Defect Processes in Germanium
In the present study both experimental and theoretical techniques have been applied to study the defect structures and stability in Ge.
The experimental studies focused on the implantation and diffusion of phosphorus (P) in Ge substrates, as a function of the protective capping layer used to passivate the Ge surface during annealing and implantation. Various capping layers were used, for example, silicon dioxide and silicon nitride. For the protected samples insignificant P diffusion was observed in Ge samples with low P concentrations. Conversely, samples with higher P concentrations exhibited enhanced concentration-dependent diffusion.
Density functional theory (DFT) calculations have been used to predict the structures and relative energies of defect clusters that form in Ge, between lattice vacancies an extensive range of dopants (p-type, isovalent and n-type dopants). For comparison, equivalent defect clusters were considered in Si. The wide range of dopants investigated illuminates similarities and differences that exist between defect structures in Ge and Si.
The transport of dopants can be influenced by defect species other than intrinsic interstitials and vacancies, in particular, extrinsic carbon (C). Using DFT simulations, the influence of C on the stability of boron-vacancy, P-vacancy and arsenic-vacancy complexes in Ge and Si was predicted and important differences in the defect chemistry of Ge and Si were highlighted. These results were used to interpret differences between predicted migration activation energies for P in Ge via a vacancy mechanism, with and without the presence of C.