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Charge collection close to the Si-Si0 2 interface of silicon strip sensors

Charge collection close to the Si-Si0 2 interface of silicon strip sensors. Thomas Pöhlsen, Eckhart Fretwurst , Robert Klanner , Sergej Schuwalow , J ö rn Schwandt, Jiaguo Zhang University of Hamburg. O verview. Introduction Charge collection close to the Si-Si0 2 interface

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Charge collection close to the Si-Si0 2 interface of silicon strip sensors

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  1. Charge collectionclosetothe Si-Si02interfaceofsiliconstripsensors Thomas Pöhlsen, Eckhart Fretwurst, Robert Klanner, Sergej Schuwalow, Jörn Schwandt, JiaguoZhang University of Hamburg

  2. Overview Introduction Charge collectionclosetothe Si-Si02interface Weighting potential Time resolvedsignals Integrated signals Results: Charge losses vs. humidityandbiashistory Conclusions Outlook Charge collection close to the Si-Si02 interface of silicon strip sensors

  3. Motivation – whysurfacestudies? Surfaceeffects: Relevant forsensorstability (breakdown, stabilityofdarkcurrent, etc.) Charge carrierlosses Humidityfoundtoinfluencetheelectricfield in sensor Electricfieldattheinterface ? (surfacecharges, surface potential, oxidecharges, etc. => boundaryconditions ?) humidity H20, H+ OH-, dirt passivation aluminium aluminium n type Si p+ implant p+ implant Si02 Si02 Si02 Charge collection close to the Si-Si02 interface of silicon strip sensors Seite 3

  4. Sensors andirradiation Irradiation: Non-irradiated Irradiated(1 MGyx-rays, 12 keV) surface damage only fixedoxidecharge: Nox = ~ 2 1012cm-2 surfacecurrent: Isurf= ~ 6 µA cm-2 Atmosphereduringmeasurement: Humid (> 50% humidity) Dry (nitrogen, < 5% humidity) T = ~24 °C (roomtemperature) passivation al al n type p+ p+ * + 2 µm Al overhang Si02 Si02 Si02 Charge collection close to the Si-Si02 interface of silicon strip sensors

  5. Measurement procedure (redlaser TCT) Redlaser light (front illumination,  = 660 nm, penetrationdepth ~ 3 µm) Sub ns-pulses (FWHM 100 ps, 1 kHz, 30 000 to 500 000 eh-pairs) Focus:  = 3 µm (+ tails) Readout: 2 strips + 1 rearcontact MiteqAM-1309 currentamplifiers Tektronixoscilloscope, 2.5 GHz bandwidth Neighbourstrips on ground (via 50 W) Charge Q calculatedoffline: Q = ∫I(t) dt Currentsignal I(t) laser al al n type p+ p+ Si02 Si02 Si02 Charge collection close to the Si-Si02 interface of silicon strip sensors

  6. Accumulationlayerandelectricfield (simulation) Electronaccumulationlayer 1 MGyirradiation (surfacedamage) Nox = 2 1012 cm-2 , Isurf = 6.4 µA cm-2 Electronaccumulationlayerpresent electrons leaving 2 1012/cm2 200 V ∆ • Electronlosses ! b.c.:  = 0 • Influencestheweighting potential w, j • Calculatew, junderbias: readoutstrip j: 0 V otherstrips: 0 V rearside: 200 V read out strip j: 1 V otherstrips: 0 V rearside: 200 V w,j= also see Hamel, Julien NIMA 597(2008), 207 Charge collection close to the Si-Si02 interface of silicon strip sensors

  7. Weighting potential (simulation) readout j Electronaccumulation 1 MGyirradiation (surfacedamage) Nox = 2 1012 cm-2 , Isurf = 6.4 µA cm-2 Electronaccumulationlayerpresent electrons leaving 2 1012/cm2 200 V 2 1012/cm2 200 V ( µm ) • Influencestheweighting potential w, j • Calculatew, junderbias: readoutstrip j: 0 V otherstrips: 0 V rearside: 200 V read out strip j: 1 V otherstrips: 0 V rearside: 200 V w,j= also see Hamel, Julien NIMA 597(2008), 207 Charge collection close to the Si-Si02 interface of silicon strip sensors

  8. Weighting potential andinducedcurrent Charge carriers (q) driftin theelectricfield: vdr = µ E Inducedcurrent: Ij = q Ew,j· vdr , Collectedcharge : Qj = ∫Ijdt 12 µm ∫ I dt ~ 70 000 e nolosses Ew, j = w, j readout j 38µm laser 12µm laser ∆ Weighting potential 38 µm h h nolosses e e ∫ I dt ~ 0 Charge collection close to the Si-Si02 interface of silicon strip sensors

  9. Weighting potential andinducedcurrent Charge carriers (q) driftin theelectricfield: vdr = µ E Inducedcurrent: Ij = q Ew,j· vdr , Collectedcharge : Qj = ∫Ijdt 12 µm electronlosses (~97 %) nolosses Ew, j = w, j readout j 38µm laser 12µm laser ∆ Weighting potential 38 µm h h e e nolosses electronlosses (~97 %) Charge collection close to the Si-Si02 interface of silicon strip sensors

  10. Weighting potential andinducedcurrent 12 µm Charge carriers (q) driftin theelectricfield: vdr = µ E Inducedcurrent: Ij = q Ew,j· vdr , Collectedcharge : Qj = ∫Ijdt electronlosses (~97 %) Ew, j = w, j nolosses Weighting potential, rear h e ∆ 38 µm electronlosses (~97 %) nolosses readout j Charge collection close to the Si-Si02 interface of silicon strip sensors

  11. Collectedcharge vs. laserposition NL Rear Assumptions: w = constataccumulationlayer, linear else Holes: collectedatcloseststrip Light profile: gaussianwiths=2 µm L R Model Data 0 Gy, humid 0 Gy, driedat500 V laser 1 MGy, driedat 0 V Strip L strip R laserposition [µm] + hole diffusion Fit results 1 MGydriedat 500Vhumid electr. 1k 35k33k holes 29k 7k31k acclayer 38 µm 30 µm- Charge collection close to the Si-Si02 interface of silicon strip sensors

  12. Collectedcharge vs. laserposition NL Rear Assumptions: w = constataccumulationlayer, linear else Holes: collectedatcloseststrip Light profile: gaussianwiths=2 µm L R Data Model laser Strip L strip R laserposition [µm] readout Fit results 1 MGydriedat 500Vhumid electr. 1k 35k33k holes 29k 7k31k acclayer 38 µm 30 µm- Charge collection close to the Si-Si02 interface of silicon strip sensors

  13. Collectedcharge vs. laserposition NL Rear Assumptions: w = constataccumulationlayer, linear else Holes: collectedatcloseststrip Light profile: gaussianwiths=2 µm L R Data Model laser Strip L strip R laserposition [µm] Readout: rearcontact Fit results 1 MGydriedat 500Vhumid electr. 1k 35k33k holes 29k 7k31k acclayer 38 µm 30 µm- Charge collection close to the Si-Si02 interface of silicon strip sensors

  14. Results on humidityandbiashistory humid: steadystate* reached after < 5 min dry: steadystate* reached after >> 1 hour (hoursordays) ( time constantsdepend on manyparameters ) same steadystatefor all humiditiesandbiashistories! * steadystate in respecttochargelossbehavior Charge collection close to the Si-Si02 interface of silicon strip sensors

  15. Results on humidityandbiashistory humid: steadystate* reached after < 5 min dry: steadystate* reached after >> 1 hour (hoursordays) ( time constantsdepend on manyparameters ) same steadystatefor all humiditiesandbiashistories! Time dependent surface charges ? Danglingbonds ? * steadystate in respecttochargelossbehavior Charge collection close to the Si-Si02 interface of silicon strip sensors

  16. Summary andconclusions Charge collection close to the Si-Si02 interface was investigated in TCT setup and described succesfully by model. Significantlossesofelectronsand / orholesobserved. Charge losses depend on applied voltage, humidity, bias history andirradiation. Charge collection close to the Si-Si02 interface of silicon strip sensors

  17. Outlook: Saturation ofelectronlosses burstmodeoperation (20 shots) 20 shots, seperatedby 12.5 ns 1 mslater: next 20 shots electronlossesdissapear* * forlatershots • methodtoestimatethe maximal amountofelectronlosses in thegap • andpotentiallydeptrapping time Charge collection close to the Si-Si02 interface of silicon strip sensors

  18. Charge collection close to the Si-Si02 interface of silicon strip sensors

  19. Saturation ofelectronlosses burstmodeoperation (20 shots) 20 shots, seperatedby 12.5 ns 1 mslater: next 20 shots electrons holes Charge collection close to the Si-Si02 interface of silicon strip sensors

  20. Collectedchargeforcarrierlosses Fullchargecollection: Collection: holesatstrip L, electronsatrearside QL = #holes·qo = 3 qo Qrear= - 3 qo QR,NL,NR = 0 Charge losses(not collectedat end ofintegration time): Qind,j = ± q · w, j ( final position) QL < 3 |Qrear| < 3 QR,NL,NR > 0 for hole losses < 0 forelectronlosses Laser electron hole Charge collection close to the Si-Si02 interface of silicon strip sensors

  21. Collectedchargeforcarrierlosses Fullchargecollection: Collection: holesatstrip L, electronsatrearside QL = #holes·qo = 3 qo Qrear= - 3 qo QR,NL,NR = 0 Charge losses(not collectedat end ofintegration time): Qind,j = ± q · w, j ( final position) QL < 3 |Qrear| < 3 QR,NL,NR > 0 for hole losses < 0 forelectronlosses Qind 0 3 0 -3 Laser electron hole Charge collection close to the Si-Si02 interface of silicon strip sensors

  22. Collectedchargeforcarrierlosses Fullchargecollection: Collection: holesatstrip L, electronsatrearside QL = #holes·qo = 3 qo Qrear= - 3 qo QR,NL,NR = 0 Charge losses(not collectedat end ofintegration time): Qind,j = ± q · w, j ( final position) QL < 3 |Qrear| < 3 QR,NL,NR> 0 for hole losses < 0 forelectronlosses Qind 0.05 2.4 0.3 -2.9 Laser electron hole Charge collection close to the Si-Si02 interface of silicon strip sensors

  23. Collectedchargeforcarrierlosses Fullchargecollection: Collection: holesatstrip L, electronsatrearside QL = #holes·qo = 3 qo Qrear= - 3 qo QR,NL,NR = 0 Charge losses(not collectedat end ofintegration time): Qind,j = ± q · w, j ( final position) QL < 3 |Qrear| < 3 QR,NL,NR > 0 for hole losses < 0 forelectronlosses Qind * -0.05 2.6 -0.3 -2.1 Laser electron hole Charge collection close to the Si-Si02 interface of silicon strip sensors

  24. Boundaryconditions ~ humid ? boundaryconditions: constant potential: f = 0 V (Dirichlet) zeroelectricfieldcomponent: Ey = 0 (Neumann) ~ ifdriedat 0 V ? FelDirichletb.c. Fel Neumann b.c. + - - + Charge collection close to the Si-Si02 interface of silicon strip sensors

  25. Charge collection close to the Si-Si02 interface of silicon strip sensors

  26. Time dependence after 1 MGy Charge collection close to the Si-Si02 interface of silicon strip sensors

  27. 200 V, 1 MGy, driedat 0 V Charge collection close to the Si-Si02 interface of silicon strip sensors

  28. Messablauf für Elektronenverluste n-Typ Messablauf: Sensor getrocknet bei 0 V → 200 V Was passiert im Detektor? 0 V : Oxidladungen kompensiert durch freie Ladungsträger 200 V : Oxidladungen unzureichend kompensiert freie Ladungsträger (Elektronen) + + + - - - - freie Ladungsträger (Elektronen) Charge collection close to the Si-Si02 interface of silicon strip sensors

  29. Messablauf für Elektronenverluste Messablauf: Sensor getrocknet bei 0 V → 200 V Was passiert im Detektor? 0 V : Oxidladungen kompensiert durch freie Ladungsträger 200 V : Oxidladungen unzureichend kompensiert Charge collection close to the Si-Si02 interface of silicon strip sensors

  30. Übersicht der Ladungsverluste nonirradiated after 1 MGyphotons driedat 500 V Hole losses driedat 500 V humid, steadystate 6 h humid Electronlosses Electronlosses driedat 0 V dried0 V Charge collection close to the Si-Si02 interface of silicon strip sensors

  31. Measuredsignalcomparedtocalculation Free parameters: numberofelectrons numberofholes diffusionofholes:sdiff stripposition accumulationlayerwidth Fixed parameters: light profiles1=3µm + tailss2=9µm fN=0.35, fNN =0.05, frear =0.06 stripwidth = 12 µm Charge collection close to the Si-Si02 interface of silicon strip sensors

  32. Measuredsignalcomparedtocalculation measurementand fit Free parameters: numberofelectrons numberofholes diffusionofholes:sdiff stripposition accumulationlayerwidth Fixed parameters: light profiles1=3µm + tailss2=9µm fN=0.35, fNN =0.05, frear =0.06 stripwidth = 12 µm read out strip 1 MGydriedat 500Vhumid electr. 1k 35k33k holes 29k 7k31k acclayer 38 µm 30 µm- rearside Charge collection close to the Si-Si02 interface of silicon strip sensors

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