APPLIED PHYSICS LETTERS
VOLUME 83, NUMBER 24
15 DECEMBER 2003
AlGaAsÕGaAs quantum wires with high photoluminescence
thermal stability
X.-Q. Liu,a) X.-L. Wang, and M. Ogura
Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST),
Tsukuba Central 2, Tsukuba 305-8568, Japan
T. Guillet, V. Voliotis, and R. Grousson
Groupe de Physique des Solides, CNRS, Universités Pierre et Marie Curie et Denis Diderot,
2 Place Jussieu, F-75251 Paris Cedex 05, France
共Received 13 March 2003; accepted 23 October 2003兲
We report a 5 nm thick V-shaped AlGaAs/GaAs single quantum wire 共QWR兲 that showed high
photoluminescence 共PL兲 thermal stability as a result of our recent progress in fabrication techniques.
The integrated PL intensity of the QWR sample was quenched only by a factor of about 2.5 when
the temperature was increased from 5 to 300 K. This sample also showed higher PL thermal stability
over the whole temperature range than a 5 nm thick single quantum well reference sample grown
under similar conditions. © 2003 American Institute of Physics. 关DOI: 10.1063/1.1633679兴
Semiconductor quantum wires 共QWRs兲 have been predicted to show various superior optical and electronic properties compared with their higher-dimensional counterpart:
quantum wells 共QWLs兲.1,2 Despite the large number of experimental works devoted to the realization of high-quality
QWRs over the past two decades,3 it remains difficult to
obtain effects from QWRs superior to those typically observed from QWLs. A few years ago, we developed a QWR
fabrication technique: the use of flow rate modulation epitaxy 共FME兲 in selective growth of AlGaAs/GaAs on
V-grooved substrates.4,5 This gave rise to significant improvements in wire quality over conventional techniques,
and we were able to demonstrate several superior optical
effects that originated from the one dimensionality of QWRs,
including smaller thermal broadening of the photoluminescence 共PL兲 linewidth 共and a much narrower PL linewidth at
room temperature兲 than that of a QWL,6 and weaker temperature dependence of the laser threshold current.7 Very recently, we developed two other techniques: the use of a new
arsenic source material 关tertiarybutylarsine 共TBAs兲兴 and an
improved V-groove stripe alignment process. These techniques further improved the wire quality in a dramatic way,
especially in terms of interface quality8,9 and crystal purity.10
We found that the use of TBAs can greatly reduce the concentration of nonradiative recombination centers around the
QWR compared with growth using AsH3 ; the roomtemperature integrated PL intensity of a 5 nm TBAs-grown
QWR was more than 3000 times stronger than that of a 5 nm
AsH3 -grown sample.10 TBAs can also suppress step bunching on the high-index (311)A facets of QWRs as revealed by
atomic force microscope observation and thus improve the
interface uniformity.8 On the other hand, use of the V-groove
stripe alignment process during photolithography allows us
to align the V grooves to the exact 关01-1兴 crystallographic
direction with precision greater than 3.7⫻10⫺3 deg. This is
improvement of approximately 27-fold over our old fabrication process. We have characterized the interface quality of
a兲
Electronic mail: xqliu@unity.ncsu.edu
our new-generation QWRs using scanning micro-PL
measurements.9 We show in Fig. 1 a typical scanning
micro-PL intensity image of a 5 nm QWR sample grown
using these improved techniques measured at 10 K. In this
case, the wires are essentially an assembly of real onedimensional monolayer-step islands whose average lengths
were estimated to be about 0.4 ␮m by statistically analyzing
their micro-PL images.9 The longest islands can be as long as
a few microns like, for example, the one at Z⫽5 ␮ m shown
in Fig. 1. This is in striking contrast to our old-generation
QWRs which showed monolayer thickness fluctuation on the
order of 20–100 nm.11 Moreover, the PL peak of each of
these one-dimensional islands can be fitted perfectly by a
single Lorentizan curve, suggesting that excitonic states are
coherently extended in these islands.9 In this letter, we report
FIG. 1. Scanning micro-PL intensity image of a new-generation 5 nm
AlGaAs/GaAs QWR measured at 10 K. The image was obtained by scanning the microscope objective lens along the wire axis on the sample surface
and recording the micro-PL spectrum at each position. The spatial resolution
of the micro-PL system was about 1 ␮m.
0003-6951/2003/83(24)/5059/3/$20.00
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© 2003 American Institute of Physics
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Appl. Phys. Lett., Vol. 83, No. 24, 15 December 2003
higher PL thermal stability observed from these newgeneration QWRs compared with QWLs grown under similar conditions.
The QWR sample studied in this work was grown at
630 °C by a low-pressure metalorganic vapor phase epitaxy
共MOVPE兲 system. Trimethylaluminum, triethylgallium, and
TBAs were used as group III and group V precursors, respectively. More details on the growth conditions can be found in
previous work.5,8 The V grooves, with a pitch of 4 ␮m, were
formed on 共001兲-oriented GaAs substrates by standard photolithography and wet chemical etching procedures. The
sample structure consists of a 0.3 ␮m GaAs buffer layer, a
0.9 ␮m Al0.42Ga0.58As lower barrier layer, a FME-grown 5
nm GaAs QWR layer, a 0.2 ␮m Al0.42Ga0.58As upper barrier
layer, and a 10 nm GaAs cap layer. For comparison, a 5 nm
single QWL with a similar layer structure was also grown
under similar conditions.
The luminescence thermal stability of the QWR sample
was characterized by temperature-dependent PL measurements. The sample was fixed on the cold finger of a He-flow
cryostat whose temperature was controlled within accuracy
of ⫾0.5 K. A continuous wave 共cw兲 Ti:sapphire laser was
focused onto the sample surface to a spot size of about 100
␮m at power of about 10 mW. The laser wavelength 共700 nm
at 5 K兲 was tuned to be between the emission wavelength of
the QWR and that of the sidewall QWL 共⬃680 nm at 5 K兲 so
as to excite the QWR selectively. The excitation wavelength
was also adjusted with increasing temperature to maintain a
constant energy separation 共140 meV兲 of the excitation laser
from the PL emission position in order to avoid possible
change in the laser absorption coefficient at high temperatures. The PL signal was detected with a Jobin-Yvon 64 cm
triple monochromator and a liquid nitrogen-cooled charge
coupled device camera.
Figures 2共a兲 and 2共b兲 show the PL spectra of the QWR
and the QWL reference samples measured in a temperature
range of 5–300 K, respectively. At 5 K, the PL peak from the
QWR was observed around 1.63 eV with a full width at half
maximum 共FWHM兲 of about 9 meV. A weak shoulder, indicated by a vertical arrow in Fig. 2共a兲, was also observed with
an energy separation of about 15.5 meV from the main peak.
The intensity of this peak was very sensitive to the temperature; it disappears completely at 20 K. A similar peak was
sometimes observed in the new-generation QWR samples,
but its origin was not very clear. It is surprising to observe
that the PL can be measured easily up to room temperature
共300 K兲 without changing any measurement conditions. The
5 nm QWL reference sample showed a ground state emission
peak at 1.624 eV at 5 K with a FWHM of about 8.8 meV. We
then calculated the integrated PL intensity of these spectra in
order to evaluate the luminescence thermal stability. Figure 3
shows the integrated PL intensity of the QWR as a function
of the temperature together with the results from the 5 nm
QWL reference sample for comparison, in which the PL intensities were normalized by the values at 5 K.
The integrated PL intensity of the 5 nm QWR first
showed a weak increase, followed by a weak decrease, with
an increase in temperature in the range of 5–50 K, then
became independent of the temperature up to 130 K. The PL
intensity started to decrease slowly with further increases in
Liu et al.
FIG. 2. Temperature-dependent PL spectra of 共a兲 the 5 nm QWR and 共b兲 the
5 nm QWL reference samples.
temperature beyond 130 K, indicating that scattering of excitons by nonradiative recombination centers begins to take
effect from 130 K. It is remarkable to notice that the overall
quenching of the integrated PL intensity from 5 to 300 K is
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Liu et al.
Appl. Phys. Lett., Vol. 83, No. 24, 15 December 2003
FIG. 3. Integrated PL intensities of the QWR 共open circles兲 and the QWL
共open triangles兲 samples as a function of the measurement temperature. The
solid lines are a visual guide for the eye.
only about 2.5-fold. This is, to the best of our knowledge, the
highest PL thermal stability ever reported for any kind of
QWRs. The PL intensity of the 5 nm QWL sample showed
no temperature-independent region; it began to quench with
increases in temperature from 5 K with quenching rates
higher than those of the QWR sample over the whole temperature range. At room temperature, the integrated PL intensity of the QWR is about 2.7 times stronger than that of the
QWL. We believe this may be the first QWR that showed
higher PL thermal stability than an equivalent QWL sample.
The PL intensity of a semiconductor quantum structure
is considered to be determined by three factors: 共1兲 the radiative recombination rate, 共2兲 the concentration of nonradiative recombination centers, and 共3兲 the scattering rate of
excitons/free carriers by nonradiative recombination centers.
A V-grooved QWR is usually surrounded by three crystallographic facets: one 共001兲 facet at the center and two 兵 311其 A
facets on the two sides.8 A 兵 311其 A facet is unlikely to give
rise to a lower concentration of nonradiative recombination
centers compared with growth on the 共001兲 surface in
MOVPE.12 We tentatively attributed the higher PL thermal
stability observed in our new-generation QWR sample to a
higher radiative recombination rate and/or a lower exciton/
free-carrier scattering rate by nonradiative recombination
centers in QWRs than in QWLs. In other words, what we
observed is most likely an intrinsic effect of a high-quality
QWR. A higher PL thermal stability is also an effect theo-
5061
retically expected for a high-quality QWR. First, a higher
radiative recombination rate is expected from a QWR than
from a QWL due to the square root dependence on the temperature of the radiative recombination lifetime of a QWR9,13
compared with the linear dependence of that of a QWL.13–15
Second, a lower longitudinal optical 共LO兲 phonon scattering
rate for excitons and free carriers was also predicted theoretically for a QWR compared with a QWL.16 However, further experiments are necessary to confirm the above speculations.
In summary, we reported a 5 nm AlGaAs/GaAs
V-shaped QWR that showed very high PL thermal stability
for a QWR as a result of our recent progress in fabrication
techniques. The integrated PL intensity of the QWR sample
was quenched only by a factor of 2.5 from 5 to 300 K. This
QWR showed a higher PL thermal stability than an equivalent QWL sample, and was tentatively considered an intrinsic
effect of a high-quality QWR. These results suggest that the
QWR as a material for high-efficiency optoelectronic devices
is superior to a conventional QWL structure.
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For recent progresses, see, for example, Abstracts of the First International
Workshop on Quantum Nonplanar Nanostructures and Nanoelectronics,
Tsukuba, Japan, 2001.
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Free excitons in a QWL should show a much faster low-temperature radiative recombination lifetime than those in a QWR due to the reduced
coherence volume in a QWR 共Refs. 13 and 14兲. However, a comparable
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at low temperatures in practical QWL samples.
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