## 3 \(n\)-Type Dopants  
Interestingly, although Ge shares the same crystal structure to Si, its defect properties are distinctively different. A key issue in Ge is the excessive \(n\)-type dopant diffusion. This in turn poses a technological difficulty to form well-defined \(n\)-type doped regions for devices. It has also been established that donor atoms such as arsenic can form clusters with vacancies (As\({}_{n}V\) defect clusters, refer to Figure 1) [4]. The community aims to confront these issues by considering defect engineering strategies that can address both the fast diffusion and the clustering of the donor atoms. It should be noted that, in the past, a common defect engineering strategy was to introduce co-dopants in the material [84; 85; 86; 87; 88]. The purpose is that the co-dopants will act competitively, attracting native point defects (such as vacancies) and in that respect, will allow the annihilation and/or control of the dopants and their clusters. Experimental and theoretical investigations used isovalent C or Sn to co-dope \(n\)-type doped Ge (refer to Reference [36] and references therein). It was determined that isovalent co-doping will impact the diffusion of the donor atoms; however, it did not affect the deactivation of the donor profile [41; 44]. In essence the picture is that the isovalent atoms associate with the migrating vacancies and (or donor-vacancy pairs) effectively immobilizing a proportion of them. These in turn, are not available to associate with the donor atoms and facilitate their migration, hence, less donor atoms will migrate, and this leads to a decline in their diffusivity [44]. The association, however, of the isovalent atoms with the donor-vacancy pairs results in defect clusters which still lead to the deactivation of the donor atoms [44]. Therefore, although isovalent co-doping solves the fast diffusion of the donor profile issue, it does not impact the deactivation issues [36; 41; 44]. To address both these issues concurrently, double-donor doping and doping with aliovalent dopants was considered [2; 89]. Double-donor doping is the process where a second \(n\)-type dopant is introduced in the lattice aiming to impact the electronic properties and defect-dopant processes [90; 91; 92; 93; 94]. [PERSON] et al. [90] investigated the diffusion and activation of P and as co-doped Ge using conventional thermal annealing (600-750 \({}^{\circ}\)C). It was determined using Secondary Ion Mass Spectrometry (SIMS ) that As diffusion was retarded, whereas P diffusion remained unaffected or even somewhat enhanced [90]. Additionally, double-donor doping was not beneficial concerning the level of activation [90]. [PERSON] et al. [92] considered P and N co-doping but with millisecond non-melt laser annealing. It was determined that co-doping with N reduces the diffusion of P but with a lower activation level [92]. Therefore, the conclusions for double-donor doping are similar to those for isovalent doping: fast diffusion is reduced but there is no benefit concerning the activation of the donor  
Figure 1: A schematic of the arsenic-vacancy (As\({}_{n}V\)) defect clusters in Ge. Black circles represent the As atoms, white circles the Ge atoms, and cubes the vacancies [4]. atoms. At any rate it should be considered that in other classes of materials (for example oxides), these defect engineering strategies are effective [95; 96]. In these systems the key is the relaxation manipulation offered by the two co-dopants, whereas here electronic effects are also important. To the best of our knowledge the primary study that considered the impact of F on donor dopants in Ge was the DFT study by [PERSON] et al. [97] nearly a decade ago. The promising DFT results motivated a series of experimental studies to determine the impact of F on the electrical activation of P and As in Ge [2; 98; 99; 100; 101; 102; 103; 104]. The introduction of F in Ge was inspired by analogous experimental and theoretical work of F in Si, where it was employed to control the transient diffusion of B [105; 106; 107].