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Optical and Electronic Properties of WO3 and Zn Chalcogenides Alloys: A Theoretical study

Time: Fri 2020-01-24 09.30

Location: Kollegiesallen, Brinellvägen 8, Stockholm (English)

Subject area: Physics, Material and Nano Physics

Doctoral student: Gustavo Baldissera , Materialvetenskap

Opponent: Dr Naoto Umezawa,

Supervisor: Clas Persson, Materialvetenskap

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In this thesis, optical and electronic properties of WO3 and Zn related alloys are analyzed by means of density functional theory (DFT). Beyond-DFT methods such as the GW approximation and hybrid functional are employed in order to minimize the error generated by the low band gap obtained with conventional DFT functionals. WO3 has six different thermodynamic stable phases in different temperature regions. A triclinic to monoclinic transition occurs near room temperature, and therefore experimental samples often contain both phases. Calculations of these two structures show similarities in absorption and band structure, with a small difference of 0.1 eV between the absorption onset. This value is related to the band gap difference between the two phases. The low temperature monoclinic phase presents a different band dispersion and a wider band gap, affecting mainly the absorption onset. In the three cases, the joint density of states have onsets in a lower energy when compared to the absorption due to forbidden transitions at low photon energies. Modeling tungsten vacancies in supercells of WO3 reveals magnetic moments in some the crystalline phases, with the effect being stronger in the low symmetry structures, triclinic and monoclinic. The magnetic moment arises from the unpaired electrons of oxygen atoms adjacent to the vacancy. This effect, however, is localized and does not generate a hole-mediated ferromagnetic phase in the material. The study of zinc alloys is performed with supercells to reach the desired mixing of elements. For Zn(O,S) and Zn(O,Se) alloys, substantial reductions in the band gap by ∼1 eV are found for concentrations close to 50%. To describe the band gap behavior of these alloys, an approach combining two different methodologies was suggested, where the region close to the binaries is described by the band anti-crossing model, while the intermediate region is represented by the alloy band bowing model. ZnO-GaN alloys also display a band gap bowing and the results obtained by the calculations are in good agreement with experimental observations. ZnTe exhibits an intermediate band when doped with a III-nitrides compound, such as GaN, AlN and InN. This effect is believed to be the result of the resonance between the ZnTe states and the states originated from the dopants.