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

Abstract

We report on the experimental and theoretical determination of the energy offsets between a silicon substrate and a monolayer of alkyl chains chemically grafted on it (self-assembly technique). Internal photoemission experiments show that energy offsets between the silicon conduction band and the lowest unoccupied molecular orbital are 4.1--4.3 eV. Similarly, the energy offsets between the silicon valence band and the highest occupied molecular orbital of the alkyl chains are 4.1--4.5 eV, irrespective of the alkyl chain length (from 12 to 18 carbon atoms). These results are confirmed by theoretical calculations (the local-density approximation and tight-binding methods). These rather similar values are explained by the fact that the carbon ${\mathrm{sp}}_{3}$ level tends to align with the silicon ${\mathrm{sp}}_{3}$ level to achieve the charge neutrality and that the band structures of the carbon and silicon are almost centered on their respective ${\mathrm{sp}}_{3}$ level. These results validate the proposed concept making use of these self-assembled monolayers as ultrathin insulator in nanometer-scale electronic devices [C. Boulas et al., Phys. Rev. Lett. 76, 4797 (1997); D. Vuillaume et al., Appl. Phys. Lett. 69, 1646 (1997)].