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/cm<sup>2</sup> 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/cm<sup>2</sup> 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.
Emerging short wavelength infrared (SWIR) LIght Detection And Ranging (LIDAR) and long range laser rangefinder systems, require large optical aperture avalanche photodiodes (APDs) receivers with high sensitivity and high bandwidth. A large optical aperture is critical to increase the optical coupling efficiency and extend the LIDAR sensing range of the above systems. Both APD excess noise and transimpedance amplifier (TIA) noise need to be reduced in order to achieve high receiver sensitivity. The dark current and capacitance of large area APDs increase with APD aperture and thus limit the sensitivity and bandwidth of receivers. Spectrolab has been developing low excess noise InAlAs/InGaAs APDs with impact ionization engineering (I<sup>2</sup>E) designs for many years and has demonstrated APDs with optical gain over 100 utilizing multiple period I<sup>2</sup>E structures in the APD multiplier. These high gain I<sup>2</sup>E APDs have an excess noise factor less than 0.15. With an optical aperture of 200 μm, low excess noise multiple periods I<sup>2</sup>E APDs have capacitances about 1.7 pF. In addition, optical gains of InAlAs based APDs show very little temperature dependence and will enable APD photoreceivers without thermal electric cooling.
Recently Spectrolab has successfully demonstrated a compact 32x32 Laser Detection and Range (LADAR)
camera with single photo-level sensitivity with small size, weight, and power (SWAP) budget for threedimensional
(3D) topographic imaging at 1064 nm on various platforms. With 20-kHz frame rate and 500-
ps timing uncertainty, this LADAR system provides coverage down to inch-level fidelity and allows for
effective wide-area terrain mapping. At a 10 mph forward speed and 1000 feet above ground level (AGL),
it covers 0.5 square-mile per hour with a resolution of 25 in2/pixel after data averaging. In order to increase
the forward speed to fit for more platforms and survey a large area more effectively, Spectrolab is
developing 32x128 Geiger-mode LADAR camera with 43 frame rate. With the increase in both frame rate
and array size, the data collection rate is improved by 10 times. With a programmable bin size from 0.3 ps
to 0.5 ns and 14-bit timing dynamic range, LADAR developers will have more freedom in system
integration for various applications. Most of the special features of Spectrolab 32x32 LADAR camera, such
as non-uniform bias correction, variable range gate width, windowing for smaller arrays, and short pixel
protection, are implemented in this camera.
Three-dimensional (3D) imaging with Short wavelength infrared (SWIR) Laser Detection and Range (LADAR) systems have been successfully demonstrated on various platforms. It has been quickly adopted in many military and civilian applications. In order to minimize the LADAR system size, weight, and power (SWAP), it is highly desirable to maximize the camera sensitivity. Recently Spectrolab has demonstrated a compact 32x32 LADAR camera with single photo-level sensitivity at 1064. This camera has many special features such as non-uniform bias correction, variable range gate width from 2 microseconds to 6 microseconds, windowing for smaller arrays, and short pixel protection. Boeing
integrated this camera with a 1.06 μm pulse laser on various platforms and demonstrated 3D imaging. The
features and recent test results of the 32x128 camera under development will be introduced.
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 (I<sup>2</sup>E) structures and recently, APDs with excess noise factor of
0.15 have been demonstrated using an I<sup>2</sup>E design. Single channel photoreceivers built using low noise I<sup>2</sup>E 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.
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 (I<sup>2</sup>E) structures based on InAlAs and InGaAlAs
heterostructures as avalanche layers. These I<sup>2</sup>E APDs showed an excess noise factor of 0.15. A photoreceiver based on
the I<sup>2</sup>E 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 I<sup>2</sup>E 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
Future robots and autonomous vehicles require compact low-cost Laser Detection and Ranging (LADAR) systems for
autonomous navigation. Army Research Laboratory (ARL) had recently demonstrated a brass-board short-range eye-safe
MEMS scanning LADAR system for robotic applications. Boeing Spectrolab is doing a tech-transfer (CRADA) of this
system and has built a compact MEMS scanning LADAR system with additional improvements in receiver sensitivity,
laser system, and data processing system. Improved system sensitivity, low-cost, miniaturization, and low power
consumption are the main goals for the commercialization of this LADAR system. The receiver sensitivity has been
improved by 2x using large-area InGaAs PIN detectors with low-noise amplifiers. The FPGA code has been updated to
extend the range to 50 meters and detect up to 3 targets per pixel. Range accuracy has been improved through the
implementation of an optical T-Zero input line. A compact commercially available erbium fiber laser operating at 1550
nm wavelength is used as a transmitter, thus reducing the size of the LADAR system considerably from the ARL brassboard
system. The computer interface has been consolidated to allow image data and configuration data (configuration
settings and system status) to pass through a single Ethernet port. In this presentation we will discuss the system
architecture and future improvements to receiver sensitivity using avalanche photodiodes.
Three-dimensional (3D) topographic imaging using Short wavelength infrared (SWIR) Laser Detection and
Range (LADAR) systems have been successfully demonstrated on various platforms. LADAR imaging
provides coverage down to inch-level fidelity and allows for effective wide-area terrain mapping. Recently
Spectrolab has demonstrated a compact 32×32 LADAR camera with single photon-level sensitivity with
small size, weight, and power (SWAP) budget. This camera has many special features such as non-uniform
bias correction, variable range gate width from 2 microseconds to 6 microseconds, windowing for smaller
arrays, and shorted pixel protection. Boeing integrated this camera with a 1.06 μm pulse laser on various
platforms and had demonstrated 3D imaging. In this presentation, the operation details of this camera and
3D imaging demonstration using this camera on various platforms will be presented.
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.
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 (I<sup>2</sup>E) 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
I<sup>2</sup>E 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
Furthermore, Spectrolab is developing low excess noise APDs with Impact Ionization Engineering (I2E). The I<sup>2</sup>E 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 I<sup>2</sup>E APDs show lower keff-value than InAlAs
and very low dark currents. Values as low as k<sub>eff</sub><0.1 have been demonstrated. These I<sup>2</sup>E 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.
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.
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)<sup>1</sup>. 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
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.
Aluminum nanowire-grid polarizers and polarizing beam splitters with a fixed pitch (i.e., period) of ~146 nm but a wide range of linewidths (from < 60 nm to 90 nm) and heights (from 150 nm to 200 nm) are studied. Immersion interference lithography, UV-nanoimprint lithography and aluminum reactive ion etching were used to fabricate the nanowire-grid polarizers. Optical performance of the nanowire-grid polarizers was characterized in a broad spectral range from UV (< 400 nm) to near infrared (> 1700 nm). The performance trade-off between transmittance/reflectance and extinction ratio is investigated in details. The developed high-performance large-area broadband nanowire-grid polarizer opens the potential for many optical applications particularly integrated optics.
InP/In<SUB>0.53</SUB>Ga<SUB>0.47</SUB>As avalanche photodiodes (APDs) have been widely deployed in high-bit-rate, long-haul fiber optic communication systems due to the higher sensitivity, relative to a PIN photodiode, afforded by internal gain of the APD. Owing to their materials and structural limitations it is uncertain whether the performance of InP-based APDs will be adequate for 10 GB/s systems and subsequent higher- speed systems. One of the impediments for the InP-based APDs is the fact that InP has roughly equal electron and hole ionization rates. This result in a symmetric multiplication process with relatively high multiplication noise and the gain-bandwidth product of an APD are primarily determined by the structure of the multiplication region. Recently, it has been reported that submicron scaling of the multiplication region thickness leads to lower multiplication noise and higher gain-bandwidth products. This is due to the nonlocal nature of impact ionization, which can be neglected if the thickness of the multiplication region is much greater than the 'dead length', the distance over which carriers gain sufficient energy to impact ionize. The advantage of thin multiplication regions, i.e., those for which, the dead space accounts for a significant portion of the total thickness, is that the number of ionization chains that result in multiplication greatly in excess of the average gain is reduced, which in turn yields lower noise for a given gain. In this paper we describe materials and structural modifications to the thin multiplication regions that result in even lower excess noise. For gains <EQ 20 APDs with thin Al<SUB>x</SUB>Ga<SUB>1-x</SUB>As multiplication layers have achieved excess noise factors less than twice the shot noise. We have also shown that ultra low noise can be achieved with an Impact-Ionization-Engineered approach that utilizes heterojunctions to incorporate adjacent regions with low and high ionization rates.