Defects in Tetrahedrally Coordinated Amorphous Semiconductors.
UTAH UNIV SALT LAKE CITY DEPT OF PHYSICS
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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 electron-electron correlation energies Ueff which are positive, although some tight-binding 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 density of paramagnetic dangling bonds. For large spin densities ns greater than or equal to 10 to the 17th power per cu cm this increase is probably due to the creation of new defects, but it is possible that at lower densities ns less than or equal to 10 to the 16th power per cu cm 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 NO-2 molecules, singly-coordinated oxygen atoms and E centers. Keywords include Hydrogenated amorphous silicon, Amorphous silicon alloys, Defects, States in the gap, and Impurities.
- Solid State Physics