Configurationally bistableCcenter in quenched Si:B: Possibility of a boron-vacancy pair

The C center is an unusual defect found in ultra-fast-quenched (cw laser irradiated) boron-doped silicon. This center introduces two deep donor-hole traps in the band gap, at EV+0.50 eV (H₁) and EV+0.36 eV (H₂), as revealed by deep-level transient spectroscopy (DLTS). We find that each C center may contribute to either of the two hole-emission signals H₁ and H₂ in a DLTS scan, but not both, and that the one it contributes to depends upon its charge state during sample cooling down to low temperatures. We present a simple double-site configuration-coordinate model of the defect that explains these unusual observations. In this model, the C center can exist in either of two configurations in both of its charge states; each configuration is stable in one charge state. No very large lattice relaxation effect is involved. The C center is then tentatively identified as a boron-vacancy pair (B-V), a defect which has eluded DLTS detection so far. We show how this microscopic model is supported by the results of previous defect studies in ultra-fast-quenched and electron-irradiated silicon. The two donor levels H₁ and H₂ are thus tentatively ascribed to next-nearest- and nearest-neighbor B-V pairs, respectively.