## 2 Results
### Production of VO Defect as a Function of the Isovalent Dopant Covalent Radius  
Figure 3 shows the production of the VO defect as a function of the isovalent dopant covalent radius. Information about the used samples is presented in Table 2. From Figure 3, taking the low carbon C\({}_{\text{L}}\) sample concentration as a base, we see that the sample doping with C and Ge results in an increase in the VO production, although doping with Sn and Pb results in a decrease in the VO concentration. More specifically, the following observations could be made. Regarding C, its ability to enhance VO production is due to its tendency [16] to act as a preferential trap for self-interstitials, Si\({}_{\text{I}}\)'s. In the course of irradiation, V and Si\({}_{\text{I}}\) are produced. Most of them annihilate (V + Si\({}_{\text{I}}\) \(\rightarrow\) \(\mathcal{O}\)). When C\({}_{\text{s}}\) is present in the lattice, it competes with vacancies in capturing Si\({}_{\text{I}}\) (C\({}_{\text{s}}\) + Si\({}_{\text{I}}\) \(\rightarrow\) C\({}_{\text{I}}\)) and this leads to a decrease in the rate of the annihilation reaction. In other words, the availability of V increases, allowing the possibility for more vacancies to be captured by O atoms [44]. Equivalently, VO production is increased (compare the C\({}_{\text{L}}\) and C\({}_{\text{H}}\) samples in Figure 3). Considering the Ge dopant, its tendency to act as an effective temporary trapping site for the V formed by the irradiation, restrains [142] their direct annihilation by the Si\({}_{\text{I}}\)'s. In essence, the association of V with Ge leads to the formation of transient GeV pairs [69] which are unstable above room temperature. This trapping and release of V by Ge atoms at T above room temperature, in essence, prevents some of the V to be annihilated by Si\({}_{\text{I}}\)'s. Equivalently, the number of available vacancies to be captured by O atoms increases, and, therefore, the VO production is enhanced. Since the Ge-doped sample contains an adequate amount of carbon, the final VO concentration is even higher than of the C\({}_{\text{H}}\) sample (Figure 3). Regarding the Sn dopant, in room temperature irradiation, SnV pairs are formed, which are stable at these temperatures [88; 100]. In essence, Sn competes with O in capturing V, and as a result, the production of VO pairs decreases. SnV pair has no IR \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Dopant (cm\({}^{-3}\))** & **[C\({}_{\text{s}}\)]\({}_{\text{o}}\)\(\times\) 10\({}^{16}\) (cm\({}^{-3}\))** & **[O\({}_{\text{i}}\)]\({}_{\text{o}}\)\(\times\) 10\({}^{17}\) (cm\({}^{-3}\))** \\ \hline (C\({}_{\text{L}}\)) & \(\cdots\) & 5 & 9.5 \\ (C\({}_{\text{H}}\)) & \(\cdots\) & 22 & 9.3 \\ (Ge) & \(2\times 10^{20}\) & 8.6 & 6.5 \\ (Sn) & \(9\times 10^{18}\) & 20 & 9.2 \\ (Pb) & \(1\times 10^{18}\) & 19 & 2.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The concentrations of Ge, Sn, and Pb-doped samples, as well as the concentrations of the O\({}_{\text{i}}\) and C\({}_{\text{s}}\) of the used samples in Figure 3[143]. Figure 3: The production of the VO defects as a function of the isovalent dopant covalent radius Reproduced with permission from [143]. American Institute of Physics, 2012.  
signal in the spectra and its presence is monitored [52; 88; 100] in the spectra by the increase in VO signal as a result of SnV dissociation around 170 \({}^{\circ}\)C (Figure 2b,c). However, the final concentration of VO is smaller than of the C\({}_{\text{L}}\) sample (Figure 3). This indicates that some of the released V from the SnV dissociation participates in other reaction channels [100], as, for instance, in the formation of the V\({}_{2}\) defect. Regarding the Pb dopant, the situation is similar to that of Sn. Indeed, due to the expected formation of PbV pairs, as a result of the competition between O and Pb in capturing V, in the course of room temperature irradiation, fewer VO are formed.