  Titanium silicide islands have been formed by the ultrahigh vacuum deposition
of thin films of titanium (< 2 nm) on atomically clean Si(100) substrates
followed by annealing to ~800 degrees C. Scanning tunneling microscopy (STM)
and scanning tunneling spectroscopy have been performed on these islands to
record current-voltage (I-V) curves. Because each island forms a double barrier
tunnel junction (DBTJ) structure with the STM tip and the substrate, they would
be expected to exhibit single electron tunneling (SET) according to the
orthodox model of SET. Some of the islands formed are small enough (diameter <
10 nm) to exhibit SET at room temperature and evidence of SET has been
identified in some of the I-V curves recorded from these small islands. Those
curves are analyzed within the framework of the orthodox model and are found to
be consistent with that model, except for slight discrepancies of the shape of
the I-V curves at current steps. However, most islands that were expected to
exhibit SET did not do so, and the reasons for the absence of observable SET
are evaluated. The most likely reasons for the absence of SET are determined to
be a wide depletion region in the substrate and Schottky barrier lowering due
to Fermi level pinning by surface states of the clean silicon near the islands.
The results establish that although the Schottky barrier can act as an
effective tunnel junction in a DBTJ structure, the islands may be unreliable in
future nanoelectronic devices. Therefore, methods are discussed to improve the
reliability of future devices.
