| Vanadium oxide(VOx) thin films can be applied in many aspects, such as infrared detection, electro-optic modulation, energy storage, etc. Therefore, the investigation of its preparation technology and properties attracted extensive attentions around the world in the past few decades.In this dissertation, VOx thin films were prepared by pulsed reactive magnetron sputtering. The influence of the parameters such as duty cycle, pulse frequency and argon flow rate(when the power supply worked in high pulse frequency mode); on the deposition rate, composition, surface morphology, microstructure, crystallinity, optical and electrical properties of the deposited VOx thin films, has been investigated by means of various instruments and methods. Based on the experimental results and the known characteristics of the pulsed magnetron plasma, the mechanisms for the duty cycle, pulse frequency and argon flow rate(when the power supply worked in high pulse frequency mode) in affecting the reactive sputtering process, the formation of VOx and the growth mode of the deposited films have been analyzed and discussed. The main results and important conclusions of this dissertation are shown as follows:1. In the atmosphere that have different oxygen/argon flow rate ratio, the influence of the duty cycle on the growth process and properties of the pulsed reactively sputtered VOx thin films has been investigated. The experimental results showed that with the reduction of the duty cycle, the deposition rate will decrease and the oxidation of the vanadium will be promoted. And with the increase of oxygen flow rate, the composition and the deposition rate of the films will become more sensitive to the duty cycle. Surface morphology analysis showed that decreasing the duty cycle or increasing the oxygen flow leaded to the decrease of the root mean square(RMS) roughness of the films. And due to the capability of the duty cycle in affecting the composition of the VOx, thus we can control and adjust the optical and electrical properties of the films by the duty cycle, even other parameters such as reactive atmosphere, sputtering power, et al keep the same. With the decrease of the duty cycle, which cause the decrease of the refractive index and extinction coefficient and increase of the transmittance and optical band gap of the films. On the other hand, the room temperature resistivity and temperature coefficient of resistance of the deposited films can be adjusted conveniently in a large range by the duty cycle. Based on these experimental phenomena, we discussed and analyzed the mechanisms for the duty cycle in affecting the competition between the formation and sputtering etch of the VOx on the target surface, the reactivity of the plasma and the film growth, et al.2. The influence of the pulse frequency on the growth process and properties of the VOx thin films has also been studied. It is found that with the increase of pulse frequency that will lead to the decrease in the deposition rate and root mean square(RMS) roughness value of the deposited films, and the evolution of the microstructure from strongly columnar to dense structure. The oxidation of the vanadium will also be promoted by increase pulse frequency. Especially, when the pulse frequency higher than 300 k Hz, the sputtered vanadium will be oxidized to V5+ valent state rapidly, even the amount of oxygen in the atmosphere is very low. Due to the influence of pulse frequency on the composition and microstructure of the deposited films, the optical and electrical properties of the films has also been changed significantly by the pulse frequency. With the increase of the pulse frequency, which lead to the optical gap increased but the extinction coefficient dropped. And the transmittance will increased rapidly, when the pulse frequency higher than 300 kHz. Electrical measurements indicated that with the pulse frequency increased from 0 kHz(DC) to 350 kHz, the room temperature resistivity of the VOx films increased almost 6 orders and TCR has also been improved significantly. According to the experimental phenomena we have observed, and combined with the known pulsed magnetron plasma’s characteristics, the mechanisms for the pulse frequency in affecting the reaction between the vanadium and oxygen, and energy balance between the plasma and growth interface has been discussed and analyzed.3. The influence of the argon flow rate on the growth process and properties of the VOx thin films has been investigated, when the power supply worked at 350 kHz pulse frequency. It is found that in the range of the argon flow rate that we studied in our experiments, the deposition rate of the deposited films was in proportion to the argon flow rate. And the deposition rate will decrease more rapidly with the reduction of the argon flow, when there was oxygen in the sputtering atmosphere. In comparison to the columnar structure growth in the films can be suppressed by increase the pulse frequency, when the argon flow rate was low. With the increase of argon flow rate, the microstructure of the deposited VOx films shows an evolution from dense, crack free to columnar like, even the power supply worked at 350 kHz high pulse frequency mode. That means the capability of the high pulse frequency in inhibit the columnar structure growth will be weakened due to the influence of the argon flow rate increasement on the atomic processes on the growth interface and ion/atom flow ratio arrived at the growing films. On the other hand, high pulse frequency makes the plasma show more reactivity, but with the increase of the argon flow rate, the sputtered vanadium would no longer be fully oxidized to V5+ valent state. The variation of the argon flow rate has also greatly affected the optical and electrical properties of the films. The optical gap of the VOx films was found would increase gradually with the decrease of argon flow rate. And the films deposited at the argon flow rate higher than 100 sccm, all shows low transmittance and high absorptance. The resistivity and TCR of the deposited films will increase with the reduction of the argon flow rate.. |
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