Invited Paper
Geiger-mode avalanche photodiode focal plane arrays for
three-dimensional imaging LADAR
Mark A. Itzler*, Mark Entwistle, Mark Owens, Ketan Patel, Xudong Jiang, Krystyna Slomkowski,
and Sabbir Rangwala
Princeton Lightwave Inc., 2555 US Route 130 South, Cranbury, NJ 08512
Peter F. Zalud, Tom Senko, John Tower, and Joseph Ferraro
Sarnoff Corp., 201 Washington Road, Princeton, NJ 08543
ABSTRACT
We report on the development of focal plane arrays (FPAs) employing two-dimensional arrays of InGaAsP-based
Geiger-mode avalanche photodiodes (GmAPDs). These FPAs incorporate InP/InGaAs(P) Geiger-mode avalanche
photodiodes (GmAPDs) to create pixels that detect single photons at shortwave infrared wavelengths with high
efficiency and low dark count rates. GmAPD arrays are hybridized to CMOS read-out integrated circuits (ROICs) that
enable independent laser radar (LADAR) time-of-flight measurements for each pixel, providing three-dimensional
image data at frame rates approaching 200 kHz. Microlens arrays are used to maintain high fill factor of greater than
70%. We present full-array performance maps for two different types of sensors optimized for operation at 1.06 ȝm
and 1.55 ȝm, respectively. For the 1.06 ȝm FPAs, overall photon detection efficiency of >40% is achieved at <20 kHz
dark count rates with modest cooling to ~250 K using integrated thermoelectric coolers. We also describe the first
evalution of these FPAs when multi-photon pulses are incident on single pixels. The effective detection efficiency for
multi-photon pulses shows excellent agreement with predictions based on Poisson statistics. We also characterize the
crosstalk as a function of pulse mean photon number. Relative to the intrinsic crosstalk contribution from hot carrier
luminescence that occurs during avalanche current flows resulting from single incident photons, we find a modest rise
in crosstalk for multi-photon incident pulses that can be accurately explained by direct optical scattering.
Keywords: avalanche photodiode, single photon detector, photon counting, Geiger-mode, ladar, three-dimensional
imaging, InP, InGaAsP
1. INTRODUCTION
Through the use of laser radar (LADAR) to obtain high-resolution range measurements at each pixel of an imager, it is
possible to obtain image data in three spatial dimensions that provides information about imaged scenes and objects
which is superior to that provided by traditional two-dimensional intensity images. Whereas intensity images require
that object shapes be inferred from edge examination and complex image processing algorithms that generally require
assumptions that are not implicit in the image data, three-dimensional data removes the ambiguity of edge determination
and provides far more definitive information concerning imaged objects. Such three-dimensional imaging techniques
[1,2] enable the acquisition of data that is generally not obtainable using two-dimensional imaging technology,
including the imaging of objects behind camouflage netting, tree foliage, or other foreground obscurants.
The use of arrays with pixels consisting of Geiger-mode avalanche diodes (GmAPDs) provides several benefits for
LADAR three-dimensional imaging applications. [3] These detectors have single photon sensitivity and thus provide
high efficiency detection for highly attenuated return signals. With single photon sensitivity at the imager pixels, the
pulse energy requirements for launched LADAR pulses are greatly reduced (on the order of 100X) and lead to much
more deployable and reliable system solutions. Moreover, the the Geiger mode detection process is inherently digital:
in these detectors, a single photon induces a macroscopic pulse of current that can be readily sensed with appropriate
threshold detection circuitry. Given the digital nature of this detection mechanism, the detection process is noiseless;
*mitzler@princetonlightwave.com; tel: 1.609.495.2551; www.princetonlightwave.com
This work was sponsored by the Defense Advanced Research Projects Agency under contract HR0011-08-C-0021. Approved for
Public Release, Distribution Unlimited. The views, opinions, and/or findings contained in this article are those of the author and
should not be interpreted as representing the official views or policies, either expressed or implied, of the Defense Advanced
Research Projects Agency or the Department of Defense.
Infrared Remote Sensing and Instrumentation XVIII, edited by Marija Strojnik, Gonzalo Paez,
Proc. of SPIE Vol. 7808, 78080C · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.861600
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the only FPA-level noise is from the shot noise associated with false counts resulting from background or dark count
mechanisms. This imager architecture is also realized with efficient “photons-to-bits” digital pixel circuitry that
provides excellent prospects for scaling to smaller pitch and larger formats. A constraint of current implementations of
GmAPDs in 3-D imaging FPAs is the limitation to 1 bit of intensity information per pixel per frameʊany given pixel
can count no more than one single photon per frame. Although this prevents the collection of scene image intensity
information with a single pulsed laser “flash”, intensity information can be readily obtained by collecting multiple
frames. This multi-frame operation is aided by the very high frames rates that we have designed for our FPAs.
In this paper, we describe focal plane arrays (FPAs) for 3-D imaging systems with single photon sensitivity employing
two-dimensional arrays of InGaAsP-based Geiger-mode avalanche photodiodes (GmAPDs). In Section 2, we describe
the FPA design, in which GmAPD arrays are hybridized to CMOS read-out integrated circuits (ROICs) that enable
independent laser radar time-of-flight measurements for each pixel, providing three-dimensional image data at frame
rates approaching 200 kHz. Other module design elements include a microlens array (MLA) used to maintain high fill
factor of greater than 70%, a thermoelectric cooler to establish chip operating temperatures of 240 K, and hermetic
packaging to provide a high reliability for use in harsh environments. In Section 3, we present a summary of
fundamental FPA performance parameters. This summary includes full-FPA performance maps for dark count rate
(DCR) and photon detection efficiency (PDE) for two different types of sensors optimized for operation at 1.06 ȝm and
1.5 ȝm, respectively. DCR distributions are also shown for both types of FPAs. For the 1.06 ȝm FPAs, the dependence
of DCR on PDE illustrates that PDE >40% is achieved at <20 kHz dark count rates, even with all optical losses
associated with MLA fill factor and other transmission losses. In Section 4, we describe the performance of the FPAs
when multi-photon pulses are incident on single pixels. The effective PDE for multi-photon pulses shows excellent
agreement with predictions based on Poisson statistics. We also characterize the pixel-to-pixel crosstalk as a function of
the average photon number in incident pulses. Relative to the intrinsic crosstalk contribution from hot carrier
luminescence that occurs during avalanche current flows resulting from single incident photons, we find a modest rise
in crosstalk for multi-photon incident pulses that can be accurately explained by direct optical scattering. Finally, in
Section 5, we summarize this work.
2. FOCAL PLANE ARRAY DESIGN CONCEPT
The GmAPD FPA sensors described in this paper consist of a number of essential components. Single photon detection
is enabled by a photodiode array (PDA) with GmAPD detectors in each pixel. A CMOS readout integrated circuit
(ROIC) provides pixel-level electrical interfacing, such as GmAPD biasing and active quenching circuitry; in-pixel
counters for time-of-flight timestamp data; and the control of FPA-level functions such as clock generation and
distribution and frame readout. A GaP microlens array (MLA) is aligned and attached to the back-illuminated PDA to
ensure high fill factor, and various packaging sub-components are used to define electrical, mechanical, and optical
interfaces to the overall sensor module. To enable flexible, turnkey operation of the module using a standard personal
computer, we have also designed an FPGA-based evaluation interface board. In this section, we discuss some details of
the PDA, the ROIC, the overall module construction, and the interface board.
2.1 Geiger-mode avalanche photodiode (GmAPD) array
When an avalanche photodetector is biased above its breakdown voltage Vb, the creation of a single electrical carrier
can induce a run-away avalanche that gives rise to a detectable macroscopic current. In this mode of operation, often
referred to as Geiger mode, the detector is sensitive to the absorption of a single photon. (For this reason, these
detectors are also often referred to as single photon avalanche diodes, or SPADs.) We have developed InGaAs/InP
avalanche diode structures specifically for single photon detection in the wavelength range of 0.92 to 1.67 ȝm [4], and
we have optimized this structure for shorter wavelength operation 1.06 ȝm by employing a quaternary InGaAsP
absorber in place of the longer wavelength ternary InGaAs absorber. [5] We have described in detail the design,
simulation, and characterization of discrete GmAPD devices employing both InGaAs and InGaAsP absorption regions
in previous publications. [4 – 9]
A schematic depiction of the GmAPD device structure is illustrated in Figure 1. Photon absorption occurs in either a
ternary InGaAs layer (Eg ~ 0.75 eV) or a quaternary InGaAsP layer (Eg ~ 1.03 eV) for optimized detection of single
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photons at either ~1.55 ȝm or 1.06 ȝm, respectively. The absorption layer is spatially separated from a wider bandgap
InP region (Eg ~ 1.35 eV) in which avalanche multiplication occurs. A primary goal of the design ʊ the separate
absorption and multiplication (SAM) region structure [10] ʊ is to maintain low electric field in the narrower bandgap
absorber (to avoid dark carriers due to tunneling) while maintaining sufficiently high electric field in the multiplication
region (so that impact ionization leads to significant avalanche multiplication). The creation of a single electron-hole
pair by photoexcitation in the absorber layer results in the injection of the hole into the high-field InP multiplication
region in which impact ionization results in avalanche gain. With the GmAPD biased above Vb in its armed state, the
resulting avalanche gives rise to a macroscopic current pulse that is sufficiently large to be sensed by a threshold
detection circuit contained in the readout integrated circuit, described below. Unlike linear mode APDs operated below
Vb, the GmAPD detection process is inherently digital, and with appropriately designed detectors and threshold circuits,
the detection process is noiseless.
100 ȝm pitch
GmAPD Photodiode Array
N-type InP substrate
InGaAs(P) absorber
multiplier
InP
diffused region
Indium
Bump
Etched
trench
Readout integrated circuit (ROIC)
Figure 1. Schematic representation of InP-based planar-geometry diffused-junction GmAPD photodiode array hybridized
to a CMOS readout integrated circuit by indium bump flip-chip bonding. The composition of the absorber layer
determines the spectral response of the detectors, with InGaAsP used for 1.06 ȝm operation and InGaAs used for 1.55 ȝm
operation. Avalanche multiplication takes place in an InP multiplier between the diffused region and the absorber.
The lateral structure of our design employs a buried p-n junction to guarantee edge breakdown suppression, low
perimeter leakage, and high reliability. The active area of this planar geometry device [11] is determined by the
patterning of a SiN dielectric passivation layer to create a diffusion mask for a subsequent diffusion of Zn dopant atoms
to create a p+-InP region within the i-InP cap layer. To suppress electric field enhancement at the edge of this planar
structure, we use two diffusions to tailor the p-n junction profile [12] so that the junction is deeper in the central part of
the active area than it is in the junction periphery. This design ensures that the gain profile across the center part of the
active region is uniform and that the gain is reduced in the peripheral region of the device. For the 32 x 32 format
arrays described in this paper, the optical active region diameter was 34 ȝm in each 100-ȝm pitch pixel.
In the fabrication of arrays of GmAPD devices, a key consideration that does not play a role in discrete device
performance is that of optical crosstalk. An avalanche of electrical carriers in one pixel gives rise to hot carrier
luminescence, and even if the number of emitted photons is small (previous studies indicate 1 emitted photon for every
~105 to 106 avalanche carriers), the single photon sensitivity of neighboring pixels makes them susceptible to correlated
crosstalk counts triggered by these luminescence photons. To reduce the line-of-sight coupling between nearestneighbor pixels, we have etched isolation trenches along the pixel boundaries, as shown schematically in Figure 1. The
GmAPD array fabrication also includes back-side anti-reflection-coated apertures aligned to the pixel active regions and
front-side patterning of indium bumps to facilitate hybridization of the detectors to mating 32 x 32 CMOS ROICs; see
Figure 1.
In addition to our work on planar-geometry GmAPD FPAs described here and in other papers [13,14], arrays of related
mesa-geometry InP-based GmAPD have been extensively studied in pioneering work by researchers at MIT-Lincoln
Laboratory [15 –17], and significant additional work on mesa-geometry devices has also been recently reported. [18,19]
These devices are based on an epitaxial structure similar to that used for our planar geometry devices, but instead of
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using dopant diffusions to define the active region p-n junctions, mesa-geometry devices have active areas defined by
the etching of mesas in which all epi-grown material is removed outside the intended active region.
2.2 CMOS Readout integrated circuit (ROIC) functionality
The basic functions of our 3-D imaging FPA are performed by a custom CMOS ROIC mated to the GmAPD PDA by
indium bump hybridization. In the disarmed state, every pixel of the detector array is biased slightly below the
breakdown voltage of the GmAPD using a single external low-noise voltage supply with voltages on the order of 70 to
80 V. Each imaging frame begins with the arming of all of the detector pixels by the ROIC, which applies an excess
bias of up to 5 V. The period during which the pixels remained armed is the range gate, which is typically on the order
of a few microseconds. In a LADAR imaging system, the beginning of the range gate is synchronized (perhaps up to a
fixed delay) with the launch of an optical pulse from which reflected photons will be detected. Every pixel contains a
pseudorandom counter that provides detection timing information on a per-pixel basis. Upon asserting the master clock
enable (MCE) signal, all pixel counters begin counting. Within each ROIC pixel, there is a threshold detection circuit
that is triggered when an avalanche event occurs in the corresponding armed pixel of the photodiode array. When a
detection occurs, an active quenching circuit removes the excess bias from the fired GmAPD pixel to disarm it, and the
in-pixel counter is stopped so that the time of detection within the range gate is recorded. At the end of each range gate,
pixels that do not sense an avalanche event record the terminal counter value, indicating that no event has occurred at
that pixel. The frame readout then consists of scanning out all of the pixel counter values. This FPA architecture
provides range resolution corresponding to the timing resolution of the pixel counters, whereas the intensity resolution
is one bit per frame since each pixel can return only a single time-stamp value per frame. Intensity information is
obtained by accumulating multiple frames.
The ROIC incorporates a phase lock loop (PLL) circuit to generate highly stable clock signals for all timing operations.
An overall 13-bit timing resolution is obtained using 11-bit pseudorandom counters with two additional vernier bits
created by using a copy of the clock with a 90 degree phase shift. [3] At the end of each range gate, frame data is
scanned out using dedicated I/O ports for each of the 32 rows in the array, and high-speed scan circuitry achieves the
read-out of the 32 pixels in each row in 3.36 ȝs. With a typical range gate duration of 2 ȝs, the full frame duration of
5.36 ȝs corresponds to a very high frame rate of 186 kHz. The range gate duration is user-selectable and can have any
value between 1 ns and 2 ȝs. For very short (i.e., ns scale) range gates, the frame rate is determined entirely by the 3.36
ȝs readout and is on the order of 300 kHz. In addition to the internally supplied clock, the FPA can also be run with an
external clock input if desired for system synchronization.
The design strategy of the pixel-level circuitry for this ROIC emphasized forward compatibility with a next generation
of higher resolution 50 ȝm pitch 32 x 128 FPAs currently under development. Scaling considerations dictated the use
of a 0.18 ȝm CMOS technology as well as the use of the very space-efficient pseudorandom counter in each pixel. The
ROIC design also targeted minimal power dissipation, and for conventional 3-D imaging frame rates on the order of 20
kHz, the ROIC dissipates less than 40 mW. Even for frame rates as high as 200 kHz, the power dissipation is only ~320
mW. The power dissipation of the entire FPA module is therefore dominated by the dissipation of the integrated twostage thermoelectric cooler used to establish a temperature differential ǻT between the ambient case temperature and
the operating temperature of the hybridized chip stack. The thermal design of the FPA will support ǻT up to 55˚C, and
to operate the FPA with the PDA chip at -35˚C, the FPA module dissipates about 5 W.
2.3 Module design summary
Following the hybridization of the GmAPD photodiode array (PDA) to the CMOS ROIC (see Figure 1), we align and
attach a GaP microlens array (MLA) to the exposed back surface of the PDA. Based on the characterization of FPA
pixel-level photon detection efficiency before and after MLA attachment, we estimate that the effective fill factor for
broad illumination of the FPA is ~75%. (Note that the fill factor in the absence of the MLA is given by the ratio of the
34 ȝm diameter GmAPD active region to the 100 ȝm pitch pixel area, or ~9%.) Once the MLA has been attached to the
PDA, the resulting chip stack of ROIC+PDA+MLA is attached to a ceramic interposer, which provides electrical
routing of signals from the ROIC to the package interconnection pins. The interposer is then placed on a two-stage
thermoelectric cooler (TEC), which has been previously mounted to the ceramic housing. A schematic illustration of
this FPA architecture is given in Figure 2(a). The solid body cut-away diagram in Figure 2(b) shows a scale model of
the module, including additional elements such as a CuW heat sink, the housing pin grid array with 175 pins for
electrical connections, and a hermetically sealed lid with an anti-reflection coated sapphire window. As with the ROIC
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design, the design
d
of this housing
h
incorpporated elemennts to ensure foorward compattibility with fuuture FPA geneerations,
including sufficient pins fo
or ROICs with a larger numbeer of I/O conneections.
Lid
(a)
MLA
(b))
GmAPD
D
PDA
ROIC
Ceramic
c interposer
TEC
Housing
Figure 2. (a) Notional co
oncept for GmAP
PD focal plane array,
a
consistingg of the hybridizzed chip stack with
w readout inteegrated
ROIC), Geiger-m
mode APD photoodiode array (PD
DA), and microllens array (MLA
A) attached to a ceramic interposser for
circuit (R
electricall signal routing and cooled by a thermoelectric cooler (TEC). (b)
( Solid body model
m
of FPA prototype
p
module with
cut-awayy showing, from
m bottom to top, CuW heatsink (gold),
(
ceramic package (gray),, TEC (white), interposer (light blue),
hybridizeed ROIC+PDA+
+MLA chip stackk (dark blue), annd package winddow (clear).
200 MB
B/s digital
I/O port
Packaaged FPA
FPGA controller
c
Figure 3. Photograph
P
of ev
valuation interfaace board providding full control to
t the GmAPD focal
f
plane arrayy (FPA) via a Naational
Instrumeents card in a perrsonal computerr. Firmware in the FPGA providdes on-board higgh-speed controll of the FPA, andd high
data throughput is manag
ged using a 200 MB/s digital I/O
O port. Additionnal electrical inteerfaces provide low-noise high voltage
v
a
control off the thermoelecttric cooler, and auxiliary
a
signals for the FPGA annd ROIC.
dc bias foor the GmAPD array,
2.4 Evaluattion interface board
To provide for
f flexible, tu
urn-key operatiion of the FPA
A with camera--level functionality, we have designed an interface
i
board (see Figure
F
3) to co
ontrol all featurres of the FPA
A and manage interfaces witth support instrrumentation. Primary
interface boaard functions are
a managed by
b an Altera Cyclone
C
III FPG
GA controller and include: FPA clock genneration
and scaling; data de-convo
olution, bufferinng, and high-sppeed output to a digital I/O card;
c
programm
mable timing annd pulse
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width of FPA control signals; device configuration registers; and laser trigger output. This board manages real-time
data retrieval from the FPA, and the FPGA executes all required buffering and formatting of the pseudorandom counter
timestamp data obtained from the sensor. The board also has simple electrical connections for support instrumentation
(specifically, a high-voltage power supply to provide the APD bias voltage and a standard TEC controller). The board
is operated by a comprehensive graphical user interface on a personal computer (PC), and communication between the
board and the computer is via National Instruments digital I/O cards. The high frame rate capability of the FPA is
supported by a 200 MB/s digital I/O port to establish high-throughput data transfer from the FPGA to a PC.
3. FUNDAMENTAL PERFORMANCE PARAMETER SUMMARY
FPA sensors were characterized in a dark enclosure via computer control using the interface board just described in subsection 2.4. Fixturing within the enclosure provided for array illumination at either 1.06 ȝm or 1.55 ȝm. Photon
detection efficiency (PDE) measurements could be obtained with optical pulses focused to a single pixel or using broad
illumination that uniformly filled an entire 32 x 32 array. Single-pixel measurements were facilitated by an automated
three-axis translation system used to scan the focused output of an optical fiber (8 ȝm at the 1/e2 points) with
translations as small as 1 ȝm. For array-level PDE characterization under broad illumination, the fiber source was
backed away from the FPA and collimated so that the optical intensity variation across the array was no more than ±5%.
Dark count rate (DCR) measurements were obtained with no illumination.
Measurements reported in this section were taken with the FPA case temperature at ~293 K and the GmAPD PDA
stabilized at temperatures close to 250 K using the two-stage TEC inside the hermetic FPA module. Data presented
below was obtained as a function of excess bias voltage for values up to 4.0 V. Illuminated measurements were made
using pulsed lasers with a pulse width of ~150 – 200 ps. The optical pulse timing could be set to have pulse arrivals at
any time within the 2 ȝs range gate, and most data was acquired with pulse arrivals near the midpoint of the range gate
(i.e., for pixel counter timestamps corresponding to ~ 1 ȝs). For single pixel measurements, the pulse intensity was
typically attenuated to a mean photon number of Ș = 0.1 so that the Poisson probability of there being two photons in
any given pulse was less than 1%. For some measurements, Ș = 1 was used to increase the data acquisition rate, in
which case the resulting illuminated count measurements were corrected by assuming Poisson statistics to calculate the
intrinsic PDE. To reduce statistical uncertainty in DCR and PDE measurements, the data presented below are based on
the collection of between 10,000 and 25,000 frames of data.
3.1 Dark count rate (DCR) and photon detection efficiency (PDE) characterization
The most important characteristic of the GmAPD FPA is the fundamental tradeoff between DCR and PDE. Higher
PDE can be achieved by operating the GmAPD array pixels at higher excess bias voltage, but only at the expense of
increased DCR. The optimal operating point for managing this trade-off depends on various factors dictated by the
specific imaging application. As an example, for operation with high background count rates (such as daytime
imaging), higher dark count rates will be tolerable as long as they do not exceed the background count rate. In this case,
FPA operation with higher PDE will be desirable since the accompanying increase in DCR will not negatively impact
image quality (i.e., as long as it remains background-limited). Conversely, in dark conditions with no background,
DCR will dominate the noise performance of the FPA, and a different operating point for PDE may be optimal.
3.1.1 DCR and PDE performance for InGaAsP/InP FPAs at 1.06 ȝm
To characterize the DCR and PDE performance of our GmAPD FPAs, we record array-level data to create performance
maps of all 1024 pixels of the 32 x 32 array. The map in Figure 4(a) illustrates DCR performance for a FPA designed
for detection at 1.06 ȝm, with the quantitative value of DCR for each pixel indicated in kHz. The map in Figure 4(b)
illustrates accompanying PDE performance for the same FPA, with quantitative values for each pixel indicated in
percentage (%). Data were obtained at an operating temperature of 253 K and an excess bias voltage of 3.5 V. These
data demonstrate perfect yield, with 100% hybridization (i.e., all connections between PDA and ROIC are present) and
100% pixel operability (i.e., all pixels within target specifications for DCR and PDE performance).
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(b)
(a)
Figure 4. Performance maps of all 1024 pixels of a 32 x 32 InGaAsP/InP (1.06 ȝm) GmAPD FPA operating with an excess
bias of 3.5 V at 253 K. (a) Dark count rate (DCR) in kHz for all pixels. All pixels are < 20 kHz; there are no high DCR
pixels in this FPA. (b) Photon detection efficiency (PDE) in % for all pixels, where the average pixel PDE of 39%
includes all optical losses related to the microlens array and other sources of insertion loss.
100
400
(b)
T = 253 K
(a)
300
10
250
DCR (kHz)
Number of Pixels
350
Average PDE = 39%
T = 253 K
200
150
pixel (4,28)
pixel (20,20)
pixel (12,12)
pixel (4,4)
pixel (18,18)
pixel (14,14)
pixel (22,24)
pixel (16,16)
pixel (28,28)
1
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More
0.1
0
Pixel Dark Count Rate (kHz)
0.1
0.2
0.3
Effective PDE
0.4
0.5
Figure 5. (a) Histogram of pixel dark count rates for the DCR map data presented in Figure 4 for InGaAsP/InP FPA at 1.06
ȝm. This FPA has no leaky or open pixels. The DCR distribution at 253 K has an average of 13.6 kHz with a standard
deviation of 2.2 kHz. Data were obtained at an excess bias of 3.5 V, which corresponds to an average PDE of 39%.
(b) Dependence of DCR on the effective PDE for a random sample of pixels from the performance maps presented in
Figure 4. Regardless of position on the FPA, pixels show very consistent DCR vs. PDE behavior.
The DCR distribution for the entire array has an average of 13.6 kHz and a standard deviation of 4.2 kHz. Additionally,
all 1024 pixels in the array exhibit a DCR of less than 20 kHz for the specified operation conditions. Details of the
DCR distribution are illustrated by the histogram in Figure 5(a). The average PDE across the entire array is 39%, and
the standard deviation of the distribution of PDE values is 6.3%. From the maps in Figure 4, it is apparent that variation
in DCR and PDE across the array is quite systematic, with very little random fluctuation. This suggests that non-
Proc. of SPIE Vol. 7808 78080C-7
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uniformities are the result of systematic variation in device properties, with the most important factor being the variation
in APD breakdown voltage Vb. Since a single applied voltage Va is supplied to the entire array, the excess bias
Vex = Va – Vb will vary among different pixels if they have different values of Vb. A smaller Vex will result in smaller
values for both DCR and PDE, and we see that the two variables tend to trend together at different locations across the
array. Other sources of performance variation are also possible and may affect DCR and PDE differently. For instance,
the rotational misalignment of the microlens array lenses relative to the PDA active areas can induce gradients in the
PDE performance but has no impact on the DCR values. We note that the non-uniformity in PDE giving can be easily
corrected using non-uniformity correction factors. However, in many applications, the FPA is scanned within the scene
being imaged, and the scanning tends to remove the impact of PDE non-uniformity without additional correction.
As described above, the relationship between DCR and PDE presents the most fundamental tradeoff in GmAPD
operation. To illustrate this relationship in more detail, we have plotted in Figure 5(b) the dependence of DCR on PDE
for a random selection of pixels from the FPA maps in Figure 4. Although there is some variation among these pixels,
at any given value of PDE, they all exhibit DCR values consistent to within a factor of ~2. Each point on a DCR vs.
PDE curve is obtained by measuring these two parameters at a single value of the excess bias. The collection of DCR
vs. PDE curves in Figure 5(b) shows even more consistent behavior than is apparent in the full-array performance maps
in Figure 4 because the mapping data includes the additional variation imposed by spatial non-uniformities in Vb (and
therefore Vex) across the array. For the DCR vs. PDE curves, since both parameters are measured at the same Vex for a
given pixel, array-level variation in Vex is not a factor.
3.1.2 DCR and PDE performance for InGaAs/InP FPAs at 1.55 ȝm
In addition to FPAs intended for use with 1.06 ȝm laser sources, we have also designed and fabricated very similar
FPAs for use at 1.55 ȝm. Three-dimensional imaging at this wavelength is preferred for a variety of applications in
which the relative eye-safety of the pulsed laser source is of concern as well as cases in which greater covertness is
provided by the use of a wavelength that is beyond the spectral sensitivity of many imagers in use today (e.g., visible
and near-infrared). Because the longer wavelength sensitivity of these FPAs requires a smaller bandgap absorber (i.e.,
InGaAs), these devices are more prone to thermal carrier excitation and thus have higher DCR for at any fixed set of
operating conditions (e.g., excess bias and operating temperature) relative to the 1.06 ȝm FPAs. Aside from the
difference in GmAPD PDAs, all other module elements (ROIC, MLA, packaging sub-components, etc.) used to
fabricate FPAs for use at 1.55 ȝm are identical to those used for the 1.06 ȝm FPAs.
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(b)
Figure 6. Performance maps of all 1024 pixels of a 32 x 32 InGaAs/InP (1.55 ȝm) GmAPD FPA operating with an excess
bias of 3.25 V at 253 K. (a) Dark count rate (DCR) in kHz for all pixels. All pixels are < 50 kHz; there are no high
DCR pixels in this FPA. (b) Photon detection efficiency (PDE) in % for all pixels, where the average pixel PDE of
22% includes all optical losses related to the microlens array and other sources of insertion loss.
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32
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As seen in the performance maps in Figure 6, we have established excellent performance for these longer wavelength
FPAs. Just as in the case of the 1.06 ȝm FPAs, these maps demonstrate perfect yield, with 100% hybridization and
100% pixel operability. The DCR map in Figure 6(a) shows that all 1024 pixels have DCR less than 50 kHz, and the
average DCR is 28 kHz with a standard deviation of 6.5 kHz. The detailed DCR distribution is illustrated by the
histogram in Figure 7(a). For the PDE performance map in Figure 6(b), the mean PDE is 22.2%, with a standard
deviation of 4.6%. The array-level non-uniformity in DCR and PDE for these FPAs is comparable to that found for the
1.06 ȝm FPAs; somewhat lower values of DCR and PDE near the edges of the FPA are explained by process-related
variations in Vb, as opposed to the wafer-level variability in Vb described above caused by epi-growth parameter
gradients.
In Figure 7(b), we have plotted the dependence of DCR on PDE for a random selection of pixels from the 1.55 ȝm FPA
maps in Figure 6. This characteristic is similar to that shown in Figure 5(b) for the 1.06 ȝm FPA, with the primary
difference being that the longer wavelength pixels show DCR to be larger by about a factor of 5X.
300
(a)
200
(b)
T = 253 K
100
DCR (kHz)
Number of Pixels
250
1000
Avg PDE = 22%
T = 253 K
150
100
pixel (4,28)
pixel (4,4)
pixel (28,28)
pixel (12,12)
pixel (20,20)
pixel (18,18)
pixel (22,24)
pixel (14,14)
pixel (16,16)
10
50
0
More
Pixel Dark Count Rate (kHz)
60
55
50
45
40
35
30
25
20
15
5
10
1
0
0.1
0.2
0.3
Effective PDE
0.4
0.5
Figure 7. (a) Histogram of pixel dark count rates for the DCR map data presented in Figure 6 for InGaAs/InP FPA at 1.55
ȝm. This FPA has no leaky or open pixels. The DCR distribution at 253 K has an average of 28 kHz with a standard
deviation of 6.5 kHz. Data were obtained at an excess bias of 3.25 V, which corresponds to an average PDE of 22%.
(b) Dependence of DCR on the effective PDE for a random sample of pixels from the performance maps presented in
Figure 6. Regardless of position on the FPA, pixels show consistent DCR vs. PDE behavior.
3.2 Summary of crosstalk performance
Another GmAPD FPA performance parameter that is generally of considerable importance to 3-D imaging applications
is crosstalk. The detection of single photons in GmAPDs involves macroscopic current flows that can cause optical
crosstalk due to hot carrier luminescence, in which the acceleration of charge in a high-field avalanche region gives rise
to photon emission at the rate of one photon per 105 – 106 carriers that flow through the avalanche region. Because all
pixels of the FPA are sensitive to single photons, the coupling of emitted photons to neighboring active areas can cause
correlated spurious dark counts at these neighbors that are defined as crosstalk events. With reference to Figure 1,
luminescent photon emission can couple from an avalanching pixel to a nearest neighbor pixel by direct “line-of-sight”
paths or to more distant neighbors by back-reflection from the rear surface of the GmAPD chip. Etched trenches
between nearest neighbor pixels are fabricated to reduce the probability for direct line-of-sight coupling.
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Crosstalk characterization can be obtained from wafer-level frame data for avalanches of any origin, so dark or lit
measurements can be used. Spatial correlations between avalanches are readily determined from pixel positions, and
temporal correlations are provided by the pixel-level timestamp data. To obtain crosstalk data from DCR
measurements, we store 1000 frames of DCR data from the entire 32 x 32 FPA and search each frame for “trigger”
events and any correlated neighboring events occuring with 10 ns of the “trigger”. From this analysis, we can compile
the probability of a crosstalk event as a function of the distance between the “trigger” pixel and the crosstalk pixel. The
highest probability exists for crosstalk events at neighboring pixels (i.e., nearest neighbors) and is just under 1% for an
operating point at which PDE ~ 35% for the full FPA. The data show a decrease in crosstalk with increasing interpixel
distance D that is roughly consistent with an expected 1/D2 rolloff, although there is non-monotonic behavior that
depends quite sensitively on the precise location of a particular crosstalk pixel. [13] The probability that more than one
crosstalk event occurs in response to a primary avalanche is ~2% when PDE ~ 35%; and the total integrated crosstalk
probabilityʊi.e., the probability that any crosstalk event takes place at any pixel in response to a primary avalancheʊis
under 15% when PDE ~ 35%. [14]
For avalanche events induced by intentional single photon illumination at a specific “trigger” pixel, we can apply the
same analysis as used for the DCR data, although we can restrict our search to a much narrower range of pixels
surrounding the illuminated pixel. To within measurement uncertainties, the resulting crosstalk for this “lit pixel”
scenario is identical to that found for the dark count data. [13] This is expected since there is no difference between
dark carrier-induced and photon-induced avalanches. More details pertaining to crosstalk performance will be
described in the next section with reference to multi-photon pulse illumination experiments.
4. FPA PERFORMANCE WITH MULTI-PHOTON PULSES
Although GmAPD FPAs provide the distinctive capability of having pixels sensitive to single photons, they can be
employed to great advantage in applications for which LADAR pulse returns may consist of multiple photons arriving
simultaneously at a given pixel. The principle performance improvement in such a scenario is much higher detection
efficiency for these pulses since any single photon in a multi-photon wave packet can trigger a detectable avalanche
event. With appropriate operating conditions, “pulse detection efficiency” (PuDE) can be well over 90% for multiphoton pulses with 10 or more photons, and even for photon numbers less than 10, PuDE values are markedly higher
than the intrinsic PDE for single photon detection. In this section, we analyze the expected behavior of GmAPD pixels
in response to multi-photon pulses and compare experimental measurements with model expectations. We also report
measurements of the FPA crosstalk with multi-photon pulse detection and explain these results using a theoretical
model that describes the quantitative impact on crosstalk of direct optical scattering.
4.1 Theoretical response to multi-photon pulses
To calculate the effective pulse detection efficiency (PuDE) for multi-photon pulses, we assume that multiple photons
arrive simultaneously and conform to Poisson statistics with optical pulses having a mean photon number ȝ. In this
case, the probability of any given pulse containing N photons is given by the Poisson distribution P(N;ȝ) = (ȝN e-ȝ)/N!.
We then consider that since the probability of detecting one photon is given by PDE, the complementary probability of
not detecting one photon is (1 – PDE). For an optical pulse containing N photons, the probability of not detecting any
of the N photons is (1 – PDE)N. Next, the complementary probability of detecting at least one of the N photons is
Pdet(N) = 1 – (1 – PDE)N. Finally, to find the pulse detection efficiency PuDE(ȝ) for pulses described by mean photon
number ȝ, we consider the sum over N for the product of P(N;ȝ) and Pdet(N):
—ሺߤሻ ൌ σே ሺܰǢ ߤሻ†‡– ሺܰሻ ൌ σே
ఓಿ ௘ షഋ
ேǨ
ሾͳ െ ሺͳ െ ሻே ሿ.
(1)
4.2 Measured multi-photon pulse detection efficiency
To experimentally determine PuDE for multi-photon pulses, we used the apparatus described earlier that allows us to
focus an incident optical pulse onto a single pixel with an optical fiber with appropriate focusing optics at the fiber
facet. With calibrated adjustable attenuation following a calibrated 1.06 ȝm optical source, we set the optical pulse
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intensity to any desired mean photon number ȝ. We then used the FPA to measure the detection probability for various
values of ȝ to determine PuDE(ȝ). The filled data points in Figure 8 show the measured PuDE for ȝ = 1, 2, 3, 4, 5, and
10 at three different excess bias values of Vex = 1, 2, and 3 V corresponding to DCR values of 0.6, 3, and 12 kHz,
respectively. The figure shows that PuDE increases dramatically for ȝ increasing from 1 to 10, with ~98% detection
probability for a 10-photon pulse at operating conditions that given 12 kHz DCR. With multi-photon pulses incident on
the detector, it is also possible to operate at much lower DCR values while maintaining reasonably high PuDE if
specific applications benefit from lower DCR.
The solid lines in Figure 8 indicate the model prediction described by Eq. (1) and show excellent agreement with the
experimentally measured data points. The value of PDE was determined by independent measurement of the pixel
being tested at a low ȝ value of 0.1.
100%
DCR ~ 12 kHz
Vex = 3.0 V
Effective Multi-photon PDE
90%
80%
DCR ~ 3 kHz
Vex = 2.0 V
70%
60%
DCR ~ 0.6 kHz
Vex = 1.0 V
50%
40%
30%
20%
10%
0%
0
1
2
3
4
5
6
7
8
Average Photons per Pulse
9
10
11
Figure 8. Effective pulse detection efficiency for multi-photon pulses as a function of the mean photon number ȝ per pulse
for three different excess bias voltages, with corresponding dark count rates indicated. Filled data points are
experimental measurements, and solid curves are from model calculations described in the text.
4.3 Measured crosstalk with multi-photon pulses
To further characterize the performance of our GmAPD FPAs in illumination by multi-photon pulses, we measured the
crosstalk as a function of mean photon number ȝ. Given our previous measurement of the dependence of PuDE on ȝ,
we then plotted the total probability for all crosstalk events as a function of the multi-photon pulse detection efficiency
PuDE. Each set of solid points of a specific shape and color corresponds to illumination with a particular value of ȝ and
shows the expected increase in total crosstalk and PuDE for increasing excess bias voltage. The solid lines are a
phenominological fit to these sets of points to better illustrate the trend in the data for each ȝ. The dashed lines indicate
the trend in the data for increasing ȝ at a fixed excess bias. The increase in multi-photon PuDE with increasing ȝ is
expected based on the results presented in Figure 8.
However, these data show that the total crosstalk probability also increases with increasing ȝ. This is an effect that can
not be related to hot carrier luminescence described above in sub-section 3.2 because the GmAPD bias circuitry dictates
the size of a detection avalanche. This avalanche magnitude will not be dependent on how many photons are absorbed
by the pixel, and so hot carrier emission and its consequent crosstalk effects can not depend on photon number ȝ. We
therefore consider the impact of optical scatteringʊe.g., from the seams of the microlens array in the FPAʊto
determine whether such scattering could quantitatively account for the observed increase in total crosstalk with
increasing ȝ.
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Since crosstalk events are defined by their correlation to an initial primary avalanche, crosstalk detection can only occur
if a detection event has occurred at a primary pixel. If a particular input optical pulse contains only one photon, it is not
possible to detect optical scattering. If the one photon triggers a primary avalanche, then it is annihilated and any
crosstalk must be from photon re-emission. If the one photon is scattered before it can trigger a primary avalanche, then
even if it is absorbed elsewhere, it will not be counted as a crosstalk event because there was no primary avalanche to
correlate with. So optical scattering can only be detected if a particular input pulse contains two or more photons.
(Note that this corresponds to the case of N • 2, which is still possible with finite probability for mean photon number
ȝ ” 2.) Additionally, even for multi-photon input pulses with N • 2, the total crosstalk probability Pxt will still contain a
contribution Pxt,0 from hot carrier luminescence.
To calculate the total crosstalk probability including the effects of optical scattering with multi-photon incident pulses,
we again assume Poisson statistics for the pulses with probability P(N;ȝ) of having N photons given a mean photon
number of ȝ. Of the N photons in any given pulse, we then assume that m photons are scattered with a 1-photon
scattering probability ȕ. To observe a crosstalk event, we must have a primary avalanche, which requires that we detect
one of the (N – m) non-scattered photons; this probability is 1 – (1 – PDE)N-m. At the same time, we must also detect
one of the m scattered photons at a neighboring pixel; this includes the likelihood that scattering occurs in the first place
as well as the likelihood that one of the m photons is then detected. This total probability is ȕm[1 – (1 – PDE)m]. The
total crosstalk probability is then Pxt,0 added to a term that sums over all possible photon numbers N for N • 2 with a
nested sum over all possible scattered photon numbers m of the product of probabilities describing the detection of a
primary avalanche and the detection of a scattered photon at a neighboring pixel.
25%
T= 248 K
4 ph/pulse
Total crosstalk probability
SN167 (16,16)
3 ph/pulse
20%
2 ph/pulse
1 ph/pulse
15%
Vex = 3.5
Vex = 3
10%
Vex = 2
5%
Vex = 1.5
Vex = 1
5 ph/pulse
10 ph/pulse
0%
0%
10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Effective Multi-photon PDE
Figure 9. Total crosstalk probability as a function of the effective multi-photon pulse detection efficiency for different
excess bias voltages and over a range of mean photon number ȝ per pulse (i.e., ȝ = 1, 2, 3, 4, 5, and 10). Filled data
points are experimental results, and the solid curves in phenomenological fits to indicate data trends for each mean
photon number ȝ. The dashed lines indicate trends across different values of ȝ for fixed excess bias voltages.
Using this model, for which additional details will be published in another paper, we find that we can accurately fit the
total crosstalk dependence on PuDE illustrated in Figure 9 with a single value of ȕ ~ 15% for the optical scattering
probability. Both the model and experimental measurements show that the total crosstalk probability for 10-photon
pulses is ~70% larger than it is for 1-photon pulses. Extracted values of the hot carrier luminescence contribution Pxt,0
for specific excess bias values (and consequent single photon PDE values) agree well with results for low mean photon
number measurements (i.e., ȝ = 0.1) [13]. The consistency of these results for the two free parameters ȕ and Pxt,0
suggest that optical scattering is in fact responsible for the observed increase in crosstalk with mean photon number for
multi-photon pulses. It is also interesting to compare the experimentally determined microlens array fill factor loss of
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~25 – 30% with the value of ~15% determined for the optical scattering probability ȕ. These results suggest that about
half of the photons lost due to the imperfect fill factor are being scattered to neighboring pixels at which they can be
detected as spurious counts. This finding warrants further consideration relative to the performance of GmAPD FPAs
even with single photon incident pulses. We believe that this optical scattering originates at the seams between
individual lenses in the microlens array where the lens geometry deviates from the intended aspheric design and can
misdirect incident photons to PDA active areas other than the active area corresponding to the illuminated microlens.
5. CONCLUSIONS
In this paper, we have described the performance of focal plane arrays based on Geiger-mode APDs with single photon
sensitivity that function as the sensor engine for 3-D LADAR imaging systems. Time-of-flight measurements are
captured on a per-pixel basis using a custom CMOS ROIC which also controls GmAPD avalanche quenching and reset
operations and can read out 3-D image data at frame rates as high as 186 kHz for 2 ȝs range gates. These 32 x 32 x 100
ȝm pitch arrays include GaP microlens arrays to provide high fill factors of ~75%. Robust hermetic packaging ensures
reliable operation in harsh environments, and FPGA-controlled interface boards have been developed to provide turnkey
camera-level electronic functionality.
We have designed two different versions of the basic FPA. One version employs a quaternary InGaAsP absorber in the
GmAPD photodiode array optimized for use in systems with 1.06 ȝm laser sources, which the other employs a ternary
InGaAs absorber for longer wavelength operation near 1.55 ȝm. For both versions of this FPA, we have demonstrated
perfect yield for FPAs, with 100% operable pixels and excellent FPA-level distributions of dark count rate and photon
detection efficiency. For the 1.06 ȝm FPA, the mean DCR was 14 kHz at 39% PDE and 250 K operation, with all
pixels having less than 20 kHz DCR. For the 1.55 ȝm FPA, the mean DCR was 28 kHz at 22% PDE and 250 K
operation, with all pixels having less than 50 kHz DCR.
We have also reported the first evaluation of GmAPD FPAs for the detection of multi-photon optical pulses with mean
photon numbers in range from ȝ = 1 to ȝ = 10. For 10-photon pulses, the pulse detection efficiency is ~98% under
operating conditions consistent with just 12 kHz DCR. We model the PuDE assuming Poisson statistics for the optical
pulses and find excellent agreement between the measured and calculated values for any photon number and excess bias
condition. We have also analyzed the dependence of the total crosstalk probability on photon number, and we have
found that we can accurately describe the increase in crosstalk with ȝ with a model based on direct optical scattering in
the presence of multi-photon pulses. Fitting of this model to our measured data yields an optical scattering probability
of ~15%, which accounts for approximately one-half of our measured fill factor loss. The model also indicates a
crosstalk contribution from photon emission due to hot carrier luminescence of ~10% at an operating PDE ~35%,
consistent with earlier crosstalk results focused on the characterization of these hot carrier emission effects.
ACKNOWLEDGMENTS
We are grateful to Dr. Monte Turner for his support throughout this work, for which we wish to acknowledge funding
from DARPA. We also greatly appreciate the collaborative efforts of many colleagues at MIT Lincoln Laboratory,
including Gary Smith, Alex McIntosh, Simon Verghese, Brian Aull, Brian Tyrrell, and Joe Funk.
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