Role of Native Defects in Fe-Doped β-Ga2O3

Iron impurities are believed to act as deep acceptors that can compensate for the n-type conductivity in as-grown Ga2O3, but several scientific issues, such as the site occupation of the Fe heteroatom and the complexes of Fe-doped beta-Ga2O3 with native defects, are still lacking. In this paper, based on first-principle density functional theory calculations with the generalized gradient approximation approach, the controversy regarding the preferential Fe incorporation on the Ga site in the beta-Ga2O3 crystal has been addressed, and our result demonstrates that Fe dopant is energetically favored on the octahedrally coordinated Ga site. The structural stabilities are confirmed by the formation energy calculations, the phonon dispersion relationships, and the strain-dependent analyses. The thermodynamic transition level Fe3+/Fe2+ is located at 0.52 eV below the conduction band minimum, which is consistent with Ingebrigtsen's theoretical conclusion, but slightly smaller than some experimental values between 0.78 eV and 1.2 eV. In order to provide direct guidance for material synthesis and property design in Fe-doped beta-Ga2O3, the defect formation energies, charge transitional levels, and optical properties of the defective complexes with different kinds of native defects are investigated. Our results show that V-Ga and O-i can be easily formed for the Fe-doped beta-Ga2O3 crystals under O-rich conditions, where the +3 charge state FeGaGai and -2 charge state FeGaOi are energetically favorable when the Fermi level approaches the valence and conduction band edges, respectively. Optical absorption shows that the complexes of FeGaGai and FeGaVGa can significantly enhance the optical absorption in the visible-infrared region, while the energy-loss function in the beta-Ga2O3 material is almost negligible after the extra introduction of various intrinsic defects.