Correlation between microstructure, electrical and optical properties of nanocrystalline NiFe1.925Dy0.075O4 thin films

Dysprosium-doped nickel-ferrite (NiFe1.925Dy0.075O4) thin films were fabricated using sputter-deposition using a stoichiometric bulk target prepared by the solid state chemical reaction. The structural, electrical and optical properties of NiFe1.925Dy0.075O4 thin films were studied in detail. The grain-size (L) and lattice-expansion effects are significant on the electrical and optical properties of NiFe1.925Dy0.075O4 films. Air annealing (T-a) at 450-1000 degrees C results in the formation of nanocrystalline NiFe1.925Dy0.075O4 films, which crystallize in the inverse spinel structure, with L = 5-40 nm. The lattice constant of NiFe1.925Dy0.075O4 increases compared NiFe2O4 due to Dy-doping. Electrical conductivity of NiFe1.925Dy0.075O4 films (at 300 K) decreases from 1.07 Omega(-1) m(-1) to 3.9 x 10(-3) Omega(-1) m(-1) with increasing T-a (450 to 1000 degrees C). Conductivity was found to decrease exponentially with decreasing the temperature from 300 K to 120 K indicating the characteristic semiconducting nature of all the films. Band gap increases from 3.17 to 4.08 eV for NiFe1.925Dy0.075O4 films with increasing T-a from 450 to 1000 degrees C. A correlation between grain-size, electrical conductivity and optical band gap in nanocrystalline NiFe1.925Dy0.075O4 films is established.