Conducting spinel oxide films with infrared transparency

Abstract Mixed transition metal spinel oxide (AB2O4) films deposited from solution or by means of magnetron sputtering are found to exhibit resistivities as low as 3.0 mΩ cm and optical transparency to wavelengths near 15 μm. These remarkable properties are achieved when metal cations selected from group VIII in the periodic table are resident in the spinel lattice. Both cation charge state and site occupancy significantly influence properties in these relatively high refractive index and chemically robust films. Post-deposition annealing of deposited films in air always led to decreased resistivity but little change in the optical transmission was seen. In order to verify the small polaron charge transport mechanism that drives the conductivity, Raman spectra and both van der Pauw and Seebeck measurements were acquired as a function of film temperature and cation composition. Results indicate the importance of cation disorder on the conductivity and along with results from electronic structure simulations suggest processing avenues to further tailor film properties. These include partial substitution of lithium for cations resident on the tetrahedral lattice sites and gross replacement of first row transition metal cations with those deeper in the periodic table.