SCI:NANO:small structure conductivity processes

Eugene Leitl (
Sun, 12 Jan 1997 16:15:33 +0100 (MET)

K. Eric Drexler, "Nanosystems", Wiley & Sons (1992), pp. 333-334.

11.6.1 Conducting paths

a. Conductivity As the diameter of a wire becomes comparable to the
electron mean free path, surface scattering can increase resistance to
above the value suggested by scaling laws and bulk resistivity. This can
be avoided by supressing surface scattering. In particular, structures
can be chosen that have matching lattice spacings across the interface
between metal insulator, measuring the spacings along the axis of current
flow (matching usually requires strain). Electrons with wave vectors that
enable propagation without reflection from metallic lattice planes will
then experience no degradation of their longitudinal momentum as a result
of diffractive scattering from the metal-insulator interface; in the
longitudinal direction, specular reflection will dominate.

Doped graphite and organic polymeric materials can exhibit conductivities
comparable to or greater than that of copper (Kivelson, 1988). Organic
polymeric materials such as doped polyacetylene are termed
quasi-one-dimensional conductors and presumably show little degradation
of conductivity through surface scattering.

By engineering semiconductor structures with suitable band gaps and
doping patterns, conducting filaments can be constructed that are narrow
enough that electron wave functions having a transverse node are
substantially higher in energy that those lacking a transverse node.
These filaments, termed quantum wires, strongly suppress the small-angle
scattering processes that dominate the degradation of electron momentum
in ordinatry conductors; they can accordingly exhibit unusually high
conductivities (Sakaki, 1980; Timp et al., 1990).

Experimental and theorectical work on conducting polymers and other
low-dimensional conducting structures suggests that eutactic electronic
structures can be built that provide conductive paths of substantially
lower resistance than would be estimated from their dimensions and bulk
metallic resistivities. Indeed, it is conceivable that the ability to
build a far wider range of metastable structures will lead to the
discovery of superconductors with critical temperatures substantially
higher than those now known. [...]

b. Current density In integrated circuits, high current densities can
cause electromigration, in which metal is redistributed from one region
of a conductor to another, eventually breaking circuit continuity. This
phenomenon limits acceptable current densities.

Electromigration results from biased diffusion of metal atoms and
vacancies, chiefly along dislocations and grain boundaries. It is
strongly suppressed in fully-dense, single-crystal metallic structures
where the energy required to form a vacancy is large (e.g., >150 maJ in
Cu). The required energy is still larger if the displaced atom cannot
occupy a normal lattice site (in a slightly expanded crystal), but is
instead forced to occupy a high-energy interstitutional site.
Accordingly, a single-crystal metal wire surrounded by a strongly-bonded,
lattice-matched sheath should be stable at far greater current densities
than a conventional wire of the same material. The structural stability
of (some) conducting polymers should likewise minimize electromigration
and similiar phenomena. [...]