Infrared transparent spinel films with p-type conductivity

Abstract

Spinel oxide films containing at least two transition metal cations were found to exhibit p-type conductivity with high optical transparency from the visible to wavelengths near 15 μm. Resistivities as low as 0.003 Ω cm were measured on 100-nm thick rf sputter deposited films that contained nickel and cobalt. Optical spectra, Raman scattering and XPS measurements indicated the valency of nickel localized on octahedral sites within the spinel lattice determines these properties. A resistivity minimum was found at the composition NiCo 2O 4 deposited from aqueous or alcoholic solutions followed by subsequent annealing at 400°C in air. Solution deposited films richer in nickel than this stoichiometry, were always found to phase separate into nickel oxide and a spinel phase with concomitant loss in conductivity. However, the phase stability region could be extended to higher nickel contents when rf-sputter deposition techniques were used. Sputter deposited spinel films having a cobalt to nickel ratio <2 were found to exhibit the highest conductivity. Results suggest that the phase stability region for these materials can be extended through appropriate choice of deposition conditions. A possible mechanism that promotes high conductivity in this system is thought to be charge transfer between the resident di- and trivalent cations that may be assisted by the magnetic nature of the oxide film.