ROLE OF RADIOLYTIC OXYGEN IN THE X-RAY-PRODUCTION AND THERMAL ANNEALING OF DEFECTS IN HIGH-PURITY AMORPHOUS SIO2

Direct experimental evidence for the presence and x-ray dose dependenc e of radiolytically displaced oxygen in high-purity, oxygen-deficient a-SiO2 is presented. Anneal-interrupted x-irradiation and electron spi n resonance measurements were used to investigate the x-ray dose depen dence of the radiation response of two types of low-OH a-SiO2, i.e., o xygen-excess and oxygen-deficient materials. The production and therma l annealing of paramagnetic defects in these two materials were compar ed to explore the role played by dissolved and radiolytically displace d oxygen in these processes. The multiple, reversible interconversions of E' centers and peroxy radicals proved to be very sensitive to the presence of free oxygen in the a-SiO2 network. Such interconversions a re very probable in the oxygen-excess material due to the presence of dissolved oxygen. It follows that such interconversions an not expecte d in the oxygen-deficient material, at least at low dose, in agreement with experimental data. However, our results show that such interconv ersions take place above a certain threshold dose in oxygen-deficient material. In fact, we observed peroxy radicals in oxygen-deficient mat erial. Their production is attributed to the presence of radiolyticall y displaced oxygen in this material. A simple physical model is presen ted to explain the results. The model involves the random creation of displaced oxygen atoms which undergo elastically driven recombination into oxygen molecules. It is these molecules which participate in the interconversions in the oxygen-deficient material. A threshold is obse rved because the recombination does not occur unless the distance betw een the oxygen atoms is less than some characteristic length, i.e., a correlation radius of an elastically coherent nanoregion in the amorph ous network. The experimental results and the model imply a universal radiation response at high dose when the density of radiolytically dis placed oxygen exceeds that of any precursors.