(C-H) based shallow thermal donors

  The observation of STDs in material that is nominally N free [168] suggests that other families of STDs may exist. Annealing Cz-Si between 450 and 600$^\circ$C produces a number of PL bands [197] some of which, e.g. the T-line, are due to defects containing interstitial C-H complexes [196]. The hydrogen is present as an unintentional impurity and the concentrations of these centres are comparable with those of thermal donors. These interstitial defects are to be contrasted with the marginally stable substitutional carbon-hydrogen centres [194,195].

(C-H)_i is believed to be a fast diffusing species in silicon [215] which is highly stable in either a bond centred or three-fold coordinated site depending on whether the carbon is sp² or sp³ hybridised. In the three-fold coordinated site it sits as a [100] split-interstitial similar to C_i. The C-H bond is parallel to [011] and lies perpendicular to the plane containing the three C-Si bonds. The C-H bond length is exceptionally small, 1.1 Å, and as this unit is isovalent with N, it suggests that an STD might arise if N in Fig. 8.1 was replaced by C-H. We therefore tried replacing N_i with (C-H)_i in the STD model.

There are three primary structures to be considered. Firstly the (CH)_iO_2i defects. In direct analogy with the N_iO_2i defects these have two possible structures, with either one oxygen atom on each side of the (CH)_i, or with both oxygen atoms forming a dimer pair on one side of the (CH)_i. We therefore investigated both of these. Then there is also the (CH)_iO_4i defects, i.e. (CH)_i flanked by two oxygen pairs, one on each side.