Electrochromic nickel oxide thin films deposited under different sputtering conditions

In this work, non stoichiometric nickel oxide (NiOx) thin films were deposited by r.f. reactive sputtering of a metallic nickel target in an O-2-Ar atmosphere. A systematic variation of two deposition parameters was done: the oxygen flux (phi) and the r.f. power (P). The electrochemical characterization of the films was performed in aqueous electrolyte. The spectral transmittance measurements, as well as the X-ray diffraction analysis were performed ex-situ, while monochromatic transmittance and stress measurements were performed in situ. Samples deposited at low oxygen flux (or high power) are transparent, in contrast to those deposited at high oxygen flux (or low power), which are dark brown. The films were cubic NiO, with preferred orientation in the (111) direction. Lattice parameters increase with increasing oxygen flux during deposition, but decrease with increasing power. For all samples, the ratio O/Ni was greater than 1, as determined by Rutherford back-scattering analysis. Also, an important hydrogen content was found in the films. The relationship between optical, electrochemical, mechanical, structural and morphological behaviour of the above mentioned films will be reported and discussed in this work.