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Experimental Evidence

  Lindström and Hallberg annealed Cz-Si samples at 370$^\circ$C and 450$^\circ$ for 25 hours, and the variation in absorption of the various observed LVMs provide strong support for assignment of the 1012 cm-1 mode to the oxygen dimer. Their results are shown in Figure 6.11 and are reproduced from Reference [239]. Note that the bands at 975, 988 and 999 cm-1 are associated with the early thermal donors, TD1, TD2 and TD3 respectively.

At 370$^\circ$C where Oi is not mobile, there is a gradual decrease in the 1012 cm-1 dimer band from its residual levels, accompanied with an associated increase in the 975-1006 cm-1 bands. These changes have been directly correlated [160], and are consistent with a thermal donor model constructed from dimers (see Chapter 9). There is no notable change in the Oi levels, suggesting the available dimers are becoming depleted as they form higher order complexes, but the lack of Oi migration means that new dimers are not being formed and hence the dimer concentration is dropping.

At 450$^\circ$C when oxygen is mobile (Figure 6.11b), the story is very different. There is a rapid initial drop in [Oi], accompanied with a rapid rise in dimer concentration. Meanwhile there is also an increase in the other modes. The dimer concentration then tails off, before reaching a steady state growth along with a similar growth in the 975-1006 modes, and an associated drop in [Oi]. This can be understood in terms of an initial glut of dimer formation due to Oi atoms which are close to the capture radius required to form a dimer. However once these are exhausted, there is then a drop to steady state Oi diffusion from a more homogenous background level; the behaviour of the 975-1006 modes is then simply a knock-on from the fluctuation in dimer levels (note that this is different from the explanation given in Reference [239]).

Figure: The initial changes of the bands 975-1012 cm-1 and [Oi] for annealing at (a) 370$^\circ$C (top) and (b) 450$^\circ$C (bottom). Taken from Reference [239]. In the 450$^\circ$C run, the 975 cm-1 signal was too weak for inclusion, and has a maximum that appears and disappears too quickly to appear on this scale.
\ \hspace{2cm} 
\psfig {file=oxygen/dimer/hallberg.eps,width=14cm}\end{figure}

next up previous contents
Next: Dimer interaction with other Up: Dimer Formation Previous: Dimer Formation
Chris Ewels