Neutron Induced Radiation Damage of KETEK SiPMs M. Centis Vignali 1 c V. Chmill b E. Garutti a R. Klanner a M. Nitschke a J. Schwandt a S. Sonder a a Hamburg University b Samara University c CERN 22.11.2016 1 matteo.centis.vignali@cern.ch
SiPMs, MPPCs > Silicon PhotoMultipliers > Multi Pixel Photon Counters > Array of avalanche photodiodes (APD) operated in Geiger mode > Quenching resistor ( R q ) in series to stop avalanche > APD in series with R q ≡ pixel > Light intensity sensitivity → count firing pixels > Pixel size: 5 − 100 µ m (side) > Avalanche zone: 0 . 5 − 2 µ m > Operating voltage: < 100 V > Gain: 10 5 − 10 6 22.11.2016 Neutron Induced Radiation Damage of KETEK SiPMs 2 / 18
IV Characteristic IV curve for a non-irradiated SiPM Different illumination conditions Breakdown voltage ( V bd ) Minimum bias voltage to achieve Geiger discharges in the pixels. The SiPM is operated a few voltages above the breakdown voltage 22.11.2016 Neutron Induced Radiation Damage of KETEK SiPMs 3 / 18
Charge Spectra Measurements Pixels firing with LED illumination Pixel model C pix R q Distribution of the integrated pulses > The capacitor is charged by the bias voltage > A Geiger discharge discharges the capacitor through R q G = C pix ( V − V ∗ bd ) / q 0 22.11.2016 Neutron Induced Radiation Damage of KETEK SiPMs 4 / 18
Dark Count Rate, Cross Talk, After Pulse Dark count rate (DCR) Charge carriers reach the avalanche region and cause a Geiger breakdown. Thermal generation, tunneling, diffusion, etc. Cross talk (XT) Photons are emitted during a Geiger avalanche and interact in a neighboring pixel. After pulse(AFP) Charge carriers created during an avalanche are trapped and released after some time causing a second avalanche. 22.11.2016 Neutron Induced Radiation Damage of KETEK SiPMs 5 / 18
Motivation Strategy HEP applications of SiPMs > Determine relevant parameters > Calorimetry, timing, ... > Build models > Are SiPMs rad. hard enough? > Improve design Measurement C pix R q V bd DCR G CN High fluence Charge (QDC) spectra X X X X X X No IV - X X X X X Yes CV-f X X - - - - Yes Pulse height measurements do not work for highly irradiated SiPMs This talk focus on IV and CV-f measurements V > V bd > G → gain > I dark = DCR G ( 1 + CN ) q 0 > CN → correlated noise (XT, AFP) > I LED = N γ A ∗ prob G ( 1 + CN ) q 0 > N γ → LED photon rate > G = C pix ( V − V ∗ bd ) / q 0 > A ∗ prob → avalanche prob. for light The results are specific to a SiPM model, the methods are general 22.11.2016 Neutron Induced Radiation Damage of KETEK SiPMs 6 / 18
Sensors Sensor > Produced by KETEK > 1 mm diameter, 4384 pixels > Pitch: 15 µ m in each direction > Depletion depth ≈ 0 . 75 µ m > V bd ≈ 27 V Irradiation > Bare Si die (unpackaged SiPM) > Neutrons → only bulk damage > Φ eq = 10 9 , ..., 10 12 cm − 2 22.11.2016 Neutron Induced Radiation Damage of KETEK SiPMs 7 / 18
C pix and R q Extraction Data measured at 25 ◦ C, 26.5 V ( ≈ V bd − 0 . 5 V) Series resistance Parallel capacitance Pixel model C pix R q C p = C pix N pix R s = R q / N pix Parameters optimized till agreement The full electrical model describes the frequency dependence of the data The relevant parameters can be extracted Circuit model: C. Xu et al. NIM A762 (2014) 149-161 An additional inductance is needed to describe high freq. 22.11.2016 Neutron Induced Radiation Damage of KETEK SiPMs 8 / 18
C pix and R q vs Φ eq Ratio of the parameters after/before irradiation Φ eq = 0 , 10 9 , 10 10 , 10 11 , 5 · 10 11 , 10 12 cm − 2 Quenching resistance R q Pixel capacitance C pix 1.1 1.1 = 0) = 0) 1.08 1.08 eq eq Φ Φ 1.06 1.06 ( ( q pix ) / R ) / C 1.04 1.04 eq 1.02 eq 1.02 Φ Φ ( q ( R 1 pix 1 C 0.98 0.98 0.96 0.96 0.94 0.94 0.92 0.92 0.9 0.9 9 10 11 12 9 10 11 12 10 10 10 10 10 10 10 10 Φ -2 Φ -2 [cm ] [cm ] eq eq > R q (Φ eq = 0 ) ≈ 575 k Ω G = C pix ( V − V ∗ bd ) / q 0 > 3% increase at Φ eq = 10 12 cm − 2 > C pix (Φ eq = 0 ) ≈ 18 fF > Systematic shift for all fluences → effect not due to irradiation C pix and R q change by less than 3% with irradiation V ∗ bd can be different than the breakdown voltage 22.11.2016 Neutron Induced Radiation Damage of KETEK SiPMs 9 / 18
IV measurements Φ eq = 10 12 cm − 2 , LED off T = 0 ◦ C, LED off − 2 10 − Current [A] 10 2 Current [A] 30 C − 3 10 − 10 3 25 C − 4 Φ -2 10 = 0 cm − 4 10 20 C eq Φ − 9 -2 10 5 = 10 cm − 5 15 C eq 10 Φ 10 − = 10 cm -2 6 − 10 C 10 6 eq 10 Φ 11 -2 − = 10 cm 7 − 5 C 10 eq 7 10 Φ 12 -2 = 10 cm − 0 C 8 eq − 10 8 10 -5 C − 9 10 − 9 10 -10 C − 10 10 − 10 10 -15 C − 11 10 − 11 -20 C 10 − 10 12 − -25 C 12 10 − -30 C 13 − 10 13 10 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 Bias [V] Bias [V] > IV characteristics measured as a function of T, Φ eq , LED on/off > Extraction of: V bd , photodetection signal, DCR > Investigation of DCR origin 22.11.2016 Neutron Induced Radiation Damage of KETEK SiPMs 10 / 18
Breakdown Voltage Φ eq = 0 , 10 9 , 10 10 , 10 11 , 5 · 10 11 , 10 12 cm − 2 � − 1 � d ln ( I ) ILD = dV 0.1 ) [V] 0.08 eq Φ 0.06 ( bd = 0) - V 0.04 0.02 eq 0 Φ ( bd − 0.02 V − 0.04 − 0.06 − 0.08 − 0.1 9 10 11 12 10 10 10 10 Φ -2 [cm ] eq 27.8 [V] bd 27.6 V G = C pix ( V − V ∗ bd ) / q 0 27.4 27.2 > ILD of LED photocurrent 27 Φ -2 = 0 cm eq > min ( ILD ) ⇔ V = V bd Φ 9 26.8 = 10 cm -2 eq Φ 10 -2 = 10 cm eq 26.6 Φ 11 -2 > No irradiation effects = 10 cm eq Φ 12 -2 = 10 cm 26.4 eq > Linear dependence on T 26.2 − − − 30 20 10 0 10 20 30 Temperature [C] The breakdown voltage measured using LED photocurrent is not affected by irradiation 22.11.2016 Neutron Induced Radiation Damage of KETEK SiPMs 11 / 18
Signal from Photodetection Measurements: I dark , I ∗ LED I LED → LED photocurrent I norm LED → charge / 1 photon LED = I LED ( V ) / I LED ( 7 . 5 V | M ∗ = 1 ) I norm V < V bd I LED = N γ M ∗ q 0 N γ → photon rate M ∗ → multiplication LED light I norm LED = M ∗ > I norm V > V bd LED spans several orders of magnitude I LED = N γ A ∗ prob G ( 1 + CN ) q 0 > Variation of I norm LED (Φ eq = 10 12 cm − 2 ) G → gain < 9 % I norm LED (Φ eq = 0 ) CN → correlated noise (XT, AFP) > A ∗ prob G ( 1 + CN ) changes < 9 % prob → avalanche prob. LED light A ∗ I norm LED = A ∗ prob G ( 1 + CN ) Photon detection changes by less than 9% with irradiation 22.11.2016 Neutron Induced Radiation Damage of KETEK SiPMs 12 / 18
Dark Count Rate from IV measurements Φ eq = 0 , 10 12 cm − 2 Measurements: I dark , I ∗ 10 11 LED I LED → LED photocurrent 10 10 I norm LED → charge / 1 photon 20°C 0 neq 10 9 LED = I LED ( V ) / I LED ( 7 . 5 V | M ∗ = 1 ) 20°C 1E12 neq I norm DCR / A* [Hz] 0°C 0 neq 10 8 0°C 1E12 neq 10 7 V > V bd -20°C 0 neq -20°C 1E12 neq 10 6 I dark = DCR G ( 1 + CN ) q 0 10 5 I norm LED = G ( 1 + CN ) A ∗ prob 10 4 G → gain 10 3 CN → correlated noise (XT, AFP) 25 30 35 40 A ∗ prob → avalanche prob. LED light Voltage [V] I dark = DCR > At 20 ◦ C DCR increases of 5 orders of q 0 I norm A ∗ magnitude LED prob > 10 11 Hz ⇒ 10 4 pulses / 100 ns gate DCR strongly affected by irradiation Using IV, rates higher than 10 7 Hz are accessible 22.11.2016 Neutron Induced Radiation Damage of KETEK SiPMs 13 / 18
Interlude > The photodetector characteristics of a SiPM are not affected by irradiation > The DCR increases significantly with irradiation DCR scaling using QDC spectra If the DCR has been measured before irradiation (QDC spectra) ⇒ The DCR of the irradiated SiPM is I dark (Φ eq ) DCR (Φ eq ) = DCR (Φ eq = 0 ) I dark (Φ eq = 0 ) Factor 2 difference wrt method shown in previous slide What is the origin of increased DCR? Bulk generation? Diffusion? Surface generation? 22.11.2016 Neutron Induced Radiation Damage of KETEK SiPMs 14 / 18
Arrhenius Plot V < V bd V = 5 , 15 V, Φ eq = 0 , 10 12 cm − 2 V = 5 V ⇒ M = 1 − 9 10 Current [A] − 9 10 Current [A] − 10 10 − 10 10 − 10 11 − 11 10 − 10 12 − 12 10 Φ -2 = 0 cm eq − 13 Φ 9 10 = 10 cm -2 Φ -2 − 13 = 0 cm , 5 V bias 10 eq eq Φ 10 = 10 cm -2 Φ -2 eq = 0 cm , 15 V bias Φ eq − 11 -2 14 = 10 cm 10 eq − Φ 12 10 14 = 10 cm -2 , 5 V bias Φ 12 = 10 cm -2 eq eq Φ 12 = 10 cm -2 , 15 V bias − eq 15 10 − 38 40 42 44 46 48 10 15 38 40 42 44 46 48 1/kT [eV -1 ] -1 1/kT [eV ] > Identify origin of DCR using > No description for Φ eq = 10 12 cm − 2 , 15 V activation energy E a (or trap level) > Fit function from SRH bulk → multiplication effects generation → eh gen. in multiplication region � � Eg > No multiplication for Φ eq = 0 cm − 2 Eg E a − I = I 0 T 2 e − 2 kT / cosh 2 kT → eh gen. outside mult. region Activation energy changes by 0 . 034 ± 0 . 006 eV with irradiation For Φ eq = 0 cm − 2 current generation mainly outside multiplication region 22.11.2016 Neutron Induced Radiation Damage of KETEK SiPMs 15 / 18
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