Defect states in amorphous silicon

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Abstract
A novel mechanism for spin-pairing at defect centres in amorphous silicon is proposed and discussed in terms of simple chemical bond arguments. Unlike the case of chalcogenide glasses (or amorphous arsenic), the presence of non-bonding (lone-pair) orbitals is found to be unnecessary in producing a negative effective Hubbard energy in this material. Instead, a mechanism is proposed in which dehybridization occurs at the negatively charged centre, the predominant bonding changing from sp3-like to p-like. Various experimental data strongly point to the presence of such spin-paired defects in amorphous silicon, and these observations are discussed in the light of the present model. The implications for a.c. conduction are considered, and it is concluded that, as in chalcogenide glasses, bipolaron hopping is the predominant mechanism.