# Toward Defect Engineering Strategies to Optimize Energy and Electronic Materials
## 4 Doping Strategies in Silicon and Germanium  
### Isovalent Doping in Silicon  
For numerous years silicon has been the dominant electronic material in the semiconductor industry. This has been achieved by its beneficial material properties and the versatile silicon dioxide that allowed  
Figure 4: The molecular dynamics derived oxygen diffusivity with respect to uniaxial stain in undoped ceria [26].  
Figure 3: The influence of uniaxial strain, doping, and temperature on oxygen diffusivity in ceria [26].  
for the efficient miniaturization of microelectronic devices. Silicon dioxide has better properties and is more stable than germanium dioxide and this has allowed silicon to prevail over germanium at the dawn of the semiconductor industry, although the first transistor was made of germanium.  
Isovalent doping in Si is technologically important (i.e., photovoltaics) and it can be used to improve the radiation hardness of Si [38]. In particular, it was previously determined that isovalent doping can suppress the formation of oxygen- and carbon-related radiation defects in Si, thus allowing its application in photovoltaics [38]. The key is to immobilize vacancies as it is the key component of the vacancy-oxygen pair (VO or A-center) [39; 40; 41]. With the incorporation of large isovalent codopants in the silicon, elastic strains are induced around the defects due to the larger tetrahedral covalent radii of the dopants as compared to that of the host atom. These strains can be relieved by the attraction of a vacancy that in essence provides space for the lattice and the dopant to relax [38].  
Sn is an isovalent dopant that is considerably larger than Si, so it can strongly bind with vacancies and thus it can influence the vacancy-related defects and the generation rate of interstitial-related defects [42; 43; 44; 45]. It was previously determined that Sn doping and in particular high concentration Sn doping will significantly impact the formation and diffusion of A-centers in Si (refer to Figure 5) [39; 40; 41; 46]. The key is the formation of the highly bound SnVO clusters, which are far less mobile than VO [47]. Alongside SnVO clusters, SnV and SnC\({}_{\text{i}}\) defects may also form in irradiated Si [48]. The main purpose to introduce Sn atoms is to capture vacancies and consequently to improve the radiation hardness of Si in applications where this is required (for example, solar cells and detectors) [38]. This is because less vacancies will be unbound and available to form vacancy-related defects such as VO, VO\({}_{2}\), and PV (in \(n\)-type doped Si), which will have a deleterious impact on the output and reliability of devices [38]. Sn doping can also influence the generation of oxygen thermal donors, and the aggregation and precipitation processes in Si. At any rate, however, the SnV defect induces electrical levels in the gap [49; 50; 51].  
Figure 5: The thermal evolution of the VO (cyan circles), VO\({}_{2}\) (dark blue squares), and SnVO (magenta diamonds) defects for poorly Sn-doped (open symbols) and highly Sn-doped (solid symbols) Si. The upper part is a schematic representation of the defects using blue and green circles for the Sn impurity and Si atoms, square and dotted squares for the V and semi-vacancy, and red circles for the oxygen interstitials. The bottom part is the thermal evolution of the oxygen interstitials for Sn-poor (open triangle) and Sn-rich (solid triangle) conditions. Reproduced with permission from [39]. American Institute of Physics, 2011.