# Toward Defect Engineering Strategies to Optimize Energy and Electronic Materials
## 4 Doping Strategies in Silicon and Germanium
### Fluorine Doping in Germanium  
The introduction of alternative oxides in microelectronics and in particular high dielectric constant oxides (high-\(k\) dielectrics such as hafnium oxide) has allowed the consideration of alternative substrates including germanium and III-V compounds [52; 53; 54; 55]. Although Ge is isostructural to Si, its defect processes were considerably different and relatively neglected up until a decade ago [56; 57; 58]. The main issue that needs to be considered in Ge is the excessive \(n\)-type dopant diffusion, which can be significant as it is difficult to control so as to form appropriate \(n\)-type doped regions for devices. A way to address this issue is via defect engineering strategies that can be employed to limit the harmful impact of defects, dopants, or their clusters in Ge (refer to Figure 6). A way to proceed is by the introduction of codopants or competing native defects at concentrations that will annihilate or control the defects, dopants, and their clusters. [PERSON] et al. [59] considered proton irradiation to form a supersaturation of Ge self-interstitials to annihilate the Ge vacancies. The formation of Ge interstitials is important as under equilibrium conditions, vacancies in Ge are the prevalent native point defects [3]. Vacancies mediate and are responsible for the fast diffusion of \(n\)-type dopants (P, As, or Sb) and the formation of large dopant-vacancy clusters (refer to Figure 6) that lead to the deactivation of a large proportion of the dopants introduced and therefore need to be controlled [60; 61; 62]. As in the case of Si, a more straightforward path towards the control of the defect processes in Ge is through the incorporation of codopants. Previous experimental and theoretical studies considered codoping of \(n\)-type doped Ge with isovalent atoms such as C or Sn, aliovalent dopants such as fluorine, or a second \(n\)-type dopant (double donor doping) [3]. Density functional theory (DFT) and experimental studies consistently revealed that isovalent codoping and double donor doping in Ge do not effectively reduce the formation of dopant-vacancy clusters that can lead to deactivation [63; 64]. The introduction of fluorine in Si and Ge is motivated as it is a dopant that can passivate the dangling bonds formed by the vacancies [65; 66; 67; 68; 69; 70]. The key is the high electronegativity of F and via the saturation of the dangling bonds, and it essentially immobilizes the lattice vacancies. This is important as the vacancies are the key to control both the diffusion of donor atoms and the formation of the large donor atom-vacancy clusters that lead to the deactivation of the donor profile. In a DFT study which employed mass action analysis, it was shown that F will m  
Figure 6: Schematic representation of the As\({}_{\text{n}}\)V clusters in Ge. Cubes represent the vacancies, black circles represent the As atoms, and white circles represent the Ge atoms. Reproduced with permission from [8]. American Institute of Physics, 2007.  
vacancies (forming F\({}_{\rm n}\)V\({}_{\rm m}\) clusters) as compared to donor atoms and this will limit the concentration of the donor-vacancy clusters [68]. This theoretical study motivated [PERSON] et al. [69] to investigate the effect of F on As diffusion in Ge. It was determined that F does reduce As diffusion in Ge (refer to Figure 7) [69]. Thereafter, the study of [PERSON] et al. [70] was consistent and concluded that implanted F atoms can passivate the vacancies at around 500 \({}^{\circ}\)C leading to an enhancement in Ge-metal-oxide semiconductor-field-effect transistor (MOSFET) performance [70]. In a recent study, [PERSON] et al. [71] investigated the local structure of F doped Ge by ion implantation and annealing using X-ray absorption near edge structure (XANES) spectroscopy. This study determined that most of the F in Ge forms F\({}_{6}\)V\({}_{2}\) complexes [71].