20th RD50 workshop Bari May 2012 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

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

Thomas Pöhlsen 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 May

Thomas Pöhlsen Transient current technique (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 diode capacitance of ~4 pF for used diodes with d=150 µm Life time determination of free charge carriers in irradiated silicon sensors May np+n+ light pulse h e

Thomas Pöhlsen Difficulties for Life Time Determination Trapping probability 1/ depends on: Density of traps Occupation probability of traps Density of free electrons Density of free holes Temperature Electric field Capture cross section trap Velocity of charge carriers Electric field Electric field not known Model assumptions to estimate an effective trapping time Time constant of the electronics: O (charge collection time) Life time determination of free charge carriers in irradiated silicon sensors May V. Eremin NIM A535 (2004) 622–631 simulated electric field

Thomas Pöhlsen Charge Correction Method (CCM) Life time determination of free charge carriers in irradiated silicon sensors May µm, n-Typ, epitaxial after cm -2 neutrons CCM by G. Kramberger. Doctoral Thesis, Ljubljana, 2001.

Thomas Pöhlsen Limitations of the Charge Correction Method Life time determination of free charge carriers in irradiated silicon sensors May ·10 14 cm cm -2 TCT with particles after 23 GeV proton irradiation J. Lange, 2008 Comparison of CCE from CCM and CCE=Q/Q 0 measured

Thomas Pöhlsen Modified Charge Correction Method Life time determination of free charge carriers in irradiated silicon sensors May =4ns =5ns cm -2 1 · cm -2 4·10 15 cm -2 2 · cm -2 non-irradiated Life time [ns] 3·10 15 cm -2

Thomas Pöhlsen Comparison of CCM and mCCM for model calculated TCT signals Life time determination of free charge carriers in irradiated silicon sensors May V Evangelos Nagel mCCM CCM Simulated Evangelos Nagel CCM mCCM Simulated

Thomas Pöhlsen Comparison of CCM and mCCM on data Life time determination of free charge carriers in irradiated silicon sensors May modified CCM (600 V) CCM FZ 200 µm

Thomas Pöhlsen Ansätze for life time determination Input dataAssumptions Charge Correction Method (CCM) time resolved current I(t) = const modified CCMtime resolved current I(t) non-irradiated Q 0 = (V) Life time determination of free charge carriers in irradiated silicon sensors May

Thomas Pöhlsen Ansätze for life time determination Input dataAssumptions Charge Correction Method (CCM) time resolved current I(t) modified CCMtime resolved current I(t) non-irradiated Q 0 Model calculation with measured drift velocity Model calculation with linear field E(x) Model calculation with max. drift velocity v sat Life time determination of free charge carriers in irradiated silicon sensors May

Thomas Pöhlsen Life time comparison for different methods Life time determination of free charge carriers in irradiated silicon sensors May Classical CCM underestimates modified CCM (600 V) lower limit on (600 V) (v dr =v sat ) CCM FZ 200 µm

Thomas Pöhlsen Life time comparison for different methods Life time determination of free charge carriers in irradiated silicon sensors May Classical CCM underestimates. All other methods: max. difference < 20 % in life time exact knowledge on electric field not neccessary to extract life time within 20% uncertainty (if CCE known) FZ 200 µm v dr = const (600 V) modified CCM (600 V) linear E(x) from V fd (600 V) linear E(x), N eff from I(t) fit (600 V) lower limit on (600 V) (v dr =v sat ) CCM

Thomas Pöhlsen Summary and Conclusion Life time determination of free charge carriers in irradiated silicon sensors May

Thomas Pöhlsen Outlook (x) Life time determination of free charge carriers in irradiated silicon sensors May Simulated electric field

Thomas Pöhlsen N eff from TCT pulses 23 MeV protons (KIT) 23 GeV protons (PS) May 2012 Life time determination of free charge carriers in irradiated silicon sensors 17 N eff at V=300 V extracted from I(t) (TCT, hole signal) non-irradiated I t 200 µm MCz n type

Thomas Pöhlsen N eff from TCT pulses and from CV 23 GeV protons (PS) May 2012 Life time determination of free charge carriers in irradiated silicon sensors 18 N eff at V=300 V extracted from I(t) (TCT, hole signal) non-irradiated 23 MeV protons (KIT) 200 µm MCz n type

Thomas Pöhlsen Life time determination of free charge carriers in irradiated silicon sensors May 2012 Seite 19

Thomas Pöhlsen Model calculation Life time determination of free charge carriers in irradiated silicon sensors May 2012 Seite 20 Induced current Drift velocity # free carriers Electric field Electronic circuit

Thomas Pöhlsen Extraction of physical quantities Reference diode with collected charge Q 0 => eff Extraction method: least 2 fit of model calculation to measured TCT pulse model calculation with N 0 = Q 0 /q 0 drifting charge carriers at t=0 Life time determination of free charge carriers in irradiated silicon sensors May 2012 Seite 21 I t eff t0t0 d Q0Q0

Thomas Pöhlsen Least 2 -fit results for MCz 200 µm, after cm -2 23GeV protons and 8 Life time determination of free charge carriers in irradiated silicon sensors May 2012 Seite 22 eff =5.5ns area = Q 0 d=200 µm Data points used for least 2 fit: n = 35 4 free fit parameters: eff, N eff, t 0, d degrees of freedom: ndf = 31 V = 300 V t0t0

Thomas Pöhlsen V fd and N eff in the presense of double junction TCT: assuming const. space charge: slope of the electric field at V > V fd (e.g. 300 V) CV: N eff according to depletion behaviour at V fd (< 150 V) How to extract N eff ? Or: what does N eff mean if extracted via CV? How to define the sign of N eff ? Life time determination of free charge carriers in irradiated silicon sensors May 2012 Seite 23 V fd = 80 V ? =>

Thomas Pöhlsen Life time determination of free charge carriers in irradiated silicon sensors May 2012 Seite 24 rear MeV protons V fd goes down (from CV measurements) but: higher voltages needed to deplete rear side! 10 60°C 80 60°C

Thomas Pöhlsen 2 matrices for MCz 200 µm after cm GeV protons, 8 80°C N eff d correlated d 200 µm (~198 µm from CV) Additional uncertainty in Q 0 N eff [10 12 cm -3 ] = 5.1 ± 0.3 stat ± 0.3 Q0 ? V fd [V] = 158 ± 10 stat ± 10 Q0 May 2012 Life time determination of free charge carriers in irradiated silicon sensors Seite 25

Thomas Pöhlsen Vergleich von verschiedenen Methoden Life time determination of free charge carriers in irradiated silicon sensors May 2012 Seite 26 Up to fluences of cm -2 according to G. Kramberger*: 1/ = here: 1/ at fixed voltage not proportional to fluence! FZ 200 µm EPI 150 µm CCM ober Grenze für 1/ (600 V) * G. Kramberger. Doctoral Thesis, Ljubljana, G. Kramberger [ cm 2 ns -1 ] Modellrechnung unteres Limit Modellrechnung v=konst mit gemessener Modellrechnung mit linearem E(x)

Thomas Pöhlsen Vergleich von verschiedenen Methoden Life time determination of free charge carriers in irradiated silicon sensors May 2012 Seite 27 Classical CCM gives life times incompatible with CCE. All other methods give consistant results for life time max. difference ~ 15 %. | CCE > | method => exact knowledge on electric field not neccessary to extract life time gemessene mittlere Driftgeschwindigkeit (600 V) Modifizierte CCM (600 V) Lineares E-Feld (600 V) untere Grenze für (600 V), aus CCE CCM FZ 200

Thomas Pöhlsen HPK-campaign, mixed irradiation Life time determination of free charge carriers in irradiated silicon sensors May 2012 Seite 28

Thomas Pöhlsen Comparison of N eff for MCz n type 23 MeV (KIT) 23 GeV (PS) May 2012 Life time determination of free charge carriers in irradiated silicon sensors Seite 29

Thomas Pöhlsen V fd and N eff in the presense of double junction TCT: assuming const. space charge: slope of the electric field at V > V fd (e.g. 300 V) CV: N eff according to depletion behaviour at V fd (< 150 V) How to extract N eff ? Or: what does N eff mean if extracted via CV? How to define the sign of N eff ? Life time determination of free charge carriers in irradiated silicon sensors May 2012 Seite 30 V fd = 80 V ? =>

Thomas Pöhlsen Conclusions N eff could be extracted from TCT current measurement and is found to strongly depend on: annealing (1 min to °C -> N eff = cm -3 ) proton energy (23 GeV vs. 23 MeV) Differences to |N eff | 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 = const for given voltage, i.e. position dependence (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 May 2012 Seite 31

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Thomas Pöhlsen Voltage dependence 200 VN eff [10 12 cm -3 ] = V N eff [10 12 cm -3 ] = 5.1 ± 0.3 stat ± 0.3 Q0 ( d = 200 µm ) 400 VN eff [10 12 cm -3 ] = 5.7for d = 200 µm fixed 6.5 for d = 196 µm free 700 Vfit not possible with given electronic circuit & drift model -> to optimize! Life time determination of free charge carriers in irradiated silicon sensors May 2012 Seite V 700 V 400 V