First-principles study of Fe and FeAl defects in SiGe alloys

First-principles, spin-polarized local-density-functional calculations are used to model interstitial iron (Fe_i) and its complexes with substitutional aluminum in dilute Si_xGe_1−x alloys (x<8%). We considered both the effect of direct bonding between Fe_i or Fe_iAl with Ge atoms in the x→0 limit and the evolution of the defect properties with the alloy composition. It is found that Fe_i prefers Si-rich regions, but when placed near a Ge atom, its (0/+) level is shifted toward the conduction band. However, the ionization energy of Fe^(+/+2)–Al⁻ is only slightly changed by the presence of neighboring Ge atoms in the proximity. It is also found that indirect alloying effects shift the donor levels of Fe_i and FeAl at a fast rate toward the valence band. The acceptor levels, however, remain approximately at the same distance from E_v.