## 2 Results
### Production of VO Defect Versus C Concentration
Upon irradiation, carbon readily captures self-interstitials converted to carbon interstitial, C\({}_{\text{i}}\) (Cs + Si\({}_{\text{i}}\)\(\rightarrow\) C\({}_{\text{i}}\)). In the latter form, carbon is very mobile even at room temperatures, and reacts strongly with O\({}_{\text{i}}\) and C\({}_{\text{s}}\) leading to the formation of C\({}_{\text{i}}\)O\({}_{\text{i}}\) and C\({}_{\text{i}}\)C\({}_{\text{s}}\) pairs, respectively, which are the main C-related defects formed. On heavier irradiation, C\({}_{\text{i}}\)O\({}_{\text{i}}\)(Si\({}_{\text{i}}\)), C\({}_{\text{i}}\)C\({}_{\text{s}}\)(Si\({}_{\text{i}}\)), and C\({}_{\text{i}}\)(Si\({}_{\text{i}}\)) complexes are also formed [16], as are even higher members of the families C\({}_{\text{i}}\)O\({}_{\text{i}}\)(Si\({}_{\text{i}}\))\({}_{\text{n}}\) and C\({}_{\text{i}}\)C\({}_{\text{s}}\)(Si\({}_{\text{i}}\))\({}_{\text{n}}\), as well as C\({}_{\text{i}}\)(Si\({}_{\text{i}}\))\({}_{\text{n}}\). There are theoretical predictions [154] for the existence of C\({}_{\text{s}}\)V defects but there is no positive experimental evidence so far that carbon captures vacancies. The IR signals from the above families are very low, and it is difficult to reach established conclusions. It needs samples with high C, Ge, Sn and Pb content to be irradiated at very large fluencies in order to have measurements for comparison purposes. This is scheduled for a future experiment. In the present work, in terms of the C-related defects, we mainly concentrated on C\({}_{\text{i}}\)O\({}_{\text{i}}\) and C\({}_{\text{i}}\)C\({}_{\text{s}}\) pairs. We additionally note that in [100] (in Figure 7 in particular) we compared, measurements for the C\({}_{\text{i}}\)O\({}_{\text{i}}\)(Si\({}_{\text{i}}\)) complex between Sn\({}_{\text{H}}\) and Sn\({}_{\text{i}}\) in Si-doped material. We have not included this information in the present work since we also need comparisons with Ge-doped and Pb-doped Si to achieve a clear picture for the behavior of this defect. Figure 16 presents [44] the IR spectra of Si for the S22 sample after irradiation and at certain characteristic temperatures in the course of isochronal anneals (data from Table 4). Sample S22 was a Cz-Si sample without any isovalent dopant and was irradiated at a fluence of \(1\times 10^{18}\) cm\({}^{-2}\). Signals from various O-related and C-related defects were present. From the latter family we shall focus mainly on C\({}_{\text{i}}\)O\({}_{\text{i}}\) (862, 739 cm\({}^{-1}\)) and C\({}_{\text{i}}\)C\({}_{\text{s}}\) (546 cm\({}^{-1}\)) defects. Signals from C\({}_{\text{s}}\)O\({}_{\text{i}}\) (585, 637, 684 cm\({}^{-1}\)) and C\({}_{\text{i}}\)O\({}_{\text{i}}\) (Si\({}_{\text{i}}\)) (936, 1020 cm\({}^{-1}\)) defects also appeared, as well as a band at 1048 cm\({}^{-1}\) attributed to the C\({}_{\text{s}}\)O\({}_{\text{2i}}\) defect. Notably, Figure 16 was given to complete Figure 1b reporting respective spectra of isovalent-doped Si samples. Figure 8:0 IR spectra of the S22 sample after irradiation and at characteristic temperatures in the course of the isochronal anneals. Reproduced with permission from [44]. Figure 17 shows the production of C\({}_{\rm i}\)O\({}_{\rm i}\) defect as a function of the covalent radius of the isovalent dopants for some representative samples (data from Table 5). As the initial carbon concentration was not the same in the various isovalent-doped samples, it was difficult to extract safe conclusions about the effect of different dopants on the production of the C\({}_{\rm i}\)O\({}_{\rm i}\) defect. However, certain observations could be made. For instance, the C\({}_{\rm i}\)O\({}_{\rm i}\) concentration was largely suppressed in the Pb-doped Si sample. This may be related to the tendency of Pb dopant in Si to pair with C and form PbC complexes [101; 108; 155]. In such a case, the available C to pair with O is reduced, leading to a decrease in the C\({}_{\rm i}\)O\({}_{\rm i}\) concentration.