Bismuth spheres grown in self-nested cavities in a silicon wafer

Article abstract of DOI:10.1021/ja0547634

We have developed a one-step, hydrofluoric acid-free hydrothermal etching method that not only produces bismuth nano/micrometer-sized spheres but also prepares porous silicon with vertical holes. By controlling the heating temperature and time, nanoscale vertical-channeled porous silicon can be received. Our result indicated that the Bi clusters were formed first on the wafer surface. Then the etching of the Bi to the wafer creates the holes. Later, the Bi spheres went into the holes and expedited the etching process. A formation mechanism and chemical process have been proposed on the basis of experimental data. This simple chemistry approach may be of great scientific and technological importance for preparing porous silicon wafer.

Full text of DOI:10.1021/ja0547634

Published on Web 09/30/2005
Bismuth Spheres Grown in Self-Nested Cavities in a Silicon
Wafer
Hong Liu†,‡ and Zhong Lin Wang*,‡
Contribution from the State Key Laboratory of Crystal Materials, Shandong UniVersity,
Jinan 250100 P. R. China, and School of Materials Science and Engineering,
Georgia Institute of Technology, Atlanta, Georgia 30332-0245
Received July 15, 2005; E-mail: zhong.wang@mse.gatech.edu
Abstract: We have developed a one-step, hydrofluoric acid-free hydrothermal etching method that not
only produces bismuth nano/micrometer-sized spheres but also prepares porous silicon with vertical holes.
By controlling the heating temperature and time, nanoscale vertical-channeled porous silicon can be received.
Our result indicated that the Bi clusters were formed first on the wafer surface. Then the etching of the Bi
to the wafer creates the holes. Later, the Bi spheres went into the holes and expedited the etching process.
A formation mechanism and chemical process have been proposed on the basis of experimental data.
This simple chemistry approach may be of great scientific and technological importance for preparing porous
silicon wafer.
1. Introduction
which were created by the bismuth microballs or nanospheres
grown in situ inside the “nests” of the etched holes. Both the
growth of the Bi spheres and the formation of nano/micropores
occurred simultaneously. The chemical reactions and processes
for this process have been proposed.
Bismuth is a semimetal with unusual electronic properties
that results from its highly anisotropic Fermi surface, low carrier
concentrations, and small effective carrier mass. Because of its
large Fermi wavelength and long carrier mean free path, Bi has
been extensively investigated for quantum transport and finite-
size effects.1 Bismuth nanostructures have been extensively
pursued in studies of classical and finite size quantum effects
for more than 20 years because of their very large magnetore-
sistance (MR)2 and excellent thermoelectric properties.3 In recent
years, synthesis and property characterization of bismuth
nanostructures are attracting a lot of interest because of their
potential applications as magnetic field sensor, thermoelectric
cooler or power generator. Some groups have synthesized
bismuth thin films, nanowires, nanotubes, nanoplates, and
nanoballs through different synthesis routes.4
In this paper, we report a distinct and novel approach that
integrates the synthesized Bi spheres with silicon wafer through
a self-initiated etching process without using HF acid. This
single step process not only receives the as-expected Bi struc-
tures, but also presents a simple etching method for preparing
silicon surfaces that is different from the traditional methods
for etching silicon. During the etching process, no current and
no hydrofluoric acid was used. During the hydrothermal
synthesis process, cavities were created inside a silicon wafer,
2. Experimental Section
The synthesis of the bismuth-silicon structure was conducted
in a 22-mL Teflon-lined stainless steel autoclave. The preparation
process comprises three steps. (1) The silicon wafers [(001) direc-
tion, 10 mm × 15 mm × 1 mm] were cleaned sequentially using
H2SO4/H2O2 [H2SO4 (97%) + H2O2 (30%)] (90 °C, 30 min), de-
ionized water (ultrasonic bath 10 min, 4-6 times, to pH 7), and eth-
anol (ultrasonic bath 10 min). (2) Bismuth hydroxide (0.01 mol/L)
suspension was prepared. Analytical grade Bi(NO3)3‚5H2O (0.0005
mol, 98%, Alfa) was dissolved in 50 mL of deionized water to get
bismuth nitric solution. Excessive ammonia hydroxide solution was
dropped into the bismuth nitric solution to get bismuth hydroxide to
precipitate. The precipitate was filtrated and washed with deionized
water several times until the pH reached 7. Then, 0.01 mol/L bis-
muth hydroxide suspension was prepared by dispersing all of the
precipitate in deionized water under ultrasonic irradiation for 60
min, and deionized water was added to 50 mL. (3) The hydrothermal
process was conducted. The silicon wafer from step 1 was placed
vertically into the bottom of an autoclave. Bismuth hydroxide suspen-
sion (2 mL) and 18 mL of deionized water were added into the
autoclave. The autoclave was sealed and put into a furnace, which was
preheated to 160-180 °C. After heating for different times, the
autoclave was taken out and cooled to room temperature. The silicon
wafer was taken out of the autoclave and washed with deionized water
and then alcohol.
† Shandong University.
‡ Georgia Institute of Technology.
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Science 1999, 284, 1335-1337.
Characterization. X-ray diffraction was performed on a Philips
X-ray diffractometer. Scanning electron microscopy (FE-SEM) (field
emission LEO1530 at 5-10 K) and energy-dispersive X-ray spectros-
copy (EDS) were used to investigate the chemical composition of the
as-synthesized samples.
(4) Zhang, Z.; Sun, X.; Dresselhaus, M. S.; Ying, J. Appl. Phys. Lett. 1998,
73, 1589-1591. (b) Heremans, J.; Thrush, C. M.; Lin, Y. M.; Cronin, S.;
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10.1021/ja0547634 CCC: $30.25 © 2005 American Chemical Society

Bismuth Spheres Grown in Self-Nested Cavities
A R T I C L E S
Figure 2. SEM images of the sample on Si(001) synthesized at 175 °C
for 24 h. (a, b) Top view of the ball-hole structures on the wafer at different
magnifications. (c, d) Top view of two individual balls. (e) SEM image
acquired from a fractured sample synthesized at 175 °C for 48 h. Inset:
Cylinders found at the bottom of the holes (marked by arrows).
Figure 1. XRD patterns of the samples reacted at (a) 175 °C for 24 h, (b)
160 °C for 24 h, and (c) 160 °C for 48 h.
3. Results and Discussions
larger than the opening ends of the holes and they are “trapped”
inside (Figure 2d). This observation indicates that the balls were
formed inside the holes and/or they were responsible for creating
the holes as they grew, but they cannot be placed in or fall in
after the holes were formed. Thus, the balls and the holes were
formed simultaneously as the reaction proceeds. Figure 2e is
the SEM image acquired at a fraction side of the sample
synthesized at 175 °C for 48 h. We can see that the etching
depth is about 5 µm. From the inset of Figure 2e, we can see
that there is a cylinder about 100 nm in diameter at the bottom
of each etching hole.
To confirm the chemical composition of the balls and the
surface of the wafer of the sample, EDS was performed in SEM.
Figure 3 shows EDS spectra from the balls at different
circumstances. The only detectable element by EDS from the
balls/polyhedra is bismuth. This supports the XRD result that
the balls are hexagonally structured bismuth crystals. At the
rough area between the holes of the wafer, besides of the main
element Si, there is a small amount of oxygen and bismuth. A
small amount of silicon oxide should exist at the surface of the
silicon wafer, and the minor Bi could come from the tiny Bi
sphere below the surface and/or the Bi balls located away from
the beam illuminated area, due to the large interaction volume
(∼1-2 µm) of the electron beam with the sample. Carbon
element detected by EDS should be from ethanol molecules
absorbed on the surface of the samples during washing with
alcohol.
X-ray diffraction (XRD) patterns from the samples synthe-
sized at 160 and 175 °C for 24 h show that the samples are
composed of hexagonal bismuth (JCPD-85-1330) and single-
crystal silicon with diamond structure (JCPD-80-0018). From
the data presented in Figure 1, we can see that, besides of the
{400} peak of silicon ((001) silicon wafer), all of the peaks in
the XRD patterns are consistent with those of the standard XRD
pattern of hexagonal bismuth. No other related compound, such
as bismuth oxide or bismuth silicate, has been detected in the
samples. In the pattern of the sample obtained at 160 °C for 48
h (Figure 1c), no crystalline compound was detected besides
of silicon substrate.
Figure 2 shows SEM images of the sample on Si(001)
synthesized at 175 °C for 24 h. Figure 2a is the morphology of
the as-synthesized sample with low magnification. Many small
balls 1.5-2 µm in diameter are distributed homogeneously on
the surface of the silicon wafer. Each ball locates inside a holey
nest created in the silicon. From Figure 2b, we can find that
the holes are perfect circles, and all of the balls are trapped in
the holes. The surface of silicon wafer looks like a lotus seedpod.
On the surface of a ball grown in a hole (Figure 2c), we can
see some rough areas with dots and some smooth areas with
well-shaped patterns, which indicates that the ball may be
crystalline. The smooth patterned area may be the growing
planes with low-index facets, and the rough places may be the
growing planes with higher index. The diameters of most balls
are approximately the same, which are slightly smaller than the
opening ends of the holey nests. For some balls, their sizes are
To receive nanosized bismuth balls and holes, samples were
prepared at lower temperature for different times (Figure 4).
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A R T I C L E S
Liu and Wang
Figure 4. Top view of the samples reacted at 160 °C for (a) 15 h, (b) 24
h, and (c) for 48 h.
Figure 5. (a, b) Cross sectional view of the sample reacted at 160 °C for
48 h and (c) uniform, vertically etched holes obtained at 165 °C for 72 h
through a well-dispersed suspension of bismuth hydroxide.
160 °C for 50 h, which have many deep holes of ∼200 nm in
diameters on the surface of the wafer, but surprisingly there
are no Bi spheres on the surface. Then, the silicon wafer was
fractured to see its cross-section (Figure 5a,b). The depth of
the etched holes is over 30 µm, and nanosized Bi particles of
about 100-150 nm in size, as indicated by circles, were found
at the bottoms of the holes. The diameter of the spheres is
slightly smaller than that in Figure 4b, and the lower half of
the nanospheres is not a standard sphere shape. At the same
time, the diameter of the hole at the bottom is slightly smaller
than that at the opening end. By adjusting the etching condition,
we can get uniform etched holes in the silicon wafer. The
conditions for successfully achieving the etching are as fol-
lows: the concentration of bismuth hydroxide suspension is
0.005 mol/L, and before being added into autoclave, the
suspension was dispersed with ultrasonic irradiation for 40 min.
The autoclave was heated at 165 °C for 72 h. The diameters of
most of the vertical holes (over 90%) are 150-190 nm, and
Figure 3. Energy dispersion spectra acquired in SEM from (a) a ball, (b)
the rough surface area between the holes, and (c) a cylinder at bottom of
an etched hole.
After reacting at 160 °C for 15 h (Figure 4a), small spheres
150-200 nm in diameter can be found on the surface of the
silicon wafer. However, no obvious etching holes can be found.
After reacting for 24 h (Figure 4b), the etching holes were
formed, and the spheres directly sat on the holes. The depth of
the holes increases with the increase of hydrothermal time.
Figure 4c shows the surface of the silicon wafer synthesized at
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Bismuth Spheres Grown in Self-Nested Cavities
A R T I C L E S
Therefore, the local concentration of OH- at the surface of
the wafer, where the bismuth hydroxide particles are adhered,
should be much higher than that at other places (Figure 6b).
Therefore, reaction between silicon and hydroxide ion should
occur immediately on the surface of the wafer.
Si + 2OH- + 2H2O f Si(OH)4 + H2v
(2)
The electrochemical etching mechanism of hydroxide ions
on silicon wafer with applied voltage has been investigated
extensively.7 In the present work, however, we did not apply
any electric field on the wafer, and all of the processes were
conducted under a hydrothermal condition.
The hydrogen atoms formed from reaction 2 have strong
reducing ability. It is easily to reduce the Bi3+ ions formed in
reaction 1. The electrode standard potential of Bi3+ is +0.317,8
thus, it is easily reduced by hydrogen:
2Bi3+ + 3H2 f 2Bi + 6H+
(3)
Bismuth atoms reduced by hydrogen accumulate at the surface
of the silicon and become crystalline nuclei (Figure 6c). At the
same time, hydrogen ions formed in reaction 3 enter the solution.
With the hydrothermal process continuing, the pH value
deceases (dropping to 5-6 after hydrothermal process).
The as-forming soluble Si(OH)4 molecules released from the
surface enter the water. Because Si(OH)4 is not stable in water,
it is easily condensed into xSiO2‚yH2O.9
Figure 6. A schematic model illustrating the formation of bismuth spheres
and the bismuth-sphere-catalyzed chemical etching of silicon (see the text
for details).
the thicknesses of the walls between two adjacent holes are
20-50 nm (see Figure 5c). This process can give a similar
etching result comparing to that of the metal-catalysis etching,
for which a layer of metal was deposited on the surface of the
silicon wafer, and the metal layer was transferred as a layer of
nanoballs by heating the wafer. But the traditional corrodent,
HF, was still used for etching. Vertical nanoholes of 50-150
nm in diameters are formed because of catalysis effect of the
metal nanoballs.5 The etching method proposed in the current
paper is much simpler and less toxic than the traditional ones.
Si(OH)4 f H2SiO3 + H2O
2Si(OH)4 f H2Si2O5 + 3H2O
3Si(OH)4 f H4Si3O8 + 4H2O
(4)
(5)
(6)
In fact, H2SiO3 could not exist at temperatures over 0 °C,
and H4Si3O8 is difficult to form during a very short period of
time.18 Therefore, the Si(OH)4 from reaction 2 should become
H2Si2O5. But H2Si2O5 is not stable at a temperatures over 160
°C, and it decomposes into water and silicon oxide at higher
temperatures:
Our result indicated that the Bi spheres were formed first on
the wafer surface. Then the etching of the Bi sphere to the wafer
creates the holes. Later, the Bi spheres fell into the holes and
continued the etching process. On the basis of the data, we
proposed a formation mechanism (Figure 6) for the etched holes
and bismuth spheres in the holes.
H2Si2O5 f 2SiO2 + H2O
(7)
The silicon oxide formed from reaction 7 should be amor-
phous, and a small amount of amorphous silica should adhere
on the surface of the wafer (Figure 6d). This is the reason we
When bismuth hydroxide suspension is added into the water
in the autoclave, a layer of amorphous bismuth hydrothermal
particles may be adsorbed on the surface of the wafer, which is
located at the bottom of the autoclave (Figure 6a). When the
autoclave is placed into the furnace, which is preheated to 160
or 175 °C, water in autoclave is overheated and forms
supercritical water. The dissolvability of bismuth hydroxide
particles in the supercritical water is thus increased because of
the novel dissoluble ability of supercritical water.6
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Phase Diagrams; American Society of Metals: Metals Park, OH, 1996;
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Bi(OH)3 f Bi3+ + 3OH-
(1)
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Palmer, R. E. J. Phys. D 1998, 31, L21-L24.
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Liu and Wang
can detect a small amount of oxygen, but no crystalline silicon
oxide can be detected by XRD on the rough place on the surface
of the silicon wafer. It should be noted that the amorphous silica
forms in the solution, and the adhered amorphous silica is
located on the surface of the wafer, which does not lower the
etching speed.
From the bismuth-silicon binary phase diagram,10 we can
see that the melting point of bismuth is only 271 °C, which is
much lower than that of silicon (1412 °C). The as-reduced
bismuth atoms should be very active, and at the same time,
silicon atoms on the surface of the silicon wafer in supercritical
water should be more active than those under a normal
condition; therefore, the silicon atoms will be “attracted” out
of its silicon lattice by the active as-reduced bismuth atoms to
form temporary congruent melting clusters:
of the hole decreases slightly at the bottom of the hole after
reaction for 48 h (see Figure 5). In addition, because the
formation and decompounding of temporary congruent melting
clusters, some bismuth atoms are moved from the front area to
the part behind the ball. Therefore, the shape of the balls should
change. For the big balls, the diameters shrink and they become
cylinders. For the nanospheres, their diameter should change
slightly, and they become thick disks.
The experimental procedure demonstrated above opens a new,
simple chemical approach for etching silicon wafers, which
offers some advantages in comparison to the existing etching
technology that normally uses HF. The bismuth element left in
the holes can be removed by nitrate acid, because silicon dose
not dissolved in nitrate acid and bismuth dose. As is well-known,
standard and advanced lithography are based on hydrofluoric
acid etching.11 The corrodents of most electrochemical etching,
chemical etching, and macromachining technology on silicon
wafer are also related to HF.12,14 Although hydroxide solution
is often used as a corrodent in electrochemical etching of silicon
wafers,15 a layer of oxide is easily formed on the surface,
resulting in a sharp drop in etching rate and the anodic current;16
the etching process has to be interrupted to remove the oxide
with HF. Although careful control of the potential of the n-type
silicon wafer during the etching process can prolong the time
before forming the passivation layer on the etched surface, it is
still difficult to totally prevent the formation of the oxide layer.17
As of now, almost all of the etching methods for silicon wafer
cannot totally avoid the use corrosive and toxic hydrofluoric
acid, which are toxic not only for the operators but also for
environment. Therefore, the demonstration of a simple and
nontoxic etching method without hydrofluoric acid is an
important part of the current work.
mBi + nSi f BimSin
(8)
From the Bi-Si phase diagram, we found that silicon and
bismuth cannot form a stable compound. Therefore, silicon
atoms in the bismuth-silicon clusters are easily released and
react with OH-, as shown in reaction 2, resulting in more
hydrogen atoms.
BimSin + 2nOH- + 2nH2O f nSi(OH)4 + nH2v + mBi (9)
The bismuth atoms disposed of as the bismuth ball and can
repeat reaction 8 at the bottom of the holes. During the further
etching process, bismuth atoms attract and release silicon atoms
and play the role of an etching catalyst. Bi3+ ions transported
by supercritical water are neutralized by the charges from the
hydrogen atoms, forming bismuth elemental atoms, which
epitaxially deposit on the surface of bismuth nuclei, increasing
their sizes to small nanospheres (Figure 6d).
4. Conclusions
All of the above reactions repeat during the hydrothermal
process, and the surface of the silicon wafer was etched to form
holes gradually (Figure 6e). Because of the catalysis function
of the bismuth spheres, silicon atoms in contact with the spheres
are easily etched, and the depth increases with increasing etching
time (Figure 6f). The diameter and depth of the holes increase,
and at the same time, the bismuth nanospheres grow with
increasing reaction time (Figure 6g). During the reaction process,
the sphere preserves its position at the bottom of the etched
hole. Finally, the concentration of H+ in the hole is much higher
than that outside of the hole, preventing Bi3+ from entering into
the hole. At equilibrium, although the etching processing can
continue and lead to an increase in hole depth, the diameter of
the hole and the size of the sphere in the hole do not increase.
At last, a porous structure of silicon is received. The hole is
deep, and the bismuth sphere sinks to the bottom of the hole.
Therefore, we cannot detect bismuth in the sample prepared at
160 °C for 48 h. Because there are a small amount of bismuth
atoms left on the wall of the hole and no additional bismuth
atoms coming into the hole during the “catalysis” process of a
bismuth ball, the ball becomes smaller after a long etching. This
may be the reason the ball becomes smaller and the diameter
In this paper, we have developed a one-step, hydrofluoric
acid-free hydrothermal etching method that not only produces
bismuth nano/micrometer-sized spheres but also prepares porous
silicon with vertical holes. By controlling the heating temper-
ature and time, nanoscale vertical-channeled porous silicon can
be received. Our result indicated that the Bi clusters were formed
first on the wafer surface. Then the etching of the Bi on the
wafer creates the holes. Later, the Bi spheres went into the holes
and expedited the etching process. A formation mechanism and
chemical process have been proposed on the basis of experi-
mental data. This simple and nontoxic chemistry approach may
be of great scientific and technological importance for preparing
porous silicon wafers.
Acknowledgment. We are grateful for the support from the
National High Technology Research and Development Program
of China (863 program, No. 2002AA31110), NSF (DMI
0403671), the NASA Vehicle Systems Program and Department
of Defense Research and Engineering (DDR&E), and the
Defense Advanced Research Projects Agency (Award No.
N66001-04-1-8903).
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