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

Abstract

This study focuses on the effect of the substrate temperature (T_S) on the quality of VO₂ thin films prepared by DC magnetron sputtering. T_S 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 T_S below 500 °C favors the growth of V₂O₅ phase, whereas higher T_S (≥500 °C) facilitates the formation of the VO₂ phase. Optical characterization of the as-prepared VO₂ films displayed a reduced optical transmittance ($\tilde{T}$) across the near-infrared region (NIR), reduced phase transition temperature (T_t), and broadened hysteresis width (ΔH) through the phase transition region. In addition, a decline of the luminous modulation $(\mathsf{\Delta }{\tilde{T}}_{\mathrm{lum}})$ and solar modulation ($\mathsf{\Delta }{\tilde{T}}_{\mathrm{sol}}$) 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 T_S lower than 650 °C presents significant challenges.

1. Introduction

Vanadium dioxide (VO₂) 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 VO₂ 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 VO₂ 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 VO₂ 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 VO₂ thin films at low T_S (down to around 300 °C) by applying a buffer layer such as ZnO, AZO, TiO₂, SnO₂ or VO [21,22,23]. Other pronounced efforts typically include substrate biasing, applying high power pulse, and post-annealing [24,25,26,27]. In order to reduce the T_S, a clear understanding of the effect of the synthesis (substrate) temperature on the stoichiometry, structure and phase transition properties is critical. Some previous works have already revealed the effects of T_S on the microstructure, stress state, and transition temperatures of VO₂ thin films [16,28,29,30,31]. Nevertheless, these works are not comprehensive and do not necessarily signify a clear relation of the underlying mechanism related to physical changes due to the T_S.

In our earlier works [17,32], we have reported the preparation of stoichiometric, near-epitaxial VO₂ thin films at T_S of 650 °C by DC reactive magnetron sputtering, where the films showed highly discernable optical and electrical phase transition properties (i.e., high optical transmittance ($\mathsf{\Delta }\tilde{T}$) and electrical conductivity (Δσ) contrasts, sharp transitions and near-zero optical transmission in the metallic phase). We have also investigated the effect of substrate biasing on the quality of VO₂ thin films [33]. In the present work, we have undertaken the deposition of VO₂ films at T_S varying from 350 to 600 °C and examined the effects on the microstructure, surface morphology, roughness, optical and electrical phase transition properties. We compared these results with the films deposited at 650 °C in our previous works [17,32]. In addition, we extracted and analyzed the $\mathsf{\Delta }\tilde{T}$ along with the luminous $(\mathsf{\Delta }{\tilde{T}}_{\mathrm{lum}})$ and solar ($\mathsf{\Delta }{\tilde{T}}_{\mathrm{sol}}$) optical modulation efficiencies. Furthermore, we estimated the electrical bandgap of the films and compared the results with those of high-quality VO₂ thin films prepared at 650 °C reported previously [32]. Additionally, we compared our results with other works that studied the effect of T_S on the properties of VO₂ thin films.

2. 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 VO₂ 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 (O₂) flow with constant rates of 100 sccm and 1.3 sccm were maintained, respectively. The selected Ar and O₂ flow rates facilitate the growth of pure VO₂ 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. Deposition conditions of VO₂ thin films on Si and sapphire substrates.

Substrate Temperature (TS) (°C)	Oxygen Flow Rate (sccm)	Sputtering Power (W)	Pressure (Pa)	Duration (Hour)	Phase
350	1.3	100	1.33	2	V2O5
400	1.3	100	1.33	2	V2O5
450	1.3	100	1.33	2	V2O5
500	1.3	100	1.33	2	VO2
550	1.3	100	1.33	2	VO2
600	1.3	100	1.33	2	VO2
650 [17]	1.3	100	1.33	2	VO2

.

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, VO₂-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 Chaux-de-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 ($\tilde{T}$) 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.

3. Results

3.1. 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 Figure 1 and Figure 2, respectively. These figures show that the pure monoclinic VO₂ 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 VO₂ [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 VO₂ [35]. For T_S = 450 °C, the Raman peaks of pure VO₂ 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 V₂O₅. As the T_S is further decreased down to 350 °C, the (001) peak of V₂O₅ dominates the XRD spectrum as can be seen in Figure 2a,b. The Raman peaks at 145, 195, 284, 303, 405, 483, 528, 701, and 996 cm⁻¹ in Figure 1a,b, signify the presence of V₂O₅ 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 V₂O₅ 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.

3.2. Surface Morphology and Roughness Characterization by SEM and Profilometry

Figure 3 presents the SEM surface images of the as-prepared VO₂ 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 V₂O₅ nanostructured films with smooth surfaces. When the T_S is lowered to 350 °C the V₂O₅ grains become clearly discernable in the films. Surface profilometry shows that T_S also affects the surface roughness of VO₂ films. Figure 4 shows the surface profile images of the VO₂ films deposited at various T_S-values. These images are taken at a surface area of approximately 0.6 × 0.6 mm² 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 VO₂ 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 VO₂ 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. Average grain size (GS_avg), average (R_a) and root-mean square (R_q) roughness of VO₂ thin films prepared at various substrate temperatures (T_S).

Substrate Temperature (TS) (°C)	GSavg (nm)	Ra (nm)	Rq (nm)	GSavg (nm)	Ra (nm)	Rq (nm)
	Si			Sapphire
500	104 ± 20	2	10	108 ± 16	4	8
550	116 ± 26	5	5	124 ± 27	2	6
600	145 ± 51	3	4	153 ± 59	2	3
650 [17]	114 ± 24	2	3	~Flat	2	2

and Figure 5 present the average grain size (GS_avg), R_a and R_q of the VO₂ 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 VO₂ 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.

3.3. Optical Characterization by Spectrophotometry

To understand the effect of T_S on the structural VO₂ phase transition properties, the optical transmission ($\tilde{T}$) spectrum of VO₂ 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 $\tilde{T}$ spectra of these VO₂ thin films. In practical terms, no notable difference can be found in the trend of the $\tilde{T}$ spectra for samples prepared at T_S = 600 and 550 °C. In Figure 6a, for the film prepared at T_S = 500 °C, ${\tilde{T}}_{2500\mathrm{nm}}$ is observed to be the lowest among other examined films, which is around 34% in the insulating phase. In Figure 6b,c the $\tilde{T}$ of films deposited at T_S = 550 and 600 °C on sapphire are observed to be very close. The drop of the $\tilde{T}$ for films prepared at 500 °C can be related to the increased defects and the inferior crystal quality of VO₂ thin films, which can be clearly observed in Figure 3d. At 368 K, all the VO₂ films on both Si and sapphire substrates transition into a near-zero transmittance in the metallic phase.

Table 3. Optical transmittance ($\tilde{T}$ ) and transmittance contrast ($\mathsf{\Delta }\tilde{T}$ ) values of VO₂ films prepared at different substrate temperatures (T_S) obtained at 300 K (insulating) and 368 K (metallic) for λ = 1500 nm, 2000 nm and 2500 nm.

Substrate Temperature (TS) (°C)	λ (nm)	${\tilde{\mathit{T}}}_{300\mathbf{K}}$ (%)	${\tilde{\mathit{T}}}_{368\mathbf{K}}$ (%)	$\mathbf{\Delta }\tilde{\mathit{T}}$ (%)	${\tilde{\mathit{T}}}_{300\mathbf{K}}$ (%)	${\tilde{\mathit{T}}}_{368\mathbf{K}}$ (%)	$\mathbf{\Delta }\tilde{\mathit{T}}$ (%)
			Si			Sapphire
500	2500	34.5	0.5	34.0	35.0	0.6	34.4
	2000	27.6	0.5	27.1	33.3	0.7	32.6
	1500	16.9	0.5	16.4	24.4	0.7	23.5
550	2500	51.9	0.2	51.7	44.5	0.2	44.3
	2000	45.0	0.2	44.8	46.7	0.3	46.4
	1500	30.7	0.3	30.4	41.1	0.3	40.8
600	2500	48.2	0.1	48.1	43.7	0.1	43.6
	2000	42.0	0.1	41.9	48.7	0.1	48.6
	1500	27.3	0.1	27.2	31.8	0.2	31.6
650 [17]	2500	54.9	0.3	54.6	51.3	0.5	50.8
	2000	49.3	0.4	48.9	45.2	0.5	44.7
	1500	35.5	0.5	35.0	39.9	0.8	39.1

presents the details of $\tilde{T}$ values at 300 K and 368 K along with the optical transmittance contrast ($\mathsf{\Delta }\tilde{T}$) of the as-prepared VO₂ films obtained at NIR wavelengths of 1500, 2000 and 2500 nm. $\mathsf{\Delta }\tilde{T}$ is defined as the difference in the optical transmittance of the insulating and metallic phases.

Figure 7a gives the temperature dependence of ${\tilde{T}}_{2500\mathrm{nm}}$ of the VO₂ thin films prepared at various T_S-values. Figure 7b–e show the $\mathrm{d}\tilde{T}/\mathrm{d}T$ vs. T plots which give the critical temperatures (T_IMT and T_MIT) and sharpness (full width at half maximum, FWHM) of the IM and MI phase transitions. It is important to emphasize that the narrow and sharp hysteresis of IM and MI transitions in VO₂ 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 (ΔH) 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:

$$T_t=\frac{T_{\mathrm{IMT}}+T_{\mathrm{MIT}}}{2}$$

(1)

By definition, T_t depends on the hysteresis width (ΔH). 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 VO₂ 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 ΔH and the full width half maximum (FWHM) of the IM and MI transitions of the as-prepared VO₂ thin films. The increasing FWHM can be interpreted as the degrading sharpness of the phase transitions. Figure 8c introduces the comparison of transmittance contrast ($\mathsf{\Delta }\tilde{T}$) 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 ΔH becomes broader. The luminous (visible) and solar transmission characteristics of the as-prepared VO₂ films deposited on sapphire substrates are also analyzed. The luminous transmittance (${\tilde{T}}_{\mathrm{lum}}$), solar transmittance (${\tilde{T}}_{\mathrm{sol}}$), the luminous modulation $(\mathsf{\Delta }{\tilde{T}}_{\mathrm{lum}})$ and solar modulation ($\mathsf{\Delta }{\tilde{T}}_{\mathrm{sol}}$) efficiencies are commonly considered properties for smart window applications [44,45,46,47,48,49,50]. ${\tilde{T}}_{\mathrm{lum}}$ measures the amount of visible light passing through a semi-transparent material. ${\tilde{T}}_{\mathrm{sol}}$ is considered as an index that characterizes the transmission of sunlight within the visible and near-infrared spectrum. ${\tilde{T}}_{\mathrm{lum}}$ and ${\tilde{T}}_{\mathrm{sol}}$ can be determined from Equation (2) below:

$${\tilde{T}}_{\mathrm{lum},\mathrm{sol}}=\frac{{\displaystyle {\int }_{}^{}{\phi }_{\mathrm{lum},\mathrm{sol}}(\lambda )\tilde{T}(\lambda )d\lambda }}{{\displaystyle {\int }_{}^{}{\phi }_{\mathrm{lum},\mathrm{sol}}(\lambda )d\lambda }}$$

(2)

${\tilde{T}}_{\mathrm{lum}}$ is obtained for a wavelength range between 380 and 780 nm and ${\tilde{T}}_{\mathrm{sol}}$ is obtained for wavelengths between 300 and 2500 nm. ${\phi }_{\mathrm{lum}}(\lambda )$ is the spectral sensitivity of the light adapted human eye [51], ${\phi }_{\mathrm{sol}}(\lambda )$ is the solar irradiance spectrum for air mass of 1.5 (37° sunlight above the horizon of earth) [52], and $\tilde{T}(\lambda )$ is acquired from the transmittance spectrum of the as-prepared films. $\mathsf{\Delta }{\tilde{T}}_{\mathrm{lum}}$ and $\mathsf{\Delta }{\tilde{T}}_{\mathrm{sol}}$ are calculated from the Equation (3) given below:

$$\mathsf{\Delta }{\tilde{T}}_{\mathrm{lum}, \mathrm{sol}}={\tilde{T}}_{\mathrm{lum}, \mathrm{sol}}(300 \mathrm{K})-{\tilde{T}}_{\mathrm{lum}, \mathrm{sol}}(368 \mathrm{K})$$

(3)

The results of the calculations of ${\tilde{T}}_{\mathrm{lum},\mathrm{sol}}$ and $\mathsf{\Delta }{\tilde{T}}_{\mathrm{lum},\mathrm{sol}}$ values are summarized in

Table 4. Luminous and solar modulation efficiencies of VO₂ films prepared on sapphire substrates at various T_S.

Substrate Temperature (TS) (°C)	${\tilde{\mathit{T}}}_{\mathbf{lum}}(\%)$ (300 K)	${\tilde{\mathit{T}}}_{\mathbf{lum}}(\%)$ (368 K)	$\mathbf{\Delta }{\tilde{\mathit{T}}}_{\mathbf{lum}}(\%)$	${\tilde{\mathit{T}}}_{\mathbf{sol}}(\%)$ (300 K)	${\tilde{\mathit{T}}}_{\mathbf{sol}}(\%)$ (368 K)	$\mathbf{\Delta }{\tilde{\mathit{T}}}_{\mathbf{sol}}(\%)$
500	9	6.8	2.2	12	3.6	8.4
550	8.6	5.8	2.8	15.91	3.2	12.71
600	5.8	3.5	2.3	12.64	2	10.64
650 [17]	17.2	9.6	7.6	22.88	5.7	17.18

. Figure 9 gives the distribution of the data presented in Table 4. In Figure 9a, ${\tilde{T}}_{\mathrm{lum}}$ values obtained for VO₂ 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 ${\tilde{T}}_{\mathrm{lum}}$ values in the metallic phase (368 K) show a similar characteristic. Since the difference between ${\tilde{T}}_{\mathrm{lum}}$ values obtained at 300 and 368 K is not large due to the high optical transmittance between 450 and 700 nm, the $\mathsf{\Delta }{\tilde{T}}_{\mathrm{lum}}$ values are small. Figure 9b presents ${\tilde{T}}_{\mathrm{sol}}$ and $\mathsf{\Delta }{\tilde{T}}_{\mathrm{sol}}$ values of the as-prepared VO₂ films which are also lower than those obtained for the film prepared at 650 °C [17]. However, the ${\tilde{T}}_{\mathrm{sol}}$ 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 ${\tilde{T}}_{\mathrm{sol}}$ values at 300 and 368 K is reflected on the large values of $\mathsf{\Delta }{\tilde{T}}_{\mathrm{sol}}$.

3.4. Electrical MIT Characterization

Figure 10a shows the temperature (T) dependence of the electrical conductivity (σ) of the VO₂ 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 ΔH. On the other hand, the electrical phase transition properties of VO₂ 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 VO₂ 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 ΔH does not seem to be greatly affected from T_S. As the T_S is lowered from 650 to 550 °C, the ΔH broadens from 9 to 11 K, respectively. However, for T_S = 500 °C, the ΔH drops to 8.4 K which shows narrower ΔH compared to the other VO₂ 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 (Δσ). 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 VO₂ films prepared at 650 °C (Ref. [32]), the Δσ of the films prepared at T_S = 600 °C and below displays a substantial drop from the order of 4 to 2.

4. Discussion

We begin our discussion by considering the structural investigation of VO₂ thin films. The results presented in Figure 1, Figure 2, Figure 3 and Figure 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 VO₂ 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 VO₂ phase, whereas lower temperature growth favors the formation of the V₂O₅ phase as can be referred to in Table 1. Since the V₂O₅ 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 $\tilde{T}$ 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 (Δσ) 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 VO₂ films. The Arrhenius plot of ln(σ) vs. 1000/T is presented in Figure 12 where it is apparent that the conductivity (σ) of VO₂ is thermally activated. We can represent the thermally activated behavior by,

$$\sigma ={\sigma }_{\mathrm{o}}\exp \left(\frac{\mathsf{\Delta }{E}_{}}{2k_{B}T}\right)$$

(4)

where ΔE = E_g if we assume the sample is intrinsic. Equation (4) yields ΔE of the VO₂ 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 ΔE = E_g as in Equation (4) or from the ionization of defects or impurities in the film, in which case ΔE is not the bandgap but the magnitude of the energy difference between the transport band and the defect or impurity level. The ΔE values appear to be lower than that of high-quality VO₂ films, which is 0.64 ± 0.01 eV from Ref. [36]. The ΔE values seem to follow a similar trend to that of Δσ with respect to T_S in Figure 11b. The drop in Δσ and the apparent narrowing of ΔE 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 VO₂ (011) at ~28° to a lower angle as T_S decreases. This signifies an enlarged lattice parameter of VO₂ due to defects and lattice strain, which is also suggested in Ref. [56]. It is worth comparing ΔE = 0.49–0.64 eV for these VO₂ films with the indirect (smallest) optical bandgap reported for VO₂ 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 ΔE is very close to these values.

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 VO₂ 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 VO₂ thin films at low substrate temperatures. Figure 14 presents the phase transition properties of VO₂ thin films obtained by means of optical transmittance ($\tilde{T}$) and electrical conductivity (σ) measurements. Figure 14a,b show the comparison of the T_t and ΔH 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. VO₂ 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 VO₂ 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 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 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 ΔH 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 ΔH around 10 K. The T_t values obtained from Ref. [57] are constant around 343 K. In addition, the ΔH drops with decreasing T_S 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 T_S. 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.

5. Conclusions

The effect of substrate temperature (T_S) on the structural, optical and electrical properties of VO₂ 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 VO₂ films. It is found that T_S below 500 °C favors the growth of the V₂O₅ phase where T_S of 500 °C and higher facilitate the growth of stoichiometric VO₂ films. The lowering of T_S from 600 to 500 °C have shown reduced optical transmittance ($\tilde{T}$) and modulation efficiencies, broadened hysteresis width (ΔH), 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 VO₂ samples, where the effect of T_S has been examined, indicates significant challenges remain in the fabrication of high-quality VO₂ films below a T_S of 650 °C.