Detectivity of mid-wave infrared (MWIR) detectors based on InAs/GaSb type II strained layer superlattices (T2SLs) can be significantly enhanced at select wavelengths by integrating the detector with a back-side illuminated plasmonic coupler. The application of a simple metal-T2SL structure directly on the GaSb substrate can result in radiation losses into the substrate due to the low refractive index of T2SL layer. However, insertion of a higher refractive index material, such as germanium (Ge), into the metal-SLS structure can confine the surface plasmon waveguide (SPW) modes to the surface. In this work, metal (Au)-Ge-T2SL structures are designed with an approximately 100 nm thick Ge layer. The T2SL layer utilized a p-i-n detector design with 8 monolayers (MLs) InAs/8 MLs GaSb. A plasmonic coupler was then realized inside the 300 μm circular apertures of these single element detectors by the formation of a corrugated metal (Au) surface. The T2SL single element detector integrated with an optimized plasmonic coupler design increased the quantum efficiency (QE) by a factor of three at an operating temperature of 77 K and 3 to 5 μm illumination wavelength, compared to a reference detector structure, and each structure exhibited the same level of dark current.
Raytheon Vision Systems (RVS) has developed scanning, high-speed (<3klps), all digital, with on-chip Analog-to-Digital Conversion (ADC), mid-wave infrared (MWIR 3-5mm) focal plane arrays (FPA) with excellent modulation transfer function (MTF) performance. Using secondary ion mass spectrometry (SIMS) data and detailed models of the mesa geometry, RVS modeled the predicted detector MTF performance of detectors. These detectors have a mesa structure and geometry for improved MTF performance compared to planar HgCdTe and InSb detector structures and other similar detector structures such as nBn. The modeled data is compared to measured MTF data obtained from edge spread measurements and shows good agreement, Figure 1. The measured data was obtained using a custom advanced test set with 1µm precision alignment and automatic data acquisition for report generation in less than five minutes per FPA. The measured MTF values of 83 unique parts, Figure 2, had a standard deviation of 0.0094 and a mean absolute deviation of 0.0066 at half Nyquist frequency, showing excellent process repeatability and a design that supports high MTF with good repeatability.
The unique linear avalanche properties of HgCdTe preserve the Poisson statistics of the incoming photons, opening up
new opportunities for GHz bandwidth LADAR and space communications applications. Raytheon has developed and
previously reported (1) unique linear mode photon counting arrays based on combining advanced HgCdTe linear mode
APDs with their high gain SB415B readout. Their use of HgCdTe APDs preserves the Poisson statistics of the incoming
photons, enabling (noiseless) photon counting. This technology is of great potential interest to infrared astronomy but
requires extension of noiseless linear HgCdTe avalanching down to much lower bandwidths (100 to 0.001 Hz) with
corresponding reductions in dark count rate.
We have hybridized the SB415B readout to SWIR HgCdTe APDs optimized for low dark count rate and have
characterized their photon counting properties at bandwidths down to 1 KHz. As bandwidth is reduced, the performance
becomes limited by the intrinsic properties of the SB415B readout, particularly readout glow, stability and 1/f noise.
We report the results of these measurements and the status of hybrid arrays utilizing a newly developed readout which
draws on Raytheon’s astronomical readout heritage, specifically the Virgo charge integrating source follower, as a path
to much lower dark count rate photon counting operation.
This paper investigates arrays of HgCdTe photon trapping detectors. Performance of volume reduced single mesas is
compared to volume reduced photon trap detectors. Good agreement with model trends is observed. Photon trap
detectors exhibit improved performance compared to single mesas, with measured noise equivalent temperature
difference (NEDT) of 40 mK and 100 mK at temperatures of 180 K and 200 K, with good operability. Performance as a
function of temperature has also been investigated.
Raytheon is developing NIR sensor chip assemblies (SCAs) for scanning and staring 3D LADAR systems. High
sensitivity is obtained by integrating high performance detectors with gain, i.e., APDs with very low noise Readout
Integrated Circuits (ROICs). Unique aspects of these designs include: independent acquisition (non-gated) of pulse
returns, multiple pulse returns with both time and intensity reported to enable full 3D reconstruction of the image.
Recent breakthrough in device design has resulted in HgCdTe APDs operating at 300K with essentially no excess noise
to gains in excess of 100, low NEP <1nW and GHz bandwidths and have demonstrated linear mode photon counting.
SCAs utilizing these high performance APDs have been integrated and demonstrated excellent spatial and range
resolution enabling detailed 3D imagery both at short range and long ranges. In the following we will review progress in
real-time 3D LADAR imaging receiver products in three areas: (1) scanning 256 × 4 configuration for the Multi-Mode
Sensor Seeker (MMSS) program and (2) staring 256 × 256 configuration for the Autonomous Landing and Hazard
Avoidance Technology (ALHAT) lunar landing mission and (3) Photon-Counting SCAs which have demonstrated a
dramatic reduction in dark count rate due to improved design, operation and processing.
Linear mode photon counting (LMPC) provides significant advantages in comparison with Geiger Mode (GM) Photon
Counting including absence of after-pulsing, nanosecond pulse to pulse temporal resolution and robust operation in the
present of high density obscurants or variable reflectivity objects. For this reason Raytheon has developed and
previously reported on unique linear mode photon counting components and modules based on combining advanced
APDs and advanced high gain circuits. By using HgCdTe APDs we enable Poisson number preserving photon counting.
A metric of photon counting technology is dark count rate and detection probability. In this paper we report on a
performance breakthrough resulting from improvement in design, process and readout operation enabling >10x
reduction in dark counts rate to ~10,000 cps and >104x reduction in surface dark current enabling long 10 ms
integration times. Our analysis of key dark current contributors suggest that substantial further reduction in DCR to
~ 1/sec or less can be achieved by optimizing wavelength, operating voltage and temperature.
Oceanit Laboratories Inc. is collaborating with Raytheon Vision Systems (RVS) to develop a novel HgCdTe-based
position sensitive detector (PSD) that can ultimately be implemented in target detection and tracking or target
interception applications in the infrared spectral region.
Raytheon has developed SWIR and Visible-SWIR Focal Plane Arrays (FPAs) with over one million pixels that meet the
demanding requirements of astronomy, night vision, and other low background systems. FPA formats are 1280 × 1024,
1024 × 1024 and 2048 × 2048, with detector elements on 20 μm pitch. This paper describes recent results on SWIR
HgCdTe detectors, low-noise Readout Integrated Circuits (ROICs), and FPA imaging. SWIR HgCdTe detectors have
been fabricated with cutoff wavelengths of 1.7 and 2.5 μm and have demonstrated high quantum efficiency and flat
spectrals, including visible response to 400 nm. We compare InGaAs and HgCdTe detectors, and show HgCdTe
passivation improvements which increase carrier lifetime fourfold over existing processes
Resonant cavity enhanced HgCdTe structures have been grown by molecular beam epitaxy, and photoconductors
have been modelled and fabricated based on these structures. Responsivity has been measured and shows a
peak responsivity of 8 x 104 V/W for a 50 X 50 μm2 photoconductor at a temperature of 200K. The measured
responsivity shows good agreement with the modelled responsivity across the mid-wave infrared window (3-5μm).
The measured responsivity is limited by surface recombination, which limits the effective lifetime to ~15ns. The
optical cut-off of the detector varies with temperature as modelled from 5.1 um at 80K to 4.4 um at 250K.
There is strong agreement between modelled and measured peak responsivity as a function of temperature from
Current infra-red detectors are limited to detect broad windows in
transmission. By adding Fabry-Perot filtering to these detectors
multi- and hyper-spectral detectors can be fabricated. However,
filtering will reduce the signal available to the detector. In
order to decrease the noise (thereby increasing the signal to
noise ratio), the detector can be moved into the resonant cavity
of the filter. The design of the mirrors is changed by placing the
detector with the resonant cavity. Materials for the design of a
resonant cavity enhanced mercury cadmium telluride detector are
investigated in this paper.