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Shallow Thermal Donors in Silicon

  Many experimental groups have observed a family of shallow thermal donors (STDs) in Si doped with both O and N. Our calculations predict that a defect consisting of a single Ni surrounded by two Oi acts as a STD [183]. Isolated Ni possesses a deep donor state localised on a neighbouring Si. However in Ni(Oi)2 this donor level is electrostatically squeezed  by the Oi, which pushes it towards the conduction band to form the observed shallow level. The mechanism is not restricted to Ni based defects and also occurs in the defect (C-H)iO4i with a similar defect core. We show that these structures may be responsible for two of the the `NL10' defects observed in Si, and discuss some of the ramifications of such a deep-to-shallow conversion mechanism.

In addition we show that (C-H)iO4i is able to structurally re-arrange when switching from the neutral to the -1 charge state, by rotating one of the oxygen atoms away from the defect core. This suggests that the defect could act as a deep acceptor to become -1 charged, or a shallow donor to become +1 charged. The structure of the neutral charge state defect is almost identical to that of the +1 defect.



 
next up previous contents
Next: Background Up: Chris Ewels' PhD Thesis Previous: Conclusions and Discussion
Chris Ewels
11/13/1997