(color online) (a) Side and (b) top views of 4H-SiC (0001) surface and (c), (d) four adsorption sites on this surface. The on-top site (OT) is located on the top of a surface Si atom, the bridge site (BR) is a two-fold coordinated site between two surface Si atoms, the three-fold hollow site (H3) is above a carbon of the third bilayer, and the four-fold hollow site (T4) is above a carbon of the first bilayer. The orange, grey, magenta, and cyan spheres denote silicon, carbon, phosphorus, and hydrogen atoms, respectively.