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Exploration des alliages III-V-Nitrures (AlGaN, BGaN et BAlN) par algorithme évolutionnaire couplé à la théorie de la fonctionnelle de la densité.

Abstract : This thesis focuses on the study of the optical band gap dependence and the determination of the band-gap bowing parameter of the ternary AlGaN, BGaN and BAlN alloys, which are respectively formed by the combination of GaN-AlN, GaN-BN and AlN-BN systems. We used for this study a new theoretical approach based on the coupling of the evolutionary algorithm (EA) and density functional theory (DFT), in order to select the most stable phases from the unique knowledge of the chemical composition of the system. According to the evolutionary algorithm, the Darwinian concept of selection allowed us - through the Materials Project database - to determine three stable phases for each alloy, for example, for BAlN, we obtained two cubic phases: B3AlN4 and BAl3N4 (which are appropriate to alloy compositions: x = 0.25 and x = 0.75, with space group P-43m, and a tetragonal phase: BAlN2 (which is appropriate to alloy composition: x = 0.50, with space group P-4m2. Using the pseudopotential method implemented in the Quantum ESPRESSO code, we calculated the structural, thermodynamic, mechanical, and dynamic properties as well as the optoelectronic properties of the predicted alloys. The study of the thermodynamic stability is carried out by the calculation of the enthalpy of formation H, the results obtained suggest that our alloys are only marginally unstable with respect to the binary compounds wz-GaN, wz-AlN or wz-BN. However, the analysis of calculated elastic constants and phonon dispersion curves reveals that all phases are mechanically and dynamically stable. Using the LDA-1/2 approach, we determined the variation of the optical band gap with respect to the composition of our alloys. We have found that the band-gap bowing parameter is strongly related to the alloy composition x, and the values obtained using the Zunger approach (denoted bZung) are very close to those obtained by the quadratic interpolation of the curves of the optical band gap variation as a function of the composition x (denoted bfitt), so that bfitt is equal to 1.39 eV, 10.76 eV and 9.38 eV for AlxGa1-xN, BxGa1-xN and BxAl1-xN respectively. We found that these results are in good agreement with other theoretical calculations and experimental measurements. From the analysis of the physical origin of the band-gap bowing parameter b, we found that this parameter comes mainly from the chemical effect through the exchange of charges for all alloys, also from the contribution of structural deformation in volume through the disagreement of the lattice parameter of parent compounds for BxGa1-xN and BxAl1-xN.
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Submitted on : Tuesday, June 23, 2020 - 8:23:35 AM
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Hamza Maiz Hadj Ahmed. Exploration des alliages III-V-Nitrures (AlGaN, BGaN et BAlN) par algorithme évolutionnaire couplé à la théorie de la fonctionnelle de la densité.. Science des matériaux [cond-mat.mtrl-sci]. Université des Sciences et de la Technologie d'Oran Mohamed Boudiaf, 2020. Français. ⟨tel-02878288⟩

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