Post on 31-Dec-2015
22 October 2009 FCAL workshop, Geneve 1
Polarization effects in the radiation damaged
scCVD Diamond detectors
Sergej Schuwalow, DESY Zeuthen
On behalf of FCAL collaboration
22 October 2009 FCAL workshop, Geneve 2
Diamond properties
Density 3.52 g cm-3
Dielectric constant 5.7 Breakdown field 107 V cm-1
Resistivity >1011 Ω cm Band gap 5.5 eV Electron mobility 1800 (4500) cm2 V-1 s-1
Hole mobility 1200 (3800) cm2 V-1 s-1
Energy to create e-h pair 13.1 eV Average signal created 36 e μm-1
*High purity single crystal CVD diamond
22 October 2009 FCAL workshop, Geneve 3
MIP Response of scCVD Diamond
Diamond
90Sr
Scint.
PM1
PM2
discr
discr
PA delay
&
AD
C
90Sr Source
Sensor box & Preamplifier
Trigger box
ADC spectrum
22 October 2009 FCAL workshop, Geneve 4
‘Ideal’ crystal charge collection
Charge collection efficiency depends on E
Full chargecollection
HV=0
Recombination
HV≠0
Charge collection
+
-
CCE = Qcollected/Qproduced CCD = CCE*d
d
22 October 2009 FCAL workshop, Geneve 5
Radiation damaged crystal
Radiation causes local damages of the lattice structure
These local damages (traps) are able to capture free charge carriers and release them after some time
Assumptions we are using:
Trap density is uniform (bulk radiation damage)
Traps are created independently (linearity vs dose)
electrons
holes
Ionization
22 October 2009 FCAL workshop, Geneve 6
Irradiation of scCVD Diamond
After 5 MGy dose diamond detector is operational
CCD is decreasing with the absorbed dose
Generation of trapping centers due to irradiation
Traps release? CCDcurrent < CCDMIP? Too high ‘missing charge’
~Natoms in the sample Pure trapping mechanism
is contradictory Recombination is
important Polarization?
‘missing charge’
DALINAC, TU-Darmstadt June 2007
CCD from 90Sr setup
CCD from Isens
22 October 2009 FCAL workshop, Geneve 7
Irradiation of scCVD Diamond
No annealing! 1.5 years, a lot of
tests with 90Sr Source, UV-light, several TSC measurements
After 10 MGy absorbed dose MIP signal is still detectable
Leakage current is very small ~pA
Continued in December 2008
Jun 2007 Data
Dec 2008 Data
22 October 2009 FCAL workshop, Geneve 8
Polarization origin
Uniform generation of e-h pairs Asymmetry is introduced by the
applied electric field Specific free charge carrier Specific free charge carrier
density is largest near detector density is largest near detector edgesedges
Asymmetric trap filling according Asymmetric trap filling according to charge carrier densityto charge carrier density
Space charge creation in the bulk Space charge creation in the bulk of the detectorof the detector
Compensation of the external Compensation of the external field by space chargefield by space charge
PolarizationPolarization
+ -Eext
22 October 2009 FCAL workshop, Geneve 9
Polarization origin
Uniform generation of e-h pairs Asymmetry is introduced by the
applied electric field Specific free charge carrier
density is largest near detector edges
Asymmetric trap filling according Asymmetric trap filling according to charge carrier densityto charge carrier density
Space charge creation in the bulk Space charge creation in the bulk of the detectorof the detector
Compensation of the external Compensation of the external field by space chargefield by space charge
PolarizationPolarization
+ -Eext
22 October 2009 FCAL workshop, Geneve 10
Polarization origin
Uniform generation of e-h pairs Asymmetry is introduced by the
applied electric field Specific free charge carrier
density is largest near detector edges
Asymmetric trap filling according Asymmetric trap filling according to charge carrier densityto charge carrier density
Space charge creation in the bulk Space charge creation in the bulk of the detectorof the detector
Compensation of the external Compensation of the external field by space chargefield by space charge
PolarizationPolarization
+ -Eext
22 October 2009 FCAL workshop, Geneve 11
Polarization origin
Uniform generation of e-h pairs Asymmetry is introduced by the
applied electric field Specific free charge carrier
density is largest near detector edges
Asymmetric trap filling according to charge carrier density
Space charge creation in the bulk Space charge creation in the bulk of the detectorof the detector
Compensation of the external Compensation of the external field by space chargefield by space charge
PolarizationPolarization
+ -Eext
22 October 2009 FCAL workshop, Geneve 12
Uniform generation of e-h pairs Asymmetry is introduced by the
applied electric field Specific free charge carrier
density is largest near detector edges
Asymmetric trap filling according to charge carrier density
Space charge creation in the bulk of the detector
Compensation of the external field by space charge
Polarization
Epol
+ -Eext
Polarization origin
E
E00 depth
22 October 2009 FCAL workshop, Geneve 13
Model: 340 μm scCVD diamond after 5 MGy CCD time dependence
time
Space charge Charge collection distance
Electric fieldExpected Signal shape
Neg Pos
Initial field
Low field,recombination
Effective charge collection regions
22 October 2009 FCAL workshop, Geneve 14
Model: 340 μm scCVD diamond after 5 MGy CCD time dependence
time
Space charge Charge collection distance
Electric fieldExpected Signal shape
Neg Pos
Zero field
E
22 October 2009 FCAL workshop, Geneve 15
Model: 340 μm scCVD diamond after 5 MGy CCD time dependence
time
Space charge Charge collection distance
Electric fieldExpected Signal shape
Neg Pos
To be confirmed!
22 October 2009 FCAL workshop, Geneve 16
Long-term signal evolution
Try to minimize an influence of the measurement onto the filled trap distribution
Use the source only for short CCD evaluation runs
Polarization is seen even after 1 month after the initial pumping – long living traps, possibility to fill all of them!
t0 = 35 days
22 October 2009 FCAL workshop, Geneve 17
Damaged Sensor under 90Sr Source: CCD vs time
Illuminate by UV-light to free all traps
Apply HV and source 90Sr
-HV
22 October 2009 FCAL workshop, Geneve 18
Damaged Sensor under 90Sr Source: CCD vs time
90Sr
±HV
Illuminate by UV-light to free all traps
Apply HV and source
22 October 2009 FCAL workshop, Geneve 19
Damaged Sensor under 90Sr Source: CCD vs time
90Sr
±HV
Illuminate by UV-light to free all traps
Apply HV and source
22 October 2009 FCAL workshop, Geneve 20
Damaged Sensor under 90Sr Source: CCD vs time
90Sr
±HV
Illuminate by UV-light to free all traps
Apply HV and source
Sh
ort
liv
ing
tra
ps
22 October 2009 FCAL workshop, Geneve 21
Beam Pumping Test
Trigger Box Linear Drive
90Sr Source
Collimator
Faraday Cup
Detector+Preamp Box
Collimator
Beam
Use intensive beam to fill up short living traps
Move (remotely) detector/preamp box to the low intensity 90Sr line
Measure signal evolution with time since beam-off
22 October 2009 FCAL workshop, Geneve 22
Beam Pumping Test
Clear indication to the presence of fast decaying traps. Additional polarization due to shallow defects filling
5 MGy10 MGy
Dose rate ~ 100 x highest dose rate @ ILC detector
22 October 2009 FCAL workshop, Geneve 23
TSC measurements
At least 3 levels are visible:
trap1 trap2 trap3
Ec-ET
[eV]
1.144
+0.002
0.851
+0.002
0.746
+0.006
nT0
[1014cm-3]
5.7 1.5 0.2
After 5 MGy
Trap concentration ~ 1015 cm-3 (still 8 orders of magnitude less than normal atom density)
22 October 2009 FCAL workshop, Geneve 24
Summary
The performance of scCVD Diamond sensor was studied as a function of absorbed dose up to 10 MGy
Strong polarization effects are observed in the radiation damaged scCVD Diamond detector
Polarizaton significantly decreases the detector charge collection efficiency in addition to pure trapping mechanism
A simple model is developed in order to understand and describe observed phenomena
Method of routinely switching bias HV polarity is proposed to suppress bulk polarization of long-living traps
Beam pumping tests indicate that short-living traps are responsible for the residual detector inefficiency
22 October 2009 FCAL workshop, Geneve 25
Thank you