## 3 Summary and Future Directions  
The effect of isovalent dopants on the properties of radiation defects in silicon has been investigated for numerous years in order to defect-engineer silicon and enhance its functionality for electronic devices. The present review focused on the effect of isovalent dopants on oxygen-related (mainly VO and VO\({}_{2}\) centers) and carbon-related (mainly C\({}_{i}\)O\({}_{1}\) and C\({}_{i}\)C\({}_{\rm s}\) centers) defects in irradiated Si. Regarding oxygen-vacancy centers, it was previously determined that isovalent dopants affect the production of VO defect and its conversion to VO\({}_{2}\) defect. More specifically, the experiments showed that the presence of Ge and C enhanced VO production, although the presence of Sn and Pb suppressed it,  
Figure 27: DFT-derived minimum energy structures for carbon related defects: (**a**) C\({}_{\rm i}\) (Si\({}_{\rm I}\)), (**b**) C\({}_{\rm i}\)O\({}_{\rm l}\), (**c**) C\({}_{\rm i}\)O\({}_{\rm l}\), (**d**) C\({}_{\rm i}\)O\({}_{\rm i}\)(Si\({}_{\rm I}\))\({}_{\rm b}\), (**e**) A-type C\({}_{\rm i}\)C\({}_{\rm s}\), (**f**) B-type C\({}_{\rm i}\)C\({}_{\rm s}\), and (**g**) C-type C\({}_{\rm i}\)C\({}_{\rm s}\). The blue spheres represent the Si atoms, yellow spheres represent the C atoms, and red spheres represent the O atoms Reproduced with permission from [22].  
Concerning the conversion of VO to VO\({}_{2}\), it was determined that the larger the covalent radius of the dopant, the less VO\({}_{2}\) formed. Studying specifically the effect of C, it was concluded that VO\({}_{2}\) decreased as the concentration of C increased. Additionally, the thermal stability of VO andVO\({}_{2}\) defects was affected.  
Regarding carbon-related centers, it was determined that the presence of Pb substantially suppressed the production of C\({}_{1}\)O\({}_{1}\) and C\({}_{1}\)C\({}_{5}\) pairs. The same effect occurred for Sn and Ge. However, the respective effect of Ge and Sn seemed to be more complicated, depending on the concentration of the latter two dopants and the C content of the material. The conversion of C\({}_{1}\)O\({}_{1}\) to the C\({}_{8}\)O\({}_{2}\) complex was slightly affected by the isovalent dopants. The experiments showed that the presence of Ge and Pb slightly enhanced C\({}_{8}\)O\({}_{2}\) formation, although Sn reduced it slightly. The thermal stability of the C\({}_{1}\)O\({}_{1}\) pair was less affected by the isovalent dopants in comparison with the VO defect. Specifically, the effect of Ge was almost negligible. Importantly it was found that isovalent dopants differently affected the thermal stability of the C\({}_{1}\)O\({}_{1}\) and C\({}_{1}\)C\({}_{5}\) pairs. The T\({}_{\rm ann}\) of the C\({}_{1}\)O\({}_{1}\) pair was reduced, although the T\({}_{\rm ann}\) of the C\({}_{1}\)C\({}_{5}\) pair was increased.  
Thus, an assessment of the hardening potential of the isovalent impurities in Si regarding the radiation defects was performed. It was demonstrated that the formation of O-related and C-related defects was suppressed in Si material. This allows the enhancement of radiation hardness of Si-based devices operated in radiation environments, as, for instance, radiation detectors and devices for aerospace. Overall, the doping of Si with electrically inactive impurities such as C, Ge, Sn and Pb, together with O, provides a tool for the electronic industry to address problems and requirements for improved devices, enhanced mechanical strength with increased radiation hardness, and thermal stability.  
This research received no external funding.