Enhanced Phase Transition Properties of VO2 Thin Films on 6H-SiC (0001) Substrate Prepared by Pulsed Laser Deposition

For growing high quality epitaxial VO2 thin films, the substrate with suitable lattice parameters is very important if considering the lattice matching. In addition, the thermal conductivity between the substrate and epitaxial film should be also considered. Interestingly, the c-plane of hexagonal 6H-SiC with high thermal conductivity has a similar lattice structure to the VO2 (010), which enables epitaxial growth of high quality VO2 films on 6H-SiC substrates. In the current study, we deposited VO2 thin films directly on 6H-SiC (0001) single-crystal substrates by pulsed laser deposition (PLD) and systematically investigated the crystal structures and surface morphologies of the films as the function of growth temperature and film thickness. With optimized conditions, the obtained epitaxial VO2 film showed pure monoclinic phase structure and excellent phase transition properties. Across the phase transition from monoclinic structure (M1) to tetragonal rutile structure (R), the VO2/6H-SiC (0001) film demonstrated a sharp resistance change up to five orders of magnitude and a narrow hysteresis width of only 3.3 °C.


Introduction
Among various vanadium oxides, vanadium dioxide (VO 2 ), a narrow bandgap semiconductor with 0.6 eV, has been extensively studied because of its conspicuous metal-insulator transition (MIT) characteristics at the critical phase transition temperature (T C ) of about 68 • C [1]. When the temperature reaches T C , VO 2 will change from the monoclinic structure (M1, P21/c) to tetragonal rutile structure (R, P42/mnm), accompanied by significant changes in optical and electrical properties [2]. For example, the resistance of VO 2 will be greatly decreased during the MIT process with the change up to 3-5 orders of magnitude. Due to the peculiar phase change characteristics of VO 2 , which can be triggered by temperatures or some other routes such as electric excitation [3]. VO 2 shows many promising applications in the field of optoelectronics, such as steep-slope transistors [4], antennas [5] and photoelectronic devices [6].
The performance of VO 2 based optoelectronic devices is closely associated with the quality of VO 2 films. As a result, the high quality VO 2 film preparation is still a focusing point and essential issue. Many previous experiments have been reported about the VO 2 epitaxial film growth on various substrates such as c-plane sapphire [7][8][9], TiO 2 [10,11], GaN [12] and ordinary quartz glass [13], directly or with a buffer layer [14,15], For instance, Wang et al. deposited VO 2 film on p-GaN/Al 2 O 3 (0001) substrate and explored the heterojunction devices. The growth of VO 2 films on the ZnO/glass substrate provided a new research direction for the application of electronically controlled smart windows [16]. However, compared with substrates of Si, Al 2 O 3 , GaN, TiO 2 and glass, which are commonly used to epitaxially grown VO 2 thin films, SiC has outstanding performance in terms of thermal conductivity [17], electron mobility, thermal stability, and chemical stability. These excellent properties expand the application environment of VO 2 thin films, such as strong acid and alkali, high temperature and other extreme conditions, while, until now, there have been few reports of epitaxial growth of VO 2 films on SiC single-crystal substrates.
In this work, VO 2 thin films were directly grown on 6H-SiC (0001) single-crystal substrates by PLD, the microstructures and electrical properties were also systematically investigated upon the growth temperature and film thickness. Ultra-high quality VO 2 film with pure monoclinic phase structure and excellent phase transition properties were obtained, which demonstrated a sharp resistance change up to five orders of magnitude and a narrow hysteresis width during the MIT process. The success combination of high quality VO 2 epitaxial film with the third-generation semiconductor SiC crystal will have an important impact on future devices based on excellent phase transition characteristics.

Preparation of VO 2 /6H-SiC Films
The 6H-SiC (0001) single-crystal substrate (double-side polished slice with the resistance of 0.02-0.1 ohm-cm and the size of 5 mm × 5 mm × 0.5 mm, Hefei Yuanjing Technology Materials Co., Ltd.) was firstly ultra-sonic cleaned by acetone solution and deionized water, and then blown dry by pure Nitrogen gas before put into the vacuum growth chamber. A VO 2 ceramic target (99.99% purity, 25 mm diameter, 2 mm thickness, Zhongnuo New Material (Beijing) Technology Co., Ltd. Haidian, Beijing, China) was fixed at the position perpendicular to the substrate with the distance of 55 mm. VO 2 films were deposited on the 6H-SiC (0001) substrate by PLD. The pulsed laser (Kr: F laser, λ = 248 nm, 10 Hz) energy density was about 1.6 J/cm 2 . During the deposition and post-annealing process, the oxygen pressure was kept at 4.0 Pa with the flux flow rate of 20 sccm.
In order to explore the optimum growth conditions of VO 2 , the experiment was divided into two groups of S 1 and S 2 . The samples of S 1 group were named S 1-1 , S 1-2 , S 1-3 and S 1-4 , respectively, with corresponding substrate temperatures (deposition time) of 450 • C (30 min), 500 • C (30 min), 550 • C (30 min) and 650 • C (30 min); the samples of S 2 group were fixed at 500 • C to improve the continuity of the VO 2 film surfaces by changing the deposition time of 10 min, 30 min, 50 min and 70 min (named S 2-1 , S 2-2 , S 2-3 and S 2-4 , in turn). All samples were annealed for 20 min after deposition and then naturally cooled to room temperature.

Materials Characterization
The crystal structure of all samples was characterized by X-ray diffractometer (XRD; PANalytical, Empyrean). Field Emission Scanning Electron Microscopy (FE-SEM; HiTACHI Regulus 8220) was used to observe the surface morphologies. X-ray photoelectron spectroscopy (XPS; Thermo, escalab 250XI) was also performed to analyze the chemical states of the obtained samples. All the binding energies were corrected by calibrating the C 1s peak at 284.6 eV. Raman spectra of the VO 2 /6H-SiC films were acquired using the XploRA™ Raman spectrometer (HORIBA Scientific, Ltd. Hefei, Anhui, China), and a 532 nm laser with power of 0.25 mW was used as the excitation source. The resistance versus temperature curve of all samples was measured using a four-probe measurement system with variable temperature ranging from 30 • C to 95 • C.

Results and Discussion
3.1. Influence of Substrate Temperatures and Deposition Time on the Properties of VO 2 /6H-SiC Films Figure 1 shows the XRD pattern in typical θ-2θ scanning mode for the VO 2 /6H-SiC films. The well-resolved sharp reflection at 35.64 • , corresponds to a classic reflections from (0006)-oriented 6H-SiC (PDF#74-1302) substrate [18]. A unique M-phase (020) diffraction peak attributed to VO 2 at 39.93 • was observed, consistent with the report by Liao et al. [19]. No other VO 2 diffraction peaks were observed in the range of angles tested. According to Figure 1a, the crystallinity of sample S 1-2 was the best among the four samples in group S 1 . Therefore, it can be inferred that the substrate temperature corresponding to the growth of high quality VO 2 films on 6H-SiC (0001) was 500 • C. On the basis of the previous experiments, the continuity of the surface of the VO 2 films was improved by setting different deposition times (10 min, 30 min, 50 min, 70 min). Figure 1b shows that the relative intensity of the VO 2 (020) diffraction peak at 39.93 • increases with increasing deposition time. This indicates that as the deposition time increases, the surface of the VO 2 film becomes more complete and continuous, which leads to a gradual increase in the quality of the crystal. were observed in the range of angles tested. According to Figure 1a, the crystallinity of sample S1-2 was the best among the four samples in group S1. Therefore, it can be inferred that the substrate temperature corresponding to the growth of high quality VO2 films on 6H-SiC (0001) was 500 °C. On the basis of the previous experiments, the continuity of the surface of the VO2 films was improved by setting different deposition times (10 min, 30 min, 50 min, 70 min). Figure 1b shows that the relative intensity of the VO2 (020) diffraction peak at 39.93° increases with increasing deposition time. This indicates that as the deposition time increases, the surface of the VO2 film becomes more complete and continuous, which leads to a gradual increase in the quality of the crystal. were X-ray diffractometer (XRD) patterns of the samples of S1 and S2 group, respectively. Inset: enlarged view of the corresponding (020) diffraction peak of the VO2/6H-SiC films at different deposition times.
As a non-destructive detection method, Raman spectroscopy can quickly analyze and identify the phase of matter. We performed Raman spectroscopy on all samples including the substrate at room temperature and the results are shown in Figure 2. In order to distinguish the characteristic peaks of VO2 and 6H-SiC more clearly, we added the Raman spectrum of 6H-SiC in Figure 2a. For the 6H-SiC substrates, three first-order peaks of Si-C vibration were detected at 765, 787 and 965 cm -1 corresponding to E2 (TO), E2 (TO) and A1 (LO) mode respectively [20]. The weaker intensity peaks in 6H-SiC are an E2 planar acoustic mode at 147 cm −1 [21]. The data in Figure 2a [22]. Meanwhile, no other peaks were observed, which proved that high quality VO2 films can be grown on 6H-SiC substrate. The result is consistent with that of XRD pattern of VO2/6H-SiC films. As a non-destructive detection method, Raman spectroscopy can quickly analyze and identify the phase of matter. We performed Raman spectroscopy on all samples including the substrate at room temperature and the results are shown in Figure 2. In order to distinguish the characteristic peaks of VO 2 and 6H-SiC more clearly, we added the Raman spectrum of 6H-SiC in Figure 2a. For the 6H-SiC substrates, three first-order peaks of Si-C vibration were detected at 765, 787 and 965 cm -1 corresponding to E2 (TO), E2 (TO) and A1 (LO) mode respectively [20]. The weaker intensity peaks in 6H-SiC are an E2 planar acoustic mode at 147 cm −1 [21]. The data in Figure 2a [22]. Meanwhile, no other peaks were observed, which proved that high quality VO 2 films can be grown on 6H-SiC substrate. The result is consistent with that of XRD pattern of VO 2 /6H-SiC films. four different substrate temperature correspond to samples in the S1 group. It can be seen that the VO2 crystal nucleus on the surface of the 6H-SiC substrate gradually grows with increasing temperature. However, the continuity of the VO2 films gradually deteriorates after the substrate temperature exceeds 500 °C. In particular, VO2 deposited on the surface of the 6H-SiC substrate formed only a discontinuous island-like particle after the substrate temperature reached 650 °C. This is due to the reflection phenomenon in the process of depositing VO2 particles onto the 6H-SiC (0001) substrate, and then becoming more disconnected with an increase in temperature. It can be seen that the VO 2 crystal nucleus on the surface of the 6H-SiC substrate gradually grows with increasing temperature. However, the continuity of the VO 2 films gradually deteriorates after the substrate temperature exceeds 500 • C. In particular, VO 2 deposited on the surface of the 6H-SiC substrate formed only a discontinuous island-like particle after the substrate temperature reached 650 • C. This is due to the reflection phenomenon in the process of depositing VO 2 particles onto the 6H-SiC (0001) substrate, and then becoming more disconnected with an increase in temperature. Figure 3e-h are FE-SEM images of S 2-1 , S 2-2 , S 2-3 and S 2-4 samples in the S 2 group, respectively. There is no doubt that the number of holes on the surface of the VO 2 /6H-SiC film will gradually decrease as the deposition time increases, which leads to a tighter connection between the VO 2 grains. This is consistent with the change in the relative intensity of the VO 2 (020) characteristic peak exhibited by the S 2 group samples in Figure 1b. In addition, it can be seen from Figure 2b that the Raman characteristic peak of the 6H-SiC substrate weakens or even disappears as the VO 2 deposition time increases.
It is well known that among the many vanadium oxides (such as VO 2 , VO, V 2 O 3 and V 2 O 5 ), only VO 2 (M) has the MIT characteristic that appears after heating the sample to the phase transition temperature. Therefore, it is very important to determine the valence state of the V atom in the VO 2 /6H-SiC films for determining the composition of the sample. We studied the precise valence state of vanadium (V) in the sample by XPS. The XPS spectrum of the VO 2 /6H-SiC films is shown in Figure 4a.
In order to more clearly show the approximate composition of the oxide film, we used a high-resolution XPS spectrum centered on O 1 S and V 2 P3/2 , as shown in Figure 4b. All elements in the composition can be well identified in the XPS measurement spectrum compared to previously reported literature [23]. The O 1s peak can be curled into two peaks at 530.0 eV and 532.1 eV. And the former van be considered to be related to vanadium oxide and the latter being mainly from hydroxide or carbonate contamination. It is worth noting that the combined energy span of 516.1 eV to 530.0 eV was calculated to be 13.90 eV, which is consistent with previous reports [24]. Meanwhile, the V 4+ 2 P3/2 and V 4+ 2 P1/2 were located at the binding energies of 516.1 eV and 523.46 eV, [25] respectively. It was confirmed that the valence state of V of the VO 2 /6H-SiC films was mainly composed of V 4+ . Furthermore, the peak at 517.5 eV belongs to the oxidation state of V 5+ . According to the XRD and Raman signals of Figures 1 and 2, V 2 O 5 was not observed. Generally, the surface free energy of the film enhances the activity of surface atoms, and the oxidation state of V 5+ is more stable than V 4+ . In combination with previous reports [26,27], we believed that V 5+ formed by the spontaneous surface oxidation of VO 2 stored in the air.  There is no doubt that the number of holes on the surface of the VO2/6H-SiC film will gradually decrease as the deposition time increases, which leads to a tighter connection between the VO2 grains. This is consistent with the change in the relative intensity of the VO2 (020) characteristic peak exhibited by the S2 group samples in Figure 1b. In addition, it can be seen from Figure 2b that the Raman characteristic peak of the 6H-SiC substrate weakens or even disappears as the VO2 deposition time increases.
It is well known that among the many vanadium oxides (such as VO2, VO, V2O3 and V2O5), only VO2 (M) has the MIT characteristic that appears after heating the sample to the phase transition temperature. Therefore, it is very important to determine the valence state of the V atom in the VO2/6H-SiC films for determining the composition of the sample. We studied the precise valence state of vanadium (V) in the sample by XPS. The XPS spectrum of the VO2/6H-SiC films is shown in Figure 4a.
In order to more clearly show the approximate composition of the oxide film, we used a high-resolution XPS spectrum centered on O 1S and V 2P3/2, as shown in Figure 4b. All elements in the composition can be well identified in the XPS measurement spectrum compared to previously reported literature [23]. The O 1s peak can be curled into two peaks at 530.0 eV and 532.1 eV. And the former van be considered to be related to vanadium oxide and the latter being mainly from hydroxide or carbonate contamination. It is worth noting that the combined energy span of 516.1 eV to 530.0 eV was calculated to be 13.90 eV, which is consistent with previous reports [24]. Meanwhile, the V 4+ 2P3/2 and V 4+ 2P1/2 were located at the binding energies of 516.1 eV and 523.46 eV, [25] respectively. It was confirmed that the valence state of V of the VO2/6H-SiC films was mainly composed of V 4+ . Furthermore, the peak at 517.5 eV belongs to the oxidation state of V 5+ . According to the XRD and Raman signals of Figures 1 and 2, V2O5 was not observed. Generally, the surface free energy of the film enhances the activity of surface atoms, and the oxidation state of V 5+ is more stable than V 4+ . In combination with previous reports [26,27], we believed that V 5+ formed by the spontaneous surface oxidation of VO2 stored in the air.

Variation of VO2/6H-SiC Thin Films
Resistance in S1 and S2 Groups During MIT Figure 5 shows resistance-temperature curves for all VO2/6H-SiC thin films. Figure 5a-d corresponds to S1-1, S1-2, S1-3 and S1-4, respectively. The resistance variations in the samples of S1-1, S1-2  Figure 5 shows resistance-temperature curves for all VO 2 /6H-SiC thin films. Figure 5a-d corresponds to S 1-1 , S 1-2 , S 1-3 and S 1-4 , respectively. The resistance variations in the samples of S 1-1 , S 1-2 and S 1-3 were 3-4 orders of magnitude, and the phase transition temperatures were at about 69 • C. However, it can be seen from Figure 5d that the S 1-4 sample does not show resistance-temperature curves of a typical VO 2 films with temperature change. The appropriate substrate temperature can promote the epitaxy growth of thin films on the substrate. During the preparation of the S 1-4 sample, the high substrate temperature causes a large amount of reflection during the deposition of VO 2 into the substrate, thus forming discontinuous islands, as shown by the FE-SEM image of the sample. In order to more clearly reflect the phase transition temperature of the sample, d(R)/dT curves for the samples of S2-1, S2-2, S2-3 and S2-4 were presented, respectively. According to the resistance-temperature curves of the samples in the S2 group, the phase transition temperatures of all VO2/6H-SiC films were in the range of 69-70 °C, which is consistent with the previous report by Zhou, et al. [28]. The reason for the higher phase transition temperature of the sample is the slight lattice mismatch between the VO2 films and 6H-SiC substrates. We calculated the resistance change and the hysteresis width (defined as Tc(heat)-Tc(cool)) of the derivative curve in the MIT process for the samples in the S2 group, and listed the data in the following Table 1: Obviously, it can be seen from Figure 6a-d that the resistance corresponding to the VO2 (R) film in the S2 group decreases gradually with the increase of deposition time. Figure 3e-h shows the In order to more clearly reflect the phase transition temperature of the sample, d(R)/dT curves for the samples of S 2-1 , S 2-2 , S 2-3 and S 2-4 were presented, respectively. According to the resistance-temperature curves of the samples in the S 2 group, the phase transition temperatures of all VO 2 /6H-SiC films were in the range of 69-70 • C, which is consistent with the previous report by Zhou, et al. [28]. The reason for the higher phase transition temperature of the sample is the slight lattice mismatch between the VO 2 films and 6H-SiC substrates. We calculated the resistance change and the hysteresis width (defined as Tc(heat)-Tc(cool)) of the derivative curve in the MIT process for the samples in the S 2 group, and listed the data in the following Table 1: Obviously, it can be seen from Figure 6a-d that the resistance corresponding to the VO 2 (R) film in the S 2 group decreases gradually with the increase of deposition time. Figure 3e-h shows the change trend of surface continuity of the VO 2 films with the increase of deposition time. In fact, integrated surface is facilitated to the conduction of free electrons on the VO 2 (R) films. As a result, the variation multiples of the phase change resistance of the sample in the S 2 group increased gradually. It is worth noting that the amplitude of the resistivity ratio during the MIT of VO 2 film with optimized condition is up to 5.66 × 10 4 , and the corresponding hysteresis width was only 3.3 • C. The excellent MIT characteristics of VO 2 /6H-SiC films are better than the growth of VO 2 film on other substrates, such as p-GaN/sapphire [29], Si [30], ZnO [31] and sapphire substrates [32]. Based on the above discussion, we believed that the appropriate substrate temperature and deposition time were critical for growing high quality VO2 films. On the one hand, the lower substrate temperature was not conducive to the growth of VO2 films on the substrate surface; on the other hand, the high substrate temperature increases the reflection of VO2 molecules deposited onto the substrate surface, resulting in the inability to the formation of highly continuous films. With the increase of deposition time, meanwhile, the VO2 grain size on the surface of the substrate becomes larger, and the grain boundary decreases, so that the compactness of the film continuously increased. VO2 films with high continuity were more effective to the conduction of electrons, which shrinking the hysteresis width in the process of MIT and reducing the resistance of VO2 (R). The interface between VO2 (020) and 6H-SiC (0006) caused the stress of VO2 film. However, according to the variation trend of VO2 (020) diffraction peak in Figure 1b, it could be clearly concluded that with the prolongation of VO2 deposition time, the stress of the film was relaxed, which can improve the phase transition characteristics of VO2 films.

Conclusions
VO2 thin films with excellent phase transition properties were successfully prepared on 6H-SiC (0001) substrate. Optimized substrate temperature and extended deposition time of VO2 particles Based on the above discussion, we believed that the appropriate substrate temperature and deposition time were critical for growing high quality VO 2 films. On the one hand, the lower substrate temperature was not conducive to the growth of VO 2 films on the substrate surface; on the other hand, the high substrate temperature increases the reflection of VO 2 molecules deposited onto the substrate surface, resulting in the inability to the formation of highly continuous films. With the increase of deposition time, meanwhile, the VO 2 grain size on the surface of the substrate becomes larger, and the grain boundary decreases, so that the compactness of the film continuously increased. VO 2 films with high continuity were more effective to the conduction of electrons, which shrinking the hysteresis width in the process of MIT and reducing the resistance of VO 2 (R). The interface between VO 2 (020) and 6H-SiC (0006) caused the stress of VO 2 film. However, according to the variation trend of VO 2 (020) diffraction peak in Figure 1b, it could be clearly concluded that with the prolongation of VO 2 deposition time, the stress of the film was relaxed, which can improve the phase transition characteristics of VO 2 films.

Conclusions
VO 2 thin films with excellent phase transition properties were successfully prepared on 6H-SiC (0001) substrate. Optimized substrate temperature and extended deposition time of VO 2 particles can effectively improve the quality of the samples and help to reduce the resistance of VO 2 in the R phase, which was as low as 13.6 Ω. During the MIT of VO 2 /6H-SiC film, the maximum amplitude of resistivity change was 5.66 × 10 4 , and the width of the resulting hysteresis loop was only 3.3 • C. This ultra-high quality VO 2 /6H-SiC (0001) thin film preparation was attributed to the good lattice matching as well as the high thermal conductivity properties between the substrate and the epitaxial film. The wide bandgap and great compatibility with metal oxide semiconductors make the combination of 6H-SiC as well as VO 2 films helpful in exploring new MIT-related finite components.