Exposure to proton radiation degrades the performance of wavelength infrared (MWIR) and long wavelength infrared (LWIR) HgCdTe photodetectors to varying degrees depending on the dose and energy of the incident particles. We report an experimental characterization of test devices of multiple sizes and configurations designed to investigate the effect proton radiation has on detector performance. Photodetector devices, from test element devices to fully functional focal plane arrays, are processed into MWIR and LWIR HgCdTe material grown by molecular beam epitaxy (MBE), in both single and two-color architectures, on CdZnTe and CdTe-buffered Si substrates. The devices receive doses of 30 krad(Si) and 100 krad(Si) from an incident beam of 63 MeV protons. The lower dose induces negligible degradation. At the higher dose, MWIR detectors begin to show reduced activation energy for higher temperatures, while LWIR detectors are more strongly affected with the activation energy being halved following proton irradiation.
We report the development of high performance low cost SWIR infrared detectors from MBEgrown HgCdTe on 3-inch CdTe-buffered silicon substrates. The experimental findings demonstrate that despite the large lattice mismatch between HgCdTe and Si substrate, the materials and detector performances are sufficiently better than those reported for III-V mixed crystals. High minority carrier lifetime of the order 3 μs at room temperature was measured on the as grown material. Photodetectors fabricated from this material produced low dark current densities on the order of 10<sup>-6</sup> A/cm<sup>2</sup> and 10<sup>-3</sup> A/cm<sup>2</sup> at 200K and 300K. Quantum efficiency exceeding 70% at 2.0 μm, without antireflective coating, was measured on single element detectors. Further, 320 X 256, 30 μm pitch FPA’s have been fabricated with this HgCdTe on Si material and dark current operability of ~ 99.5% (mean dark current of 30 pA/Pixel) at 200K has been demonstrated.
Imaging spectrometry can be utilized in the midwave infrared (MWIR) and long wave infrared
(LWIR) bands to detect, identify and map complex chemical agents based on their rotational and
vibrational emission spectra. Hyperspectral datasets are typically obtained using grating or
Fourier transform spectrometers to separate the incoming light into spectral bands. At present,
these spectrometers are large, cumbersome, slow and expensive, and their resolution is limited
by bulky mechanical components such as mirrors and gratings. As such, low-cost, miniaturized
imaging spectrometers are of great interest. Microfabrication of micro-electro-mechanicalsystems
(MEMS)-based components opens the door for producing low-cost, reliable optical
systems. We present here our work on developing a miniaturized IR imaging spectrometer by
coupling a mercury cadmium telluride (HgCdTe)-based infrared focal plane array (FPA) with a
MEMS-based Fabry-Perot filter (FPF). The two membranes are fabricated from silicon-oninsulator
(SOI) wafers using bulk micromachining technology. The fixed membrane is a standard
silicon membrane, fabricated using back etching processes. The movable membrane is
implemented as an X-beam structure to improve mechanical stability. The geometries of the
distributed Bragg reflector (DBR)-based tunable FPFs are modeled to achieve the desired
spectral resolution and wavelength range. Additionally, acceptable fabrication tolerances are
determined by modeling the spectral performance of the FPFs as a function of DBR surface
roughness and membrane curvature. These fabrication non-idealities are then mitigated by
developing an optimized DBR process flow yielding high-performance FPF cavities. Zinc
Sulfide (ZnS) and Germanium (Ge) are chosen as the low and the high index materials,
respectively, and are deposited using an electron beam process. Simulations are presented
showing the impact of these changes and non-idealities in both a device and systems level.