Stoichiometry Controlled Bipolar Conductivity in Nanocrystalline Ni_xCd_1-xO1_+δ Thin Films

Wide-gap oxides exhibiting both n- and p-type bipolar conductivity are extremely desirable for the development of next generation transparent optoelectronics. While most metal oxides show a propensity for n-type conductivity, nickel oxide (NiO) is one of the few that exhibits p-type conductivity without intentional doping. We synthesize nanocrystalline-Ni_xCd_1-xO_1+δ-alloy thin films over the entire composition range with variable crystal stoichiometry using RT radio-frequency magnetron sputtering. Optical and electronic properties of the films are investigated with a variety of experimental techniques, including spectroscopic ellipsometry, the Seebeck effect, and the variable-temperature Hall effect as well as x-ray photoelectron spectroscopy. We find that the conduction type of Ni_xCd_1-xO_1+δ-alloy thin films depends on the alloy composition and oxygen stoichiometry. Stoichiometric Ni_xCd_1-xO alloys are n type in the composition range of 0 ≤ x ≤ 0.52 and insulating for higher x. On the other hand, O-rich alloys are p-type conducting for x ≥ 0.38. This demonstrates that in the alloy composition range of 0.38 < x < 0.52, n-type and p-type Ni_xCd_1-xO alloys can be synthesized by controlling the oxygen stoichiometry. 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. The results offer an interesting opportunity for using Ni_xCd_1-xO_1+δ alloys for transparent devices that require bipolar conductivity.