Negative-U Properties for Point Defects in Silicon

Experimental evidence is presented in support of a recent suggestion by Baraff, Kane, and Schl\"uter that the isolated lattice vacancy in silicon is an Anderson negative-$U$ system. The second donor level (+/++) (of charge state + if defect level is occupied by an electron; ++, if unoccupied) is located at ${E}_{v}+0.13$ eV, above a first donor level (0/+) at ${E}_{v}+0.05$ eV. Evidence is presented that interstitial boron has negative-$U$ properties with a single donor level (0/+) at ${E}_{c}\ensuremath{-}0.13$ eV, above a single acceptor level (-/0) at ${E}_{c}\ensuremath{-}0.45$ eV.

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