The VO Centre

Irradiation of Si at room temperature produces mobile vacancies and interstitials. The former trap O_i yielding A-centres, or VO complexes [75]. These are the primary oxygen-related defect in radiation damaged Cz-Si, and form irrespective of the irradiation method (electrons, protons, neutrons, $\alpha$-particles, $\gamma$-rays, etc.) The structure of the negatively charged centre has been established by EPR [76], whereas uniaxial stress studies on the local vibrational mode (LVM) at 836 cm^-1 (4.2K) has shown that the neutral defect has a similar structure [77]. This consists of an O atom bridging a pair of Si neighbours of the vacancy (Figure 5.1). The defect has associated vibrational modes at 836 and 885 cm^-1 for the neutral and negatively charged species respectively. The defect has an acceptor level at E_c-0.17 eV, possibly due to the strained Si-Si bond between the other pair of Si atoms bordering the vacancy. The lines are, however, broad and no ²⁹Si isotope shifts are seen [78]. The reorientation energy of the centre is surprisingly low, at only 0.38 eV [75]. Their barrier to migration is 1.46$\pm$0.29 eV [79] (also given as 1.86 eV [80,31]), allowing them to migrate at lower temperatures than O_i.