**Resonant Tunneling Through Individual Molecules on Silicon:
Implications for Electronics and Chemical Spectroscopy**

**Mark Hersam**

***Materials Science & Engineering Dept.***

***Northwestern University***

In recent years, substantial progress has been made in the emerging
field of molecular electronics. In particular, metal-molecule-metal
junctions have been widely studied. In this talk, charge transport
through molecule-semiconductor junctions is considered. The presence of
the energy band gap in semiconductors provides opportunities for
resonant tunneling through individual molecules, leading to interesting
effects such as room temperature negative differential resistance (NDR).
Furthermore, by doping the substrate, the majority charge carrier can be
tailored, thus allowing asymmetry to be intentionally designed into the
current-voltage characteristic. Through judicious choice of the
molecular species, the bias voltage of the NDR can also be controlled.
By demonstrating these effects on the Si(100) surface,
semiconductor-based molecular electronic devices have the potential of
being directly interfaced to conventional silicon integrated circuit
technology.

As an introduction, this talk will summarize recent theoretical \[1\]
and experimental \[2\] work on silicon-based molecular resonant
tunneling diodes. In particular, the ultra-high vacuum scanning
tunneling microscope (STM) has allowed individual molecules to be
imaged, addressed, and manipulated on the Si(100)-2×1 surface with
atomic resolution at room temperature. While previous work has focused
on the characterization of individual styrene and
2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) molecules, this talk will
also detail previously unpublished results for isolated cyclopentene
molecules and full monolayers of TEMPO and cyclopentene. In all cases,
STM current-voltage characteristics on individual molecules mounted on
degenerately n-type Si(100) show multiple NDR events at negative sample
bias. On the other hand, at positive sample bias, the current-voltage
characteristics do not show NDR, although discontinuities in the
differential conductance are observed. When the Si(100) substrate is
changed to degenerate p-type doping, multiple NDR events are observed at
positive sample bias while the discontinuities in the differential
conductance occur at negative sample bias. These empirical observations
can be qualitatively explained by considering the energy band diagram
for a semiconductor-molecule-metal junction.

For full organic monolayers on degenerately doped Si(100), similar
behavior is observed except that the degree of NDR is significantly
attenuated compared to charge transport measurements on isolated
molecules. Possible explanations for this behavior include broadening of
molecular energy levels through intermolecular interactions or
concurrent tunneling paths through multiple molecules. This talk will
conclude by describing recent efforts to quantify and understand the
critical parameters that dictate charge transport through individual
molecules and organic monolayers on Si(100) surfaces.

\[1\] T. Rakshit, G.-C. Liang, A. W. Ghosh, and S. Datta,
*arXiv:Cond-Mat*, 0305695v1 (2003).

\[2\] N. P. Guisinger, M. E. Greene, R. Basu, A. S. Baluch, and M. C.
Hersam, *Nano Letters*, **4**, 55 (2004).