Sub-band-gap-excited luminescence of localized states in SiO2-Si and SiO2-Al glasses

Silica glass samples doped with extra silicon (SiO2-Si: artificial oxygen deficiency) and with aluminum (SiO2-Al: Al-doped without accompanying alkali ions) were studied. The luminescence properties of these two samples are compared in the range of temperature 15-290 K under excitation of ArF excimer laser (193 nm). In both samples the luminescence of oxygen deficient centers (ODCs) is detected, i.e., emission bands in the blue at 440 nm and the UV at 280 nm. Cooling of the both samples led to strong increases of luminescence intensity down to 80 K with much smaller increases for still lower temperatures. At 290 K in SiO2-Si a luminescence similar to that of twofold-coordinated silicons in stoichiometric silica glasses was detected, i.e., displaying exponential decay of emission bands in the blue (τ = 10.3 ms) and UV (τ = 4.5 ns). In both samples emission at 440 nm occurs in times shorter that ∼400 μs, according to a non-exponential decay law. These decay times are much faster than the 10.3 ms exponential decay typical of the twofold-coordinated silicon center in pure undoped silica. Conversely, the decay of the UV band possesses an additional decay ranging from 2 to 5 μs, that is much slower than 4.5 ns typical of lone twofold-coordinated silicons in stoichiometric silica. Prolongation of the decay time of UV emission can only be explained in terms of electron-hole recombination processes. Moreover, the observed diminishing of the luminescence intensity concomitant with acceleration of decay times with increasing temperature above 90 K is found to be correlated with thermally activated recombination of self-trapped holes in pure silica [D.L. Griscom, J. Non-Cryst. Solids 149 (1992) 137]. It is concluded that the ArF-laser induced electronic processes of recombination luminescence in SiO2-Si (and SiO2-Al) are related to trapping of an electron on in a localized state related to oxygen deficiency (or to an Al in the SiO2 network) and nearby trapping of a hole on a normal bridging oxygen, forming an STH.