(C-H)i is believed to be a fast diffusing species in silicon  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  split-interstitial similar to Ci. The C-H bond is parallel to  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 Ni with (C-H)i in the STD model.
There are three primary structures to be considered. Firstly the (CH)iO2i defects. In direct analogy with the NiO2i 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)iO4i defects, i.e. (CH)i flanked by two oxygen pairs, one on each side.