X-Ray Reflectometry for electrochromic thin film characterization: WO3 as an example
Résumé
Electrochromic (EC) materials and devices are able to modify their optical properties under the appliance of a voltage. This technology is recognized as one of the key green technology for sustainability and energy saving. The ability of electrochromic materials to maintain their coloured state once the applied voltage is withdrawn associated to its reversibility is called the memory effect. This unusual property on metal oxides allows to reduce power consumption of devices by avoiding to apply a voltage permanently in order to keep the desired colour.
WO 3 thin films of ~100 nm were grown by radio-frequency magnetron sputtering using a WO3 ceramic target in an argon and oxygen atmosphere. The control
of sputtering conditions as power density, total pressure and oxygen partial pressure allows to adjust the memory effect on WO 3 films. By varying the total pressure from 1 Pa to 2 Pa and 4 Pa, WO3 thin films with an irreversible colouration, a high memory effect or a poor memory effect can be obtained. The presentation will make a focus on WO 3 films deposited at 2 Pa.
X-Ray Reflectometry (XRR) is a non-destructive technique which is a useful tool to evaluate thickness and the thin film density, thus the composition. From XRR, density, thickness, surface and interface roughness can be obtained. XRR measurements were performed at the Metrology beamline at SOLEIL synchrotron using CASTOR device. The reflectivity was recorded on a WO 3 thin film deposited on silicon wafer at 8 keV photon energy. Simulation of XRR curves were performed using model based on different layers as substrate, film/substrate interface and thin film.
Mots clés
material characterisation
thin film
X-Ray Reflectometry
Electrochromic material
optical properties
coloured state
metal oxide
radio-frequency magnetron sputtering
ceramic target
sputtering conditions
irreversible colouration
density
thickness
surface
interface roughness
Metrology
SOLEIL
synchrotron
CASTOR
ionizing radiation
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