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 CB in ultrahighly doped Si(001):B layers grown by gas-source molecular beam epitaxy from B₂H₆/Si₂H₆. For CB≤2.5×10²⁰cm^-3, all B atoms reside on tetrahedrally bonded electrically active substitutional Si sites. At higher CB, 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.