Epitaxially fabricated MS systems offer the best opportunity to
understand the dependence of the electronic properties on the
structure of a MS interface. The SBH at an epitaxial MS
interface is frequently homogeneous and, therefore, can be
directly correlated with the interface atomic structure.
In addition, the atomic structure at single crystal MS
interfaces can be determined by experimental means and then
used to calculate the expected SBH. A comparison of the
experimental and theoretical SBH allows a test of the validity
of theories of the SB. Therefore, even though the formation
mechanism of the SBH at ordinary MS interfaces is of the most
technological interest, its deduction almost certainly has to
come from the simpler, epitaxial, "model" MS systems.
Silicides are ordered metal-Si compound phases and have
wide applications in Si integrated circuits as contacts
and gate electrodes. Silicide thin films are usually formed
by the deposition of metal on a Si substrate, followed by a
high temperature treatment to induce solid-phase reactions.
Because part of the substrate participates in the reaction
of silicide thin films, the location of the final silicide-Si
interface is usually well inside the original Si surface.
This ensures intimate contact between the silicide and the Si,
with transport behavior essentially independent of the initial
conditions of the Si surface. A few silicides, most notably
NiSi2 and CoSi2, have lattice structures
similar to that of Si and close lattice matches with Si.
As a result of these favorable conditions, these silicides
can form epitaxially on silicon and the quality of these
silicide-Si interfaces has been the best among MS interfaces
ever fabricated. The SBH measured from single crystal silicide
interfaces shows a remarkable dependence on the interface structure.
The best known of these results is the dependence of the SBH
on silicide orientation at the NiSi2/Si(111) interfaces.
High quality single crystals of NiSi2 may be grown
on Si(111) with either type A
or type B orientation by a
proper choice of template growth condition. These two orientations
differ only by an azimuthal rotation: the type A silicide has
the same orientation as the silicon substrate, and the type B
silicide shares the surface normal <111> axis with Si, but is
rotated 180o about this axis with respect to the Si.
The atomic structure of both type A and type B
NiSi2/Si(111) interfaces have been studied by various
experimental techniques and found to have the
Intriguingly, type A and type B NiSi2 have been found
to have distinctively different SBH's, as illustrated in
these current-voltage curves.
The atomic structure of the planar NiSi2/Si(100)
interface has been studied experimentally by HREM and shown
to be consistent with having either 6-fold or 8-fold coordination.
Theoretical calculations tend to favor the 8-fold model over
the 6-fold model. Because extra streaks at the
(0 1/2 1/2)
related positions have been observed at flat
NiSi2/Si(100) interface by TED, it is clear that
the atomic structure at this interface is much more complicated
than these basic models. Its exact details notwithstanding,
the flat NiSi2/Si(100) interface obviously has an atomic
structure which is entirely different from either of the two
NiSi2/Si(111) interfaces. One thus expects to
find the flat NiSi2/Si(100) interface with an SBH
of its own. Indeed, uniform NiSi2/Si(100) interfaces
show a SBH which is much lower on n-type Si than either of
the two NiSi2/Si(111) interfaces, as shown in
One notes that the low SBH of 0.4 eV measured from uniform
NiSi2 layers is also very different from the value
of 0.6 - 0.7 eV usually observed for polycrystalline
NiSi2 on Si.
The overall picture of the formation of the SBH at single
crystal MS interfaces, presented by experimental investigations,
is identical to that described earlier from theoretical studies.
In the few cases where theoretical calculations have enough
accuracy for a meaningful comparison with experimental SBHs,
most notably on the difference of SBHs between the type A and
the type B NiSi2/Si(111) interfaces, good numerical
agreement was found. The message is unmistakable: the SBH
depends on the atomic structure of epitaxial MS interfaces.
This dependence is inconsistent with SBH models that ignore
interaction between the metal and the semiconductor.
It also disagrees with the basic notion of CNL-based models
that the distribution of interface states is independent of
the metal. It is in good agreement with the spirit of the
bond polarization theory. However, one notes that the
tight-binding picture of the bond polarization theory would
not be able to explain the A-B dependence of the NiSi2
SBH either. Before one rules out any SBH model based on
the experimental data observed from epitaxial MS interfaces,
however, one needs to appreciate the complexity of the SBH problem.
Even after the entire interface electronic structures are known
for a number of epitaxial MS interfaces from calculations,
researchers are still at a loss as to the single most important
feature or parameter that determines the SBH. There is none,
other than the "charge density distribution of the entire
interface". The formation of the SBH has always contained a
bulk term and an interface term.
Results described in this chapter give strong indication that
the interface term depends on the interaction between the metal
and the semiconductor. This should be regarded as the most
important message from these studies. If we were to ask ourselves
the question "given all the information one now knows about the
electronic properties of epitaxial MS interfaces, what is the
most convenient way to view the formation of the interface dipole
(at any MS interface)?" The answer cannot be anything other than