Carrier recombination in 1.3 μ m Ga As Sb ∕ Ga As quantum well lasers
K. Hild, S. J. Sweeney, S. Wright, D. A. Lock, S. R. Jin, I. P. Marko, S. R. Johnson, S. A. Chaparro, S.-Q. Yu,
and Y.-H. Zhang
Citation: Applied Physics Letters 89, 173509 (2006); doi: 10.1063/1.2369649
View online: http://dx.doi.org/10.1063/1.2369649
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APPLIED PHYSICS LETTERS 89, 173509 共2006兲

Carrier recombination in 1.3 ␮m GaAsSb/ GaAs quantum well lasers
K. Hild, S. J. Sweeney,a兲 S. Wright, D. A. Lock, S. R. Jin, and I. P. Marko
Advanced Technology Institute, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom

S. R. Johnson, S. A. Chaparro, S.-Q. Yu, and Y.-H. Zhang
MBE Group, Arizona State University, Tempe, Arizona 85287

共Received 14 August 2006; accepted 19 September 2006; published online 26 October 2006兲
In this letter the authors present a comprehensive study of the threshold current and its temperature
dependence in GaAsSb-based quantum well edge-emitting lasers for emission at 1.3 ␮m. It is found
that at room temperature, the threshold current is dominated by nonradiative recombination
accounting for more than 90% of the total threshold current density. From high hydrostatic pressure
dependence measurements, a strong increase in threshold current with pressure is observed,
suggesting that the nonradiative recombination process may be attributed to electron overflow into
the GaAs/ GaAsP barrier layers and, to a lesser extent, to Auger recombination. © 2006 American
Institute of Physics. 关DOI: 10.1063/1.2369649兴
Lasers emitting close to 1.3 ␮m are of considerable importance for the development of metro-area networks.1
Progress in this area has been hindered largely by the need to
significantly reduce the cost of the laser module itself. The
incumbent InGaAsP quantum well 共QW兲 material system
used to make such lasers suffers from two problems: Firstly,
the fact that they are grown on InP makes it difficult to
produce vertical cavity surface emitting lasers 共VCSELs兲
which are vastly more cost effective but are better suited to
GaAs substrates. Secondly, the devices are highly susceptible
to temperature variations resulting in the need to incorporate
sophisticated temperature control electronics into the package, leading to an order of magnitude increase in cost.
Hence, there has been considerable effort devoted to the development of GaAs-based laser active regions which emit at
1.3 ␮m. InAs quantum dots2 and GaInNAs-based QWs have
been the subject of extensive research. However, the properties of even the best quantum dot lasers are far from ideal,
since their threshold current density increases quickly with
temperature around room temperature due to nonradiative
recombination resulting in a low characteristic temperature,
T0 ⬃ 50 K 关T0 = 共d ln Ith / dT兲−1兴, similar to that of standard
1.3 ␮m QW based lasers.3 P-doped quantum dot lasers can
exhibit very high T0 values 共even infinite over a narrow temperature range兲, but this is achieved at the expense of higher
threshold currents compared with undoped devices.4 For
GaInNAs-based QW lasers, it has been shown that even for
the best 1.3 ␮m devices available, approximately 50% of the
threshold current at room temperature may be attributed to
defect-related recombination.5 The implications of this on
long-term device stability have yet to be fully addressed.
Another possibility is the use of GaAsSb/ GaAs QWs.6 Lasers based upon this material have been produced,7 but little,
if any, research has been undertaken to assess the carrier
recombination and temperature dependent processes occurring in such devices. The aim of this letter is to consider the
characteristics of GaAsSb/ GaAs-based edge-emitting lasers
and to explore the potential of GaAsSb/ GaAs active regions
for use in 1.3 ␮m VCSELs.
a兲

Author to whom correspondence should be addressed; electronic mail:
s.sweeney@surrey.ac.uk

The devices in this study consist of a triple
GaAs0.9P0.1 / GaAs/ GaAs0.7Sb0.3 / GaAs/ GaAs0.9P0.1 共9 nm/
5 nm/ 7 nm/ 5 nm/ 9 nm兲 strain compensated QW active region grown at 585 ° C by molecular beam epitaxy. The nominal 30% Sb concentration is estimated based on photoluminescence measurements. The GaAsP layers provide strain
compensation to increase the maximum number of highly
strained QWs that can be grown pseudomorphically and to
reduce strain driven in-plane Sb segregation that leads to
inhomogeneous linewidth broadening. The active region is
sandwiched between two 20 nm Al0.25Ga0.75As layers, two
150 nm graded-index 共GRIN兲 AlGaAs layer with Al concentration linearly increased from 25% to 65%, one 2 ␮m
n-type 共Si doped, 2 ⫻ 1018 cm−3兲 Al0.65Ga0.35As cladding
layer followed by 500 nm GaAs buffer layer at bottom, and
one 2 ␮m p-type 共Be doped, 2 ⫻ 1018 cm−3兲 Al0.65Ga0.35As
cladding layer followed by 100 nm GaAs cap layer at top.
The doping concentration is decreased from 2 ⫻ 1018 to
1 ⫻ 1017 in both GRIN layers from cladding layer to active
region and is increased to 2 ⫻ 1019 in GaAs cap layer from
cladding layer to surface. The devices are fabricated using a
typical broad contact edge-emitting laser process. The device
ridges 共50 and 100 ␮m wide兲 were defined using photolithography and inductively coupled plasma dry etching. By
etching down through the p-GaAs contact layer and stopping
about 0.1 ␮m above the active region, these ridges provide
current confinement as well as waveguiding. Ti/ Pt/ Au
p-contact stripes ranging from 40 to 90 ␮m were deposited
using a second mask, after which the wafers are lapped down
to 100 ␮m and AuGe/ Ni/ Au n-metal contacts are deposited
on the backside of the substrate; this is followed by rapid
thermal annealing for both metal contacts. The devices were
measured as cleaved. Their room temperature emission
wavelength was measured to be 1.27 ␮m.
Temperature dependence measurements were performed
with a standard closed cycle cryostat setup over the temperature range of 60– 290 K. The lasers were driven under pulsed
operation with 200 ns long pulses at a duty cycle of 10 kHz
in order to minimize Joule heating effects. We investigated
the temperature and current dependencies of the spontaneous
emission spectra from which we extracted the radiative current 共since the integrated spontaneous emission is propor-

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FIG. 2. Jth and Jrad as a function of temperature 共normalized at 60 K兲. The
inset shows the temperature dependence of Jrad on an expanded scale from
which it can be seen to exhibit an ideal QW-like linear temperature dependence 共indicated by the dashed line兲.
FIG. 1. Integrated spontaneous emission as a function of current at 110 K
共solid diamonds兲 and 260 K 共open squares兲. At 110 K Lspon increases linearly with current, indicating that it is dominated by radiative recombination. The contrasting sublinear behavior at higher temperatures indicates the
presence of a nonradiative process. Note that at each temperature, Lspon has
been normalized to its value at threshold 共Lpin兲. The lines are guides to
the eye.

We find that the devices are highly temperature sensitive
with a characteristic temperature T0 ⬃ 60– 70 K at RT comparable to values that have been reported elsewhere for
GaAsSb/ GaAs devices7 and InGaAs共P兲 / InP devices8 emitting around 1.3 ␮m. From measurements on probed broad
area devices at 300 K, we estimate Jth to be ⬃1.6 kA cm−2
Also shown in Fig. 2 is the temperature dependence of the
tional to the radiative current兲. To measure the spontaneous
radiative current density Jrad 共squares兲, which was found
emission, we milled a window in the n side of the devices
from the pinning level of the integrated spontaneous emisand aligned an optical fiber to collect the spontaneous emission, Lpin, as seen in Fig. 1, since at threshold Lpin ⬀ Jrad.
sion. This technique is described in detail elsewhere.8 HydroFrom this, we estimate that at 290 K the radiative recombistatic pressure measurements were also performed on the denation process accounts for ⬃10% of the total threshold curvices. The application of high hydrostatic pressure causes an
rent. Clearly, nonradiative processes dominate and account
increase in the direct band gap, mimicking the effect of alfor ⬃90% of Jth close to RT. From the inset in Fig. 2, it
loying, thereby allowing one to investigate wavelength decan also be seen that Jrad has the ideal QW linear temperapendent properties of semiconductor lasers. It can also be
ture dependence,10 leading to the conclusion that the loss
used to vary the alignment between direct 共⌫兲 and indirect
mechanism共s兲 must increase superlinearly with increasing
共X, L兲 bands and to alter the band offsets between layers
temperature.
which have different pressure coefficients.9 In this work, hyFurther evidence for the importance of nonradiative redrostatic pressure measurements were performed over the
combination may be found from high pressure measurerange of 0 – 8 kbar using a gas compressor system.
ments. Figure 3 shows the measured room temperature presThe integrated spontaneous emission 共Lspon兲 versus cursure dependence of Jth for the GaAsSb devices 共circles兲. Also
rent at two different temperatures, 110 and 260 K, is shown
shown is the ideal expected variation of Jrad ⬀ E2g 共Ref. 11兲
in Fig. 1. In both cases the integrated spontaneous emission
共dot-dot-dashed line兲, where Eg is the band gap 共taken from
pins at threshold due to the fact that the carrier density is
Eg = hc / ␭lasing, where ␭lasing is the measured lasing waveclamped by the lasing process. The two curves have been
length兲. Note that in each case, Jth and Jrad have been nornormalized to the value of Lspon at threshold 共=Lpin兲, so that
malized to their respective values at atmospheric pressure.
the subthreshold shape of both curves can be easily comClearly, it can be seen that Jth increases more rapidly with
pared and contrasted. There is a clear difference in the bepressure than the ideal Jrad. Preliminary pressure measurehavior of the device at the two temperatures. At 110 K, the
ments suggest that the pressure dependence of Jrad in our
subthreshold Lspon versus current curve is linear, suggesting
devices is very close to ideal.12 From the earlier temperature
that the primary current path flowing through the laser may
dependence measurements, we found that a nonradiative probe associated with radiative recombination. Furthermore, the
cess dominated at room temperature forming ⬃90% Jth. We
fact that the curve remains linear down to low currents sugtherefore conclude that the observed strong increase of Jth
gests that defect-related recombination makes a relatively
with pressure is due to nonradiative recombination. The consmall contribution to the threshold current. In stark contrast,
duction band offset of the GaAsSb/ GaAs interface 共for 30%
the sublinear behavior of the Lspon versus current curve at
Sb兲 is still a matter of debate with reports in the literature of
260 K suggests that a nonradiative process is present and
both weak type I 共Ref. 13兲 and type II alignments.14 This
that this process has a stronger carrier density dependence
indicates that the conduction band offset is close to zero. Due
than the radiative current.
to the slightly larger pressure coefficient of the GaAsSb 共Ref.
In Fig. 2 we show the normalized 共at 60 K兲 temperature
15兲 QW ⌫ minimum compared to the GaAs barrier ⌫
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dependence of the threshold current density Jth 共diamonds兲.
minimum16 共difference= + 1.2 meV/ kbar兲, occupation of
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Appl. Phys. Lett. 89, 173509 共2006兲

Hild et al.

rier leakage and Auger recombination. However, based on
our data, it is not yet possible to quantify the role of each of
these nonradiative current paths.
In summary, from our first studies of 1.3 ␮m GaAsSb
lasers, we find that at room temperature the threshold current
is dominated by nonradiative recombination. The nonradiative process is also responsible for the poor temperature sensitivity of the devices resulting in low T0 values around room
temperature. From our pressure dependence measurements,
we observe that the threshold current increases with pressure,
consistent with the transfer of carriers into the GaAs/ GaAsP
barrier layers due to the decrease of the band offset with
pressure. This suggests that carrier overflow into the GaAs
and/or GaAsP barrier layers together with Auger recombination forms a large nonradiative current path in these devices
at normal operating temperatures.

FIG. 3. Measured pressure dependence of Jth for the GaAsSb devices
共closed circles兲 and ideal Jrad 共E2g dependence兲 共dot-dot-dashed line兲, normalized at atmospheric pressure. The dotted line indicates the calculated
pressure dependence of carrier leakage into the GaAs ⌫ minimum, while the
dot-dashed line indicates the pressure variation of carrier leakage into the X
minima of the GaAsP layers. For comparison, the pressure dependence of Jth
for a 1.3 ␮m InGaAsP device is shown 共open squares兲 together with the
calculated variation in the Auger current with pressure for the InGaAsP
device 共solid line兲 共derived from Ref. 17兲.

electrons in the GaAs barrier states will increase with increasing pressure, as shown by the calculated dotted line in
Fig. 3. Furthermore, due to the small or possibly negative
共type II兲 GaAsSb/ GaAs band offset, the GaAsP layers act as
additional barriers. With increasing pressure, the X minima
in the GaAsP layers move to lower energy and may provide
an additional leakage path, as shown by the dot-dashed line
in Fig. 3. Thus, for a fixed quasi-Fermi-level splitting, the
occupation of the barrier layers will increase exponentially
with increasing pressure leading to an increased leakage current. While carrier leakage into the GaAs and/or GaAsP barrier layers offers a possible explanation for the increase of Jth
with pressure, it is clear that this in itself is insufficient. The
lower rate of increase of Jth with pressure dependence compared to the leakage paths may be explained if carrier leakage and a nonradiative process that decreases with pressure
are both present. In Fig. 3 for comparison we plot the pressure dependence of Jth 共open squares兲 and the Auger current
共solid line兲 for a typical 1.3 ␮m InGaAsP device 共taken from
Ref. 17兲. For the InGaAsP devices, Auger recombination is
the dominant nonradiative process8,10,17,18 causing Jth to reduce with pressure due to the decrease in the Auger current
with pressure. We therefore propose that the nonradiative
recombination current which dominates Jth for the GaAsSb
devices at room temperature is due to a combination of car-

The authors thank E. P. O’Reilly and S. B. Healy 共Tyndall Institute, Ireland兲 for useful discussions. They also gratefully acknowledge EPSRC 共UK兲 for supporting this project
under Grant No. GR/T21516/01.
1

T. P. McGarty and G. J. Clancy, Jr., IEEE J. Sel. Areas Commun. 1, 816
共1983兲.
2
O. B. Shchekin and D. G. Deppe, IEEE Photonics Technol. Lett. 14, 1231
共2002兲.
3
I. P. Marko, A. R. Adams, S. J. Sweeney, I. R. Sellers, D. J. Mowbray, M.
S. Skolnick, H. Y. Liu, and K. M. Groom, IEEE J. Sel. Top. Quantum
Electron. 11, 1041 共2005兲.
4
I. P. Marko, N. F. Masse, S. J. Sweeney, A. D. Andreev, A. R. Adams, N.
Hatori, and M. Sugawara, Appl. Phys. Lett. 87, 211114 共2005兲.
5
S. Tomic, E. P. O’Reilly, R. Fehse, S. J. Sweeney, A. R. Adams, A. D.
Andreev, S. A. Choulis, T. J. C. Hosea, and H. Riechert, IEEE J. Sel. Top.
Quantum Electron. 9, 1228 共2003兲.
6
T. Anan, K. Nishi, S. Sugou, M. Yamada, K. Toukutome, and A. Gomyo,
Electron. Lett. 34, 2127 共1998兲.
7
M. Yamada, T. Anan, K. Toukutome, A. Kamei, K. Nishi, and S. Sugou,
IEEE Photonics Technol. Lett. 12, 774 共2000兲.
8
S. J. Sweeney, A. F. Philips, A. R. Adams, E. P. O’Reilly, and P. J. A.
Thijs, IEEE Photonics Technol. Lett. 10, 1076 共1998兲.
9
D. J. Wolford, T. F. Kuech, J. A. Bradley, M. A. Gell, D. Ninno, and M.
Jaros, J. Vac. Sci. Technol. B 4, 1043 共1986兲.
10
E. P. O’Reilly and M. Silver, Appl. Phys. Lett. 63, 3318 共1993兲.
11
A. R. Adams, M. Silver, and J. Allam, Semicond. Semimetals 55, 301
共1998兲.
12
K. Hild, S. J. Sweeney, I. P. Marko, S. R. Jin, S. R. Johnson, S. A.
Chaparro, S. Yu, and Y.-H. Zhang, Phys. Status Solidi B 共to be published兲.
13
J.-B. Wang, S. R. Johnson, S. A. Chaparro, D. Ding, Y. Cao, Yu. G.
Dadofyev, Y.-H. Zhang, J. A. Gupta, and C. Z. Guo, Phys. Rev. B 70,
195339 共2004兲.
14
Q. Liu, S. Derksen, A. Lindner, F. Scheffer, W. Prost, and F.-J. Tegude, J.
Appl. Phys. 77, 1154 共1994兲.
15
A. D. Prins, D. J. Dunstan, J. D. Lambkin, E. P. O’Reilly, A. R. Adams, R.
Pritchard, W. S. Truscott, and K. E. Singer, Phys. Rev. B 47, 2191 共1993兲.
16
D. J. Wolford and J. A. Bradley, Solid State Commun. 53, 1069 共1985兲.
17
A. F. Phillips, S. J. Sweeney, A. R. Adams, and P. J. A. Thijs, Phys. Status
Solidi B 211, 513 共1999兲.
18
M. Silver, E. P. O’Reilly, and A. R. Adams, IEEE J. Quantum Electron. 9,
1557 共1998兲.

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