## 1 Introduction
C atoms, due to their smaller size than Si, introduce tensile strains into the Si lattice and have a tendency to react with Si\({}_{\rm{I}}\). In the course of irradiation, the produced Si\({}_{\rm{I}}\)'s are readily captured by substitutional C\({}_{\rm{s}}\) impurities. As a result, the C\({}_{\rm{s}}\) atoms switch at interstitial sites, by the Watkins replacement mechanism [42], and the produced C\({}_{\rm{i}}\) atoms react with other impurities, giving rise to the production of numerous defects complexes as mentioned above [16; 17; 18; 19; 20; 21; 22; 23; 24; 25], and providing possibilities to elucidate the mechanisms of various defect reactions especially those involving oxygen impurities [43; 44]. C as a codopant also affects the diffusion of various dopants such as, for example, B and P in Si [45; 46; 47], and suppresses crystal defects in Si-based structures and generally in other semiconductors [48; 49]. C codoping with other isovalent impurities, such as Ge, Sn and Pb, is also used [50; 51; 52; 53; 54; 55] in material processing in certain circumstances. Ge has a covalent radius slightly larger than Si. The introduction of Ge into the Si lattice improves the material, especially for microelectronic and photovoltaic applications as there is an increase in electron mobility. Additionally, it has been determined that Ge dopants enhance the mechanical strength of Cz-Si wafers, improving the production yield of devices [56]. Appropriate concentrations of Ge also enhance oxygen precipitation in Si, thus increasing internal gettering capability for metallic contamination of wafers [57]. Ge suppresses the formation of thermal donors, which is beneficial for the stable electrical properties of wafers [58]. Furthermore, Ge substantially reduces the formation of crystal originated particles (COPs) which has a beneficial effect for the crystal and wafer quality [59; 60]. Importantly, Ge leads to a reduction in boron-oxygen complexes in boron-doped Cz-Si. This leads to the suppression of light-induced degradation (LID), a phenomenon particularly harmful for the efficiency of solar cells [61; 62]. Additionally, Ge suppresses [63] the generation of dislocations in boron-doped Si and affects [64] the minority carrier lifetime in gallium-doped Si. Ge, as well as other impurities, for example, B, suppresses the formation of [113] defects which affect [38; 65] the unwanted transient enhanced diffusion (TED) phenomena of dopants in irradiated Si. The covalent radius of Ge is larger than Si by \(\sim\)4.3%, and it introduces local strains into the lattice. As a result, it could affect [66] the solubility of impurities in crystalline materials. For instance, it has been reported [67] that the concentration of As dopant in Si was enhanced when Ge was alloyed with Si. This is an important procedure in semiconductors since it affects [66; 67] the free charge carriers in the material. Ge also could affect [68] the distribution of B and P during the thermal oxidation of Si. Additionally, in irradiated Si, Ge improves [69] the radiation hardness of the wafers and, therefore, of the relevant devices. Notably, Ge in Si acts as an efficient trap for vacancies although its interaction with (SiI)s is very low, except for large Ge content [51; 69; 70; 71]. Its tendency to pair with vacancies leads to the formation of GeV pairs in low temperature irradiated Si. Although GeV pairs are electrically inactive, it has been determined that they anneal out [69] at about 200 \({}^{\circ}\)C, releasing the trapping vacancies. Apparently, Ge dopant due to this tendency to interact with intrinsic defects, would affect the generation of irradiation induced defects in Si. The exact knowledge of this behavior will offer opportunities for defect-engineering strategies for improving the wafers quality thus enhancing the functionality of Si-based devices. Sn has a significantly larger covalent radius than Si (by \(\sim\)20.5%) and, therefore, its introduction induces strains in the lattice.