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Oxidation of vanadium nitrides thin films : a novel method to form efficient thermochromic VO₂

Abstract : In this thesis, we study the formation of vanadium dioxide (VO₂) from a two-step procedure. We demonstrate for the first time that high-quality thermochromic VO₂ can be obtained from the air oxidation of sputter-deposited VN and V₂N polycrystalline films. Depending on the annealing conditions (duration and temperature) VO₂ and V₂O₅ are the two principal phases formed during the oxidation process of the nitride films. The first oxide obtained is VO₂(M), which coexists with the precursor for a prolonged period at 450°C or suddenly disappeared at 525 and 550°C. For optimized durations at each temperature VO₂(M) is the predominant phase. For higher durations, the VO₂ phase is oxidized into V₂O₅. The thermochromic performance of the vanadium oxide films is strongly influenced by the oxidation duration. TEM analysis indicates that both VN and V₂N precursors follow different oxidation kinetics. Concerning the VN oxidation, there is an abrupt interface between the polycrystalline VN films and the formed VO₂. On the other hand, EELS results for the V₂N oxidation evidence a formation of a VN layer at the interface between the remaining V₂N layer and the growing VO₂ one. Besides, the influence of the V₂N thickness on the optical modulation properties of the formed VO₂(M) is studied. The oxidation of 100 nm-thick V₂N films on Si substrate yields a negative emissivity switch (Δε). On the contrary, the annealing of 445 nm V₂N samples produces a positive value for Δε. These findings indicate the possibility to control the sign of the optical modulation of the formed VO₂ depending on the initial thickness of the V₂N precursor. Furthermore, we investigate the oxidation of epitaxial VN thin films, which follow a different path than polycrystalline VN films, caused by the absence of grain boundaries in the epitaxial layers. This process depicts the epitaxial growth of a VO₂(B) layer on the remaining VN, as shown by TEM results. Finally, we apply a simple criterion to transpose the deposition conditions between the two sputtering machines used in the thesis based on the thermochromic performance.
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Submitted on : Tuesday, June 21, 2022 - 2:45:31 PM
Last modification on : Wednesday, June 22, 2022 - 3:39:53 AM
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Alexis C. García-Wong. Oxidation of vanadium nitrides thin films : a novel method to form efficient thermochromic VO₂. Materials Science [cond-mat.mtrl-sci]. Université de Lorraine, 2021. English. ⟨NNT : 2021LORR0098⟩. ⟨tel-03356443⟩



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