Photon counting imaging applications requires low noise from both detector and readout integrated circuit (ROIC) arrays. In order to retain the photon-counting-level sensitivity, a long integration time has to be employed and the dark current has to be minimized. It is well known that the PIN dark current is sensitive to temperature and a dark current density of 0.5 nA/cm2 was demonstrated at 7 °C previously. In order to restrain the size, weight, and power consumption (SWaP) of cameras for persistent large-area surveillance on small platforms, it is critical to develop large format PIN arrays with small pitch and low dark current density at higher operation temperatures. Recently Spectrolab has grown, fabricated and tested 1024x1280 InGaAs PIN arrays with 12.5 μm pitch and achieved 0.7 nA/cm2 dark current density at 15 °C. Based on our previous low-dark-current PIN designs, the improvements were focused on 1) the epitaxial material design and growth control; and 2) PIN device structure to minimize the perimeter leakage current and junction diffusion current. We will present characterization data and analyses that illustrate the contribution of various dark current mechanisms.
Daniel Law, J. Boisvert, E. Rehder, P. Chiu, S. Mesropian, R. Woo, X. Liu, W. Hong, C. Fetzer, S. Singer, D. Bhusari, K. Edmondson, A. Zakaria, B. Jun, D. Krut, R. King, S. Sharma, N. Karam
Recent progress in III-V multijunction space solar cell has led to Spectrolab’s GaInP/GaAs/Ge triple-junction, XTJ, cells with average 1-sun efficiency of 29% (AM0, 28°C) for cell size ranging from 59 to 72-cm2. High-efficiency inverted metamorphic (IMM) multijunction cells are developed as the next space solar cell architecture. Spectrolab’s large-area IMM3J and IMM4J cells have achieved 33% and 34% 1-sun, AM0 efficiencies, respectively. The IMM3J and the IMM4J cells have both demonstrated normalized power retention of 0.86 at 5x1014 e-/cm2 fluence and 0.83 and 0.82 at 1x1015 e-/cm2 fluence post 1-MeV electron radiation, respectively. The IMM cells were further assembled into coverglass-interconnect-cell (CIC) strings and affixed to typical rigid aluminum honeycomb panels for thermal cycling characterization. Preliminary temperature cycling data of two coupons populated with IMM cell strings showed no performance degradation. Spectrolab has also developed semiconductor bonded technology (SBT) where highperformance component subcells were grown on GaAs and InP substrates separately then bonded directly to form the final multijunction cells. Large-area SBT 5-junction cells have achieved a 35.1% efficiency under 1-sun, AM0 condition.
There is a strong interest in developing sensitive Short Wavelength Infrared (SWIR) avalanche photodiodes (APDs) for
applications like eye safe laser ranging and robotic vision. The excess noise associated with the avalanche process is
critical in dictating the sensitivity of APDs. InGaAs APDs that are commonly used in the SWIR region have either InP
or InAlAs as an avalanche layer and these materials have excess noise factor of 0.5 and 0.22, respectively. Earlier,
Spectrolab had developed APDs with impact ionization engineering (I2E) structures based on InAlAs and InGaAlAs
heterostructures as avalanche layers. These I2E APDs showed an excess noise factor of 0.15. A photoreceiver based on
the I2E APD exhibited an noise equivalent power (NEP) of 150 fW/rt(Hz) over 1 GHz bandwidth at 1.06 μm. In this
paper, a new multiplier structure based on multiple stages of I2E is studied. The APDs show optical gains over 100
before device breakdown. The increased gain and low excess noise will improve the sensitivity of InGaAs APDs based
photoreceivers.
Future NASA light detection and ranging (LIDAR) mapping systems require multi-channel receivers with high
sensitivity and bandwidth operating at 1-1.5 μm wavelengths. One of the ways to improve the system performance is to
improve the sensitivity of photoreceiver. InGaAs avalanche photodiode (APD) sensor technology is considered for this
wavelength region because of high reliability. However, commercially available InGaAs APDs have low sensitivity due
to the high excess-noise of InP material. Spectrolab has been developing low excess noise InGaAs avalanche
photodiodes (APDs) with impact ionization engineering (I2E) structures and recently, APDs with excess noise factor of
0.15 have been demonstrated using an I2E design. Single channel photoreceivers built using low noise I2E APDs show a
noise equivalent power (NEP) of 150 fW/rt(Hz) over a bandwidth of 1 GHz, a record for InGaAs based APDs. A 16
channel GHz SWIR photoreceiver was designed and built at Spectrolab. The photoreceiver was designed to work with a
custom fiber bundle which couples the light from telescope to detectors. The photoreceiver shows a system level NEP
less than 300 fW/rt(Hz) with 1 GHz bandwidth.
Topographic mapping lidar instruments must be able to detect extremely weak laser return signals from high altitudes
including orbital distance. The signals have a wide dynamic range caused by the variability in atmospheric transmission
and surface reflectance under a fast moving spacecraft. Ideally, lidar detectors should be able to detect laser signal return
pulses at the single photon level and produce linear output for multiple photon events. Silicon avalanche photodiode
(APD) detectors have been used in most space lidar receivers to date. Their sensitivity is typically hundreds of photons
per pulse, and is limited by the quantum efficiency, APD gain noise, dark current, and preamplifier noise. NASA is
pursuing three approaches for a 16-channel laser photoreceiver for use on the next generation direct-detection airborne
and spaceborne lidars. We present our measurement results and a comparison of their performance.
Ping Yuan, Rengarajan Sudharsanan, Xiaogang Bai, Joseph Boisvert, Paul McDonald, Eduardo Labios, Bryan Morris, John Nicholson, Gary Stuart, Harrison Danny, Stephen Van Duyne, Greg Pauls, Stephen Gaalema
The performance of Geiger-mode LAser Detection and Ranging (LADAR) cameras is primarily defined by individual
pixel attributes, such as dark count rate (DCR), photon detection efficiency (PDE), jitter, and crosstalk. However, for the
expanding LADAR imaging applications, other factors, such as image uniformity, component tolerance,
manufacturability, reliability, and operational features, have to be considered. Recently we have developed new 32×32
and 32×128 Read-Out Integrated Circuits (ROIC) for LADAR applications. With multiple filter and absorber structures,
the 50-μm-pitch arrays demonstrate pixel crosstalk less than 100 ppm level, while maintaining a PDE greater than 40%
at 4 V overbias. Besides the improved epitaxial and process uniformity of the APD arrays, the new ROICs implement a
Non-uniform Bias (NUB) circuit providing 4-bit bias voltage tunability over a 2.5 V range to individually bias each
pixel. All these features greatly increase the performance uniformity of the LADAR camera. Cameras based on these
ROICs were integrated with a data acquisition system developed by Boeing DES. The 32×32 version has a range gate of
up to 7 μs and can cover a range window of about 1 km with 14-bit and 0.5 ns timing resolution. The 32×128 camera can
be operated at a frame rate of up to 20 kHz with 0.3 ns and 14-bit time resolution through a full CameraLink. The
performance of the 32×32 LADAR camera has been demonstrated in a series of field tests on various vehicles.
Next generation LIDAR mapping systems require multiple channels of sensitive photoreceivers that operate in the
wavelength region of 1.06 to 1.55 microns, with GHz bandwidth and sensitivity less than 300 fW/√Hz. Spectrolab has
been developing high sensitivity photoreceivers using InAlAs impact ionization engineering (I2E) avalanche photodiodes
(APDs) structures for this application. APD structures were grown using metal organic vapor epitaxy (MOVPE) and
mesa devices were fabricated using these structures. We have achieved low excess noise at high gain in these APD
devices; an impact ionization parameter, k, of about 0.15 has been achieved at gains >20 using InAlAs/InGaAlAs as a
multiplier layer. Electrical characterization data of these devices show dark current less than 2 nA at a gain of 20 at room
temperature; and capacitance of 0.4 pF for a typical 75 micron diameter APD. Photoreceivers were built by integrating
I2E APDs with a low noise GHz transimpedance amplifier (TIA). The photoreceivers showed a bandwidth of 1 GHz and
a noise equivalent power (NEP) of 150 fW/rt(Hz) at room temperature.
There is strong interest in developing Short Wavelength Infrared (SWIR) photo receivers for applications like laser
ranging and robotic vision. Recently, Spectrolab has developed a first generation low noise receiver for NASA. The
receiver shows a bandwidth of 180 MHz, presently limited by the transimpedance amplifier (TIA). The first generation
photoreceiver has InP avalanche photodiode (APD). The overall photoreceiver noise equivalent power (NEP) is less than
300 fW/√Hz.
Furthermore, Spectrolab is developing low excess noise APDs with Impact Ionization Engineering (I2E). The I2E low
noise APDs were built from baseline InAlAs APDs with a keff value of 0.22. A thin layer of InGaAlAs alloy was
incorporated into the InAlAs multiplication layer in these devices. All the I2E APDs show lower keff-value than InAlAs
and very low dark currents. Values as low as keff<0.1 have been demonstrated. These I2E APDs will be used in
Spectrolab's second generation photoreceiver. A Noise Equivalent Power (NEP) of 300 fW/√Hz is expected over a
1GHz response bandwidth.
Ping Yuan, Rengarajan Sudharsanan, Xiaogang Bai, Joseph Boisvert, Paul McDonald, Eduardo Labios, Michael Salisbury, Gary Stuart, Harrison Danny, Angel Portillo, Alric Roybal, Stephen Van Duyne, Greg Pauls, Steve Gaalema
For the wide applications of LAser Detection and Ranging (LADAR) imaging with large format Geiger-mode (GM)
avalanche photodiode (APD) arrays, it is critical and challenging to develop a LADAR camera suitable to volume
production with enough component tolerance and stable performance. Recently Spectrolab and Black Forest
Engineering developed a new 32x32 Read-Out Integrated Circuit (ROIC) for LADAR applications. With a specially
designed high voltage input protection circuit, the ROIC can work properly even with more than 1 % of pixels
shorted in the APD array; this feature will greatly improve the camera long-term stability and manufacturing
throughput. The Non-uniform Bias circuit provides bias voltage tunability over a 2.5 V range individually for each
pixel and greatly reduces the impact of the non-uniformity of an APD array. A SMIA high speed serial digital
interface streamlines data download and supports frame rates up to 30 kHz. The ROIC can operate with a 0.5 ns
time resolution without vernier bits; 14 bits of dynamic range provides 8 μs of range gate width. At the meeting we
will demonstrate more performance of this newly developed 32x32 Geiger-mode LADAR camera.
LAser Detection And Ranging (LADAR) is a promising tool for precise 3D-imaging, which enables field
surveillance and target identification under low-light-level conditions in many military applications. For the time
resolution and sensitivity requirements of LADAR applications, InGaAsP/InP Geiger-mode (GM) avalanche
photodiodes (APDs) excel in the spectrum band between 1.0~1.6 μm. Previously MIT Lincoln Laboratory has
demonstrated 3D LADAR imaging in the visible and near infrared (1.06 μm) wavelengths with InP/InGaAsP GM-APD
arrays. In order to relieve the design tradeoffs among dark count rate (DCR), photo detection efficiency (PDE),
afterpulsing, and operating temperature, it is essential to reduce the DCR while maintaining a high PDE. In this
paper we will report the progress of GM-APD detectors and arrays with low DCR and high PDE at 1.06 μm.
In order to improve both DCR and PDE, we optimized the multiplication layer thickness, substrate, and
epitaxial growth quality. With an optimized InP multiplier thickness, a DCR as low as 100 kHz has been
demonstrated at 4V overbias at 300 °C. and at 240 K, less than 1 kHz DCR is measured. A nearly 40% PDE can be
achieved at a DCR of 10 kHz at the reduced temperature.
This paper will review the development of single photon counting sensors at Boeing Spectrolab. Future development
over the next five years will be discussed in the context of sensor requirements that have been established and will
be established. Greater sensitivity through lower false event rates, higher bandwidths, lower after-pulsing rates,
higher operating temperatures, and better uniformity are figures-of-merit that will be discussed in this presentation.
We will present performance of large format InP/InGaAs Geiger mode avalanche photodiode arrays operating at
1.06 μm and 1.55 μm.
J. Asbrock, S. Bailey, D. Baley, J. Boisvert, G. Chapman, G. Crawford, T. de Lyon, B. Drafahl, J. Edwards, E. Herrin, C. Hoyt, M. Jack, R. Kvaas, K. Liu, W. McKeag, R. Rajavel, V. Randall, S. Rengarajan, J. Riker
Advanced LADAR receivers enable high accuracy identification of targets at ranges beyond standard EOIR sensors. Increased sensitivity of these receivers will enable reductions in laser power, hence more affordable, smaller sensors as well as much longer range of detection. Raytheon has made a recent breakthrough in LADAR architecture by combining very low noise ~ 30 electron front end amplifiers with moderate gain >60 Avalanche Photodiodes. The combination of these enables detection of laser pulse returns containing as few as one photon up to 1000s of photons. Because a lower APD gain is utilized the sensor operation differs dramatically from traditional "Geiger mode APD" LADARs. Linear mode photon counting LADAR offers advantages including: determination of intensity as well as time of arrival, nanosecond recovery times and discrimination between radiation events and signals. In our talk we will present an update of this development work: the basic amplifier and APD component performance, the front end architecture, the demonstration of single photon detection using a simple 4 × 4 SCA and the design and evaluation of critical components of a fully integrated photon counting camera under development in support of the Ultra-Sensitive Detector (USD) program sponsored by AFRL-Kirtland.
Boeing Spectrolab has grown, fabricated and tested InGaAs PIN arrays with less than 1 nA/cm2 dark current density at 280 °K. The PIN diodes display greater than 1 A/W responsivity at -100 mV reverse bias with about 50 fF of diode capacitance.
We have designed, fabricated and characterized InGaAs/InP Geiger-mode avalanche photodiode (APD) 32 x 32 arrays
optimized for operation at both 1.06 and 1.55 μm wavelengths Single element devices with a thick multiplication layer
thickness showed dark count rate as low as 60 kHz at a 3 V overbias, while photon detection efficiencies at a wavelength
of 1.55 μm exceed 30% at 2 V overbias. Back illuminated 32 x 32 detector arrays exhibited breakdown uniformity of
greater than 97% and excellent dark current uniformity. Detector arrays were integrated with low-noise read-out
integrated circuits for an imaging demonstration. 3D imaging was demonstrated using 1.06 micron detector arrays.
Recent developments in three-dimension imaging, quantum cryptography, and time-resolved spectroscopy
have stimulated interest in single-photon counting avalanche photodiodes (APD) operating in the short wavelength
infrared region. For visible and near infrared wavelengths, Silicon Geiger-mode APDs have demonstrated excellent
photon detection efficiency (PDE) and low dark current rate (DCR)1. Recently, MIT Lincoln Laboratories, Boeing
Spectrolab, and Boeing SVS have demonstrated Geiger-mode (GM) APD focal plane arrays (FPA) operating at 1.06
μm. However for longer wavelength sensitivity around 1.55 μm, GM-APDs have to be cooled to 180~240 K to
achieve a usable DCR. Power consumption, package weight and size and APD PDE all suffer with this cooling
requirement.
In this paper we report the development of an InP/InGaAs GM-APD structure with high PDE and low DCR
at 273K. The photon collection efficiency was optimized with a single step-graded quaternary layer and a 3.5 μm
InGaAs absorption layer, which provides a broadband coverage from 0.95 μm to 1.62 μm. The InP multiplication
layer and the charge layer are carefully tailored to minimize the DCR and maximize the PDE. Despite having a low
bandgap absorber layer InGaAs, these APDs demonstrated excellent dark current, optical responsivity, and superior
DCR and PDE at 1.55 μm. The DCR and PDE were evaluated on 25 μm diameter APDs at 273 K. DCRs as low as
20 kHz have been measured at a 2 V overbias, while PDEs at 1.55 μm exceed 30% at 2 V overbias.
32×32 element InGaAsP/InP avalanche photodiode arrays operating at 1.06 μm have been fabricated and characterized.
Material characterization data on uniformity and layer quality have been correlated to array performance using the
McIntyre model. Sheet resistivity maps, Hall mobility, dark current, capacitance and gain data are presented. These
devices have showed gain as high as 75 with low dark current. Both device and materials uniformity characterization
data will be presented.
Large-area APDs operating in the wavelength region of 1- 1.5 micron are useful for many low light level applications. Present commercially available InGaAs based APDs are small, (<500 micron diameter size) and thus limit the field of view. We report here on low dark current density, large-area (1 mm diameter) InGaAs APDs. InGaAs APD device structures with InP and InAlAs multiplication layers were grown by metaloragnic vapor deposition method. The combination of good quality material and a proprietary passivation process yielded 1 mm APD devices with low dark current density and high gain. Devices exhibited gain as high as 30 and dark current density as low as 0.5 microamperes per square centimeter.
InAlAs/InGaAs avalanche photodiodes (APDs) have been fabricated and characterized in linear mode operation as photon counting detectors. Dark current, dark count, dynamic range, pulse shapes, and counting statistics are presented on 75 μm and 200 μm diameter APDs.
40x40 element InAlAs/InGaAs APD arrays have been fabricated and characterized for performance in short wave infrared (SWIR) applications. Characterization data collected to date indicate that the arrays have >99% operability at operating gains of 10. The median un-multiplied dark current for an array element is about 170 pA, and the un-multiplied responsivity at 1550 nm is about 0.75 A/W.
Low cost germanium photodetectors for sensing applications in the 900-1600 nm spectral region have been developed. By varying the Ge substrate resistivity as well as device area, photodetector properties such as reverse leakage current, capacitance, and shunt resistance have been engineered. Low leakage current devices of various sizes up to 1 cm2 have been fabricated and have consistently exhibited exceptionally high shunt resistances and excellent linearity. Over 5000 hours of active stress testing have left the ultra-low leakage currents unchanged. These data were measured in accordance with Telcordia 468-CORE requirements at 85°C, 125°C and 175°C. The results indicate that these mesa photodetectors meet telecommunication industry requirements for reliability. These devices are comparable to commercially available Ge photodetectors, and can be readily substituted for more complex InGaAs photo-detectors in applications such as laser monitor diodes.
Germanium (Ge) photodetectors are fabricated by growing epitaxial III-V compounds on Ge substrates and by in-situ formation of the PN junction by MOVPE. After material growth, Ge photodetectors are mesa-etched using conventional optoelectronic device processing techniques. By varying the Ge substrate resistivity and the device area, Ge photodetector properties such as reverse leakage current, capacitance, and shunt resistance have been engineered. Such devices have demonstrated leakage currents below 50(mu) A/cm2 at -0.1 V bias. For optoelectronic applications that require high temperature operation, high shunt resistance detectors exhibit leakage currents below (mu) A/cm2 at 80 degree(s)C. Low capacitance devices have measured as little as 275 pF at 0V bias for a 1 mm diameter detector. High shunt resistance devices are a low cost alternative to conventional InGaAs photodiodes in applications such as laser monitor diodes.
The spectral content of radiation incident on a bulk photoconductor can significantly affect the measured photocurrent. We show that front-illuminated Si:As bulk photoconductors operated at low temperature (approximately 10 degree(s)K) are sensitive to the spectral characteristics of the background, as well as signal fluxes. Our analysis is applied to spectral response measurements on bulk photoconductors themselves, and to filter transmission measurements that have been made utilizing these devices as IR detectors.
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