The Mechanism of Oxygen Diffusion in Vitreous SiO2

Abstract

Oxygen diffuses in vitreous as network O atoms and molecules. The latter process is also responsible for the growth of films on silicon during thermal oxidation. Since the distance between solubility sites of molecules is ∼2.6 nm, the molecules either diffuse along randomly oriented structural channels resembling the ordering along the c‐axis in tridymite crystal, or they cover this distance by several random jumps of ∼0.26 nm (the O‐O distance). In the latter case, the incorporation of an molecule at a jump site affects the structure of so that no further solution of can occur within a volume element of ∼10−20 cm3 around it. This process is associated with structural ordering as indicated by the unusually large negative entropy term. Analysis of reported results on the growth of films on silicon in ambient shows that the diffusivity of is about three times larger than in fused . From the observed concentration profile of 18O atoms we imply that the diffusivity of network oxygens is about three orders of magnitude less than in fused ; this may be due to the very low contamination level and/or increased structural ordering in the film. This effect is another aspect of the influence of oxygen diffusion upon the structure of . Under these conditions the and network oxygen diffusion processes are not related to each other in the manner they are in fused. It is suggested that increased structural ordering in films on silicon involves preferential alignment of structural channels more or less perpendicular to the Si surface. It is also suggested that the structural channels consist of rings haying 7–8 tetrahedra; these channels cannot be considered as pores which are gross imperfections.