Hole confinement in quantum islands in Ga ( As Sb ) ∕ Ga As ∕ ( Al Ga ) As
heterostructures
S. Horst, S. Chatterjee, K. Hantke, P. J. Klar, I. Nemeth, W. Stolz, K. Volz, C. Bückers, A. Thränhardt, S. W.
Koch, W. Rühle, S. R. Johnson, J.-B. Wang, and Y.-H. Zhang
Citation: Applied Physics Letters 92, 161101 (2008); doi: 10.1063/1.2913767
View online: http://dx.doi.org/10.1063/1.2913767
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/92/16?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Effects of GaAs(Sb) cladding layers on InAs/AlAsSb quantum dots
Appl. Phys. Lett. 102, 023107 (2013); 10.1063/1.4776221
Complex emission dynamics of type-II GaSb/GaAs quantum dots
Appl. Phys. Lett. 95, 061102 (2009); 10.1063/1.3202419
Effects of two-dimensional electron gas on the optical properties of InAs/GaAs quantum dots in modulationdoped heterostructures
Appl. Phys. Lett. 86, 021916 (2005); 10.1063/1.1849853
Microstructural and optical properties of InAs/GaAs quantum dots embedded in modulation-doped Al x Ga 1−x
As/GaAs heterostructures
J. Appl. Phys. 91, 5195 (2002); 10.1063/1.1459752
Experimental and theoretical study of strain-induced AlGaAs/GaAs quantum dots using a self-organized GaSb
island as a stressor
J. Appl. Phys. 86, 2001 (1999); 10.1063/1.371000

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
209.147.144.20 On: Sat, 07 Feb 2015 22:50:31

APPLIED PHYSICS LETTERS 92, 161101 共2008兲

Hole confinement in quantum islands in Ga„AsSb… / GaAs/ „AlGa…As
heterostructures
S. Horst,1,a兲 S. Chatterjee,1 K. Hantke,1 P. J. Klar,1,b兲 I. Nemeth,1 W. Stolz,1 K. Volz,1
C. Bückers,1 A. Thränhardt,1 S. W. Koch,1 W. Rühle,1 S. R. Johnson,2 J.-B. Wang,2 and
Y.-H. Zhang2
1

Faculty of Physics and Material Sciences Center, Philipps-Universität Marburg, Renthof 5,
D-35032 Marburg, Germany
2
Center for Solid State Electronics Research and Department of Electrical Engineering, Arizona State
University, Tempe, Arizona 85287-6206, USA

共Received 13 March 2008; accepted 3 April 2008; published online 21 April 2008兲
Formation of self-organized Ga共AsSb兲 quantum islands during growth is shown to occur in a series
of Ga共AsSb兲 / GaAs/ 共AlGa兲As heterostructures, resulting in an in-plane hole confinement of several
hundred meV. The shape of the in-plane confinement potential is nearly parabolic and, thus, yields
almost equidistant hole energy levels. Transmission electron microscopy reveals that the quantum
islands are 100 nm in diameter and exhibit an in-plane variation of the Sb concentration of more
than 30%. Up to seven bound hole states are observed in the photoluminescence spectra.
Time-resolved photoluminescence data are shown as function of excitation density, lattice
temperature, and excitation photon energy and reveal fast carrier capture into, and relaxation within,
the quantum islands. The advantages of such structures as active laser material are discussed.
© 2008 American Institute of Physics. 关DOI: 10.1063/1.2913767兴
One of the remaining challenges of today’s optoelectronics is the fabrication of emitters in the wavelength regime of
1.3– 1.55 ␮m on GaAs substrates. One approach is to grow
self-assembled quantum dots where material systems containing Sb have proven to be promising candidates: high dot
densities1 resulting in large peak modal gain2 have been obtained. Problems related with the GaSb/ GaAs system are the
type-II alignment of the bands2,3 and the subtle change between Stranski–Krastanov and interfacial-misfit growth
mode.4
We study a series of 7 nm GaAs0.64Sb0.36 layers in between GaAs spacers of varying thicknesses. The spacer layer
thicknesses are 0, 1, 2, 3, 6, and 9 nm for the six different
samples which are labeled as samples 0, 1, 2, 3, 6, and 9,
respectively. Carrier confinement layers of 75 nm
Al0.25Ga0.75As and 50 nm Al0.5Ga0.5As on either side complete the structure. Details of sample structure and growth by
solid-source molecular beam epitaxy are described in Refs. 5
and 6. Here, we show that this leads to the formation of
quantum islands 共QIs兲 in the Ga共AsSb兲 layer.
Time-resolved photoluminescence 共PL兲 data are taken as
function of excitation density and lattice temperature. The
samples are excited by a 100 fs Ti:sapphire laser with an
excitation energy of 1.476 eV, corresponding to a wavelength of 840 nm. Thus, the Ga共AsSb兲 layers are directly
excited for temperatures below 170 K. The laser has a repetition rate of 80 MHz and its average power is varied from
3 to 250 mW. It is focused to a spot of 30 ␮m diameter at
full width at half maximum. The excitation density is, thus,
approximately varied between 4 ⫻ 1011 and 3 ⫻ 1013 cm−2,
taking into account reflection losses and assuming an absorption coefficient of 104 cm−1.

The sample temperatures are set to 10, 30, and 290 K.
The PL is imaged with three times magnification onto the
30 ␮m wide, vertical entrance slit of an imaging spectrometer where the PL is spectrally dispersed with a resolution of
3 nm. The horizontal entrance slit of the streak camera at the
exit of the spectrometer is 30 ␮m wide, yielding a temporal
resolution of 15 ps. The vertical and horizontal slits, thus, cut
out an area of 10⫻ 10 ␮m2 of the excitation spot on the
sample. The QIs of one sample are also resonantly excited
into the third excited state of the QIs at 1.094 eV using a
synchronously pumped optical parametric oscillator 共OPO兲
with all other experimental parameters kept identical.
In Fig. 1, we present a cross-section transmission electron microscopy 共TEM兲 image of sample 6 for better understanding of the structures under investigation. It was performed in 关010兴 projection using a JEOL JEM 3010 at an
acceleration voltage of 300 kV. Dark field imaging using the
chemically sensitive 共002兲-reflection was applied to derive
the chemical composition of the mixed III/V alloys. The
GaAs spacer layers are darker compared to Sb-rich material
under the imaging conditions used. The brightness fluctuations in the quantum well 共QW兲 between the dark GaAs
spacer layers indicate a strongly inhomogeneous Sb profile
in the QW plane, with up to 100 nm wide regions of high Sb
concentration 共⬃36% 兲, the QIs, separated from each other
by about 50 nm wide, Sb-poor regions 共⬃0 % 兲. The fluctua-

a兲

Electronic mail: swantje.horst@physik.uni-marburg.de.
Present address: I. Physikalisches Institut, Justus-Liebig-Universität
Gießen, Heinrich-Buff-Ring 16, D-35392 Gießen, Germany.

b兲

FIG. 1. Cross-section TEM of sample 6.

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
0003-6951/2008/92共16兲/161101/3/$23.00
92, 161101-1
© 2008 American Institute of Physics
209.147.144.20 On: Sat, 07 Feb 2015 22:50:31

161101-2

Horst et al.

FIG. 2. Time-integrated photoluminescence spectra of the series of samples
for lattice temperatures of T = 10 K and an excitation power of 共a兲
␳exc = 3 mW and 共b兲 ␳exc = 250 mW, respectively. The data are normalized to
their relative maximum.

tions of Sb concentration cause a strong confinement of the
holes in the Sb-rich regions. In contrast, the conduction-band
offset of about 40 meV7 is small, but yields a weak type-II
confinement. The electrons are, therefore, only confined between the 共AlGa兲As layers and/or by the attractive Coulomb
potential of the localized holes.
The structures behave like inhomogeneously broadened
QWs in PL measurements and electroabsorption3,7 when the
samples are excited on a large area with low spatial resolution. However, taking a “closer look” with 10 ␮m spatial
resolution reveals structured PL emission at high excitation
densities and/or high temperatures, respectively. Figure 2 depicts time-integrated spectra of the various samples at 共a兲
low and 共b兲 high excitation densities. The data at low carrier
density show, as a general trend, a decrease in transition
energy with increasing GaAs spacer width. Sample 1 slightly
deviates from that trend and shows a smaller transition energy than expected, which can be explained by internal
electric fields as described in Refs. 7 and 8. Only one line
is resolved for all samples at a photon density of
1.5⫻ 1013 cm−2/pulse 共a兲, whereas, up to seven almost

Appl. Phys. Lett. 92, 161101 共2008兲

equidistant lines are resolved at the highest density of
1.2⫻ 1015 cm−2 per pulse 共b兲. As a general trend, the energy
splitting between the almost equidistant lines decreases with
increasing thickness of the GaAs spacer layer.
We now discuss the spatial variation of the PL across
one sample. The corresponding spectra 共not shown here兲 are
taken at different sample positions close to the waver center
at distances of 100 ␮m so that the excitation spots do not
overlap. The distance and the relative intensities of the various peaks in the PL lines change with the position on the
sample. This demonstrates an inhomogeneous size or shape
distribution of the QIs. No structured PL is observed at the
edges of the 2 in. wafers. For sample 9, one observes up to
seven peaks, covering an overall energy range of 100 meV
共not shown here兲, corresponding to recombination involving
excited states. The peak positions are roughly equidistant in
energy. Therefore, the shape of the additional confinement
potential within the Ga共AsSb兲 layer should be close to a
paraboloid. Similar results hold for all samples.
All these experimental features corroborate the interpretation of the various lines in the high density PL as recombination from the ground and excited states of holes in a QI
confinement within the Ga共AsSb兲 layer. We now estimate the
energy spacings of the excited hole states assuming a parabolic confinement potential, a hole mass of 0.08m0 共corresponding to the effective mass of a heavy hole in the
GaAs0.64Sb0.36 plane, the light hole is ⬃200 meV lower than
the heavy hole, and is therefore, not taken into account兲 and
a variation of Sb concentration from 0% to 36% 共the corresponding band gap difference between GaAs and
GaAs0.64Sb0.36 is 0.49 eV at 30 K兲. Widths of a parabolic
potential from 50 to 100 nm then deliver equidistant energies of the excited states of 15– 40 meV. We, thus, get
good agreement between the TEM results and the PL
measurements.
Next, we investigate the carrier relaxation in the QI
structures. Figure 3共a兲 depicts time traces taken at the spectral positions indicated by the dashed lines and symbols in
the spectrum, which is shown in the inset. The excited state
emission at higher energies decays faster. The lines at lower
energy initially show a constant intensity caused by the re-

FIG. 3. 共Color online兲 Time-resolved
photoluminescence data of sample 6.
共a兲 Temporal evolution of the photoluminescence at T = 30 K for different
energies, excited by the Ti:sapphire laser at 1.476 eV. The corresponding
energetic positions are shown in the
inset. 共b兲 Excited at 1.094 eV 共䊏兲 and
1.476 eV with different photon densities 共䊊, 쐓, 탊兲 at T = 10 K. The data
are normalized to their relative
maximum.

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
209.147.144.20 On: Sat, 07 Feb 2015 22:50:31

161101-3

Appl. Phys. Lett. 92, 161101 共2008兲

Horst et al.

laxation of holes from the excited, high energy states to
states at lower energy. This effect is well known and becomes less pronounced at higher temperatures since then the
excited states are also thermally occupied.
Figure 3共b兲 shows the rise of the normalized PL of
sample 6 excited with 共i兲 the Ti:sapphire laser at 1.476 eV
for three different photon densities and 共ii兲 with the OPO at
1.094 eV for highest photon density. The rise time gets faster
with increasing photon density in the case of 1.476 eV excitation. This observation corroborates that carrier-carrier scattering or Auger processes play a role in the capture process.
The excitation at 1.094 eV yields a density independent, fast
rise time. We conclude that the relaxation within the QI is
dominated by phonon emission. This fast capture and relaxation is also corroborated by the effect that at low densities,
only the ground state emission is observed.
We find the clear evidence for the QI formation only in
the center region of the 2 in. wafers. This inhomogeneity on
a large scale might be caused by a slightly inhomogeneous
radial growth temperature profile in combination with the
sensitivity of Sb-containing layers to changes of growth
parameters.4
Finally, we discuss the potential advantages of the QI
systems investigated here. The TEM measurements show
distances of 100 nm of the QIs, yielding a QI density of
about 1010 cm−2 and, thereby, a high density of states per
volume. Such a high density obviously leads to a high modal
gain of up to 200 cm−1, which was recently obtained with
exactly these structures containing only one active layer9 and
which is much larger than the 45 cm−1 recently reported by
Tatebayashi et al.2 or the 30 cm−1 reported by Borri et al.10
Second, the QIs have a strong confinement due to the large
in-plane variation of the Sb concentration. The high confinement potential of more than 100 meV is necessary for room
temperature operation of QI lasers. Third, the spacing of the
QI states is large, in the regime of a few tens of meV. In
particular, the excited states in the QI are almost equidistant
due to a parabolic confinement potential. We, therefore, expect that relaxation of holes between the QI states is fast
since the problem of the phonon bottleneck in intraisland
carrier relaxation is strongly reduced.11 Relaxation of holes
via emission of high energy phonons is very efficient and all
energy spacings are neither too small nor larger than the
maximum phonon energy. Fourth, carrier-carrier or Auger
scattering enables fast capture of holes into the QIs at higher
carrier densities. Fifth, the overlap between the wave functions of electrons and holes remains reasonably high since
we have the additional confinement of the electrons in
growth direction by the additional 共AlGa兲As barriers and by
the variation of Sb concentration in the plane of the

Ga共AsSb兲 layer and/or the attractive Coulomb potential of
the localized holes which localizes the electrons at the
GaAs/ Ga共AsSb兲 interface.2 This could be beneficial for
both, large oscillator strength and fast capture of holes into
the QIs via carrier-carrier scattering. The detrimental influence of a type-II structure can thus be reduced. A direct
comparison of our QIs with conventional QW structures
shows that actually, radiative recombination is comparably
fast and efficient.
In summary, we conclude that the formation of Sb-rich
QIs leads to a deep, nearly parabolic confinement potential
for holes in the plane of a Ga共AsSb兲 layer surrounded by
GaAs spacer layers. The energy separations of the excited
states are in the order of a few tens of meV. The QIs have
small distances with respect to each other. Barriers of
共AlGa兲As, the variation of Sb within the Ga共AsSb兲 layer,
and/or the attractive Coulomb potential of the strongly localized holes lead to an additional confinement of the electrons.
These effects, together, might prove as beneficial for a potential laser performance.
Financial support by the German Science Foundation,
Topical Research Group 483: “Metastable Compound
Semiconductor Systems and Heterostructures” and the
Optodynamics Research Centre is gratefully acknowledged.
1

P. Aivaliotis, L. R. Wilson, E. A. Zibik, J. W. Cockburn, M. J. Steer, and
H. Y. Liu, Appl. Phys. Lett. 91, 013503 共2007兲.
2
J. Tatebayashi, A. Khoshakhlagh, S. H. Huang, G. Balakrishnan, L. R.
Dawson, D. L. Huffaker, D. A. Bussian, H. Htoon, and V. Klimov, Appl.
Phys. Lett. 90, 261115 共2007兲.
3
G. Blume, T. J. C. Hosea, S. J. Sweeney, S. R. Johnson, J.-B. Wang, and
Y.-H. Zhang, IEE Proc.: Optoelectron. 152, 110 共2005兲.
4
G. Balakrishnan, J. Tatebayashi, A. Khoshakhlagh, S. H. Huang, A.
Jallipalli, L. R. Dawson, and D. L. Huffaker, Appl. Phys. Lett. 89, 161104
共2006兲.
5
S. R. Johnson, C. Z. Guo, S. Chaparro, Yu. G. Sadofyev, J. Wang, Y. Cao,
N. Samal, J. Xu, S. Q. Yu, D. Ding, and Y.-H. Zhang, J. Cryst. Growth
251, 521 共2003兲.
6
J.-B. Wang, S. R. Johnson, S. A. Chaparro, D. Ding, Y. Cao, Yu. G.
Sadofyev, Y.-H. Zhang, J. A. Gupta, and C. Z. Guo, Phys. Rev. B 70, 1
共2004兲.
7
C. Bückers, G. Blume, A. Thränhardt, C. Schlichenmaier, P. J. Klar, G.
Weiser, S. W. Koch, J. Hader, J. V. Moloney, T. J. C. Hosea, S. J.
Sweeney, J.-B. Wang, S. R. Johnson, and Y.-H. Zhang, J. Appl. Phys. 101,
033118 共2007兲.
8
G. Blume, Ph.D. thesis, Novel materials for semiconductor micro-cavities
Modulation and other Spectroscopy, University of Surrey, UK 共2006兲.
9
C. Lange, M. Schwalm, S. Chatterjee, W. W. Rühle, N. C. Gerhardt, S. R.
Johnson, J.-B. Wang, and Y.-H. Zhang, Appl. Phys. Lett. 91, 191107
共2007兲.
10
P. Borri, W. Langbein, S. Schneider, U. Woggon, R. L. Sellin, D. Ouyang,
and D. Bimberg, Phys. Rev. Lett. 89, 187401 共2002兲.
11
T. Inoshita and H. Sakaki, Phys. Rev. B 46, 7260 共1992兲.

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
209.147.144.20 On: Sat, 07 Feb 2015 22:50:31