1. Radiation damage in silicon sensors
Topics:
Damage due to protons and neutrons
Radiation induced defects
Annealing
Donor removal (type inversion)
Alternatives to p-n
Damage due to electrons, photons,…
In case of any questions:
Jaap Velthuis (University of Bristol)

2. Jaap Velthuis (University of Bristol)
Quick review
Semiconductor detectors are used close to primary vertex to
Limit occupancy and reduce ambiguities
Give very precise space point
Need trick to remove free charge carriers
Use high band gap semiconductor
Cool to cryogenic temperatures
Build p-n junction and deplete detector
Jaap Velthuis (University of Bristol)

3. Jaap Velthuis (University of Bristol)
Quick review (II)
Energy loss described by Behte-Bloch equation
Minimum ionizing particle
Energy loss (=signal) is Landau distributed
Particles scatter in matter, so need to have thin detectors
MIP yields 8900 e-h pairs per 100 m Si
If pitch ~ charge cloud, charge is shared. Need lots of strips.
Trick intermediate strip using C-charge sharing, but non-linear charge sharing
Jaap Velthuis (University of Bristol)

4. Jaap Velthuis (University of Bristol)
Charged … matter
Bethe-Bloch describes average energy loss
Collisions stochastic nature, hence energy loss is distribution instead of number.
First calculated for thin layers was Landau. Hence energy loss is Landau distributed.
 is most probable value
Jaap Velthuis (University of Bristol)

5. Jaap Velthuis (University of Bristol)
-electrons - +
Some of generated carriers have so much kinetic energy that they will free more charge carriers
Lots of signal
Do not know where hit was
Responsible for tails signal distribution
Number of ’s dependent on material: EG high-> less 
Jaap Velthuis (University of Bristol)

6. Jaap Velthuis (University of Bristol)
Charge collection
If pitch > charge cloud all charge collected on 1 strip
In this case analog signal value not importantchose digital or binary readout
To do better need to share charge over more strips need pitch20m for 300 m thick sensor
Problem: connecting all strips to readout channel yields too many strips
Jaap Velthuis (University of Bristol)

7. Strip pitch & Analogue vs binary
Can improve position reconstruction by using neighbour signal. Simplest: CoG
But the further away, the more effect on . So S/N neighbour limits resolution.
Now choice:
use many strips to get enough S/N on neighbour and use analogue readout
Live with limitations, spread out strips and readout binary
Jaap Velthuis (University of Bristol)

8. Strip pitch & Analogue vs binary
If analogue, need fancy chip that consumes lots of power to process signal.
Need: shaper, pipeline, ADC & Storage of pulse heights
Advantages: high precision, good understanding noise
Disadvantages: high power, lots of processing, loads of infrastructure
ATLAS chose binary. 50m pitch thus 17m precision and easy read out
Only this needed
for binary
Jaap Velthuis (University of Bristol)

9. Jaap Velthuis (University of Bristol)
sLHC radiation dose
5 year radiation dose close to beam pipe ~1016 neq/cm2
too high for state-of-the-art standard silicon sensors
Most radiation hard material: diamond
High bandgap
Displacement energy 43eV (13-20 for Si)
Jaap Velthuis (University of Bristol)

10. Radiation with protons/neutrons
Silicon crystal is organised in a Face-Centred-Cubic lattice (or diamond lattice)
Remember: impurities yield lattice sites with different number of valence e-  doping
Energetic radiation knocks atoms out of lattice: similar thing
Jaap Velthuis (University of Bristol)

11. Radiation with protons/neutrons
Energy needed to displace atom from lattice=15eV
Damage energy dependent
~<2keVisolated point defect
2-12keVdefect cluster
~>12keVmany defect clusters
This damage is called Non-Ionizing Energy Loss (NIEL)
Results scaled to 1MeV neutrons
Electrons and photons don’t make defects!
Jaap Velthuis (University of Bristol)

12. Jaap Velthuis (University of Bristol)
Radiation … neutrons
Displacement changes band structure
Levels in middle of band gap arise
Can capture electrons/holes (trapping)
Can donate electrons/holes
Can increase leakage current by two-step transitions from valence to conduction band
Can act as recombination centre
Role dependent on neighbours and present impurities
Conduction band +
Valence band -
Jaap Velthuis (University of Bristol)

13. Jaap Velthuis (University of Bristol)
Annealing
Annealing is process in which crystal recovers from radiation damage
Atoms occupy vacancies
Defect complexes change in different complexes
New defect complexes can also worsen damage (reverse annealing)
Processes highly temperature dependent
“Not very well understood”
Depends on which (unintentional) impurities present
Which defects are formed
Alchemy!
Jaap Velthuis (University of Bristol)

14. Radiation damage: Leakage current
I = Volume
Material independent
linked to defect clusters
Annealing material independent
Scales with NIEL
Temp dependence
 = 3.99  0.03 x 10-17Acm-1
after 80minutes annealing at 60C
Jaap Velthuis (University of Bristol)

15. Jaap Velthuis (University of Bristol)
Type inversion
Dopants may be captured into defect complexes.
Donor removal and acceptor generation
type inversion: n  p
depletion width grows from n+ contact
Increase in full depletion voltage
P-strips in p-bulk
= 0.025cm-1 measured after
beneficial anneal
Jaap Velthuis (University of Bristol)

16. Partially depleted detectors
undepleted
Undepleted region like high-ohmic resistor
If detector partially depleted
from strip side
 only charge in depleted region contributes  smaller signal, similar spatial resolution
from backplane
carriers travel towards strips, but don’t reach it signal spread over many strips poor spatial resolution
undepleted
Jaap Velthuis (University of Bristol)

17. Example: NA60 type inversion
After type inversion Vdep increases with dose smaller radii
Lowering Vbias leaves larger area undepleted
Depletion from backside  layer almost dead
Jaap Velthuis (University of Bristol)

18. Example: NA60 type inversion
After type inversion Vdep increases with dose smaller radii
Lowering Vbias leaves larger area undepleted
Depletion from backside  layer almost dead
Jaap Velthuis (University of Bristol)

19. Example: NA60 type inversion
After type inversion Vdep increases with dose smaller radii
Lowering Vbias leaves larger area undepleted
Depletion from backside  layer almost dead
Jaap Velthuis (University of Bristol)

20. Example: NA60 type inversion
After type inversion Vdep increases with dose smaller radii
Lowering Vbias leaves larger area undepleted
Depletion from backside  layer almost dead
Jaap Velthuis (University of Bristol)

21. Example: NA60 type inversion
After type inversion Vdep increases with dose smaller radii
Lowering Vbias leaves larger area undepleted
Depletion from backside  layer almost dead
Jaap Velthuis (University of Bristol)

22. Example: NA60 type inversion
After type inversion Vdep increases with dose smaller radii
Lowering Vbias leaves larger area undepleted
Depletion from backside  layer almost dead
Jaap Velthuis (University of Bristol)

23. Example: NA60 type inversion
After type inversion Vdep increases with dose smaller radii
Lowering Vbias leaves larger area undepleted
Depletion from backside  layer almost dead
Jaap Velthuis (University of Bristol)

24. Jaap Velthuis (University of Bristol)
Thermal runaway
Problem with donor removal:
Need higher voltages to deplete detector
Higher voltage  higher leakage current
Higher leakage current  more power dissipated in detector
More power  heating
heating more leakage current
“Solution”: cool detectors
ATLAS will operate at –7oC
Jaap Velthuis (University of Bristol)

25. Solutions radiation damage
Start with n+ strips in n-type detector
After inversion  substrate p type  depletion now from strip side (LHC-b)
Build p-type detectors with n-strips
Different crystal orientation
Less dangling bonds at Si-SiO2 interface
Material Engineering
Operate very cold
Use different material (e.g. CVD diamond)
3D-structures
Jaap Velthuis (University of Bristol)

26. Jaap Velthuis (University of Bristol)
N+-on-n detectors
Standard: p-strips on n-bulk
Problem: n-bulk becomes p-type 
pn-junction moves from strip-bulk to bias contact-bulk interface
Only good spatial resolution when fully depleted
Solution: make n+-strips in n-bulk
After radiation: n+-strips and p-bulk
Disadvantages:
strips not well isolated before radiation. Need p-strips (or spray) between n-strips.
Need guard rings at bottom (expensive)
n+-strips
N-type bulk
becomes p-type
After radiation
n+ bias
contact
Jaap Velthuis (University of Bristol)

27. Jaap Velthuis (University of Bristol)
P-type detectors
P-type bulk with n-type strips
Collect electrons instead of holes
Electron mobility ~3> hole mobility  less trapping
Depletion starts from strip side
Even at partial depletion good spatial resolution
No need for guard rings on backside  cheaper than n+-on-n
1E15cm-2 10 years
Dose for ATLAS strips
Jaap Velthuis (University of Bristol)

28. Jaap Velthuis (University of Bristol)
Material engineering
Diffusing oxygen suppresses V2O formation
V + O  VO
V + VO  V2O
V2O  reverse annealing
Still alchemy…
Jaap Velthuis (University of Bristol)

29. Jaap Velthuis (University of Bristol)
Lazarus Effect
Remember:
Radiation induces (more) traps. Capture and emission very temperature dependent.
Cooling makes sure traps stay filled  no trapping
Can actually operate forward biased
Downside: need to keep detector in cryostat
Jaap Velthuis (University of Bristol)

30. Jaap Velthuis (University of Bristol)
CVD Diamond
Remember:
Limit background charge by using large bandgap material
Si: EG=1.12 eV  1.5E10 free carriers/cm3
Diamond: EG=5.47 eV  6E-28 free carriers/cm3  no need for pn-junction
Diamond lattice very strong  very radiation hard
Note: large EG  only few eh pairs produced (3600 vs 8900 per 100 m), but also lower noise
Jaap Velthuis (University of Bristol)

31. Jaap Velthuis (University of Bristol)
Diamond growth
Trap- and recombination centers limit charge collection
Trapped charge at grain boundaries builds up a polarization field superimposed on the biasing field
Substrate and growth side must be ground and polished for good quality
Jaap Velthuis (University of Bristol)

32. ATLAS CVD diamond pixel module
Sensor:
Active area: 61x16.5mm2
Thickness 800µm
Pixel size 400(600)x50µm2
46k pixels
ATLAS frontend chip FE-I3
0.25µm IBM
Radiation tolerant >50MRad
Designed for Si sensors
Binary/low res. analog readout
Noise same as bare FE-chip
Noise ≈137e-
Threshold ≈1454e-
Threshold spread ≈25e-
Jaap Velthuis (University of Bristol)

33. CVD diamond radiation hardness
Still S/N≈18-25 after 1.8x1016 p/cm2 (~500 Mrad) depending on field
No problem operating in sLHC conditions
Noise limited by electronics, NOT by sensor capacitance or leakage current
E=2V/µm
E=1V/µm
Jaap Velthuis (University of Bristol)

34. Single crystal diamond
Largest single crystal diamond 14x14mm
No grain boundaries → no trapping
Produced 400 µm thick single chip sensor using ATLAS FE chip
Jaap Velthuis (University of Bristol)

35. Diamond signal spectrum
S/N≈99
N=136.8 e-
Jaap Velthuis (University of Bristol)

36. CVD diamond as radiation monitor
Babar uses diamond radiation monitors for almost 2 years
Response is fast and material doesn’t die
Jaap Velthuis (University of Bristol)

37. Czochralski silicon (Cz)
Czochralski Growth
Pull Si-crystal from a Si-melt contained in a silica crucible while rotating.
Silica crucible is dissolving oxygen into the melt  high concentration of O in CZ
Material used by IC industry (cheap)
Recent developments (~2 years) made
CZ available in sufficiently high purity (resistivity) to allow for use as particle detector.
Ask Michael if I can use the Siltronix photo
Jaap Velthuis (University of Bristol)

38. Czochralski silicon (Cz)
Standard FZ silicon
type inversion at ~ 21013 p/cm2
strong Neff increase at high fluence
Oxygenated FZ (DOFZ)
reduced Neff increase at high fluence
CZ silicon and MCZ silicon
no type inversion in the overall fluence range  donor generation overcompensates acceptor generation in high fluence range
24 GeV/c proton irradiation
Jaap Velthuis (University of Bristol)

39. Jaap Velthuis (University of Bristol)
3D detectors 3D
standard
Problems standard sensors:
After radiation high voltage needed to fully deplete  high currents  high noise & thermal runaway
Need guard ring structures lot of wasted space
Make “strips” vertical inside bulk
Jaap Velthuis (University of Bristol)

40. Jaap Velthuis (University of Bristol)
3D detectors 3D
standard
P=50, 100 or 200 m
Physical
edge
Sideways depletion: smaller distance between electrode and strips  lower depletion voltage
Sideways charge collection
Edgeless (dead edge < 5 m)
Still use full 300 m thickness
Rapid charge collection (~2 ns)
Radiation hardness
Jaap Velthuis (University of Bristol)

41. Jaap Velthuis (University of Bristol)
3D detectors
S/N source test=13 with 121 m thick detector
Jaap Velthuis (University of Bristol)

42. Jaap Velthuis (University of Bristol)
3D detectors
Efficiency loss underneath electrodes
P-type loss 66%
N-type loss 43%
Signal: 10-90% <5µm !
Standard sensors need 5-6mm
Used for TOTEM
electrodes
Jaap Velthuis (University of Bristol)

43. Summary radiation hardness
Radiation damage in sensors mainly bulk damage
Atoms knocked out of their lattice position extra levels in band gap 
Effectively donor removal (type inversion)
High leakage currents 
High noise
Thermal runaway
Problems to get full depletion
Jaap Velthuis (University of Bristol)

44. Summary radiation hardness (II)
Solutions:
n+-on-n or even better n-on-p detectors
Material engineering (oxygenated Si/CZ)
Cool
ATLAS at –7oC
cryogenic temperatures (Lazarus effect)
Use different materials like diamond
Use different detector type like 3D
Jaap Velthuis (University of Bristol)