Materials Sciences and Applicatio ns, 2011, 2, 649-653
doi:10.4236/msa.2011.26089 Published Online June 2011 (http://www.SciRP.org/journal/msa)
649
Photoluminescence Properties of LaF3-Coated
Porous Silicon
Sinthia Shabnam Mou, Md. Johurul Islam, Abu Bakar Md. Ismail*
Department of Applied Physics and Electronic Engineering, Rajshahi University, Rajshahi, Banglaesh.
Received November 12th, 2010; revised January 8th, 2011; accepted May 17th, 2011.
ABSTRACT
This work reports the coating of porous silicon (PS) with LaF3 and its influence on the photoluminescence (PL) prop-
erty of PS. PS samples, prepared by electrochemical etching in a solution of HF and ethanol, were coated with e-beam
evaporated-LaF3 of different thicknesses. It was observed that the thin LaF3 la yer on PS led to a good enhancement of
PL yield of PS. But with the increasing thickness of LaF3 layer PL intensity of PS was decreasing along with a small
blue-shift. It was also observed that all the coa ted samples showed degradation in PL intens ity with time, but annealing
could recover and stabilize the d egra ded PL.
Keywords: Porous Silicon, Anodic Etching, Photoluminescence , S urf ace C oat i n g, Photonics
1. Introduction
Porous silicon (PS) can be considered as a silicon (Si)
crystal having a network of voids in it [1]. The nanosized
voids in the Si bulk result in a sponge-like structure of
pores and channels surrounded with a skeleton of crystal-
line Si nanowires. Because of its versatility and tunable
characteristics, PS has attracted much attention in opto-
electronic industry due to its considerably strong photo-
luminescence (PL) behavior at room temperature apart
from its various applications such as sensor and MEMS.
As it has a potential to emit visible light, it is expected
that this material can be effectively used to fabricate
Si-based visible light emitting devices and optical inter-
connections. The PL phenomena of PS were reported in
the red light region first, while orange, yellow and green
PL was also observed latter. The wide spectral tunability
of the PL of PS that extends from near infrared (IR)
through the whole visible range to the near ultraviolet
(UV) make this material a very interesting one. A disad-
vantage of this material is the aging, that is, the slow
spontaneous oxidation of PS [2]. A native oxide layer
forms on the surface of the pores and the thickness of this
oxide layer grows with time. Due to the aging effect, the
structural and optical properties of PS show continuous
change with storage time [2]. That is, many of its proper-
ties, such as PL, are age dependent and unstable, and
why PS is still can not be applied in photonics. One pos-
sible way to reduce the aging effect could be “passiva-
tion” of PS [3]. This surface coating can be achieved
from various chemical adsorbates by several techniques
[4-6].
In this work, an attempt has been made to coat PS sur-
face by Lanthanum Fluoride (LaF3), aiming to obtain
stable and enhanced luminescence of PS layers. LaF3 is a
large band-gap (about 10.3 eV) [7] material having a
hexagonal crystal structure with a refractive index of
1.61 [8]. Lanthanum fluoride is a promising vacuum ul-
traviolet (VUV) transparent material similar to the other
large band-gap fluorides such as GdF3 and MgF2. Due to
its high band-gap and refractive index, higher than those
of the other VUV transparent films, LaF3 is a useful ma-
terial for VUV optics. Therefore, the PL properties of
anodically etched PS with and without LaF3-coating have
been compared and the influence of LaF3-coating on the
PL has been discussed in this article. The experimental
results show that coating the PS with optimum LaF3
thickness could provide stabilization of PL of PS, which
is very important for PS to be a potential material in
photonic devices.
2. Experimental
PS was prepared by anodic etching of p+-type single
crystal silicon wafer having <100> orientation, 10 - 24 Ω
cm resistivity and 200 ± 20 μm thickness. Etching was
done in a 3:1 (v/v) solution of 48% HF and absolute
ethanol. PS samples with various current densities and

Photoluminescence Properties of LaF-Coated Porous Silicon 650 3
etching times were prepared. Among them, the samples
with 15 mA/cm2 current density and 10 min etching time
were coated with LaF3 of three different thicknesses (70
± 10 nm, 120 ± 10 nm and 180 ± 10 nm). The etching or
electrochemical dissolution of Si wafer was done using
single tank cell arrangement, where the Si itself was used
as the anode and a platinum (Pt) electrode was used as
the cathode. The back contact of the Si wafer was done
by depositing a thick silver (Ag) layer by electron-beam
(e-beam) evaporation. The anhydrous LaF3 with 99.999%
purity was bought from the Kishida Co., Japan, and was
used without further purification. LaF3 layers were de-
posited by e-beam evaporation in a vacuum coating unit.
A high vacuum of 10–6 Torr was maintained during
evaporation of LaF3. PL study was done by Spectro
Fluorophotometer where the excitation wavelength was
350 nm.
Just after anodic etching the PS samples were dried
and put inside the vacuum coating unit for LaF3 deposi-
tion.
3. Results and Analysis
The fabricated PS samples with LaF3 layers were inves-
tigated with a scanning electron microscope (SEM).
Figures 1 and 2 show the SEM image of the surface and
cross-section of a LaF3-coated PS sample with a 180 ± 10
nm LaF3 layer. Since LaF3 covers the whole surface the
pore tips are not visible. The LaF3/PS/Si heterostructure
(from left to right in the image) is clearly visible in Fig-
ure 2. For both sets of the as-grown PS sample, single
PL peak appeared around 695 nm, which is similar to the
reported value [9,10]. The oxygen content of the film
was very low which is evident from the single-peak PL
band of PS [11]. The PL spectra of PS coated with LaF3
layer of three different thicknesses with an uncoated PS
sample has been compared and shown in Figure 3.
It is clearly seen from the figure that coating PS with
thin (e.g. 70 nm) LaF3 layer could protect the PS from its
PL intensity-degradation, which may be related to the
coating effect of the LaF3 that prevents the PS surface
chemistry. But with the increase in the thickness of LaF3,
the PL intensity is decreased. And for the sample with
LaF3 of 180 ± 10 nm thickness, the PL intensity of PS is
decreased even below the PL intensity of the uncoated
PS. The decrease in the intensity of the PL emission with
increasing thickness of LaF3 can be explained by the ab-
sorption of light by the LaF3 shell. The light emitted by
the PS is absorbed by the LaF3 when it passes through
the LaF3 layer. The absorption is proportional to the
thickness of the LaF3 layer. Therefore, the intensity of
the PL emission decreases with the increase of the LaF3
thickness. The important thing is the shape of the PL
response that did not change due to coating with LaF3.
Figure 1. SEM image of LaF3-coated PS surface.
Silicon (Si)
Porous-Si
LaF
3
Figure 2. Cross-sectional SEM image of PS sample coated
with LaF3.
Figure 3. Comparative PL spectra of PS with and without
LaF3 coating.

Photoluminescence Properties of LaF-Coated Porous Silicon651
3
The full width at half maxima (FWHM) of the Gussian
shaped PL response of the coated-PS remained almost
constant at ~12 nm. This signifies that the LaF3 layer
behaves just as a capping layer that keeps the Pb-center
of PS and do not interfere with the PS surface chemistry
inside the pore.
The peak wavelengths of the PL spectra of the samples
shifted a little towards blue with the increasing thickness
of LaF3. This nature of blue-shift has also been reported
by Gokarna et al. [12] for the PS/CdS and ZnS system.
This shift in the wave length of the PL can be attributed
to the shrinking of nanocrystallites due to thicker LaF3
layer [13]. The shift has been shown in Figure 4. The
FWHM of the PL responses of the LaF3-coated PS was
very narrow (~12 nm) that implies the homogeneity of
the nanocrystal.
LaF3-coated PS samples were then stored in air for one
month and the aging effect on the PL intensity was ob-
served. It was observed that the PL intensity of LaF3-
coated PS also decreased due to the aging effect (Figure
5). But all the three samples did not show same amount
of degradation in PL intensity. PS sample coated with
LaF3 of less thickness (70 ± 10 nm) suffered more from
aging effect than those coated with thicker LaF3, i.e. the
lower the thickness of LaF3, the more was the aging ef-
fect.
It appeared from the SEM that the LaF3 layer only de-
posited as a capping layer and did not deposit into the
pore of the porous silicon to passivate the silicon skele-
ton. The aging effect of the LaF3-coated PS may be due
to the trapped oxygen in the pore of the porous silicon
that slowly oxidizes the silicon skeleton. As-deposited
LaF3 layer shows non-stoichiometry [14] and reach in
pores [15]. The thicker layer of the LaF3 could provide
protection from the further oxidation of the silicon
skeleton from the ambient oxygen, but the thinner layer
failed because of pores in the LaF3. That is why the deg-
radation in the PL intensity was more for the thinner
layer of as-deposited LaF3 on PS.
But these pores can be removed, and stoichiometry of
LaF3 layer can be partially recovered by annealing that
may cause the PL stabilization of PS. The degradation of
PL and recovery of PL has been compared in the Figure
6. As shown in Figure 6, annealing recovers the PL in-
tensities for all the coated samples. The influence of an-
nealing on the PL of LaF3-coated PS will be reported
elsewhere.
4. Discussion
Coating of PS with e-beam evaporated LaF3 has been
investigated for the first time in order to see the role of
LaF3 as a coating layer on the optical properties of PS
such as, in enhancing and stabilizing the PL of PS. The
Figure 4. Small blue-shifts with the LaF3 thickness.
Figure 5. Aging of PL intensity of LaF3-coated PS with time.
Figure 6. Partial recovery of PL intensity by annealing.

Photoluminescence Properties of LaF-Coated Porous Silicon 652 3
thickness of the LaF3 appeared to have an important in-
fluence on the PL characteristics of PS. Thinner layer of
LaF3 could enhance the PL intensity without destroying
the Gaussian shape of the PL response. This enhance-
ment of PL is believed to be due to the coating effect of
LaF3 that prevented oxidation of PS. But thicker layer of
LaF3 lowered the PL intensity that may be due to the
degradation of refractive index of the LaF3 layer induced
by the mechanical stress which depends on the film
thickness. The LaF3-coated un-annealed samples showed
aging on the PL intensity. Generally the as-deposited
evaporated LaF3 layer contains pores which when ex-
posed to the air water vapor enters into the pore due to
capillarity. This water vapor replaces the Si-H bonds
with Si-O bond which seems responsible for the degrada-
tion of the PL intensity. As the thicker layer of
as-deposited LaF3 contains more water vapor the degra-
dation of PL intensity was more. When the coated sam-
ples were annealed at 400˚C for 10 minutes the PL inten-
sity was partially recovered and the degradation of the
recovered PL intensity was minimum for PS cited with
thicker LaF3 layer. Experimental results suggest that
coating PS with e-beam evaporated LaF3 and subsequent
annealing could enable the PS to have an enhanced and
stable PL.
5. Conclusions
All the studies done in this work can be summarized as;
coating PS with LaF3 of optimum thickness followed by
annealing could lead to better luminous stability of PS.
From these experimental results it can be concluded that
coating with LaF3 of optimum thickness could provide
the PS an enhanced and stable photoluminescence that
will enable its use in the fabrication of optical devices,
such as light-emitting diodes (LED), lasers and solar
cells.
6. Acknowledgements
The authors wish to thank the COMSTEC, TWAS, Tri-
este, Italy, the Bangladesh University Grants Commis-
sion, and the Faculty of Science of Rajshahi University
for providing financial assistance.
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