1. ELECTRON- AND HOLE- AVALANCHE HgCdTe PHOTODIODE ARRAYS FOR ASTRONOMY Donald N. B. Hall Institute for Astronomy University of Hawaii

2. 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.

3. 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.

4. 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

5. 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

6. 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.

7. 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, 77K & 6V REVERSE BIAS

8. 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 approaches pure h-APD at Eg = eV – the ideal SAM layer.

9. e-APD GAIN - SUMMARY
T=200K

10. 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 eV RESONANCE
e- IONIZATION IS VERY HIGH
THUS FOR EB < 0.6 eV (λC > 2 μm) ONLY e- AVALANCHE (k = 0)

11. 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.

12. 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 M-LAYER, 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.

13. 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 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?

14. J. ROTHMAN SUMMARY

15. 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

16. M KINCH_JEM_V37N9P1453_2008 page 1454 Fig. 2

17. M KINCH_JEM_V37N9P1453_2008 page 1454 Fig.1.(a)

18. M KINCH, EAPDs, page 122, Fig. 7.13

19. 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.

20. e-APDs by CEA LETI, DRS, BAE & TIS
Company
Process
Geometry
Use
CEA-LETI
LPE & MBE
Plane
(Width)
MWIR PAI
1.5μm LADAR
DRS
MBE
Cylinder
BAE
LPE
TIS
PHOTON COUNTING

21. e-APD ARCHITECTURE - DEFIR
caption

22. e-APD ARCHITECTURE - DSL
caption

23. THREE COMPLIMENTARY TIS APPROACHES

24. e-APD GAIN - SUMMARY
T=200K

25. DEFIR F VALUES (J. ROTHMAN)

26. e-APD GAIN σ - DRS
caption

27. e-APD GAIN σ - DRS
caption

28. e-APD GAIN σ - DEFIR
caption

29. e-APD GAIN (CUM) - DEFIR
caption

30. e-APD GAIN vs TEMP - SUMMARY

31. e-APD GAIN vs TEMP - DEFIR
caption

32. e-APD GNDC - DEFIR
caption

33. e-APD GNDC vs TEMP - DEFIR
caption

34. e-APD PULSE PROFILE - DEFIR
caption

35. e-APD PULSE RISE TIME - DEFIR
caption

36. e-APD PULSE DECAY TIME - DEFIR

37. 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.

38. h-APD DEVELOPMENT
RAYTHEON (RVS, HRL & RMS) HAS DEMONSTRATED SWIR (1.55 μm) e-APD BASED LADAR OPERATING AT 300K.
THEY REPORT NO EXCESS NOISE TO GAINS >100, NEP < 1nW & GHZ BANDWIDTH.
CZT => 6” Si WAFER PROCESSING.

39. PERFORMANCE OF 90 RANDOMLY SELECTED APDs - RAYTHEON
Jack et al, Proc of SPIE V6542, P65421A (2007)

40. 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.

41. APPROACH SIMILAR MASKS FOR e-APD & h-APD HgCdTe INCLUDE:
- PROCESS EVALUATION CHIPS (PECs).
- FOUR 256 x 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.

42. CONCEPTUAL “TADPOLE” LAYOUT
KSPEC MODIFICATIONS
CONCEPTUAL “TADPOLE” LAYOUT
Diodes in the 64um-500um range aligned along two parallel lines

43. UH-TIS HAWAII Heritage
Guide mode &
read/reset opt.
On-chip butting
Reference pixels
Stitching
1024 x 1024 pixels
3.4 million FETs
0.8 µm CMOS
18 µm pixel size
HAWAII - 1
1994
2048 x 2048 pixels
13 million FETs
0.8 µm CMOS
18 µm pixel size
1998
HAWAII - 2
HAWAII - 1R
2000
WFC 3
1024 x 1024 pixels
3.4 million FETs
0.5 µm CMOS
18 µm pixel size
HAWAII - 1RG
2001
1024 x 1024 pixels
7.5 million FETs
0.25 µm CMOS
18 µm pixel size
HAWAII - 2RG
2002
2048 x 2048 pixels
29 million FETs
0.25 µm CMOS
18 µm pixel size
Smaller pixels, Improved performance, Scalable resolution
HAWAII-4RG-15
HAWAII-4RG-10
2011 (proposed)
2006
SIDECAR ASIC
15µm pixels
2003
4096 x 4096
110 million FETs
0.25 / 0.18 µm CMOS
15 µm pixel size
4096 x 4096
110 million FETs
0.25 µm CMOS
10 µm pixel size
Control chip for H1RG, H2RG and H4RG-10/15

44. DARK CURRENT vs TEMPERATURE FOR 2.5 AND 5 UM MATERIAL

45. 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

46. 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) p-on-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 eV M-LAYER.

47. 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).

48. 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.

49. KSPEC MODIFICATIONS
Sphere Assembly
Cryo ASIC
Detector Module

50. KSPEC X-SECTION
LEDS
APERATURE
ASIC
DETECTOR

51. PHOTON COUNTING WITH H1RG
SUB-ARRAY #
PIXEL
FRA
μ-sec ME
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
8 x 8 64
6.4
160
4 x 4 16
1.6
625
HYBRIDIZE 256 x 256 SUB-ARRAY TO OUTPUTS 0 – 3 IN CORNER OF H1-RG.
SIDECAR ASIC 10 Mpxl/SEC.
50 – 60 RMS e- CDS READ NOISE.
FRAME RATES:

52. A LOOK INTO THE CRYSTAL BALL
DISCRETE APDs FOR INTENSITY INTERFEROMETRY, ADAPTIVE OPTICS & FRINGE TRACKING IN 1 -2 YEARS.
MODEST ARRAYS KHz FRAME RATE WITH ONE ASIC.
H-2RG, H-4RG-15 FOR LOW BACK-GROUND SPECTROSCOPY & SPACE.
SPECIALIZED READOUTS – TIME TAGGING PHOTONS.

53. CURRENT STATUS
END

54. A B

55. e-APD GAIN - DEFIR
caption

56. e-APD GAIN - DSL
caption

57. e-APD GAIN – TIS 2004

58. e-APD GAIN - BAE
T=200K
caption