1 research outputs found
Effect of Internal Heteroatoms on Level Alignment at Metal/Molecular Monolayer/Si Interfaces
Molecular
monolayers at metal/semiconductor heterointerfaces affect
electronic energy level alignment at the interface by modifying the
interface’s electrical dipole. On a free surface, the molecular
dipole is usually manipulated by means of substitution at its external
end. However, at an interface such outer substituents are in close
proximity to the top contact, making the distinction between molecular
and interfacial effects difficult. To examine how the interface dipole
would be influenced by a single atom, internal to the molecule, we
used a series of three molecules of identical binding and tail groups,
differing only in the inner atom: aryl vinyl ether (<b>PhO</b>), aryl vinyl sulfide (<b>PhS</b>), and the corresponding molecule
with a CH<sub>2</sub> groupî—¸allyl benzene (<b>PhC</b>). Molecular monolayers based on all three molecules have been adsorbed
on a flat, oxide-free Si surface. Extensive surface characterization,
supported by density functional theory calculations, revealed high-quality,
well-aligned monolayers exhibiting excellent chemical and electrical
passivation of the silicon substrate, in all three cases. Current–voltage
and capacitance–voltage analysis of Hg/PhX (X = C, O, S)/Si
interfaces established that the type of internal atom has a significant
effect on the Schottky barrier height at the interface, i.e., on the
energy level alignment. Surprisingly, despite the formal chemical
separation of the internal atom and the metallic electrode, Schottky
barrier heights were not correlated to changes in the semiconductor’s
effective work function, deduced from Kelvin probe and ultraviolet
photoemission spectroscopy on the monolayer-adsorbed Si surface. Rather,
these changes correlated well with the ionization potential of the
surface-adsorbed molecules. This is interpreted in terms of additional
polarization at the molecule/metal interface, driven by potential
equilibration considerations even in the absence of a formal chemical
bond to the top Hg contact