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Materials Science and Engineering A 528 (2011) 78317834

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Materials Science and Engineering A

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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: dibyendu3108@gmail.com(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.

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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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