Growth of a-axis oriented vanadium dioxide polycrystals on glass substrates
Introduction
Bulk VO2 undergoes a fully reversible metal to insulator transition (MIT) at a critical temperature Tc (68 °C), which was first reported by Morin in 1959 [1]. Actually, this fascinating transition is a first order phase transformation from monoclinic (space group P21/c, M) to tetragonal (space group P42/mmm, R) symmetry [2], accompanied by noteworthy reversible jumps in electrical resistance, optical transmittance and reflectance in the infrared region. These features make VO2 suitable for applications in thermo-chromic coating [3], [4], ultrafast switching devices [5], sensors and micromechanical systems [6], etc.
The desired properties of VO2 thin films are high resistance, reflectance change during phase transition and a narrow hysteresis width. It has been proven that using single crystal substrates is effective to obtain VO2 thin films with a large electrical during phase transition and a narrow hysteresis width because metal–insulator domain wall propagation of highly oriented VO2 is faster. Orientation control is an interesting topic in thin film growth; highly oriented VO2 films have been obtained on sapphire and TiO2 single crystals [7], [8], [9]. It is found that the conductivity exhibits a variation of more than four orders of magnitude for the highly (100) texture of VO2 thin films and only three orders of magnitude for the highly (010) texture of sample with a hysteresis behavior upon heating and cooling through the transition [9]. However, to the best of our knowledge, the work concerning the synthesis of a-axis oriented VO2 thin films on glass substrates has rarely been reported. In our previous work, we reported the high visible transmittance of preferred orientation vanadium dioxide with acicular nano-structure on glass slide substrates [4]; however, the MIT characteristics of these VO2 films are negligible. Therefore, in the present work, we report the successful preparation of a-axis oriented VO2 films with the obvious MIT characteristics.
Section snippets
Experimental
VO2 thin films were deposited on glass substrates, from radio-frequency reactive sputtering technique, using a V2O5 target of diameter of 49 mm. The distance between the substrate and the target for sputtering was 80 mm. The vacuum chamber was evacuated to 9.0×10-4 Pa and then back-filled with a mixture of Ar and oxygen to a certain total gas pressure. Ar and oxygen were pre-mixed in a small chamber at a positive pressure before being led into a vacuum chamber to maintain a sputtering pressure.
It
Results and discussion
Fig. 1 shows the XRD spectra of the films of as-deposited and annealing. As shown in Fig. 1(a), one diffraction (2θ=19.96°) peak which matches to V2O5 phase is observed. However, as it is shown in Fig. 1(b), the patterns show peaks due to thin layers at angles of 18.36°, and 37.15° which are very similar to values reported in [9], [10]. Following the calculated pattern description of the monoclinic structure of VO2, these peaks can be indexed as the reflections on the (100) and (200) planes
Conclusions
VO2 polycrystals were deposited on soda–lime glass substrates by using RF-magnetron sputtering technique in which the control of post-deposition parameters enhances the quality of the films׳ structure. XRD data suggest that the VO2 thin films exhibited a highly (100) texture. However, it was found that a recrystallization process took place after annealing, which led to a preferential growth along the a-axis of the monoclinic VO2. The sharpness and the hysteresis width, ΔT, of T-dependent
Acknowledgments
This work was financially supported by the Ministry of Science and Technology of the People׳s Republic of China (No. 2010DFR10720) and National Natural Science Foundation of China (No. 11374080)
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2021, Solar Energy Materials and Solar CellsCitation Excerpt :From this and by applying Eq. (6) the critical transition temperature (TC) was calculated and found to be 37.1°Cand 33.1 °C for the 30 nm and 90 nm thick film, respectively. These values are close to those at room temperature and significantly lower from the values reported in literature for undoped VO2 films deposited by rf sputtering technique on uncoated or pre-coated glass substrates, at low deposition temperature (300 °C) [15,27,32,38–40] or films grown by other techniques on various substrates [18,41]. The very low critical transition temperature is attributed to the low crystallinity of the films caused by the low deposition temperature as well as to the non-stoichiometry of the film, which both introduce defects in the films’ structure [42] that assist the transition of the VO2structure from monoclinic to tetragonal rutile phase decreasing the required energy, thus decreasing the transition temperature.