Vacancy-arsenic clusters in germanium

A Chroneos, RW Grimes, BP Uberuaga, S Broztmann, H Bracht
Applied Physics Letters 91, 192106 (2007)

Electronic structure calculations are used to investigate the structures and relative energies of defect clusters formed between arsenic atoms and lattice vacancies in germanium and, for comparison, in silicon. It is energetically favorable to form clusters containing up to four arsenic atoms tetrahedrally coordinated around a vacancy. Using mass action analysis, the relative concentrations of arsenic atoms in different vacancy-arsenic clusters, unbound arsenic atoms, and unbound vacancies are predicted. At low temperatures the four arsenic-vacancy cluster is dominant over unbound vacancies while at higher temperatures unbound vacancies prevail. In terms of concentration, no intermediate size of cluster is ever of significance.

Carbon, dopant, and vacancy interactions in germanium

A Chroneos, BP Uberuaga, RW Grimes
Journal of Applied Physics 102, 083707 (2007)

Electronic structure calculations have been used to study the interaction of carbon with isolated substitutional dopants (boron, phosphorus, or arsenic), vacancies, and dopant-vacancy pairs in germanium. For comparison, equivalent defects were examined in silicon. The calculations employed a plane-wave basis set and pseudopotentials within the generalized gradient approximation of density functional theory. The results predict a range of different association preferences, with carbon being strongly bound in some cases and unbound in others. For example, in germanium, the carbon-vacancy cluster is weakly bound whereas in silicon it is more strongly bound. Conversely, dopant-carbon pairs are not stable in either germanium or silicon compared to their isolated components. If, however, they are formed during implantation, they will act as strong vacancy traps. Details of clusters comprised of a dopant, carbon, and vacancy are also discussed with respect to their formation by the association of a vacancy or cluster pair.