Chemical vapor deposition of Si nanowires nucleated by TiSi2 islands on Si

Article abstract of DOI:10.1063/1.125852

Silicon nanowires can be formed by chemical vapor deposition of Si onto Si substrates on which nanometer-scale, Ti-containing islands have been grown. At the growth temperatures used, the Ti-containing islands remain solid and anchored to the substrate, while the Si nanowires grow out from the islands, which remain at their bases. The nanowire growth mechanism, therefore, differs from the usual vapor-liquid-solid process and provides a potential route for the formation of oriented Si nanostructures or semiconductor-metal-semiconductor structures compatible with Si integrated circuits.

Full text of DOI:10.1063/1.125852

Chemical vapor deposition of Si nanowires nucleated by TiSi 2 islands on Si
T. I. Kamins, R. Stanley Williams, Y. Chen, Y.-L. Chang, and Y. A. Chang
Citation: Applied Physics Letters 76, 562 (2000); doi: 10.1063/1.125852
View online: http://dx.doi.org/10.1063/1.125852
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APPLIED PHYSICS LETTERS
VOLUME 76, NUMBER 5
31 JANUARY 2000
Chemical vapor deposition of Si nanowires nucleated by TiSi2 islands on Si
T. I. Kamins,a) R. Stanley Williams, Y. Chen, Y.-L. Chang, and Y. A. Changb)
Hewlett-Packard Laboratories, Palo Alto, California 94304
͑Received 23 August 1999; accepted for publication 6 December 1999͒
Silicon ‘‘nanowires’’ can be formed by chemical vapor deposition of Si onto Si substrates on which
nanometer-scale, Ti-containing islands have been grown. At the growth temperatures used, the
Ti-containing islands remain solid and anchored to the substrate, while the Si nanowires grow out
from the islands, which remain at their bases. The nanowire growth mechanism, therefore, differs
from the usual vapor-liquid-solid process and provides a potential route for the formation of oriented
Si nanostructures or semiconductor-metal-semiconductor structures compatible with Si integrated
circuits. © 2000 American Institute of Physics. ͓S0003-6951͑00͒02305-6͔
With the constantly decreasing feature sizes of
integrated-circuit devices, the need for increasingly fine,
lithographically defined patterning is limiting further ad-
vances of the technology. Consequently, a growing amount
of effort is being devoted to self-assembly techniques to
form switching elements without fine-scale lithography.1 The
self-assembled switching elements may be integrated on top
of a Si integrated circuit so that they can be driven by con-
ventional Si electronics in the underlying substrate. To ad-
dress the switching elements, interconnections or wires, pref-
erably also formed by self-assembly, are needed. The self-
assembled wires connecting the conventional electronics to
the self-assembled switching elements should be anchored at
locations defined by the underlying circuitry and should be
composed of materials compatible with Si integrated-circuit
processing.
Recent reports have shown that catalytic decomposition
of a Si-containing gas by a metal, such as Au or Fe, can form
long ‘‘nanowires.’’ 2,3 These studies were based on the ear-
lier work of Wagner and Ellis,4,5 who developed a technique
frequently called the vapor-liquid-solid ͑VLS͒ mechanism. A
liquid alloy droplet containing the metal and Si is located at
the tip of the wire and moves along with the growing end of
the wire. The wires may either be formed in the gas phase or
anchored at one end on a substrate.6,7 However, Au and Fe
migrate into Si rapidly and create deep levels, which can
degrade devices, such as addressing circuitry and other por-
tions of the system formed by conventional Si integrated-
circuit technology.
Ti-containing islands were formed by chemical vapor
deposition ͑CVD͒ on 150-mm diameter, Si͑001͒, and Si͑111͒
substrates in a commercially available, lamp-heated, single-
wafer reactor using TiCl4 in argon as the precursor for the Ti
͑Ref. 10͒ and a H2 ambient. The partial pressure of TiCl4 was
0.06 Pa (4.5ϫ10Ϫ4 Torr), and the total reactor pressure was
670 pa ͑5 Torr͒. Although the Ti deposition process is
selective,11,12 unpatterned wafers were used in this demon-
stration. In most cases, only TiCl4 was introduced from the
gas phase, with the Si being supplied from the Si substrate.
In a few cases, a Si-containing gas (SiH4 or SiH2Cl2) was
added during the deposition of the Ti to minimize consump-
tion of the Si substrate during TiSi2 formation. At the low
deposition temperatures of 640–670 °C used, the islands are
likely to be Ti-rich compared to TiSi2. After deposition, the
islands were sometimes annealed at a higher temperature
(ϳ920 °C) to reduce their density. On unpatterned wafers,
the island density after annealing is determined primarily by
the amount of Ti initially deposited, and the island size is
determined by the annealing temperature.10 The island com-
position after annealing at 920 °C is assumed to be stoichio-
metric TiSi2.
After Ti deposition and possible annealing, the tempera-
ture was set to the silicon deposition temperature ͑generally
640 or 670 °C), and the islands were exposed to a Si-
containing gas. SiH4 and SiH2Cl2 were used in different ex-
periments, with SiH2Cl2 having the potential advantage of
allowing selective Si deposition. After removing the sub-
strate from the reactor, the surface was examined by scan-
ning electron microscopy and/or atomic-force microscopy.
The location of the Ti within the wire was measured by
field-emission Auger analysis.
Titanium and TiSi2 are compatible with integrated-
circuit technology and are frequently used in Si circuits to
reduce resistance of silicon and polycrystalline-silicon con-
ducting regions. Although Ti forms deep levels in Si, its
solubility and diffusion coefficient in Si are low,8,9 and the
deep levels are not at midgap. With suitable handling, Ti is
generally accepted in integrated-circuit facilities. This report
demonstrates the formation of nanowires nucleated by Ti-
containing islands. The formation mechanism will be shown
to differ from the VLS mechanism generally reported.
Figure 1͑a͒ shows a sparse array of TiSi2 islands formed
on a Si͑001͒ substrate by Ti deposition at 640 °C, followed
by annealing at 920 °C for 30 min; Fig. 1͑b͒ shows corre-
sponding islands formed on a Si͑111͒ substrate. After expos-
ing similar substrates with annealed TiSi2 islands to a partial
pressure of 70 Pa ͑0.54 Torr͒ of SiH2Cl2 in a 2.7 kPa ͑20
Torr͒ hydrogen ambient at 640 °C, Si nanowires extended
from the TiSi2 islands, as shown in Fig. 2͑a͒ for Si͑001͒ and
in Figs. 2͑b͒ and 2͑c͒ for Si͑111͒. The Si nanowires nucle-
ated at the TiSi2 islands, rather than on the bare Si. Most of
the wires are curved, suggesting that they contain defects.
a͒
Electronic mail: kamins@hpl.hp.com
On leave from the Department of Materials Science and Engineering, Uni-
b͒
versity of Wisconsin, Madison, WI 53706.
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0003-6951/2000/76(5)/562/3/$17.00 562 © 2000 American Institute of Physics
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Appl. Phys. Lett., Vol. 76, No. 5, 31 January 2000
Kamins et al.
563
FIG. 3. Scanning-electron micrograph of long, dense Si nanowires nucle-
ated on Ti-containing islands grown at 670 °C on Si͑001͒ and not annealed.
The Si wires were grown at 670 °C for 395 s using SiH4.
FIG. 1. Atomic-force micrographs of TiSi2 islands formed by chemical
vapor deposition at 640 °C and annealed at 920 °C on ͑a͒ Si͑001͒ and ͑b͒
Si͑111͒. ͑Image size: 1 ␮m; z scale: 10 nm.͒
Some wires branch along their length. Less than 1 nm of Si
would be grown on bare Si under these conditions.
branch examined. These observations, together with the fact
that TiSi2 and Si are essentially insoluble in each other,8,9
lead to the conclusion that the wire is pure Si, with no ap-
preciable Ti at the tip to catalyze the decomposition of the
Si-containing gas. A straight wire was examined at both
ends, and a line scan was taken. Ti was observed at the
darker end of the wire ͑assumed to be the base͒, and not at
the brighter end ͑assumed to be the free end͒, confirming that
the Ti remains at the base of the wire for Si͑111͒. The Auger
signal from wires on the Si͑001͒ substrate was weaker, but
the measurements again suggest that Ti remains at the base
of the wire.
Because Ti remains at the base of the wire, Ti-catalyzed
decomposition of the Si-containing gas probably occurs,
with wire growth by extrusion from the base region. This
behavior differs from wire formation by Au and Fe catalysis
of the Si decomposition reaction, where the growth occurs at
the tip by the VLS process.4,5 In the VLS process, the metal
reacts with Si from a vapor source to form a low-melting-
temperature liquid droplet. This droplet then dissolves addi-
tional Si from the vapor source. When the solubility of Si in
the liquid droplet reaches saturation, Si precipitates from the
Figure 3 shows a Si͑001͒ substrate with a dense array of
nucleating islands deposited at 670 °C and subsequently ex-
posed to a partial pressure of 5 Pa ͑0.038 Torr͒ SiH4 at a total
pressure of 2.7 kPa ͑20 Torr͒ at 670 °C for an extended time
of 395 s. ͓The Ti-containing islands were not annealed be-
fore Si wire growth in this case and were much denser than
those shown in Fig. 1͑a͒.͔ A dense array of curved wires and
a few exceedingly straight wires were formed. The straight
wires are likely to be free of defects. From the scanning
electron micrographs, the median wire diameter appears to
be ϳ25 nm. One of the straight wires is 1.3 ␮m long and
ϳ22 nm in diameter, corresponding to an aspect ratio of
60:1.
To determine the composition of the nanowires, field
emission Auger measurements were performed. After mild
sputter cleaning ͑equivalent to removing about 2 nm of
SiO2) to reduce the amount of C and O on the sample sur-
face, a clear Ti Auger signal was seen for Si wires on a
Si͑111͒ substrate. Two wires were examined in detail with
both spot analysis and line scans. On a Y-shaped, branched
wire the base of the Y is very likely the end attached to the
substrate, with the two branches of the Y possibly arising
from coalescence of wires forming from two adjacent nucle-
ation sites on the same TiSi2 island during growth. As shown
in Fig. 4, a strong Ti Auger signal was observed at the base
of the Y. No Ti was detected at the growing end of the
FIG. 2. Scanning-electron micrographs of Si nanowires nucleated on an-
FIG. 4. Ti signal from Auger measurement ͑white line͒ superposed on scan-
ning electron micrograph of Y-shaped Si wire on Si͑111͒, showing Ti at the
wire base, but not at the tip.
nealed TiSi islands on ͑a͒ Si͑001͒ and ͑b͒ and ͑c͒ Si͑111͒. The Si wires
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were grown at 640 °C for ͑a͒ 160 s and ͑b͒ and ͑c͒ 40 s using SiH2Cl2.
128.123.35.41 On: Tue, 02 Sep 2014 13:06:13

564
Appl. Phys. Lett., Vol. 76, No. 5, 31 January 2000
Kamins et al.
liquid, leading to the growth of a thin Si wire, with the liquid
moving along with the tip of the wire. In the present case of
Ti and Si, the lowest temperature for a liquid eutectic to form
is ϳ1300 °C, far above the 640–670 °C deposition tempera-
ture of the Si. The enhanced, rapid decomposition of the
silicon containing gas (SiH4 or SiH2Cl2) presumably arises
from a catalytic effect of the small Ti-containing islands. The
decomposed Si immediately attaches to the Ti-containing
surface. The added Si probably first satisfies any Si defi-
ciency in the Ti-containing island to form stoichiometric
TiSi2. Additional incoming Si then diffuses to the surround-
ing Si, leading to the growth of the nanowire. Nucleation of
Si on the Ti-containing islands is consistent with the small
Ti/TiSi2 interfacial energy.13,14
If the decomposition reaction occurs at the base of the
wire, and Si grows on the surrounding Si with its normal
rate, the Ti-containing island is likely to eventually become
completely surrounded by the Si growing on the adjacent Si.
At that time, the Si-containing gas will no longer be able to
reach the Ti-containing island. The accelerated reaction will
cease, and the wire length will no longer increase rapidly.
The length of the wire will, therefore, be limited by the rela-
tive reaction rates on TiSi2 and on Si. This ratio is likely to
be greater at lower temperatures and to decrease as the tem-
perature increases, consistent with the observed limited wire
growth and thicker wires formed at the higher Si deposition
temperature of 920 °C. On the other hand, if deposited Si
atoms weakly bound to the surrounding Si surface diffuse to
the wire, growth may continue for an extended period of
time.
surface to form Si nanowires anchored at one end to the
substrate. The Ti appears to remain at the base of the grow-
ing wire. These wires are also likely to nucleate on Ti-
containing regions formed by conventional techniques on a
Si integrated circuit, allowing interconnection of conven-
tional circuitry with self-assembled elements.
The authors would like to thank M. Juanitas and G. Gi-
rolami for assistance with the measurements, Hewlett-
Packard’s ULSI Laboratory for use of deposition equipment,
and Dr. N. Shamma for CVD TiSi2 process information.
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