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Life time determination of free charge carriers in irradiated silicon sensors

Life time determination of free charge carriers in irradiated silicon sensors. Thomas Poehlsen , Doris Eckstein*, Joachim Erfle , Eckhart Fretwurst , Erika Garutti , Jörn Lange, Evangelos Nagel, Coralie Neubueser , Georg Steinbrueck Hamburg University *DESY. O verview.

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Life time determination of free charge carriers in irradiated silicon sensors

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  1. Life time determination of free charge carriers in irradiated silicon sensors Thomas Poehlsen, Doris Eckstein*, Joachim Erfle, Eckhart Fretwurst , Erika Garutti, JörnLange, Evangelos Nagel, Coralie Neubueser, Georg Steinbrueck Hamburg University *DESY

  2. Overview Motivation Transient current technique (TCT) Methods to determine the life time: Charge Correction Method (CCM) modified Charge Correction Method (mCCM) Comparison of CCMand mCCMon model calculations and on data Model calculations of TCT pulses Conclusion Outlook Life time determination of free charge carriers in irradiated silicon sensors

  3. Motivation LHC upgrade: 10 x higher radiation damage after high luminosity upgrade radiation hard material needed Radiation induced trapping centers → charge losses, signal reduction Aim: understand and describe signal reduction in irradiated silicon (HL-LHC fluences) Find a standard method to extract life times in irradiated silicon (e.g. for HPK-campaign) Life time determination of free charge carriers in irradiated silicon sensors

  4. Transient currenttechnique (TCT) red laser light pulse: 670 nm, 3 µm penetration depth FWHM 40 ps generates N = ~ 1 million e-h pairs induced current (pad sensor) : readout: digital oscilloscope (bandwidth 1 GHz, 512 averages) 10 x Phillips current amplifier diodecapacitanceof ~4 pFforuseddiodeswithd=150 µm light pulse p+ n n+ e h Life time determination of free charge carriers in irradiated silicon sensors

  5. Difficultiesfor Life Time Determination simulatedelectricfield Trappingprobability 1/tdepends on: Densityof traps Occupationprobabilityof traps • Densityoffreeelectrons • Densityoffreeholes • Temperature • Electricfield Capture crosssectionstrap • Velocityofchargecarriers • Electricfield Electricfield not known Model assumptionstoestimate an effectivetrapping time t Time constantoftheelectronics: O(chargecollection time) V. Eremin NIM A535 (2004) 622–631 Life time determination of free charge carriers in irradiated silicon sensors

  6. Charge Correction Method (CCM) Number of drifting charge carriers N reduces due to trapping: Drift velocityfielddependent charge collection time voltagedependent Trappingcorrectedsignal: Trappingfullycorrected: (fullchargecollection, CCE=1) Correct all voltageswiththe same until linear fit ofQc(V) givesslope = 0 Noreferencemeasurement (Q0) needed! (i.e. noknowledge on CCE needed) CCM was usedforfluencesupto a few 1014cm-2neq. andvoltagesclosetoVfd 150 µm, n-Typ, epitaxial after 1015 cm-2neutrons CCM by G. Kramberger. Doctoral Thesis, Ljubljana, 2001. Life time determination of free charge carriers in irradiated silicon sensors

  7. Limitationsofthe Charge Correction Method 1.) CCE calculated: withtfrom CCM (lines) 2.) CCE measured: CCE = Q / Q0 (Symbols) Comparison: using CCM leadstooverestimationofchargelossesatvoltages >> Vfd ? Comparisonof CCE from CCM and CCE=Q/Q0measured 3·1014 cm-2 1015 cm-2 TCT withaparticles after 23 GeVprotonirradiation J. Lange, 2008 Life time determination of free charge carriers in irradiated silicon sensors

  8. ModifiedCharge CorrectionMethod non-irradiated Collected charge: Use collectedchargeof non-irradiateddiode Q0 Charge collectionefficiencyCCE = Q / Q0 Foreachvoltageindependently: Current is correcteduntil (i.e. ) Extractedlife time depends on voltage: life time field dependent: ? 1·1015 cm-2 2·1015 cm-2 Life time t [ns] t=5ns t=4ns 1015 cm-2 4·1015 cm-2 3·1015 cm-2 Life time determination of free charge carriers in irradiated silicon sensors

  9. Comparisonof CCMandmCCMformodelcalculated TCT signals Model calculation: , electronic distortions calculated in SPICE Vfd = 100 V, Neff = const, d = 200 µm b) a) 600 V CCM mCCM mCCM CCM Evangelos Nagel Evangelos Nagel Simulated Simulated • CCM underestimates mCCM give proper results. Life time determination of free charge carriers in irradiated silicon sensors

  10. Comparison of CCM and mCCM on data FZ 200 µm modified CCM (600 V) CCM • CCM underestimates Life time determination of free charge carriers in irradiated silicon sensors

  11. Ansätze forlife time determination Life time determination of free charge carriers in irradiated silicon sensors

  12. Ansätze forlife time determination Life time determination of free charge carriers in irradiated silicon sensors

  13. Life time comparisonfor different methods modified CCM (600 V) FZ 200 µm lowerlimit on t(600 V) (vdr=vsat) CCM Classical CCM underestimatest Life time determination of free charge carriers in irradiated silicon sensors

  14. Life time comparisonfor different methods vdr = const (600 V) modified CCM (600 V) linear E(x) fromVfd (600 V) linear E(x), Nefffrom I(t) fit (600 V) FZ 200 µm lowerlimit on t(600 V) (vdr=vsat) CCM • Classical CCM underestimatest. • All othermethods:max. difference < 20 % in life time t • exactknowledge on electricfield not neccessarytoextractlife time within 20% uncertainty • (if CCE known) Life time determination of free charge carriers in irradiated silicon sensors

  15. Summary andConclusion Classical Charge Correction Method fails at high fluences (> 1015 cm-2) for electron collection in studied sampled. A modified Charge Correction Method was developed taking into account the charge collection efficiency (CCE). Extracted life times are compatible with model calculations within systematic uncertainties of ~ 20% even without exact knowledge on E(x): t can be estimated with uncertainties of the charge collection efficiency must be taken into account Effective life times are voltage dependent: (higher life times for higher voltages). Life time determination of free charge carriers in irradiated silicon sensors

  16. Outlook t(x) can be extracted without knowledge on . Can we also extract if we know ? May we then draw conclusions on field dependence? Will we see filled electron traps at the n+ side? Combinedstudywithboth, classicalTCT and edge TCT withintheframeworkofthe CMS HPK campaign. Simulatedelectricfield Life time determination of free charge carriers in irradiated silicon sensors

  17. Nefffrom TCT pulses Neff at V=300 V extracted from I(t) (TCT, hole signal) 23 GeV protons (PS) non-irradiated I t 23 MeV protons (KIT) 200 µm MCz n type t Life time determination of free charge carriers in irradiated silicon sensors

  18. Nefffrom TCT pulsesandfrom CV Neff at V=300 V extracted from I(t) (TCT, hole signal) 23 GeV protons (PS) non-irradiated 23 MeV protons (KIT) 200 µm MCz n type Vfd from CV (0°C, 1 kHz) sign according to TCT Life time determination of free charge carriers in irradiated silicon sensors

  19. Life time determination of free charge carriers in irradiated silicon sensors

  20. Model calculation Electricfield Drift velocity Inducedcurrent # freecarriers Electronic circuit Life time determination of free charge carriers in irradiated silicon sensors

  21. Extractionofphysicalquantities Reference diodewithcollected charge Q0=> teff Extraction method: least c2fit ofmodelcalculationtomeasured TCT pulse modelcalculationwithN0= Q0 /q0 drifting charge carriers at t=0 Q0 teff I t0 t d Life time determination of free charge carriers in irradiated silicon sensors

  22. Least c2-fit resultsforMCz200 µm, after 3.9∙1014 cm-223GeV protons and 8 min @80°C := 0.3 µA • least = 35 teff=5.5ns d=200 µm area = Q0 • Data points used for least c2 fit: • n = 35 • 4 free fit parameters: teff, Neff, t0, d • degrees of freedom: ndf = 31 t0 V = 300 V baseline before pulse: ≈ 0.25 µA Life time determination of free charge carriers in irradiated silicon sensors

  23. Vfdand Neff in thepresenseof double junction TCT: assumingconst. space charge: slopeoftheelectricfieldat V > Vfd (e.g. 300 V) CV: Neffaccordingtodepletionbehaviourat Vfd (< 150 V) Howtoextract Neff? Or: whatdoes Neffmeanifextracted via CV? HowtodefinethesignofNeff? Vfd = 80 V ? => Life time determination of free charge carriers in irradiated silicon sensors

  24. rear 10 min @ 60°C MeV protons 80 min @ 60°C Vfdgoes down (from CV measurements) but: highervoltagesneededtodepleterearside! Life time determination of free charge carriers in irradiated silicon sensors

  25. c2matricesforMCz 200 µmafter 4∙1014 cm-2 23 GeV protons, 8 min @ 80°C Neff <-> d correlated d ≈ 200 µm (~198 µm fromCV) Additional uncertainty in Q0 Neff [1012 cm-3] = 5.1 ± 0.3stat ± 0.3Q0 ? Vfd [V] = 158 ± 10stat ± 10Q0 Life time determination of free charge carriers in irradiated silicon sensors

  26. Vergleich von verschiedenen Methoden Modellrechnung unteres Limit CCM G. Kramberger [b = 4.1∙10-16 cm2 ns-1 ] Modellrechnung v=konst mit gemessener ober Grenze für 1/t(600 V) FZ 200 µm EPI 150 µm Modellrechnung mit linearem E(x) Uptofluencesof 2∙1014 cm-2accordingto G. Kramberger*: 1/t = b f here: 1/tatfixedvoltage not proportional tofluence! * G. Kramberger. Doctoral Thesis, Ljubljana, 2001. Life time determination of free charge carriers in irradiated silicon sensors

  27. Vergleich von verschiedenen Methoden gemessene mittlere Driftgeschwindigkeit (600 V) Modifizierte CCM (600 V) Lineares E-Feld (600 V) FZ 200 untere Grenze für t(600 V), aus CCE CCM Classical CCM giveslifetimesincompatiblewith CCE. All othermethodsgiveconsistantresultsforlife time t, max. difference ~ 15 %. Dt|CCE > Dt|method=> exactknowledge on electricfield not neccessarytoextractlife time Life time determination of free charge carriers in irradiated silicon sensors

  28. HPK-campaign, mixedirradiation HPK-campaign mixed irradiation with protons and neutrons according to expected ratio between charged and neutral hadrons. So far: 23 MeV protons from Karlsruhe (KIT) Type inversion in MCz n-type material after 23 MeV proton irradiation observed (not expected). Effect due to the proton energy? |Neff| may be extracted from CV measurements via full depletion voltage Vfd. For non-irradiated diodes: well understood but: only the absolute value |Neff| is accessible For irradiated diodes further limitations: depletion behavior unclear (double junction), frequency and temperature dependent =(V) (space charge is voltage dependent) Life time determination of free charge carriers in irradiated silicon sensors

  29. Comparisonof NeffforMCz n type TCT: extracted at 0°C, V=300 V from dE/dx CV: extracted at 0°C, 1 kHz from Vfd sign according to TCT fit result! 23 GeV (PS) 23 MeV (KIT) Life time determination of free charge carriers in irradiated silicon sensors

  30. Vfdand Neff in thepresenseof double junction TCT: assumingconst. space charge: slopeoftheelectricfieldat V > Vfd (e.g. 300 V) CV: Neffaccordingtodepletionbehaviourat Vfd (< 150 V) Howtoextract Neff? Or: whatdoes Neffmeanifextracted via CV? HowtodefinethesignofNeff? Vfd = 80 V ? => Life time determination of free charge carriers in irradiated silicon sensors

  31. Conclusions Neff could be extracted from TCT current measurement and is found to strongly depend on: annealing (1 min to 10 min @ 80 °C -> DNeff = 3.5 ∙ 1012 cm-3) proton energy (23 GeV vs. 23 MeV) Differences to |Neff| extracted from CV measurements observed open questions: Impact of a voltage dependent space charge on CV and TCT interpretation? (depletion behaviour unclear, TCAD simulation of double junction and CV?) How good is the assumption t = const for given voltage, i.e. position dependence t(x) negligible? (combined edge-TCT / TCT study) Systematic impact of electronic circuit? (description improvable?) Life time determination of free charge carriers in irradiated silicon sensors

  32. front Life time determination of free charge carriers in irradiated silicon sensors

  33. rear Life time determination of free charge carriers in irradiated silicon sensors

  34. Life time determination of free charge carriers in irradiated silicon sensors

  35. Life time determination of free charge carriers in irradiated silicon sensors

  36. Life time determination of free charge carriers in irradiated silicon sensors

  37. Life time determination of free charge carriers in irradiated silicon sensors

  38. Voltagedependence 8@80°C 200 V Neff [1012 cm-3] = 4 300 V Neff[1012 cm-3] = 5.1 ± 0.3stat ± 0.3Q0 ( d = 200 µm ) 400 V Neff [1012 cm-3] = 5.7 for d = 200 µm fixed 6.5 for d = 196 µm free 700 V fit not possiblewithgiven electronic circuit& driftmodel -> tooptimize! 700 V 400 V 200 V Life time determination of free charge carriers in irradiated silicon sensors

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