Electronic structure of a heterostructure of an alkylsiloxane self-assembled monolayer on silicon

We report on the experimental and theoretical determination of the energy o ffsets between a silicon substrate and a monolayer of alkyl chains chemical ly grafted on it (self-assembly technique). Internal photoemission experime nts show that energy offsets between the silicon conduction band and the lo west unoccupied molecular orbital are 4.1-4.3 eV. Similarly, the energy off sets between the silicon valence band and the highest occupied molecular or bital of the alkyl chains are 4.1-4.5 eV, irrespective of the alkyl chain l ength (from 12 to 18 carbon atoms). These results an confirmed by theoretic al calculations (the local-density approximation and tight-binding methods) . These rather similar values are explained by the fact that the carbon sp( 3) level tends to align with the silicon sp(3) level to achieve the charge neutrality and that the band structures of the carbon and silicon are almos t centered on their respective sp(3) level. These results validate the prop osed concept making use of these self-assembled monolayers as ultrathin ins ulator in nanometer-scale electronic devices [C. Boulas el nl., Phys. Rev. Lett. 76, 4797 (1997); D. Vuillaume ct al., Appl. Phys. Lett. 69, 1616 (199 7)]. [S0163-1829(98)03638-0].