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.
Fraunhofer IAF can look back on many years of expertise in developing high-performance infrared photodetectors. Since
pioneering the InAs/GaSb type-II superlattice detector development, extensive capabilities of epitaxy, process
technology, and device characterization of single element detectors and camera arrays for the mid- and longwave
infrared (MWIR and LWIR) have been established up to the level of small-scale production. Bispectral MWIR/MWIR
and MWIR/LWIR cameras based on type-II superlattices or HgCdTe are key topics at Fraunhofer IAF. Moreover, the
development of InGaAs-based short-wave infrared (SWIR) photodetectors for low-light-level applications has recently
In this contribution, we report on the status of recent photodetector development activities at IAF, covering detector
design, epitaxial growth, process technology, and most recent electro-optical characterization results of focal plane
arrays as well as single element detectors especially for the SWIR based on InGaAs material system.
We report on materials and technology development for short-wave infrared photodetectors based on InGaAs p-i-n and avalanche photodiodes (APDs). Using molecular beam epitaxy for the growth of thin layers with abrupt interfaces, which are required for optimized APD structures, excellent crystalline quality has been achieved for detector structures grown on 3-inch InP substrates. For the fabrication of focal plane detector arrays, we employed a mesa etching technology in order to compare the results with the commonly utilized planar technology. Camera detector arrays as well as test structures with various sizes and geometries for materials and process characterization are processed using a dry-etch mesa technology. Aspects of the process development are presented along with measured dark-current and photo-current characteristics of the detector devices.
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.
InAs/GaSb-based type-II superlattice photodiodes have considerably gained interest as high-performance infrared
detectors. Beside the excellent properties of InAs/GaSb superlattices, like the relatively high effective electron mass
suppressing tunneling currents, the low Auger recombination rate, and a high quantum efficiency, the bandgap can be
widely adjusted within the infrared spectral range from 3 - 30 μm depending on the layer thickness rather than on
composition. Superlattice growth and process technology have shown tremendous progress during the last years. Fully
integrated superlattice cameras have been demonstrated by several groups worldwide.
Within very few years, the InAs/GaSb superlattice technology has proven its suitability for high-performance infrared
imaging detector arrays. At Fraunhofer IAF and AIM, the efforts have been focused on developing a mature fabrication
technology for bispectral InAs/GaSb superlattice focal plane arrays for a simultaneous, co-located detection at 3-4 μm
and 4-5 μm in the mid-wavelength infrared atmospheric transmission window. A very low number of pixel outages and
cluster defects is mandatory for dual-color detector arrays. Sources for pixel outages are manifold and might be caused
by dislocations in the substrate, the epitaxial growth process or by imperfections during the focal plane array fabrication
process. Process refinements, intense root cause analysis and specific test methodologies employed at various stages
during the process have proven to be the key for yield enhancements.
Improving the crystal quality of AlGaN epitaxial layers is essential for the realization of efficient III-nitride-based light
emitting diodes (LEDs) with emission wavelengths below 365 nm. Here, we report on two different approaches to
improve the material quality of AlGaN buffer layers for such UV-LEDs, which are known to be effective for the
MOVPE growth of GaN layers. Firstly, we grew AlGaN on thin GaN nucleation islands which exhibit a threedimensional
facetted structure (3D GaN nucleation). Lateral overgrowth of these islands results in a lateral bending of
dislocation lines at the growing facets. Secondly, in-situ deposited SiN<sub>x</sub> interlayers have been used as nano-masks
reducing the dislocation density above the SiN<sub>x</sub> layers. Both approaches result in reduced asymmetric HRXRD ω-scan
peak widths, indicating a reduced edge-type dislocation density. They can be applied to the growth of AlGaN layers with
an Al concentration of at least 20%, thus suitable for LEDs emitting around 350 nm. On-wafer electroluminescence
measurements at 20 mA show an increase in output power by a factor of 7 and 25 for LED structures grown on 3D GaN
nucleation and SiNx interlayer, respectively, compared to structures grown on a purely 2D grown low Al-content AlGaN
nucleation layer. Mesa-LEDs fabricated from the LED layer sequences grown on buffers with SiN<sub>x</sub> interlayer exhibit a
low forward voltage of 3.8 V at 20 mA and a maximum continuous wave (cw) output power of 12.2 mW at 300 mA.
InAs/GaSb short-period superlattices (SL) have proven their large potential for high performance focal plane array
infrared detectors. Lots of interest is focused on the development of short-period InAs/GaSb SLs for mono- and bispectral
infrared detectors between 3 - 30 μm. InAs/GaSb short-period superlattices can be fabricated with up to 1000
periods in the intrinsic region without revealing diffusion limited behavior. This enables the fabrication of InAs/GaSb SL
camera systems with very high responsivity, comparable to state of the art CdHgTe and InSb detectors. The material
system is also well suited for the fabrication of dual-color mid-wavelength infrared InAs/GaSb SL camera systems.
These systems exhibit high quantum efficiency and offer simultaneous and spatially coincident detection in both spectral
An essential point for the performance of two-dimensional focal plane infrared detectors in camera systems is the
number of defective pixel on the matrix detector. Sources for pixel outages are manifold and might be caused by the
dislocation in the substrate, the epitaxial growth process or by imperfections during the focal plane array fabrication
process. The goal is to grow defect-free epitaxial layers on a dislocation free large area GaSb substrate. Permanent
improvement of the substrate quality and the development of techniques to monitor the substrate quality are of particular
importance. To examine the crystalline quality of 3" and 4" GaSb substrates, synchrotron white beam X-ray topography
(SWBXRT) was employed. In a comparative defect study of different 3" GaSb and 4" GaSb substrates, a significant
reduction of the dislocation density caused by improvements in bulk crystal growth has been obtained. Optical
characterization techniques for defect characterization after MBE growth are employed to correlate epitaxially grown
defects with the detector performance after hybridization with the read-out integrated circuit.
InAs/GaSb short-period superlattices (SL) based on GaSb, InAs and AlSb have proven their great potential for high
performance infrared detectors. Lots of interest is currently focused on the development of short-period InAs/GaSb SLs
for advanced 2nd and 3rd generation infrared detectors between 3 - 30 μm. For the fabrication of mono- and bispectral
thermal imaging systems in the mid-wavelength infrared region (MWIR) a manufacturable technology for high
responsivity thermal imaging systems has been developed. InAs/GaSb short-period superlattices can be fabricated with
up to 1000 periods in the intrinsic region without revealing diffusion limited behavior. This enables the fabrication of
InAs/GaSb SL camera systems with high responsivity comparable to state of the art CdHgTe and InSb detectors. The
material system is also ideally suited for the fabrication of dual-color MWIR/MWIR InAs/GaSb SL camera systems with
high quantum efficiency for missile approach warning systems with simultaneous and spatially coincident detection in
both spectral channels.
Near-UV LEDs emitting at around 400 nm can be used e.g. as pump light source in tri-phosphor RGB white
luminescence-conversion LEDs with high color rendering.<sup>1</sup> Although non-thermal roll-over decreases towards shorter
emission wavelengths in GaInN-based LEDs, this effect still limits the efficiency of 400 nm emitting LEDs at current
densities above 50 A/cm<sup>2</sup>. One way to overcome non-thermal roll-over is to combine a GaInN wide-well active region
with the growth on low dislocation density (DD) substrates. Single-well LEDs with GaInN layer widths between 3 nm
and 18 nm were grown (a) directly on sapphire substrates with a resulting DD of 10<sup>9</sup> cm<sup>-2</sup>, (b) on low DD GaN templates
on sapphire (DD of 10<sup>8</sup> cm<sup>-2</sup>), and (c) on freestanding GaN substrates (FS-GaN, DD of 4×107 cm<sup>-2</sup>). At low current
densities (pulsed mode operation) the LEDs with a 3 nm GaInN QW active region showed the highest efficiency,
irrespective of the substrate. However, the electroluminescence (EL) efficiency peaks at around 50 A/cm<sup>2</sup> and shows a
clear non-thermal roll-over towards higher current densities. The efficiency of LEDs with well widths >3 nm grown on
sapphire decreases with increasing well width over the whole range of current densities (≤300 A/cm<sup>2</sup>). However, when
grown on low DD GaN templates or FS-GaN, the efficiency of the LEDs with 11 and 18 nm wide GaInN layers
surpasses that of the conventional LEDs (well widths ≤6 nm) for current densities ≥250 A/cm<sup>2</sup>, yielding the highest EL
efficiency of all LED-structures.