Download M. E. Lee, Y. C. Yeh, Y. H. Chung, C. L. Wu, C. S. Yang, W. C. Chou

ARTICLE IN PRESS
Physica E 26 (2005) 422–426
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Carrier capture and relaxation of self-assembled ZnTe/ZnSe
quantum dots prepared under Volmer–Weber and
Stranski–Krastanow growth modes
M.-E. Leea, Y.-C. Yehb, Y.-H. Chungb, C.-L. Wub, C.-S. Yangc, W.-C. Chouc,
C.-T. Kuob, D.-J. Jangb,
a
Department of Physics, National Kaoshiung Normal University, Kaoshiung, 80264 Taiwan, ROC
Department of Physics, National Sun Yat-sen University, 70 Lienhai Road, Kaoshiung, 80441 Taiwan, ROC
c
Department of Electrophysics, National Chiao-Tung University, Hsinchu, 30056 Taiwan, ROC
b
Available online 24 November 2004
Abstract
The carrier capture and relaxation of type II ZnTe/ZnSe quantum dots have been investigated with ultrafast timeresolved photoluminescence upconversion. The carrier capture times were 7 and 38 ps for the Volmer–Weber mode and
Stranski–Krastanow mode, respectively. We found that the carrier relaxation of QDs exhibits faster decay under the
Volmer–Weber growth mode than under the Stranski–Krastanow growth mode. We attribute the difference of carrier
relaxation to the wetting layer formed in the Stranski–Krastanow growth mode.
r 2004 Elsevier B.V. All rights reserved.
PACS: 78.47.+p; 73.63. b; 78.67. n
Keywords: Type II quantum dots; Time-resolved photoluminescence; Carrier capture; Carrier relaxation; Auger process
Semiconductor quantum dots (QDs) have been
extensively studied for potential applications in
optoelectronic devices and for their novel electronic and optical properties in 3-D confined nanostructures [1,2]. Carrier capture and relaxation are
among the important properties that affect the
device performance such as threshold current and
Corresponding author. Fax: 886-75253709.
E-mail address: [email protected] (D.-J. Jang).
temperature stability. Recently, the impact of the
wetting layer on the carrier capture and relaxation
in QDs has been investigated. Several reports
indicated that the wetting layer is important for
carriers to relax excess energy easily through a
continuum tail of wetting layer defect states [3].
The carrier dynamics in the wetting layer in
relation to the capture into the QDs have also
been investigated [4]. However, recently, a report
found that the two-dimensional character of the
1386-9477/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.physe.2004.08.092
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M.-E. Lee et al. / Physica E 26 (2005) 422–426
wetting layer is not relevant in determining the QD
capture and relaxation [5]. While the details of the
mechanism of carrier capture and carrier relaxation remain controversial and most of the studies
were focused on type I QDs of III–V materials, the
present work demonstrate that the wetting layer is
significant to the carrier dynamics in type II QDs
of II–VI materials.
In order to identity the impact of wetting layers
on the carrier capture and relaxation, we investigate the carrier capture and carrier relaxation of
self-assembled ZnTe quantum dots grown in a
ZnSe matrix by molecular beam epitaxy with two
growth modes. One of the growth methods is the
Stranski–Krastanow (SK) mode that a layer was
grown first in two dimensions then followed by
three-dimensional islands after a critical thickness.
The other growth mode is the Volmer–Weber
(VW) mode in which no two-dimensional layer
was formed before three dimension islands were
grown. The type-II QD structure, with electrons
and holes confined in different spatial regions, also
exhibits interesting physical properties, such as
slower radiative lifetime, tunability of emission
energy, and reduction of electron–hole interaction.
While holes are confined in the ZnTe layer,
electrons are localized in ZnSe barriers. We study
the carrier capture and relaxation with ultrafast
time-resolved photoluminescence (PL) that provides a temporal resolution of better than 300 fs.
The high temporal resolution is essential to study
the fast carriers captured by QDs and the PL
decay at the first 20 ps after photoexcitation. The
carrier capture times were determined from the
time-resolved photoluminescence to be 7 and 38 ps
for the VW mode and SK mode, respectively. We
found that carrier relaxation of QDs in the VW
growth mode exhibits faster decay than that of
QDs in the SK growth mode due to the wetting
layer in the SK mode providing a pathway for
carriers to diffuse and migrate from large (small)
to small (larger) QDs.
Self-assembled ZnTe QDs were grown in a ZnSe
matrix on the GaAs substrate by using a Riber 32P
molecular beam epitaxy system with two growth
modes of VW and SK. The VW mode of QDs with
2.9 MLs coverage were capped with a ZnSe
thickness of 5 nm and the SK mode of QDs with
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3.0 MLs coverage were capped with a ZnSe
thickness of 50 nm. Atomic force microscopy
(AFM) showed that two families with different
sizes of diameters, large and small, of QDs were
grown for both samples. The AFM also indicated
that mainly small size QDs were grown for small
ZnTe deposition (thickness of 1.3 MLs). The
dependence of the sizes of QDs on ZnTe deposition thickness were also reported for InAs/GaAs
QDs [6]. Large QDs with dot densities of 3.6 108
and 1.5 109 cm 2 for the VW mode and SK
mode, respectively, were estimated by atomic force
microscopy as well as the small QDs with dot
densities of 4.0 109 and 1.0 1010 cm 2 for the
VW mode and SK mode, respectively. The details
and growth parameters of these two modes are
given elsewhere [7,8]. To study the carrier capture
and relaxation of two different growth mode of
QDs, the all optical PL up-conversion spectroscopy [9,10] was used to measure the time-resolved
photoluminescence of these samples. This upconversion technique, in contrast to a pump
probe, covers a very wide spectral range of PL
without the need of a tunable laser source. A beam
with a 150 fs pulsewidth from a Kerr-lens modelocked Ti:sapphire laser at a repetition rate of
76 MHz was split into two beams. One of the two
beams was frequency-doubled with a 4 betabarium-borate (BBO) nonlinear crystal and was
used to illuminate the samples. For the present
work, 100 mW of average power was focused on a
50 mm spot on the surface of the sample mounted
in a low-vibration closed cycle cryostat kept at
temperature of 35 K for the present study. The PL
was collected and focused by a pair of off-axial
parabolic mirrors onto another BBO nonlinear
crystal, where the PL was mixed with the fundamental beam that was sent through an optical
delay. The signal of the sum frequency was
generated by angle-tuning the BBO crystal to the
spectrum of interest according to the phasematching condition. The up-converted signal was
filtered with a band-pass filter to remove any
contribution that might come from either the
fundamental or frequency-doubled beams. The
signal was then dispersed by a monochromator of
30 cm focal length to enhance the spectral resolution before collection by a thermal-electrically
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cooled GaAs photomultiplier. Eventually, we used
a standard photon-counting instrument to analyze
the signal.
The time-integrated PL of the QDs photoexcited
nonresonantly with the frequency-doubled light
from the fundamental of the Ti:sapphire laser
shows that the peaks of the PL for the VW mode
and SK mode QDs are 2.33 and 2.22 eV, respectively. The difference of the peak energies of these
two similar height QDs is the result of the wetting
layer and larger QDs formed, whose energies are
lower, in the SK mode. The width of 40 nm for the
time-integrated PL of the VW mode and 30 nm for
the SK mode are due to the size distribution of
QDs. The time-resolved PL of both samples for
energies at the barriers and the peaks of the timeintegrated PL are shown in Fig. 1. The PL
intensity of the barrier for the VW mode QDs
(Fig. 1(a)) increases rapidly with a time scale of
about 0.5 ps and is followed by a decay with a time
scale of 7 ps. The PL of the barrier for the SK
mode QDs reveals a slower rise time of 3 ps and a
decay time of 38 ps. The differences of the PL rise
times for these two different growth modes may be
due to the different thicknesses of the capping
layers. The ratio of the rise time to the decay time
is consistent for both samples, which may indicate
Fig. 1. The time-resolved PL of ZnTe/ZnSe QDs grown by VW
mode ( triangle down) and SK mode (circle) at energy of barrier
(a) and at the peak of the time-integrated PL (b). The inset
shows the time-resolved PL in the first 100 ps. The sample’s
temperature is 35 K. The lines are for visual guidance and the
vertical scale is linear.
that only the thickness of the capping layer affects
the increase in rise and decay times in the SK
mode. Fig. 1(b) shows the time-resolved PL at
peak energies of the time-integrated PL. For the
VW QDs, we observe that the PL intensity
increases with a risetime of 2 ps and the decay of
the PL can be characterized bi-exponentially with
two time constants: a fast decay time of 20 ps and a
slower decay time of 1.6 ns. The figure also shows a
much slower PL risetime of 15 ps and a PL lifetime
of more than 4 ns for the SK growth mode. The
times of carrier capture by QDs, determined from
the lifetimes of the barriers, are 7 and 38 ps for the
VW mode and SK mode, respectively. We
attribute the fast PL decay of the VW mode
within the first 20 ps after photoexcitation to the
Auger processes due to the higher carrier density
generated inside the dots [11]. As the carrier
density decreases with delay time, the Auger
process is less effective. Because of the nature of
the type II QDs structure of ZnTe/ZnSe, holes
were confined in the ZnTe dots and electrons are
localized in the ZnSe layer around the dots. The
spatial separation of the wave functions of holes
and electrons accounts for the slow PL decay time
of 1.6 ns for the VW mode. In comparison, we
found that for the SK mode the PL decay time is
more than 4 ns, which is about 3 times slower than
that for the VW mode. We attribute this slow
decay, in addition to the spatial separation of wave
function of holes and electrons, to the wetting
layer formed in this structure. The carriers
generated from the capping layer by photoexcitation are funneled into the wetting layer before they
are captured by QDs. Once the carriers reach the
wetting layer, either from barriers or QDs, they
will not escape easily to the barrier as a result of
the confinement by the barrier, whose band gap
energy is larger than that of the wetting layers.
This could also partially explain the slow PL rise
time of the SK mode compared to that of the VW
mode in Fig. 1(b). In contrast, the carriers in
the QDs of the VW mode are not confined before
they are captured by the QDs and, thus, the
probability of escaping from the QDs increases.
Therefore, the wetting layer in the SK mode plays
an important role in carrier capture and relaxation. The slow decay lifetime for both samples are
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M.-E. Lee et al. / Physica E 26 (2005) 422–426
slower/comparable to that of type I CdSe/ZnSe
QDs [12,13].
In order to understand the role of wetting layer
in carrier capture and relaxation, we measured the
decay of PL as a function of energy for both
samples, as shown in Fig. 2. The PL decay of the
SK mode at energies above the peak of the timeintegrated PL exhibits bi-exponential decay. For
comparison, we determined the PL decay at all
energies with two time constants, one refers to the
slow decay and the other refers to the fast decay of
the PL although the PL shown in Fig. 1(b) can
easily be explained by a single exponential. As
shown in Fig. 2, the slow decay time of the SK
mode decreases with energy. We attribute the
decrease of slow decay times also to the wetting
layer because the carriers in the small QDs, whose
energy states are larger, can be excited to the
wetting layer either by the Auger process or by
thermal emission and will later relax to the large
QDs. Carriers in the confined wetting layer
migrate from small QDs to the large QDs by
diffusion. On the contrary, the slow decay times of
the VW mode level around 1.6 ns indicates that the
probabilities of carrier capture into the QDs are
equal and thus the carriers excited by the Auger
process and thermal emission will relax to large
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and small QDs with the same probabilities since
no path is provided for carriers to diffuse and
migrate from large (small) to small (larger) QDs.
The same probabilities of carrier capture in the
VW mode is confirmed by studying the PL rise
times of the QDs and we found that they were all
about 2 ps, regardless the detected energy. The
above argument could also explain why the slow
PL decay times for the SK mode are smaller than
the VW mode at energies above 2.3 eV. The
carriers escape away from small QDs in the SK
mode and diffuse to the large QDs through the
wetting layer. Therefore, fewer carriers are relaxed
to the bandgap of the small QDs than to the large
QDs and the slow decay times are thus smaller
than those of the VW mode.
In conclusion, we studied the carrier capture
and relaxation of ZnTe/ZnSe QDs grown in the
VW and SK modes with ultrafast PL upconversion. We found that the carrier capture times were
7 and 38 ps for the VW mode and SK mode,
respectively. We attribute the fast PL decay of the
VW mode within the first 20 ps after photoexcitation to the Auger processes. We attribute the slow
decay of the PL for the SK mode, compared to
that of the VW mode, in addition to the spatial
separation of wave function of holes and electrons,
to the wetting layer formed in this structure. We
have demonstrated the wetting layer plays an
important role in carrier capture and relaxation of
Type II ZnTe/ZnSe QDs.
This work was supported in part by National
Science Council, ROC under Grant No. NSC 922112-M-110-015.
References
Fig. 2. The fast and slow PL decay times of ZnTe/ZnSe QDs.
Open (solid) down triangle are the fast (slow) PL decay times
for SK mode. Open (solid) circles are the fast (slow) PL decay
times for VW mode. The scales for data points to the left and
right of the break are on the left and right vertical axes,
respectively. The lines between data points are for visual
guidance. The dashed and solid lines are the normalized timeintegrated PL of VW mode and SK mode, respectively.
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