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 Eᵥ+0.13 eV, above a first donor level (0/+) at Eᵥ+0.05 eV. Evidence is presented that interstitial boron has negative-U properties with a single donor level (0/+) at Ec-0.13 eV, above a single acceptor level (-/0) at Ec-0.45 eV.