Long wavelength (1.3 and 1.5 μm) photoluminescence from InGaAs/GaPAsSb quantum
wells grown on GaAs
P. Dowd, W. Braun, David J. Smith, C. M. Ryu, C.-Z. Guo, S. L. Chen, U. Koelle, S. R. Johnson, and Y.-H.
Zhang
Citation: Applied Physics Letters 75, 1267 (1999); doi: 10.1063/1.124663
View online: http://dx.doi.org/10.1063/1.124663
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APPLIED PHYSICS LETTERS

VOLUME 75, NUMBER 9

30 AUGUST 1999

Long wavelength „1.3 and 1.5 ␮m… photoluminescence
from InGaAs/GaPAsSb quantum wells grown on GaAs
P. Dowd,a) W. Braun,b) David J. Smith,c) C. M. Ryu,
C.-Z. Guo,d) S. L. Chen,e) U. Koelle, S. R. Johnson,
and Y.-H. Zhang
Department of Electrical Engineering & Center for Solid State Electronics Research,
Arizona State University, Tempe, Arizona 85287-6206

共Received 1 February 1999; accepted for publication 2 July 1999兲
Room temperature photoluminescence at wavelengths between 1.2 and 1.5 ␮m has been observed
in samples consisting of InGaAs/GaPAsSb quantum well structures grown on GaAs. The emission
wavelength is varied primarily by changing the composition within the GaPAsSb layer. It is
proposed that such long wavelength emission results from a spatially indirect interband transition in
the type-II quantum wells where the electron and hole wave functions have large spatial overlap.
© 1999 American Institute of Physics. 关S0003-6951共99兲00235-1兴

Vertical-cavity surface-emitting lasers 共VCSELs兲 operating at 1.3 ␮m are desirable for low cost optical telecommunication systems and data links. Realization of these devices
may enable applications such as ‘‘fiber to the home’’, which
operate over distances of only a few kilometers. Due to the
potentially large market for 1.3 ␮m VCSELs, much research
has been carried out to develop devices using different approaches based primarily on two substrates, InP and GaAs.
When grown using InP/InGaAsP, VCSELs have poor performance due to the high thermal sensitivity and refractive index properties of the materials.1 One of the most promising,
albeit complex, approaches has been to use wafer fusion, in
which the active region grown on an InP substrate and the
distributed Bragg reflectors grown on GaAs are bonded together to form the VCSEL.2
In order to ensure reliability and reproducibility, and to
overcome the limitations of the InP/InGaAsP material system, there is interest in developing alternative structures
based on GaAs, especially since GaAs-based technology is
generally more advanced than that of InP. However, it is not
straightforward to find materials that can be grown on GaAs
with band gaps that are suitable for 1.3 ␮m emission. In one
approach, quantum dot 共QD兲 structures have been developed.
InGaAs QDs have shown photoluminescence 共PL兲 at 1.3
␮m,3 and a resonant cavity photodiode operating at 1.27 ␮m
has been realized.4 More recently, an edge-emitting QD laser
operating close to 1.3 ␮m has also been demonstrated.5
Room temperature 共RT兲 PL at 1.3 ␮m has been observed
using strained GaAsSb quantum wells 共QWs兲 and lasing has
been reported in an edge-emitting device at 1.27 ␮m.6 PL
wavelengths of up to 1.33 ␮m have also been observed in
GaAsSb/InGaAs bi-layer QW samples, with a type-II bandedge alignment.7 The approach that was able to achieve the

longest emission wavelength so far in a GaAs-based VCSEL
structure used a single GaInNAs QW, and RT pulsed operation was achieved with an emission wavelength of 1.18 ␮m.8
In order to avoid the limitation of the lasing wavelength
being determined by the band gap of a single material, we
have developed InGaAs/GaPAsSb QWs grown on GaAs.
This letter describes the optical and structural characteristics
of the QWs, in particular demonstrating that the peak of the
PL emission spectrum can be shifted from 1.2 to 1.5 ␮m,
depending on the composition of the GaPAsSb layer. Theoretical modeling shows that the proposed QW structure has a
type-II band-edge alignment, in which the electron and hole
wave functions have large spatial overlap.
A schematic of the band-edge alignment of the type-II
QWs is shown in Fig. 1. The proposed structure consists of
thin QWs of InGaAs and GaPAsSb for the electrons and
holes, respectively, embedded in GaAs. A theoretical twoband model based on the envelope wave function approximation has been used to calculate the transition energies and
overlaps between electron and hole wave functions for the
type-II QW structures. The wave functions of the lowest
electron energy state and the lowest hole energy state are
also shown schematically in Fig. 1. The band offsets have

a兲

Electronic mail: phil.dowd@asu.edu
Now at the Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz
5-7, D-10117, Berlin, Germany.
c兲
Center for Solid State Science and Department of Physics and Astronomy,
Arizona State University, Tempe AZ 85287-1704.
d兲
On leave from Beijing University, Beijing, People’s Republic of China.
e兲
On leave from Tsinghua University, Beijing, People’s Republic of China.
b兲

FIG. 1. Schematic band-edge diagram of InGaAs/GaPAsSb/InGaAs type-II
QW embedded in GaAs. Schematic electron and hole wave functions are
also shown.

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© 1999 American Institute of Physics
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Dowd et al.

Appl. Phys. Lett., Vol. 75, No. 9, 30 August 1999

TABLE I. Room temperature transition wavelengths between electronheavy hole and the wave function overlaps as a function of the layer thickness of the type-II QW.
Thickness of each layer

Transition wavelength
共e-hh兲 共␮m兲
Wave function overlap
共%兲

15 Å

30 Å

60 Å

1.154

1.265

1.357

75.5

63.8

37.6
FIG. 2. Dark field electron micrograph showing cross section of an InGaAs/
GaPAsSb/InGaAs QW structure. The total well region layer thickness is 9.0
nm.

been calculated using the model-solid theory9 and the effects
of strain on the band structure have also been included. Nonquantum confinement as is the case for conventional quanparabolicities for both the conduction band and the valence
tum dots, in which electrons are confined in a threeband have been taken into account. Table I summarizes the
dimensional dot. The interfaces between the InGaAs layers
results obtained for different thicknesses of an
and the GaPAsSb layer are not very well defined in the elecIn0.4Ga0.6As/GaP0.15As0.4Sb0.45 /In0.4Ga0.6AsQW
structure
tron micrographs. This is due at least partially to the lack of
embedded in GaAs. For 3.0-nm-thick layers, the calculated
contrast between the different group-V element atoms. Inhotransition wavelength is close to 1.3 ␮m and there is a large
mogeneous strain contrast is also visible extending into the
overlap of the electron and hole wave functions. Such a large
GaAs cladding layers on either side of the QW. The strain
electron-hole wave function overlap is the foundation of this
field variations are believed to result from thickness and
structure design.
compositional variations within the QW itself. Electron miBased on these calculations, samples with QW thickcrographs of sample B showed undulating contrast within the
nesses of approximately 3.0 nm have been grown and charQW structure, confirming these variations. High resolution
acterized. All samples have been grown by molecular beam
lattice imaging showed no visible sign of structural defects.
epitaxy 共MBE兲 on GaAs substrates at a growth temperature
Chemical composition within this QW cannot be determined
of 505 °C. The samples for PL measurements were grown on
easily and further experiments using other characterization
a 20-period AlAs–GaAs superlattice followed by a 220 nm
tools are currently under way.
GaAs buffer layer, and they were capped by a 10-nm-thick
Room temperature PL measurements have been perGaAs layer. The InGaAs layers of the active region were
formed using the 514 nm line of an argon-ion laser at excinominally 3.0 nm thick, with ⬃40% In composition, while
tation densities between about 40 and 1000 W/cm2. Samples
the GaPAsSb layer was also nominally 3.0 nm thick. The
with different compositions for the GaPAsSb layer show PL
maximum strain per layer was calculated to be less than 3%.
emission wavelength varying over a broad range up to 1.5
Two samples 共labeled here as A and B兲 with different compositions for the middle layer were chosen for further investigation. Samples containing only the GaPAsSb layer or only
the two InGaAs layers separated by a 3 nm GaAs layer were
also grown for comparison.
Reflection high-energy electron diffraction 共RHEED兲
measurements were carried out during growth. These
showed that both InGaAs layers displayed a transition to a
faceted surface morphology, very close to the completion of
the layer. It is thus expected that the first InGaAs layer has
formed a rough uneven surface for deposition of the GaPAsSb layer. Deposition of the GaPAsSb layer appeared to
smoothen out the growth surface, resulting in a relatively
smooth surface at the end of the 3.0 nm growth on which the
second InGaAs layer was then deposited.
Transmission electron microscopy observations of crosssectional samples have been carried out to characterize the
layer structure of the samples. Figure 2 shows a dark field
transmission electron micrograph obtained for sample A.
This image clearly shows two well defined interfaces with a
separation of 9.0 nm, which is in excellent agreement with
the total thickness determined from the growth rate for the
InGaAs/GaPAsSb QW structure. This finding demonstrates
that the as-grown sample has indeed a QW structure. The top
FIG. 3. Room temperature PL spectra of two different InGaAs/GaPAsSb/
InGaAs/GaAs interface shows a certain degree of roughness,
InGaAs QW structures embedded in GaAs. 共a兲 Sample A at excitation den2
which is consistent with the RHEED observation. However,
sities
1000at:W/cm
, 共ii兲 250 W/cm2, 共iii兲 100 W/cm2, Downloaded
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this roughness does not provide substantial additional lateral
W/cm . 共b兲 Sample B at an excitation density of 1000 W/cm2.
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Dowd et al.

Appl. Phys. Lett., Vol. 75, No. 9, 30 August 1999

FIG. 4. Temperature dependency of the integrated PL intensity and the peak
PL wavelength of sample A at an excitation density of 50 W/cm2.

␮m. Figure 3 shows the PL spectra obtained for the two
samples described in this letter. For sample A, the peak of
the PL spectrum was observed at 1.3 ␮m and the linewidth,
defined as the full width at half maximum 共FWHM兲, was
measured to be 110 meV. As the excitation intensity increased, a blueshift of the PL spectrum of 13 meV was observed. For sample B, the peak wavelength was close to 1.5
␮m, with a FWHM of 130 meV. These samples also displayed a long wavelength spectral tail, which is believed to
be caused by compositional variations within the GaPAsSb
layer. For the samples without any GaPAsSb layer, PL emission between 1.07 and 1.15 ␮m was observed, with no long
wavelength spectral tail. No blueshift of the spectrum with
increasing excitation density was seen. These results indicate
that the long wavelength emission arises from a spatially
indirect transition between the InGaAs and the GaPAsSb
layer. At low temperature 共2 K兲, a larger blueshift of 30 meV
was observed as the excitation density was increased from 5
to 250 W/cm2. This is primarily attributed to the band filling
of lower energy states which arise from the compositional
fluctuations within the GaPAsSb layer. The observed blueshift has also been seen in type-II structures.10,11
Temperature-dependent measurements have shown that
the integrated PL intensity increases by a factor of 15 on
cooling from 300 to 2 K, which is shown in Fig. 4 for sample
A. This occurs primarily as the temperature is cooled to
about 120 K, after which the intensity increase becomes
much smaller. The spectral peak wavelength was also found
to vary much less below 120 K, and increased at a rate of

1269

approximately 3.7 Å/K above 120 K. The wavelength
change, together with the absolute wavelength, are best modeled by considering the temperature dependency of the electron and hole energy level shifts in a type-II band-edge
aligned QW. The spectral linewidth also decreased to a minimum value of 74 meV below 120 K. The broad linewidths
are believed to be dominated by the fluctuations in composition of the GaPAsSb layer and the widths of the QW
layers.12 Additional broadening mechanisms could include
the strong band-filling effect in type-II structures,13 as well
as InGaAs QW asymmetry resulting from segregation in the
growth direction.
In conclusion, we have demonstrated a material system
grown on GaAs that is suitable for optoelectronic devices
operating at long wavelength. PL emission at wavelengths
between 1.2 and 1.5 ␮m has been achieved. Theoretical
modeling has suggested that this emission is due to a type-II
transition, where there is a large spatial overlap of the electron and hole wave functions.
Electron microscopy was carried out at the Center for
High Resolution Electron Microscopy at Arizona State University.

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