Omnidirectional antireflective properties of porous tungsten oxide films with in-depth variation of void fraction and stoichiometry

We report on the fabrication of porous hot-wire deposited WO_x (hwWO_x) films with omnidirectional antireflective properties coming from in-depth variation of both (i) void fraction from 0% at the Si substrate/hwWO_x interface to 30% within less than 7 nm and to higher than 50% at the hwWO_x/air interface, and (ii) x, namely hwWO_x stoichiometry, from 2.5 at the Si/hwWO_x to 3 within less than 7 nm. hwWO_x films were deposited by means of hw deposition at rough vacuum and controlled chamber environment. The films were analyzed by Spectroscopic Ellipsometry to extract the graded refractive index profile, which was then used in a rigorous coupled wave analysis (RCWA) model to simulate the antireflective properties. RCWA followed reasonably the experimental reflection measurements. Void fraction and x in-depth variation, controlled by the hw process, greatly affect the antireflective properties, and improve the omnidirectional and broadband characteristics. The reflection suppression below 10% within the range of 500-1000 nm for angles of incidence up to more than 60° is demonstrated.