Abstract P-type silicon has been patterned using highenergy protons beam prior to electrochemical etching in hydrofluoric acid. The ion beam selectively damages the silicon lattice, resulting in an increase in the local resistivity of the irradiated regions. It is found that the photoluminescence intensity of the irradiated regions increases with proton irradiation into a 0.02 .cm resistivity p-type silicon. By immersing the etched sample into potassium hydroxide, the porous silicon is removed to reveal the underlying three-dimensional structure of the patterned area

A A Bettiol

E J Teo

F J T Champeaux

· D J Blackwood

· F Watt

· M B H Breese

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J Porous Mater (2006) 13: 259–261
DOI 10.1007/s10934-006-8013-6
Fabrication of patterned porous silicon using high-energy
ion irradiation
E.J. Teo · M.B.H. Breese · A.A. Bettiol ·
F.J.T. Champeaux · F. Watt · D.J. Blackwood
C© Springer Science + Business Media, LLC 2006
Abstract P-type silicon has been patterned using high-
energy protons beam prior to electrochemical etching in hy-
drofluoric acid. The ion beam selectively damages the silicon
lattice, resulting in an increase in the local resistivity of the
irradiated regions. It is found that the photoluminescence in-
tensity of the irradiated regions increases with proton irradia-
tion into a 0.02 .cm resistivity p-type silicon. By immersing
the etched sample into potassium hydroxide, the porous sil-
icon is removed to reveal the underlying three-dimensional
structure of the patterned area.
Keywords Porous silicon . Photoluminescence . Ion
irradiation . Electrochemical etching
Introduction
Bulk silicon, due to its indirect bandgap is a poor emitter of
light. By electrochemical etching of silicon, it is discovered
that the porous silicon (PoSi) formed is capable of emitting
strong visible photoluminescence (PL) at room temperature
[1]. This is due to the quantum confinement effects produced
by the nano-sized silicon columns remaining after etching.
Since then, there has been tremendous interest in developing
E.J. Teo () · D.J. Blackwood
Department of Materials Science and Engineering, 9 Engineering
Drive 1, National University of Singapore, Singapore 117546
e-mail: msetej@nus.edu.sg
M.B.H. Breese · A.A. Bettiol · F.J.T. Champeaux · F. Watt
Centre for Ion Beam Applications, Physics Department, National
University of Singapore, Singapore 117542
the material for fabrication of optoelectronic devices, due to
its compatibility with standard microelectronics processes.
In order to incorporate PoSi-based light emitters into a
standard optical/microelectronic circuit, it is important to
be able to pattern PoSi with sufficiently high spatial res-
olution. There are many ways of producing patterned PoSi,
with photolithography being the most common approach [2].
Fluorescent images have been produced in n- and p-type sil-
icon by projecting a black and white image onto the surface
during etching [3]. There have also been many previous ion
beam irradiation studies on the production of PoSi. Most of
the irradiation with heavy ions showed that light emission is
preferentially quenched at the irradiated regions [4].
In this work, 2 MeV protons are used to create patterned
areas of light emitting porous silicon. According to SRIM
simulation, the penetration depth of 2 MeV protons is 48 μm
and this is much higher than that of keV ions used in previous
irradiation, allowing for better control of the PL emission [5].
Experimental
P-type silicon with resistivity of 0.02 .cm were first irra-
diated with a focused 2 MeV proton beam at the nuclear
microprobe facility at National University of Singapore [6].
Electrical contact was made to the backside of the silicon
using a copper wire and In-Ga eutectic paint. In order to
protect the wire from the HF attack, a thick layer of epoxy
was applied to cover the wire. The samples were then elec-
trochemically etched in a solution of HF: H2O: ethanol in
the ratio of 1:1:2 at constant current density of 37 mA/cm2
for 5 min. After etching, the sample was washed in an equal
mixture of ethanol and distilled water and transferred to a
vacuum chamber immediately.
Springer
260 J Porous Mater (2006) 13: 259–261
Fig. 1 (a) Ring pattern
irradiated with a dose of
1 × 1015/cm2. (b) Micro PL
spectrum of the irradiated and
unirradiated regions
The micro-photoluminescence measurements were car-
ried out using a 532 nm diode laser that was focused through
a Leica DMLM microscope with a 100 × objective. The PL
signals were then detected using a CCD Ocean Optics spec-
trometer via an optical fiber. A 550 nm bandpass filter was
placed in front of the detector to cut off the laser light. All
spectra had been corrected for response using a calibrated
deuterium light source.
The etched sample was further immersed in diluted potas-
sium hydroxide for 2–4 min to remove the porous silicon and
the underlying structures are then analyzed using scanning
electron microscopy (SEM).
Results and discussion
A circular ring pattern was irradiated into a low resistivity ma-
terial of 0.02 .cm with a fluence of 1.0×1015 protons/cm2.
From Fig. 1. it is clearly seen that the irradiated regions
exhibit higher emission than the surrounding unirradiated
regions. Micro-PL measurement of the irradiated regions
shows that the emission has a broad FWHM of 200 nm with
a peak wavelength of 750 nm. This suggests the presence
of a wide distribution of pore sizes. Very low PL signal is
observed from the unirradiated regions.
Figure 2 shows the SEM micrograph of the morphology of
the porous silicon in the irradiated and unirradiated regions.
It can be clearly seen that the regions irradiated by the proton
beam is made more porous than the unirradiated regions.
Previous work shows that anodization of low resis-
tivity wafers produces a significant fraction of meso-
porus/macroporous silicon which results in a weaker, red-
orange PL emission [7, 8]. The PL intensity is maximum
for wafers with moderate resistivity of 0.1–10 .cm, where
a large fraction of microporous silicon is formed with sizes
less than 2 nm [7]. Quantum confinement effects are expected
to dominate when electrons are confined to a volume smaller
than the exciton radius, equal to 4.9 nm for silicon. By irradi-
ation with proton beam, vacancies are created along the ion
Fig. 2 Close up SEM of the irradiated and unirradiated regions of
porous silicon
Fig. 3 SEM micrograph showing the underlying structure of the irra-
diated pattern in Fig. 1
path and these defect acts to trap the holes from migrating
to the electrolyte under anodic bias. This results in an in-
crease in resistivity and reduction in current flow through the
irradiated regions. According to Yamaguchi et. al. [9], the re-
sistivity is expected to increased by 1–2 orders of magnitude
Springer
J Porous Mater (2006) 13: 259–261 261
for a dose of 1015/cm2. This resistivity change explains the
observed increase in PL intensity.
The patterned PoSi seen in Fig. 1 is subsequently removed
using diluted KOH to reveal the three dimensional underlying
structure. Due to the thinner layer of PoSi formed over the
irradiated regions, the structure is raised by about 2 μm.
This technique provides a direct write method for producing
complex three-dimensional structures [10].
Conclusions
High energy proton irradiation has been used to produce light
emitting porous silicon patterns in low resistivity p-type sili-
con. The irradiated regions exhibit stronger emission and this
is associated to the higher porosity formed. This result shows
that increasing the local resistivity of the material with the ion
beam increases the PL intensity of porous silicon, consistent
with previous measurements on bulk silicon. By removing
the PoSi layer, a 3D structure of the irradiated pattern
is revealed.
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Springer


Fabrication of patterned porous silicon using high-energy ion irradiation

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Fabrication of patterned porous silicon using high-energy ion irradiation

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