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8/22/2019 A novel method of fabricating porous silicon.pdf
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Materials Science and Engineering A 528 (2011) 78317834
Contents lists available at ScienceDirect
Materials Science and Engineering A
journa l homepage: www.elsevier .com/ locate /msea
Rapid communication
A novel method offabricating porous silicon
Dibyendu Chakravarty a,, B.V. Saradaa, S.B. Chandrasekhara, K. Saravananb, T.N. Raoa
a International Advanced Research Center for PowderMetallurgy andNewMaterials (ARCI), Balapur P.O., Hyderabad500005, AndhraPradesh, Indiab Materials Research Group, IndiraGandhiCenterfor Advanced Research (IGCAR), Kalpakkam603102, Tamil Nadu, India
a r t i c l e i n f o
Article history:
Received 17 May2011
Received in revised form 4 July 2011
Accepted6 July 2011Available online 18 July 2011
Keywords:
X-ray diffraction
Porous materials
Sintering
Graingrowth
Powder metallurgy
a b s t r a c t
Porous silicon was fabricated using the spark plasma sintering technique. High porosity 1050% and
strength5060MPa wasobtained by tailoringthe SPS variables.XRD andRaman investigationsshowed
presence ofpure silicon and exhibition ofphotoluminescenceunder visible light at wavelength
520nmconfirmed the presence ofporous silicon.
2011 Elsevier B.V. All rights reserved.
1. Introduction
Porous silicon is extensively used in electronic and optoelec-
tronic applications due to its strong visible photoluminescencefrom near IR to near UV regime [14]. It is also used as optical and
electrochemical transducers, supports in chemical and biochem-
ical applications, sacrificial layers in Si lithography, cell interface
for bone tissue engineering and support for growth of animal cells
[512]. Porous silicon is fabricated by electrochemical anodization
of silicon wafer in a hydrofluoric acid electrolyte [13]. Theporosity,
porediameter and microstructuredepends on various anodization
conditions such as current density, etching time, HF concentra-
tion, wafer doping, ambient temperature and humidity [13]; each
of these anodization conditions influences the formation process
immensely. Besides, the electrolysis process involved is lengthy
and may lead to contamination by reaction between metals and
HF.
An alternate, novel method for fabricating bulk porous silicon
is the powder metallurgy route. Conventional powder metallurgy
processes involve prolonged time to develop porous structures
without imparting sufficient strength due to insufficient necking
between powder particles. A rapid sintering technique retaining
a porous matrix with sufficient strength is the key for success-
ful development of porous materials. In this context the spark
plasma sintering (SPS) technique becomes appropriate as (i) it
Corresponding author. Tel.: +91 40 24441075; fax: +91 40 24442699.
E-mail address: [email protected](D. Chakravarty).
is possible to develop porous microstructures with sufficiently
high strength in very short times (2min) and (ii) amount of
porosity in the sample, pore size distribution and strength can
be controlled by tailoring the SPS variables [14]. The presentstudy was undertaken with the specific objective of fabricating
porous silicon samples using SPS starting with commercial sil-
icon powders, making way for the expensive silicon wafers as
starting materials and the lengthy electrolysis process involving
HF.
2. Experimental
Nano powders of silicon were obtained by wet milling of
commercial silicon powders. These powders were subjected to
SPS in a Dr. Sinter 1050 apparatus (SPS Syntex Inc., Japan)
under a vacuum of 6Pa, with a pulse duration of 3.3ms and
a pulse on:off ratio 12:2. The sintered samples were polished
progressively with diamond paste upto 1m for microstructural
observations using a scanning electron microscope (Model S-
4300SE/N, Hitachi, Tokyo, Japan). They were chemically etched
using an etchant prepared with acetic acid (55mL), nitric acid
(35mL) and hydrofluoric acid (20 mL). Porosity was estimated
from the SEM micrographs using the Analysis Five image ana-
lyzer software. Pore size distribution was estimated by mercury
intrusionporosimetry (POREMASTER, Quantachrome Instruments,
FL). Strength of the samples was measured using the diametral
compressive test in a universal testing machine (UTM Model No:
5500R, INSTRON, Grove City, PA). X-ray diffractometer (Bruker
AXS, Germany) was used for phase analysis over the 2 range
0921-5093/$ seefrontmatter 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.msea.2011.07.004
http://localhost/var/www/apps/conversion/tmp/scratch_6/dx.doi.org/10.1016/j.msea.2011.07.004http://localhost/var/www/apps/conversion/tmp/scratch_6/dx.doi.org/10.1016/j.msea.2011.07.004http://www.sciencedirect.com/science/journal/09215093http://www.elsevier.com/locate/mseamailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_6/dx.doi.org/10.1016/j.msea.2011.07.004http://localhost/var/www/apps/conversion/tmp/scratch_6/dx.doi.org/10.1016/j.msea.2011.07.004mailto:[email protected]://www.elsevier.com/locate/mseahttp://www.sciencedirect.com/science/journal/09215093http://localhost/var/www/apps/conversion/tmp/scratch_6/dx.doi.org/10.1016/j.msea.2011.07.0048/22/2019 A novel method of fabricating porous silicon.pdf
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7832 D. Chakravarty et al. / MaterialsScience andEngineering A 528 (2011) 78317834
Fig. 1. (a) X-ray diffraction showing of Si phase in samples sintered at 1375K, no
other major peaks areobserved. (b)Ramaninvestigationshowingspectrumof single
crystal silicon at 521.6cm1 andthe shift in spectra andpeak broadening in porous
silicon samples sintered at 1175, 1275 and1325 K. The peak position of thesample
sintered at 1375 K is similar to thesingle crystal.
1080 using Cu-K radiation with wavelength 1.5405A. Raman
spectrometer (Jobin Yvon, Horiba HR 800) with Argon ion laser
(514nm) as the excitation source was used and the spectra were
obtainedbetween450cm1 and 550 cm1. Forphotoluminescence
studies, etched samples were excited with Argon ion laser of
wavelength 457nm with power of 20mW and the signal was
collected on a triple grating spectrometer (Jobin Yvon, Horiba
T64000).
3. Results and discussions
X-ray diffraction (XRD) was carried out to analyze the phases
present in the samples sintered at different temperatures. XRD
of the sample sintered at the highest temperature (1375K) is
shown in Fig. 1a as a representative sample, because of the
propensity of silicon to get oxidized and form oxide or sub-oxide
phases at higher temperatures. The figure shows the presence
of pure silicon without any trace of oxides, sub-oxides or other
impurity phases. To corroborate the presence of silicon, Raman
investigations were carried out on the sintered samples along
with a silicon single crystal as reference. Raman spectra of the
reference sample showed a symmetric Raman band with the
peak centered at 521.6 cm1; for the sintered samples a dis-
tinct shift in the spectrum with temperature towards lower
wave number was observed, as shown in Fig. 1b. This was
attributed to the decrease in phonon energy due to disturbances
in the silicon lattice because of high porosity in the samples
[15].
Theevolutionofmicrostructure inthesiliconsamples with tem-
perature under constant stress of 50MPa and hold time of 2min
is shown in the SEM micrographs, Fig. 2. The sample sintered at
1175K, being at the initial stage of sintering, was highly porous
with the silicon powders loosely bonded and just starting to form
necks without any grain formation. With gradual increase in tem-
peraturethe intermediatestageof sinteringwasreached wherethe
grains slowlybegan to develop andtheporosity started decreasing
retaining a continuous pore phase. At 1275K the microstructure
predominantly consisted of fine, well defined grains; fine pores
were located between these grains forming a continuous porous
opennetwork. At 1325K similar microstructurewasobserved but
thegrains were slightlybiggerin size retainingthe porousnetwork.
With further increase in temperature the grains became distinct,
porosity decreased further and pores began to shrink and form
a string-like continuous structure sitting along the grain edges.
At 1375K extensive necking was observed and a more closed
microstructurewas obtained.Thedecrease inporosities inthesam-
ples with temperatures is shown in Fig. 3.
The pore size distribution of all the samples was evaluated. The
samples sintered at 1175 and1225K showedwide pore size distri-
butionrangingfrom20to 1500nm and 10to1000nm,respectively.
The sample sintered at 1325K exhibited a good combination of
strength and porosity and was observed to give a much narrower
pore size distributionvaryingbetween 10and200 nmdiameters in
themercury porosimeter under an intrusionpressure of 60,000psi
(414MPa) as shown in Fig. 4. For the sample at 1375K the porosity
wasbetween8 and10%,poreswerenot continuous, consistentwith
the SEM observations. The porosimetry data along with the SEM
micrographs conclusively prove the presence of a micro porous
structure after sintering.Standard flexural strength measurement using 3-point or
4-point bending test requires rectangular samples of size
40mm3mm2 mmasperASTMstandards, standardtensile test
specimens also need to be machined into a dog-bone shape. Due
to experimental limitations with respect to the size of samples
that can be prepared using the SPS technique it was not feasible
to prepare samples for either of these two standard test measure-
ments. Instead, thediametralcompressiontestwasadopted as this
can accommodate samples of all sizes. It is speculated that dur-
ing SPS, sparks are generated with possible formation of plasma at
the points of contact between powder particles removing oxides
from the powder surfaces keeping them clean and nascent and
also leading to local temperature increments at points of contact
between particles [16]. This leads to formation of strong necks atthese junctions with a concomitant increase in strength in the sin-
tered compacts at lower sintering temperatures and shorter times
compared to conventional methods of sintering and hot pressing.
The tensile strength of the spark plasma sintered porous silicon
samples showed a monotonically increasing trend with temper-
ature, shown in Fig. 5, attributed to the increase in necking with
temperature.
Photoluminescence (PL) study was carried out as a confirma-
tory test forporoussilicon over thewavelength range480620nm.
A broad PL spectrum was observed at 520nm when the sam-
ple was chemically etched for 20s, as shown in Fig. 6. The PL
spectrum was observed only for the etched samples and could
be attributed to the H and OH bonds adsorbed on the surface of
porous silicon during chemical etching, creating emission centers
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D. Chakravarty et al. / Materials Science andEngineering A 528 (2011) 78317834 7833
Fig.2. (a) SEMmicrographsof silicon samplesspark plasma sinteredat 1175K showinglooselybonded silicon particles, (b)SEMmicrographsof silicon samples spark plasma
sintered at 1275K showing development of grainswith an openmicrostructure. (c) SEM micrographs of silicon samples sparkplasma sintered at 1325K showing an open
microstructure but with grains larger than in (b). (d) SEM micrographs of silicon samples spark plasma sintered at 1375K showing well developed grains with a closed
microstructure.
essential for the exhibition of PL around 520 nm, as also noted
elsewhere [17]. This fact is confirmed from the XPS spectra of O
1s and Si 2p which clearly show formation of hydroxide peaksat 101.12 and 533.26eV and an oxy-hydroxide peak at 102.58eV
for the etched sample. The Si-based oxy-hydroxide essentially has
a structure starting with a Si O type core attached to H or OH
species; the oxidation of the oxygen dissolved in HF breaks the
Si bonds during etching to form SiF bonds which are hydrolyzed
Fig. 3. Variation of porosity with sintering temperature in porous silicon samples.
to SiOH, a compound known to exhibit visible PL[18]. Increasing
the etching time progressively to 40, 60 and 80s severely dented
the photoluminescence as the porous structure collapsed due toover etching. Thus 20s is considered to be the optimum etching
time. Further work is being carried out to make a quantitative
estimate of surface functionalization on the exhibited photolumi-
nescence.
Fig. 4. Pore size distributionof thesiliconsample sinteredat 1325 K andchemically
etched for 20s.
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7834 D. Chakravarty et al. / MaterialsScience andEngineering A 528 (2011) 78317834
Fig. 5. Variation of strengthwith sintering temperature in porous silicon samples.
Fig. 6. Photoluminescence spectrum of porous silicon sample sintered at 1325 K
and thesame samplechemically etchedfor 20s.
4. Conclusion
Porous silicon was fabricated from commercial silicon powders
for the first timeusing the SPS technique in the temperature range
11751375K under applied stress of 50MPa in 2min.
Raman investigations showed a gradual spectral shift in the
sintered samples (521.6515.8cm1) compared to silicon single
crystal (521.6cm1) due to increase in porosity with decreasing
temperature.
Low temperature and short sintering time (2 min) enabled
retention of porous microstructures with the amount of porosity
decreasing from 50% at 1175 K to 10% at 1375 K. The sam-
ple sintered at 1325K revealed a pore size distribution of 10
200nm.
Extensive necking in SPS has led to the development of bulk
porous silicon samples with tensile strength 5060MPa, suffi-
ciently high for use as substrates in chemical and biochemical
applications.
Exhibition of photoluminescence in chemically etched spark
plasma sintered samples at a wavelength 520nm conclusively
proves theexistence of porous silicon andmakes them suitable for
use in electronic and optoelectronics applications.
Acknowledgement
Dr. G.V.M. Kiruthika, CECRI, Karaikudi, is gratefully acknowl-
edged for carrying out the XPS analysis.
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