1. 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

2. Avalanche gain in HgCdTe
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.
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
Quite a useful component!

3. Avalanche gain v. bias volts and cutoff wavelength
HgCdTe avalanche photodiodes at 77K
Cut-off wavelength
[μm]

4. Avalanche gain v. bias volts and cutoff wavelength
HgCdTe avalanche photodiodes at 77K
Cut-off wavelength
[μm]
Used for Burst Illumination LIDAR (BIL) imaging
Potential for low background flux astronomy

5. HgCdTe technology options for APDs
LPE HgCdTe layer grown on CdZnTe substrate
HgCdTe monolith bonded to ROIC
APD array using via-hole process
LPE material + via-hole hybrid technology
- Currently gives best breakdown voltages
Bump bonded to ROIC
Multi-level APD design
MOVPE HgCdTe layer grown on 75mm GaAs substrate
MOVPE material + mesa hybrid technology
- Under development for APDs

6. Silicon multiplexer (ROIC) options
ME770 – Dual Mode
256x320 on 24µm pitch
Thermal imaging OR BIL imaging
Thermal image BIL image
ME780 - Swallow 3D
256x320 on 24µm pitch
3D intensity and range per pixel
BIL intensity image BIL range image
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

7. Pixel to pixel uniformity of avalanche gain
No avalanche gain
Gate ns
Avalanche gain - 4.6
Gate - 800ns
Avalanche gain
Gate - 300ns
Avalanche gain - 38
Gate - 100ns
Short and long range uniformity of avalanche gain – no issue for data acquisition

8. Noise after avalanche gain
Noise proportional to: Gain . sq rt (gate time . noise figure)
Detailed measurements give noise figure of 1.3 up to x97 gain
Extra noise due to avalanche process negligible

9. Array operability performance – BIL compared with SW
Noise spatial distribution for typical BIL detector
Temp - 100K
Wavelength – 4.5 μm
Gate time - 160ns
Ava. gain - x25
Very few defects due to short gate time
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

10. 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]

11. Allows for photon noise
Noise-equivalent-photons (NEPh) sensitivity figure of merit for APDs
Allows for photon noise

12. SELEX APD Pre-development Programme for ESO
ME770 – Dual Mode
2.50 μm
3 variable jn hybrids
5 full hybrids
2.54 μm
2 FPAs to ESO in flatpacks
ME780 - Swallow 3D
2.64 μm
2 variable jn hybrids
4 full hybrids
SW LPE HgCdTe layers
2 FPAs to ESO in flatpacks

13. Experimental hybrid with variable junction diameters

14. Result of variable junction diameter experiment
Better signal with smaller junction
No effect on avalanche gain
Conclusion: use small junction diameters on further arrays

15. ESO measurements on variable jn diameter array
Data:
Integration time – 3ms
Temperature – 60K
Cut-off – 2.64 μm
ESO measurements show strong S/N benefit from using small junctions

16. Target dark current specification is <1e-11 A/cm2 (360 e/s)
NEPh v. Bias Volts as function dark current - to set dark current specification
Dark current
(A/cm2)
Data:
Integration time – 5ms
Temperature – 70K
Wavelength – 2.5 μm
Target dark current specification is <1e-11 A/cm2 (360 e/s)

17. Comparison of SELEX and ESO measurements of dark current v. temperature
Target spec <1e-11 A/cm2
Array data:
Cut-off wavelength – 2.64um
Trap-assisted tunnelling behaviour
ESO measurements
Shows dark current specification is met for temperatures below 90K

18. ESO Electro-Optic Test Rig

19. Typical output from ESO Test Rig
Signal
Noise
Shows that noise is limited by photon shot noise

20. ESO measurement of uniformity under moderate gain
ROIC – ME784
Bias – 7.1V
Temperature – 70K
TBB - 100ºC-50ºC

21. ESO measurement of Avalanche Gain – comparison with model
Measured data for 2.64 μm diode
Fitted: APD Gain = *2(Vbias/1.126)+0.905
Model for 2.64 μm diode (green)
Model for 2.5 μm diode (red)
ROIC – ME770
Temperature – 70K

22. ESO measurement of Quantum Efficiency – 70%
ROIC – ME770
Bias – 8.63V
Gain - 16x
Temperature – 70K

23. 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

24. ESO measurement of noise at gain of 6.4
ROIC – ME784
Temperature – 60K
Aval. gain – 6.4
Integration time – 5ms

25. ESO measurement of noise at gain of 6.4
Theory for custom ROIC
Theory for ME784
ROIC – ME784
Temperature – 60K
Aval. gain – x6.4
Integration time – 5ms

26. Reducing temperature reduces the number of high dark current pixels
Dark current defect map under extreme conditions – effect of temperature
45K 60K K 80K
Reducing temperature reduces the number of high dark current pixels

27. Low photon flux imaging using avalanche gain
Readout with avalanche gain of x1.5
Readout with avalanche gain of x7
FPA at 60K
Average of 10 frames
6 electrons imaging

28. Modelled sensitivity based on measured data and with a custom ROIC
Integration time – 5ms
Temperature – 77K
Cut-off – 2.5um
Avalanche gain offers an order improvement in NEPh

29. 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