Mercury cadmium telluride (HgCdTe or MCT) is the material of choice for infrared avalanche photodetectors (APDs) owing to its desirable qualities including high quantum efficiency and low excess noise factor. Recent advancements in growth techniques have allowed for bandgap engineered MCT films that further enhance the performance of MCT APDs. Monte Carlo has been a widely used method for simulating the multiplication process within avalanche photodiodes (APDs) due to its ability to accurately simulate non-equilibrium transport. In this work, we demonstrate how the gain, excess noise, and bandwidth of bandgap engineered MCT APDs can be accurately modeled in 3-D using Monte Carlo.
In this paper we report on recent advancements in the development of linear-mode photon-counting (LMPC) electron-initiated avalanche photodiodes (e-APDs) at Leonardo DRS. The Hg1-xCdxTe linear-mode e-APD fills a gap in single-photon detectors from near- to mid-infrared wavelengths and enables several new space lidar and laser communication applications. The combination of high e-APD gain and near unity excess noise factor enables robust, single-photon detection. Another important feature of the Hg1-xCdxTe e-APD is that there is no dead time or latency between successive photon detection events. Since the inception of the device, Leonardo DRS has sought to improve the performance of these e-APDs by: increasing linear gains to greater than 1000; decreasing single photon jitter; reducing ROIC glow contributions to dark counts; and decreasing intrinsic detector dark currents. To these ends, we begin by showing that ROIC glow contributions to the false-event rate (FER) can be significantly reduced using an improved, photon blocking shield. We continue by examining the performance of focal-plane arrays (FPAs) with two differing material cutoff wavelengths, demonstrating record low FERs at high photon detection efficiencies (PDEs); this improvement in performance is assisted in part to the successful integration of micro-lens arrays (MLAs) onto the detectors. We conclude our study by integrating one detector unit into a tactical, Integrated Dewar Cooler Assembly (IDCA) and comparing performance prior and following this integration.
We report new results on Linear Mode HgCdTe electron Avalanche Photodiodes (e-APDs) for applications in the 500 - 3000 nm band at 200-240K with high gain, low excess noise factor, low dark current density and high quantum efficiency. Results from two classes of devices will be reported: first the conventional HgCdTe e-APD grown by LPE with homogeneous composition and second bandgap engineered HgCdTe e-APDs grown by MBE. Detailed characterization data of the e-APDs including gain, dark current density and excess noise factor will be reported and compared with modeling results.
A HgCdTe avalanche photodiode (APD) focal plane array assembly with linear mode photon-counting capability has been developed for space lidar applications. An integrated detector cooler assembly (IDCA) has been built using a miniature Stirling cooler. A microlens array has been included to improve the fill factor. The HgCdTe APD has a spectral response from 0.9- to 4.3-μm wavelengths, a photon detection efficiency as high as 70%, and a dark count rate of <250 kHz at 110 K. The mass of the IDCA is 0.8 kg and the total electrical power consumption is about 7 W. The HgCdTe APD arrays have been characterized at NASA Goddard Space Flight Center. A series of environmental tests have been conducted for the IDCAs, including vibration, thermal cycling, and thermal vacuum tests. A description of the device and the test results at NASA are given in this paper.
A linear mode photon counting HgCdTe avalanche photodiode (APD) focal plane array (FPA) detector was developed for space lidar applications. An integrated detector cooler assembly (IDCA) was manufactured using a miniature Stirling cooler. The HgCdTe APD demonstrated a greater than 60% photon detection efficiency from 0.9 to 4.3 μm wavelength and a dark count rate less than 250,000/s. The IDCA cooled the FPA to 110K from ambient room temperature at a total electrical power of 7 W. The IDCA has passed environmental tests, including vibration, thermal cycling and thermal vacuum tests.
Results of characterization data on linear mode photon counting (LMPC) HgCdTe electron-initiated avalanche photodiode (e-APD)focal plane arrays (FPA) are presented that reveal an improved understanding and the growing maturity of the technology. The first successful 2x8 LMPC FPA was fabricated in 2010 [1]. Since then a process validation lot of 2x8 arrays was fabricated. Five arrays from this lot were characterized that replicated the previous 2x8 LMPC array performance. In addition, it was unambiguously verified that readout integrated circuit (ROIC) glow was responsible for most of the false event rate (FER) of the 2010 array. The application of a single layer metal blocking layer between the ROIC and the detector array and optimization of the ROIC biases reduced the FER by an order of magnitude. Photon detection efficiencies (PDEs) of greater than 50% were routinely demonstrated across 5 arrays, with one array reaching a PDE of 70%. High resolution pixel-surface spot scans were performed and the junction diameters of the diodes were measured. The junction diameter was decreased from 31 μm to 25 μm resulting in a 2x increase in E-APD gain from 470 on the 2010 array to 1100 on one of the 2013 FPAs. Mean single photon signal to noise ratios of >12 were demonstrated at excess noise factors of 1.2-1.3. NASA Goddard Space Flight Center (GSFC) performed measurements on the delivered FPA that verified the PDE and FER data.
A linear mode photon counting focal plane array using HgCdTe mid-wave infrared (MWIR) cutoff electron initiated avalanche photodiodes (e-APDs) has been designed, fabricated, and characterized. The broad spectral range (0.4 to 4.3 μm) is unique among photon counters, making this a “first of its kind” system spanning the visible to the MWIR. The low excess noise [F(M)≈1] of the e-APDs allows for robust photon detection while operating at a stable linear avalanche gain in the range of 500–1000. The readout integrated circuit (ROIC) design included a very high gain-bandwidth product resistive transimpedance amplifier (3×1013 Ω-Hz) and a 4 ns output digital pulse width comparator. The ROIC had 16 high-bandwidth analogs and 16 low-voltage differential signaling digital outputs. The 2×8 array was integrated into an LN2 Dewar with a custom leadless chip carrier and daughter board design that preserved high-bandwidth analog and digital signal integrity. The 2×8 e-APD arrays were fabricated on 4.3 μm cutoff HgCdTe and operated at 84 K. The measured dark currents were approximately 1 pA at 13 V bias where the measured avalanche photodiode gain was 500. This translates to a predicted dark current induced dark count rate of less than 20 KHz. Single photon detection was achieved with a photon pulse signal-to-noise ratio of 13.7 above the amplifier noise floor. A photon detection efficiency of 50% was measured at a photon background limited false event rate of about 1 MHz. The measured jitter was in the range of 550–800 ps. The demonstrated minimum time between distinguishable events was less than 10 ns.
A 16 element HgCdTe e-APD detector has been developed for lidar receivers that has significant improvements in
sensitivity in the spectral range from < 1μm to 4 μm. A demonstration detector consisting of a 4x4 APD detector array, with 80 μm square elements, a custom CMOS readout integrated circuit (ROIC), a closed cycle cooler-Dewar, and
support electronics has been designed, fabricated, and tested. The custom ROIC design provides > 6 MHz bandwidth
with low noise and 21 selectable gains. Ninety-six arrays were fabricated with 69% of the arrays meeting the dark
current spec in the center 4 pixels at 10 V bias where the APD gain was expected to be around 150. Measurements to 12
V on one array showed APD gains of 654 with a gain normalized dark currents of 1.2 fA to 3.2 fA. The lowest dark
current array showed a maximum dark current of 6.2 pA at 10 V and 77 K. The 4.4 μm cutoff detector was characterized
at an operating temperature of 77K with a 1.55 μm, 1μs wide, laser pulse. The photon conversion efficiency at unity
gain was 91%. The mean measured APD gain at 77 K was 308 at 11V, the responsivity was 782 μV/pW, the average
NEP was 1.04 fW/Hz1/2. The bandwidth was 6.8 MHz, and the broadband NEP was 2.97 pW. This detector offers a
wide spectral response, dynamic range, and substantially improved sensitivity and lifetime for integrated path
differential absorption (IPDA) lidar measurements of atmospheric trace gases such as CO2 and CH4.
A linear mode photon counting FPA using HgCdTe MWIR cutoff e-APDs has been designed, fabricated, and characterized. The broad spectral range (0.4 μm to 4.3 μm) is unique among photon counters, making this a "first of its kind" system spanning the visible to the MWIR. The low excess noise ((F(M) ≈ 1) of the e-APDs allows for robust photon detection while operating at a stable linear avalanche gain in the range of 500 to 1000. The ROIC design included
a very high gain-bandwidth product RTIA (3x1011 Ohm-Hz) and a 4 ns output digital pulse width comparator. The ROIC had 16 high bandwidth analog and 16 LVDS digital outputs. The 2x8 array was integrated into an LN2 Dewar with a custom LCC and daughter board design that preserved high bandwidth analog and digital signal integrity. The 2x8 e-APD arrays were fabricated on 4.3 μm cutoff HgCdTe and operated at 84 K. The measured dark currents were
approximately 1 pA at 13 V bias where the measured APD gain was 500. This translates to a predicted dark current induced dark count rate of less than 20 KHz. Single photon detection was achieved with a photon pulse SNR of 13.7 above the amplifier noise floor. A photon detection efficiency of 50% was measured at a background limited false event rate (FER) of about 1 MHz. The measured jitter was in the range of 550 ps to 800 ps. The demonstrated minimum time
between distinguishable events was less than 10 ns.
The operation of the mid-wave infrared (MWIR) HgCdTe cylindrical electron injection
avalanche photodiode (e-APD) is described. The measured gain and excess noise factor are
related to the to the collection region fill factor. A 2D diffusion model calculates the time
dependent response and steady state pixel point spread function for cylindrical diodes, and
predicts bandwidths near 1 GHz for small geometries. A 2 μm diameter spot scan system
was developed for point spread function and crosstalk measurements at 80 K. An electron
diffusion length of 13.4 μm was extracted from spot scan data. Bandwidth data are shown
that indicate bandwidths in excess of 300 MHz for small unit cells geometries. Dark current
data, at high gain levels, indicate an effective gain normalized dark density count as low as
1000 counts per μs per cm2 at an APD gain of 444. A junction doping profile was
determined from capacitance-voltage data. Spectral response data shows a gain independent
characteristic.
The next generation of IR sensor systems will include active imaging capabilities. One example of such a system is a gated-active/passive system. The gated-active/passive system promises long-range target detection and identification. A detector that is capable of both active and passive modes of operation opens up the possibility of a self-aligned system that uses a single focal plane. The detector would need to be sensitive in the 3-5 μm band for passive mode operation. In the active mode, the detector would need to be sensitive in eye-safe range, e.g. 1.55 μm, and have internal gain to achieve the required system sensitivity. The MWIR HgCdTe electron injection avalanche photodiode (e-APD) not only provides state-of-the-art 3-5 μm spectral sensitivity, but also high avalanche photodiode gain without minimal excess noise. Gains of greater than 1000 have been measured in MWIR e-APDs with a gain independent excess noise factor of 1.3. This paper reports the application of the mid-wave HgCdTe e-APD for near-IR gated-active/passive imaging. Specifically a 128x128 FPA composed of 40 μm pitch, 4.2 μm to 5 μm cutoff, APD detectors with a custom readout integrated circuit was designed, fabricated, and tested. Median gains as high as 946 at 11 V bias with noise equivalent inputs as low as 0.4 photon were measured at 80 K. A gated imaging demonstration system was designed and built using commercially available parts. High resolution gated imagery out to 9 km was obtained with this system that demonstrated predicted MTF, precision gating, and sub 10 photon sensitivity.
Hyperspectral imaging in the infrared bands is traditionally performed using a broad spectral response focal plane array,
integrated in a grating or a Fourier transform spectrometer. This paper describes an approach for miniaturizing a
hyperspectral detection system on a chip by integrating a Micro-Electro-Mechanical-System (MEMS) based tunable
Fabry Perot (FP) filter directly on a photodetector. A readout integrated circuit (ROIC) serves to both integrate the
detector signal as well as to electrically tune the filter across the wavelength band. We report the first such
demonstration of a tunable MEMS filter monolithically integrated on a HgCdTe detector. The filter structures, designed
for operation in the 1.6-2.5 μm wavelength band, were fabricated directly on HgCdTe detectors, both in photoconducting
and high density vertically integrated photodiode (HDVIP) detectors. The HDVIP detectors have an architecture that
permits operation in the standard photodiode mode at low bias voltages (≤0.5V) or in the electron avalanche photodiode
(EAPD) mode with gain at bias voltages of ~20V. In the APD mode gain values of 100 may be achieved at 20 V at 200
K. The FP filter consists of distributed Bragg mirrors formed of Ge-SiO-Ge, a sacrificial spacer layer within the cavity
and a silicon nitride spacer membrane for support. Mirror stacks fabricated on silicon, identical to the structures that will
form the optical cavity, have been characterized to determine the optimum filter characteristics. The measured full width
at half maximum (FWHM) was 34 nm at the center wavelength of 1780 nm with an extinction ratio of 36.6. Fully
integrated filters on HgCdTe photoconductors with a center wavelength of approximately 1950 nm give a FWHM of
approximately 100 nm, and a peak responsivity of approximately 8 × 104 V/W. Initial results for the filters on HDVIP
detectors exhibit FWHM of 140 nm.
Hyperspectral imaging in the infrared bands is traditionally performed using a broad spectral response focal plane array, integrated in a grating or a Fourier transform spectrometer. This paper describes an approach for miniaturizing a hyperspectral detection system on a chip by integrating a Micro-Electro-Mechanical-System (MEMS) based tunable Fabry Perot (FP) filter directly on a photodetector. A readout integrated circuit (ROIC) serves to both integrate the detector signal as well as to electrically tune the filter across the wavelength band. We report the first such demonstration of a tunable MEMS filter monolithically integrated on a HgCdTe detector. The filter structures, designed for operation in the 1.6-2.5 μm wavelength band, were fabricated directly on HgCdTe detectors, both in photoconducting and high density vertically integrated photodiode (HDVIP) detectors. The HDVIP detectors have an architecture that permits operation in the standard photodiode mode at low bias voltages (≤0.5V) or in the electron avalanche photodiode (EAPD) mode with gain at bias voltages of ~20V. In the APD mode gain values of 100 may be achieved at 20 V at 200 K. The FP filter consists of distributed Bragg mirrors formed of Ge-SiO-Ge, a sacrificial spacer layer within the cavity and a silicon nitride spacer membrane for support. Mirror stacks fabricated on silicon, identical to the structures that will form the optical cavity, have been characterized to determine the optimum filter characteristics. The measured full width at half maximum (FWHM) was 34 nm at the center wavelength of 1780 nm with an extinction ratio of 36.6. Fully integrated filters on HgCdTe photoconductors with a center wavelength of approximately 1950 nm give a FWHM of approximately 100 nm, and a peak responsivity of approximately 8×104 V/W. Initial results for the filters on HDVIP detectors exhibit FWHM of 140 nm.
A low temperature MEMS process integrated with an infrared detector technology has been developed. The integrated microsystem is capable of electrically selecting narrow wavelength bands in the range from 1.6 to 2.5 μm within the short-wavelength infrared (SWIR) region of the electromagnetic spectrum. The integrated fabrication process is compatible with two-dimensional infrared focal plane array technology. The demonstration prototypes consist of both HgCdTe SWIR photoconductive as well as high density vertically integrated photodiode (HDVIP®) detectors, two distributed Bragg mirrors formed of Ge-SiO-Ge, an air-gap optical cavity, and a silicon nitride membrane for structural support. The tuning spectrum from fabricated MEMS filters on photoconductive detectors indicates a wide tuning range and high percentage transmission. Tuning is achieved with a voltage of only 7.5 V, and the FWHM ranged from 95-105 nm over a tuning range of 2.2 μm to 1.85 μm. The same MEMS filters, though unreleased, and with the sacrificial layer within the optical cavity, have been fabricated on planarised SWIR HDVIP® photodiodes with FWHM of less than 60 nm centred at a wavelength of approximately 1.8 μm. Finite element modelling of various geometries for the silicon nitride membrane will also be presented. The modelling is used to optimize the filter geometry in terms of fill factor, mirror displacement versus applied voltage, and membrane bowing.
A monolithically integrated low temperature MEMS and HgCdTe infrared detector technology has been implemented and characterised. The MEMS-based optical filter, integrated with an infrared detector, selects narrow wavelength bands in the range from 1.6 to 2.5 μm within the short-wavelength infrared (SWIR) region of the electromagnetic spectrum. The entire fabrication process is compatible with two-dimensional infrared focal plane array technology. The fabricated device consists of an HgCdTe SWIR photoconductor, two distributed Bragg mirrors formed of Ge-SiO-Ge, a sacrificial spacer layer within the cavity, which is then removed to leave an air-gap, and a silicon nitride membrane for structural support. The tuning spectrum from fabricated MEMS filters on photoconductive detectors shows a wide tuning range and high percentage transmission is achieved with a tuning voltage of only 7.5 V. The FWHM ranged from 95-105 nm over a tuning range of 2.2 μm to 1.85 μm. Finite element modelling of various geometries for the silicon nitride membrane will also be presented. The modelling is used to determine the best geometry in terms of fill factor, voltage displacement prediction and membrane bowing.
Electron injection avalanche photodiodes in SWIR to LWIR HgCdTe show gain and excess noise properties indicative of a single ionizing carrier gain process. The result is an electron avalanche photodiode (EAPD) with "ideal" APD characteristics including near noiseless gain. This paper reports results obtained on long-wave, mid-wave, and short wave cutoff infrared HgCdTe EAPDs that utilize a cylindrical "p-around-n", front side illuminated, n+/n-/p geometry that favors electron injection into the gain region. These devices are characterized by a uniform, exponential, gain voltage characteristic that is consistent with a hole-to-electron ionization coefficient ratio, k, of zero. Gains of greater than 1000 have been measured in MWIR EAPDS without any sign of avalanche breakdown. Excess noise measurements on MWIR and SWIR EAPDs show a gain independent excess noise factor at high gains that has a limiting value less than 2. At 77 K, 4.3 μm cutoff devices show excess noise factors of close to unity out to gains of 1000. The excess noise factor at room temperature on SWIR EAPDs, while still consistent with the k = 0 operation, approaches a gain independent limiting value of just under 2. The k = 0 operation is explained by the band structure of the HgCdTe. Monte Carlo modeling based on the band structure and scattering models for HgCdTe predict the measured gain and excess noise behavior. A noise equivalent input of 7.5 photons at a 10 ns pulsed signal gain of 964, measured on an MWIR APD at 77 K, provides an indication of the capability of the HgCdTe EAPD.
This paper reports new results on multi/hyper-spectral IR detectors based on GaAs/AlGaAs multiple quantum well (MQW) materials. The wavelength tuned quantum-well IR photodetectors are obtained by patterning three-dimensional diffractive resonant optical cavities into the MQW structures. Control of the peak wavelength within the absorption band of the MQW material is achieved by adjusting the optical cavity dimensions. This approach enables highly effective optical coupling into the MQW material, produces narrow spectral response, improved detector quantum efficiency (QE) and reduced dark currents. Results are reported on the performance of multispectral detectors that are designed to operate in the 8-14 μm spectral band. Spectrally broadened MQW material absorption is achieved by engineering a triple coupled quantum well design that produces absorption bands with full width at half maximum (FWHM) of less than or equal to 3 μm. Two back-to-back MQW stacks have been employed to achieve spectral coverage in the range of 8-14 μm. As many as 52 different detector designs were fabricated on a single chip with 32x64 pixels. The spectrally tuned multispectral detector arrays exhibit excellent spectral dispersion and spectral response FWHM as narrow as ~ 0.4 μm at 8.5 μm peak wavelength. The detectors exhibit QE values in the range of 15-50%. The background limited detectivity measured at 40 K with a 295 K background and f/2.5 is in the range of (0.8-1.5)x1011 cm-Hz0.5/W. These high performance QWIPs are the first multispectral detectors fabricated over the 8-14 μm wavelength band.
This paper reports recent results from a novel approach for fabricating RI detectors from GaAs/AlGaAs multiple quantum well (MQW) materials. It involves the fabrication of quantum-well IR photodetectors (QWIPs) by patterning a 3D diffractive resonant optical cavity into the MQW structure. This approach, called the Enhanced QWIP (EQWIP), enables highly effective optical coupling into the MQW material resulting in narrow spectral response, improved detector quantum efficiency (QE), reduced dark currents, and improved photoconductive gain. EQWIP arrays operating in the 8-10 μm spectral band are reported in this paper. The peak response wavelength of the detectors, within the absorption band of the MQW material, is tuned by controlling the dimensions of the resonant structure. EQWIPs with peak spectral response at 8.6 μm exhibit peak responsivity as high as 2.6 A/W and quantum efficiency (QE) as high as 57%. Total conversion efficiency is as high as 35%. The background limited, peak detectivity measured at 9.0μm and 55 K with a 295 K background at f/2.5 is greater than or equal to 1 × 1011 cm-Hz0.5/W.
Recent advances in metalorganic vapor phase epitaxy (MOVPE) of HgCdTe are reviewed that have impacted in situ growth of bandgap engineered IR detector devices. MOVPE can now be readily used to grow multilayer HgCdTe structures with complete flexibility in iodine donor and arsenic acceptor doping and tight control of alloy composition. 100% activation of both donor and acceptor dopants has been achieved and the mobilities and lifetimes of HgCdTe grown on lattice-matched CdZnTe are comparable to the best values achieved in HgCdTe by any epitaxial growth technique. The defects measured by etch pit density counts in multilayer structures with n-type and p- type regions are reported. Single-band IR detector device results are reported that have been grown in situ, for operation in the long wavelength (LW, 8 - 12 micrometer) and medium wavelength (MW, 3 - 5 micrometer) IR spectral bands. Their material characteristics and detector performances are reviewed and compared with theoretical modeling results.
We report results for 64 X 64 simultaneous MW/LW dual-band HgCdTe Focal Plane Arrays (FPAs). The MW and LW average cutoff wavelengths at 78 K are in the 4.27 - 4.35 micrometer and 10.1 - 10.5 micrometer ranges respectively. The unit cell size is 75 X 75 micrometer2. These staring dual-band FPAs exhibit high average quantum efficiencies (MW: 79%; LW:67%), high median detectivities (MW: 4.8 X 1011 cm- (root)Hz/W; LW: 7.1 X 1010 cm-(root)Hz/W), low median NE(Delta) Ts (MW: 20 mK; LW: 7.5 mK for TSCENE equals 295 K and f/2.9), large dynamic ranges (MW: 77 dB; LW: 75 dB), and 87% stare efficiencies for both the MW and LW spectral bands. The dual-band HgCdTe detector array is fabricated from a four- layer P-n-N-P film grown in situ by MOVPE. The dual-band silicon CMOS input circuit utilizes a unique floating-direct- injection approach to achieve separate and simultaneous integration of both bands within each unit cell. There are two Compact Signal Averager circuits in each unit cell, to average subframes within every frame for each spectral band, allowing full stare efficiency in both spectral bands, as well as variable band-independent transimpedance gains. These data confirm that all key features of our P-n-N-P dual-band HgCdTe detector and our dual-band input circuit function as designed.
Recent developments in MOCVD growth of Hg1-xCdxTe photodiodes using the interdiffused multilayer process are reported. Iodine doping of HgCdTe is described using ethyl iodide. Using ethyl iodide, the iodine doping level can be controlled in the range of 7 X 1014 - 2 X 1018 cm-3 without any memory effect. Activation of the iodine as a singly ionized donor is near 100% at concentrations < 1 X 1017 cm-3. Ethyl iodide was not found to react with the other organometallic precursors and abrupt dopant profiles are obtained. The iodine doped HgCdTe films exhibit 80 K electron mobilities >= 1 X 105 cm2/V-s, auger limited lifetimes of approximately 1 microsecond(s) for concentrations of (1-3)X1015 cm-3, and x-values approximately 0.22. LWIR p-on-n heterojunctions have been grown in situ using iodine doping for the n-type absorber layer and arsenic doping for the p- type cap layer. Detailed characterization data for the photodiodes are reported.
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