Self-assembly of TiSi nanowires on TiSi2 thin films by APCVD

Article abstract of DOI:10.1016/j.jallcom.2011.04.108

Titanium silicide thin films and nanowires (NWs) were prepared on a glass substrate using the APCVD method. Gaseous SiH₄ and TiCl₄ were used as precursors for Si and Ti, respectively. TiSi₂ thin films were precipitated on

Full text of DOI:10.1016/j.jallcom.2011.04.108

Journal of Alloys and Compounds 509 (2011) 7519–7524
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Self-assembly of TiSi nanowires on TiSi thin films by APCVD
2
∗
Zhaodi Ren, Peng Hao, Jun Du, Gaorong Han, Wenjian Weng, Ning Ma, Piyi Du
State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Titanium silicide thin films and nanowires (NWs) were prepared on a glass substrate using the APCVD
method. Gaseous SiH4 and TiCl4 were used as precursors for Si and Ti, respectively. TiSi2 thin films were
precipitated on the glass substrate first, and then single-crystal TiSi NWs were grown on the TiSi2 thin
films as-prepared by controlling the concentrations of the source gases, SiH4 and TiCl4, without using
any catalysts. The TiSi NWs were typically 1–2 ␮m long and 15–50 nm thick. The growth direction of
the NWs was perpendicular to the (1 1 0) plane, in which a competition in growth rate among different
crystallographic planes of the TiSi crystalline phase occurs. The oriented growth of the TiSi crystalline
phases is responsible for the formation of the NWs.
Received 3 December 2010
Received in revised form 19 April 2011
Accepted 20 April 2011
Available online 27 April 2011
Keywords:
Nanostructured materials
Titanium silicide
Vapor deposition
Crystal growth
Anisotropy
©
2011 Elsevier B.V. All rights reserved.
1
. Introduction
found to exhibit excellent field emission properties, despite Ti5Si3
showing a slightly higher resistivity [5].
Metal silicides (MS) have received great attention for years
The field emission properties of the TiSi NWs covering the TiSi2
thin films are expected to be improved because the resistivity of
due to their application in very-large-scale integration (VLSI) tech-
nology and their low resistivity and high thermal stability [1–3].
Among the metal silicides, titanium silicides exhibit the lowest
resistivity and higher chemical stability than the others. Thus, they
are used not only as an ohm contact material but also as a cold
cathode material in field emission devices [4,5]. Actually, several
different phases of Ti Si, Ti5Si , Ti5Si , TiSi, and TiSi are formed
the TiSi phase is lower than that of Ti5Si . Therefore, it is impor-
3
tant to investigate the formation of TiSi NWs on TiSi2 thin films.
Furthermore, the CVD method is one of the most widely used
methods for preparing nanostructures with high purity [14]. Many
freestanding titanium silicide NWs, such as TiSi2 [15], TiSi [4,16],
and Ti5Si3 NWs, have been successfully synthesized by the CVD
method. The atmospheric pressure CVD (APCVD) method accepts
full gaseous sources instead of consuming the Si or Ti substrate
used normally in the CVD method and represents a cost-effective
process for continuous large-scale production on many required
substrates. Self-assembled NWs, such as TiSi NWs on Ti5Si3 thin
films/glass substrates, have been successfully synthesized by the
APCVD method [16–19]. In this paper, TiSi NWs synthesized on a
3
4
3
2
under different conditions as shown in the Ti–Si phase diagram [6].
These titanium silicide phases show different structural and electri-
cal properties. TiSi [7] has a face-centered orthorhombic structure
2
and a resistivity as low as 16–20 ␮ꢀ cm. It is mainly used as an
interconnect. Ti5Si [8–10], however, has a hexagonal structure and
3
shows a resistivity of 50–120 ␮ꢀ cm, which is slightly higher than
that of TiSi . It is a promising material for high-temperature appli-
2
cations. Another compound, TiSi [4], has an orthorhombic structure
TiSi base layer on a glass substrate by the APCVD method are inves-
2
and is found to have a resistivity of 60 ␮ꢀ cm, which is lower than
tigated in detail. The growth mechanisms for the formation of TiSi
NWs on the TiSi2 thin film are also considered.
that of Ti5Si . With the development of device miniaturization, one-
3
dimensional nanostructures have received much attention over
the past decades because of their unique properties and potential
applications [11–13]. Thus, titanium silicide nanostructures have
attracted extensive interest due to their low resistivity, high ther-
mal stability, large curvature at the top of the nanowires (NWs),
and large surface area. TiSi2 thin films covered by Ti5Si3 NWs are
2
.
Experimental details
Gaseous SiH4 and liquid TiCl4 were used as the Si and Ti precursors and gaseous
◦
N2 was used as the dilution gas, respectively. Liquid TiCl4 was heated to 38–41 C via
water bath until it vaporized enough, and then the vapor was carried by N2 flow for
the reaction. The SiH4, TiCl4 and N2 were transported initially into a stainless steel
chamber with a diameter of 5 cm and a length of 26.5 cm to mix for 40 s. Then they
were introduced onto the glass substrates via a long quartz tube with a showerhead
at its end for deposition. The samples were deposited in a vacuum chamber with a
4
pressure of 7–9 × 10 Pa. The deposition process can be divided into two steps: first,
∗ Corresponding author. Tel.: +86 571 87952324; fax: +86 571 87952324.
E-mail address: dupy@zju.edu.cn (P. Du).
a titanium silicide thin film was deposited, and second, titanium silicide NWs were
formed, and the sample was kept in the vacuum chamber during the deposition. In
0
925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.04.108

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Z. Ren et al. / Journal of Alloys and Compounds 509 (2011) 7519–7524
Fig. 2. Cross-sectional image of sample II. The white arrows indicate the NWs grown
on the thin film. The white cross represents the point at which the EDX analysis is
probed (see Fig. 3 below).
Fig. 1. XRD patterns of the titanium silicide thin films and nanowires (NWs) of
sample I and sample II. The inset shows the XRD pattern of sample II with 2ꢁ between
3
Fig. 4a, there are no spherical particles at the tip of the nanowire,
which is normally reported for NWs grown by the VLS mechanism
◦ ◦
5 and 55 .
[
20]. It can be deduced that no catalysts were used during the for-
the first step, the gases were transported onto the glass substrate at a temperature of
◦
mation processes. The high-resolution TEM image of the nanowire
is shown in Fig. 4b. The inset in Fig. 4b shows the selected area
electron diffraction (SAED) pattern of the NWs, showing reciprocal
lattice peaks along the different lattice directions, which is indexed
to be orthorhombic single-crystal TiSi phase. The growth front of
the NWs is along the direction that is perpendicular to its (1 1 0)
plane, as shown in Fig. 4a and b. The TEM morphology of a single
nanowire of sample II is exhibited in Fig. 4c. No particle is observed
at the top of the nanowire. Fig. 4d shows the high-resolution TEM
image of the nanowire of sample II. The inset in Fig. 4d shows the
selected area electron diffraction (SAED) pattern of the nanowire. It
exhibits the formation of the orthorhombic TiSi single-crystal NWs.
The growth front of the NWs is along the specific direction that is
perpendicular to their (1 1 0) planes, as exhibited in Fig. 4c and d.
As a result, the TiSi2 thin films were formed on the glass substrates,
and then the TiSi single-crystal NWs were successfully formed on
the TiSi2 thin films as-prepared.
Fig. 5a–c shows the SEM images of sample I at different magni-
fications while Fig. 5d shows the morphology of sample II. A large
area of NWs was formed on the thin films, as shown in Fig. 5a. The
NWs had a length of 1–2 ␮m, as shown in Fig. 5b and c for sam-
ple I and in Figs. 2 and 5d for sample II. The thickness of the NWs
was between 15 and 50 nm, as shown in Fig. 5c for sample I and
Fig. 5d for sample II. Both the thickness and the length of the NWs
for sample I and sample II have no obvious differences, although
the deposition conditions were varied.
7
4
20 C for 3 min for sample I and 15 s for sample II, with a mole ratio of SiH4/TiCl4 of
for the deposition of the TiSi2 thin films. The flow rates of SiH4, TiCl4, and N2 were
maintained at 21.2 sccm, 5.3 sccm, and 973.5 sccm, respectively, for sample I and
2.8 sccm, 13.2 sccm, and 734 sccm, respectively, for sample II. The flow rates of SiH4
5
and N2 were controlled by a rotameter and a mass control flowmeter, respectively,
and the flow rate of TiCl4 was controlled by the flow rate of the N2 carrier gas. In
the second step, the total gas flow rate and the deposition temperature were kept
the same as those in the first step, while the ratio of SiH4 to TiCl4 was modulated
to approximately 3. The flows of the reactive gases of SiH4 and TiCl4 were stopped
while the N2 was introduced into the gas mixing chamber to maintain the total gas
flow rate of 1000 sccm for sample I and 800 sccm for sample II for additional reaction
times of 15 min and 1 min, respectively. The concentrations of the reactive gases of
SiH4 and TiCl4 were diluted naturally and exponentially during the second step.
After the two deposition steps, the NWs covering on the thin films were deposited
on the glass substrate.
Field emission-scanning electron microscopy (FE-SEM, SIRON, FEI) was used to
observe the morphology of the samples. A Rigaku D/max diffractometer with Cu
K␣ radiation was used to characterize the structures of the NWs. The morphology
of the NWs was observed by TEM. The microstructure of the NWs was evaluated
using a JEOL-2010 high-resolution transmission electron microscopy (HRTEM). The
chemical composition of the samples was analyzed using energy dispersive X-ray
spectroscopy (EDX). The samples of the NWs for observation by TEM and HRTEM
were prepared as follows: first, the samples were immersed in an ethanol solution
and dispersed by an ultrasonic oscillator for 15 min; second, the dispersed solutions
containing the NWs were dropped on a carbon-covered copper grid; and third, the
copper grids were used for observation after drying.
3
. Results and discussion
Fig. 1 shows the XRD patterns of sample I and sample II deposited
at a flow rate ratio of SiH /TiCl4 of 4. The inset in Fig. 1 shows
4
◦
◦
the XRD pattern of sample II with 2ꢁ between 35 and 55 . A TiSi
2
crystalline phase with face-centered orthorhombic structures was
formed in these two samples, as shown in Fig. 1. In addition, a small
amount of TiSi crystalline phase obviously formed in sample I, as
shown in Fig. 1. The quantity of the TiSi crystalline phase in sample
2
I is higher than that of sample II, as exhibited in Fig. 1. Fig. 2 shows
the cross-sectional image of sample II as-deposited. The three lay-
ers of the glass substrate, the thin film and the NWs are present
from bottom to top, and the thickness of the thin film is approxi-
mately 500 nm. This implies that the thin films form on the glass
substrates in the first stage, and then the NWs grow on the thin
films during the second stage. Fig. 3 shows the EDX spectrum of
the thin film. The probe position is marked as a “+” in Fig. 2. The
results of the EDX reveal that the atomic ratio of Si to Ti in the thin
film is 2.3:1, which shows the formation of the TiSi2 phase in the
thin films in addition to the TiSi phase as the Si content in the TiSi2
phase is higher than that of the TiSi phase. The TEM morphology of
a single nanowire of the samples is exhibited in Fig. 4. As shown in
Fig. 3. EDX analysis at a point marked by a “+” as shown in Fig. 2.

Z. Ren et al. / Journal of Alloys and Compounds 509 (2011) 7519–7524
7521
Fig. 4. The TEM images of a single TiSi nanowire of (a), (b) sample I and (c), (d) sample II. The insets in Fig. 4(b) and (d) show the selected area electron diffraction (SAED)
patterns of sample I and sample II, respectively.
During the first step, source gases with a flow rate ratio of
SiH /TiCl of 4 were used for deposition. The reactions probably
occurred thermodynamically as shown below.
The Gibbs free energy of that reaction is −1037.412 KJ at 1000 K,
which is also very low. The Ti5Si3 formed from reaction (3) most
likely serves as one of the reactants in reaction (4) to form ulti-
4
4
mately TiSi . Therefore, the TiSi2 thin films are finally formed on
the glass substrates in this case.
2
4
SiH (g) + TiCl (g) = TiSi (s) + SiH Cl(g)
4
4
2
3
+
SiHCl (g) + 6H (g)
(1)
(2)
Moreover, as exhibited in Fig. 1, the quantity of the TiSi2 crys-
talline phases in sample I is higher than that of sample II. This can be
attributed, in fact, to a thickness limitation with regard to measur-
ing the crystallinity in the thin film. Actually, there is a limitation on
the detection of thickness when using XRD technology to measure
thin films. If the thin film dimension is larger than the limitation,
then the sample thickness is not an indicator of crystallinity. How-
ever, the thicknesses of the thin films that were prepared are on
the order of several micrometers, which did not reach the thickness
limitation for XRD measurements in this work. The peak intensity in
the XRD pattern will thus increase with the increase in the thickness
of the thin films rather than with the crystallinity. In fact, the thick-
ness of the thin films formed in sample I is approximately 2 ␮m,
which is about 4 times thicker than that of sample II. The larger
quantity of the crystalline phase in sample I is attributed appar-
ently to the greater thickness of the thin film with a long deposition
time, although the concentration of the source gases in sample I is
lower than that of sample II. Because the quantity of the crystalline
phase is mainly generated by the increase in thickness, the TiSi2
thin films deposited at the first stage under different conditions
3
2
4
4
SiH (g) + TiCl (g) = TiSi (s) + SiCl (g) + Si(s) + 8H (g)
4
4
2
4
2
SiH (g) + TiCl (g) = 1/5Ti5Si (s) + 17/5SiH Cl(g)
4
4
3
3
+
3/5HCl(g) + 13/5H (g)
(3)
2
The Gibbs free energies [21] of reactions (1)–(3) are −449.079 KJ,
−
504.419 KJ, −414.973 KJ at 1000 K, respectively. Although all of
the reactions shown above are likely to occur, the reaction shown
in Eq. (2) is most likely to happen thermodynamically compared to
those of the other equations due to its negative free energy with
the largest absolute value of −504.419 KJ at 1000 K. Furthermore,
no Ti5Si crystalline phase was found other than the TiSi2 phase
3
as shown in Fig. 1. This indicates that the reaction below probably
occurs following reaction (3)
Ti5Si (s) + 9SiH (g) + TiCl (g) = 6TiSi (s) + 4HCl(g)
3
4
4
2
+
16H (g)
(4)
2

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Z. Ren et al. / Journal of Alloys and Compounds 509 (2011) 7519–7524
Fig. 5. SEM images of (a)–(c) sample I with different magnifications and (d) sample II, respectively.
likely show the same quantity of the TiSi crystalline phase in their
nano-islands are thus formed widely on the TiSi2 nucleating layer.
The TiSi nano-islands are very active as the dimension of the TiSi
islands is on the nano-scale, and thus, they present a quasi-liquid
state character and dissolve atoms, as demonstrated in our previous
papers [16,17]. The Ti and Si atoms can be dissolved into the quasi-
liquid nano-islands easily with continuous feeding of the source
2
surfaces. Therefore, the TiSi crystalline phase-contained thin films
2
deposited in the first step are used as a layer supplying nucleation
site, inducing the formation of stable NWs on the thin layers in a
way almost independent of the deposition conditions used in this
work.
After forming the crystalline layer of TiSi , source gases with a
gases of SiH4 and TiCl . Finally, the TiSi phases grow continuously
2
4
flow rate ratio of SiH /TiCl4 of 3 were used, and the concentrations
of the reactive gases of SiH4 and TiCl4 in the total gas were diluted
naturally and exponentially. As a second step, the reaction is likely:
by supersaturating the Ti and Si atoms in the separating TiSi nano-
islands, except for some that grow with the depositing of a few
atoms on the surface of the TiSi phases.
4
In fact, it is very difficult for the nuclei to grow up and form a
continuous thin film in this case due to the anisotropic growth of
the crystalline phase. The lower the surface energy is, the easier
the surface will stretch out as an outer plane. As a result, the TiSi
phase would be formed with the lowest surface energy. When the
TiSi crystalline phase begins to form on the nano-islands, a compe-
tition in the growth rate among different crystallographic planes
begins. The larger the number of atoms per unit area of the plane
is, the lower the surface energy of the crystallographic plane will
be. As seen in Fig. 7, it has been calculated that the numbers of
atoms per unit area of the (0 0 2) and (1 1 0) planes of the TiSi crys-
talline phase are the largest and the second largest, respectively.
Therefore, the surface-free energies of the (0 0 2) and (1 1 0) planes
are the lowest and the second lowest, correspondingly. As the TiSi
nucleus forms, any of the crystallographic planes will be exposed
as an outer surface plane of the crystalline phase. However, a plane
that has the lowest surface energy is the easiest to form and the
easiest to grow as an outer surface. As the TiSi crystalline phase
forms, the (0 0 2) and the (1 1 0) planes are expected to grow as
outer surfaces more easily compared to other planes. However, the
surface-free energy of the (0 0 2) plane is lower than that of the
(1 1 0) plane. The increase in surface area of the (0 0 2) plane rather
than of the (1 1 0) plane will lower the surface energy per unit area
of the TiSi crystalline phase. Therefore, the surface area of the (0 0 2)
plane increases spontaneously and rapidly, while the surface area of
1
5SiH (g) + 5TiCl (g) + 2TiSi (s) = 7TiSi(s) + 12SiH Cl(g)
4
4
2
3
+
8HCl(g) + 8H (g)
(5)
2
where the Gibbs free energy of the reaction at 1000 K is
1455.899 KJ [21]. This means that the reaction is thermodynami-
−
cally favorable.
In traditional VLS mechanisms for NW preparation, metal cata-
lysts often tend to form a liquid alloy droplet with a relatively low
solidification temperature. A metal-containing liquid nanoparticle
normally forms at the growth frontier of the NWs, acting as a cat-
alytic active site. However, there are no particles being observed at
the ends of the NWs in this work, as shown above. In the system
of Ti and Si, the lowest liquid eutectic point of Ti and Si shown on
◦
the equilibrium phase diagram is 1330 C, which is markedly above
◦
7
20 C, the temperature at which the titanium silicide NWs form in
this work. It seems that the liquid alloy droplet cannot be formed if
there are no catalysts used in this case. Therefore, the growth of the
TiSi NWs cannot be explained by the traditional VLS mechanisms,
which are widely used to illustrate the growth of NWs.
Actually, as shown in Fig. 6, when the source gases of SiH4 and
TiCl4 are introduced onto the nucleating layer of TiSi2 during the
second step, the TiSi nuclei are most likely formed on the TiSi2
layer, according to reaction (5). The TiSi nuclei serve unambigu-
ously as seeds for further growth of the TiSi crystalline phase. TiSi

Z. Ren et al. / Journal of Alloys and Compounds 509 (2011) 7519–7524
7523
Fig. 6. Proposed growth process for the formation of the titanium silicide thin films and NWs. (a) Deposition of the preliminary layer of TiSi2 thin films on the glass substrate,
(
b) Formation of the TiSi nano-islands on the TiSi2 thin films, (c) “self-catalysis” growth of the TiSi NWs and (d) the final TiSi NWs formed on the TiSi2 thin films.
ating layer of the face-centered orthorhombic TiSi crystalline layer
2
by a two-step APCVD method without using any catalysts. The for-
mation of the TiSi NWs originates from the TiSi nano-islands that
are induced by the nucleating layer of the synchronously deposited
TiSi crystalline phase under favorable thermodynamic conditions.
2
As the source gases dissolve in the Ti–Si nano-islands, the TiSi crys-
talline phases precipitate and grow continuously with both the
supersaturation of Ti and Si atoms in the nano-islands and the
deposition of Ti and Si atoms directly on the surfaces of the TiSi
crystalline phases that form. The anisotropic growth of the TiSi
crystalline phases occurs due to the different surface-free energy,
which result in the formation of the NWs and the growth of the NWs
along the directions perpendicular to the (1 1 0) planes. The “self-
catalysis” mechanism dominates the growth of the TiSi NWs in the
case of the APCVD processes. The TiSi NWs combined with the TiSi2
thin films are expected to be used as assembled nano-electrodes in
future multifunctional nano-devices.
Fig. 7. Schematic representation of the TiSi structured cell.
Acknowledgments
This work is supported by NSFC (Grant No. 50672084),
the National Key Scientific and Technological Project (Grant
No.2009CB623302) and Fundamental Research Funds for the Cen-
tral Universities, China.
the (1 1 0) plane, being perpendicular to the (0 0 2) plane, increases
only slightly. Consequently, served by the quasi-liquid Ti–Si nano-
islands as seeds, the TiSi NWs form and grow along the direction
perpendicular to the (1 1 0) planes of the TiSi crystalline phases with
an increase in the surface area of the (0 0 2) planes, as exhibited in
Fig. 4b and d. Simultaneously, Ti and Si atoms will deposit on the
outer surface of the NWs directly and contribute to the continuous
growth of the NWs. The formation process of the TiSi NWs by the
two-step method is graphically represented in Fig. 6. The formation
of the TiSi NWs without using any catalysts is called “self-catalysis”
growth, which is similar to that of the “VS” mechanism [22].
Finally, a large quantity of TiSi NWs is formed by the two-step
method, as shown in Fig. 5. The NWs are formed in the second step,
in which the nucleation of the NWs in the thin film relates to the
modulation of both the total concentrations of the source gases and
the deposition times within a certain range in the first step. Neither
the thicknesses nor the lengths of the NWs for sample I and sample
II show any difference between the two samples, despite the depo-
sition conditions being varied. This shows that the morphology of
the NWs formed in this case is almost stable.
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