The microscopic behavior of Fe in relaxed Si1–xGex alloy was addressed here, where various new aspects were highlighted. In p-type materials 2 types of defect involving Fe could coexist under equilibrium; the isolated form, Fei, and the Fe-acceptor pair, Fei–As. The latter complex was favored over the former because it was thermodynamically more stable. In each case the Fe atom stabilizes at the interstitial tetrahedral site. When B was the acceptor impurity, both the isolated and the paired forms introduce donor-like levels, distant from each other by 0.28eV. In the relaxed Si1–xGex bulk alloy, these levels were shown to remain separated by the same amount. However, they shift toward the valence band much faster than the shrinkage of the band gap when the Ge content was increased. The consequence was that the pair-related donor level merges with the valence band at a fairly low alloy composition (x  7%) while the Fe donor level was predicted to disappear from the gap for x  25%. It was also shown that neither the entropy nor the enthalpy of migration of free Fe, whose experimental determination requires one to take into account the above-mentioned shift, were affected by alloying. Therefore, the fast diffusing character, attributed to Fe in Si, still holds in the alloy. Finally, the major technological implication emerging from the new findings was addressed. In particular, it was shown that both the gettering by segregation, routinely used in Si, and the field-induced out-diffusion, established in n-type Si ten years ago, were totally inefficient in the Si1–xGex alloy.

Iron in Relaxed Si1–xGex Alloy - Band Gap Related Levels, Diffusion and Alloying Effects. A.Mesli, B.Vileno, C.Eckert, A.Slaoui, C.Pedersen, A.Nylandsted Larsen, N.V.Abrosimov: Physical Review B, 2002, 66[4], 045206 (12pp)