Metal-semiconductor transition materials : FeS and VO2 thin films by RF reactive sputtering

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2007

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DOI:
http://dx.doi.org/10.22029/jlupub-9557

Abstract

MST materials show the abrupt change of some physical properties. The switching devices triggered by heat, pressure, etc., can be realized by detecting the variation of these physical properties. For example, VO2 thin film shows different transmittance between the semiconducting and metallic phases. It is one of the most potential materials for the application as smart window. But this application is restricted by the low luminous transmittance and uncomfortable color of VO2 thin film. Another MST material, FeS, shows an abrupt change on two orders of magnitude in the electrical conductivity at T. Accompanied the change of the electrical conductivity, the optical transmittance and reflectance probably changes also. But it has never been investigated. The prerequisite for studying the optical properties is the successful preparation of FeS films. In the present work, two MST systems, FeS and VO2 thin films were investigated. Iron sulfide thin films over a range of composition were prepared by reactive sputtering. The influence of the substrate, sputter power, substrate temperature and stoichiometry on the structure and MST of iron sulfide films was investigated. Iron sulfide films deposited at different temperatures show temperature dependent structure and MST. FeS films on float glass show (110) and (112) orientations when the substrate temperature is 200 and 500 oC, respectively. The transition temperature and width of the hysteresis loop determined from the temperature dependent conductivity curves of iron sulfide films decrease with the substrate temperature, which should be caused by the decrease of the residual stress inside the films. The sputter power has little influence on the structure and MST of FeS films. Fe and S excess in FeS films both result in the decrease of the transition temperature and width of the hysteresis loop. The vacuum-annealing affects the MST of FeS films significantly. When FeS films were annealed below the deposition temperature, the transition temperature decreases; otherwise increases. The residual stress plays an important role during the annealing process. The higher the residual stress inside the FeS films is, the higher the transition temperature of FeS films. With the increase of the annealing temperature, the residual stress in FeS films is first released and then enhances, which gives rise first to the decrease and then increase of the transition temperature of FeS films. This explains why the transition temperature of FeS films increases with increasing substrate temperature. At high substrate temperatures, the residual stress is higher. Consequently, the transition temperature of FeS films increases. In addition, the MST of FeS films was influenced by the ambient aging. With the increase of the aging time, the transition temperature first increases and then decreases. The subsequent decrease of the transition temperature is caused by the release of the residual stress inside the FeS films. Why the transition temperature first increases after a certain time of ambient aging is unclear, but it is probably not due to the gas adsorption onto FeS films. MST materials show the abrupt change of some physical properties. The switching devices triggered by heat, pressure, etc., can be realized by detecting the variation of these physical properties. For example, VO2 thin film shows different transmittance between the semiconducting and metallic phases. It is one of the most potential materials for the application as smart window. But this application is restricted by the low luminous transmittance and uncomfortable color of VO2 thin film. Another MST material, FeS, shows an abrupt change on two orders of magnitude in the electrical conductivity at T. Accompanied the change of the electrical conductivity, the optical transmittance and reflectance probably changes also. But it has never been investigated. The prerequisite for studying the optical properties is the successful preparation of FeS films. In the present work, two MST systems, FeS and VO2 thin films were investigated. Iron sulfide thin films over a range of composition were prepared by reactive sputtering. The influence of the substrate, sputter power, substrate temperature and stoichiometry on the structure and MST of iron sulfide films was investigated. Iron sulfide films deposited at different temperatures show temperature dependent structure and MST. FeS films on float glass show (110) and (112) orientations when the substrate temperature is 200 and 500 oC, respectively. The transition temperature and width of the hysteresis loop determined from the temperature dependent conductivity curves of iron sulfide films decrease with the substrate temperature, which should be caused by the decrease of the residual stress inside the films. The sputter power has little influence on the structure and MST of FeS films. Fe and S excess in FeS films both result in the decrease of the transition temperature and width of the hysteresis loop. The vacuum-annealing affects the MST of FeS films significantly. When FeS films were annealed below the deposition temperature, the transition temperature decreases; otherwise increases. The residual stress plays an important role during the annealing process. The higher the residual stress inside the FeS films is, the higher the transition temperature of FeS films. With the increase of the annealing temperature, the residual stress in FeS films is first released and then enhances, which gives rise first to the decrease and then increase of the transition temperature of FeS films. This explains why the transition temperature of FeS films increases with increasing substrate temperature. At high substrate temperatures, the residual stress is higher. Consequently, the transition temperature of FeS films increases. In addition, the MST of FeS films was influenced by the ambient aging. With the increase of the aging time, the transition temperature first increases and then decreases. The subsequent decrease of the transition temperature is caused by the release of the residual stress inside the FeS films. Why the transition temperature first increases after a certain time of ambient aging is unclear, but it is probably not due to the gas adsorption onto FeS films. MST materials show the abrupt change of some physical properties. The switching devices triggered by heat, pressure, etc., can be realized by detecting the variation of these physical properties. For example, VO2 thin film shows different transmittance between the semiconducting and metallic phases. It is one of the most potential materials for the application as smart window. But this application is restricted by the low luminous transmittance and uncomfortable color of VO2 thin film. Another MST material, FeS, shows an abrupt change on two orders of magnitude in the electrical conductivity at T. Accompanied the change of the electrical conductivity, the optical transmittance and reflectance probably changes also. But it has never been investigated. The prerequisite for studying the optical properties is the successful preparation of FeS films. In the present work, two MST systems, FeS and VO2 thin films were investigated. Iron sulfide thin films over a range of composition were prepared by reactive sputtering. The influence of the substrate, sputter power, substrate temperature and stoichiometry on the structure and MST of iron sulfide films was investigated. Iron sulfide films deposited at different temperatures show temperature dependent structure and MST. FeS films on float glass show (110) and (112) orientations when the substrate temperature is 200 and 500 oC, respectively. The transition temperature and width of the hysteresis loop determined from the temperature dependent conductivity curves of iron sulfide films decrease with the substrate temperature, which should be caused by the decrease of the residual stress inside the films. The sputter power has little influence on the structure and MST of FeS films. Fe and S excess in FeS films both result in the decrease of the transition temperature and width of the hysteresis loop. The vacuum-annealing affects the MST of FeS films significantly. When FeS films were annealed below the deposition temperature, the transition temperature decreases; otherwise increases. The residual stress plays an important role during the annealing process. The higher the residual stress inside the FeS films is, the higher the transition temperature of FeS films. With the increase of the annealing temperature, the residual stress in FeS films is first released and then enhances, which gives rise first to the decrease and then increase of the transition temperature of FeS films. This explains why the transition temperature of FeS films increases with increasing substrate temperature. At high substrate temperatures, the residual stress is higher. Consequently, the transition temperature of FeS films increases. In addition, the MST of FeS films was influenced by the ambient aging. With the increase of the aging time, the transition temperature first increases and then decreases. The subsequent decrease of the transition temperature is caused by the release of the residual stress inside the FeS films. Why the transition temperature first increases after a certain time of ambient aging is unclear, but it is probably not due to the gas adsorption onto FeS films.

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