Multiple thermal switching effect on the stability of vanadium dioxide films
Ngaruiya, James Mbiyu
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The study aimed at establishing the effect of multiple thermal switching on the stability of vanadium dioxide films. The films were produced in an atmosphere of a mixture of argon and oxygen. On optimization of oxygen flow rate, films, which were approximately 80nm thick and showed metal to semiconductor phase transition were obtained. Electron diffraction pattern showed the films to be polycrystalline. The sheet resistance was found to change by three orders of magnitude upon heating the films from room temperature to temperatures above transition temperature. The spectral normal transmittance over the wavelength range of 300 to 2500nm showed a monotonic increase in transmittance at 30°C to 69% and a decrease in the infrared to about 16% on running the spectrum at 100°C. The refractive index, n and the extinction coefficient, k, for the as-deposited films were 2.85 and 1.47 for the semiconducting state respectively. Values of 2.62 and 1.37 were obtained for the films in the metallic state for n and k respectively. Upon multiply switching the films the sheet resistance was found to increase by a small magnitude at both semiconducting and metallic states. The magnitude of change of sheet resistance decreased by a small magnitude but retained the same order. From the temperature dependent transmittance the transition temperature was found to decrease and tend to stabilise at 61°C from 69°C for the as-deposited. The hysteresis loop width was found also to decrease from 15°C to 10°C upon cycling. The spectral normal transmittance showed no appreciable change upon cycling. After one thousand thermal cycles n increased to 3.08 and k decreased to 1.08 at semiconducting state. In the metallic phase both n and k decreased to 2.46 and 0.64 respectively, after cycling.