Efficient indirect exciton luminescence in silicon nanowires

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Physica E 17 (2003) 183 – 184
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Ecient indirect-exciton luminescence in silicon nanowires
S. Nihonyanagi, Y. Kanemitsu
∗
Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma, Nara 630-0192, Japan
Abstract
We have studied the near-infrared photoluminescence (PL) spectrum and dynamics in crystalline silicon nanowires with
a triangular cross-section of about 65 nm side length. Time-resolved PL measurements clearly show that the nanowire has a
shorter PL lifetime than bulk crystalline silicon. The PL decay curve of the nanowire consists of the fast and slow components.
The temperature dependence of the fast component intensity agrees with that of the integrated PL intensity. This agreement
suggests that non-radiative recombination is suppressed in the nanowire. The short PL lifetime and no signiÿcant thermal
quenching of the PL intensity result in the enhancement of the radiative recombination in the nanowire.
? 2002 Elsevier Science B.V. All rights reserved.
PACS: 78.55.Ap; 71:35: − y; 78.66.Db; 78.67.Lt
Keywords: Photoluminescence; Nanowire; Crystalline silicon; Indirect exciton
1. Introduction
In order to realize optical interconnections in an
integrated circuit (IC), it is necessary to fabricate
an ecient light emitter compatible with standard,
Si-based IC technologies. In the last decade, many
researchers have reported ecient photolumines-
cence (PL) and electroluminescence from Si-based
nanostructures, such as porous Si and Si nanocrys-
tals [1,2]. However, because of their inhomogeneous
structures, the observed PL is broad. Recently, pro-
gresses of silicon processing techniques have made
it possible to fabricate high-quality nanometer-sized
Si-based nanostructures. It opens opportunities not
only to realize ecient light-emitting devices also to

∗
Corresponding author. Graduate School of Materials Science,
Nara Institute of Science and Technology, 8916-5 Takayama,
Ikoma, Nara 630-0101, Japan. Fax: +81-74372-6019.
E-mail address: (Y. Kanemitsu).
study electronic and optical properties in crystalline
Si (c-Si) nanostructures hidden behind structural in-
homogeneity. In our previous paper [3], we have fab-
ricated large c-Si nanowires and the near-infrared PL
is observed even at room temperature. In this study,
we performed time-resolved PL measurements and
discuss the mechanism of ecient PL in the c-Si
nanowires.
2. Sample and experimental setup
The c-Si nanowires were fabricated from c-Si
substrate by anisotropic etching and thermal oxida-
tion techniques [3]. The nanowire has a triangular
cross-section of about 65 nm side length and are fully
surrounded by silicon dioxide. The same c-Si substrate
was used as the bulk c-Si sample and its surface was
oxidized under the same condition. Time-resolved PL
was measured by using a photon-counting method
1386-9477/03/$ -see front matter ? 2002 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1386-9477(02)00753-1
184 S. Nihonyanagi, Y. Kanemitsu /Physica E 17 (2003) 183–184
under 400 and 150 fs laser pulse excitation. The PL
signal was detected by an InP/InGaAsP photomulti-
plier. The samples were mounted on the cold ÿnger of
a temperature-variable closed cycle He gas cryostat.
3. Results and discussion

The Si nanowire shows several phonon-assisted PL
bands, which are quite similar to those in bulk c-Si
at low temperatures [3]. The PL decay curves were
measured at 1:099 eV, which corresponds to the peak
energy of the TO-phonon-assisted PL band. The inset
in Fig. 1 shows the PL decay curves at 40 K both in
the nanowire and bulk c-Si. The PL decay curve in
the nanowire consists of two exponential decay com-
ponents. The PL lifetime of the fast component in the
nanowire is about 0:4 s, which is shorter than that
in bulk c-Si. The lifetime of the slow component is
comparable with that in bulk c-Si. Fig. 1 shows the
temperature dependence of the fast-decay PL inten-
sity and the integrated PL intensity in the nanowire
and bulk c-Si. In the nanowire, the temperature de-
pendence of the fast-decay PL intensity is in good
050100
10
–2
10
–1
10
0
0 10
Temperature (K)
PL Intensity (arb. units)
NW
Bulk
PL Intensity (arb. units)
Time (µs)

40 K
fast–decay
NW
Integrated Intensity
NW Bulk
Fig. 1. Temperature dependence of the fast-decay intensity in the
nanowire (NW) (
•
) and the integrated intensity both in the NW
and bulk c-Si ( and
). The inset shows PL decay curves at
40 K.
agreement with that of the integrated intensity, com-
pared with that in bulk c-Si. This agreement suggests
that in the nanowire the fast radiative recombination
process is dominant. The temperature dependence of
the integrated PL intensity in the nanowire is com-
pletely dierent from that in bulk c-Si. Considering the
weak temperature dependence of the PL intensity, the
radiative recombination is enhanced in the nanowire.
It is likely that in the nanowire the enhancement of the
radiative combination is caused by the spatial conÿne-
ments of excitons due to the strains near the Si=SiO
2
interface [4], rather than the quantum conÿnements of
excitons in the nanowire, because no blueshift of the
PL energy is observed in the nanowire.
4. Conclusion
We have studied PL dynamics in the large c-Si
nanowires fully surrounded by silicon dioxide.

Time-resolved PL measurements show that in the
nanowire the PL decay curve consists of the fast
and slow components and the PL lifetime of the fast
component is shorter than that in bulk c-Si. It is con-
sidered that the radiative recombination of excitons
is enhanced in the nanowire. The temperature depen-
dence of the PL intensity shows that the PL eciency
is determined by the fast-decay components.
Acknowledgements
The authors would like to thank Prof. Y. Hirai of
Osaka Pref. Univ. for sample preparations and discus-
sions. This work was supported in part by The Foun-
dation of Nara Institute of Science and Technology,
and a Grant-in-Aid for Scientiÿc Research from Japan
Society for the Promotion of Science.
References
[1] Y. Kanemitsu, Phys. Rep. 263 (1995) 1.
[2] L. Pavesi, L. Dal Negro, C. Mazzoleni, G. FranzÂo, F. Priolo,
Nature 408 (2000) 440.
[3] Y. Kanemitsu, H. Sato, S. Nihonyanagi, Y. Hirai, Phys. Stat.
Sol. (A) 190 (2002) 755.
[4] S. Nihonyanagi, Y. Kanemitsu, unpublished data.