Negative-UProperties for Interstitial Boron in Silicon

Electrical detection of a single donor level of interstitial boron at $E_c-0.13\mathrm{}$ eV using novel photo—deep-level transient spectroscopy techniques is reported. Complementary behavior between this level and the acceptor level at $E_c-0.45\mathrm{}$ eV establishes that the defect levels have negative-$U$ ordering. The donor level exhibits a large Poole-Frenkel effect which, when properly accounted for, provides a direct and unambiguous connection to the EPR-identified interstitial boron atom. This represents the first definitive indentification of negative-$U$ properties for a defect in any solid.