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

(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