InAs/(GaIn)Sb short-period superlattices for focal plane arrays

Copyright 2005 Society of Photo-Optical Instrumentation Engineers
InAs/(GaIn)Sb short-period superlattices for focal plane arrays
Robert Rehm 1a , Martin Walther a , Johannes Schmitz a , Joachim Fleißner a , Frank Fuchs a ,
Wolfgang Cabanski b , and Johann Ziegler b
a Fraunhofer-Institut für Angewandte Festkörperphysik, Freiburg i. Br., Germany
b AEG Infrarot-Module GmbH (AIM), Heilbronn, Germany
ABSTRACT
An infrared camera based on a 256x256 focal plane array for the Mid-IR spectral range (3-5 µm) has been realized for
the first time with InAs/GaSb short-period superlattices. The detector shows a cut-off wavelength of 5.4 µm and reveals
a quantum efficiency of 30%. The noise equivalent temperature difference (NETD) reaches 9.4 mK at 73 K with F/2
optics and 6.5 ms integration time. Excellent thermal images with low NETD values and a very good modulation
transfer function are presented. Furthermore, a new method to passivate InAs/GaInSb superlattice photodiodes for the
8-10 µm regime is demonstrated. The approach is based on the epitaxial overgrowth of wet-etched mesa diodes using
lattice matched AlGaAsSb. A complete suppression of surface leakage currents in small sized test diodes with 70 µm
diameter is observed.
Keywords: infrared, focal plane array, superlattice, GaSb, thermal imaging
1. INTRODUCTION
In the past years, several groups have demonstrated promising photodiodes based on InAs/(GaIn)Sb superlattices
[1,2,3]. Since the demonstration of the first fully operational 256x256 Mid-IR (MWIR) camera exhibiting an excellent
performance [4], the interest in InAs/(GaIn)Sb p-i-n superlattice photodiodes for the fabrication of focal plane arrays
(FPA) for the second (3-5 µm) and third (8-12 µm) atmospheric transmission window has increased strongly. The high
quantum efficiency of p-i-n superlattice photodiodes allows to achieve a very low noise-equivalent temperature
difference at a short integration time.
InAs/(GaIn)Sb short-period superlattices show a broken band gap type-II band alignment leading to spatially indirect
transitions between hole states localized in the (GaxIn1-x)Sb layers and delocalized electronic states in the InAs layers
VB
InAs
GaSb
InAs
GaSb
InAs
GaSb
InAs
E G
Figure 1. Schematic band structure of an InAs/GaSb short-period superlattice.
1 Robert.Rehm@iaf.fraunhofer.de, phone (+49)-761-5159-353, fax (+49)-5159-353-851

(see Figure 1). The effective band gap EG of these structures can be adjusted from 0.3 eV to values below 0.1 eV by
varying the thickness of the InAs layer or the indium mole fraction in the (GaxIn1-x)Sb. The growth is performed by
Molecular Beam Epitaxy (MBE) on 2” GaSb substrates which are commercially available with low defect densities.
During growth, the strain can be kept sufficiently low by precisely controlling the interfaces between the InAs and the
(GaIn)Sb. The MBE growth process offers promising prospects for future dual color / dual band sensors for 3 rd
generation thermal imaging.
For the realization of high performance IR-camera systems, Fraunhofer IAF cooperates with AIM Infrarot Module
GmbH (AIM) located in Heilbronn, Germany. Fraunhofer IAF is committed to the development and fabrication of the
detector chip, i.e., the MBE growth, process technology and electro-optical characterization. IAF’s fully processed
detector chips are hybridized at AIM with a custom-designed silicon readout integrated circuit (ROIC) using flip-chip
indium solder bump technology. These detector hybrids are mounted into a detector cooler assembly with a 1 W linear
Stirling cooler, which is part of a complete IR imaging camera system. Subsequently, an overview of the Fraunhofer
IAF / AIM superlattice technology will be presented.
2. InAs/GaSb SHORT-PERIOD SUPERLATTICES FOR THE MID-IR SPECTRAL RANGE
Sensors for the Mid-IR spectral range between 3-5 µm are based on binary InAs/GaSb short-period superlattices. MBE
growth starts with a 500 nm thick Al0.5Ga0.5As0.04Sb0.96 lattice matched buffer layer followed by 700 nm GaSb:Be
(3x10 18 cm -3 ) acting as a p-type contact layer. Subsequently, 190 periods of a 9 monolayer (ML) InAs / 10 ML GaSb
superlattice are grown. The SL region is terminated by a 20 nm InAs:Si (1x10 17 cm -3 ) cap layer representing an ohmic
n-contact layer. For the formation of p-i-n photodiodes the lower 90 periods of the SL are p-doped with 1x10 17 cm -3 Be
in the GaSb layers. The next 40 SL-periods are not intentionally doped followed by 60 periods with a n-type doping of
1x10 17 cm -3 Si in the InAs layers. The tensile strain caused by the lattice mismatch between InAs and GaSb, was
compensated by the formation of InSb-like bonds at the interfaces in the SL by an appropriate shutter sequence during
growth [5]. Growth conditions, i.e. growth temperature, V/III beam equivalent pressure ratios and shutter sequences
have been optimized in order to improve material quality and to establish a reproducible and reliable growth
process [6].
Figure 2. Cross-section schematic of a 9 ML InAs / 10 ML GaSb p-i-n
superlattice photodiode for the Mid-IR spectral region.
256 x 256 FPAs with 40 µm pitch are processed as full wafers using standard optical lithography with a mask set which
also contains various test diodes with different size and geometry. On a single 2” GaSb wafer four FPAs are realized,
each with 11.2 x 11.2 mm 2 area. Processing starts with the deposition of the ohmic n-contact metalization, followed by
a dry etching process for mesa definition using Chemically Assisted Ion Beam Etching (CAIBE). Subsequently, the

Figure 3. SEM image of a 256 x 256 MWIR InAs/GaSb superlattice FPA after CAIBE dry etching of the mesas (a)
and at the end of the process prior to hybridization with the silicon ROIC (b).
etch damage is removed by wet chemical means. After the evaporation of the p-contact metalization, the samples are
passivated with a SiO2-based approach. Figure 2 shows a schematic cross-section of a diode after the subsequent
fluorine-based reactive ion etching of the dielectric passivation layer on top of the contact metalization. In Figure 3a, a
scanning electron microscope (SEM) image prior to the deposition of the passivation layer is shown. The high
selectivity of the wet chemical etchant used for the etch damage removal produces a small step between the GaSb:Be ptype
contact layer and the superlattice. After the deposition of further metalization layers, which are important for the
subsequent hybridization process, the wafers are diced to separate the FPAs. Figure 3b shows a section of a completely
processed 256 x 256 InAs/GaSb superlattice FPA with 40 µm pitch.
Using standard indium solder bump technology the FPAs are flip-chip bonded to the ROIC. In order to reduce thermally
induced stress and to decrease free carrier absorption in the residual p-type GaSb substrate, the substrate is removed by
R 0 A [Ωcm 2 ]
10 7
10 6
10 5
10 4
10 3
FPA pixel, area = (37 µm) 2 , 5%-Cut-off: 5.4 µm
77 K
(a)
0 5 10 15
DIODE #
77 K
(b)
10 -7
10 -9
10 -11
-0.2 -0.1 0.0 0.1 10-13
BIAS (V)
Figure 4. (a) R0A product of 15 randomly selected, fully processed 256x256
FPA diodes with a cut-off wavelength of 5.4 µm and a size of (37 x 37 µm 2 ) at
77 K. (b) Corresponding total dark current of such a FPA diode at 77 K.
DARK CURRENT (A)

Figure 5. (a) Responsivity and corresponding quantum efficiency at 77 K. (b) Large area test diode with optical
window in the top contact metalization for electro-optical characterization.
a combination of mechanical lapping, wet-chemical polishing and wet-chemical etching to a thickness of about 20 µm.
The detector hybrid is anti-reflection coated and finally integrated into a stand-alone camera system.
The I-V characteristics of the diodes were measured on test
diodes of the same size and geometry (37 x 37 µm 2 ) as the FPA
diodes. Figure 4b shows the total dark current vs. bias voltage of
a diode with a cut-off wavelength (5%) of 5.4 µm. At 0 V bias,
the total dark current is below 1 pA at 77 K. The diodes are
generation-recombination limited and show background-limited
performance (BLIP). Figure 4a illustrates the R0A product of a
set of 15 randomly selected diodes. The value of the R0A
product equals 4x10 5 Ωcm 2 (a)
at 77 K and shows a very
homogeneous behavior apart from diode #15 which is
presumably damaged by a bulk defect. At 67 K R0A beyond
(b)
Figure 6. Histogram of the NETD (a) and mapping
of dead pixels (b) in a 256 x 256 InAs/GaSb shortperiod
superlattice FPA integrated on IDCA.
1x10 6 Ωcm 2 have been measured.
To determine responsivity and quantum efficiency electrooptical
measurements have been performed under front side
illumination on large area test diodes with an optical window in
the top contact metalization (see Figure 5b). Due to the absence
of an optical reflector in this configuration, the infrared
radiation passes the diode only once. Caused by the mirror
metalization on top of each pixel, responsivity and quantum
efficiency are roughly doubled in FPAs which are illuminated
from the backside in the camera system. The spectrally resolved
current responsivity of such a test diode is plotted in Figure 5a
for single optical path. A current responsivity of 0.7 A/W was
measured at a wavelength of 5 µm, resulting in a quantum
efficiency of η = 17 % for single optical path. Thus, the
quantum efficiency is estimated to be around η = 30 % in FPA
detector pixels under backside illumination.
After flip-chip bonding and removal of the GaSb substrate, the
detector chips were mounted in an integrated detector cooler
assembly with a 1 W linear Stirling cooler. An important figure
of merit is the noise-equivalent temperature difference (NETD)
of the camera system. The histogram over 256 x 256 detector
pixels at an integration time of 6.5 ms and with F/2 optics is

Figure 7. Thermal images of the first fully integrated 256 x 256 Mid-IR superlattice camera with 9.4 mK NETD.
plotted in Figure 6a. Excellent spatial uniformity is obtained over the array, resulting in a median NETD value of
9.4 mK at a detector temperature of 73 K. For an integration time of 1.25 ms a NETD value of 25 mK has been
measured.
The very good homogeneity of the NETD across the entire 256 x 256 array is illustrated in Figure 6b which shows a
mapping of defective pixels. In total, pixel outages are well below 1% and appear statistically distributed,
predominantly as single pixel faults without large clusters.
The excellent image quality delivered from the InAs/GaSb SL mid-IR camera system is shown in Figure 7. The fine
structure of the clothing proves the good spatial resolution and the low cross talk between neighboring detector
elements in the FPA. With this camera demonstrator, it is obvious, that InAs/GaSb SLs are in the position to provide
low NETD values at short integration times comparable with HgCdTe detectors. The excellent performance of the
camera system demonstrates the high potential of InAs/GaSb SLs for the fabrication of future IR imaging systems.
3. InAs/GaInSb SHORT-PERIOD SUPERLATTICES FOR 8-10 µm
The main technological challenge for the fabrication of large format FPAs with pixel sizes of 40 µm and less is the
occurrence of surface leakage currents. Since surface leakage currents in low band gap devices are mostly due to
tunneling of electrons, these currents increase exponentially when the band gap of the device shifts to longer
wavelengths. Besides efficient suppression of surface leakage currents, a passivation layer suitable for production
purposes must withstand various treatments occurring during the subsequent processing and integration up to the
camera level. Therefore, ammonium-sulfide treatments, which have been investigated by several groups [7,8], seem
difficult to be incorporated in a production process. Up to now the reproducibility and long-term stability achieved by
the deposition of a dielectric passivation layer is not sufficient for photodiodes in the 8 – 12 µm range. Therefore, we
investigated the epitaxial overgrowth of InAs/(GaIn)Sb short-period superlattice mesa devices with lattice matched
AlxGa1-xAsySb1-y by MBE [9].
The approach is demonstrated using an InAs/(Ga0.75In0.25)Sb short period superlattice p-i-n photodiode with a cut-off
wavelength around 10 µm at 77 K grown by MBE on undoped 2” (001)-GaSb substrate. MBE growth started with a
500 nm thick Al0.5Ga0.5As0.04Sb0.96 lattice matched buffer layer followed by a p-type contact layer comprised of 700 nm
GaSb:Be (3x10 18 cm -3 ). The active region contains 190 periods of an 8.6 ML InAs / 5.0 ML (Ga0.75In0.25)Sb superlattice
structure. The active region is terminated by a 30 nm GaSb cap layer. The lower 90 periods of the SL were p-doped
with 1x10 17 cm -3 Be in the Ga0.75In0.25Sb layers. The next 40 SL-periods were not intentionally doped followed by 60
periods with an n-doping of 1x10 17 cm -3 Si in the InAs layers.

Figure 8. Cross-section schematic of a InAs/GaInSb superlattice for the 8-12 µm spectral
range passivated by MBE overgrowth with lattice matched Al0.5Ga0.5As0.04Sb0.96.
A full wafer process using standard optical lithography with a mask set containing various test diodes of different size
and geometry was used to process the wafer. A cross section scheme of a completely processed mesa detector is
presented in Figure 8. The processing sequence starts with the mesa definition which was done wet-chemically using
citric acid. In order to ensure homogeneous wet etching and overgrowth of both sidewalls of rectangular shaped mesas,
the mesa mask was aligned at 45 degree to the natural cleavage planes of the crystal. In this way two slightly positive
facets for both the (100) and (010) oriented mesa sidewalls were achieved. After mesa etching, great care was taken to
remove the resist completely. The sample was then re-inserted into the MBE chamber and the residual surface oxide
was thermally desorbed. Subsequently, epitaxial overgrowth of the mesa-structured sample with a 150 m thick
Al0.5Ga0.5As0.04Sb0.96 passivation layer was performed. During oxide desorption and overgrowth group-V stabilization of
the semiconductor surface was ensured by a sufficiently high Sb-flux.
Figure 9. (a) SEM image of a 256x256 InAs/GaInSb superlattice FPA with 40 µm pitch for 8-12 µm directly after
overgrowth with lattice-matched Al0.5Ga0.5As0.04Sb0.96. (b) Edge of such a FPA pixel at higher magnification. The
smooth surface morphology without polycrystalline phases is evident.

Figure 9a shows a SEM image of mesas with 40 µm pitch in a 256x256 FPA arrangement after epitaxial overgrowth
with the 150 nm thick Al0.5Ga0.5As0.04Sb0.96 layer. In Figure 9b the corner of such an FPA pixel is shown at higher
magnification. The excellent surface morphology without polycrystalline phases is evident. In order to prevent
oxidation of the Al-containing passivation layer, a 200 nm thick silicon nitride layer was deposited after removing the
sample from the MBE system. Since the Fermi level at the p-n junction on the diode sidewall is defined by the
Al0.5Ga0.5As0.04Sb0.96 layer, the electrical properties of the photodiode are not affected by this dielectric layer. The
silicon nitride was used as an etch mask for wet chemical etching of the Al0.5Ga0.5As0.04Sb0.96 passivation and the GaSbcap
layer. The wet chemical etchant chosen for this process step offers a sufficiently high selectivity to the Incontaining
SL. Finally, ohmic n- and p-contact metalization layers were deposited.
Various test diodes were wire-bonded after processing and characterized by dark I-V measurements. Since surface
leakage currents occur at the mesa sidewalls, dark current measurements in dependence of the perimeter-to-area (P/A)
ratio were carried out to separate the surface leakage currents from the bulk contributions. A convenient measure for
this investigation is the R0A product, since it is inversely proportional to the square of the limiting Johnson noise
contribution [10].
(R 0 A) -1 (Ω -1 cm -2 )
10 3
10 2
10 1
10 0
10 -1
10 -2
EDGE LENGTH OF
QUADRATIC MESA (µm)
800 700 600 500 400 300200 200 100
Dielectric passivation
Passivation by overgrowth
λ c = 10 µm
(a) 77 K (b)
77 K
0.02 0.04 0.06
PERIMETER-TO-AREA (µm -1 )
70 µm edge length
P/A = 0.057 µm -1
10 3
10 2
10 1
10 0
10 -1
0 2 4 6 8 10 12 10-2
DIODE #
Figure 10. (a) Inverse R0A product vs. P/A ratio of InAs/GaInSb superlattice photodiodes with 10 µm cut-off at
77 K. (b) Histogram of the (R0A) -1 product of the diodes from (a) with 70 µm edge length (P/A = 0.057 µm -1 ).
Figure 10a shows the (R0A) -1 in dependence of the P/A ratio on a logarithmic scale at 77 K. Since the P/A ratio depends
on the size and geometry of the mesa, the edge length of a quadratic mesa is depicted on the upper scale for
convenience. The (R0A) -1 data of the sample passivated by overgrowth is compared with a dielectrically passivated
sample with 10 µm cut-off wavelength. R0A values obtained with a dielectric passivation are low and the inverse R0A
increases linearly with increasing P/A ratio. Obviously the employed dielectric layer is not well suited for the
passivation of FPA pixels with 40 µm diameter and less. In contrast, the R0A product obtained with the passivation by
AlGaAsSb overgrowth is more than three orders of magnitude larger compared to the dielectric passivation.
Furthermore, the best diodes align along the dashed line corresponding to R0A ≈ 10 Ωcm 2 which is the known bulk
value of this material. The independency of (R0A) -1 on the (P/A) ratio indicates suppression of surface leakage currents
(R 0 A) -1 (Ω -1 cm -2 )

on the mesa sidewalls. Some larger diodes show significantly higher (R0A) -1 values which are attributed to growth
defects in the bulk of the diode. This interpretation is in accordance with the histogram of 12 quadratic diodes with 70
µm edge length (P/A = 0.057 µm -1 ) from Figure 10a, which is shown in Figure 10b. Only one diode (#11) out of 12
randomly selected diodes differs significantly from R0A ≈ 5 – 10 Ωcm 2 . Further experimental investigations, which are
beyond the scope of this paper, do not suggest any problems with the long-term stability or reproducibility of the
Al0.5Ga0.5As0.04Sb0.96 passivation.
4. CONCLUSIONS
Important progress on short-period p-i-n superlattice photodiodes based on the InAs/GaInSb material system could be
achieved in both atmospheric transmission windows in the infrared spectral range. In the 3-5 µm regime the first fully
operational 256x256 camera system was demonstrated exhibiting a NETD performance of 9.4 mK at 6.5 ms integration
time with F/2 optics at 73 K. The excellent homogeneity of the electro-optical performance combined with the very low
pixel outages without any large cluster defects constitute an important advantage over state-of-the-art HgCdTe detectors
at comparable performance. In the 8-10 µm region a method based on the epitaxial MBE overgrowth with high bandgap
AlGaAsSb offers a stable method to passivate InAs/GaInSb superlattice photodiodes. No surface leakage currents were
observed in test diodes as small as 70 µm diameter. Since the illustrated approach is compatible with standard device
processing, the realization of the first superlattice FPA in the 8-10 µm window seems within reach.
ACKOWLEDGMENTS
The authors are grateful to K. Schwarz, H. Güllich, L. Kirste and K. Windscheid for characterization of detector
structures. We also express our thanks to P. Koidl and G. Weimann for fruitful discussions. Financing support by the
Federal Ministry of Defense is gratefully acknowledged.
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