Effect of the Ni²⁺/Ni³⁺ Concentration on the Optoelectronical Properties of Lithium-Doped Nickel Oxide Films

Lithium-doped nickel oxide (LNiO) films were prepared through radio-frequency magnetron sputtering. The optoelectronic and structural properties of the LNiO films were investigated with respect to varying ratios of argon–oxygen (Ar/O2) and argon–hydrogen (Ar/H2) reaction gases. The resulting LNiO films were polycrystalline, displaying a NaCl-type structure with a grain orientation in the (111) and (200) directions. An increase in the Ni3+ electron bonding states led to a decrease in the resistivity of the LNiO films under the Ar/O2 reaction gas. Nevertheless, the Ni3+ ions in LNiO films induced brown or black coloration and a decrease in transmittance. The high-transmittance LNiO films were deposited with varying ratios of Ar/H2 as the reaction gas. However, the generation of OH bonds promoted the adsorption of water molecules. The electrons provided by the adsorbed water molecules on the LNiO film reduced the number of Ni3+ ions, and this reaction reduced the carrier concentration and resistivity of the film. Therefore, the LNiO/Ag/LNiO multilayer structure with top and bottom LNiO films deposited with the Ar/H2 reaction gas exhibited impressive resistivity and transmittance.