Session G28: Infrared Sensing and Imaging

Mid-IR GeSn alloys with narrow band gaps beyond 8 μm

GeSn alloys with dilute Sn concentrations around the near-infrared indirect to direct transition have been thoroughly investigated in recent years. However, the GeSn alloy system, in much the same way as the HgCdTe alloy, is expected to have a continuum of band gaps that reach a value of zero. Covering this IR spectral range requires much higher Sn concentrations, which are not obviously attainable given the thermodynamic metastability of the Ge-Sn system. Sporadic reports of such alloys have appeared in the literature, but in most cases the samples were unsuitable for band gap determinations.

In this presentation we discuss recent work¹ characterizing the band gap and electronic structure of GeSn alloys which Sn concentrations as high as 33%. These materials have direct band gaps approaching 0.15 eV (~ 8 mm), thus reaching well into the mid-IR spectral range.
The high-quality GeSn films required for optical characterization were grown directly on Si substrates by CVD reactions of polygermanes and stannanes at temperatures between 240-290°C. The structural properties of the films are similar to those of their dilute counterparts, in spite of the fact that the fraction of Sn-Sn bonds is expected to be significant. For example, we find that the lattice parameter of the alloys still follows Vegard’s law, as reported for lower Sn concentrations.

Ellipsometric measurements of the complex dielectric function make it possible to determine the band gap by modeling the absorption edge. Our model includes excitonic, band filling (Burstein-Moss), and non-parabolicity effects. We find that the compositional dependence of the band gap cannot be described by a simple quadratic polynomial, as is the case in many alloy systems. The implications of this finding for the ability of the GeSn system to cover the 8-12 μm mid-IR window will be discussed in detail.

¹ C. Xu et al, Appl. Phys. Lett. 114 212104 (2019).   

Antimonides T2SLS Infrared Focal Plane Arrays for Space Remote Sensing Applications

Ga-free type-II strained layer superlattice (T2SLS) mid-wave infrared (MWIR) and long-wave infrared (LWIR) barrier infrared detector (BIRD) such as nBn and xBn focal plane arrays (FPAs) could easily operate at higher operating temperature (i.e. compared to InSb and quantum well infrared photodetectors (QWIP) and FPAs) due to the strong suppression of G-R dark current due to SRH processes as explained earlier. We have successfully fabricated MWIR HOT-BIRD FPA and integrated with a dewar cooler assembly for the CubeSat Infrared Atmospheric Sounder (CIRAS) 6U CubeSat. We have also successfully fabricated LWIR BIRD FPA for the Hyperspectral Thermal Imager (HyTI) 6U CubeSat and it will be integrated with a dewar cooler assembly as well. These antimonides based T2SLS BIRDs outperform existing thermal infrared detectors such as QWIPs and InSb. Another 20-30K higher operating temperature advantage can be achieved when we further improve the performance by hybridizing the T2SLS LWIR BIRD detector array to the high-dynamic range in-pixel digital ROICs. Based on III-V compound semiconductors, the T2SL BIRDs offer a breakthrough solution for the realization of low cost (high yield), highperformance FPAs with excellent uniformity and pixel-to-pixel operability. Therefore, T2SLS MWIR and LWIR BIRD FPA technology is very attractive to Earth and planetary remote sensing instruments.
  

Low-cost infrared imaging technology development at the Air Force Research Laboratory

Infrared imaging technology has numerous commercial, military, and scientific applications. As performance requirements increase, so do the costs of fabricating and operating infrared imaging systems. While many high-performance applications can be met using the very well-developed mercury cadmium telluride material system, the overall manufacturing and operational costs of the imaging system as a whole can be prohibitively expensive. The Air Force Research Laboratory (AFRL) is actively exploring various technologies using different materials and device physics to achieve adequate performance for the given application at significantly lower costs. A sample of different technologies will be reviewed with an emphasis on fundamental science and basic research needs.   

Photonics for Mobile Handset through Automotive 3D Sensing Consumer Applications

Machine learning and predictive algorithms are increasingly providing new opportunities for technology to directly participate in our interaction with the physical world around us. A crucial element of this interaction is the detailed digital characterization of that physical environment as an input to these algorithms. While digital visible light cameras have advanced dramatically, optical sensor systems able to add accurate depth measurements to 2D images within practical consumer environments are being developed to enable new consumer applications. Primary examples of these sensor driven applications include biometric authentication on mobile phone handsets and LIDAR scene mapping for autonomous vehicles. This presentation will provide an overview of the architectures of these 3D sensor systems, their underlying enabling photonic technologies, including VCSEL arrays, narrow linewidth semiconductor lasers, MEMS and optomechanical packaging, and how such complex technological solutions are being successfully brought to the consumer market.   

Mid-Wave Infrared Resonant Cavity Detectors

The dominant issue confronted by designers and users of mid-wave infrared (MWIR) detectors is
their large dark current and its noise-producing fluctuations. MWIR dark current is more than a million times that of short-wave (telecom) IR detectors. Several decades of work on dark current reduction via attention to material quality and epitaxial structure are nearing maturity. Significant improvements in dark current require new approaches. Our approach reduces dark current by reducing the volume of the detector’s absorber layer. A thin absorber layer is located inside an optical cavity formed by two mirrors – the so-called Resonant Cavity Detector (RCD). Optical absorption of the thin layer is typically ~1%, which would produce unacceptably small quantum efficiency (QE) in a single-pass detector. In an RCD, the light passes through the thin absorber many times (~50-100) as it re-circulates within the optical cavity, which produces good QE, and also low dark current. RCDs with QE~50% have achieved nearly 10× reduction in dark current and we expect to eventually reduce dark current by 100×.The RCD approach also affects frequency bandwidth (speed) and spectral bandwidth. RCDs absorbers are ~100× thinner than conventional detectors, which reduces minority carrier transit time, thus the frequency bandwidth is expected to increase to > 10 GHz. The spectral bandwidth, Δ, of RCDs is greatly reduced due to interference of the recirculating light with itself. RCDs with Δ ~ 20 nm have been produced, and Δ ~ 3 nm seems feasible. The narrow Δ and low dark current makes RCDs ideal for narrow band applications, such as detection of lasers or sensing spectrally sharp absorption features of gases. The RCD’s spectral response can be tuned 50-100 nm with detector temperature or the light’s angle of incidence. Finally, we discuss the tolerance on the epitaxial growth, which is significantly tighter for RCDs (1% layer thickness control) than for conventional MWIR detectors.