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Defects in amorphous Si are universally viewed as due to undercoordinated atoms. The dominant EPR-active center is identified as threefold-coordinated Si (dangling bond), but evidence is inconclusive. I introduce a new viewpoint, motivated by results on defects in crystalline Si, and conclude that overcoordination defects are as likely. I propose that most EPR-active centers are fivefold-coordinated Si, with an electron in a state that I label "floating bond." This new analysis is favored by experiments and theory and leads to new predictions.
Sokrates T. Pantelides (Mon,) studied this question.