Atomic structures of boron-induced protrusion features on Si(100) surfaces

It is known that ultrahigh doping can be realized for boron on Si(100) substrates, while boron-induced features on a heavily boron-doped Si(100) surface cannot form any periodic structure. Here, we demonstrate that boron-induced features actually result from the adsorption of boron-silicon addimers, owing to the underneath substitutional boron atoms at the second layer. Furthermore, more closely arranged boron atoms at the second layer make the energy of the (2x1) surface lower, and the whole second layer can be completely occupied by boron atoms while the surface is still (2x1) reconstructed.