Etude de la microstructure et des transitions de phases électroniques et cristallines de couches épitaxiales de VO₂ déposées sur différents substrats

Abstract : The research presented in this manuscript deals the study of the effect of strain (epitaxial or thermal) on the structural and the electrical transitions of vanadium dioxide. VO₂ thin films have been synthesized by e-beam deposition and Pulsed Laser Deposition methods. The strain geometry is controlled by modifying, on the one hand, the nature of the substrates and, on the other hand, the thickness of thin films. In the case of (001) sapphire substrates (Al₂ O₃ ), the important lattice mismatch leads to a domain matching epitaxial growth mechanism, so that the residual strain solely result from the film/substrate thermal expansion mismatch. The study of the structural phase transition, using X-ray diffraction, and the study of the metal-insulator transition, using a 4-probes device, showed that the tensile strain along the cᵣ axis leads to an increase of the transition temperature (> 68◦ C). The appearance of an intermediate phase was demonstrated during the study of the structural phase transition. Growth on (001)- and (111)-TiO₂ substrates is characterized by a weaker lattice mismatch (∼ 1%), with a critical thickness of 4 nm, from which dislocations are created to relax the elastic energy. The study of electrical and structural transitions has shown that the evolution of transitions results from a competition between epitaxial distorsion, thermal distorsions and the presence of oxygen vacancies at the interface.
Keywords : Vanadium dioxide
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Virginie Thery. Etude de la microstructure et des transitions de phases électroniques et cristallines de couches épitaxiales de VO₂ déposées sur différents substrats. Micro et nanotechnologies/Microélectronique. Université de Limoges, 2017. Français. ⟨NNT : 2017LIMO0059⟩. ⟨tel-01661485⟩

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