Negative-U Properties for Point Defects in Silicon

Experimental evidence is presented in support of a recent suggestion by Baraff, Kane, and Schlüter 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 $E_v+0.13\mathrm{}$ eV, above a first donor level (0/+) at $E_v+0.05\mathrm{}$ eV. Evidence is presented that interstitial boron has negative-$U$ properties with a single donor level (0/+) at $E_c-0.13\mathrm{}$ eV, above a single acceptor level (-/0) at $E_c-0.45\mathrm{}$ eV.