## 1 Introduction
Sn doping can enhance the radiation hardness and thermal stability of devices, therefore, it is particularly useful for space applications and in nuclear medicine. Being electrically neutral and larger than Si, it is used for stress compensation in layers of heavily doped elements such as B and P, which are smaller than Si [72; 73; 74]. Sn dopant can serve to monitor mechanical stresses in Si, originating from the presence of structural defects, impurities and growth defects [75; 76]. Furthermore, it has been determined that Sn improves [77] the minority carrier lifetime in Fe-contaminated Si, influences [78] the recombination characteristics of \(\gamma\)-irradiated Si, and accelerates [79] the degradation conductivity of electron irradiated Si. Sn doping has also been used to monitor [80] the interaction between extended dislocation loops and impurities in Cz-Si. It affects the diffusion of oxygen, as well as the formation of thermal donors and oxygen precipitates in Si [39; 74; 81; 82; 83]. Importantly, Sn alloying with Si provides materials applicable to bandgap and lattice parameter engineering, which are key elements for optoelectronic devices such as lasers and light-emitting diodes [84; 85]. Interestingly, SiSn alloys are expected to be important for infrared brain imaging detectors [86]. As the Sn atom is larger than Si, it has the tendency to trap vacancies to relax the strains induced by its presence in the lattice. This property of Sn was recognized long ago and has been used to study vacancy and divacancy generation and some of their properties and behavior in Si [87; 88; 89; 90; 91; 92]. Notably, the SnV pairs formed by irradiation anneal out [93] at about 170 \({}^{\circ}\)C, releasing the trapping vacancies. SnV defect is IR inactive and its presence is indirectly concluded by the increase [94] of the concentration of the VO and V\({}_{2}\) centers. As a result of the efficient trapping of vacancies by Sn, its presence in Si affects the production of radiation damage centers and has the ability to enhance the radiation tolerance of Si for the manufacturing of solar cells and detectors [95; 96; 97; 98]. The formation of standard radiation defects as VO and V\({}_{2}\), as well as C\({}_{1}\)O\({}_{1}\) and C\({}_{1}\)C\({}_{8}\), is significantly reduced in Sn-doped Si [99; 100]. Additionally, Sn and C codoped Si has been investigated and suggested as a promising material for radiation hard devices [52; 53]. Among the isovalent dopants considered here, Pb is the element with the largest covalent radius (by \(\sim\)22.2% compared with Si). Therefore, Pb introduces the largest compressive strains into the lattice. The association of an oversized isovalent atom such as Pb with V is energetically favorable, as it results in the compensation of the induced strains following Si, and the accommodation of the Pb atom at a lattice site in Si. However, there are no indications of any electrical signals originating from the PbV pair, and there are no reports, so far, of any optical signal related with this structure or with any Pb related structures [54; 101]. In any case, since Pb is even larger than Sn, it is expected that more vacancies are captured by Pb than Sn atoms. It has been reported [102] that PbV pairs do not dissociate below 280 \({}^{\circ}\)C. Evidently, Pb competes with O in capturing vacancies, and, therefore, suppresses VO formation. It is a potential agent for enhancing the radiation tolerance of devices. Usually, Pb is codoped with C in Si. Since C is smaller than Si and Pb, their simultaneous presence in the Si lattice leads to a strain compensation, stabilizing the Pb atoms at lattice sites, thus preventing their precipitation [54; 103; 104]. Additionally, it hasbeen determined [105] that the presence of Pb reduces the density of growth of micro defects, influences the concentration of dislocations, increases the lifetime of non-equilibrium current carriers and generally affects the recombination parameters of Si.