APPLIED PHYSICS LETTERS
VOLUME 81, NUMBER 19
4 NOVEMBER 2002
Exciton formation dynamics in crescent-shaped Quantum Wires
T. Yasuhira,a),b) K. Komori,b) R. Akimoto,b) X. L. Wang, M. Watanabe, and M. Ogura
National Institute of Advanced Industrial Science and Technology Tsukuba Central 2, 1-1-1 Umezono,
Tsukuba 305-8568, Japan
共Received 5 June 2002; accepted 5 September 2002兲
We have investigated the ultrafast carrier dynamics in optically high-quality V-grooved GaAs/
AlGaAs quantum wires using a highly sensitive nonlinear spectroscopy technique. It is found that,
with resonant excitation, there is a blueshift due to exciton–exciton interaction within 0.3 ps
followed by a redshift due to exciton relaxation. With nonresonant excitation, the redshift occurs due
to the phonon emission during exciton formation. As a result, the exciton formation time and the
exciton lifetime in the quantum wires are found to be 5 and 110 ps, respectively. © 2002 American
Institute of Physics. 关DOI: 10.1063/1.1517721兴
Semiconductor nanostructures are expected to become
materials for ultrafast devices because of their high differential gain and optical nonlinearity. The nonlinear response of
excitons confined in GaAs/AlGaAs quantum wells has been
extensively studied both theoretically and experimentally.1 In
particular, the progress made on femtosecond spectroscopic
techniques has enabled us to investigate the ultrafast nonlinear response of photo-excited carriers and excitons in quantum wells. In contrast, the new method fabricating crescentshaped quantum wires on a V-grooved substrate by the flow
rate modulation epitaxy method 共FME兲, which can obtain
little roughness on heterojunctions, was developed and the
characteristic of high-quality quantum wires2–5 with narrower photoluminescence 共PL兲 linewidth was reported compared with those of other methods. However, the low density
of conventional V-grooved quantum wires has limited the
use of nonlinear spectroscopy. In this work, we fabricated
quantum wires with high optical quality, high density, and a
flat surface, which enabled us to study them by using nonlinear spectroscopy.
We investigated the ultrafast dynamics of the excitonic
optical nonlinearities in GaAs/AlGaAs V-grooved quantum
wires under conditions of resonant and nonresonant excitation by using highly-sensitive and highly-time-resolved
reflection-type pump-probe nonlinear spectroscopy. This
study provides information on the exciton formation time,6
exciton lifetime, and the interaction between photo-excited
carriers and excitons.
We fabricated two samples under the same growth conditions but with periods of 10 and 15 for the scanning electron microscope 共SEM兲 micrograph and experiment, respectively. Figure 1共a兲 shows a cross-sectional SEM micrograph
of 10 periods of GaAs/AlGaAs quantum wires. We fabricated the sample used in this experiment as follows.
V-grooves with periods of 4 ␮m were fabricated on a GaAs
substrate and an AlGaAs buffer layer, 15 periods of GaAs
quantum wires AlGaAs barrier layers, and an AlGaAs cladding layer were successively grown on the V-grooves by
using the FME method.2,4,5 After the wet chemical etching of
共100兲 quantum film layers on the mesa-top between the
V-grooves, an AlGaAs cladding layer is overgrown on the
sample and the regrown surface is polished mechanically in
order to obtain a sample with a flat surface. The quantum
wires were 11 nm thick and 46 nm wide 共full width兲. The
distance between each uncoupled quantum wire was 50 nm.
Figure 1共b兲 shows the PL and the PL excitation 共PLE兲 spectra of the quantum wires measured at the low temperature of
T⫽6 K. In the PLE curves, we observed the 1e–1hh exciton
peak at 800 nm and the 1e–1lh exciton peak around 793 nm,
with excitation laser polarization parallel and vertical to the
quantum wires. This polarization dependence is due to the
one-dimensional band mixing effect and is an intrinsic phenomenon to one-dimensional nanostructures.7 The luminescence from the quantum wires in the bottom of the V-groove
at about 1.5498 eV dominates the PL spectrum and that from
the 共111a兲 sidewall quantum wells is very weak.2 The Stokes
shift of 2 meV means that this sample consists of optically
high-quality quantum wires that have little roughness of the
heterojunction.
We used an ultrashort pulse from a Ti3⫹ : sapphire laser
as the pump and the probe pulse. A weak probe pulse of
about 100 fs duration is reflected by the sample at various
time delays between the pump and the probe pulse and is
detected with the highly-sensitive lock-in system. The polar-
a兲
Electronic mail: t-yasuhira@aist.go.jp
Also at: CREST-Japan Science and Technology Corporation 共JST兲, 4-1-8,
Kawaguchi, Saitama, 332-0012, Japan.
b兲
FIG. 1. 共a兲 Cross-sectional SEM micrograph of multi-layered quantum
wires. 共b兲 PL and PLE characteristics of quantum wires.
0003-6951/2002/81(19)/3642/3/$19.00
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© 2002 American Institute of Physics
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Appl. Phys. Lett., Vol. 81, No. 19, 4 November 2002
FIG. 2. Differential reflectivity spectra for various delay times 共a兲 under
resonant excitation (␭pump⫽800 nm) and 共b兲 under nonresonant excitation
(␭pump⫽795 nm).
ization of the pump pulse and the probe pulse is parallel and
vertical to the quantum wires, respectively.
Figure 2 shows the differential reflectivity ⌬R spectra
for the various delay times ␶ of ⫺0.4 to 100 ps. The full
width at half maximum 共FWHM兲 of the pump pulse spectrum was set the same as the linewidth 共3 nm兲 of 1e–1hh
shown in Fig. 1共b兲 in order to reduce the effect of free carriers under resonant excitation. Figs. 2共a兲 and 2共b兲 show the
differential reflectivity spectra ⌬R for the resonant excitation
at the 1e–1hh absorption line and those for the nonresonant
excitation at 795 nm, respectively. The arrows indicate the
center positions of the wavelengths of the half maxima in the
spectra. They show the spectral shift for the various delay
times. The spectra labeled ⫺0.4 and ⫺0.3 ps correspond to
cases where the probe pulse proceeded to the pump pulse on
the sample. The symmetric spectra, for which negative signals appear on the high-energy sides and positive signals
appear on the low-energy sides of the pump spectra at minus
delays, are said to indicate the coherent effect according to
Foing et al.8,9 However, in our experiment, whether these
Yasuhira et al.
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FIG. 3. Delay time dependence of 共a兲 the peak values, 共b兲 shifted wavelengths, and 共c兲 FWHM of ⌬R spectra. Solid curves show those under
resonant excitation and dotted curves show those under the nonresonant
excitation.
spectra at negative delay are due to a coherent effect or an
artifact is not clear. Further experiments will be undertaken
on this phenomenon in the near future.
Figures 3共a兲 and 3共b兲, respectively, show the peak value
and the shifted wavelength of ⌬R spectra in Fig. 2 versus the
delay time. Under resonant excitation, as shown in Figs. 2共a兲
and 3共b兲, only the blueshift occurs within 0.3 ps, and after
that the redshift occurs. This blueshift is due to the mutual
exciton–exciton interaction caused by adjusting the pump
pulse spectrum inside the line width of exciton resonance for
the resonant excitation and generating relatively high-density
excitons.1,10 In the second step, spectrum redshift occurs due
to the relaxation of K⫽0 excitons in the quantum wires to
K⫽0 excitons, namely excitonic relaxation.11 In the third
step, the radiative decay, namely the decrease of the exciton
population, occurs. This causes the decrease of the peak
value of the spectrum. In contrast, Ambigapathy et al. have
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3644
Yasuhira et al.
Appl. Phys. Lett., Vol. 81, No. 19, 4 November 2002
also observed the blueshift and subsequent redshift.12 In their
case, the carrier density was 3⫻106 cm⫺1 , which is about
one order larger than our experimental condition (2.6
⫻105 cm⫺1 ). Therefore, band-gap renormalization is the
dominant effect in their experiment, rather than the exciton–
exciton interaction found in Refs. 1 and 10.
Under nonresonant excitation, as shown in Figs. 2共b兲 and
3共b兲, there is only a redshift in the peaks of the ⌬R spectra
from the excitation wavelength to the PLE peak of 1e–1hh
after the excitation. From the decay rate of the peak values of
the ⌬R spectra, the exciton lifetime is found to be about 110
ps. Oberli et al. obtained an exciton radiative lifetime of 350
to 400 ps with GaAs/AlGaAs quantum wires.13 The thickness of their quantum wires was from 1.5 to 5 nm, which is
half the thickness of the wires we used 共11 nm兲. For this
reason, we consider the excitons in their quantum wires to be
localized due to interface disorder. This would result in the
fluctuations in the confining potential along the wire axis and
thus in the occurrence of local potential minima where exctions are bound.11 Therefore, their exciton lifetime 350 to
400 ps13 at low temperature is considered to be due to these
localized excitons. In contrast, as the thickness of our quantum wire is 11 nm, we consider the excitons in our quantum
wires to be almost free and not to be localized. A onedimensional exciton is theoretically expected to have a short
lifetime of 150 ps.14 Experimentally, Bellessa et al. report
that localized excitons have a long lifetime of 300 ps and
free excitons show a very short lifetime of 150 ps.3 Our
experimental result is consistent with the lifetime of these
free excitons.
Under nonresonant excitation, free electrons and holes
are initially generated and become exciton states. This is why
the peaks reach their maximum 5 ps, as shown in Fig. 3共a兲,
after the excitation. Kumar et al. obtained an exciton formation time of 30 ps with a difference of 0.274 eV between the
excitation energy and the exciton energy.11 On the other
hand, we obtained an exciton formation time of 5 ps for a
difference of 10 meV between the excitation energy and the
exciton energy. The relaxation time per one phonon emission
calculated from the number of phonons in Ref. 10 suggests
that our exciton formation time should be of about picosecond order. Figure 3共c兲 shows the FWHM of the ⌬R spectrum
共namely, the linewidth of the exciton兲 versus the delay time.
Under resonant conditions, the FWHM of the exciton line
increases rapidly between 0 and 0.6 ps and becomes constant
after 0.6 ps. The rapid broadening is due to the exciton–
exciton interaction that occurs during the optical excitation.
With nonresonant excitation, the spectrum is considered to
become initially broad as the free carriers and the excitons
are excited at 795 and 800 nm. Subsequently, these free carriers form excitons and rapid spectral narrowing occurs. The
exciton–exciton scattering rather than the exciton-free carrier scattering becomes dominant, and these cause the spectral narrowing.
In conclusion, we have studied the ultrafast carrier dynamcs in optically-high-quality quantum wires using highlysensitive and highly-time-resolved pump-probe measurements. As a result, under resonant excitation a blueshift
occurred within 0.3 ps, and after that a redshift occurred.
These blue- and redshifts are considered to be the manybody effect due to the exciton–exciton interaction and that
due to exciton relaxation, respectively. With nonresonant excitation, the red shift occurs from the nonresonant excitation
wavelength. Prior to exciton formation, the redshift is due to
the phonon emission to form excitons. With nonresonant excitation, the exciton formation time is 5 ps for an energy
difference of 10 meV between the excitation energy and the
exciton energy. With resonant excitation, the exciton lifetime
is about 110 ps. This exciton lifetime corresponds to the free
excitonic lifetime, which is close to the theoretical intrinsic
lifetime. Both the short exciton lifetime and the small
Stokes-shift reveal the characteristic of high-quality quantum
wires, which is little interface roughness.
The authors thank Dr. N. Kobayashi, Director of the
Photonics Research Institute of National Institute of Advanced Industrial Science and Technology 共AIST兲 for his
encouragement. The authors also thank Dr. M. Foerst, Dr. T.
Dekorsy, and Prof. H. Kurz of the Institute of Semiconductor
Electronics, RWTH Aachen for helpful discussions.
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