Sol-gel preparation and optical characterization of NiO and Ni1-xZnxO thin films

NiO and Ni1-xZnxO thin films have been prepared on Al2O3 (0 0 0 1) substrates by a sol-gel method using nickel and zinc acetates as precursors. Polycrystalline films were obtained by spin-coating the precursor solutions followed by annealing in air in the 300-900degreesC range with the best crystalline quality being achieved near 800degreesC. For Ni1-xZnxO (x less than or equal to 0.31) films the cubic lattice constant is found to increase linearly with the Zn composition. The optical constants of the NiO and Ni1-xZnxO films have been obtained by spectroscopic ellipsometry in the 2.0-4.5eV photon energy region. The optical band gap due to a charge-transfer transition between the 0 p-like bands and the Ni d-like bands is found to decrease with increasing Zn composition from that of NiO (3.65eV). A new absorption structure is observed for Ni1-xZnxO below the band-gap edge. It is attributable to transition between the O p-like bands and the Zn s-like bands, created inside the band gap by the Zn substitution. (C) 2003 Elsevier B.V. All rights reserved.