Effect of thermal annealing on the photoluminescence of Er doped Ge–Se–Ga glasses

Er³⁺ doped Ga_x(Ge₃₀Se₇₀)_100−x and (GeSe₂)_0·76(Ga₂Se₃)_0·24 glasses (x=6 and 9 at%) were prepared by melt quenching methods. The samples were annealed at various times and temperatures T _a over a wide range, including the glass transition temperature T _g, but still being below the crystallisation temperature (T _c). We have examined the changes in the intensity of 1550 nm photoluminescence (PL) excited by a 976 nm laser, and in Raman scattering as a function of annealing temperature and time. The intensity of Er³⁺ ions absorption spectra did not change after the annealing process. The photoluminescent (PL) intensity increased with increasing T _a and reached a maximum at a certain annealing temperature that is very close to T _g, but still below T _c. PL spectra of these samples are broader than as quenched glass, but Raman spectra did not show any noticeable change in structure. We argue that increase in the intensity and width of PL spectra are related to a more random redistribution of Er³⁺ ions and to a decrease in the probability of nonradiative transitions in the host glass. The latter is due to the reduction of defects in the glass structure during annealing. When the annealing temperature is above T _g (close to the crystallisation temperature), the PL intensity drops sharply. Probably, small amounts of a crystalline phase forming above T _g may act to quench the PL intensity.