In this chapter we examine the interaction between oxygen and vacancies in Si. Vacancies are introduced into Si in large quantities through irradiation treatments such as electron irradiation or ion implantation. Although most of the resultant vacancies and self-interstitials immediately recombine, large quantities remain. Annealing causes a mixture of recombination and complexing with other impurities in the crystal. In Cz-Si grown wafers there is a radial band of high vacancy concentration with radially varying vacancy and self-interstitial concentration. In all of these cases it is important to know how vacancies behave in the presence of oxygen. An attraction between V and Oi would be expected due to lattice strain compensation arguments.
The most common oxygen-vacancy complex is the VO centre (or `A' centre), consisting of a single off-site substitutional oxygen. Annealing at 300C leads to the gradual loss of VO centres, as they are replaced, initially with a defect with vibrational modes at 914 and 1000 cm-1, and then with a defect with a mode at 889 cm-1. One of the primary objectives of this section is to positively identify this defect, and we show it to be VO2, a prediction which has since been verified by experiment. Annealing at 450C and above leads to higher order Vm-On defects that form from the 889 cm-1 defect.
We show here that the VO2 defect has D2d symmetry and only one calculated O-related high frequency IR active mode at 807 cm-1. The VO3 defect has three high frequency IR active modes. The V2O defect has one such LVM, at lower frequency. These results provide strong support for the assignment of the 889 cm-1 (300 K) local vibrational mode to VO2.
We also discuss the annealing behaviour of the VOn centres including intermediate species that form between VO and VO2, the role of oxygen dimers, and the possibilities for higher order VOn species.