MACROSCOPIC AND MICROSCOPIC EFFECTS OF RADIATION IN AMORPHOUS SIO2

Microscopic and macroscopic changes in the a-SiO2 network induced by radiation are reviewed. In the former case, ten intrinsic network defects are identified, most having been examined structurally by electron spin resonance. The presence of the different defects is dependent upon manufacturing processes. For example, it is demonstrated that oxygen-vacancy-like and non-bridging hole center defects may be produced in a correlated manner as a result of bond scission in strained networks. Other defects, such as Si microclusters, are observed in oxygen deficient materials again resulting from preparation conditions. Macroscopic density variations less-than-or-equal-to 3% are induced by radiation. It is demonstrated that this densification cannot be attributed to stable defect creation and cannot be likened to pressure/temperature induced plastic densification. Analysis of refractive index, Raman scattering and infrared absorption data indicates that radiation induces a unique ''structure'' in which there is a substantial reduction in the mean Si-O-Si bridging bond angle (of approximately 10-degrees) but in which the network density is only marginally changed. This is the exact opposite of the situation observed in pressure/temperature induced plastically densified a-SiO2. We argue that the structural modification involves the creation of large voids or open spaces analagous to the cage networks found in the crystalline phases known as porosils.