Transparent Ni_xCd_1−xO_1+δ alloy thin films with bipolar conductivity

NiO-CdO alloy films over the entire composition range were synthesized by RT magnetron sputtering in both pure Ar (Ni_xCd_1-xO) and Ar +O2(Ni_xCd_1-xO_1+δ) environments. These alloy films are nanocrystalline with rocksalt structure. We demonstrated that in the alloy composition range of 0.38 ≤ x ≤ 0.52, Ni_xCd_1-xO_1+δ films with bipolar (i.e., n- or p-type) conductivity can be achieved by controlling the oxygen stoichiometry. Within this composition range the band gap of the alloy is ~3-3.3 eV with transmittance >50% in the visible range. The unusual electrical and optical properties of Ni_xCd_1-xO_1+δ alloy thin films can be explained by the modifications of the electronic band structure due to anticrossing interactions between localized Ni d levels and extended valence and conduction band states of the alloy. In addition, we studied the transport mechanism of p-type Ni_xCd_1-xO_1+δ alloy films and found that the hole mobility increases with temperature in the range of 400-450 K. This is consistent with hole transport via a small polaron hopping process with an activation energy Ea of 0.174 eV (x=1) to 0.331 eV (x=0.47). These results strongly suggest that the nanocrystalline Ni_xCd_1-xO_1+δ alloy system has great technological potential for applications in transparent optoelectronic device.