AVS 49th International Symposium
    Thin Films Monday Sessions
       Session TF-MoA

Paper TF-MoA5
Raman Spectra of Cobalt Nickel ITCO Films

Monday, November 4, 2002, 3:20 pm, Room C-101

Session: Transparent Conductive Coatings
Presenter: C.F. Windisch Jr., Pacific Northwest National Laboratory
Authors: C.F. Windisch Jr., Pacific Northwest National Laboratory
K.F. Ferris, Pacific Northwest National Laboratory
G.J. Exarhos, Pacific Northwest National Laboratory
S.K. Sharma, University of Hawaii
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Cobalt-nickel spinel oxide films have recently shown promise as infrared transparent conducting oxide (ITCO) materials, with resistivity as low as 10@super -3@ ohm cm and transmittance (for a 100 nm-thick film) approaching 80% at 5 µm. While our current research is focused on optical behavior, the materials have been the subject of previous study, mainly due to their capacity to catalyze the water electrolysis reaction. Attempts to characterize the underlying structural and mechanistic causes for the remarkable electrical and magnetic properties, however, were only partly successful. In particular, there is still uncertainty regarding not only the electrical conduction mechanism, but also the identities of the various charge states of the Ni and Co ions and their distributions among the octahedral and tetrahedral sites in the spinel lattice. In all likelihood, the characteristics of the ions and the specifics of the conduction mechanism are intimately related. In order to understand how the unique electrical and optical properties of the cobalt-nickel spinel oxide films result from chemical structure, Raman spectra were obtained as a function of temperature and composition. Shifts in peak frequencies and changes in bandwidth as a function of temperature and laser power, particularly under cryogenic conditions, could not be explained by simple heating effects alone. Details of this behavior, as well as the spectral changes observed as a function of the Co/Ni ratio in the spinel, point to the presence of a localization of charge states and the important role of small polaron hopping in the electrical conduction mechanism. This work was supported by the Materials Sciences Division of Basic Energy Sciences through the DOE Office of Science and the ARO through DARPA contract AO J209/00. Pacific Northwest National Laboratory (PNNL) is operated by Battelle Memorial Institute for the U.S. Department of Energy under Contract DE-AC06-76RLO 1830.