Thermodynamic stability of Vo2-Metal thin films

Dissertation presented in fulfilment of the requirements of the degree of MASTER OF SCIENCE to the DEPARTMENT OF PHYSICS AND ENGINEERING UNIVERSITY OF ZULULAND, 2009.

Thermodynamic stability of VO2 in contact with metals was conducted on solid thin metal films. The VO2 film was synthesized on a corning glass under optimized condition parameters using rf magnetron reaction sputtering. The samples with structure glass/VO2 were then taken for optical characterization and the results obtained correlate well with theoretical behavior of VO2 thin film. UV-VIS-NIR Spectrometry was used to measure the optical behavior of the VO2 film. The structures on which a solid thin metal film of thermodynamic stability of VO2 in contact with metals was conducted on solid thin metal films of Pd, Ni, Hf, Pt, Co, and Al were e-beam deposited on VO2/glass. This resulted in structure of the form glass/VO2/metal. The deposited samples were annealed in a furnace at different temperatures and time intervals keeping some of the as-deposited metals as virgin samples. Using Rutherford Backscattering Spectrometry (RBS), the samples were investigated if an interaction between VO2 and metal has taken place by comparing the as-deposited (no heat) samples with annealed ones. RBS analysis was conducted using a 2 MeV beam of alpha particles. In case a reaction has taken place, X-Ray Diffraction (XRD) was used to identify the compound phases formed. The only metals found to show a reaction are Hf and Al when each was in contact with VO2 and annealed. The metals (Pd, Ni, Pt, and Co) seem not to react with VO2 at temperature up to 700°C annealed for one hour. For all the metals investigated heats of reaction were calculated for all the possible combinations. Our experimental observations were found to be in agreement with the theoretical predictions. In such cases ternary phase diagrams were constructed and found also to be in agreement with the experimental results. Metals with an average electro-negativity less than 1.70 (the cross over point) in Pauling scale were found to react with VO2 while those with average electro-negativity greater than 1.70 were found not to react with VO2. The Miedema electro-negativity parameter was also used and metals whose values are less than the cross over point of 4.67 V were found to react while those metals greater than 4.67 V were found not to react. These predictions based on average electro-negativity and Miedema electro-negativity correlate well with our experimental results.

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