OPTICAL PROPERTIES OF InAs QUANTUM DOTS WITH InAlAs/InGaAs COMPOSITE MATRIX Wei–Sheng Liu and Jen-Inn Chyi Department of Electrical Engineering, National Central University Chung-Li, Taiwan 32054, R.O.C.
[email protected] The influence of InAlAs/InGaAs composite overgrown layer on the optical properties of InAs quantum dots is investigated. Quantum dots with narrow photoluminescence linewidth and wide state-separation at 1.3 µm can thus be obtained. Recently, great effort has been focused on improving 1.3
InGaAs
strain-reducing
layer.
Although,
InAlAs
and
µm quantum dot (QD) lasers due to its potential application in
InAlAs/InGaAs overgrown layers were used to improve the
optical fiber communication systems (1). Typical approaches to
luminescence properties of InAs QDs, the detailed overgrowth
extend the emission wavelength of InAs QDs are overgrown
mechanism on InAs QDs is still not clear. In this paper, we
InGaAs layers or dots in InGaAs well. However, these
systemically study the optical properties of QDs in different
structures have lower barrier height and smaller state separation
matrices and clarify the correlation between the optical
between the ground state and the excited state in QDs, which
properties of InAs QD and its overgrown indium-contained
leads to a lower characteristic temperature when the laser is
layers.
operated above room temperature. Therefore, raising the
The samples studied in this work were grown in a Riber
temperature stability of QD laser is one of the major tasks in
32P molecular beam epitaxy system. InAs QDs were deposited
this research area currently. Since the overgrown InAlAs layer
onto a 300 nm-thick GaAs buffer layer, followed by
is expected to increase the confinement potential, reduce the
indium-contained overgrown layer, i.e. the strain reducing layer.
compressive strain and suppress the indium segregation of InAs
To investigate the changes in the QD electronic states with
QDs, a few studies have been performed on the effects of
different combinations of strain reducing layers, samples with,
InAlAs and InAlAs/InGaAs overgrowth (2-4). Wei et al. (2)
6
capped the InAs QDs with varied InAlAs thickness, followed
InGaAs/InAlAs overgrown layers were prepared. A sample of
by InGaAs overgrowth, and demonstrated the largest state
QDs without the strain reducing layer was also grown for
separation of 108 meV in QDs emitting at 1.3 µm. Zhang et al.
reference. All the samples were covered by a layer of 100
(3) deposited different mole fractions of AlAs in the overgrown
nm-thick GaAs for photoluminescence (PL) studies. The
InAlAs to show that extending InAs QD emission wavelength
samples can be grouped into two series. The basic layer
to 1.3 µm involves both the suppression of indium segregation
structure of these two series of samples is depicted in Fig 1.
and the strain reduction by the indium-contained overgrown
The detailed overgrowth material and sequence of the first and
layer. In addition, Jia et al. (4) revealed the enhanced optical
second series of samples is listed in Table I and Table II.,
intensity and state separation with a combination of InAlAs and
respectively. The indium composition of all the overgrown
nm-thick
InAlAs,
InGaAs,
InAlAs/InGaAs,
and
layers in the first series was 16 %, while the composition was
with those in the first series. Since strain relaxation effect had
11 % for InGaAs and 20 % for InAlAs in the second series. The
been studied by growing QDs within different matrices (9), the
InAs monolayer for QD growth in the second series was
spatial distribution of the strain-relaxation inside the QDs is
designed to be lower than that in the first series to show the
still not clear. In order to investigate the predominant strain
strain-reduction effect for extending the emission wavelength to
relaxation region in QDs after capping with strain reducing
1.3 µm. The 488 nm line of an Ar ion laser was used as the
layer, samples #8, #9, and #10, which represent different levels
excitation source for the photoluminescence (PL) measurement,
of strain relief at the upper and lower parts of QDs, are
performed at room temperature. A cooled InGaAs detector was
prepared. Sample #8 has a 3 nm-thick In0.11Ga0.89As overgrown
used to measure the signal dispersed by a 0.5-meter
layer at QD lower part and 3 nm-thick In0.20Al0.80As layer at
monochromator via conventional lock-in techniques.
upper part, while sample #9 has a reverse sequence of the
+
The room temperature PL spectra of the first and second
overgrown layers. The QDs of sample #10 are buried in a 6
series samples are shown in Fig. 2(a), and Fig. 2(b),
nm-thick In0.20Al0.80As matrix. According to the reported strain
respectively. From Fig. 2(a), the emission peak wavelength of
distribution in QDs, the strain at the top of the QDs is relaxed
QDs in the GaAs matrix (#1) is limited to 1272 nm due to the
much more than that at the lower part (10-11); therefore it is
hydrostatic compressive strain (5) and strain induced In-Ga
believed that the residual compressive strain in the bottom of
intermixing (6) in QDs. When the QDs were capped by a 6
the QDs is the dominant factor in QD emission wavelength. It
nm-thick In0.16Ga0.84As overgrown layer (#2), the ground state
is worth noting that samples #8 and #9, both of which have
transition line red shifted to 1360 nm. This was attributed to the
In0.11Ga0.89As (3 nm) and In0.20Al0.80As (3 nm) overgrown layers
reduction of both the compressive strain and In-Ga intermixing
but in reverse deposition sequence, exhibit different emission
in QDs. Besides, the so-called activated alloy phase separation
wavelengths, in contrast to the results of samples #3 and #4.
effect (7) on the increase of QD volume via the migration of
Since the composite matrices of both samples #8 and #9 are the
indium adatoms toward to InAs QDs was also proposed for the
same, it is expected that the total amount of strain relaxation of
emission wavelength extension. As shown in Fig. 2 (a), the
QDs is identical for these two samples. However, sample #8
samples with composite (#3, #4) and In0.16Al0.84As (#5)
shows shorter peak wavelength than #9 due to the less indium
overgrown layers, which were designed to provide the same
content in In0.11Ga0.89As overgrown layer at the lower part of
degree of strain relief to QDs, exhibit interestingly similar
the QDs.
emission peak wavelengths near 1.3 µm. The implication of
have a In0.20Al0.80As overgrown layer at the lower part of the
this result is two fold. First, in contrast to the InGaAs case,
QDs, exhibit similar peak wavelengths at 1.3 µm regardless of
indium segregation or phase separation is indeed suppressed by
the type of the upper overgrown layer. This is consistent with
the InAlAs overgrown layer (8). Second, the insensitivity of the
the behavior of samples #3 and 4, which have same In content
emission wavelength of QDs to the growth sequence of the
in the overgrown layer at the lower part of the QDs. This
InGaAs/InAlAs composite overgrown layers, which were
implies that the same degree of strain relaxation at the QD
expected to result in different degrees of indium segregation,
bottom results in similar emission wavelength among these
shows that strain reduction mechanism dominates the emission
samples. It implies that relaxing the compressive strain at the
wavelength extension for InGaAs overgrown layers of this
QD bottom is an essential way to extend the QD emission
thickness and composition.
wavelength.
In contrast to sample #8, samples #9 and #10, which
Fig. 2(b) shows the PL spectra of the second series of
To gain insight into the optical properties of the QDs with
samples designed to exert different stress on QDs compared
InAlAs / InGaAs composite matrices , the detailed spectral
features are summarized in Table I, and Table II. The peak
results in different degree of redshift for the ground state and
emission wavelengths, full widths at half maximum (FWHM),
the first excited state when the lower overgrown layer is
and state separation between the ground state and the first
replaced from InGaAs InAlAs (2). Besides, the absence of
excited state of these two series of samples are included in the
In-Ga
tables. As compared with the GaAs capped QDs, the narrow
QD/overgrown layer interface and contributed to the deeper
FWHM spectra of QDs with InGaAs overgrown layer can be
confining potential (14). Again, because the ground state
accounted for by the InAs QD height increment induced by
transition happens near the bottom of QDs, the influence of
phase separation, which decreases the sensitivity to QD volume
potential barrier height on emission wavelength is expected to
variation (7, 12). However, the phase separation effect would
be more pronounced at the same place.
intermixing
could
also
give
rise
to
sharper
prevail at certain energetically favorable centers; a composition
In conclusion, our results show that the strain reducing
fluctuation is expected in InAs dot ensemble. In contrast to the
effect is more dominant in extending the emission wavelength
InGaAs, InAlAs overgrown layer reduces the indium
of QDs when the overgrown layer is thin. Since InAlAs layer
segregation of QDs and preserves better composition
suppresses indium segregation and increases potential barrier
uniformity in the dot ensemble. Therefore, QDs with InAlAs
height, narrower PL linewidth and larger state separation results
overgrown layer exhibit narrower PL spectra linewidth than
are revealed by QDs with InAlAs overgrown layer at the QD
those with InGaAs one. In our PL results, QDs with InAlAs
lower part. Further quantitative studies are necessary, but the
overgrown layer positioned at QD lower part show narrower
mechanisms discussed here could be constructive in realizing
FWHM than it was placed at the upper part of QDs, as
long-wavelength QD LDs with high differential gain and
observed in the samples with composite matrix (#3, #4, and #8,
characteristic temperature as operated above room temperature.
#9). Since reported calculations and experiments reveal that the ground state charge distributions are located in the bottom region of QDs (13). InAs QDs with InAlAs overgrown layer at lower part (#4, and #9) could show narrower linewidth than those with the reverse layer sequence (#3 and #8), due to the preserved composition uniformity at QD bottom. Compared with 6 nm-thick InGaAs capped QDs (sample
Acknowledgement The authors would like to thank Dr. N.-T. Yeh for his assistance and fruitful discussions on MBE growth and characterization. Support from the MBE laboratory of the
Optical Sciences
Center at National Central University is acknowledged. This work was supported by the National Science Council of the R.O.C. under contract NSC 91-2120-E-008-001.
#2); QDs with a 6 nm-thick InAlAs overgrown layer (sample #5) exhibit narrower linewidth of 23.5 meV and larger state separation. According to the reported results, InAs QDs with
Reference 1.
InGaAs overgrown layer showed decreased state separation because of the lower potential barrier height (2). The InAlAs
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Z. Y. Zhang, B. Xu, P. Jin, X. Q. Meng, Ch. M. Li, X. L. Ye, and Z. G. Wang, J. Appl. Phys. 92, 511 (2002)
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GaAs cap layer
10
Upper Part: 3 nm Lower Part: 3 nm
GaAs buffer layer Fig. 1. Basic layer structure of the first series and second series of samples Table I. The layer sequence and spectra features of the first series of samples. The indium composition of both of the InGaAs and InAlAs are 16%. Noting that the denoted symbol “U” means upper part, and “L” means lower part in Fig. 1, respectively. Above dots
Peak position FWHM State Separation
#1 #2 U: GaAs U: InGaAs L: GaAs L: InGaAs 1272 nm 1360 nm 30 mev 83 mev
26 mev 79 mev
29 mev 75 mev
24.5 mev 100 mev
23.5 mev 92 mev
Peak position FWHM State Separation
#6 #7 U: GaAs U: InGaAs L: GaAs L: InGaAs 1230 nm 1286 nm 33 mev 73 mev
29 mev 77 mev
#8 #9 #10 U: InAlAs U: InGaAs U: InAlAs L: InGaAs L: InAlAs L: InAlAs 1254 nm 1298 nm 1300 nm 29 mev 70 mev
26 mev 95 mev
GaAs cap InGaAs: 6nm U: InAlAs + L: InGaAs U: InGaAs + L: InAlAs InAlAs: 6nm
(a)
6 4 2 0
1150 1200 1250 1300 1350 1400 1450 Wavelength (nm)
#3 #4 #5 U: InAlAs U: InGaAs U: InAlAs L: InGaAs L: InAlAs L: InAlAs 1294 nm 1292 nm 1294 nm
Table II. The layer sequence and spectra features of the second series of samples. The indium composition of InGaAs is 11%, and 20% in InAlAs, respectively. Noting that the denoted symbol “U” means upper part, and “L” means lower part in Fig. 1, respectively. Above dots
8
PL Intensity (a.u.)
InAs QDs
10
PL Intensity (a.u.)
7.
8
(b)
GaAs cap InGaAs: 6nm U: InAlAs + L: InGaAs U: InGaAs + L: InAlAs InAlAs: 6nm
6 4 2 0 1100
1150
1200
1250
1300
1350
Wavelength (nm)
24 mev 91 mev
Fig. 2. Room temperature photoluminescence spectra of (a) first series, and (b) second series of samples
