Optical bandgap and luminescence in Er³⁺ doped oxyfluoro-germanate glass-ceramics

We determine the optical bandgap energy and the Urbach energy in trivalent erbium (Er³⁺) doped oxyfluoro-germanate glass-ceramics from the ultraviolet-visible absorption spectra. The glass-ceramics are produced by subjecting the as-prepared glass to heat treatment at various temperatures above T_g (494 °C) for 1 hour. The calorimetric measurements show that the nanocrystals formed during heating are stable and the composition of the remaining glass matrix remains unchanged. It is found that both the density and the bandgap energy of the glass ceramics vary non-linearly with increasing heat-treatment temperature. This non-linear trend accords with that for the upconversion luminescence of these materials. Furthermore, the indirectly allowed transition in bandgap is observed in all the studied glass ceramics. An exponential dependence of absorption coefficient on the photon energy is observed, indicating that the Urbach rule is obeyed. The results derived from this study will be potentially useful for designing the rare earth ion doped glass-ceramics with a strong luminescence effect.