Effect of as-deposited residual stress on transition temperature of VO 2 films
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HH2.8.1
Effect of as-deposited residual stress on transition temperature of VO2 films Kuang Yue Tsai, Tsung-Shune Chin, *Han-Ping D. Shieh 2 and **Cheng Hsin Ma Department of Materials Science and Engineering, National Tsing Hua University, 101, Sec-2. Kuang-Fu Rd., Hsinchu 30043, TAIWAN; *Institute of Electro-Optical Engineering, National Chiao-Tung University, 1001, Ta-Hsueh Rd., Hsinchu 30010, TAIWAN **Materials Science and Engineering, and Frederick Seitz Materails Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL, USA. Abstract VO2 is a material with reversible thermo-chromic properties. The reversible phase transition of a strain-free single-crystalline VO2, at a transition temperature (Tt) 68oC, is accompanied with changes in crystal structure, optical and electrical properties. With different processing conditions during thin film deposition, different transmittance loops will be resulted upon thermal cycling. The residual stress of the thin films with poor crystallinity, as determined from X-ray diffractometry, is found to be an important factor responsible for the Tt that increases with increasing residual stress. Residual stress affects the hysteresis span of the transmittance loop. The relationship between residual stress of as-deposited VO2 films and the relative positions between vanadium and oxygen under the residual stress are also delineated. Introduction VO2 with a rutile structure is a well known thermochromic material with a reported reversible phase transition around 68oC1,2), especially for bulky single crystalline one. With different deposition processing, optical switching performance of VO2 films is also different. The hysteresis width of each transmittance – temperature curve is also different.3,4,6) Regardless of measurement methods, the variations in the transition temperatures have been suggested to arise from the inhomogeneity and strained structure.6,7) During phase change, the process of atom movement of VO2 is affected by the relative force among atoms so that initial structure of VO2, under the external residual stress, is an important issue worthy of investigation. Especially, by adjusting the residual stress and processing condition, it is possible to modify the optical switching property of VO2 without doping impurity. This study aimed to understand quantitatively the relationship between the residual stress and the optical switching properties such as transition temperature and hysteresis width of polycrystalline VO2 films prepared by rf-magnetron sputtering.
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Experimental V2O5 targets were prepared by powder compaction and then sintered in air at 650 oC for 30 minutes and cooled down to room temperature in air. Films were deposited by rf magnetron sputtering with a target of 2-inch in diameter. The distance between the substrate and the target for sputtering was 10 cm. The vacuum chamber was evacuated to 10-5 Torr and then back-filled with a mixture of Ar and oxygen to a certain total gas pressure. Ar and oxygen were pre-mixed in a small cha
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