Direct growth of amorphous silica nanowires by solid state transformation of sio2 films

Direct growth of amorphous silica nanowires
by solid state transformation of SiO
2
films
Ki-Hong Lee
a,
*
, Hyuck Soo Yang
a
, Kwang Hyeon Baik
a
, Jungsik Bang
a
,
Richard R. Vanfleet
b
, Wolfgang Sigmund
a
a
Materials Science and Engineering Department, University of Florida, Gainesville, FL 32611, USA
b
Advanced Materials Processing and Analysis Center, University of Central Florida, Orlando, FL 32816, USA
Received 16 September 2003; in final form 5 November 2003
Published online: 5 December 2003
Abstract
Amorphous silica nanowires (a-SiONWs) were produced by direct solid state transformation from silica films. The silica
nanowires grow on TiN/Ni/SiO
2
/Si substrates during the annealing in H
2
or a H

2
:CH
4
mixture at 1050 °C. Titanium nitride (TiN)
films were used to induce a solid state reaction with silica (SiO
2
) films on silicon wafers to provide silicon atoms into growing
nanowires. The TiN layers induce the diffusion of silicon and oxygen to the surface by a stress gradient built inside the films. The
nickel diffuses to the surface during the TiN deposition and acts as a nucleation site for the a-SiONWs.
Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction
Formation of a liquid phase has been an essential
factor for the growth of one-dimensional nanowires by
the vapor–liquid–solid (VLS) [1–3] or the solid–liquid–
solid (SLS) mechanism [4]. The liquid phase acts as a
source for dissolution and re-precipitation of compo-
nents for the growth of nanowires. Amorphous semi-
conducting materials, such as Si–C–H, can be synthesized
with various compositions, to manipulate the optical
properties in an extremely wide range [5]. Amorphous
silica is widely used in silicon based integrated devices
and can also be produced as nanowires. Yu et al. [6]
showed that a-SiONWs emit blue light and might hence
be applied in integrated optical devices.
The VLS and the SLS mechanism have been an act-
ing mechanism for the growth of silica nanowires [7,8].
In this work, a novel growth mechanism for a-SiONWs
is presented via direct solid state transformation from
silica films. Titanium nitride (TiN) films were used to
induce a solid state reaction with the silica (SiO

2
) films
on silicon wafers to provide silicon atoms into growing
nanowires. The TiN layers induce the diffusion of silicon
and oxygen to the surface by a stress gradient built in-
side the films. The nickel diffuses to the surface during
the TiN deposition and acts as a nucleation site for the
a-SiONWs.
2. Experimental
N-type silicon h100i wafers (3 X cm, 1 Â 1 cm) were
used as substrates for the growth of SiONWs. After
thermally oxidizing the Si substrates, Ni films of 5 nm
were deposited on the oxide layer by e-beam evapora-
tion. TiN films were deposited on the nickel films by
laser ablation of a TiN target (99.9%). The ablation was
carried out using a KrF excimer laser (k ¼ 248 nm) at a
fluence of 1 J/cm
2
, and a repetition rate of 10 Hz. The
TiN film (30–50 nm) was deposited at 650 °C under
evacuated atmosphere (<8 Â 10
À6
Torr) without flowing
nitrogen gas.
A quartz tube furnace with diameter of 1.5
00
was used
for silica nanowire synthesis. Annealing of the substrates
Chemical Physics Letters 383 (2004) 380–384
www.elsevier.com/locate/cplett

*
Corresponding author. Fax: +13528463355.
E-mail address: khonglee@ufl.edu (K H. Lee).
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2003.11.056

was carried out in two conditions. After annealing in a
H
2
:Ar (200:150 sccm) mixture for 10 min at 1050 °C,
then a H
2
:CH
4
(200:10 sccm) mixture incorporated into
the system for 10 min. Another approach is to flow a
H
2
:Ar (200:150 sccm) mixture gas for longer times
without the incorporation of CH
4
. The annealing time
was increased to 80 min in this case. Ar (800 sccm) was
kept flowing through the quartz tube to purge the sys-
tem during the heating and cooling.
A field emission scanning electron microscope (FE-
SEM, JEOL 6335F) was used to investigate the growth
characteristics of a-SiONWs on the substrates. A
transmission electron microscope (TEM, JEOL 2010F)

equipped with an energy dispersive spectroscope (EDS),
was used for structure and composition analysis. Elec-
tron energy loss spectroscopy (EELS, Tecnai F30) was
carried out for further characterization of the nano-
wires. Surface analysis of the substrates was carried out
by Auger electron spectroscopy (AES, Perkin–Elmer
PHI 660) at an acceleration voltage of 8 keV. The sub-
strates for the AES analysis were transferred to the
system after exposing to air.
3. Results and discussion
A-SiONWs were synthesized by simply annealing
TiN/Ni/SiO
2
/Si substrates in a H
2
:Ar or a CH
4
:H
2
mixture gas. Fig. 1 show FESEM photo graphs after
annealing the substrates at 1050 °C in two gas condi-
tions. Fig. 1a shows silica nanowires grown on the
substrate after annealing in H
2
for 10 min followed by
CH
4
:H
2
for 10 min. More process time is necessary for

the synthesis of the nanowires without CH
4
. Large
density of the nanowires was achieved without CH
4
with
a longer process time, as shown in Fig. 1b. The seed
particles are attach ed to the top of the grown nanowires
as shown in the insets.
The phase and the structure of the nanowires are
identified by high resolution transmission electron mi-
croscopy (HRTEM), electron dispersive spectroscopy
(EDS), as well as electron energy loss spectroscopy
(EELS). One structural difference of the nanowires be-
tween the two gas mixture conditions is that an amor-
phous carbon shell is formed around silica nanowires
when CH
4
is incorporated into the system. Fig. 2a shows
a nanowire of Fig. 1a in scanning TEM (STEM) mode
used for the EELS analysis. The EELS line scanning
profiles show composition changes for silicon and car-
bon across the nanowire (Fig. 2b). The fine structure
EELS of the silicon 2p edge from the inner phase reveals
the formation of amorphous silica nanowire (Fig. 2c);
Carbon 1s edge band form the outside shell shows an
amorphous carbon phase (Fig. 2d). No titanium and
nitrogen were detected by the EELS in the nanowires.
The carbon on the shell is supplied by thermal decom-
position of CH

4
by nickel. No carbon shells were ob-
served from the silica nanowires grown in the Ar:H
2
gas
mixture (not shown here).
Titanium nitride films should play an important role
on the growth of silica nanowires. No nanowires were
observed without TiN films on the substrates with the
same annealing processes. Titanium nitride is reduced to
Ti in hydrogen atmosphere at high temperature. Tita-
nium has a higher tendency to oxidize than silica films,
resulting in formation of a titanium oxide (TiO
x
) phase
by removal of oxygen from the silica layer [9]. The re-
duction of TiN seems to be a critical factor in the growth
kinetics of a-SiONWs. The growth of silica nanowires
was limited by introducing ammonia (NH
3
) into the
system to suppress the decomposition of TiN films. The
Ni islands act as a nucleation site for the a-SiONW
growth, and expedite the reduction of TiN by supply-
ing extra hydrogen by thermal decomposition of
Fig. 1. FESEM photographs of a-SiONWs nanowires grown on the substrates after annealing at 1050 °C in: (a) H
2
(200 sccm) for 10 min followed by
CH
4

:H
2
(10:200 sccm) for 10 min; (b) H
2
:Ar (200:150 sccm) for 80 min. The arrows represent the seed particles attached to the top of nanowires.
K H. Lee et al. / Chemical Physics Letters 383 (2004) 380–384 381
CH
4
, which explains the faster growth of a-SiONWs
with CH
4
.
Nickel atoms diffuse out to the surface during the
deposition of TiN. The AES profile of the substrate
surface right after the laser ablation reveals the existence
of nickel on the surface, as shown in Fig. 3. Silicon at-
oms diffuse out to the surface and form the nanowires
during the annealing at 1050 °C. The AES of the sub-
strate after annealing at 1050 °C for 20 min in Ar:H
2
shows the appearance of silicon at the surface. Silica
nanowires nucleate on the nickel islands and grow on
the surface by silicon diffusion from the under layer SiO
2
films. Fig. 4 shows a nanowire grown on the substrate
and EDS spectra showing compositions in each layer in
the structure at the same process condition with Fig. 3.
As shown in the TEM photographs, the nanowires begin
to grow at this stage even though they are not observed
using the FESEM.

The growth behavior of the nanowires and the EDS
spectra of the layers in Fig. 4 show several facts which
would not be observed by the VLS or the SLS mecha-
nism. As shown in Fig. 4a (also can be seen in Fig. 1),
the metal particles are attached to the end of the
nanowires, supporting the top growth mode. Catalyst
Fig. 3. AES profiles from the substrates after the deposition of TiN
films, and after annealing in the H
2
:Ar (200:150 sccm) mixture for
20 min. The insets show the Ni (LMM) transition peak.
Fig. 2. EELS spectrum profiles of a nanowire synthesized in the CH
4
:H
2
mixture: (a) a STEM photograph of the nanowire on a TEM grid used for
the EELS analysis. The arrow shows a carbon wire grown from the surface amorphous carbon by a focused electron beam scanning cross the wire;
(b) intensity profiles of Si and C cross the wire showing the composition variation cross the wire. Oxygen has a similar profile with Si (not shown
here); (c) a Si 2p EELS profile from inside the nanowire indicating the formation of an amorphous silica phase; (d) a carbon 1s profile from the
outside shell indicating a amorphous carbon phase.
382 K H. Lee et al. / Chemical Physics Letters 383 (2004) 380–384
materials should be at the surface of the substrate in
order that source atoms dissolve and precipitate as a
nanowire on a catalyst liquid droplet, called Ôbase
growthÕ, as in the SLS mechanism. The existence of
metal particles on the top of the nanowires illustrates
that the growth is not established by the SLS mecha-
nism. Silicon is present after annealing in the original
TiN layer; otherwise, nickel is not present in the layer
(Fig. 4d). The parti cle is composed of nickel mainly

(Fig. 4b), which is unlikely to form a liquid phase at the
process temperature. Silicon was not detected in the
particles attached to the nanowires by the EELS (not
shown here). As a result, there is little possibility to form
a liquid phase either in the supporting layers (the TiN
films) or in the seed particles. In addition, silicon sources
were not incorporated into the system directly from
vapor phase. These facts show that it is unlikely for the
nanowires to grow by the VLS mechanism or the SLS.
The under layer silicon oxide film is the only Si source
for the growth of a-SiONWs in the system. Silicon
should be supplied to growing silica nanowires by solid
state diffusion through the TiN layer. The TiN layers
decompose into Ti and form an oxide by the substitu-
tion reaction with silica. Reduced silicon can form a
nickel silicide with Ni remaining in the interface (Fig. 4e)
or diffuse out to the surface to form the silica nanowires.
(Fig. 4c) The Si diffusion can be derived by a stress
variation built in the TiN layer during the annealing
processes. The substitutional reaction initiates at the
interface between the TiN and the silica, thereby, it
builds a compressive stress in the interface region and a
tensile stress in the surface. NH
3
gas suppresses the
decomposition of TiN, as a result, it limits the growth of
a-SiONWs nanowires by the mechanism. Oxygen seems
Fig. 4. (a) A TEM photograph of a cross section of the substrate with the same treatment condition as Fig. 3; (b)–(f) show EDS data from each layer
indicated in (a). It is difficult to define the existence of nitrogen in the TiN layer since it is close to the oxygen and the Ti(L) peak. However, nitrogen is
considered to be present in the layer at this annealing stage by the AES in Fig. 3.

Fig. 5. A schematic diagram showing a growth mechanism of silica
nanowires by the solid–solid transformation.
K H. Lee et al. / Chemical Physics Letters 383 (2004) 380–384 383
to be supplied either from the silica or the vapor
phase. Fig. 5 shows a schematic diagram showing the
growth mechanism of a-SiONWs in our experimental
condition.
4. Conclusion
In summary, silica nanowires were synthesized by
solid state diffusion of silicon from the silica films. The
growth mechanism could be exp lained by direct solid to
solid phase transformation, so called, the SS mecha-
nism. The TiN films react with the silica films to produce
a silicon source for the nanowires and cause the silicon
diffusion by the internal stress. Our result suggests a
novel growth mechanism for growth of nanowires, and
can be applied to the synthesis of other kind of nano-
wires.
Acknowledgements
This work was supported by DARPA/Army Re-
search Office under Grant No. DAAD19-00- 1-0002
through the center for materials in sensors and actuat ors
(MINSA). The authors thank Kerry Siebein (of the
Major Analytical Instrumentation Center at the Uni-
versity of Florida) for the TEM and the EDS analysis.
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