmission characteristics of the as-prepared VO 2 films deposited on sapphire substrates are also analyzed. The luminous transmittance ( T lum ), solar transmittance ( T sol ), the luminous modulation (∆ T lum ) and solar modulation (∆ T sol ) efficiencies are commonly considered properties for smart window applications [44][45][46][47][48][49][50]. T lum measures the amount of visible light passing through a semi-transparent material. T sol is considered as an index that characterizes the transmission of sunlight within the visible and near-infrared spectrum. T lum and T sol can be determined from Equation (2) below:      T lum is obtained for a wavelength range between 380 and 780 nm and T sol is obtained for wavelengths between 300 and 2500 nm. ϕ lum (λ) is the spectral sensitivity of the light adapted human eye [51], ϕ sol (λ) is the solar irradiance spectrum for air mass of 1.5 (37 • sunlight above the horizon of earth) [52], and T(λ) is acquired from the transmittance spectrum of the as-prepared films. ∆ T lum and ∆ T sol are calculated from the Equation (3) given below: ∆ T lum, sol = T lum, sol (300 K) − T lum, sol (368 K) The results of the calculations of T lum,sol and ∆ T lum,sol values are summarized in Table 4. Figure 9 gives the distribution of the data presented in Table 4. In Figure 9a, T lum values obtained for VO 2 thin films prepared at T S = 500, 550 and 600 • C in the insulating phase (300 K) appear to be smaller from that obtained for films prepared at T S = 650 • C. The T lum values in the metallic phase (368 K) show a similar characteristic. Since the difference between T lum values obtained at 300 and 368 K is not large due to the high optical transmittance between 450 and 700 nm, the ∆ T lum values are small. Figure 9b presents T sol and ∆ T sol values of the as-prepared VO 2 films which are also lower than those obtained for the film prepared at 650 • C [17]. However, the T sol values at 368 K are indeed much lower compared to the ones at 300 K, due to the near-zero transmission in the NIR region integrated in Equation (2). This large difference between the T sol values at 300 and 368 K is reflected on the large values of ∆ T sol .      Figure 10a shows the temperature (T) dependence of the electrical conductivity (σ) of the VO 2 films prepared at T S = 500, 550, 600 and 650 [32] • C, and their phase transition properties. The critical temperatures T IMT , T MIT and the sharpness (FWHM) are obtained from the d(log(σ))/dT vs. T. plots as shown in Figure 10b-e. Here, it can be seen that there is only a slight broadening of the ∆H. On the other hand, the electrical phase transition properties of VO 2 thin films studied in this work show a prominent difference compared to the film prepared at T S = 650 • C in Ref. [32]. The difference between the hysteresis properties of the samples is due to the difference in the nature of electrical and structural (observed optically) MIT [33,53]. Figure 11 gives the critical temperatures and phase transition parameters of VO 2 films deposited on sapphire substrates obtained in σ vs. T measurements. As it is clear from the Figure 11a, the transition temperatures (T IMT , T MIT and T t ) drop with decreasing T S . In Figure 11b, the ∆H does not seem to be greatly affected from T S . As the T S is lowered from 650 to 550 • C, the ∆H broadens from 9 to 11 K, respectively. However, for T S = 500 • C, the ∆H drops to 8.4 K which shows narrower ∆H compared to the other VO 2 films. On the other hand, the effect of T S appears to be prominent in the sharpness of both IM and MI transitions and the conductivity contrast (∆σ). The FWHM of the films prepared between 500 and 600 • C are larger compared with those of the film prepared at 650 • C in both IM and MI transitions. The increase of FWHM, or loss of sharpness, in films between 500 and 600 • C indicates degradation of the abrupt phase transition. In addition, compared to the high-quality VO 2 films prepared at 650 • C (Ref. [32]), the ∆σ of the films prepared at T S = 600 • C and below displays a substantial drop from the order of 4 to 2.

Discussion
We begin our discussion by considering the structural investigation of VO 2 thin films. The results presented in Figures 1-4 clearly show that the T S has a considerable effect on the film structure, including both the crystallinity and surface roughness. For T S between 600 and 500 • C, both the XRD diffractograms and the Raman spectra show VO 2 peaks, which disappear for T S ≤ 450 • C. From this observation, it can be understood that higher T S (≥500 • C) facilitates the formation of the VO 2 phase, whereas lower temperature growth favors the formation of the V 2 O 5 phase as can be referred to in Table 1. Since the V 2 O 5 phase appears at lower T S and the root mean square roughness (R q ) increases with lower T S , one may argue that there may be a correlation between the two observations. Nonetheless, we do not have any further experimental evidence to validate the latter. Yet, it is apparent from Figure 3 that low T S leads to a polycrystalline structure of misoriented grains with dissimilar geometries. Thus, not only the smaller grain size but also the increased defects in the films at low T S (e.g., 500 • C) may explain the drop in the T spectrum. Secondly, the drop in the transition temperature (T t ) can be related to the significance of unrelaxed lattice strain [54,55]. The degradation of the crystalline quality can be seen in the worsening of the phase transition properties given in Figure 7. Moreover, considering the electrical characterization, the degradation of contrast in the conductivity between the insulator and metallic phases (∆σ) may be a result of the reduction of the current path due to increased oxygen vacancies and surface roughness [56]. We also examined the conductivity activation energy of the as-prepared VO 2 films. The Arrhenius plot of ln(σ) vs. 1000/T is presented in Figure 12 where it is apparent that the conductivity (σ) of VO 2 is thermally activated. We can represent the thermally activated behavior by, where ∆E = E g if we assume the sample is intrinsic. Equation (4) yields ∆E of the VO 2 films as 0.53 ± 0.02, 0.52 ± 0.01 and 0.49 ± 0.01 eV corresponding to prepared at T S = 600, 550 and 500 • C, respectively. The thermally activated conductivity can arise either from intrinsic conductivity (thermal generation across the bandgap) with ∆E = E g as in Equation (4) or from the ionization of defects or impurities in the film, in which case ∆E is not the bandgap but the magnitude of the energy difference between the transport band and the defect or impurity level. The ∆E values appear to be lower than that of high-quality VO 2 films, which is 0.64 ± 0.01 eV from Ref. [36]. The ∆E values seem to follow a similar trend to that of ∆σ with respect to T S in Figure 11b. The drop in ∆σ and the apparent narrowing of ∆E may too be related to the increased density of grain boundaries and defects of oxygen vacancies due to low T S which is pointed out in Ref. [57]. In addition, Figure 13 presents the shifting of the amplified XRD peak of VO 2 (011) at~28 • to a lower angle as T S decreases. This signifies an enlarged lattice parameter of VO 2 due to defects and lattice strain, which is also suggested in Ref. [56]. It is worth comparing ∆E = 0.49-0.64 eV for these VO 2 films with the indirect (smallest) optical bandgap reported for VO 2 films deposited at 650 • C, which is 0.52 eV [17]. The work in Ref. [33] puts the optical bandgap between in the range 0.55-0.60 eV; again ∆E is very close to these values.
VO2 films on sapphire substrates presented in this work drops down to 323 K as the TS is lowered to 500 °C. As can be observed in Figure 14b, reducing the TS has a significant impact on the ΔH of the films prepared in the present work which can be related to the polycrystallinity as well as increased compressive and tensile strain in the films [28,32]. Figure 14c,d present the comparison of electrical phase transition properties obtained from Refs. [28,57]. Moreover, VO2 films reported in Ref. [36] were prepared by molecular beam epitaxy (MBE) at TS = 350 °C. In Figure 14c, the Tt of the films of Ref. [28] increases from a low TS = 450 to 550 °C and then drops at high TS = 700 and 720 °C. On the contrary, in Figure 14d, the ΔH values appear to be relatively unaffected by TS, and they seem to be mostly constant around 5.5 K, only films prepared at 450 °C have a large ΔH around 10 K.
The Tt values obtained from Ref. [57] are constant around 343 K. In addition, the ΔH drops with decreasing TS from 13 K and 5 K. However, if the phase transition sharpness and Δσ are taken into consideration, these films in Refs. [28,57] do not display attributes associated with high-quality films with decreasing TS. Overall, according to these comparisons, the preparation of high-quality VO2 films at low temperatures appear to be a challenge even though different deposition techniques and deposition parameters are applied. The degradation of the film properties may essentially rise from loss of crystalline quality, the degradation of stoichiometry (e.g., if there is an interface film between VO2 and the substrate) and the presence of defects and lattice strain that arise from film-substrate mismatch.   Finally, we compare our results with other works [21,28,30,57] in which the effect of T S on the phase transition properties of VO 2 thin films have been studied. These studies have employed RF reactive inverted cylindrical magnetron sputtering [21], RF magnetron sputtering [28], inductively coupled plasma (ICP)-assisted sputtering [30], and pulsed-laser deposition (PLD) [57] for thin film preparation. We also consider selected works [26,36] which have reported the production of high-quality VO 2 thin films at low substrate temperatures. Figure 14 presents the phase transition properties of VO 2 thin films obtained by means of optical transmittance ( T) and electrical conductivity (σ) measurements. Figure 14a,b show the comparison of the T t and ∆H characteristics of the films on sapphire substrates from the present work with the films prepared in Refs. [21,30]. It should be noted that the optical transmittance has been measured at different wavelengths. VO 2 films reported in Ref. [26] were prepared at T S = 200 • C by reactive magnetron sputtering with a post-deposition annealing treatment at 300 • C, and the transition properties were obtained at λ = 2000 nm. The present work has used λ = 2500 nm, whereas λ = 1450 nm for Ref. [21] and 1100 nm for Ref. [30] were used. In Figure 14a, the films in Ref. [21] have a constant T t around 340 K, whereas the T t of films in Ref. [30] decreases from Ts = 600 to 500 • C and stays constant around 343 K until Ts = 450 • C. In contrast, the T t of the VO 2 films on sapphire substrates presented in this work drops down to 323 K as the T S is lowered to 500 • C. As can be observed in Figure 14b, reducing the T S has a significant impact on the ∆H of the films prepared in the present work which can be related to the polycrystallinity as well as increased compressive and tensile strain in the films [28,32]. Figure 14c,d present the comparison of electrical phase transition properties obtained from Refs. [28,57]. Moreover, VO 2 films reported in Ref. [36] were prepared by molecular beam epitaxy (MBE) at T S = 350 • C. In Figure 14c, the T t of the films of Ref. [28] increases from a low T S = 450 to 550 • C and then drops at high T S = 700 and 720 • C. On the contrary, in Figure 14d, the ∆H values appear to be relatively unaffected by T S , and they seem to be mostly constant around 5.5 K, only films prepared at 450 • C have a large ∆H around 10 K. The T t values obtained from Ref. [57] are constant around 343 K. In addition, the ∆H drops with decreasing T S from 13 K and 5 K. However, if the phase transition sharpness and ∆σ are taken into consideration, these films in Refs. [28,57] do not display attributes associated with high-quality films with decreasing T S . Overall, according to these comparisons, the preparation of high-quality VO 2 films at low temperatures appear to be a challenge even though different deposition techniques and deposition parameters are applied. The degradation of the film properties may essentially rise from loss of crystalline quality, the degradation of stoichiometry (e.g., if there is an interface film between VO 2 and the substrate) and the presence of defects and lattice strain that arise from film-substrate mismatch.

Conclusions
The effect of substrate temperature (T S ) on the structural, optical and electrical properties of VO 2 thin films during DC magnetron sputtering have been investigated. The results showed that the T S plays a crucial role on the microstructure and hence the optical and electrical phase transition properties of the VO 2 films. It is found that T S below 500 • C favors the growth of the V 2 O 5 phase where T S of 500 • C and higher facilitate the growth of stoichiometric VO 2 films. The lowering of T S from 600 to 500 • C have shown reduced optical transmittance ( T) and modulation efficiencies, broadened hysteresis width (∆H), and decreased transition temperature, which suggests the presence of defects and unrelaxed lattice strain in the films. The comparison of the present results with those published on similar VO 2 samples, where the effect of T S has been examined, indicates significant challenges remain in the fabrication of high-quality VO 2 films below a T S of 650 • C.  Data Availability Statement: Data is available from the corresponding author upon reasonable request.