Effect of Doping Density on the Charge Rearrangement and Interface Dipole at the Molecule-Silicon Interface

The interface level alignment of alkyl and alkenyl monolayers, covalently bound to oxide-free Si substrates of various doping levels, is studied using X-ray photoelectron spectroscopy. Using shifts in the C 1s and Si 2p photoelectron peaks as a sensitive probe, we find that charge distribution around the covalent Si-C bond dipole changes according to the initial position of the Fermi level within the Si substrate. This shows that the interface dipole is not fixed but rather changes with the doping level. These results set limits to the applicability of simple models to describe level alignment at interfaces and show that the interface bond and dipole may change according to the electrostatic potential at the interface.