Design And Fabrication Of Variable Reflectivity Mirror Based On Phase Transition Characteristics Of Vanadium Dioxide Film

Single crystal VO2 undergoes a first-order phase transformation at a critical temperature TSMT~68℃ from monoclinic state to tetragonal state. Below 68℃, VO2 is a semiconductor and exhibit some degree of transparency in the infrared(IR) region, while above 68℃, it behaves as a metal and is highly IR reflecting. The phase transition of VO2 can be triggered by many ways. Such unique characteristics make VO2 have been suggested for many potential applications, and this attracted a large number of researchers’ interesting. In this paper, a novel variable reflectivity mirror based on the phase transformation characteristics of VO2 thin films was designed and prepared, which can change the infrared reflectivity by adjusting the temperature of the mirror.Firstly, the W-doped VO2 thin films were prepared by DC-reactive magnetron sputtering on the Na Ca glass substrates, and the effect of substrate temperature on electrical and optical performances of W-doped VO2 thin films was studied. The results exhibit that the W-doped VO2 thin film was B phase under the low substrate temperature of 400℃, which exhibit no phase transition characteristics. As the substrate temperature increasing above 480℃, the crystal structure of W-doped VO2 thin film changed to M phase. And the W-doped VO2 thin films phase transition performance enhanced with substrate temperature increasing. W-doped VO2 thin films with high phase transition performance were prepared at 400°C by adding substrate negative bias in the deposition process. With substrate negative bias increasing, the grain size of the thin films increased, and the phase transition temperature decreased. The appropriate substrate bias voltage range is-100V~-200 V. The W-doped VO2 thin films prepared under this condition can show the better infrared cut-off capability.The Zn O buffer layer were prepared by DC reactive magnetron sputtering between the Na Ca glass substrates and VO2 thin films, then the influence of buffer layer thicknesses on the structure and properties of the VO2 thin films was studied. The results show that the visible transmittance and solar energy modulability of the VO2 thin film can be significantly improved by adding Zn O buffer layer. Moreover, VO2 thin films with high phase transition performance were prepared by DC reactive magnetron sputtering on the Zn O/Na Ca glass substrates under the low temperature of 400℃ with various substrate negative bias voltage, and the influence of substrate bias on the physics properties of VO2 thin films was studied. The intrinsic stresses of the thin films increased with the increasing of substrate negative bias, and the largest stresses is-576 MPa. At the same time, VO2 thin films exhibit lower phase transition temperature with the increasing of substrate negative bias, and the lowest is about 38.5℃, which is near the room temperature.At last, high refractive index thin films(VO2) and low refractive index thin films(Zn O) were used as the basic film material, and the optical constants of the thin films testd by ellipsometry for mirror designing. Based on the work in the above, a variable reflectivity mirror with a center wavelength of 1050 nm was designed, and then the variable reflectivity mirror was prepared by DC reactive magnetron sputtering on Na Ca glass substrates. The characterization results showed that the infrared reflectivity of the mirror can be changed by adjusting the temperature and the desired goal were achieved.