ELECTRON- AND HOLE- AVALANCHE
HgCdTe PHOTODIODE ARRAYS FOR
ASTRONOMY
Donald N. B. Hall
Institute for Astronomy
University of Hawaii
OUTLINE
• WHY APDs?
• CONVENTIONAL APD’S e.g. Si, Ge & GaAs.
• WHY Hg:Cd:Te – the PERFECT INFRARED
(and VISIBLE) APD MATERIAL?
• e-APD and h-APD CHARACTERISTICS of
Hg:Cd:Te.
• STATUS of the NASA FUNDED
UH/GSFC/TELEDYNE Hg:Cd:Te APD
PROGRAM.
• UH TEST and CHARACTERIZATION.
• FUTURE DEVELOPMENTS.
WHY APDs?
• THE HAWAII-2RG ARRAYS DEVELOPED FOR
JAMES WEBB APPROACH THE IDEAL
DETECTOR IN ALL BUT ONE RESPECT –
READ NOISE!
• DUE TO BASIC PHYSICS OF CMOS, READ
NOISE HAS IMPROVED LITTLE SINCE
HUBBLE NICMOS – TECHNOLOGY LARGELY
FROZEN IN TIME FOR 20 YEARS.
• READ NOISE LIMITS LOW BACKGROUND
AND/OR HIGH SPEED APPLICATIONS
• Hg:Cd:Te APDs HOLD PROMISE OF THE
SOLUTION.
EXAMPLES
• HIGH SPEED – MODEST FORMAT, RELAXED
DARK CURRENT:
- Wave-front Sensing
- Fringe Tracking
• HIGH SENSITIVITY – LARGE FORMAT,
DEMANDING DARK CURRENT:
- High Resolution Spectroscopy
- Low Background Space
• BOTH – ALSO HIGH TIME RESOLUTION:
- Time Resolved Spectroscopy
- Quantum Astrophysics
CONVENTIONAL APDs e.g. Si, Ge & GaAs
• IN CONVENTIONAL APD MATERIALS (e.g. Si, Ge and
GaAs) BOTH ELECTRONS AND HOLES AVALANCHE
(IN OPPOSITE DIRECTIONS).
• THIS SPREADS THE STATISTICAL AVALANCHE
GAIN PRODUCING EXCESS NOISE.
• McINTYRE (1968) DEFINED THE EXCESS NOISE
FACTOR:
F = (S / B)
IN
/ (S / B)
OUT
• THE THEORETICAL LIMIT FOR “F” IN THE CASE
WHERE BOTH ELECTRONS AND HOLES
AVALANCHE IS 2 BUT IT IS OFTEN >>2.
• THIS DUAL AVALANCHING ALSO SIGNIFICANTLY
STRETCHES OUT RESPONSE TIME.
• BEST CONVENTIONAL APDs REACH F VALUES ~ 2
McINTYRE MODEL
• PHOTO-IONIZATION INITIATES
AVALANCHING BY BOTH ELECTRONS AND
HOLES.
• COLLISIONS FULLY REDISTRIBUTE BOTH
ELECTRONS AND HOLES BEFORE
REACHING IONIZING ENERGY.
• EXCESS NOISE AND PULSE BLURRING
INHERRENT IN PROCESS.
• RULES OUT “NOISELESS” (F = 1) PHOTON
COUNTING IN LINEAR MODE.
• PHOTON COUNTING ONLY IN GEIGER MODE
WITH LIMITED DUTY CYCLE, AFTER-PULSES
AND REQUIREMENT FOR QUENCHING.
Hg:Cd:Te AVALANCHE CHARACTERISTICS
• IT IS WELL KNOWN THAT BY VARYING THE
“x” FRACTION OF Hg(1-x):Cd(x):Te, THE
CUT-OFF WAVELENGTH λc CAN BE VARIED
OVER THE RANGE λc < 1.3 μm TO λc > 15 μm.
• OVER THIS RANGE THERE ARE ALSO
DRAMATIC CHANGES IN THE AVALANCHE
PROPERTIES OF THE CRYSTAL LATTICE.
• THE NEXT CHART SHOWS LOG10 GAIN vs
BAND-GAP (eV) FOR LAYERS FROM LETI,
BAE, TIS & DRS, ALL @ 77K & 6V REVERSE
BIAS
e- & h- APD REGIMES OF HgCdTe
Figure 5: The distinct e-APD and h-APD regimes of HgCdTe cross over at Eg ~
0.65 eV (λco ~ 1.9 μm). At lower band-gaps the e-APD gain increases exponentially
with decreasing bandgap - material for four manufacturers shows remarkably
consistent results. To higher bandgap the ratio k = αh / αe asymptotically
e-APD GAIN - SUMMARY
1E+3
235-G
ʎco = 4.54µm at 80K
Elements 32, 44, 85
Area = 250x250 µm2
F/5
T=80K
T=120K
Gain
1E+2
1E+1
T=160K
T=200K
1E+0
0
1
2
3
4
5
6
Voltage (V)
7
8
9
10
11
AVALANCH PROPERTIES of HgCdTe
• HOLE ACCELERATION IS VERY LOW –
HIGH EFFECTIVE MASS – SLOWER.
• e- ACCELERATION IS VERY HIGH PHONON SCATTERING LOW – VERY
FAST.
• HOLE IONIZATION IS VERY LOW
EXCEPT FOR 0.938 eV RESONANCE
• e- IONIZATION IS VERY HIGH
• THUS FOR EB < 0.6 eV (λC > 2 μm)
ONLY e- AVALANCHE (k = 0)
HgCdTe as an e-APD
• AVALANCHE GAIN INCREASES EXPONENTIALLY
WITH BIAS & DECREASING EB.
• e- TRAJECTORIES ARE BALLISTIC BETWEEN
IONIZING COLLISIONS.
• DETEMINISTIC SO NO EXCESS NOISE – F ~ 1.
• VERY FAST PULSE - GAIN BANDWIDTH > 1THZ.
• THERE IS NO GEIGER BREAKDOWN AND SO NO
GEIGER MODE OPERATION.
• HOWEVER NOISELESS (F ~ 1) PHOTON COUNTING
IS POSSIBLE IN THE LINEAR (PROPORTIONAL)
MODE TO GAIN ~ 104.
• FOR ASTRONOMY, THE PRIMARY CHALLENGE IS
TO REDUCE DARK CURRENT.
APDs in MBE HgCdTe
• DEPOSITION BY MBE ALLOWS A SEPARATE
ABSORPTION-MULTIPLICATION (SAM)
STRUCTURE.
• A-LAYER GRADED INTO M-LAYER
• TO AVOID PHOTOIONIZATION IN THE MLAYER, λC FOR THE A-LAYER MUST BE
LONGER THAN λC FOR THE M-LAYER.
• MISMATCH IN CRYSTAL LATTICE
PROPERTIES MAY LIMIT THE DIFFERENCE
BETWEEN THE TWO λCs.
BAND-GAP TRADE-OFF
0.25 eV (λc ~ 4.5 μm) vs 0.5 eV (2.6 μm)
• 0.25 eV M-LAYER HAS HIGH GAIN (>5,000 @
12.5 V) WITH MATURE PROCESSING
TECHNOLOGY.
• BUT VERY SUSCEPTIBLE TO THERMAL
BACKGROUND.
• 0.5 eV M-LAYER HAS MUCH LOWER GAIN
BUT OFFSET BY MUCH LOWER
BACKGROUND.
• 0.5 eV DARK CURRENT NOT
DRAMATICALLY LOWER DUE TO TRAP
INDUCED TUNNELING CURRENT.
• OPTIMUM M-LAYER BANDGAP?
J. ROTHMAN SUMMARY
EMPIRICAL MODEL for e-APD GAIN
• BECK (2001, 2002) DETERMINED THAT
THE e-APD GAIN M VARIES WITH V AS:
M = 2 (V – VTH)/(VTH/2)
• VTH ~ 6.8 Eg FOR ALL COMPOSITIONS:
0.2 < x < 0.5
• “DEAD VOLTAGE” MODEL OF e-APD
GAIN IN HgCdTe
• FIGURE FOR VTH = 5 Eg AND ά = 1
M KINCH_JEM_V37N9P1453_2008 page 1454 Fig. 2
M KINCH_JEM_V37N9P1453_2008 page 1454 Fig.1.(a)
M KINCH, EAPDs, page 122, Fig. 7.13
e-APD DEVELOPMENT
• DEFIR (Design and Future of the IR)
INITIATIVE BRINGS TOGETHER SOFRADIR’S
R&D WITH CEA-Leti.
• MCT e-APD RESEARCH TOWARD
INDUSTRIALIZATION.
• PASSIVE AMPLIFIED IMAGING (PAI) & 3-D
LADAR.
• DRS DALLAS (WITH SELEX) - PAI & 3-D
LADAR PLUS ASTRONOMY.
• RAYTHEON - PAI & 3-D LADAR (PLUS
ASTRONOMY?).
• BAE R&D.
• TIS – ASTRONOMY.
e-APDs by CEA LETI, DRS, BAE & TIS
Company
Process
Geometry
Use
CEA-LETI
LPE &
MBE
Plane
(Width)
DRS
MBE
Cylinder
MWIR PAI
1.5μm
LADAR
MWIR PAI
1.5μm
LADAR
BAE
LPE
Plane
MWIR PAI
TIS
MBE
Plane
PHOTON
COUNTING
e-APD ARCHITECTURE - DEFIR
caption
e-APD ARCHITECTURE - DSL
caption
THREE COMPLIMENTARY TIS APPROACHES
e-APD GAIN - SUMMARY
1E+3
235-G
ʎco = 4.54µm at 80K
Elements 32, 44, 85
Area = 250x250 µm2
F/5
T=80K
T=120K
Gain
1E+2
1E+1
T=160K
T=200K
1E+0
0
1
2
3
4
5
6
Voltage (V)
7
8
9
10
11
DEFIR F VALUES (J. ROTHMAN)
e-APD GAIN σ - DRS
caption
e-APD GAIN σ - DRS
caption
e-APD GAIN σ - DEFIR
caption
e-APD GAIN (CUM) - DEFIR
caption
e-APD GAIN vs TEMP - SUMMARY
e-APD GAIN vs TEMP - DEFIR
caption
e-APD GNDC - DEFIR
caption
e-APD GNDC vs TEMP - DEFIR
caption
e-APD PULSE PROFILE - DEFIR
caption
e-APD PULSE RISE TIME - DEFIR
caption
e-APD PULSE DECAY TIME - DEFIR
h-APD APPLICATIONS TO ASTRONOMY
• 0.938 eV (λc ~ 1.32 μm) M-LAYER
COMPATIBLE WITH A-LAYER INSENSITIVE
TO ROOM TEMPERATUREBACKGROUND.
• ATTRACTIVE FOR HST-LIKE MISSIONS &
GROUND BASED APPLICATIONS.
• SUBSTRATE REMOVAL FOR VISIBLE
APPLICATIONS.
• CHALLENGES ARE DARK CURRENT &
ACHIEVING F ~ 1.
• h-APD AVLANCHE PULSE ~ 10X SLOWER.
h-APD DEVELOPMENT
• RAYTHEON (RVS, HRL & RMS) HAS
DEMONSTRATED SWIR (1.55 μm) eAPD BASED LADAR OPERATING AT
300K.
• THEY REPORT NO EXCESS NOISE TO
GAINS >100, NEP < 1nW & GHZ
BANDWIDTH.
• CZT => 6” Si WAFER PROCESSING.
PERFORMANCE OF 90 RANDOMLY
SELECTED APDs - RAYTHEON
Jack et al, Proc of SPIE V6542, P65421A (2007)
GOALS OF THE UH/GSFC/TELEDYNE
Hg:Cd:Te APD PROGRAM
• THREE YEAR PROGRAM FUNDED
PRIMARILY BY NASA “RESEARCH
OPPORTUNITIES IN SPACE AND EARTH
SCIENCES” INITIATIVE - SUPPLEMENTAL
FUNDING BY GSFC.
• WILL UTILIZE TELEDYNE’S BROAD
EXPERIENCE IN MBE Hg:Cd:Te
PROCESSING TO PRODUCE APDs
OPTIMIZED FOR ASTRONOMY.
• UH WILL MODIFY TEST FACILITIES
DEVELOPED FOR THE JWST PROGRAM TO
CHARACTERIZE ARRAYS IN PHOTON
COUNTING MODE.
APPROACH
• SIMILAR MASKS FOR e-APD & h-APD
HgCdTe INCLUDE:
-
PROCESS EVALUATION CHIPS (PECs).
FOUR 256 x 256 @ 18 μm PITCH SUB-ARRAYS
TWO “TADPOLES”
• SCREEN AND INITIAL EVALUATION OF
LAYERS USING PECs.
• CHARACTERIZE PHOTON COUNTING WITH
SUB-ARRAYS BONDED TO CORNER OF
H1RG, READ OUT WITH SIDECAR ASIC.
• “TADPOLES” FOR HIGH SPEED (QUANTUM
ASTROPHYSICS AND LADAR).
• GOAL IS LOW DARK WITH F ~ 1.
CONCEPTUAL
“TADPOLE” LAYOUT
KSPEC MODIFICATIONS
Diodes in the
64um-500um
range aligned
along two parallel
lines
UH-TIS HAWAII Heritage
On-chip butting
Guide mode &
read/reset opt.
Reference pixels
Stitching
HAWAII - 1
HAWAII - 2
HAWAII - 1R
HAWAII - 1RG
HAWAII - 2RG
1994
1998
2000
2001
2002
1024 x 1024 pixels
7.5 million FETs
0.25 µm CMOS
18 µm pixel size
2048 x 2048 pixels
29 million FETs
0.25 µm CMOS
18 µm pixel size
WFC 3
1024 x 1024 pixels
3.4 million FETs
0.8 µm CMOS
18 µm pixel size
HAWAII-4RG-15
2048 x 2048 pixels
13 million FETs
0.8 µm CMOS
18 µm pixel size
HAWAII-4RG-10
2011 (proposed) 15µm pixels
4096 x 4096
110 million FETs
0.25 / 0.18 µm CMOS
15 µm pixel size
1024 x 1024 pixels
3.4 million FETs
0.5 µm CMOS
18 µm pixel size
2006
4096 x 4096
110 million FETs
0.25 µm CMOS
10 µm pixel size
Smaller pixels,
Improved
performance,
Scalable
resolution
SIDECAR ASIC
2003
Control chip
for H1RG,
H2RG and
H4RG-10/15
DARK CURRENT vs TEMPERATURE FOR 2.5
AND 5 UM MATERIAL
UH 2.5um, UH 5.0um, and STScI 5.0um Measurements
Dark Current Logarithmic
10.000
UH 2.5um
UH 5.0um
1.000
SCA Average Dark Current (e
-
/sec, pixel)
STScI 5.0um
0.100
0.010
0.001
25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135
Tem perature (K)
CURRENT STATUS
• FIRST RUN OF n-on-p e-APDs HAD POOR
DIODE CHARACTERISTICS.
• ATTRIBUTED TO PROBLEMS WITH
SURFACE PASSIVATION.
• IN 2009 CONDUCTED AN EXTENSIVE
INVESTIGATION OF SURFACE PASSIVATION.
• READY TO PROCEED WITH 2ND RUN.
• FIRST RUN OF p-on-n h-APDs UNDERWAY.
• TESTING IN NOVEMBER.
• EVALUATION OF h-APD GAIN of TIS
HERITAGE 0.73 eV (λco ~ 1.7 μm) p-on-n PEC
h-APD GAIN of TIS HERITAGE
0.73 eV (λco ~ 1.7 μm) p-on-n PEC
• STANDARD 0.73 eV (λco ~ 1.7 μm) pon-n PEC.
• NO APD OPTIMIZATION OR SAM – ALL
SAME MATERIAL.
• GAIN & BANDGAP CONSISTENT WITH
h-APD AVALANCHING.
• PLAN TO EVALUATE IN H1RG.
• PRESENT h-APD RUN CONSISTS OF
THIS MATERIAL FOR A-LAYER WITH
0.938 eV M-LAYER.
h-APD GAIN of TIS HERITAGE
0.73 eV (λco ~ 1.7 μm). p-on-n PEC
Figure 3: Measured gain vs. reverse bias voltage for TIS heritage 0.73 eV p-on-n
material (λco ~ 1.7 μm).
KSPEC UPGRADE - CURRENT STATUS
• COMPLETELY SEALED, ULTRA LOW
BACKGROUND TEST FACILITY.
• ILLUMINATION BY IR LEDs.
• REFERENCE DETECTORS.
• HIGH GEOMETRIC ATTENUATION TO < 1
PHOTON per PIXEL per FRAME READ
• FIBER FEED OPTION FOR LASER PULSE
MEASUREMENTS.
• UP TO H2-RG.
• < + 1 mK TEMPERATURE CONTROL OVER
30K to 200K RANGE.
KSPEC MODIFICATIONS
Sphere Assembly
Cryo ASIC
Detector Module
KSPEC X-SECTION
LEDS
APERATURE
ASIC
DETECTOR
PHOTON COUNTING WITH H1RG
• HYBRIDIZE 256 x
256 SUB-ARRAY TO
OUTPUTS 0 – 3 IN
CORNER OF H1-RG.
• SIDECAR ASIC
READS @ 10
Mpxl/SEC.
• 50 – 60 RMS e- CDS
READ NOISE.
• FRAME RATES:
SUBARRAY
#
PIXEL
FRA ME
μ-sec KHz
64 x 256
16,384
1,638 0.675
64 x 64
4,096
409.6
2.5
32 x 32
1,024
102.4
10
16 x 16
256
25.6
40
8x8
64
6.4
160
4x4
16
1.6
625
A LOOK INTO THE CRYSTAL BALL
• DISCRETE APDs FOR INTENSITY
INTERFEROMETRY, ADAPTIVE OPTICS
& FRINGE TRACKING IN 1 -2 YEARS.
• MODEST ARRAYS - H-1/4RG @ 10 KHz
FRAME RATE WITH ONE ASIC.
• H-2RG, H-4RG-15 FOR LOW BACKGROUND SPECTROSCOPY & SPACE.
• SPECIALIZED READOUTS – TIME
TAGGING PHOTONS.
CURRENT STATUS
• END
A
• B
e-APD GAIN - DEFIR
caption
e-APD GAIN - DSL
caption
e-APD GAIN – TIS 2004
e-APD GAIN - BAE
1E+3
235-G
ʎco = 4.54µm at 80K
Elements 32, 44, 85
Area = 250x250 µm2
F/5
T=80K
T=120K
Gain
1E+2
1E+1
T=160K
T=200K
1E+0
0
1
2
3
4
5
6
Voltage (V)
caption
7
8
9
10
11