197
For near-no
96
For a sample
phase formed is M
2
consumed, MSi forms and then MSi
2
follows. This is because the
97
Hence, according to thermodynamics, the equilibrium silicide
phases are respectively NiSi
2
and CoSi
2
in the binary systems of Ni/Si
2
Si and Co
2
Si and
second phases formed are NiSi and CoSi. How to predict which is
silicide formation. The phase formation is a kinetic phenomenon;
formation energy is not controlling the selection of phases and the
selection cannot be predicted by equilibrium phase diagrams.
98
For
in certain cases. In Ni/Si and Co/Si systems, the silicide phases of
NiSi
2
and CoSi
2
mismatch to Si at room temperature. So the excellent epitaxy with
Si, which is central to the fabrication of semiconductor devices,
99
the sequence of phase formation have no certain answer yet, because
there are not enough data for a systematic analysis. However, the
phenomenon of single phase formation remains true in nano silicide
formation in Si nanowires.
5.2.1.2 Growth kinetics
The solid state reactions for silicide formation under steady state
and temperature dependence which takes place over a relative wide
range of temperature, and the growth proceeds laterally uniformly
with relatively sharp interface. The growth process can be either
µ anneal time
, or limited
by interfacial reaction, thickness of silicide
µanneal time
silicides that follows a
t
relation, and some early transition-metal
silicides are reported to follow linear time dependence. The di-metal
Introduction to Solid-State Phase Transformation in Thin Film
198
Formation of Epitaxial Silicide in Silicon Nanowires
and mono-metal silicides of near-noble metals such as Ni and Co all
supplies the nucleation and growth of silicide is very small. It is
substrate is much larger. We expect that the kinetics of these two
case, it is more similar to a slow deposition of metal atoms on a Si
silicide formation and consider only vapor deposition.
substrate and there is also a probability that the atom desorbs from
adatom on the surface have to be considered.
Considering the nucleation of an epitaxial nucleus of the thickness
of a single atomic layer, the nucleation of such a circular disc on the
However, when we consider the nucleation of a disc of silicide which
has the same crystal structure and orientation as the single crystal
silicide substrate, we can ignore the strain. More importantly, a
positive surface energy comes from the circumference of the disc
has to be included in the nucleation event.
100–101
This kind of disc
nucleation on a single crystal surface is referred to as homogeneous
nucleation. Why homogeneous nucleation can occur in epitaxial
growth of silicide in Si nanowires needs to be explained later. This
is rare because it cannot compete with heterogeneous nucleation.
Heterogeneous nucleation occurs with a much higher frequency or
with a much lower activation energy in most materials because it is
enhanced by imperfections, impurities, and external surfaces.
88,102
However, homogeneous nucleation is found in epitaxial silicide
growth in Si nanowires.
103
In the classic nucleation theory, the shape of a standard nucleus
only a cap-shape of nucleus (a small part of a sphere) is required
by heterogeneous nucleation. In nanoscale silicides formation in
199
nanowire of Si, owing to the fact that the growth of the epitaxial
silicide is atomic layer by atomic layer, so the nucleation event
is two-dimensional and the nucleus has the thickness of a single
atomic layer. Thus, the standard shape of the homogeneous nucleus
is a circular disc. For heterogeneous nucleation, the nucleus is just
a small part of a circular disc. It is worth mentioning that when
the disc nucleus has the thickness of an atomic layer, whether its
circumference is a surface or a line is more of a theoretical issue to
be carefully studied.
5.2.2 Examples of Silicides Formation on Si Wafers
Atoms of Ni are depositing on a Si substrate by molecular beam
reaction will occur when annealing occurs at a high temperature.
The Si substrate serves as a parent phase and the deposition
supplies metal atoms to keep the reaction going. The reaction can be
growth conditions.
104
The reaction between metal and Si can occur during metal
deposition. The surface of a semiconductor is the starting place of
of metal adatoms to surface steps on the semiconductor surface. The
surface steps provide favorable sites for atoms to bond and they serve
they are essential in homo-epitaxial and hetero-epitaxial growth.
1
planar interface.
17,99
and has no steps, nucleation of a disc is needed.
5.2.2.1 Ni silicides formation
exhibits a sequential growth of three phases: Ni
2
Si, NiSi, and NiSi
2
.
105
The growth of Ni
2
Si is initiated at the interface between Ni and Si at
temperatures from 200
o
C to 350
o
C. The growth kinetics of Ni
2
Si
follows a parabolic relation between the thickness of Ni
2
Si and the
annealing time. It was observed that the growth rate of Ni
2
Si at 275
o
C
is two times faster on (100) Si substrate than on (111) Si substrate. In
formed on (100) and (111) Si substrate. The activation energy of
Ni
2
Si formation is 1.5 ± 0.2 eV at the temperature range of 200 to
Introduction to Solid-State Phase Transformation in Thin Film
200
Formation of Epitaxial Silicide in Silicon Nanowires
350
o
C; however, the activation energies do not have remarkable
The transformation of Ni
2
Si to NiSi is initiated at the interface
between Si and Ni
2
Si. The transformation was found to accompany
with a stress change from compression in Ni
2
Si to tension in NiSi.
The formation is very fast at 350
o
C, and the phase is stable up to 750
o
C. At the temperature higher than 750
o
C, epitaxial growth of NiSi
2
was found on (111), (110), and (100) Si.
2
to form.
106
For reaction of mono-layers of metal or metal–silicon
mixtures with silicon substrates, the reduced reaction length
and stress were found to change the kinetics of the reaction, and
formation.
107–108
For example, the metastable phase -Ni
2
Si forms at
never on (001) Si. Its growth is followed by the subsequent growth
of type-A NiSi
2
at 450 °C.
109–110
2
was found to grow heavily faceted on (001) Si.
The interface between NiSi
2
and (111) Si is faceted but less than
that on (001) Si. The interface was observed to be very rough on
a microscopic scale, however, it was quite smooth on atomic scale
and at short range.
111
Defect clusters and planar defects were also
single-crystal NiSi
2
can be grown on (111) Si.
111
islands which are formed at the initial stage of deposition.
1,112–113
layers of Ni and the Si substrate even at room temperature. Under
appropriate conditions, NiSi
2
can be formed at 450–500
o
C using
few mono-layers of Ni results in a disordered layer containing three-
islands coalesce and the Ni–Si reaction slows down due to the lack of
exposed Si regions.
114
The unreacted Ni is expected .to stay on top of
the precursor silicide layer as more Ni is deposited.
5.2.2.2 Co silicides formation
2
Si. Under
non ultra-high-vacuum (UHV) deposition and annealing, the CoSi
2
201
transition from polycrystalline to epitaxial appeared to be very
sluggish and interfacial dislocations at epitaxial silicide/Si interfaces
were found with the dislocation spacing of several nm to over 100
nm.
97,115
2
can be formed on (111)
followed by rapid thermal annealing.
116
Under UHV condition, epitaxial heterostructures, Si/CoSi
2
/Si,
have been fabricated.
117–118
A series of in situ low energy electron
measurements showed that at room temperature the Co/(111)Si
interface consists of a very thin silicide layer, an intermediate silicide
in the metallic matrix and at the top surface. Upon heating to high
temperature about 600 °C, sequential silicide formation (Co
2
Si, CoSi,
CoSi
2
2
was obtained on the
(111) Si surface.
119–122
In situ
reaction occurs from isolated nucleation sites and progresses
laterally across the surface.
123
2
lattice may
2
formation temperature (400–600 °C)
CoSi
2
layers.
124–125
5.2.3 Summary
In the phase diagram of metal–Si systems, there are more than
three silicides present. Nevertheless, not all equilibrium phases
are present as dominant phases during silicide formation in thin
by the available techniques. The examples shown in Section 2.2
were chosen as representative of the three broad classes of silicide
single crystal Si: the metal-rich silicides, M
2
Si; the monosilicides,
MSi; and the disilicides, MSi
2
.
Table 5.1 shows the general pattern of silicide formation. The
examples of the phase formation for the elements shown in the
left-hand column. The phase formation proceeds from left to right
Introduction to Solid-State Phase Transformation in Thin Film
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