Modulation of the optical absorption edge of ∈- and k- Ga_2O_3 due to Co impurities caused by band structure changes: Work function measurements and first-principle calculations

摘要

Despite a wide bandgap of 4.8 eV, Ga_2O_3 has good electrical conductivity and thus has a wide range of potential applications. We previously reported that the bandgap of ∈-Ga_2O_3 is widened by Co-doping; here, we present a theoretical discussion of the changes in the electronic state induced by Co impurities. By comparing calculated and experimental absorptions, the experimentally observed optical bandgap was assigned to a transition from a bulk peak (l.0 eV below the valence band maximum) to the conduction band minimum. The photoabsorption of Ga_2O_3∶Co is not readily explained simply in terms of the bandgap of Ga_2O_3 However, the adjustable shift of the cutoff in its photoabsorption spectrum can be explained in terms of midgap impurity levels due to Co-doping, while the bandgap of Ga_2O_3 was almost unchanged even when the Co-impurity concentration was high. In addition, the work function, which was determined experimentally by photoemission spectroscopy, increased with the content of Co impurities. This was attributed to a lowering of the Fermi level induced by Co-doping.