A study of optical and structural properties of spin-coated V2O3 thin films on sapphire substrates

Abstract: Many transition metal oxides exhibit a Metal-Insulator Transition (MIT). This is an interesting phenomenon where the material undergoes a structural phase transition, and the electronic charge is redistributed in the crystal lattice. The crystallographic, optical, electrical, and magnetic properties are drastically changed when a material undergoes a MIT. Vanadium oxides are thermochromic materials, which means that the optical properties change with temperature, closely connected to the MIT. Vanadium sesquioxide (V2O3) experiences a MIT at the transition temperature (Tt) 160-170 K. Below the transition temperature, it is a monoclinic ferromagnetic insulator, and above the transition temperature it becomes a paramagnetic rhombohedral corundum-type metal. In the insulating phase, infrared (IR) light is transmitted, whereas in the metallic phase, IR light is reflected. The aim of this Master thesis is to improve the methodology to produce V2O3 thin films, characterize them, and study how different parameters affect the structural and optical properties of the film. V2O3 thin films were synthesized by sol-gel and deposited by spin-coating on sapphire substrates. Thereafter, the resulting films underwent an annealing process in a reducing atmosphere to achieve crystalline V2O3. The obtained crystal phase was examined by X-ray diffraction. The surface structure and morphology were studied with an optical microscope and scanning electron microscope. The optical transmittance in the IR region was examined by Fourier transform infrared spectroscopy. At last, a laser test was performed on one of the samples. In conclusion, a majority of the thin films consisted of polycrystalline V2O3. The MIT was studied with the optical hysteresis loop, which showed great difference among the different samples studied. The thin film that exhibited best optical properties showed a maximum transmission of 66 % below the Tt, and a minimum transmission of 6 % above the Tt. In comparison to this one, the other samples exhibited lower transmission and a decrease in Tt. This difference was attributed to the different morphologies and crystal orientations of the samples.