Thermodynamics of native point defects in alpha-Fe2O3: an ab initio study

The native point defects in Fe2O3 are theoretically investigated using ab initio methods based on the GGA + U formalism. We consider vacancies and interstitials of Fe and O atoms as well as the electron polaron as Fe-II defects at the host Fe-III site. The formation energies and charge transition levels are computed for each defect type with careful elimination of size effects of the supercell. It is found that the Fe interstitial and vacancy form donor and acceptor levels close to band edges, respectively, thereby allowing for charge carriers at room temperature. We determine the oxygen deficiency under high-temperature equilibrium conditions and find an excellent agreement with experiment. In the quenched condition, it is found that the Fermi level is pinned at similar to 0.5 eV below the conduction band minimum, which may limit the performance of Fe2O3 as a photoanode in solar water-splitting cells. Furthermore, the oxygen vacancy is mostly neutral and the Fe interstitial is responsible for electron carriers.