Synthesis and characterization of transparent conducting oxide cobalt-nickel spinel films

Cobalt-nickel oxide films of nominal 100 nm thickness, and resistivity on the order of 10(-2) Ohm CM have been deposited by spin casting from both aqueous and organic precursor solutions followed by annealing at 450 degreesC in air. Films deposited on sapphire substrates exhibit a refractive index of about 1.7 and are relatively transparent in the wavelength region from 0.6 to 10.0 mum. They are also magnetic. The electrical and spectroscopic properties of the oxides have been studied as a function Of the x = Co/(Co + Ni) ratio, An increase in film resistivity was found upon substitution of other cations (Zn2+, Al3+,...) for Ni in the spinet structure. However, some improvement in the mechanical properties of the films resulted. A combination of x-ray diffraction, x-ray photoelectron spectroscopy, UV/Vis, and Raman spectroscopy indicated that NiCo2O4 is the primary conducting component and that the conductivity reaches a maximum at this stoichiometry. When x < 0.67, NiO forms leading to an increase in resistivity, when x > 0.67, the oxide was all spinel but the increased Co content lowered the conductivity. The influence of cation charge state and site occupancy in the spinel structure markedly affects calculated electron band structures and likely influences an anomalous switch of p-type conductivity to n-type conductivity seen at x = 0.67. Electronic structure modeling studies also suggest the important role of the Ni3+ cation in the conductivity mechanism. Finally, Raman spectra of the films were relatively easy to obtain and therefore were useful as a routine tool for identifying composition and optimizing conductivity. (C) 2001 American Vacuum Society.