G. Finger, R. Dorn, S. Eschbaumer, D. Ives, L. Mehrgan, M. Meyer, J. Stegmeier

1. Recent Performance Improvements, Calibration Techniques and Mitigation Strategies for Large-format HgCdTe Arrays G. Finger, R. Dorn, S. Eschbaumer, D. Ives, L. Mehrgan, M. Meyer, J. Stegmeier

2. Introduction • Hawaii-2RG close to prefect wrt basic parameters noise, QE, darkcurrent • comparison of 4 methods to determine conversion gain • persistence of HgCdTe Hawaii-2RG arrays • mitigation strategy to reduce persistence • method to measure persistence in darkness

3. Noise comparison H2RG #119 / H2RG #184 • H2RG #119 (X-Shooter)25.3 erms on IR active pixels7.7 erms on reference pixels • H2RG #184 (KMOS)6,9 erms on IR active pixels5.8 erms on reference pixels • bond pad contact resistance improved  noise reduced from 25.3 to 6.9 erms • Readout noise < 10 erms for DCS on 5 new science arrays (KMOS and SPHERE)

4. Noise of KMOS arrays with Fowler sampling • Reduce noise with multiple nodestructive sampling • Noise 2.2 erms for 32 Fowler pairs

5. Noise map of crossdispersed spectrum • slit open / warm instrument shutter closed • integration time = 600s ( 903 nondestructive readouts) • limited by shot noise of photon background which is dominated by scattered light of K-band (5E-2 e/s/pixel) K order 11 J order 26

6. Dark current of lc =2.5 mm HgCdTe 1 2 • Crossdispersed echelle spectrum with slit closed • Cut levels : 0 - 5E-3 e/s/pixelat T=81K , Vbias=1V

7. Dark current of lc =2.5 mm HgCdTe • dark current outside optical field:4.2 E-4 e/s/pixel • dark current in J1.3 E-3 e/s/pixel • T=81K, VBIAS = 1V

8. Dark current versus temperature • Quantum efficiency high over the entire sensitive range of the array • Measurement at optical wavelengths pending

9. IPC with single pixel reset • uniformly illuminate arraywith high flux integration time 1 s • Use guide mode of Hawaii-2RG muxguide window size 1x1 • Reset single pixel before readoutintegration time < 500ms

10. IPC with single pixel reset • uniformly illuminate arraywith high flux integration time 1 s • Use guide mode of Hawaii-2RG muxguide window size 1x1 • Reset single pixel before readoutintegration time < 500ms • Observe capacitive coupling on next neighbors

11. recent improvements of IPC • improvements in multiplexer layout • resulted in reduction of coupling coefficient a:a#184=1.7% • a#226=1.4% H2RG #184 H2RG #226

12. Conversion gain and single pixel reset • Ori Fox method of classical propagation of errors: used also by Teledyne • Assuming a true variance Cij is covariancebetween pixels i and j

13. Conversion gain and single pixel reset • Ori Fox method of classical propagation of errors: used also by Teledyne • Assuming a a a a a true variance Cij is covariancebetween pixels i and j • a=0.17 : correction factor 1+8a +52 a2 =1.13

14. Conversion gain and single pixel reset • variance versus signal: 2.26 e/ADU • single pixel reset IPC correction1.96 e/ADU

15. Conversion gain from integrated autocorreolation • variance versus signal: 2.26 e/ADU • single pixel reset IPC correction1.96 e/ADU • integrated autocorrelation versus signal: 2.04 e/ADU

16. Conversion gain by capacitance comparison method sum of signal of all pixels dc level drift on external capacitor slope a = C0/Cext

17. Conversion gain by capacitance comparison method • Ccryo difficult to estimate without risk for detector:Ccryo includes capacitances of cable, preamplifier board and wirebond ceramics • Ceramic capacitors on HAWAII-2RG wirebond ceramics show strong temperature dependence:T=296 K C=1mFT=77 K C=276 nF

18. Conversion gain by capacitance comparison method a0 measured with Cext removed C0=32.8 fFCcryo=394 nF C0/e=205e/mV (in our setup: 1.89e/ADU)

19. comparison of methods to obtain conversion gain • Remarks: • first three methods are stochastic (rely on noise measurement) • single pixel reset measures coupling coefficient but assumes only coupling to next neighbors • capacitance comparison is direct and robust method • taking into account coupling to all pixels taking into account cable capacitance and cold ceramic capacitors at detector

20. Persistence: X-Shooter as test bench • with slit closed: instrumental background: 4.2E-4 e/s/pixel • ideal for persistence tests

21. Persistence: lamp on • DIT=1.65s slit open ThAr lamp

22. Persistence: slit closed • first dark exposure with DIT=128s after 2048s exposure with open slit

23. Persistence versus stimulus • Persistence of first 2 min. dark exposure is ~6.3e-4 of stimulus

24. Persistence at different wavelengths • Persistence almost the same at l=1.07 mm and l=2.2 mm

25. Persistence model of Roger Smith charge trapped when location of trap becomes undepleted and is released in next dark exposure traps populated when exposed to mobileelectrons and holes pn -junction n p

26. Persistence model of Roger Smith charge trapped when location of trap becomes undepleted and is released in next dark exposure traps populated when exposed to mobileelectrons and holes pn -junction n p

27. Mitigation of persistence: global reset detrapping charge trapped when location of trap becomes undepleted and is released in next dark exposure traps populated when exposed to mobileelectrons and holes pn -junction n p keep global reset switch closed after science exposure allow de-trapping of charge

28. Mitigation of persistence: global reset detrapping • Slit open

29. Mitigation of persistence: global reset detrapping • First 2 minute dark exposurewithout global reset de-trapping

30. Mitigation of persistence: global reset detrapping • First 2 minute dark exposurewith global reset de-trapping • Keep reset switch of all pixels • permanently closed with global reset for 128 s at the end of bright exposure to force depletion width to stay wide avoiding population of traps • de-trapping time is 128 s • close slit and return to normal operating mode taking dark exposures • Persistence in first dark exposure reduced by factor of 9

31. Mitigation of persistence: global reset detrapping • First 2 minute dark exposure with global reset always closed during bright exposure • if reset closed before switching on bright source and kept closed until slit closed again persistence is zero • global reset is an electronic shutter • which protects detector from • persistence while exposed to bright • illumination

32. Mitigation of persistence: global reset detrapping • In first 2 minute dark • exposure intensity of • persistence is reduced by • a factor of 9 with • global reset de-trapping • Duration of detrapping 128 s without global reset de-trapping with global reset de-trapping reset always closed during bright exposure

33. Method to measure persistence in darkness hypothesis: persistence is generated by the change of the voltage across pn junction of pixel diode instead of using light to shrink depletion region reduce bias voltage in selected area of array using the window mode of the Hawaii-2RG multiplexer and the global rest outside window normal operation of the array

34. Persistence electrical /optical • Generated with bias change • in selected area using global reset • Generated with light source

35. Persistence electrical /optical • red diamonds: • persistence generated with light source on /off • black triangles: • in selected area using global reset • persistence generated with bias low / high • decay with similar time constants

36. Persistence measured in darkness • Measure persistence of all 3 KMOS detectors in one go • uniformity, cosmetics, dark current. readout noise, persistence • 128 channel cryo-preamps , flex boardsand vacuum connectors • GL scientific mosaic mount

37. Persistence measured in darkness • Mosaic test facility: • no windowno optics • detector covered by black plateflux < 1E-3 e/s/pixel

38. Persistence measured in darkness • integration time 120 sec • operating temperature 66K • generated with bias change in darkness • on selected area using global reset

39. Persistence versus time • persistence lasts for > 1500s • persistence is device dependent • array # 184 is better

40. Persistence versus temperature • persistence is device dependent • persistence of devices • #211 and #212 has a maximum at T=66K • persistence of device #184 does not have this temperature maximum

41. Persistence versus detrapping time • peristence is decreases with increasing detrapping time

42. Persistence versus duration of illumination • persistence increases when detector is exposed to the bright source for a longer time

43. Persistence versus signal intensity • persistence increases with increasing stimulus • bias = DSUB – VRESET VRESET=0.5V increasing signal ( brighter light source)

44. Global reset de-trapping: on sky test • after global reset detrappingvertical stripes • in first difference images of two 1200s exposures • intensity of stripes in first difference ~ 0.03 e/s/pixel

45. Global reset de-trapping: on sky test • profile of vertical stripes • in difference images of two 1200s exposures • intensity of stripes in first difference ~ 0.03 e/s/pixel

46. Global reset de-trapping: on sky test • vertical stripes • in first difference images of two 1200s exposures • intensity of stripes in first difference ~ 0.03 e/s/pixel • stripes located at start of fast shift register

47. Global reset de-trapping: on sky test • keep clocks running during global reset detrapping • no vertical stripes • in first difference images of two 600s exposures

48. Global reset de-trapping: on sky test • profile of vertical stripes • in difference images of two 1200s exposures • intensity of stripes in first difference ~ 0.03 e/s/pixel • intensity of stripes in second difference ~ negligible • to be further investigated

49. Global reset de-trapping: on sky test • automatic flexure compensation: line intensity 45000 e/s/pixel

50. Global reset de-trapping: on sky test • automatic flexure compensation: line intensity 45000 e/s/pixel • First 1500 s dark exposure : no persistence !