Onderzoek naar het effect van spanning op de lage-temperatuur fasetransitie van V2O3: Structurele, elektrische en fonon-eigenschappen

In today's society, our heavy reliance on high-tech products like consumer electronics, computers, and smart appliances depends on basic electronic devices. To meet rising demands for efficiency, durability, and functionality, the focus is on improving these devices. Understanding their material properties rooted in fundamental physics is crucial for versatile manipulation. One of the interesting material physics issues in the strongly correlated electron material system is the metal-to-insulator transition (MIT). This MIT phenomenon describes materials, unlike classic metals or insulators that experience a drastic increase in the resistivity with respect to the temperature. However, most of these transition do not occur under atmosphere room-temperature condition, making it challenging to apply this to electronic devices in practice. Thus, understanding the mechanism behind the MIT has become a popular topic to explore whether there is a potential for future applications. Pure V₂O₃ is an archetype material with an MIT of seven orders of magnitude difference in resistivity at around 180 Kelvin. With the advantages of a high resistivity change, a stable phase, and non-toxic properties, V₂O₃ serves as an excellent platform to study the MIT. Nevertheless, because V₂O₃ shares the same crystal structure as sapphire (α-Al₂O₃), Cr₂O₃, and α-Fe₂O₃, its lattice constant can be modulated by epitaxial strain in thin film form. In this thesis, the focus is on the low-temperature MIT of V₂O₃ thin films with different biaxial strain states. High quality V₂O₃ thin films are epitaxially grown by molecular beam epitaxy (MBE) on different templates: the c-plane sapphire substrate and (FeₓCr₁₋ₓ)₂O₃ buffer layers. Owing to the gradually increasing Fe percentage in the buffer layer, a series of corundum structure templates with systematically increased a-lattice constants are obtained, providing different biaxial strains for V₂O₃ thin films. These series of samples are then characterized by four temperature-dependent techniques: (1) synchrotron-based X-ray reciprocal space mapping to observe the crystal structure at different temperatures, (2) Van der Pauw method to measure the thin film resistivity, (3) Fourier-transform infrared spectroscopy for studying the infrared reflectance spectrum, and (4) Raman spectroscopy for understanding the phonon behavior. It is found that the V₂O₃ low-temperature MIT is closely accompanied by the phase transition, more precisely, the structural phase transition (SPT) from the corundum structure to the monoclinic structure. This SPT occurs as a two-step procedure. Firstly, V₂O₃ transitions from the corundum structure to an intermediate monoclinic structure around 180K upon cooling, resulting in a starting point for a high resistivity increase. Then, from 180K to around 100K, in the case of V₂O₃ grown on the Cr₂O₃ buffer layer, co-phasing of the intermediate phase and the corundum phase is observed. Below 100K, the intermediate monoclinic structure transitions again into the final-state monoclinic structure by rearranging the monoclinic crystal orientation. This observation is confirmed with the infrared spectrum that shows the splitting of the V₂O₃ Eᵤ phonon mode into a pair of Aᵤ and Bᵤ modes during the SPT. Moreover, one of the Aᵤ (or Bᵤ) modes is on the basal plane, which is more sensitive to the rearrangement of the crystal orientation, resulting in a different reflectance infrared spectrum in the far-infrared wavelength region. Lastly, the Raman spectrum provides a more quantitative analysis of similar strained series samples, specifically in 1.5% Cr-doped V₂O₃ thin films. With a detailed analysis of the A1g phonon modes of these different strained-state samples, it is concluded that the A1g phonon frequency can be an indicator of the shortest V-V dimer distance, which is a critical parameter influencing the MIT.