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

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(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)
Page 1: Copyright 2005 Society of Photo-Opt
Page 5 and 6: Figure 7. Thermal images of the fir
Page 7 and 8: Figure 9a shows a SEM image of mesa

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

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