Effects of fluorine on structure, structural relaxation, and absorption edge in silica glass

Abstract

The effects of fluorine (F) doping on silica glass structure, structural relaxations, and vacuum ultraviolet (VUV) absorption edge were investigated by the infrared and vacuum ultraviolet absorption measurements. Linewidth of 2260 cm−1 absorption band, which is one of the good indicators of structural disorder, decreases proportionally to the fictive temperature (Tf). On the other hand, it is irrespective of F concentration. Structural relaxations are strongly stimulated by F doping. The transparency near the absorption edge depends on both F concentration and Tf. When Tf is constant, absorption edge shifts to a shorter wavelength proportionally to the F concentration. These results strongly suggest that there are two main factors that control the VUV absorption edge of F-doped silica glass; band gap widening by increasing Si–F bond and reduction of structural disorder. The latter factor is not directly caused by structural change by F doping. F enhances the structural relaxation, and it is easy to obtain samples with lower Tf, resulting in reducing structural disorder. Furthermore, a way to determine Tf of F-doped silica glass is offered.