Chemistry Reference
In-Depth Information
5.4 Conclusions
Experimental quantification of electron scattering effects in nanoscale structures is
in its infancy. Steps on macroscopic surfaces provide a useful model system for
quantifying defect scattering in nanostructures, where the large surface/volume ratio
will cause surface effects to dominate. The effects of defects can be documented
via observations of local surface resistivity [
24
] and potential drops [
26
] related
to surface steps, which provide direct electrostatic information with limited spatial
resolution. In addition, direct measurements of current flow via MFM, as discussed
above, can be scaled from micron-scale structures to assess the relative magnitudes
of behavior likely at nanoscale structures. Direct measurements of electromigration-
induced mass flow provide another window into the nature of electron scattering at
defects. In this case, the observations can be interpreted in terms of atomic-scale
mass transport, yielding structure-specific values of the electromigration force [
75
,
76
].
The observations presented above represent the two extremes of high-resolution
observations of electromigration-induced transport and low-resolution direct mea-
surements of the electrostatic effects of surface scattering at defects. Analysis of the
electromigration-induced transport on Ag(111) surfaces allowed identification of an
atomically specific force, specifically acting at kink sites. The analysis of the force
yielded a surprisingly large value, which in the ballistic electron scattering model
may arise from a combination of geometric blocking at the kink sites, changes in
electron density due to the lower coordination at the kink sites, and current crowd-
ing. The blocking effect is likely to yield an enhancement up to a factor of 2 at step
edges compared with free adatoms on the surface [
38
,
39
]. The direct measurements
of current crowding at the micron scale show, for small aspect ratio structures anal-
ogous to kink sites, that the scale of the enhancement is more typically some tens
of percent [
77
]. These classical effects thus do not predict the large enhancements
measured. However, recent atomistic calculations including non-equilibrium trans-
port explicitly may provide an explanation in terms of quantized conductance at the
atomic-scale resistivity dipole created by current flow around kink sites [
78
]. Further
experiments to assess the electromigration force for different metals and different
types of scattering sites will be needed to test this exciting possibility.
Acknowledgments
This work has been supported by the University of Maryland NSF MRSEC
under grant # DMR 05-20471, including use of the Shared Experimental Facilities. Infrastructure
support is also provided by the UMD NanoCenter and CNAM.
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, 159-231 (1997)
3. P.S. Ho, T. Kwok, Electromigration in Metals. Rep. Prog. Phys.
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