Optical properties of n-type porous silicon obtained by photoelectrochemical etching

Article abstract of DOI:10.1016/S0038-1098(99)00271-9

The optical studies of n-type porous silicon prepared by the photo-assisted chemical etching are reported here. The optical properties of samples obtained under different conditions have been investigated by photoluminescence and Fourier transform infrared absorption measurements, and they are compared with that of p-type porous silicon. Our results clearly demonstrate that the blue emission in porous silicon originates from surface compounds. From the infrared absorption measurement, we point out that the surface compounds are Si-OH complexes. This conclusion is further supported by a recent calculation which shows that Si-OH complexes can emit the photon energy in the range observed here. We show that the optical properties of the n-type porous silicon are more stable than that of the p-type porous silicon. The result provides the evidence to support the fact that the n-type porous silicon is a better candidate for the application in optoelectronics.

Full text of DOI:10.1016/S0038-1098(99)00271-9

PERGAMON
Solid State Communications 111 (1999) 681–685
Optical properties of n-type porous silicon obtained by
photoelectrochemical etching
*
C.H. Chen , Y.F. Chen
Department of Physics, National Taiwan University, Taipei, Taiwan
Received 2 March 1999; accepted 31 May 1999 by S. Ushioda
Abstract
The optical studies of n-type porous silicon prepared by the photo-assisted chemical etching are reported here. The optical
properties of samples obtained under different conditions have been investigated by photoluminescence and Fourier transform
infrared absorption measurements, and they are compared with that of p-type porous silicon. Our results clearly demonstrate
that the blue emission in porous silicon originates from surface compounds. From the infrared absorption measurement, we
point out that the surface compounds are Si–OH complexes. This conclusion is further supported by a recent calculation which
shows that Si–OH complexes can emit the photon energy in the range observed here. We show that the optical properties of the
n-type porous silicon are more stable than that of the p-type porous silicon. The result provides the evidence to support the fact
that the n-type porous silicon is a better candidate for the application in optoelectronics. ᭧ 1999 Elsevier Science Ltd. All rights
reserved.
Keywords: A. Nanostructures; A. Semiconductors; D. Recombination and trapping; E. Luminescence
Silicon is the main material of microelectronics at
present, but it is not widely used in optoelectronics.
The reason is due to the inherent nature of the indirect
transition in the band-edge emission. When the visible
photoluminescence (PL) of electrochemically etched
porous silicon was reported by Canham [1] in 1990,
the material [2–5] has been extensively studied to
clarify luminescence mechanism and to investigate
its possible use as a new material for the optical
device application. Despite of a large amount of
research, the mechanism underlying the visible lumi-
nescence is unclear. The two most widely discussed
mechanisms are the quantum confinement model and
the surface model. Recently, an improved model,
including both the effects of quantum confinement
and surface state, has been proposed [6,7]. In our
recent reports, we suggested that the lower energy
emission (ϳ1.8 eV) originates from the quantum
confinement effect and the higher energy emission is
dominated by the surface-state recombination [8,9].
Unlike most of the previous reports concentrated on
the studies of p-type porous silicon, in the paper, we
present the optical studies of n-type porous silicon
prepared by the photo-assisted chemical etching.
The obtained results will be compared with that of
the p-type porous silicon. We clearly demonstrate
that the blue emission in porous silicon originates
from surface compounds. From the infrared absorp-
tion measurement, we point out that the surface
compounds are Si–OH complexes. Our result is
further supported by the recent calculation, which
shows that Si–OH complexes can emit the photon
energy in the range observed here. In addition, we
show that the optical properties of the n-type porous
* Corresponding author.
0038-1098/99/$ - see front matter ᭧ 1999 Elsevier Science Ltd. All rights reserved.
PII: S0038-1098(99)00271-9

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C.H. Chen, Y.F. Chen / Solid State Communications 111 (1999) 681–685
Fig. 1. Photoluminescence spectra of the n-type porous silicon prepared by the photo-assisted etching.
silicon are more stable than that of the p-type porous
silicon.
blown dry by nitrogen gas immediately after electro-
chemical etching. The photoluminescence spectra
were recorded by a SPEX 0.85 m double spectro-
meter, and a photomultiplier tube. The sample was
placed inside a closed-cycle He cryostat. A He–Cd
laser working at 325 nm was used as the excitation
source. The infrared absorption spectra were
measured using a Nicolet ZDS Fourier transform
infrared spectrometer at room temperature.
Porous silicon samples were prepared by an elec-
trochemical anodization method. The silicon wafers
were n-type P-doped with resistance of 0.005–
0.01 V cm and (111) orientation. Anodization was
carried out using 60 mA/cm2 in a solution of HF
(40%):C2H5OH (1:1) for 25 min. A 100 W bulb was
used for illumination from a 15 cm distance [10].
Samples were rinsed in running deionized water and
The reaction involved in the anodic dissolution of
Fig. 2. Photoluminescence spectra of the n-type porous silicon prepared by different etching time.

C.H. Chen, Y.F. Chen / Solid State Communications 111 (1999) 681–685
683
as seen by the naked eye. Fig. 1 shows the PL spec-
trum of the as grown n-type porous silicon. Note that
without the assistance of light illumination during
etching, the as-grown n-type porous silicon does not
have a detectable PL signal. As we are aware, p-type
porous silicon, after annealing in an oxygen environ-
ment at 500ЊC for 1 h, can exhibit a similar spectrum
as shown in our previous report [9]. However, here we
show that the appearance of the blue emission in the
n-type porous silicon does not need the annealing
procedure.
Fig. 2 shows the PL spectra of different etching
time of the n-type porous silicon. We can see that
the 1.8 eV peak has a blue shift when the etching
time increases, while 2.9 eV peak remains at a fixed
position. This behavior is indeed consistent with our
previously proposed model [8,9], in which we
suggested that the 1.8 eV emission originates from
the quantum confinement effect, and the 2.9 eV blue
emission is dominated by the surface recombination.
According to this model, when the etching time
increases, electrons are confined in a narrower region,
as a result the 1.8 eV peak is blue shifted. Because the
2.9 eV peak originates from surface complexes, its
peak position is not influenced by the effect of quan-
tum confinement, and remains fixed as the etching
time is varied.
Fig. 3. Infrared absorption of the n-type porous silicon prepared by
different etching time.
siliconin HF solution can be expressed by Eqs. (1) and
(2) [10,11]
To explore further the origin of the surface
complexes, we measured the infrared absorption spec-
tra for porous silicon prepared by different etching
time. It is found that the blue emission correlates
very well with the absorption band centered around
3400 cmϪ1 as shown in Fig. 3. We can see that the
infrared absorption intensity becomes stronger as the
etching time is longer. Besides, we also observed that
the absorption intensity around 3400 cmϪ1 of the
sample prepared by the photo-assisted etching is
much stronger than that of the sample prepared with-
out light illumination. According to the reported result
on porous silicon as well as other studies [14–16], the
absorption band around 3400 cmϪ1 can be attributed
to the vibration mode of Si–OH complexes. We,
therefore, suggest that the active microstructures of
the blue emission are Si–OH surface complexes.
Further evidence to support our suggestion here can
be also found in a recent theoretical calculation [17].
It is known that due to its unstable structure, porous
silicon is subject to a significant evolution with time,
Si ϩ 4HOadϪ ϩ l ꢀ! SiO2 ϩ 2H2O ϩ ꢀ4 Ϫ l†eϪ
ꢀl Յ 4†SiO2 ϩ 6HF ! H2SiF6 ϩ 2H2O
ꢀ1†
Si ϩ 2FϪad ϩ 4HF ϩ l ꢀ! H2SiF6 ϩ H2 ϩ ꢀ2 Ϫ l†eϪ
ꢀl Յ 2†
ꢀ2†
Ϫ
Ϫ
in which HOad and Fad are the adsorbed ions, and
ꢀ and eϪ represent hole and electron, respectively.
According to these equations, the etching of the n-
type porous silicon is very difficult, because the reac-
tion needs holes to react. In order to overcome this
problem during etching, we illuminated the sample
with a 100 W bulb to produce more hole carriers.
Similar photoelectrochemical etching of the n-type
porous silicon has also been reported [12,13]. Quite
surprisingly, the as-grown sample reveals a blue color

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C.H. Chen, Y.F. Chen / Solid State Communications 111 (1999) 681–685
which directly affects its luminescent properties
[18,19]. This behavior is quite true for the p-type
porous silicon as shown in Fig. 4. We can see that
the PL intensity of the p-type porous silicon suffers
a
significant degradation under illumination.
However, under a similar condition, the PL intensity
of the n-type porous silicon decays much slower than
that of the p-type porous silicon as shown in Fig. 4.
This implies that the structure of the n-type porous
silicon is more resistant to external illumination. A
further test of the stability of the n-type porous silicon
was conducted under an external bias. In Fig. 5, we
can clearly see that under the same condition, the PL
intensity of the p-type porous sample reduces by about
65% when the bias is 40 V, while that of the n-type
porous silicon decreases only by about 15%. We,
therefore, conclude that the structural stability of the
n-type porous silicon is better than that of the p-type
porous silicon.
In conclusion, we reported an investigation of opti-
cal properties of the n-type porous silicon prepared by
the photo-assisted etching. We found that the as-
grown porous silicon can exhibit a blue emission at
room temperature. We confirmed that the blue emis-
sion originates from the surface Si–OH complex, and
the lower energy 1.8 eV peak results from the quan-
tum confinement. We also demonstrated that the
structural stability of the n-type porous silicon is
better than that of the p-type porous silicon. This prop-
erty indicates that the n-type porous silicon is a better
candidate for the application in optoelectronics than
the p-type porous silicon.
Fig. 4. The normalized PL decay intensity as a function of illumi-
nation time for the n-type and the p-type porous silicon.
Acknowledgements
This work was partly supported by the National
Science Council of the Republic of China. We thank
Professors J.S. Guan and J.G. Lin for the measurement
of infrared absorption spectra.
References
[1] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046.
[2] R.T. Smith, S.D. Collins, J. Appl. Phys. 71 (1993) R1.
[3] D.J. Lockwood, Solid State Commun. 92 (1994) 101.
[4] L. Brus, J. Phys. Chem. 98 (1994) 3735.
[5] G. Cullis, L.T. Canham, P.D.J. Calcott, J. Appl. Phys. 82
(1997) 909.
Fig. 5. The drop of the PL peak intensity versus applied lateral
voltage for the n-type and the p-type porous silicon.

C.H. Chen, Y.F. Chen / Solid State Communications 111 (1999) 681–685
685
[6] F. Koch, V. Petrova-Koch, T. Muschik, A. Nikolov, V. Gavri-
lenko, Mater. Res. Soc. Symp. Proc. 283 (1993) 197.
White, B.E. Gnade, L. Magel, Appl. Phys. Lett. 64 (1994)
1374.
[7] G.G. Qin, Y.Q. Jia, Solid State Commun. 86 (1993) 559.
[8] I.M. Chang, Y.F. Chen, J. Appl. Phys. 82 (1997) 3514.
[9] J.C.Fan,C.H.Chen,Y.F.Chen,Appl.Phys.Lett.72(1998)1605.
[10] R. Memming, G. Schwandt, Surf. Sci. 4 (1966) 109.
[11] X.G. Zhang, J. Electrochem. Soc. 138 (1991) 3750.
[12] J.L. Maurice, A. Riviere, A. Alapini, C. Levy-Clement, Appl.
Phys. Lett. 66 (1995) 1665.
[15] P. Gupta, A.C. Dillon, A.S. Bracker, S.M. George, Surf. Sci.
245 (1991) 360.
[16] M.G. Jani, L.E. Halliburton, E.E. Kohnke, J. Appl. Phys. 54
(1983) 6321.
[17] J.L. Gole, D.A. Dixon, Phys. Rev. B 37 (1998) 12 002.
[18] J.T. Lue, K.Y. Lo, S.K. Ma, C.-L. Chen, C.S. Chang, Solid
State Commun. 86 (1993) 593.
[13] A. Starovoitov, S. Bayliss, Appl. Phys. Lett. 73 (1998) 1284.
[14] J. Yan, S. Shih, K.H. Jung, D.L. Kwong, M. Kovar, J.M.
[19] I.M. Chang, S.C. Pan, Y.F. Chen, Phys. Rev. B 48 (1993)
8747.