Delayed onset of photochromism in molybdenum oxide films caused by photoinduced defect formation

We report the photochromic properties of amorphous MoO₃ films deposited by dc sputtering with different O₂ flow rates. The kinetics of film coloration under UV light irradiation is determined using optical transmission spectroscopy. Changes in the absorbance and refractive index were derived from the analysis of transmittance spectra. The absorbance spectra exhibited a growing broad peak centered around 830 nm, which was induced by the UV irradiation. In the early stages of irradiation, the absorbance of the films did not change but their refractive indices did change. This induction time was correlated with the O₂ partial pressure during the film deposition, which was controlled by the O₂ flow rate. The origins of this observation are discussed.