Optical and electrochromic properties of RF reactively sputtered WO_3 films

概要

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The effects of deposition conditions, including substrate temperature and sputtering gas　pressure, on the optical and electrochromic (EC) properties of WO_3 films prepared by RF　reactive sputtering were investigated. The maximum optical band gap energy of 3.15 eV was obtained on a room-temperature substrate temperature, and decreased with　increasing substrate temperature. However, the maximum refractive index of about 2.5 was obtained at a sputtering gas pressure of 5 mTorr and a substrate temperature of 500°C, and decreased with decreasing substrate temperature and with increasing pressure. The decrease in bandgap energy and refractive index are thought to have been caused by an increase in WO_3 cluster size and a decrease in film density, respectively. Electrochromic (EC) response times of the WO_3 films were measured in an electrolyte of 1N H_2SO_4 aqueous solution. Fast EC responses were obtained for the WO_3 films with wide band gap energies and low refractive indices. The results indicate that the optical properties of WO_3 films are useful indicators of EC response.