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 N_i surrounded by two O_i acts as a STD [183]. Isolated N_i possesses a deep donor state localised on a neighbouring Si. However in N_i(O_i)₂ this donor level is electrostatically squeezed  by the O_i, which pushes it towards the conduction band to form the observed shallow level. The mechanism is not restricted to N_i based defects and also occurs in the defect (C-H)_iO_4i 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)_iO_4i 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.