Microstructural and electrochromic properties of sputter-deposited Ni oxide films grown at different working pressures

Abstract

The electrochromic properties of Ni oxide films grown at different working pressures (2.7, 5, 15, and 20 mTorr) by rf sputtering were investigated by means of in situ transmittance measurement and the resulting data were related to the crystallographic structure, surface morphology, and film density. At working pressures of over 5 mTorr, the sputter-deposited Ni oxide films crystallized gradually due to the plasma heating effect at a low growth rate. Although the Ni oxide film grown at 2.7 mTorr had the same amorphous crystallographic structure as the film grown at 5 mTorr, the former had a considerably more inhomogeneous surface and a much lower film density due to the rapid growth rate, and resulted in a defect-rich Ni oxide film. The electrochromic properties, such as the transient cycling period, coloration efficiency, and coloring/bleaching response times, were best for the sample grown at 5 mTorr and they are discussed in terms of defect-rich and crystalline Ni oxide films.