Hexagonally-arranged-nanoporous and continuous NiO films with varying electrical conductivity

Nickel oxide (NiO) thin films have been prepared by magnetron sputtering, with different Ar/O-2 ratios in the plasma, on several substrates, including hexagonally arranged nanoporous anodic alumina membranes (AAM). The obtained films exhibit columnar growth, which makes it possible to preserve the hexagonal order of the AAM substrates in the NiO thin films. X ray diffraction patterns show a polycrystalline structure with a crystallographic texture that depends on the plasma composition. Additionally, the NiO lattice parameter increases with the oxygen content of the plasma. The presence of oxygen during deposition is responsible for these structural changes, as well as for an oxygen enrichment in the NiO films, which leads to changes in their electrical properties. The electrical resistivity of the films decreases with the oxygen content of the plasma, which suggests p-type conductivity due to oxygen enrichment in the NiO lattice. Indeed, an analysis of the EXAFS oscillations at the Ni-K edge confirms the lattice expansion and a decrease of the Ni-Ni coordination number when the oxygen content of the plasma increases, which points towards an increasing presence of Ni vacancies for larger values of the O-2/Ar ratio. (c) 2013 Elsevier B.V. All rights reserved.