Web Release Date: March 4,
Variable Density Effect of Self-Assembled Polarizable Monolayers on the Electronic Properties of Silicon
The Institute of Chemistry and the Hebrew University Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
Received October 16, 2007
Abstract:
Electronic structures at the Si/SiO2/molecule interfaces were studied by Kelvin probe techniques
(contact potential difference) and compared to theoretical values derived by the Helmholtz equation. Two
parameters influencing the electronic properties of n-type <100> Si/SiO2 substrates were systematically
tuned: the molecular dipole of coupling agent molecules comprising the layer and the surface coverage of
the chromophoric layer. The first parameter was checked using direct covalent grafting of a series of
trichlorosilane-containing coupling agent molecules with various end groups causing a different dipole with
the same surface number density. It was found that the change in band bending (
BB) clearly indicated
a major effect of passivation due to two-dimensional polysiloxane network formation, with minor differences
resulting from the differences in the end groups' capacity to act as "electron traps". The change in electron
affinity (EA) parameter increased upon increasing the dipole of the end group comprising the monolayer,
resulting in a range of 600 mV. Moreover, a shielding effect of the aromatic spacer compared with the
aliphatic spacer was found and estimated to be about 200 mV. The density effect was examined using the
4-[4-(N,N-dimethylamino phenyl)azo]pyridinium halide chromophore which has a calculated dipole of more
than 10 D. It was clearly shown that upon increasing surface chromophoric coverage an increase in the
electronic effects on the Si substrate was observed. However, a major consequence of depolarization was
also detected while comparing the experimental and calculated values.
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