222
Formation of Epitaxial Silicide in Silicon Nanowires
Table 5.5 Comparison of Co–Si and Ni–Si Systems
System Structure Activation energy (eV/atom)
Co–Si ~2.03
CoSi
2
CaF
2
~2.81
Ni–Si NiSi Orthorhombic ~1.25
NiSi
2
CaF
2
~1.5
In samples annealed at 700 °C and 800 °C, we observed that
the Co particles appeared beside the Si nanowires and on the SiO
2
window. These Co particles seemed to be stable in annealing. Here, we
and Co silicides since we found no change in the distribution of the
size of Co particles after 50 min at 800 °C. In other words, there is no
ripening among the Co particles, as shown in Fig. 5.16a,b.
Figure 5.16
50 min annealing.
5.3.6 Summary
Si nanowires transformed into CoSi and CoSi
2
by point contact
reaction between Co nanodots and Si nanowires at annealing
temperatures of 700 °C and 800 °C, respectively. The CoSi
2
has axial
between CoSi
2
and Si is very sharp. A layer by layer stepwise growth
was found in which we can resolve the nucleation stage and the
growth stage. In addition, lateral growth in the form of an atomic
step sweeping across the CoSi
2
/Si interface was observed. The
repeating nucleation stages of CoSi
2
in the epitaxial reaction have
223
been observed and the incubation time recorded. In point contact
reactions, we propose that it can be limited by the supply of Co atoms
or the dissolution of Co into the Si via the point contact area. Single
crystal CoSi
2
/Si/CoSi
2
heterostructure with sharp interfaces can be
made by this method. The heterostructures are promising as high-
performance transistors based on intrinsic Si nanowires.
Si nanowires were found to transform to NiSi at 450–700
o
C and
to NiSi
2
at 800
o
2
have epitaxial growth relation
with Si and form very sharp silicide/Si interface. Heterostrucures
of NiSi/Si/NiSi and NiSi
2
/Si/NiSi
2
interfaces by in situ
of Ni patterning sample was prepared to synthesize multiple
heterostructure of silicide/Si/silicide in a Si nanowire.
5.4 Homogeneous Nucleation of Nanoscale
Silicide Formation
5.4.1 Introduction
The studies on nanoscale silicide formations in Si nanowires in the
on the nucleation studies of epitaxial silicide formation in nanoscale
nucleation in epitaxial growth of CoSi
2
and NiSi silicides in the axial
direction of nanowire of Si will be discussed. Using high-resolution
the silicides requires nucleation. It is homogeneous nucleation
because heterogeneous nucleation has been suppressed due to the
surface oxide of the Si nanowire. Owing to the repeating events,
we can measure the distribution of incubation time of nucleation.
Knowing the incubation time, we can calculate the steady state rate
of homogeneous nucleation, that is, the number of stable critical
nucleus per unit area per unit time, when the activation energy of
nucleation is measured. Since it is homogeneous nucleation, we are
able to apply Zeldovich factor to obtain the number of molecules in
forming the critical nucleus in the steady state process. In this report,
we have correlated the theory and experiment of homogeneous
nucleation.
Homogeneous Nucleation of Nanoscale Silicide Formation
224
Formation of Epitaxial Silicide in Silicon Nanowires
5.4.2 Results and Discussions
5.4.2.1 Stepwise growth and repeating events of nucleation
The epitaxial growth mode of NiSi, NiSi
2
and CoSi
2
in (111) Si
nanowires was found to be the same.
103,143
The growth in the
axial direction occur atomic layer by atomic layer with the moving
of steps or kinks across the epitaxial interface as shown in Fig.
5.17a–b and d–e. The growth rate of the silicide, at 450 °C to 750 °C
for NiSi and at 800 °C for CoSi
2
, was measured from in situ
motion of one NiSi atomic layer across the NiSi/Si interface, and the
stepwise growth direction is the step motion in the radial direction
of the wire. Similarly, Fig. 5.17d,e shows two images of step motion
of CoSi
2
. During the growth of an atomic layer, the growth mode is
the moving of steps or kinks across the epitaxial interface. However,
there is a long period of stagnation before the next stepwise growths
of silicide can take place. When we plot the stagnation period as
stair-type curves as shown in Fig. 5.17c,f. It shows that the growth
rate or the time of growth of each silicide atomic layer is the same,
which is about 0.06 s per layer for NiSi and about 0.17 s per layer for
CoSi
2
, and we note that it is just the width of the vertical line in the
stair-type curves. In between the vertical lines (the horizontal part
incubation time of nucleation of a new layer. Since nucleation is a
time of nucleating each atomic layer can vary. Accordingly, the plots
of the distribution of incubation time are shown in the inset in Fig.
5.17c,f. The average value of the incubation time of NiSi is about 3 s
and that of CoSi
2
is about 6 s.
The average growth rate of CoSi
2
along the axial direction is
0.0365 nm/s. However, the radial growth rate, or the step velocity,
can be calculated to be about 135 nm/s. It is remarkable that the
radial growth rate is about 3700 times faster than that of the axial
growth rate. This is because the measured average axial growth rate
has included the incubation time of nucleation of every step.
Without including the incubation time of nucleation, on the
basis of the measured radial growth rate of CoSi
2
of 135 nm/s, the
225
axial growth rate of each CoSi
2
atomic layer would have been about
1.82 nm/s. This axial growth rate should have been obtained by
V = Nvh
where V is axial growth rate, v is radial growth rate, N is number
of steps per unit length, and h is the height of the step. However,
this equation fails in describing the epitaxial growth in nanowires
when there is a long period of stagnation between each silicide layer
growth.
Figure 5.17
2
/Si interfaces. The
direction of the atomic layer motion is upward from the center
of the nanowire to the edge in NiSi and downward in CoSi
2
. The
smaller numbers are in units of 1/100 s. (c, f) The stair-type
growth curves for NiSi and CoSi
2
, respectively. The insets are
the distribution curves of incubation periods of nucleation.
Homogeneous Nucleation of Nanoscale Silicide Formation
226
Formation of Epitaxial Silicide in Silicon Nanowires
in situ
the overall growth rate of CoSi
2
and NiSi layers is linear,
141
yet we
can decompose the linear curve into many stair-steps where the
step height equals to an atomic layer thickness and the step width
equal to the incubation time of nucleating a new step.
103
Therefore,
nucleation stages and growth stages can be separated and the
repeating nucleation events can be used to study nanoscale silicide
nucleation in Si nanowires experimentally and theoretically.
5.4.2.2 Supply limit reaction
The incubation time of nucleation is required to create a new critical
nucleus for the formation of a new silicide layer. The incubation
time varies slightly. When a critical nucleus was created, it grew
very quickly across the Si/silicide interface and consumed nearly all
the supersaturated Ni or Co atoms in the Si nanowire. In repeating
nanowires to reach the supersaturation in order to nucleate a new
critical nucleus. So the supply limited dissolution of Ni or Co into
the Si at the point contact may become rate limiting. The overall
reaction rate is limited by the incubation time of nucleation, so it is a
reaction-limited.
limited and interfacial-reaction-limited reactions. In point contact
reactions, we shall consider supply limited reaction since it can be
the slowest kinetic process in the reaction.
143
Let J be the dissolution
and the unit of J is the number of atoms/cm
2
-s. Let be the area
of a point contact. Thus in a period of t, the number of metal atoms
dissolved into the Si nanowire is J()t. The growth of silicide will
consume the dissolved metal atoms. If we assume the axial or cross-
sectional silicide/Si interface area is A, and is the linear growth
rate, we have by mass conservation that
JAtCtAdu
()
=
()
(5.1)
where C is the concentration of metal in the silicide, and we have
J
A
A
C
d
u
Ê
Ë
Á
ˆ
¯
˜
=
(5.2)
of J = C<> where <
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