Crystalline silicon core fibres from aluminium core preforms

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Traditional fibre-optic drawing involves a thermally mediated geometric scaling where both the fibre materials and their relative positions are identical to those found in the fibre preform. To date, all thermally drawn fibres are limited to the preform composition and geometry. Here, we fabricate a metre-long crystalline silicon-core, silica-cladded fibre from a preform that does not contain any elemental silicon. An aluminium rod is inserted into a macroscopic silica tube and then thermally drawn. The aluminium atoms initially in the core reduce the silica, to produce silicon atoms and aluminium oxide molecules. The silicon atoms diffuse into the core, forming a large phase-separated molten silicon domain that is drawn into the crystalline silicon core fibre. The ability to produce crystalline silicon core fibre out of inexpensive aluminium and silica could pave the way for a simple and scalable method of incorporating silicon-based electronics and photonics into fibres. The integration of silicon optoelectronic devices in a fibre platform has great potential, but drawing such fibres is difficult. Using a simple, low cost and scalable method, Hou et al. fabricate a metre-long crystalline silicon-core, silica-clad fibre from a preform not containing elemental silicon.

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### ARTICLE

Received 8 Aug 2014 | Accepted 9 Jan 2015 | Published 20 Feb 2015 DOI: 10.1038/ncomms7248
## Crystalline silicon core fibres from aluminium core preforms

Chong Hou 1,2,3, Xiaoting Jia 2,3, Lei Wei 4, Swee-Ching Tan 5, Xin Zhao 1, John D. Joannopoulos 2,3,6 & Yoel Fink 1,2,3,7

Traditional fibre-optic drawing involves a thermally mediated geometric scaling where both

the fibre materials and their relative positions are identical to those found in the fibre preform.

To date, all thermally drawn fibres are limited to the preform composition and geometry.

Here, we fabricate a metre-long crystalline silicon-core, silica-cladded fibre from a preform

that does not contain any elemental silicon. An aluminium rod is inserted into a macroscopic

silica tube and then thermally drawn. The aluminium atoms initially in the core reduce the

silica, to produce silicon atoms and aluminium oxide molecules. The silicon atoms diffuse into

the core, forming a large phase-separated molten silicon domain that is drawn into the

crystalline silicon core fibre. The ability to produce crystalline silicon core fibre out of

inexpensive aluminium and silica could pave the way for a simple and scalable method of

incorporating silicon-based electronics and photonics into fibres.

1 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. 2 Institute of Soldier
Nanotechnology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. 3 Research Laboratory of Electronics, Massachusetts
Institute of Technology, Cambridge, Massachusetts 02139, USA. 4 School of Electrical and Electronic Engineering, Nanyang Technological University,
50 Nanyang Avenue, Singapore 639798, Singapore. 5 Department of Materials Science and Engineering, National University of Singapore, 9 Engineering
Drive 1, Singapore 117575, Singapore. 6 Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. 7 Department
of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. Correspondence and
requests for materials should be addressed to Y.F. (email: [yoel@mit.edu).](mailto:yoel@mit.edu)

ilicon (Si) and its oxide silica (SiO2) have both played an
important role in modern day electronics and communications 1–10 . In recent years the opportunities for tighter
# S
integration between electronics and photonics have led to the
emergence of the field of Si photonics where wafer processes are
used to combine the best features of both materials 11–14 . In a
mirror effort the advanced demonstrations of incorporating Si
into fibres have been explored in electronics and photonics,
such as photodetection 15, fibre battery 16, fibre solar cell 17,18,
electro-optical modulation 19, material engineering 20, non-linear
optics 21,22, pressurized Si sphere fabrication 23 and so on. Two
disparate methods of producing Si-core silica-cladded fibres have
been reported, the first relies on the physical flow process and the
second on a chemical reaction internal to a pre-fabricated fibre. In
the first method, high-purity Si is placed inside a silica preform
and drawn down into a silica fibre with a molten Si core 23,24 . The
second involves the use of high-pressure Si precursor gas (SiH4)
to deposit Si inside a pre-fabricated hollow channel fibre via a
chemical vapour deposition (CVD) process 25 . The CVD method
is more versatile while the thermal drawing approach is more
scalable.

The placement of disparate materials thermally drawn into
close proximity at elevated temperatures presents compelling
opportunities for inducing chemical reactions between the
constituent materials 26–28 that could in principle provide the
versatility of chemical reactions with the scalability of fibre
drawing. Results of this approach that we call fibre-draw synthesis
to date have been confined to reactions in only very small
domains at an interface between materials.

In this work, we utilize the redox reaction between aluminium
(Al) and silica (SiO2) during fibre drawing to produce a fibre with
a different core composition than that of the preform. This occurs
through the following reaction:

4Al þ 3SiO2 ! 2Al2O3 þ 3Si

At high drawing temperature, the metallic Al reacts with the
cladding material SiO2 and reduces the Si. The products (Al2O3
and Si) then get phase separated, thus forming a pure Si-core in
silica fibre. The reaction, diffusion and the phase separation all
take place at an elevated temperature close to the fibre draw
temperature.

Results
Fibre fabrication and characterization. The proposed method
centres on a technique so-called preform-to-fibre fabrication by
thermally drawing a macroscopic solid-state preform into
extended lengths of uniform fibres, as shown in Fig. 1a. The
preform consists of one piece of Al wire (3 cm long, 250 mm
diameter) placed inside a silica tube (inner diameter 300 mm,
outer diameter 600 mm). This assembly is further cladded with
two concentric silica tubes (one with 1 mm inner diameter, 6 mm
outer diameter, and the other one with 6 mm inner diameter and
12 mm outer diameter), as shown in Fig. 1b. The chamber where
the Al wire resides is then pumped to 10 � 3 mbar and sealed with
silica rods on both ends to prevent Al in contact with air. The
preform with the Al core is then heated up to 2,200 �C in Argon
atmosphere and pulled into metre-long fibres with the diameters
of B500 mm. During the fibre draw, Al melts and reacts with the
surrounding silica cladding, and reduce Si atoms out of silica 29 .
Si atoms accumulate during this process and finally occupy the
entire core, resulting in a Si-core fibre with silica cladding, as
shown in Fig. 1c.

Figure 2a shows an optical image of the fibre and indicates a
change in core material as the fibre is drawn. From top to bottom,
the colour of the fibre core is seen to transit from light yellow to
black. Scanning electron microscope (SEM) and energy-dispersive

Silica

5mm

60μm

500μm

Figure 1 | Aluminium-core-silica-cladding preform drawn into
silicon-core fibre. (a) The schematic diagram of the drawing process.
A macroscopic preform with an elemental Al core surrounded with silica is
thermally drawn into a fibre with an elemental Si core and a silica cladding.
(b) The optical image of the preform before the shiny Al core is sealed in
the cladding. (c) The optical image of the drawn fibre with a black Si core.
The core diameter is about 60 mm.

X-ray spectroscopy (EDX) analyses are taken at four locations
along the length of the fibre. Figure 2b–e is SEM images of the
cross-sections at locations 1–4 marked in Fig. 2a, respectively.
Comparing these four figures, one notice there are light-grey
dotswithin the core region in Fig. 2c–e. The grey dot in Fig. 2c is
so small that is hardly noticed, but can be seen more clearly from
EDX mapping on a zoomed-in area as shown in Fig. 2o. The dots
in Fig. 2d,e are much larger. It is worth mentioning though that
these dots in Fig. 2c–e, however, are quite different in forming
mechanism and in 3D shape. In Fig. 2c the dot is actually a
precipitated spherical drop composed of a majority of Si. But in
Fig. 2e the dot is actually cross-section of the long crystalline Si
wire (Supplementary Note 1). The EDX analyses around
these areas are shown to the right of the four SEM images and
confirm that the dots in locations #3 and #4 are composed
of high-purity Si. In Fig. 2f–q, green, red and yellow colours
denote element Al, oxygen (O) and Si, respectively. In Fig. 2f,j,n
(location #1), the core is composed mainly of Al and O.
In Fig. 2g,k,o (location #2), a small agglomeration of Si appears
while Al and O still dominate the core area. In Fig. 2h,l,p
(location #3), a large Si core is formed in the centre of the core
area, surrounded by a thick layer composed of Al and O. Finally,
in Fig. 2i,m,q (location #4 which is in the black core section),
Si occupies the entire core region, leaving Al to only a thin layer
between the core and the cladding.

To further explore the differences between the light-yellow and
black core regions, transmission electron microscopy (TEM)based analyses are performed at two different cross-sections
(see Supplementary Fig. 1 for preparation of TEM samples),
corresponding approximately to locations #2 and #4, respectively.
In the core of a yellow-core region, we find that spherical drops
form scattered along the axial direction (Supplementary Fig. 2). If
we focus on one of these spherical drops, we obtain the results
shown in the top panel of Fig. 3. Figure 3a shows that the sphere
is about 100 nm in diameter and is surrounded by a circle of
different materials. The diffraction pattern in Fig. 3b shows that
the materials are mostly amorphous, and the EDX maps in
Fig. 3c–e reveal that the sphere is Si rich while the surrounding

AI O

100 μm
#1

20 μm

50 μm 50 μm

20 μm

50 μm

100 μm

Figure 2 | Transition of fibre core. (a) The optical image of a piece of fibre where a light-yellow core transits to a black core. The white dashed
arrow illustrates the fibre drawing direction. (b–e) The SEM images of the cross-sections at the four labelled locations in a. (f–q) The EDX mappings
of the marked area in b–e correspondingly. Green, red and yellow denote element aluminium (Al), oxygen (O) and silicon (Si), respectively.

1,800
O
Oxygen
Aluminium
1,500 Silicon

100 nm

Core

Clad

2 nm

300 nm 300 nm

1,200

900

600

300

0 50 100 150 200 250
Distance (nm)

{110}

200 nm

Al Si

200 nm 200 nm

220

202

0.192nm
<111>

300 nm

5,000
Oxygen
Aluminium
4,000 Silicon

3,000

2,000

1,000

0
0 100 200 300

Distance (nm)

400 500

Figure 3 | Nano-scale analyses on fibre core. On the top panel, a–f data are measured inside the core region of the fibre with a yellow core (near location
#2 in Fig. 2a). (a) TEM image, the dot is one of the many dots seen on the sample. (b) The diffraction pattern indicates that the sample is mostly
amorphous. (c–e) EDX mappings on a, red, green and yellow denote O, Al and Si, respectively. (f) EDX line scan result. The white arrow in e shows the
line scanning path corresponding to f. On the bottom panel, g–l data are measured at the core-cladding interface of the fibre with a black core (near
location #4 in Fig. 2a). In g, the yellow dashed line indicates the interface between the crystalline part (core) and the amorphous part (clad). The parallel
orange lines are {110} faces, and the distance between them is measured 0.192 nm. (h) Taken from crystalline side of the yellow line, and the zone axis has
been tilted to o1114. (i–k) EDX mappings on g, red, green and yellow denote O, Al and Si, respectively. l, EDX line scan result. The white arrow in k shows
the line scanning path corresponding to l.

halo is Al rich. A line scan result across the sphere, as shown in
Fig. 3f, confirms this observation. In the case of the black core
region (bottom panel), we find distinct properties across the corecladding interface. As shown in Fig. 3g, the dashed yellow line
separates a crystalline part from an amorphous part. Figure 3h is
the diffraction pattern taken on the crystalline side and indicates
that the material is crystalline with a diamond cubic structure
as viewed along o1114 zone axis. The two parallel lines
highlighted in Fig. 3g represent {110} faces. The distance between
the neighbouring faces is 0.192 nm, from which the lattice
constant of the crystalline material is calculated to be 0.543 nm.
This detailed lattice structure information helps identify that the
crystalline material is Si (Supplementary Fig. 3, Supplementary
Note 2). Furthermore, X-ray diffraction (XRD) and Raman
spectrum 30,31 help identify that the Si core wire is polycrystalline,
which is similar to that of the core drawn from molten-core
method 18,24 (Supplementary Figs 4 and 5). The EDX mappings
shown in Fig. 3i–k clearly demonstrate that the thickness of the
Al-rich layer surrounding the Si core is about 100 nm. Finally, the
line scan in Fig. 3l demonstrates the purity of Si in the core
region.

Incorporation of crystalline Si into fibres can enable fibre
optoelectronic applications. Current approaches include the
molten core drawing or high-pressure CVD with post-annealing
process, leading to the demonstration of in-fibre semiconductor
devices 15,17,23,30 . Here we characterize the electric performance of
the Si core fabricated by the fibre-draw synthesis method. As
shown in Fig. 4a, 20-nm-thick Al and 150-nm-thick gold (Au)
are deposited on selected locations of a piece of Si wire
(Supplementary Figs 6 and 7, Supplementary Note 3). This
piece of Si wire is taken from the fibre by etching off cladding
with 40% hydrofluoric acid. During the measurement, the 4-point
probe method is used to prevent the possible Shottky junction
between the Si wire and the electrodes. From Fig. 4b one could get
the resistance per unit distance on the Si wire. Because the Si wire
is measured to be 100 mm in diameter, the resistivity is calculated
to be 0.3 O cm (conductivity 3.4 S cm � 1 ), which is about the same
order as that of the annealed Si core from the high-pressure CVD
method and several orders of magnitude smaller than that of
intrinsic Si (refs 3,30). Though exact dopant concentration cannot
be obtained through the resistivity value alone, it is highly
possible that the Si core is doped instead of intrinsic. Further
optimization of structure and fabrication process is supposed to
achieve better control of material qualities.

Si-core forming mechanism. The mechanism for Si material to
appear in the fibre is a chemical reaction between the Al core and
silica cladding. Al has a melting temperature of 660.32 �C, so it
melts when fed into the heating zone and keeps its molten state
until oxidized 32 . Also as an active reducer, Al reduces the
surrounding silica and produces Si atoms and Al2O3 molecules 33 .
The drawing temperature of 2,200 �C is higher than alumina’s
melting point so that alumina cannot form a shield to protect Al
atoms from oxidizing as it would at room temperature. Without
the protection of alumina, the reaction between Al atoms and
silica happens very fast due to two factors: (1) as described
by Arrhenius’s law, reaction rate relates to temperature
exponentially. When the temperature increases by about
2,000 K, the reaction rate is orders of magnitude higher.
(2) The largely increased diffusivity at high temperature helps
transfer products away from the reaction site at the interface, thus
effectively decreasing the products’ local concentration and
maintaining the chemical reaction. Figure 5a is a schematic
depicting how the chemical reaction happens at the core-cladding
interface.

1 4 7
Electrodes

180
R vs distance
160

140

120

100

Si wire

9 10

0
0 1 2 3 4 5

Distance (mm)

Figure 4 | Si wire electrical characterization. (a) The 4-point probe
measurement set-up. A piece of Si wire (B3 cm long) is taken from fibre by
etching off the cladding, and then 20 nm Al and 150 nm Au are deposited
on selected locations (2 outmost electrodes for current source and 10
electrodes for voltage metre contact marked 1 to 10). During the
measurement, the probe directly contacts the Al/Au layer on top of Si wire.
Current is provided from electrodes from two ends, and voltage is
measured across neighbourhood electrodes in between the two end
electrodes. (b) The resistance across neighbour electrodes are calculated
from a and are plotted against their corresponding distance. Linear
regression on the points gives information of Si resistivity.

In addition, although the reduced Si atoms are mixed with the
other product Al2O3 and not-yet-reacted Al atoms in the
beginning, their solubility properties play an important role in
separating them and finally forming a Si core. Most importantly,
Si is completely miscible with Al in the liquid state, but their
solubility with Al2O3 is comparatively low 32,34 . As more and
more Al2O3 is produced, the excessive Al2O3 precipitates out of
molten Al. Meanwhile, Si stays with molten Al because of their
miscibility. As the density of molten Al and Si are about
2.4 g cm � 3 and are smaller than that of molten alumina that is
about 2.8 g cm � 3 (refs 34–36), molten Al-Si floats up and molten
alumina sinks to the bottom as shown in Fig. 5b. As the draw and
chemical reaction progress, more and more Al in the Al-Si melt
gets consumed hence much less Al2O3 remains in the melt, while
Si keeps accumulating in the Al-Si melt on top of alumina melt.
Finally, when Al is completely consumed in the reaction, a highly
pure Si melt is obtained on the top of the core. Further drawing
results in a highly pure Si-core fibre. Figure 5c–e depicts the
above-mentioned core composition change during the draw.

AI
Si-O

O 2–

AI 3+

O 2–

Clad Core

Al + Si

Al Al2O3

Al2O3

Stage I Stage II Stage III
Ratio Ratio Ratio

Al + Si

Si (most)

Al2O3

Location Location Location

Figure 5 | Si-core forming mechanism in fibre draw. (a) The chemical reaction at the core-cladding interface. Al atoms (blue) break Si-O bond, reduce Si
(black), release O 2 � ions (red) and change to Al 3 þ ions (green). All the products of ions and atoms dissolve in the Al melt and diffuse into the core.
(b) The Si atoms (black), because of their miscibility, diffuse and remain in the Al melt (blue), while ions of O and Al form Al2O3 molecules (yellow)
and precipitate out at the bottom. (c–e) Sketches of the evolution of the core in the heating zone throughout the drawing process. As the draw continues,
more and more Si atoms accumulate in the Al-Si melt and eventually one obtains a highly pure Si melt on top of the Al2O3 with all the Al consumed
in the redox reaction (Stage II). Further drawing then results in a highly pure Si-core fibre (Stage III). The diagrams corresponding to each figure
demonstrate the ratio of different materials along the core axially. Blue, black and yellow denote Al, Si and Al2O3, respectively. The green dashed line that
gets lower and lower during the draw marks the position of the core. The cyan shadows in b and c illustrate the location of the transition region.

Solubility not only plays an important role in obtaining a
Si-core fibre, but also explains the embedded spheres seen in
Fig. 3a as well. Those Si-rich spheres in alumina are from the
small amount of Si that dissolves in molten alumina in the
heating zone. As the fibre is pulled out of the furnace and
the temperature drops, the solubility of Si in Al2O3 decreases. The
excess Si then precipitates out and forms spheres in alumina.

Besides accumulating with the help of miscibility properties,
the Si core is also purified by going through the furnace. Because
the preform is fed into the furnace from the top side and the
fibre is pulled out of the furnace from the bottom, the difference
between the hot furnace temperature and room temperature
constructs a temperature profile in which Si melts when entering
the hot zone and solidifies when leaving the hot zone. This
temperature profile provides the zone melting condition to
further purify the Si core 37 . In addition, Si is crystallized when it
cools down from liquid state after pulling out of furnace 38 .

Discussion
Our method remains further optimization for future applications.
For example, the size of Si-core could be significantly reduced by
redesigning the core-clad ratio and adjusting the fibre draw
parameters. The redraw technique could also be applied to
achieve nanometer-scale Si wires 23 . Also the thin interface layer
that lies between the core and the cladding as seen in Figs 2i
and 3j could be of interest for the potential advantage on oxygen
gettering and surface modification for the post-drawing
process 18,39 . The smoothness of the Si is verified in the atomic
force scan (AFM) image (Supplementary Fig. 8, Supplementary
Note 3). Last but not least, the mechanism of producing Si during
fibre drawing is also applicable to other reductive metals, for
example, Titanium (Ti) or Calcium (Ca), which could reduce Si
because of their more negative reduction potentials 40 .

To conclude, we demonstrate a new way to fabricate a
high-quality crystalline Si-core fibre by thermally drawing a
preform consisting of low-cost metal. The methods described
herein set the stage for extending the materials and architecture
selections with fibre drawing, thus paving the way towards more
sophisticated fibre-based devices.

Methods
Preform fabrication and fibre drawing. For the preform preparation we use the
fused-quartz tubes and rods supplied by Technical Glass Products and the Al wire
from Alfa Aesar. The fused-quartz capillary fibre (300 mm inner diameter and
600 mm outer diameter) is from Molex Incorporated (product # TSP300794). The
coating material on the capillary fibre is burned with flame and cleaned up with
isopropanol.

The 0.25-mm-diameter Al wire is first inserted into the capillary fibre and then
put into a fused-quartz tube with an inner diameter of 1 mm and outer diameter of
6 mm (1 � 6 tube). Two 1-mm-diameter fused-quartz rod are also inserted into the
1 � 6 tube, one on top of the capillary fibre and the other in the bottom of the
capillary fibre. The whole assembly is put into a fused-quartz tube with an inner
diameter of 6 mm and outer diameter of 12 mm. With the help of propane flame,
one end of the structure is fused and the whole assembly is pumped to vacuum and
the other end is fused under vacuum.

The fibre drawing process starts at the temperature of 2,150 �C and the drawing
process continues at 2,200 �C. The preform is fed into the furnace from top with a
speed of 1 mm min � 1, and the fibre is pulled out from the bottom end of the
furnace with a speed of 660 mm min � 1 . The fibre is further coated with coating
material, which is called ‘DSM Desotech 3471-3-14(941-314) Desolite single coat
coating’, and cured by an in-line ultraviolet coating system.

Characterization instruments and methods. The SEM images and related EDX
mappings on cross-sections of the fibres are taken with the JEOL 6010LA SEM. The
TEM samples are prepared in the Helios Nanolab 600 Dual Beam Focused Ion
Beam Milling System, and the process of preparing a TEM sample is detailed in the
Supplementary Information (Supplementary Fig. 1, Supplementary Note 1). The
TEM images and related EDX scans are taken using the JEOL 2010 FEG Analytical
Electron Microscope. The Raman spectrum is taken in the Horiba Jobin Yvon
LabRAM Model HR800. The XRD spectrum is taken in Rigaku SmartLab with a
parallel-beam optics and capillary-sample set-up. The electrical characterization of
the Si wire is on a 4-point stage using the Hewlett-Packard (HP) 4155A Semiconductor Parameter Analyser. The surface smoothness of the Si wire is characterized with Digital Instruments Veeco Dimension 3100 AFM, and the data are
processed in the Nanoscope Analysis software (v1.40) from Bruker Corporation.

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Acknowledgements
This work was supported in part by the MIT MRSEC through the MRSEC Programme of
the National Science Foundation under award number DMR-0819762 and also was
supported in part by the US Army Research Laboratory and the US Army Research
Office through the Institute for Soldier Nanotechnologies, under contract number
W911NF-13-D-0001.

Author contributions
C.H. and L.W. fabricated fibre, S.T. prepared TEM samples, C.H., S.T. and X.J. conducted
TEM analysis, C.H. and X.Z. provided electrical characterization, C.H. conducted other
experiments, C.H., J.J. and Y.F. discussed the Si-core forming mechanism and C.H. and
Y.F. wrote the manuscript.

Additional information
Supplementary Information accompanies this paper at [http://www.nature.com/](http://www.nature.com/naturecommunications)
[naturecommunications](http://www.nature.com/naturecommunications)

Competing financial interests: The authors declare no competing financial interests.

Reprints and permission information is available online at [http://npg.nature.com/](http://npg.nature.com/reprintsandpermissions/)
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How to cite this article: Hou, C. et al. Crystalline silicon core fibres from aluminium
core preforms. Nat. Commun. 6:6248 doi: 10.1038/ncomms7248 (2015).

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