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Defects in semi-conductor

The intrinsic defects and impurities play an important role on electrical and optical properties of semiconductors.The role of individual defects is importantin order to engineer semiconductor materials with specific properties fordesired applications. Under the framework of densityfunctional theory (DFT)and beyond state-of-the-art methodologies, we attempt to understand the role of individualdefects in various reactive environments. The creation of a defect in the bulk or at the surface of a semiconductor changes the free energy of the system. Depending on environmental conditions viz. temperature, pressure and concentration of doping, formation of defects may be energetically favored for manysemi-conducting materials. Therefore, in those instances,the defects are inevitable and formation of defects leads to a favorable Gibbs free energy of formation. The Gibbs free energy of formation Gqf (T, p) for an isolated point defect is given by the change in free energy of the system containing the defect at a given charge state with respect to the pristine neutral system. The formation energy depends on the position of the Fermi level, i.e.concentration and type of dopants. In an experimental setup, the position of the Fermi level is often hard to determine accurately. Moreover, doping or creating non-equilibrium populations of charge carriers by optical excitation can alter its position. Therefore, it is of profound interest for the researchers to provide theoretical guidance to experiment and technology in order to understand the electronic structure of defects in semi-conductor. In a theoretical approach, any position of the Fermi level can in principle be modeled. DFT with advanced functionals has reached to an unprecedented level of accuracy. DFT when combined with ab initio thermodynamics can correctly predict the stability and concentration of defects at various experimentally relevant temperatures and realistic pressureconditions. Using hybrid density-functional theory and ab initio atomistic thermodynamics, we investigate the interplay of bond-making, bond-breaking, and charge-carrier trapping at the bulk and surfaces of variousmaterials including metal-oxides, 2D materials, energy materials, perovskites, topological insulators etc.