Interstitial boron in silicon: A negative-Usystem

An electrical level 0.45 eV below the conduction band is detected by deep-level-capacitance transient spectroscopy (DLTS) in boron-doped silicon irradiated at 4.2 K by 1.5-MeV electrons. This level is attributed to interstitial boron. Greatly enhanced annealing of the level is observed under minority-carrier injection in both $n$- and $p$-type material. A quadratic dependence of the annealing rate on the injected current density reveals that the process is a Bourgoin mechanism in which the defect migrates by jumping from one lattice configuration to another as the defect alternates its charge state between ${\mathrm{B}}_i^{+}$ and ${\mathrm{B}}_i^{-}$ Combining these results with published electron-paramagnetic-resonance information, it is proposed that interstitial boron is an example of an Anderson "negative-$U$" system, with a single donor state (0/+) at $\sim E_c-0.15\mathrm{}$ eV which is above the single acceptor state (-/0) at $E_c-0.45\mathrm{}$ eV detected in the DLTS experiments.