Electrically active and inactive b lattice sites in ultrahighly b doped si(001): An x-ray near-edge absorption fine-structure and high-resolution diffraction study

B lattice positions are determined as a function of B concentration C_B in ultrahighly doped Si(001):B layers grown by gas-source molecular beam epitaxy from B₂H₆/Si₂H₆. For C_B ≤ 2.5 × 10²⁰ cm^-3, all B atoms reside on tetrahedrally bonded electrically active substitutional Si sites. At higher C_B, inactive B is incorporated as B pairs located on single Si sites and oriented primarily along in-plane [100] and [010] directions. The B pairs are sp² bonded with trigonal coordination while substitutional single B atoms are sp³. A surface reaction path leading to inactive B incorporation is proposed.