In the tetrahedrally coordinated amorphous semiconductors the dominant defects deep in the gap are attributed to dangling bonds on the group IV atoms. These defects are commonly thought to have effective electronelectron correlation energies Ueff which are positive, although some tightbinding estimates suggest negative Ueff. Defect states near the band gap edges are invoked to account for many experimental results including the usual appearance of an Urbach absorption edge. These shallow defect states are usually attributed to strained bonds but two-fold-coordinated group IV atoms have also been suggested. The application of light of near-band-gap energies alters the deniity of paramagnetic dangling bonds. For large spin densities (ns ≥ 1017 cm−3) this increase is probably due to the creation of new defects, bui it is possible that at lower densities (ns ≤ cm−3) the rearrangement of electronic charge in existing defects is important. Impurities also contribute to the defects observed in tetrahedral amorphous semiconductors. Particular species include trapped atomic and molecular hydrogen, trapped N0−2 molecules, singly-coordinated oxygen atoms and E' centers.
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