Effect of annealing and lithium substitution on conductivity in nickel-cobalt oxide spinel films

• Published in: Surface and interface analysis 2005-04, Vol.37 (4), p.424-431
• Main Authors: Owings, Robert R., Holloway, Paul H., Exarhos, Gregory J., Windisch Jr, Charles F.
• Format: Article
• Language: English
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Electrical properties of specific thin films; Electrical properties of specific thin films and layer structures (multilayers, superlattices, quantum wells, wires, and dots); Electron states; Electronic structure and electrical properties of surfaces, interfaces, thin films and low-dimensional structures; Exact sciences and technology; lithium; nickel cobalt oxide; Physics; Polarons and electron-phonon interactions; spinel; transparent conducting oxide
• ISSN: 0142-2421; 1096-9918
• DOI: 10.1002/sia.2040

Mixed transition‐metal oxide spinels exhibit high electrical conductivity and enhanced infrared transmissivity resulting from the presence of small polarons in the NiCo2O4 lattice. Polarons are formed as a result of judicious choice of component metal cations and attendant resident cation charge states. Substitution of lithium for cobalt (Ni1+xCo2−x−zLizO4) while maintaining the spinel stoichiometry and controlled post‐deposition annealing was found to influence measured conductivity in both solution‐ and sputter‐deposited thin films. For lithium concentrations < 10% an improvement in conductivity was observed, depending upon whether the oxide film was deposited from solution (large increase) or sputter deposited (small increase). However, higher lithium concentrations degraded conductivity. Both XPS and SIMS analyses of films with high lithium concentrations (z > 0.3) revealed that lithium was concentrated near the film surface and the formation of carbonate was detected by XPS and Fourier transform infrared data. Results indicate that additions of lithium to transition‐metal spinel oxides can lead to increased conductivity by increased polaron formation. However, in high concentration, lithium is an interstitial that diffuses to the surface to form compounds that degrade electrical conductivity. Copyright © 2005 John Wiley & Sons, Ltd.