Ion-Implantation Effects in Noncrystalline SiO2

Impurity ions (H+, He+, B+, O+, A+, Kr+, and Xe+) have been implanted in high-purity vitreous silica and their production of and interaction with implantation damage was studied by optical absorption techniques. An absorption band at 245 nm (B2-band) is produced by all ions except H+. The implantation of H+ greatly reduces the B2-band absorption intensity produced either by prior or subsequent sample implantation with heavier ions. For ions other than H+, the intensity of the B2-absorption increases with increasing ion mass for a given ion fluence and suggests that the defect giving rise to this absorption is generated by ion energy going into displacement processes. The spectral position of the implantation-induced B2-band is identical with that of a band found in impure as-manufactured fused silica. The band in impure silica is believed to be due to a center consisting of a trivalunt metallic impurity (e.g., Al3+) substituting for Si4+ and an adjacent oxygen vacancy. The results of the present investigation suggest that the implantation induced defect producing the B2 band in high-purity silica is an oxygen vacancy at which the binding electrons of the two adjacent Si atoms can be trapped. If the B2-band is also associated with SiO2/Si interface states in MOS devices as has been suggested, the present results may offer a possibility for reducing the density of these states by H+-implantation.