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HgCdTe Avalanche Photodiode Arrays for Wavefront Sensing and Interferometry Applications
Ian Baker* and Gert Finger***SELEX Sensors and Airborne Systems Ltd, Southampton, UK**ESO, Garching, Germany
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Avalanche gain in HgCdTe
Avalanche photodiodes
• Voltage controlled gain at the point of absorption
• Almost no additional noise
• Near-zero power consumption
• Up to GHz bandwidth
• Requires no silicon real estate
HgCdTe – a unique material
• Electron/hole mass ratio very large – electron gets all the energy – single carrier cascade process gives low added noise
• The conduction band of HgCdTe devoid of any low-lying secondary minima, which allows for large electron energy excursions deep into the band, and hence the high probability of impact ionization, with the generation of electron-hole pairs.
Quite a useful component!
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1
10
100
1000
0 2 4 6 8 10 12 14
Bias volts
Av
ala
nc
he
ga
in 2.5 μm
3 μm
3.5 μm
4 μm
4.5 μm
Avalanche gain v. bias volts and cutoff wavelength
Cut-off wavelength
[μm]
HgCdTe avalanche photodiodes at 77K
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1
10
100
1000
0 2 4 6 8 10 12 14
Bias volts
Ava
lan
che
gai
n 2.5 μm
3 μm
3.5 μm
4 μm
4.5 μm
Avalanche gain v. bias volts and cutoff wavelength
Cut-off wavelength
[μm]
HgCdTe avalanche photodiodes at 77K
Used for Burst Illumination
LIDAR (BIL) imaging
Potential for low background flux
astronomy
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LPE HgCdTe layer grown on CdZnTe substrate
HgCdTe monolith bonded to ROIC
n p
HgCdTe technology options for APDs
APD array using via-hole process
Multi-level APD designMOVPE HgCdTe layer grown on 75mm GaAs substrate
Bump bonded to ROIC
LPE material + via-hole hybrid technology- Currently gives best breakdown voltages
MOVPE material + mesa hybrid technology- Under development for APDs
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Silicon multiplexer (ROIC) options
ME770 – Dual Mode
256x320 on 24µm pitch
Thermal imaging OR BIL imaging
ME780 - Swallow 3D
256x320 on 24µm pitch
3D intensity and range per pixel
Both ROICs can be configured to run in non-destructive readout. Parasitic capacitance is higher than a custom ROIC but results can allow for this.
Both used for ESO APD study
Thermal image BIL image
BIL intensity image BIL range image
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No avalanche gain
Gate - 3900ns
Avalanche gain - 4.6
Gate - 800ns
Avalanche gain - 13.8
Gate - 300ns
Avalanche gain - 38
Gate - 100ns
Pixel to pixel uniformity of avalanche gain
Short and long range uniformity of avalanche gain – no issue for data acquisition
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Noise proportional to: Gain . sq rt (gate time . noise figure)
Detailed measurements give noise figure of 1.3 up to x97 gain
Noise after avalanche gain
Extra noise due to avalanche process negligible
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Noise spatial distribution for typical BIL detector
Temp - 100K
Wavelength – 4.5 μm
Gate time - 160ns
Ava. gain - x25
Array operability performance – BIL compared with SW
The low pixel defect count of BIL detectors is due to the short gate time. Wavefront sensors need 3e5x longer integration time so dark current critical
Very few defects due to short gate time
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Avalanche gain for wavefront sensors
How does avalanche gain benefit wavefront sensors?
Typical requirement:
Integration time – 1.0 to 5.0 ms
Waveband – 1.0 to 2.5 µm
Multiple non-destructive readouts
Sensitivity in noise-equivalent-photons (NEPh) – 3 photons rms
[Note NEPh a better Figure of Merit for APDs]
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Noise-equivalent-photons (NEPh) - sensitivity figure of merit for APDs
5.02
..
.211
.2.
NFGT
FN
Q
NFNEPh
0
10
20
30
40
50
60
70
80
0 2 4 6 8 10 12
Diode bias volts
NE
Ph
(p
ho
ton
s r
ms
)
NEPh
NEPh with CDS
Allows for photon noise
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SW LPE HgCdTe layers
SELEX APD Pre-development Programme for ESO
3 variable jn hybrids5 full hybrids
2.64 μm
2.54 μm
2.50 μm
ME770 – Dual Mode
ME780 - Swallow 3D
2 variable jn hybrids4 full hybrids
2 FPAs to ESO in flatpacks
2 FPAs to ESO in flatpacks
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Experimental hybrid with variable junction diameters
Variable junction diameter
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0
2
4
6
8
10
12
0 2 4 6 8 10
Bias voltage
Sig
na
l (m
V)
D - 5.8
B - 6.2
F - 6.6
E - 7.0
C - 7.4
A - 7.8
0
2
4
6
8
10
12
14
0 2 4 6 8 10
Bias voltage
Gai
n
D - 5.8
B - 6.2
F - 6.6
E - 7.0
C - 7.4
A - 7.8
Result of variable junction diameter experiment
Better signal with smaller junction
No effect on avalanche gain
Conclusion: use small junction diameters on further arrays
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ESO measurements on variable jn diameter array
ESO measurements show strong S/N benefit from using small junctions
Data:
Integration time – 3ms
Temperature – 60K
Cut-off – 2.64 μm
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NEPh v. Bias Volts as function dark current- to set dark current specification
Target dark current specification is <1e-11 A/cm2 (360 e/s)
Data:
Integration time – 5ms
Temperature – 70K
Wavelength – 2.5 μm0
10
20
30
40
50
60
70
0 2 4 6 8 10 12
Diode bias volts
NE
Ph
(p
ho
ton
s r
ms
) 1.E-09
3.E-10
1.E-10
3.E-11
1.E-11
3.E-12
1.E-12
0
Dark current
(A/cm2)
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1.E-12
1.E-11
1.E-10
1.E-09
0 20 40 60 80 100 120
Temperature (K)
Da
rk c
urr
en
t (A
/cm
^2)
SELEX
ESO
Comb
Comparison of SELEX and ESO measurements of dark current v. temperature
ESO measurements
Shows dark current specification is met for temperatures below 90K
Array data:
Cut-off wavelength – 2.64um
Target spec <1e-11 A/cm2
Trap-assisted tunnelling behaviour
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ESO Electro-Optic Test Rig
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Signal Noise
Typical output from ESO Test Rig
Shows that noise is limited by photon shot noise
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ROIC – ME784
Bias – 7.1V
Temperature – 70K
TBB - 100ºC-50ºC
ESO measurement of uniformity under moderate gain
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ESO measurement of Avalanche Gain – comparison with model
ROIC – ME770
Temperature – 70K
Measured data for 2.64 μm diode
Fitted: APD Gain = 0.0782*2(Vbias/1.126)+0.905
Model for 2.64 μm diode (green)
Model for 2.5 μm diode (red)
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ESO measurement of Quantum Efficiency – 70%
ROIC – ME770
Bias – 8.63V
Gain - 16x
Temperature – 70K
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ESO measurement of electrons per ADU to calibrate the detector test – 2.21 e/ADU
ROIC – ME784
Gain of 6.4
Temperature – 80K
Signal electrons – Q
Noise electrons – Q0.5
Signal V = Q.e.T/C
(Noise V)2 = Q.(e.T/C)2
Signal/(Noise)2 in ADUs = electrons/ADU
T is pixel transfer function
C is integration cap
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ESO measurement of noise at gain of 6.4
ROIC – ME784
Temperature – 60K
Aval. gain – 6.4
Integration time – 5ms
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ESO measurement of noise at gain of 6.4
ROIC – ME784
Temperature – 60K
Aval. gain – x6.4
Integration time – 5ms
Theory for ME784Theory for custom ROIC
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Dark current defect map under extreme conditions – effect of temperature
45K 60K 70K 80K
Reducing temperature reduces the number of high dark current pixels
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Low photon flux imaging using avalanche gain
FPA at 60K
Average of 10 frames
6 electrons imaging
Readout with avalanche gain of x1.5
Readout with avalanche gain of x7
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Modelled sensitivity based on measured data and with a custom ROIC
0
10
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30
40
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70
80
0 2 4 6 8 10 12
Diode bias volts
NE
Ph
(p
ho
ton
s r
ms
)
NEPh
NEPh with CDS
Data:
Integration time – 5ms
Temperature – 77K
Cut-off – 2.5um
Avalanche gain offers an order improvement in NEPh
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Conclusions on avalanche gain for wavefront sensing applications (A-O and interferometry)
Results so far
• Avalanche gains up to x16 at 8.6V bias achieved in 2.64 μm material
• 6 electrons rms achieved with existing non-optimised ROIC and electronics
• Optimised technology could provide 2-3 photons rms
• All the aspirations of wavefront and interferometric applications can be met by APD technology
Future work
• Need to establish parameter space of APDs i.e. wavelength, temperature etc
• Need to design custom ROIC