measurement of the trapping time constants in silicon with...
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![Page 1: Measurement of the Trapping Time Constants in Silicon with ...rd50.web.cern.ch/rd50/2nd-workshop/talks/2-4-Olaf-Krasel.pdf · Transient Current Technique, Set-up • 672nm red laser](https://reader033.vdocuments.site/reader033/viewer/2022042216/5ebdc26d0b9b9e64a3171625/html5/thumbnails/1.jpg)
Measurement of theTrapping Time Constants in Silicon
with the Transient Current Technique
*O. Krasel, C. Gößling, J. Klaiber-Lodewigs, R. Klingenberg,M. Maß, S. Rajek and R. Wunstorf for the ATLAS Collaboration
Lehrstuhl f. Experimentelle Physik IV, Universität Dortmund
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Olaf Krasel, Universität Dortmund 2
• fluence up to 1015 1-MeV-neq/cm2
for design luminosityData on charge collection efficiency are needed for:• detector simulation• optimization of operation conditions
Motivation: The ATLAS Pixel Detector• innermost part of tracking system
• 3 points for tracking
• coverage 0≤η≤2.5
• 3 barrel layers, 6 wheels
• total active area ≈2m² (1744 modules with ≈80 million pixel)
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Olaf Krasel, Universität Dortmund 3
Effective Trapping Time
• Trapping leads to charge carrier loss:
• charge induced on electrodes by driftingcharge (Ramo’s theorem):
• resulting (measured) signal current:
• injection with short range laser from one sideallows to distinguish between electron andhole signal
)(with ,)(1
)( eqeffeffeff
dttQtdQ Φ=−= τττ
)/exp()()( 0effm ttv
dQ
ti τ−=
dttvdQ
dxdQ
dq )(==
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Olaf Krasel, Universität Dortmund 4
Transient Current Technique, Set-up
• 672nm red laser (3.5µm absorption length, FWHM = 44ps),• voltage source with picoamperemeter (Keithley 487)• fast pulse amplifier (10×, 100 kHz - 1.8 GHz), (current sensitive!)• oscilloscope (Tektronix TDS 784D, band width 1 GHz)• rise time of system (incl. detector) about 1 ns• PC readout system (LabVIEW)• cooling system (-20°C - +20°C, RMS 0.2°C)
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Olaf Krasel, Universität Dortmund 5
Charge Correction Method (CCM)• correction of measured current:
•
• t obtained from slope vs. t plotdepbias
cc
VV
dttiQ
>
== ∫ and correct for
const.)(
τ
( )τ/exp)()( ttiti measc +⋅=icorr(corrected)
im (measured)
lines with τ=const.
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Olaf Krasel, Universität Dortmund 6
Exponentiated Charge Crossing (ECC)
dtttiQ meas )/exp()(exp τ+= ∫• exponentiated charges from different Vbias are plotted vs. 1/τ• 1/τ is obtained from mean of intersection points of lines
icorr(corrected)
im (measured)
lines with Vbias=const.
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Olaf Krasel, Universität Dortmund 7
Samples
• irradiated with 24 GeV protons at CERN-PS,no biasing during irradiation
• 4 diodes from ATLAS-Pixel pre-production (CiS),irradiated to 1.1, 3.2, 5.0 and 8.9⋅1014 neq/cm2
annealed to minimum in Vdep at 60ºC• 2 diodes from SRD-Project (CiS),
irradiated to 4.5 and 6.5⋅1014 neq/cm2;annealed to minimum in Vdep at room temperature
• ⟨111⟩ crystal orientation,24h at 1200ºC oxygenation
For the irradiation of the samples I want to thank M. Glaser,M. Moll (CERN) and P. Sicho (Acadamy of Sciences, Prague)
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Olaf Krasel, Universität Dortmund 8
Trapping Times: Fluence Dependence
/nscm10)16.016.5( 216−⋅±=eβ /nscm10)16.004.5( 216−⋅±=hβ
• errors include only “statistical” error, systematics not yet included• comparison with results obtained by other groups (extrapolated from data taken at fluences < 2.4⋅1014neq/cm2): agreement for electrons, deviation for holes
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Olaf Krasel, Universität Dortmund 9
Trapping Times: Annealing
holes,cm/1046.4 2eq
14eq n⋅=Φ
electrons
,cm/1046.4 2eq
14eq n⋅=Φ
( ) ( )( )( )( ) 19% by increase h,59.069.1
20%,by decreaseh,80.077.2
/exp1/exp)(
,
,
0
≈±=
≈±=−−⋅+−⋅= ∞
ha
ea
aa ttt
τ
ττβτββ
• ansatz for annealing function:
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Olaf Krasel, Universität Dortmund 10
Charge Collection Efficiency
electrons
,cm/1046.4 2eq
14eq n⋅=Φ
holes
,cm/1046.4 2eq
14eq n⋅=Φ
• CCE changes during annealing due to changes in Neff and t
• charge collection efficiencies CCEmin refer to chargesinjected close to surface
• therefore they give only a lower limit for CCE interesting inHEP experiments
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Olaf Krasel, Universität Dortmund 11
Charge Collection Efficiency
• CCEmin for electrons is40-50% at Vdep50-60% at Vdep+50V (as foreseen in ATLAS)
electrons
,cm/1046.4 2eq
14eq n⋅=Φ
Vdep from chargecollection curve,? Vdep=15V
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Olaf Krasel, Universität Dortmund 12
Conclusions• the effective trapping times of holes and electrons in
silicon have been measured for fluences between 1.1 and8.9·1014 1-MeV-neq/cm2
•
• confirmed• results for electrons are consistent with extrapolation of
measurements at lower fluences (Brodbeck et al., 2000;Kramberger et al., 2000)
• trapping times for holes are not consistent with thesemeasurements
• annealing of t is negligible, for electrons it even improvesslightly (important for n+-on-n-pixels)
( ) ( )
)ts!measuremenother to(contrary
ns,/cm1016.004.5 ns,/cm1016.016.5
e
216216
h
he
ββ
ββ
≈
⋅±=⋅±= −−
eqΦ⋅= βτ1
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Olaf Krasel, Universität Dortmund 13
Finally...
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Olaf Krasel, Universität Dortmund 14
Signal Treatment
• bandwidth:
• tail (problems withreflections/oscillations):
DRC
BW
mRCmm
RC
tu
tututu
==
+→
τ
τ)(
)()()( &
parametersfit are,
point inflection
for for
)/exp()(
)(
0
0
f
ip
ip
ip
f
BWBW
tu
t
tttt
ttutu
tu
=
>≤
−→