## 1 Introduction
Pb also affects the formation of thermal donors and oxygen precipitates [106; 107]. Finally, although there is indirect evidence for the interaction [54; 104] between Pb and C, there are no reports of the experimental signal for the existence of any PbC related structures. In any case, it has been found that Pb suppresses the formation of carbon-related complexes in Si [108]. It should be noted that Pb doping does not induce the formation of any defects in as-grown Si material [109]. There are continuous efforts to extend the applications of isovalent doping in Si microelectronic and optoelectronic devices [110]. As a general comment, it is stated that the doping of isovalent impurities C, Ge, Sn and Pb, in Si, despite being electrically neutral at lattice sites, due to their different sizes to Si, give rise to stress fields inside the crystal. This affects the equilibrium concentration of V and Si, the mechanisms of the diffusion of various impurities, and the formation process of the defects produced upon material processing [35; 111; 112; 113]. As a result, the production rate, the concentration, the thermal stability and other properties of defects formed is expected to change, with an impact on the functionality of the corresponding devices. Over the past few decades, the ever-increasing computational power and resources in conjunction with methodological advances (for example, density functional theory (DFT)) have led to a complementary understanding of defects and their properties in solid state materials [114; 115; 116; 117; 118; 119]. In particular, these theoretical methods have been employed to the fundamental atomistic understanding of the structure of defects, their electronic structure and the formation of defect clusters [120; 121; 122; 123]. The prediction of these properties not only provides useful complementary insights, but can be used to construct defect-engineering strategies to control defects and their deleterious impact. For example, in Si, defect-engineering strategies are concerned with limiting the effect of oxygen-related defects via codoping with larger isovalent dopants [93; 123]. Electronic devices, on a number of occasions, operate in radiation environments, as, for instance, detectors in nuclear applications, and equipment related with accelerators and aerospace. Generally, irradiations lead to a degradation of the devices' characteristics and properties, mainly because of the introduction of defects in their active area. A convenient way to study such defects is by electron irradiation, which enables a controllable introduction of the defects for their study. One way to suppress the impact of such defects on the operation of Si-based devices is the introduction of isovalent impurities in the Si lattice. The goal of this paper is to present an overview of the role of isovalent doping on the production and evolution of O-related and C-related defects in Si. Important radiation defects such as VO, VO\({}_{2}\), VO\({}_{3}\), V\({}_{2}\)O, V\({}_{3}\)O, as well as C\({}_{1}\)O\({}_{i}\), C\({}_{i}\)C\({}_{8}\), C\({}_{1}\)O\({}_{i}\)(Si\({}_{i}\)), are discussed extensively both experimentally (mainly by IR spectroscopy) and theoretically (DFT). Defect-engineering strategies are examined, as these can enhance the capability of Si-based devices operated in environments where these defects are commonly generated. The results are expected to be useful for many applications, for example, the enhancement of radiation hardness of silicon detectors (a key issue in particle physics which has been under investigation [124] for many decades), and providing new ways of understanding and controlling defects in strained materials [125] or materials subjected to external strain [126], to mention a few. Additionally, an understanding and deeper insight into the role and behavior of isovalent dopants in the Si lattice will be attained.