1. Simulation of signal in irradiated silicon detectors
Gregor Kramberger , DESY Hamburg
Devis Contarato, University of Hamburg
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

2. Outline Motivation Basics of simulations
calculation of induced current
ATLAS silicon detectors simulation
micro-strip detectors (trapping induced charge sharing)
pixel detectors
More radiation hard detectors (towards SLHC)
thin pixel detectors
novel semi-3D design
Summary
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

3. Motivation CERN: RD48 RD39 RD50
All LHC experiments will use silicon (diamond?) for vertex detectors!
Degradation of performance of irradiated silicon detectors (bulk):
increase of |Neff|
Significant improvement in last years to improve performance:
Material: DOFZ, Czochralski, epitaxial material
Geometry: semi-3D, 3D and thin detectors
Operational conditions: cryogenic operation,
current induced devices
Increase of leakage current (independent on material)
loss of drifting charge – trapping
(TCT studies – systematic measurements of trapping times)
CERN:
RD48
RD39 RD50
Determine the signal formation in irradiated detectors!
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

4. Questions
How does the geometry of the electrodes influence the performance?
Can silicon detectors be successfully operated at fluences around 1016 cm-2 ?
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

5. Basics of simulation point charge “bucket”:
electric and magnetic field
trapping
weighting field
constant 1/D if
pad or strip dimension>>D
In general complex - highest close to collecting electrodes
Irradiation:
Calculation of electric and weighting potential
performed by custom made software and
ISE-TCAD package!
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

6. Point charge track y x y x holes electrons ionizing particle track
electron-hole pair
holes
electrons
Point charge x y
ionizing particle track
buckets 1 mm apart
track
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

7. What is not considered in simulation!
Charge generation – a uniform charge generation along the track was assumed
(of course a full GEANT simulation for non-uniform charge generation would be more appropriate for dealing with delta electrons …)
Homogenous effective dopant concentration –
(so called effect of double-junction is not taken into account – how important is it really?)
No further electronic processing of induced current
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

8. Atlas – silicon strip detectors
Detector: thickness=280 mm, strip pitch=80 mm , strip width=18 mm
weighting potential
electric potential Uw
Neff=-6x1012 cm-3
y[mm]
y[mm]
x[mm]
x[mm]
x[mm]
y[mm]
carriers drifting towards the strips contribute to a large part of the induced charge
far from constant
as it is in a diode
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

9. NOTE THAT DIODE SIGNAL IS ALWAYS THE SAME!
p+-n-n+
p+-p-n+
irrad.
Feq=5x1013 cm-2
p+,n+
n+,p+
n,p
n+-n-p+
n+-p-p+
irrad.
Feq=5x1013 cm-2
NOTE THAT DIODE SIGNAL IS ALWAYS THE SAME!
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

10.  n+ strips p+ strips just good enough
U>Vfd are simulated – detectors will be always fully depleted (|Neff| = 0.02 cm-1x Feq)
CCE for detectors with n+ strips is higher than for p+ strips (LHC~10%)
S/N~9 after 10 years of operation (U=450 V)
p+,n+
n+,p+
p bulk
just good enough 
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

11. p+ strips Small difference in CCE for detectors with n+ and p+ strips!
Loss of charge
high U: trapping
low U: diffusion
p+,n+
n+,p+
p bulk ±U x
Average over all strips yields <4% lower signal than for central strip at 450 V!
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

12. Trapping induced charge sharing
measuring
p bulk x ±U
At 450 V around 1000 e are induced on left and right neighbors for central strip!
After Feq=2x1014 cm-2
(F=3.3x1014 p cm-2)
p+ strips
n+ strips
diode
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

13. the amount of charge induced depends also on strip geometry
measuring
p bulk x ±U
trapped
charge
trapping
absence of trapping
this effect is far more important in irradiated detectors with p+ strips
the amount of charge induced depends also on strip geometry
This effect is present also in other devices – not unique to silicon!
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

14. Constant weighting field 1/D
equal charge measured in the front and in the back electrode
electrode
hit by ionizing particle
p+ - induced charge on neighboring electrodes has the same polarity as for the hit electrode
n+- induced charge on neighboring electrodes has the opposite polarity as for the hit electrode
n+ - higher signal in hit electrode p+ - wider clusters
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

15. “Segmentation” in terms of charge collection means how much weighting field deviates from constant (diode)
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

16. Atlas – pixel detectors
Pixel: 400 mm x 50 mm, 23 mm implant width, mm thick!
Electric potential
(linear electric field – diode-like)
Weighting potential
(far from constant – not diode-like)
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

17. Trapping times: te~1.8 ns, th~1.3 ns
Feq=1x1015 cm-2
U=Vfd~350 V, D=250 mm
n-type
p-type
trapping switched off in simulation
Trapping times: te~1.8 ns, th~1.3 ns
Neff = cm-1 x Feq
(DOFZ silicon used, k=0.62)
Current integrated over 25 ns!
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

18. Overdepletion becomes less important at high Feq
d/D=62% but Q(d)/Q(D)=73%
Shape of the weighting field (small at x~D) in combination with trapping results in a smaller contribution to the charge from the region at x~D.
Overdepletion becomes less important at high Feq
Around e at Feq=1015 cm-2 (most probable – not mean)
Even at 200V more than 6000 e, U>400V seems enough with DOFZ
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

19. experimental data U [V] good agreement with measured data
Vfd
experimental
data
U [V]
good agreement with measured data
only small increase of charge at U>Vfd (saturation of vdr )
clear deviation from: (more linear)
at higher fluences it is difficult to extract depletion voltage
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

20. signal on neighbors is below typical cuts applied (2000 - 3000 e)
particle
track
p+-pixels would perform better than n+-pixels even if operated at U>Vfd.
signal on neighbors is below typical cuts applied ( e)
Charge sharing caused by trapping is a very strong argument for using n+ pixels instead of p+!
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

21. Thin Pixel Detectors  
The way to cope with high fluences (1016 cm-2):
- at high Neff detectors can be fully depleted 
short collection times i.e. collection distance 
small signal: need for radiation hard low noise electronics
ATLAS pixel ~ 150 eo, can sustain 2x1015 cm-2
Other issues:
low mass – small X0
small pixel dimensions ~50mm – low capacitance – noise
fast read-out high series noise
power consumption as low as possible
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

22. No difference between n and p pixels is expected for PW/D<1!
Geometries considered (3x3 pixels array was simulated):
70 x 70 mm (50 mm implant width)
thicknesses: 25,50,75,100 mm
Only central hits were considered: diode-like electric field!
Weighting potential along the central line!
D=100 mm
No difference between n and p pixels is expected for PW/D<1!
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

23. Simulated current at Vfd !
D = 50 mm
Neff = cm-1x Feq
(DOFZ, no donors left after 1015 cm-2 )
New materials can reduce the increase of |Neff|:
50 mm thick epi-diodes (small donor removal)
Czochralski material
Simulated current at Vfd !
The charge collection times are short – so are trapping times
(at Feq=1016 cm-2 of order 0.15 ns )
What are the consequences?
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

24. p-type pixels n-type pixels At best only 1000-2000 e at high fluences
Small difference between different pixel thicknesses at 1016 cm-2
Much better performance of n-type pixels for PW/D<1
almost no difference between U=VFD and U=VFD+100 V
around 800 e more at 5·1015 cm-2
Very high VFD (<E>=30000 V/cm for 50 mm thick detector)
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

25. Trapping induced charge sharing
D=100 mm,
Pitch=70 mm,
Width=50 mm.
operated at Vfd
particle
track
Diffusion is negligible due to the short collection times
Very beneficial n-type pixels
(possible use of signals of opposite polarity to enhance S/N)
Up to 30% of the signal is induced on neighbors for p-type pixels, which is usually not enough to reach the threshold for detection – it is lost!!!!
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

26. What if we make a device that has ideal Neff~0 ?
visible light -
hole injection
High resistivity EPI material or Current Induced Devices
The signal that we can get out of thin pixel detectors after Feq=1016 cm-2 is between 1000 e – 1600 e !
Is this enough?
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

27. Novel semi-3D silicon strip detectors
Junction grows from both sides – reduction of Vfd for up to 40%!
Can they stand Feq=1015 cm-2 – also in terms of CCE ?
p+ implants
200 mm
asymmetric device
symmetric device
200 mm
n+ implants
read-out
Detectors studied: thickness=200 mm, strip pitch=120 mm
Neff=-8x1012 cm-3
electric potential
U=280 V
U=250 V
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

28. How much over-depletion is needed to obtain sufficiently large CCE?
Neff=-1.2x1013 cm-3
also detectors with p+
Smaller Vfd of semi-3D detectors for wide strips!
How much over-depletion is needed to obtain sufficiently large CCE?
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

29. Drift paths of electrons and holes in asymmetric detector
U=250 V , Feq~1.1x1015 cm-2, Neff=-8x1012 cm-3
regions with low E
central hit
20 mm from center
40 mm from center
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

30. Red: strip left of hit strip Blue: strip right of hit strip
n+ strips
Semi 3D
n+ strips
(U=280 V)
U=250 V
much larger charge spread, but also larger cluster signal as in detector with n+ strips!
U=250 V
D=200 mm,
pitch/width=120/60 mm
Neff=-8x1012 cm-3, Feq~1.1x1015 cm-2
p+ strips
Black: hit strip
Red: strip left of hit strip
Blue: strip right of hit strip
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

31. Cluster charge (3 strips) as a function of impact position
Neff=-8x1012 cm-3, Feq~1.1x1015 cm-2
Can large charge sharing in semi-3D detectors be used in non-irradiated detectors to improve position resolution?
Only Q>0 considered
Completely different picture as for conventional strip detectors.
The device can be efficiently operated also near the depletion voltage.
A large signal with respect to conventional strip detectors is anyway gained at higher operational voltages.
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

32. highest cluster charge of all in maximum – but large variations
The charge spread in symmetric detector is highly position dependent!
sensing electrodes connected together
U=280 V
Neff=-8x1012 cm-3, Feq~1.1x1015 cm-2
Higher noise if both electrodes are connected to same channel?
highest cluster charge of all in maximum – but large variations
Charge collection
is poor for central
hit!
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii

33. Conclusions
In irradiated segmented detectors it is beneficial to collect electrons (n+-strips, pixels): charge sharing mechanism.
ATLAS strip detectors: Q~15500 e, improvement at U>Vfd , significant signal induced also on neighbors, n+-strips would be a better option.
ATLAS pixel detector: a good agreement with measured values was found: CCE~60% after Feq=1x1015 cm-2 ; if operated at U<Vfd, the collected charge loss due to partial depletion is smaller than predicted by 1-d/D.
Thin pixel detector: no advantage of n+-type pixels for PW/D>1, even if detectors are operated at Neff~0 expected signals are ~ e after Feq=1x1016 cm-2.
Novel semi 3D design: interesting properties, reduction of Vfd, large charge sharing, cluster signal comparable to n+-strip detectors.
G. Kramberger, Simulation of signal in irradiated silicon detectors Vertex 2002, Nov. 3-8, 2002, Kailua-Kona, Hawaii