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

3. Noise comparison H2RG #119 / H2RG #184 l H2RG #119 (X-Shooter) 25.3 erms on IR active pixels 7.7 erms on reference pixels l H2RG #184 (KMOS) 6,9 erms on IR active pixels 5.8 erms on reference pixels l bond pad contact resistance improved  noise reduced from 25.3 to 6.9 erms l Readout noise < 10 erms for DCS on 5 new science arrays (KMOS and SPHERE)

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

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

6. Dark current of c =2.5  m HgCdTe l Crossdispersed echelle spectrum with slit closed l Cut levels : 0 - 5E-3 e/s/pixel at T=81K, V bias =1V 1 2

7. l dark current outside optical field: 4.2 E-4 e/s/pixel l dark current in J 1.3 E-3 e/s/pixel l T=81K, VBIAS = 1V Dark current of c =2.5  m HgCdTe

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

9. IPC with single pixel reset l uniformly illuminate array with high flux integration time 1 s l Use guide mode of Hawaii-2RG mux guide window size 1x1 Reset single pixel before readout integration time < 500  s

10. l uniformly illuminate array with high flux integration time 1 s l Use guide mode of Hawaii-2RG mux guide window size 1x1 Reset single pixel before readout integration time < 500  s l Observe capacitive coupling on next neighbors IPC with single pixel reset

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

12. Conversion gain and single pixel reset l Assuming a Ori Fox method of classical propagation of errors: used also by Teledyne C ij is covariance between pixels i and j true variance

13. Conversion gain and single pixel reset l Assuming a Ori Fox method of classical propagation of errors: used also by Teledyne C ij is covariance between pixels i and j  =0.17 : correction factor 1+8   =1.13 true variance    

14. Conversion gain and single pixel reset l variance versus signal: 2.26 e/ADU l single pixel reset IPC correction 1.96 e/ADU

15. Conversion gain from integrated autocorreolation l variance versus signal: 2.26 e/ADU l single pixel reset IPC correction 1.96 e/ADU l integrated autocorrelation versus signal: 2.04 e/ADU

16. Conversion gain by capacitance comparison method dc level drift on external capacitor sum of signal of all pixels slope  = C 0 /C ext

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

18. Conversion gain by capacitance comparison method C 0 =32.8 fF C cryo =394 nF C 0 /e=205e/mV (in our setup: 1.89e/ADU)  0 measured with C ext removed

19. comparison of methods to obtain conversion gain methodconversion gainC [e/ADU][e/mV][fF] variance versus signal2.2624539.2 integrated autocorrelation (Moore et al)2.0422135.4 single pixel reset (Fox et al)1.9621234.0 capacitance comparison1.8920532.8 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 l DIT=1.65s slit open ThAr lamp

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

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

24. Persistence at different wavelengths l Persistence almost the same at =1.07  m and =2.2  m

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

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

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

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

29. 29 First 2 minute dark exposure without global reset de-trapping Mitigation of persistence: global reset detrapping

30. 30 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 First 2 minute dark exposure with global reset de-trapping Mitigation of persistence: global reset detrapping

31. 31 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 First 2 minute dark exposure with global reset always closed during bright exposure Mitigation of persistence: global reset detrapping

32. 32 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 Mitigation of persistence: global reset detrapping

33. Method to measure persistence in darkness l hypothesis: persistence is generated by the change of the voltage across pn junction of pixel diode l 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 l 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 GL scientific mosaic mount 128 channel cryo-preamps, flex boards and vacuum connectors

37. Persistence measured in darkness Mosaic test facility: no window no optics detector covered by black plate flux < 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 detrapping vertical 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 !

51. l readout noise improved by a factor of 2 on new H2RG (6.9 erms single DCS) l shot noise limited operation achieved in cross dispersed spectrometer in J l PTF corrected by single pixel reset factor yields conversion gain which agrees with capacitance comparison method within 3% persistence model confirmed by experiment: traps at edge of valence and conduction bands persistence is a consequence of changing bias voltage no persistence is expected with CTIA since bias voltage constant l new method to measure persistence in darkness global reset detrapping successfully tested on sky without residuals Conclusion

52. HAWK-I first light l Tarantula Nebula THE END

53. TLI: Threshold Limited Integration l Multiple nondestructive readouts scheme l Set saturation level l If signal exceeds saturation level, readout not used to calculate slope of integration ramp l Extrapolate signal to DIT l In effect different integration times for pixels exceeding saturation level

54. TLI: Threshold Limited Integration l Multiple nondestructive readouts scheme l Set saturation level l If signal exceeds saturation level, readout not used to calculate slope of integration ramp l Extrapolate signal to DIT l In effect different integration times for pixels exceeding saturation level

55. TLI: Threshold Limited Integration l Multiple nondestructive readouts scheme l Set saturation level l If signal exceeds saturation level, readout not used to calculate slope of integration ramp l Extrapolate signal to DIT l In effect different integration times for pixels exceeding saturation level l Signal can be >> 10 7 e l Gain of > 2 orders of magnitude in dynamic range

56. TLI: Threshold Limited Integration l Multiple nondestructive readouts scheme l Set saturation level l If signal exceeds saturation level, readout not used to calculate slope of integration ramp l Extrapolate signal to DIT l In effect different integration times for pixels exceeding saturation level l Full well 1E5 electrons l With TLI it is possible to integrate > 1E7 electrons with TLI without TLI Saturation level

57. Trapping model P N - - - - - + + + + + next dark exp. (small bias reduction) The released charge reduces the bias voltage. persistence - - - + + + - - - - - + + + + + high flux signal (low bias) As signal accumulates the depletion width is reduced. Traps newly exposed to charge can capture some mobile carriers. Trapped holes Trapped electrons - - - + + + + + + + + - - - - - reset (large reverse bias) At “reset” the wider depletion region is restored, but trapped charge stays behind. Depleted Mobile electrons Mobile holes dark idle (large reverse bias) All traps have released their charge in depletion region + + + + + - - - - - -+ R.Smith, SPIE 7021-22, Marseille 2008-06-24

58. Dark current versus temperature l generation- recombination limited above 110K I dark  exp(-T eff /T) I dark  exp(-E gap /(1.7 KT)) l surface leakage and tunneling below 100K 1.7E-3 e/s/pixel