Effects of Ni d-levels on the electronic band structure of Ni_xCd_1-xO semiconducting alloys

Ni_xCd_1-xO has a ∼3 eV band edge offset and bandgap varying from 2.2 to 3.6 eV, which is potentially important for transparent electronic and photovoltaic applications. We present a systematic study of the electronic band structure of Ni_xCd_1-xO alloys across the composition range. Ion irradiation of alloy samples leads to a saturation of the electron concentration associated with pinning of the Fermi level (E_F) at the Fermi stabilization energy, the common energy reference located at 4.9 eV below the vacuum level. The composition dependence of the pinned E_F allows determination of the conduction band minimum (CBM) energy relative to the vacuum level. The unusually strong deviation of the CBM energy observed from the virtual crystal approximation is explained by a band anticrossing interaction between localized 3d states of Ni and the extended states of the Ni_xCd_1-xO alloy host. The resulting band structure explains the dependence between the composition and the electrical and optical properties of the alloys - the rapid reduction of the electron mobility as well as previously observed positive band gap bowing parameter. X-ray photoelectron spectroscopy studies confirm that the L-point valence band maximum in the Cd-rich alloys are unaffected by the interaction with Ni d-states.