Significantly improved minority carrier lifetime observed in a long-wavelength infrared
III-V type-II superlattice comprised of InAs/InAsSb
E. H. Steenbergen, B. C. Connelly, G. D. Metcalfe, H. Shen, M. Wraback, D. Lubyshev, Y. Qiu, J. M. Fastenau,
A. W. K. Liu, S. Elhamri, O. O. Cellek, and Y.-H. Zhang
Citation: Applied Physics Letters 99, 251110 (2011); doi: 10.1063/1.3671398
View online: http://dx.doi.org/10.1063/1.3671398
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APPLIED PHYSICS LETTERS 99, 251110 (2011)

Significantly improved minority carrier lifetime observed in a long-wavelength
infrared III-V type-II superlattice comprised of InAs/InAsSb
E. H. Steenbergen,1,a) B. C. Connelly,2 G. D. Metcalfe,2 H. Shen,2 M. Wraback,2
D. Lubyshev,3 Y. Qiu,3 J. M. Fastenau,3 A. W. K. Liu,3 S. Elhamri,4 O. O. Cellek,1
and Y.-H. Zhang1

1
Center for Photonics Innovation and School of Electrical, Computer and Energy Engineering,
Arizona State University, Tempe, Arizona 85287, USA
2
U.S. Army Research Laboratory, RDRL-SEE-M, Adelphi, Maryland 20783, USA
3
IQE, Inc., Bethlehem, Pennsylvania 18015, USA
4
Department of Physics, University of Dayton, Ohio 45469, USA

(Received 8 November 2011; accepted 1 December 2011; published online 22 December 2011)
Time-resolved photoluminescence measurements reveal a minority carrier lifetime of >412 ns at
77 K under low excitation for a long-wavelength infrared InAs/InAs0.72Sb0.28 type-II superlattice
(T2SL). This lifetime represents an order-of-magnitude increase in the minority carrier lifetime
over previously reported lifetimes in long-wavelength infrared InAs/Ga1xInxSb T2SLs. The
considerably longer lifetime is attributed to a reduction of non-radiative recombination centers
with the removal of Ga from the superlattice structure. This lifetime improvement may enable
background limited T2SL long-wavelength infrared photodetectors at higher operating
C 2011 American Institute of Physics. [doi:10.1063/1.3671398]
temperatures. V
The InAs/Ga1xInxSb type-II superlattice (T2SL) is the
most investigated III-V T2SL material for mid- and longwavelength infrared (MWIR and LWIR) photodetectors.
T2SLs are predicted to have a number of advantages over
the currently used bulk HgCdTe, including a decreased dependence of the bandgap on compositional non-uniformity,
the ability to leverage III-V manufacturing capabilities, the
lower cost of substrates, a higher electron effective mass
leading to smaller tunneling currents, and band-engineered
lower Auger recombination rates and thus lower dark currents.1 However, reported minority carrier lifetimes at 77 K
are 50–80 ns for MWIR InAs/Ga1xInxSb T2SLs (Refs. 2
and 3) and 10–40 ns for LWIR InAs/Ga1xInxSb T2SLs
(Refs. 2, 4 and 5) as compared to 1 ls for Hg0.78Cd0.22Te
(10 lm bandgap).2 The short minority carrier lifetime has
been attributed to Shockley-Read-Hall (SRH) recombination
and is detrimental to the device dark current and quantum efficiency.6 Calculations show that a 350-ns lifetime must be
reached for a LWIR T2SL pn homojunction photodiode to
achieve background limited operation (BLIP) at 80 K with
F/6.5 optics in a 300 K background.4
Campaigns to improve the minority carrier lifetime have
led to investigations of the InAs/Ga1xInxSb T2SL interface
type7 and density,8,9 as well as doping concentration,2,10 but
have thus far resulted in minor or no improvements in the carrier lifetime. It is hypothesized that a native defect associated
with InAs or GaSb limits the carrier lifetime.9 The measured
lifetimes of bulk InAs (325 ns) (Ref. 9) and bulk
InAs0.80Sb0.20 (250 ns) (Ref. 2) are longer than that of bulk
GaSb (100 ns),9 suggesting that defects associated with
GaSb and other Ga-related bonds limit the lifetime of InAs/
Ga1xInxSb T2SLs and that InAs/InAs1xSbx T2SLs should
have a longer minority carrier lifetime than InAs/Ga1xInxSb
T2SLs. Furthermore, InAs/InAs1xSbx T2SLs have been suca)

Electronic mail: Elizabeth.Steenbergen@asu.edu.

0003-6951/2011/99(25)/251110/3/$30.00

cessfully demonstrated for MWIR lasers and proposed for
LWIR photodetectors.11 A theoretical comparison between
LWIR InAs/InAs1xSbx and InAs/Ga1xInxSb T2SLs, which
excludes SRH recombination, found that the ideal detectivities
of the two types of T2SL devices are comparable and are both
greater than that of HgCdTe devices.12 This letter reports an
order-of-magnitude improvement of the minority carrier lifetime for a LWIR InAs/InAs1xSbx T2SL over that of a LWIR
InAs/Ga1xInxSb T2SL. We observe a carrier lifetime of
>412 ns at 77 K under low excitation for an InAs/
InAs0.72Sb0.28 T2SL as determined by time-resolved photoluminescence (TRPL) measurements. This improvement in minority carrier lifetime could now enable LWIR T2SL BLIPs
at higher operating temperatures.
The InAs/InAs0.72Sb0.28 T2SL was designed with AlSb
barriers for TRPL measurements. The AlSb barriers ensure
that the measured photoluminescence (PL) decay time is due
to carrier recombination in the T2SL and the influence of carrier transport, surface recombination, or any junction fields
within the sample is minimized. Studies of a T2SL homojunction have shown that the restoring current in a narrowbandgap junction results in an ostensibly long PL lifetime.7
The sample was grown by molecular beam epitaxy on an
undoped 2-in. GaSb substrate with a 500-nm GaSb buffer
layer. The T2SL consists of 20 periods of InAs (173 Å) and
InAs0.72Sb0.28 (72 Å), with a total thickness of 500 nm. The
T2SL is unintentionally doped n-type (3  1016 cm3 at
10 K) as determined by Hall measurements. AlSb barrier
layers (100 Å), above and below the T2SL, are used to confine
the electrons to the superlattice as well as to provide an
adequate heavy-hole barrier of over 100 meV. The entire
structure is capped with 100 Å of pþ InAs. PL measurements
show peak emission at 8.2 lm (150 meV) at 77 K.
TRPL measurements were performed on the InAs/
InAs0.72Sb0.28 T2SL sample at 11, 40, 77, 110, 150, 200, and
250 K. An ultrafast laser with 100 fs pulses at a 250 kHz

99, 251110-1

C 2011 American Institute of Physics
V

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251110-2

Steenbergen et al.

Appl. Phys. Lett. 99, 251110 (2011)

FIG. 2. (Color online) Combined temperature-dependent time-resolved photoluminescence decay measurements on the sample from Fig. 1.
FIG. 1. (Color online) Time-resolved photoluminescence measurements on
an InAs/InAs0.72Sb0.28 T2SL at 77 K for initial excess carrier densities ranging from 4.0  1015 to 1.0  1017 cm3.

repetition rate with 2 lm (0.62 eV) emission was used to
excite carriers in only the T2SL region to excess carrier densities between 1015 and 1017 cm3. The carrier concentrations were calculated using an absorption coefficient of
104 cm1 from published n-type InAs room-temperature
absorption data at 0.62 eV.13 This is a reasonable absorption
coefficient value given that the laser pump energy is well
above the SL band edge energy. The PL was detected with
an HgCdTe detector operating at 200 K with a 3 ns temporal
resolution and a 1  1 mm2 detector area. A 3.6 lm longpass
filter isolated the PL signal from the pump laser scattering.
Further details of the experimental technique can be found in
Ref. 5.
The TRPL signal at 77 K is shown in Fig. 1 for a sampling of initial excess carrier densities, dpt ¼ 0, between
4.0  1015 and 1.0  1017 cm3. For the highest dpt ¼ 0,
1.0  1017 cm3, a fast initial decay is observed in the PL
signal, and the instantaneous PL lifetime increases significantly as the signal decays. At the lowest initial excess carrier density of 4.0  1015 cm3, the PL signal approaches a
single exponential decay, indicating excitation levels are
much lower than the background doping density (approximately an order of magnitude lower), and we are approaching the low-excitation regime. As described in Ref. 5, at a
given temperature the PL intensity is only a function of the
excess carrier density, dp. Therefore, the PL data taken at
lower dpt ¼ 0 can be shifted in time to overlap with the data
taken at higher dpt ¼ 0. This shifting process provides a combined PL decay signal with an improved signal-to-noise ratio. Figure 2 shows combined curves for temperatures from
11 to 250 K with initial excess carrier densities of
1.0  1017 cm3. At each temperature, the decay rate of the
PL signal shows a strong dependence on dp, evolving from a
faster decay in the first 100 ns, which corresponds to excess
carrier densities >5  1016 cm3, to a slower, almost single
exponential decay at the tail end of the decay, which corresponds to excess carrier densities <5  1015 cm3. This
strong dependence of the carrier lifetime on the excess carrier density cannot be explained by SRH recombination
alone. Contributions from radiative or Auger recombination,
which vary strongly with excess carrier density, must also be
considered.

Under typical detector operating conditions, only very
small excess carrier densities on the order of 1012 cm3 are
expected,14 so it is important to determine the carrier lifetime
in the low-excitation regime where the lifetime is independent of the excess carrier density to predict device performance. We, therefore, fit the tail of the TRPL data, where the
excitation level is low (1015 cm3) compared to the background doping density (1016 cm3), with a single exponential decay to obtain the lifetime, s, at each temperature. The
resulting PL lifetimes from the fit are plotted in Fig. 3
(points) as a function of inverse temperature. When the PL
decay rate reaches a single exponential decay in the lowexcitation regime, the PL lifetime is equivalent to the minority carrier lifetime. At higher excitation levels, however, the
PL lifetime is shorter than the minority carrier lifetime. Since
the lowest excitation levels used in this study are just
approaching the low-excitation regime, the measured PL
lifetime represents a lower limit of the minority carrier
lifetime.
The PL lifetime is observed to increase from low temperature (11 K) to a maximum of 412 ns at 77 K. This lifetime is an order-of-magnitude longer than the SRH-limited
lifetime of 30 ns that was previously observed in LWIR
InAs/Ga1xInxSb T2SL absorber layers at 77 K.2,4,5 The temperature dependence of the lifetime can be attributed to a
combination of both SRH and radiative recombination. For
illustration, the temperature dependence of the SRH lifetime

FIG. 3. Carrier lifetimes extracted from the fits in Fig. 2 of the PL decay are
shown as points as a function of 1000/T. Also plotted is the temperature dependence of the SRH lifetime (sSRH ! T 1/2, dotted line), radiative lifetime
(sRad ! T 3/2, dashed line), and a combination of both SRH and radiative
lifetimes (solid line).

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251110-3

Steenbergen et al.

(sSRH ! T1/2, dotted line), radiative lifetime (sRad ! T 3/2,
dashed line), and a combination of SRH and radiative lifetimes (solid line) are plotted along with the data in Fig. 3.15
At temperatures below 77 K, the PL lifetime increases with
increasing temperature, indicating that the lifetime is dominated by radiative recombination and that the radiative lifetime is shorter than the non-radiative (SRH) lifetime. At
temperatures above 77 K, the PL lifetime decreases with
increasing temperature, signifying that the PL lifetime is
dominated by SRH recombination and the radiative lifetime
is longer than the SRH lifetime. Around 77 K, both radiative
and SRH recombination contribute significantly to the
lifetime.
The improved lifetime observed in this InAs/
InAs0.72Sb0.28 T2SL sample offers evidence that the constituent InAs and InAsSb layers have excellent crystalline properties, and the sample possesses a low density of non-radiative
recombination centers at the interfaces and in the layers. It is
important to note that this sample gives a strong PL signal
and has excellent structural properties with an x-ray diffraction zero-order satellite peak FWHM of 40 arc sec, ruling
out the possibility of a long carrier lifetime due to strong
localization of photogenerated carriers by interface roughness or layer thickness fluctuations. The InAs/InAs0.72Sb0.28
T2SL has a longer lifetime than even bulk InAs at 77 K due
to the decreased radiative transition probability compared to
that of a direct bandgap bulk material resulting from a
decrease in the wave function overlap in the type-II structure.
These results also shine some light on the origin of the relatively short carrier lifetime (30 ns) in LWIR InAs/
Ga1xInxSb T2SLs, which could be due to the non-radiative
recombination centers associated with Ga atoms. Furthermore, the “stabilized Fermi level” due to intrinsic point
defects in bulk GaSb and GaAs is near the valence band
edge or the midgap, respectively,16 leaving mid-gap trap
states available for SRH recombination. In comparison, in
bulk InAs, the stabilized Fermi level is above the conduction
band edge,16 rendering any mid-gap defect states inactive for
SRH processes, as demonstrated by relatively high photoluminescence efficiencies in As-rich InAs/InAsSb T2SLs.17
In summary, we observe an order-of-magnitude longer
minority carrier lifetime (>412 ns at 77 K) in the LWIR
InAs/InAs0.72Sb0.28 T2SL sample studied compared to that
observed in LWIR InAs/Ga1xInxSb T2SLs. In addition,
the observed carrier lifetime in InAs/InAs0.72Sb0.28 is longer
across all measured temperatures than that previously
reported in InAs/Ga1xInxSb T2SLs. Measurements on several other InAs/InAsSb T2SLs similar to the sample presented here also show substantially longer minority carrier
lifetimes (100’s of ns). We attribute the recombination
mechanism to both SRH and radiative recombination, with
comparable contributions from both near 77 K. This minority

Appl. Phys. Lett. 99, 251110 (2011)

carrier lifetime improvement may now enable background
limited T2SL LWIR pn photodetectors at higher operating
temperatures. It should be noted that the InAs/InAs0.72Sb0.28
T2SL sample growth and material properties have not been
optimized yet, suggesting that there is still room for
improvement in the InAs/InAs1xSbx T2SL minority carrier
lifetime. Since the non-radiative recombination rate has now
been significantly reduced, future studies can examine the
tradeoff between radiative and non-radiative recombination,
and sample designs can be optimized to balance lowering the
wave function overlap to decrease the radiative recombination rate with increasing the wave function overlap to
increase the absorption coefficient.
The authors acknowledge funding by the U. S. Army
Research Laboratory and the U. S. Army Research Office
MURI program under Grant No. W911NF-10-1-0524 and by
the AFOSR under Grant No. FA9550-10-1-0129. E.H.S.
appreciates the DOD SMART, ARCS, and SFAz scholarships
and acknowledges funding from the ASU Office of the VicePresident for Research and Economic Affairs, the Graduate
Research Support Program, and the Graduate College.
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