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The role of silicon self-interstitials

  In later oxygen precipitation, quartz precipitates are formed with an associated ejection of Sii. This has led to a general assumption that there are many Sii present in Cz-Si, and from this to the idea that they may lie in the core of the thermal donor. This has been supported by the Deák model which is based around Sii. This is further supported by the fact that carbon suppresses thermal donors, which has led to another common assumption that this is occurring through Sii absorption by the reaction

\rm C_s + Si_i \rightarrow C_i\end{displaymath}

This has led to many unusual postulates to explain experimental data, for example that there are two types of dimer complex, `normal' and `O2*' which can eject Sii atoms[266]. However these assumptions are flawed. It is known that CiOn defects (the `P-line' complexes) form in the first 15 minutes of annealing[197] at 450$^\circ$C, showing the presence of either Ci or Sii to produce Ci via the mechanism given above. However these are at extremely low concentrations, and indeed, act as evidence that Sii concentrations are low as a consequence.

In addition, as described above it is not possible as far as we are aware to construct a single Sii structure that does not possess gap states, and so thermal donor models based around a core Sii are unable to explain the electrically inactive forms of the TDs. Carbon suppression of thermal donors can occur through other mechanisms, primarily Cs absorption of Oi and O2i[267], and possibly also a self-catalysed Ci suppression mechanism (described briefly in Section 9.3.1).

Long time annealing FTIR studies[239] show absorption at 999 cm-1 associated with the thermal donors. However after long times this disappears and is replaced with broad absorptions at 1012, 1006, and finally 1015 cm-1, believed to be associated with the shallow thermal donors. This would be consistent with a picture whereby over long times the thermal donors transform into quartz, releasing large quantities of Sii. These are then able to initially transform all available Cs into Ci, leading to a glut of Ci-based STD formation (CHO4 complexes). Thereafter the remaining Sii combine to form $\langle$311$\rangle$ rod-like precipitates.

This therefore suggests that Sii are not present in large quantities in the early stages of Cz-Si annealing but are formed later when quartz precipitation occurs. In addition they are not involved in the structure of thermal donors[268].

next up previous contents
Next: Earlier Thermal Donors - Oi Up: Thermal Donors in Si Previous: Y-lids - Tri-valent oxygen
Chris Ewels