| Summary: | 博士 === 國立清華大學 === 材料科學工程學系 === 93 === The unique properties of VO2, with large reversible variations of transmittance and resistivity accompanying its phase change upon thermal cycling, provide potential for applications on display technique, data storage and optical communication. For example, a sharp change in transmittance for light of 1550 nm reaches 50% during the phase change within 1-2oC. The transition temperatures of VO2, modified by changing fabrication conditions or doping dopants, show its high flexibilities and demonstrate the importance of research on the properties of VO2. I did the researches on properties and applications of VO2 thin films, started from fabrication, then the measurements and discussion of mechanical and optical properties of the thin films. Finally, I developed a new driving method to utilize VO2 as an IR shutter. I hope the research, from the properties to the driving method of the material, can inspire broader applications of this material.
In my study, a new deposition process to attain rutile VO2 films of rutile structure from a V2O5 target was developed using reactive oxygen instead of hydrogen. Rutile VO2 films were prepared by adjusting the substrate temperature and oxygen flow ratio under optimum deposition conditions on Si and thick glass substrates. Crystalline phases analyzed by x-ray diffraction show the evolution among V4O9, V6O13, and VO2 films prepared under different deposition conditions.
Mechanical properties of thin films, used in the derivation of residual stress, can be studied by using a nano-indentation method. The variations of maximum penetration depths under the same maximum load (Pmax), resulted from the grain curvature effect, were found to be the cause of the deviations in estimated hardness and Young’s modulus. The cureves of hardness and Young’s modulus measured at lowest penetration depth, being thought to be free from effect of grain curvature, coincided very well with the curves measured by continuous stiffness measurements mode and demonstrated the model I proposed.
The transition temperatures (Tt) of VO2 thin films were though to be caused by the residual stress and inhomogeneity of the thin film. The residual stresses in the films quantitatively determined from X-ray diffractometry and inhomogeneity analyzed by ESCA were used to explain this variation. The differences in the binding energy of core electrons 2p1/2 and 2p3/2 of the vanadium atom, possibly caused by oxygen content, were proportional to Tt of the films. The bond length between vanadium and oxygen at room temperature varies with different residual stresses and inhomogineity and, furthermore, affects the movements of both atoms during phase change and hence the Tt and hysteresis span of the transmittance loop of VO2 thin films.
In order to apply VO2 thin film as in high speed IR shutter, many methods to fast drive the reversible phase change of VO2 were reported. A new high speed IR shutter based on total optical modulation, composed of Wavelength Division Multiplexing (WDM), fiber lens or convex lens system, and a glass sheet with VO2 thin film on it, had been demonstrated to utilize VO2 film without a higher power heating laser and precise optical alignment systems for two beams. The easy installation of the system implies the possibility to highly miniaturize the VO2-based optical shutter. A continuous response of IR signal is detected synchronously with the highest rate of a continuous square heating signal of 3 mW at 120 kHz. A heating pulse of 0.7 ns and 13 mW can be used to stimulate an IR pulse with fiber lens.
The dissertation has demonstrated the outstanding and stable switching properties of the VO2 material from researches of the fabrications, mechanical properties, optical properties and even driving method of the materials. Furthermore, practical application as a high speed IR shutter has been demonstrated feasible.
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