OUTLINE

1. ELECTRON- AND HOLE- AVALANCHEHgCdTe PHOTODIODE ARRAYS FOR ASTRONOMYDonald N. B. HallInstitute for AstronomyUniversity 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, ALL @ 77K & 6V REVERSE BIAS

8. e- & h- APD REGIMES OF HgCdTe Figure5: 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 = 0.938 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 0.938 eV RESONANCE • e-IONIZATION IS VERY HIGH • THUS FOR EB < 0.6 eV (λC > 2μm) 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-OFF0.25 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?

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

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 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.

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

43. UH-TIS HAWAII Heritage HAWAII - 1 HAWAII - 2 HAWAII - 1R HAWAII - 1RG HAWAII - 2RG 1994 1998 2000 2001 2002 WFC 3 1024 x 1024 pixels 3.4 million FETs 0.8 µm CMOS 18 µm pixel size 2048 x 2048 pixels 13 million FETs 0.8 µm CMOS 18 µm pixel size 1024 x 1024 pixels 3.4 million FETs 0.5 µm CMOS 18 µm pixel size 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 Guide mode & read/reset opt. Stitching On-chip butting Reference pixels 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 0.938 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