Doping in hydrogenated amorphous silicon occurs by a process of an ionised donor atom partially compensated by a haraged dangling bond. The total energies of various dopant and dopant/bonding combinations are calculated for tetrahedral amorphous carbon. It is found that charged dangling bonds are less favoured because of the stronger Coulombic repulsion in ta-C. Instead dopants can be compensated by weak bond states in the lower gap associated with odd-membered #pi#-rings or odd-numbered #pi#-chains. The effect is that the doping efficiency is low but there are not charged midgap recombination centres, to reduce photoconductivity with doping, as occurs in a-Si:H.
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