Type-II superlattices (T2SLs) are currently recognized as the sole material system offering comparable performance to HgCdTe, yet providing higher operability, stability over time, spatial uniformity, scalability to larger formats, producibility and affordability. Hence, T2SL technology is very promising for space applications. Fraunhofer IAF played a vital role in the development of III-As/Sb T2SLs right from the beginning. Mono- and bi-spectral focal plane arrays up to 640×512 pixels for the mid- and long-wavelength infrared (IR) have been demonstrated. The growth of T2SL is performed by molecular beam epitaxy (MBE) in multi-wafer reactors. We report on the excellent homogeneity and reproducibility of the growth process, established in the past years at Fraunhofer IAF. After processing this material to detector arrays, the T2SL detectors have been characterized down to low temperatures (below 40K) with promising properties regarding the dark current. For MWIR and LWIR detectors the resolution limit of the measurement setup with a dark current density of 2×10-10 A/cm2 has been reached at 77 K and 36 K, respectively.
Type-II superlattices (T2SLs) are considered the III/V alternative to HgCdTe for infrared (IR) detectors and have already reached market maturity. Fraunhofer IAF has demonstrated mono- and bi-spectral T2SL focal plane arrays up to 640×512 pixels for mid- and long-wavelength IR. In order to develop an industry-compatible T2SL technology, we have established the complete chain for detector array fabrication including design and modelling, epitaxial growth, as well as front- and backside processing. The epitaxial growth of T2SLs is performed by molecular beam epitaxy (MBE) in multi-wafer reactors. In this paper, we report on the control of growth rates during epitaxy, uniformity and reproducibility of the growth process, as well as characterization techniques to monitor the quality of the epitaxial layers. For the superlattice period, an average thickness variation far below a single atomic monolayer is required and achieved routinely. The standard deviation of the photoluminescence peak for both colors of bi-spectral IR detectors is around 0.04 μm for consecutive growth runs. With this very stable and reproducible epitaxial growth process in conjunction with our mature front- and backside processing we have been able to set up a pilot line production for bi-spectral T2SL IR detector arrays.
The fabrication and characterization of InAs/GaSb type-II superlattice long-wavelength infrared (LWIR) photodetectors for high operating temperature (HOT) are assessed regarding possible device yield. We investigate laterally-operated photoconductors with a detector cutoff wavelength in the LWIR at an operating temperature accessible with 3-stage thermoelectric cooling, realized by suitably tailoring the layer composition. Type-II superlattices with a layer composition of 14 monolayers InAs and 7 monolayers GaSb are grown on semi-insulating 3-inch GaAs substrates. We report on the growth of three different buffer layer variants that serve as growth templates for GaSb-based layers on GaAs substrates. The characterization of 75 nominally equal single element detectors per sample evidences the reliability of device processing. The electro-optical evaluation of a randomly chosen subset indicates a high uniformity of responsivity and noise of LWIR InAs/GaSb HOT photoconductors. At 210 K, the devices operate at a cutoff wavelength of 10.5 μm and achieve a mean peak spectral detectivity of 3.3 × 108 Jones.
Through the choice of appropriate layer thicknesses, the bandgap of InAs/Ga(As)Sb type II superlattices (T2SLs) can be engineered in a wide range covering the mid-wavelength and long-wavelength infrared (MWIR, 3 μm - 5 μm and LWIR, 8 μm - 12 μm) spectral regions. Using this material system, Fraunhofer IAF develops bi-spectral MWIR image sensors based on homojunction photodiodes for missile warning applications and pursues modern heterojunction approaches as well as heteroepitaxial growth of T2SLs on GaAs. We discuss topics arising from efforts to improve the manufacturability of our bi-spectral arrays and report on the progress of the integration with MWIR heterojunction designs that exhibit reduced dark currents.
Photodetectors in the non-visible region of the electromagnetic spectrum are essential for security, defense and space science as well as industrial and scientific applications. The research activities at Fraunhofer IAF cover a broad range in the infrared (IR) regime. Whereas short-wavelength IR (SWIR, <1.7 μm) detectors are realized by InGaAs/InP structures, InAs/GaSb type-II superlattice (T2SL) infrared detectors are developed for the spectral bands from mid- (MWIR, 3-5 μm) to long-wavelength IR (LWIR, 8-12 μm). We report on the extension of the superlattice empirical pseudopotential method (SEPM) to 300 K for the design of LWIR heterostructures for operation near room temperature. Recently, we have also adapted heterostructure concepts to our well established bi-spectral T2SL MWIR detector resulting in a dark current density below 2 × 10-9 A/cm2 for a cut-off wavelength close to 5 μm. Finally, we present first results obtained with a gated viewing system based on our InGaAs/InAlAs/InP avalanche photodiode arrays.
This paper reports on advances in the electro-optical characterization of InAs/GaSb short-period superlattice infrared photodetectors with cut-off wavelengths in the mid-wavelength and long-wavelength infrared ranges. To facilitate in-line monitoring of the electro-optical device performance at different processing stages we have integrated a semi-automated cryogenic wafer prober in our process line. The prober is configured for measuring current-voltage characteristics of individual photodiodes at 77 K. We employ it to compile a spatial map of the dark current density of a superlattice sample with a cut-off wavelength around 5 μm patterned into a regular array of 1760 quadratic mesa diodes with a pitch of 370 μm and side lengths varying from 60 to 350 μm. The different perimeter-to-area ratios make it possible to separate bulk current from sidewall current contributions. We find a sidewall contribution to the dark current of 1.2×10-11 A/cm and a corrected bulk dark current density of 1.1×10-7 A/cm2, both at 200 mV reverse bias voltage. An automated data analysis framework can extract bulk and sidewall current contributions for various subsets of the test device grid. With a suitable periodic arrangement of test diode sizes, the spatial distribution of the individual contributions can thus be investigated. We found a relatively homogeneous distribution of both bulk dark current density and sidewall current contribution across the sample. With the help of an improved capacitance-voltage measurement setup developed to complement this technique a residual carrier concentration of 1.3×1015 cm-3 is obtained. The work is motivated by research into high performance superlattice array sensors with demanding processing requirements. A novel long-wavelength infrared imager based on a heterojunction concept is presented as an example for this work. It achieves a noise equivalent temperature difference below 30 mK for realistic operating conditions.
For more than two decades, Antimony-based type-II superlattice photodetectors for the infrared spectral range between
3-15 μm are under development at the Fraunhofer Institute for Applied Solid State Physics (IAF). Today, Fraunhofer
IAF is Germany’s only national foundry for InAs/GaSb type-II superlattice detectors and we cover a wide range of
aspects from basic materials research to small series production in this field. We develop single-element photodetectors
for sensing systems as well as two-dimensional detector arrays for high-performance imaging and threat warning
systems in the mid-wavelength and long-wavelength region of the thermal infrared. We continuously enhance our
production capabilities by extending our in-line process control facilities. As a recent example, we present a
semiautomatic wafer probe station that has developed into an important tool for electrooptical characterization. A large
amount of the basic materials research focuses on the reduction of the dark current by the development of bandgap
engineered device designs on the basis of heterojunction concepts. Recently, we have successfully demonstrated
Europe’s first LWIR InAs/GaSb type-II superlattice imager with 640x512 pixels with 15 μm pitch. The demonstrator
camera already delivers a good image quality and achieves a thermal resolution better than 30 mK.
InAs/GaSb superlattices are characterized by a broken-gap type II band alignment. Their effective band gap can be engineered to match mid to long wavelength infrared (IR) photon energies. Fraunhofer IAF has developed image detectors for threat warning systems based on this material system that are capable of spatially and temporally coincident detection in two mid-IR wavelength ranges. We review the present status of the processing technology, report continuous improvements achieved in key areas of detector performance, including defect density and noise behavior, and present initial results for statistical characterization of ensembles of detector elements with respect to diode characteristics and noise.
To examine defects in InAs/GaSb type-II superlattices we investigated GaSb substrates and epitaxial InAs/GaSb layers
by synchrotron white beam X-ray topography to characterize the distribution of threading dislocations. Those
measurements are compared with wet chemical etch pit density measurements on GaSb substrates and InAs/GaSb type-II
superlattices epitaxial layer structures. The technique uses a wet chemical etch process to decorate threading dislocations
and an automated optical analyzing system for mapping the defect distribution.
Dark current and noise measurements on processed InAs/GaSb type-II superlattice single element photo diodes reveal a
generation-recombination limited dark current behavior without contributions by surface leakage currents for midwavelength
infrared detectors. In the white noise part of the noise spectrum, the extracted diode noise closely matches
the theoretically expected shot noise behavior.
For diodes with an increased dark current in comparison to the dark current of generation-recombination limited
material, the standard shot-noise model fails to describe the noise experimentally observed in the white part of the
spectrum. Instead, we find that McIntyre’s noise model for avalanche multiplication processes fits the data quite well.
We suggest that within high electric field domains localized around crystallographic defects, electrons initiate avalanche
multiplication processes leading to increased dark current and excess noise.
3rd generation IR modules - dual-color (DC), dual-band (DB), and large format two-dimensional arrays - require
sophisticated production technologies such as molecular beam epitaxy (MBE) as well as new array processing
techniques, which can satisfy the rising demand for increasingly complex device structures and low cost detectors. AIM
will extend its future portfolio by high performance devices which make use of these techniques. The DC MW / MW
detectors are based on antimonide type-II superlattices (produced by MBE at Fraunhofer IAF, Freiburg) in the 384x288
format with a 40 μm pitch. For AIM, the technology of choice for MW / LW DB FPAs is MCT MBE on CdZnTe
substrates, which has been developed in cooperation with IAF, Freiburg. 640x512, 20 μm pitch Focal Plane Arrays
(FPAs) have been processed at AIM. The growth of MW MCT MBE layers on alternate substrates is challenging, but
essential for competitive fabrication of large two-dimensional arrays such as megapixel (MW 1280x1024, 15 μm pitch)
FPAs. This paper will present the development status and latest results of the above-mentioned 3rd Gen FPAs and
Integrated Detector Cooler Assemblies (IDCAs).