RADIATION-DAMAGE AND THE ROLE OF STRUCTURE IN AMORPHOUS SIO2

The influence of structure on the creation efficiency of oxygen-vacancy center and non-bridging oxygen-hole center defects in amorphous SiO2 by ionizing radiation is examined. In undensified, amorphous SiO2 the defect creation depends significantly upon the OH content of the network because of self-annealing effects due to the trapping of O2 molecules (dry silica) or water-related molecules (wet silica). Oxygen vacancy defect creation is argued to be due to displacement of network O atoms - precursor transformation accounts for at most 1014 cm-3. It is suggested that strained Si-O-Si bonds may act as precursors for both oxygen vacancy and nonbridging oxygen defects. Metastable densification of the SiO2 structure results from a process of Si-O-Si intertetrahedral bridging bond angle reduction coupled with a reduction in the average number of members in a ring. Diffusion of molecules involved in defect annealing is found to be impeded by the densification process. Defect creation is found to be very much enhanced in densified, amorphous SiO2 produced by hydrostatic compression or resulting directly from deposition in chemical vapour deposited films. This is interpreted as due to a process of defect creation due to strained bond cleavage. The enhancement is found to be up to several orders of magnitude for oxygen vacancy defects and of the order of 40 for nonbridging oxygen defects. The bond cleavage interpretation appears to be valid for both wet and dry forms of densified silicas. © 1990.