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 $C_{\mathrm{B}}$ in ultrahighly doped Si(001):B layers grown by gas-source molecular beam epitaxy from ${\mathrm{B}}_2{\mathrm{H}}_6/{\mathrm{Si}}_2{\mathrm{H}}_6$. For $C_{\mathrm{B}}\le 2.5\times {10}^{20}{\mathrm{cm}}^{-3\mathrm{}}$, all B atoms reside on tetrahedrally bonded electrically active substitutional Si sites. At higher $C_{\mathrm{B}}$, 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 ${\mathrm{sp}}^2$ bonded with trigonal coordination while substitutional single B atoms are ${\mathrm{sp}}^3$. A surface reaction path leading to inactive B incorporation is proposed.