First-principles study of defects in wide band gap semiconductors
Kyrtsos, Alexandros Andreas
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Wide band gap semiconductors typically exhibit a band gap of more than 2eV and are technologically important for various applications. Such materials are essential for the development of high power, high temperature, and high frequency applications, which are not available with the current silicon based technology. Nonetheless, defects are present and unavoidable in all materials. Their effects on the physical properties of the materials are significant, determining their functionality. A typical example is degradation in devices which is often caused by point defects, which are also responsible for the performance of a device by determining the level of doping that can be achieved. Furthermore, defects may introduce energy levels within the band gap that can act as recombination centers, impeding the performance of solar panels or light-emitting diodes. In some cases, the defects introduce radiative centers responsible for the undesired luminescence of wide band gap semiconductors, such as the yellow luminescence in GaN. Therefore, the study of the effects of the defects in technological materials is essential both in the understanding of the physical properties of the material and the engineering of better devices and systems. In this work, both standard density functional theory and hybrid functional calculations are employed to investigate the material systems of GaN, Ga2O3 , and AlGaN. Specifically, this framework is used to determine the electronic properties and the migration barriers of native and carbon related point defects in GaN. The migration barriers of these defects typically range from 2 to 3eV. The migration barriers of gallium interstitials are lower, ranging from 0.7 to 1.6eV. Furthermore, the study of the migration of vacancies in Ga 2 O 3 and a search for possible p-type dopants is performed. The migration barriers of the oxygen vacancies in Ga2O3 exhibit barriers of typically more than 2eV, which are larger than the barriers of the gallium vacancies which are typically less than 2eV. The acceptor levels introduced by the substitutional dopants were found to be deep levels with activation energies of more than 1eV. Finally, the electronic and thermodynamic properties of the AlGaN alloys are investigated, in order to address the discrepancies observed in literature regarding basic properties of this system such as the band gap bowing parameter. The atomic configuration was found to affect the band gaps significantly, causing the bowing parameter to range from 0 to large positive values.