## 1 Introduction  
After many decades of research, Si continues to be an important material in the semiconductor industry. It is the main element for the fabrication of integrated circuits, solar cells, detectors, sensors, semiconductor lasers, fast-responding transistors, etc. Additionally, it is used for applications in important scientific fields which impact our daily life, such as quantum computing, nuclear medicine, and the aerospace industry [1; 2; 3; 4; 5; 6; 7; 8]. The properties of materials in general are largely affected by impurities and defects introduced into the lattice during growth and processing. In the case of Si, O and C are the two main impurities incorporated into the lattice during growth. Both are electrically inactive, but during irradiation and subsequent anneals at various temperatures they lead to the formation of a number of oxygen-related defects such as V\({}_{\rm{n}}\)O\({}_{\rm{m}}\) complexes [9; 10; 11; 12; 13; 14; 15], and carbon-related defects such C\({}_{\rm{i}}\), C\({}_{\rm{i}}\)O\({}_{\rm{i}}\), C\({}_{\rm{i}}\)S\({}_{\rm{e}}\), C\({}_{\rm{i}}\)Si\({}_{\rm{i}}\), C\({}_{\rm{i}}\)O\({}_{\rm{i}}\)Si\({}_{\rm{i}}\), and C\({}_{\rm{i}}\)C\({}_{\rm{e}}\)Si complexes [16; 17; 18; 19; 20; 21; 22; 23; 24; 25], where V stands for the vacancy, SiI for the self-interstitial, C\({}_{\rm{i}}\) for the C interstitial, O\({}_{\rm{i}}\) for the C interstitial, O\({}_{\rm{i}}\) for the oxygen interstitial, and C\({}_{\rm{e}}\) for the C substitutional. Most are electrically active, affecting the electrical properties of the material mainly in a negative way. Importantly, among the above defects, VO, V\({}_{\rm{2}}\)O and V\({}_{\rm{3}}\)O, as well as C\({}_{\rm{i}}\)O\({}_{\rm{i}}\) and C\({}_{\rm{i}}\)C\({}_{\rm{e}}\), act as recombination centersleading to severe degradation of the devices [26; 27; 28; 29; 30]. In particular, the C\({}_{\rm{i}}\)C\({}_{\rm{s}}\) pair (known as the G-center), is studied for the fabrication of light-emitting diodes in Si, and even for potential applications in quantum computing and Si-based lasers [31; 32; 33; 34]. Obviously, the ability to control the properties and behavior of these defects is very important, in order to increase the efficiency of the corresponding devices. A way to improve the properties of Si is by doping with isovalent atoms. In the case of O- and C-related defects in Si, doping with isovalent impurities (Ge, Sn, Pb and C) offers clear advantages. At first, isovalent dopants are electrically inactive and, therefore, they do not have essential influence on the electrical properties of Si. Secondly, they reside at substitutional sites, and due to their different covalent radius than Si, they introduce elastic stresses into the lattice, thus affecting the equilibrium concentration of intrinsic defects. In particular, they interact with vacancies in the case of impurities with a larger covalent radius than Si (r\({}_{\rm{Si}}\) = 1.17 A), such as Ge (r\({}_{\rm{Ge}}\) = 1.22 A), Sn (r\({}_{\rm{Sn}}\) = 1.41 A) and Pb (r\({}_{\rm{Pb}}\) = 1.44 A), and with Si\({}_{\rm{i}}\)'s in the case of impurities with a smaller covalent than Si, such as C (r\({}_{\rm{C}}\) = 0.77 A), in order to relieve the induced strains [35; 36]. As a result, the formation of defects is largely affected, due to irradiation, and/or thermal treatments and device processing, or during crystal growth. In this framework, the production and evolution of the O-related and C-related defects could be purposefully modified with the introduction of isovalent dopants, which offers possibilities for defect-engineering, towards controlling the material properties. In particular, critical characteristics such as the thermal stability and radiation hardness of Si wafers could be controlled [37; 38; 39] by isovalent dopants, leading to a substantial improvement of the material. C is the smaller isovalent impurity introduced into the Si lattice unintentionally during growth, or as a deliberate dopant for technological applications [16; 40; 41].