Effect of Molecular Binding to a Semiconductor on Metal/Molecule/Semiconductor Junction Behavior

Diodes made by (indirectly) evaporating Au on a monolayer of molecules that are adsorbed chemically onto
GaAs, via either disulfide or dicarboxylate groups, show roughly linear but opposite dependence of their
effective barrier height on the dipole moment of the molecules. We explain this by Au-molecule (electrical)
interactions not only with the exposed end groups of the molecule but also with its binding groups. We arrive
at this conclusion by characterizing the interface by in situ UPS-XPS, ex situ XPS, TOF-SIMS, and Kelvin
probe measurements, by scanning microscopy of the surfaces, and by current-voltage measurements of the
devices. While there is a very limited interaction of Au with the dicarboxylic binding groups, there is a much
stronger interaction with the disulfide groups. We suggest that these very different interactions lead to different
(growth) morphologies of the evaporated gold layer, resulting in opposite effects of the molecular dipole on
the junction barrier height.