InAs/GaSb Superlattice Photodiodes Operating in the Midwave
Infrared Range
P. Christol
1
, R. Taalat
1
, C. Cervera
1
, H. Aït-Kaci
1
, M. Delmas
1
, J. B. Rodriguez
1
, E. Giard
2
and
I. Ribet-Mohamed
2
1
Institut d’Electronique du Sud, UMR-CNRS 5214, Université Montpellier 2, Place Eugène Bataillon,
34095 Montpellier Cedex 5, France
2
ONERA, Chemin de la Hunière, 91761 Palaiseau, France
Keywords: Photodiode, Midwave Infrared, InAs/GaSb Superlattice.
Abstract: This communication reports on InAs/GaSb superlattice (SL) pin photodiodes. The SL structures, fabricated
by molecular beam epitaxy (MBE), are made of symmetrical and asymmetrical period designs adapted for
the Midwave Infrared (MWIR) domain, with cut-off wavelength near 5µm at 77K. The structures are
studied in terms of dark current-voltage measurements. Comparison of results revealed the predominance of
the asymmetric SL design with dark current densities J = 4x10
-8
A/cm
2
at 77K for V
bias
= -50mV and R
0
A
product equal to 1.5x10
6
.cm
2
at 77K, one order of magnitude higher than the symmetric SL structures.
Such result demonstrates the strong influence of the period on the electrical properties of SL MWIR
photodiodes.
1 INTRODUCTION
Infrared (IR) photodetectors based on type-II
InAs/GaSb superlattice (SL) material has been given
a lot of attention this past decade. Among the
advantages of this material system, one can cite the
possibility to span a large IR range (3 to 30 µm) by
tailoring the band-gap independently from the lattice
constant, allowing addressing many applications by
the same fabrication process and the realization of
multi-color IR sensors for high performance imaging
systems.
Since the first works of Yang and Bennet in 1994
(Yang et al., 1994), impressive progresses were
obtained on MWIR SL detectors. Indeed, the
maturity of the growth of the SL quantum structure
by molecular beam epitaxy (MBE) (Kaspi et al.,
2001); (Haugan et al., 2004); (Satpati et al.,, 2007)
and progress on the processing of SL devices (Hood
et al., 2007); (Chaghi et al.,, 2009); (Plis et al., 2011)
resulted in the demonstration of high-performance
mega-pixel focal plane arrays (FPA) in midwave
infrared (MWIR) spectral bands (Rehm et al., 2006);
(Little et al., 2007); (Gunapala et al., 2010);
(Abdollahi et al., 2011). Consequently, InAs/GaSb
SL detector can be now considered as a
complementary technology to the well-established
InSb and HgCdTe MWIR photodiodes (Rogalski et
al., 2009).
Reported SL periods for MWIR operation
usually comprise GaSb and InAs layer thicknesses in
the range of 8 to 11 monolayers (MLs), and most of
the SL periods reported are symmetric with an InAs
to GaSb thickness ratio close to one (Rodriguez et
al., 2005); (Cervera et al.,, 2009); (Rehm et al.,
2006); (Little et al., 2007); (Wei et al., 2005); (Hill
et al., 2007). The period composition affects many
material characteristics such as the effective masses
and therefore the intrinsic carrier concentration,
involved in both the diffusion (J
diff
) and generation-
recombination (J
GR
) dark-currents. It also affects the
electron-holes wave-function overlap which is
involved in the external quantum efficiency of the
device. Consequently, it is clear that the SL period
composition has implications on both the signal and
the noise of the photodetector. Its optimization
should lead to improve device performances or
temperature operation.
In this paper, we report on dark current-voltage
measurements of MWIR pin SL photodiodes grown
by molecular beam epitaxy (MBE) on p-type GaSb
substrates. Two designs of SL period were
fabricated with a 5µm cut-off wavelength at 77K. A
91
Christol P., Taalat R., Cervera C., Aït-Kaci H., Delmas M., Rodriguez J., Giard E. and Ribet-Mohamed I..
InAs/GaSb Superlattice Photodiodes Operating in the Midwave Infrared Range.
DOI: 10.5220/0004222600910095
In Proceedings of the International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS-2013), pages 91-95
ISBN: 978-989-8565-44-0
Copyright
c
2013 SCITEPRESS (Science and Technology Publications, Lda.)
symmetric SL period made of 10 InAs MLs and 10
GaSb MLs (10/10 SL) and an asymmetric period
design, with an InAs to GaSb thickness ratio
close to 2, made of 7 InAs MLs and 4 GaSb MLs
(7/4 SL). Comparison of results obtained showed the
influence of the SL period design on the electrical
performances of the SL MWIR photodiodes.
2 PHOTODIODES FABRICATION
The samples were grown in a Varian Gen II MBE
reactor equipped with tellurium and beryllium
dopant cells, and valved cracker cells for both
arsenic and antimony. The pin SL structures
(Figure 1) were grown on two inch (2") GaSb (100)
wafers, p-type doped in the 5-8x10
17
/cm
3
range,
mounted on an In-free molybdenum holder. The pin
device consisted of a 500 nm thick Be-doped
(p = 1x10
18
/cm
3
) GaSb contact layer, followed by a
non-intentionally doped (nid) absorbing layer of
1µm of 10 MLs InAs/10 MLs GaSb (10/10)
symmetric (170 periods) or 7MLs InAs/4MLs GaSb
(7/4) asymmetric (310 periods) MLs SL design and
capped with a 100nm or 20 nm thick InAs Te-doped
(n = 1x10
18
/cm
3
) top contact layer. The first and last
10 SL periods are p and n-type doped respectively to
decrease the energy band offsets existing with the
GaSb and InAs contact layers.
GaSb (p) substrate
"
GaSb p
+
= 1-2.10
18
/cm
3
, e=0.5µm
cap InAs n
+
=2.10
18
/cm
3
active zone n.i.d
10/10 SL or 7/4 SL
p
+
(Be) SL (10 periods)
n
+
(Te) SL (10 periods)
e ~ 1µm
e = 20nm
Figure 1: Schematic diagram of the InAs/GaSb pin SL
MWIR structures on p-type GaSb. The SL active zone is
composed of 10/10 symmetric or 7/4 asymmetric period
design, for a total thickness of 1µm.
Structural characterizations were routinely
performed on the SL samples. According to X-ray
diffractometry data, SL structures lattice matched to
the GaSb substrate were obtained. An example of
high resolution X-ray diffraction spectrum of a
perfectly matched SL structure on the GaSb
substrate is shown on Figure 2.
26 27 28 29 30 31 32 33 34
100
1000
10000
100000
1000000
1E7
1E8
1E9
/2 (°)
Counts/s
GaSb
SL
0
InAs/GaSb SL structure
Figure 2: HRXRD patterns of 170 periods InAs/GaSb
10/10 SL structures lattice-matched to the GaSb substrate.
Concerning optical characterizations, the 10/10
and 7/4 SL structures exhibited photoluminescence
(PL) spectra at 80K at 5µm (Figure 3).
Figure 3: Normalized photoluminescence spectra of
symmetric 10/10 (a) and asymmetric 7/4 (b) SL structures.
In inset, band diagram of the SL structure showing first
electron C1 and hole V1 minibands.
PHOTOPTICS2013-InternationalConferenceonPhotonics,OpticsandLaserTechnology
92
In particular, a set of several 7/4 SL samples with
active layer thicknesses varying from 1µm (310
periods) to 4µm (1240 periods) have been grown,
exhibiting photoluminescence emission at around
5µm at 77K (Figure 3b). This result shows the
reproducibility and the control of the MBE grown
SL structures.
From epitaxial SL material, circular mesa
photodiodes were fabricated using standard
photolithography with a mask set containing diodes
and photodiodes with several diameters, from 60µm
up to 310µm (inset of Figure 4). Metallization were
ensured by CrAu sputtering on the p-GaSb substrate
and on the n-type InAs cap layer. Mesa photodiodes
were realized by wet etching using H
3
PO
4
/H
2
O
2
/
H
2
O/C
6
H
8
O
7
solution followed by NaClO smoothing
(Chaghi et al.,, 2009).
To complete the device processing, photoresist
AZ-1518 was spun onto the sample in order to
protect the surface from ambient air. Then, the same
mask as the one used for front-side metallization
was used to open paths for wire bonding and the
photoresist was heated at 200°C for 2 hours to be
polymerized.
Finally, the samples were wire bounded and
packaged in TO-8 sub-mounts. The devices were
placed in a liquid nitrogen cryostat in order to
perform characterizations at low temperature.
Illuminating the device frontside, the spectral
response of the unbiased photodiode was measured
using a FTIR spectrometer. The asymmetric 7/4 SL
photodetector exhibits cut-off wavelength at around
5µm at 80K (Figure 4), consistent with the PL peak
positions presented in Figure 3b.
Figure 4: Photoresponse spectrum of InAs/GaSb (7/4) SL
photodiode at 80K. In inset, top view of a processed
sample, diodes (C) and photodiodes (P) with several
diameters from 60µm up to 310µm.
3 PHOTODIODES ELECTRICAL
CHARACTERIZATION
In order to perform current-voltage (I-V)
characterization in dark conditions, a KEITHLEY
6417A Electrometer was used to both apply the bias
voltage and read the current delivered by the SL
device. Figures 5a and 5b show J-V curves at 77K of
the pin symmetric 10/10 and asymmetric 7/4 SL
diodes, respectively, for a bias voltage in the range
[-1V, +0.3V]. Whatever the diode, no leakage
currents in volume at reverse bias or no surface
leakage currents in forward bias are observed.
Extracted from J(V) curves, dynamic resistance–area
products R
d
A are also plotted in Figure 5. At V=0V,
the R
0
A product is one of the figures of merit of IR
photodiodes (Rogalski et al., 2009).
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2
10
-9
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
Current Density (A/cm
2
)
Voltage (Volt)
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
10
6
10
7
10
8
InAs/GaSb 10/10 SL photodiode
R
0
A = 5.5x10
5
..cm
2
RdA (.cm
2
)
c
= 4.95 µm (77K)
J(-50mV) = 9.7 x10
-8
A/cm²
J(-1V) = 7.8 x10
-7
A/cm²
T=77K
a)
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2
10
-9
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
Current Density (A/cm
2
)
Voltage (Volt)
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
10
6
10
7
10
8
InAs/GaSb 7/4 SL photodiode
R
0
A = 3x10
6
..cm
2
RdA (.cm
2
)
c
= 5 µm (77K)
J(-50mV) = 4.1 x10
-8
A/cm²
J(-1V) = 3.5 x10
-7
A/cm²
T=77K
b)
Figure 5: Dark current density measurements J(V) at 77K
performed on symmetric 10/10 (a) and asymmetric 7/4 (b)
SL pin photodiodes. The dynamic resistance area products
R
d
A as a function of bias are extracted from J(V) curves.
For the symmetric 10/10 SL device having cut-
off at 4.95µm, results at 77K show dark current
density at V=-50mV inferior to 1x10
-7
A/cm
2
and a
R
0
A product superior to 5x10
5
.cm
2
(Figure 5a).
For the asymmetric 7/4 SL diodes, results show
InAs/GaSbSuperlatticePhotodiodesOperatingintheMidwaveInfraredRange
93
an increase of performances with
J(-50mV) = 4.1x10
-8
A/cm
2
and R
0
A as high as
3.5x10
6
.cm
2
, which is one of the best R
0
A value
reported for a diode having a cut-off wavelength at
5µm at 77K.
Figure 6 displays J(V) curves of the asymmetric
7/4 SL diode for different temperatures ranging from
77K to 300K. At room temperature, J (-50mV) = 10
A/cm
2
was measured and a good R
0
A value of
1x10
-2
.cm² was deduced from this measurement.
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0.01
0.1
1
10
100
1000
J(-50mV) = 4x10
-8
/cm
2
(77K)
c
= 5µm (77K)
300K
270K
250K
230K
200K
170K
150K
120K
110K
100K
90K
77K
Current Density (A/cm²)
Bias Voltage (V)
= 90µm
Asymmetric (7/4) SL diode
Figure 6: Asymmetric 7/4 SL pin photodiode : Dark
current density-voltage characteristics for different
operating temperature [77-300K].
Extracted from J-V measurements, dark current
densities at 50mV reverse bias are reported as a
function of inverse temperature (Arrhenius plot) in
Figure 7. Temperature dependence of the dark-
current density reveals that the 7/4 SL device is
diffusion-limited at high temperature, while it is GR-
limited below 120K.
50 100 150
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0.01
0.1
1
10
100
G-R
Diffusion
J (-50mV)
Diff
GR
Current Density (A/cm²)
q/kT
300250 200 150 100
Temperature (K)
T=120K
Figure 7: Asymmetric 7/4 SL pin photodiode : Arrhenius
plot of the dark current density (-50mV). The diffusion
and GR limited regimes are reported.
4 PHOTODIODES COMPARISON
For several symmetric and asymmetric photodiodes
operating in the MWIR domain, the results obtained,
in terms of R
0
A, are compared in Figures 8.
Significant results extracted from literature (Rehm et
al., 2006); (Wei et al., 2005); (Walther et al., 2006);
(Hill et al., 2007) have also been reported.
0.20 0.22 0.24 0.26 0.28 0.30 0.32 0.34
10
5
10
6
10
7
10
8
(10/10)
(7/4)
Symmetric
SL diode
Asymmetric
SL diode
[4]
[3]
[2]
T = 77 K
Wavelength (µm)
R
0
A (.cm
2
)
Energy (eV)
[1]
MWIR SL diodes
a)
65.5 5 4.5 4
0.16 0.18 0.20 0.22 0.24 0.26
1E-4
1E-3
0.01
0.1
Asymmetric
SL diode
Wavelength (µm)
T = 300K
R
0
A (.cm
2
)
Energy (eV)
7.5 7 6.5 6 5.5 5
Symmetric
SL diode
b)
(7/4)
MWIR SL diodes
(10/10)
Figure 8: Experimental values of R
0
A product versus cut-
off wavelength for both symmetric (circle) and
asymmetric (star) MWIR SL structures at 77K (a) and
300K (b), and comparison (square) with typical results of
SL pin diodes extracted from literature (1-Rhem et al.,
2006; 2-Wei et al., 2005; 3-Walther et al., 2006; 4-Hill et
al., 2007).
Figure 8a (77K) and Figure 8b (300K) show that,
whatever the temperature, at low temperature when
the InAs/GaSb SL diode is GR limited or at high
temperature when the SL diode is diffusion limited,
the R
0
A values of the asymmetrical SL photodiodes
are always, at least, one decade higher than the
symmetrical SL design. This improvement of R
0
A
values for the asymmetric SL is due to the reduction
PHOTOPTICS2013-InternationalConferenceonPhotonics,OpticsandLaserTechnology
94
of GaSb layer in the InAs/GaSb period. Indeed, the
asymmetric structure induces a reduction of SL
effective masses, then a decrease of the intrinsic
carrier concentration for a given temperature,
leading to an improvement of R
0
A product
(Rodriguez et al., 2010).
5 CONCLUSIONS
Dark current measurements were performed on
MWIR InAs/GaSb SL detectors with two period
designs: a symmetric (10/10) and an asymmetric
(8/8) SL periods. The two SL photodiodes, grown by
MBE on p-type GaSb substrate, showed cut-off
wavelength near 5µm at 77K. Zero-bias resistance
area product R
0
A equal to 3.5x10
6
.cm
2
were
measured at 77K on asymmetric SL diode, one
decade higher than the equivalent symmetric period
design and this predominance of the asymmetric SL
period structure still valid whatever the temperature
operation of the diode, at low temperature when the
diode is G-R limited as well as at high temperature
when the diode is diffusion limited. Such results
obtained demonstrate the strong importance of the
selected SL period to enhance electrical
performances of MWIR InAS/GaSb SL pin
photodiodes. These results have to be completed by
calibrated photoresponse measurements and will be
the subject of forthcoming studies.
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