Silicon is an ideal model for strong covalent solids because it is an elemental semiconductor amenable to many well-defined structural, mechanical and electronic experiments. It was shown some 25 years ago that electronically active dopants enhance dislocation mobility in silicon, but recently Sumino and co-workers have shown that phosphorus also strongly pins dislocations at low stresses. Almost paradoxically the same dislocations move with enhanced mobility once they are in motion at higher stresses.

We have performed local density functional pseudopotential calculations on a molecular fragment taken from the core of a 90° partial dislocation of silicon. The surface dangling bonds were hydrogenated and phosphorus was substituted for silicon in various positions. Our results suggest that there is a strong valence effect in operation that causes phosphorus to bind very strongly to three fold coordinated sites in the core, simultaneously losing electrical activity. It is this binding that inhibits dislocation motion, because motion would require the coordination of phosphorus to change from three-fold to the more energetic fourfold.