Frank Rutz

Type-II superlattices: the Fraunhofer perspective

In the past years, the development of the type-II InAs/GaSb superlattice technology at the Fraunhofer-Institute for Applied Solid State Physics (IAF) has been focused on achieving series-production readiness for third generation dualcolor superlattice detector arrays for the mid-wavelength infrared spectral range. The technology is ideally suited for airborne missile threat warning systems, due to its ability of low false alarm remote imaging of hot carbon dioxide signatures on a millisecond time scale. In a multi-wafer molecular beam epitaxy based process eleven 288×384 dualcolor detector arrays are fabricated on 3 GaSb substrates. Very homogeneous detector arrays with an excellent noise equivalent temperature difference have been realized. The current article presents the type-II superlattice dual-color technology developed at IAF and delivers insights into a range of test methodologies employed at various stages during the fabrication process, which ensure that the basic requirements for achieving high detector performance are met.

Status of mid-infrared superlattice technology in Germany

In Germany, InAs/GaSb superlattice detector technology for the mid-wavelength infrared spectral range has been intensively developed in recent years. Mid-IR InAs/GaSb superlattice photodiodes achieve a very high quantum efficiency. The worlds first high-performance infrared imagers based on InAs/GaSb superlattices were realized offering high spatial and excellent thermal resolution at short integration times. Additionally, the technology for dual-color superlattice detectors featuring simultaneous, pixel-registered detection of two separate spectral regimes in the mid-IR has been developed. Due to the ability to detect signatures of hot carbon dioxide, dual-color superlattice detectors are ideally suited for missile alerting sensors. The capability for small volume production of InAs/GaSb superlattice detectors has been established.

InAs/GaSb superlattice focal plane array infrared detectors: manufacturing aspects

InAs/GaSb type-II short-period superlattice (SL) photodiodes have been shown to be very promising for 2nd and 3rd generation thermal imaging systems with excellent detector performance. A multi-wafer molecular beam epitaxy (MBE) growth process on 3-GaSb substrates, which allows simultaneous growth on five substrates with excellent homogeneity has been developed. A reliable III/V-process technology for badge processing of single-color and dual-color FPAs has been set up to facilitate fabrication of mono- and bi-spectral InAs/GaSb SL detector arrays for the mid-IR spectral range. Mono- and bispectral SL camera systems with different pitch and number of pixels have been fabricated. Those imaging systems show excellent electro-optical performance data with a noise equivalent temperature difference (NETD) around 10 mK.

Defect density reduction in InAs/GaSb type II superlattice focal plane array infrared detectors

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 channels. 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.

Antimony-based superlattices for high-performance infrared imagers

InAs/GaSb short-period superlattices (SL) for the fabrication of mono- and bispectral thermal imaging systems in the mid-wavelength infrared region (MWIR) have been optimized in order to increase the spectral response of the imaging systems. The responsivity in monospectral InAs/GaSb short-period superlattices increases with the number of periods in the intrinsic region of the diode and does not show a diffusion limited behavior for detector structures with up to 1000 periods. This allows the fabrication of InAs/GaSb SL camera systems with high responsivity. 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 have been realized.

SWIR photodetector development at Fraunhofer IAF

For surveillance and reconnaissance applications in the short-wave infrared (SWIR) spectral range, the imaging systems have to cope with usually very low photon flux densities. Thus, dark-current and noise characteristics of the focal plane array (FPA) are demanding. On the other hand, the challenge of detecting extremely low photocurrents can be mitigated by utilizing an internal gain as provided by avalanche photodiodes (APDs). Fraunhofer IAF has recently started the development of InGaAs-based SWIR detectors. We report on the current development status covering design considerations, epitaxy, process technology and electro-optical characterization. Detector structures based on both, classical InGaAs PIN homojunction diodes as well as InGaAs/InAlAs APDs in separated-absorption-grading-charge-and-multiplication layer heterostructures, have been grown by molecular beam epitaxy on InP. Diodes structures were fabricated with a dry-etch mesa process and a subsequent dielectric passivation of the mesa sidewalls. High-resolution FPAs with 640 x 512 pixels and a 15 μm pixel pitch based on PIN diodes have been assembled to a SWIR camera system in cooperation with AIM Infrarot-Module GmbH. Design variations, in particular for the APDs, were assisted by band-edge-profile simulations. APD test structures as well as fan-out hybrids have been characterized, revealing gain values larger than 300 at room temperature.

Infrared photodetector development at Fraunhofer IAF

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 been initiated. 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.

Advanced III/V quantum-structure devices for high performance infrared focal plane arrays

A mature production technology for Quantum Well Infrared Photodetector (QWIP) focal plane arrays (FPAs) and InAs/GaSb superlattice (SL) FPAs has been developed. Dual-band and dual-color QWIP- and SL-imagers are demonstrated for the 3-5 μm and 8-12 μm atmospheric windows in the infrared. The simultaneous, co-located detection of both spectral channels resolves the temporal and spatial registration problems common to existing bispectral IRimagers. The ability for a reliable remote detection of hot CO2 signatures makes tailored dual-color superlattice imagers ideally suited for missile warning systems for airborne platforms.

Defects and noise in Type-II superlattice infrared detectors

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.

Dual-color InAs/GaSb superlattice infrared imagers

High-performance dual-color infrared imaging systems have been realized with InAs/GaSb type-II superlattices. The detectors offer a spatial resolution of 288×384 pixels with a simultaneous, co-located detection at 3–4 μm (blue channel) and 4–5 μm (red channel), respectively. Against a 300 K background, a thermal resolution of 18 mK in the blue and 10 mK in the red channel has been achieved. The detectors are well suited for low-false alarm imaging of hot carbon dioxide signatures on a millisecond time scale.