Iron in relaxedSi1−xGexalloy: Band gap related levels, diffusion, and alloying effects

Abstract

The microscopic behavior of iron in relaxed Si 1Ϫx Ge x alloy is addressed in the present work where various new aspects are highlighted. In p-type materials two types of defects involving iron may coexist under equilibrium; the isolated form, Fe i , and the iron-acceptor pair, Fe i -A s . The latter complex is favored over the former because it is thermodynamically more stable. In each case the iron atom stabilizes at the interstitial tetrahedral site. When boron is the acceptor impurity, both the isolated and the paired forms introduce donorlike levels, distant from each other by 0.28 eV. In the relaxed Si 1Ϫx Ge x bulk alloy, these levels are 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 is increased. The consequence is that the pair-related donor level merges with the valence band at a fairly low alloy composition (xу7%) while the iron donor level is predicted to disappear from the gap for xу25%. We also show that neither the entropy nor the enthalpy of migration of free iron, whose experimental determination requires one to take into account the abovementioned shift, are affected by alloying. Therefore, the fast diffusing character, attributed to iron in silicon, still holds in the alloy. The origin of spectral broadening, related to the chemical disorder, is discussed. Finally, the major technological implication emerging from our new findings is addressed. In particular, we show that both the gettering by segregation, routinely used in silicon, and the field-induced outdiffusion, established in n-type silicon ten years ago, are totally inefficient in the Si 1Ϫx Ge x alloy.

Acceleration of Diffusional Jumps of Interstitial Fe with Increasing Ge Concentration in Si1 − x Ge x Alloys Observed by Mössbauer Spectroscopy

“…However, this would result in an effect on the jump frequency opposite to that observed. For positively charged Fe i + no change in the diffusion coefficient has been measured up to x õ 0.1 [4], however, the relative increase detected here for Fe i 0 if transferred to Fe i + would be within the error margins of those measurements. We thus consider a change in the diffusion mechanism.…”

“…The Fe i donor level is known to approach the valence band with increasing Ge concentration and the band gap shrinks [4]. In view of the illumination of the samples, the band gap shrinkage and the almost constant doping levels of the samples it is therefore likely that a mixture in charge states would tend to more neutral Fe i with increasing Ge concentration.…”

“…It is interesting to note that for the series of interstitial 3d elements in silicon theory predicts a change of the most stable site from a tetrahedral site for the early elements up to Fe i to a hexagonal site for the late elements Co, Ni and Cu, the fastest diffusing elements in the series [3]. In this contribution we show that alloying of Ge increases the jump frequency of Fe i in Si 1jx Ge x , whereas the long-range diffusivity is apparently not affected [4] for x < 0.1. Semiconducting Si 1jx Ge x , an almost ideal random alloy, may be of interest as a system of particular simplicity to study the alloying effect on interstitially diffusing impurities [4] and interstitial 3d elements and Fe in particular are important, mostly harmful impurities in silicon-based technologies.…”

“…In this contribution we show that alloying of Ge increases the jump frequency of Fe i in Si 1jx Ge x , whereas the long-range diffusivity is apparently not affected [4] for x < 0.1. Semiconducting Si 1jx Ge x , an almost ideal random alloy, may be of interest as a system of particular simplicity to study the alloying effect on interstitially diffusing impurities [4] and interstitial 3d elements and Fe in particular are important, mostly harmful impurities in silicon-based technologies.…”

Composition dependence of the interband critical points in Si1−xGex alloys: A fractional–dimensional space approach

“…4 and 5͒, which may be explained by the well-known alloy effects. [46][47][48] However, further discussion on this topic is beyond the scope of this article.…”