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Development of Radiation Hard Si Detectors

Dr. Ajay K. SrivastavaOn behalf of Detector Laboratory of the

Institute for Experimental PhysicsUniversity of Hamburg, D-22761, Germany.

Ajay K. Srivastava Uni.- Hamburg 107/09/2008

22

• Activities of our group

• Motivation to develop radiation harder Si detectors

• CMS Silicon Tracker

• An overview of Si detectors (why?, types, relevant parameters,working principle)

• Radiation damage in silicon detectors* Macroscopic Radiation damage in Si detectros* Microscopic Radiation damage in Si

• Effects of microscopic defect on device performance * Strategies to develop radiation hard Si detectors* Experimental studies of radiation damage.

* Macroscopic measurements* Microscopic measurements* Kinetics of defects.

• Summary of my work done for pixel detector (HPAD) development for European Free-electron-laser XFEL at Hamburg, Germany

Ajay K. Srivastava Uni.- Hamburg07/09/2008

OutlineOutline

33

• ZEUS experiment at HERA collider, DESY, Hamburg, Germany: silicon vertex detector, physics

•CMS experiment at CERN, Geneva, Switzerland: silicon tracking detector, detector calibration, GRID computing, physics

• Radiation hard detector research & development

Since the early 1960s our group has been investigating silicon detectors for nuclear spectroscopy and high-energy physics experiments. Our research covers device physics, detector systems, radiation damage and sensor optimization

• Development of pixel detector (HPAD) for European Free-electron-laser XFEL at Hamburg, Germany

07/09/2008 Ajay K. Srivastava Uni.- Hamburg

Activity of Our GroupActivity of Our Group

44

LHC (2008), L = 1034cm-2s-1

(14 TeV pp collider, 25 ns bunch spacing)

Φ(r=4cm) ~ 3·1015cm-2

LHC upgrade:Super-LHC (?), L = 1035cm-2s-1

Φ(r=4cm) ~ 1.6·1016cm-2

• Detector for the European Free-Electron-Laser XFEL at Hamburg (start in 2013): photon fluxes up to: 1016 /cm2 ≙ 109 Gy [109 J/kg]

5 years

2500 fb-1

10 years

500 fb-1

× 5

0 10 20 30 40 50 60r [cm]

1013

51014

51015

51016

Φeq

[cm

-2] total fluence Φeqtotal fluence Φeq

neutrons Φeq

pions Φeq

other charged

SUPER - LHC (5 years, 2500 fb-1)

hadrons ΦeqATLAS SCT - barrelATLAS Pixel

Pixel (?) Ministrip (?)Macropixel (?)

(microstrip detectors)

[M.Moll, simplified, scaled from ATLAS TDR]


Main Motivations for R & D on Radiation TolerantMain Motivations for R & D on Radiation TolerantDetectorsDetectors

(2oo ns distance between pulses)

55

CMS Silicon Tracker


• Finely segmented silicon sensors (strips and pixels) enable charged particles to be tracked and their momenta to be measured. They also reveal the positions at which long-lived unstable particles decay.This part of the detector is the world's largest silicon detector. It has 205 m2

of silicon sensors (approximately the area of a tennis court) comprising 9.3 million microstrips and 66 million pixels.

CMS Silicon TrackerCMS Silicon Tracker

66

•Why Si detectors for particle tracking in High-Energy-Physics Experiments?

1. Fast response 2. High position resolution (~10μm) 3. Reliable operation

•Types: I. Si strip detector (single sided/double sided)II. Si pixel detectorIII.Si pad detector


Si DetectorSi Detector

Strip detector Pixel detector Pad detector

77

• Depletion depth and Voltage

• Reverse current = generation current

• Capacitance

• Noise

• Charge collection

Relevant parameters


Working principle of Si strip detector (AC coupled)Working principle of Si strip detector (AC coupled)

8Ajay K. Srivastava Uni.- Hamburg 8

• Bulk (crystal) damage due to Non Ionizing Energy Loss (NIEL)- displacement damage, crystal defects/microscopic defect

I. Change of effective doping concentration Neff (higher depletion voltage Vdep) II. Increase of leakage current (increase of shot noise, thermal runaway)

III.Increase of charge carrier trapping (reduced charge collection efficiency (CCE))

• Surface damage due to Ionizing Energy Loss (IEL)

I. Charge build-up in SiO2 (shift of flatband voltage Vfb, II. Traps of Si-SiO2 interface breakdown of critical corners)

III Surface generation current (increase shot noise)

07/09/2008

Macroscopic Radiation Damage in Silicon DetectorsMacroscopic Radiation Damage in Silicon Detectors

99

[TRIM-simulation, G. Davies, RD50 workshop Ljubljana 2008, modified]

Examples for defect reactions:


Microscopic Radiation Damage in SiliconMicroscopic Radiation Damage in Silicon

(Ek>25 eV)

(Ek>5 keV) I- Interstitials Cs- substitutional C

V- Vacancy Ci – Interstitials C

• Interstitials and vacancies are mobile at room temperature

• 50 keV Si ion with fluence 3 x 1016 cm-2

1010

trapping (e and h)⇒ CCE

shallow defects do not contribute at room

temperature due to fast detrapping

charged defects ⇒ Neff , Vdepe.g. donors in

upper half of band gap and acceptors

close to midgap

generation⇒ leakage current

levels close to midgap

most effective

• Influence of defects on the material and device properties


Radiation Induced Defects and Impact on Device Radiation Induced Defects and Impact on Device PerformancePerformance

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Strategies to Develop Radiation Hard Si detector Strategies to Develop Radiation Hard Si detector

I. Material/Defect engineering

- Understanding of radiation damage

• Irradiation with different particles and energies• Thermal treatment to understand kinetics• Macroscopic effects to understand detector performance• Microscopic defects and simulation

Improved sensor performance with oxygen rich material.Study different materials : DOFZ, Cz, MCz, EPI-Si


II. Device Engineering

- Simulation and study of prototype detectors• n+- in- p• 3D detectors• Thin detectors

1212

1011 1012 1013 1014 1015

Φeq [cm-2]10-6

10-5

10-4

10-3

10-2

10-1

ΔI /

V

[A/c

m3 ]

n-type FZ - 7 to 25 KΩcmn-type FZ - 7 KΩcmn-type FZ - 4 KΩcmn-type FZ - 3 KΩcm

n-type FZ - 780 Ωcmn-type FZ - 410 Ωcmn-type FZ - 130 Ωcmn-type FZ - 110 Ωcmn-type CZ - 140 Ωcm

p-type EPI - 2 and 4 KΩcm

p-type EPI - 380 Ωcm

[M.Moll PhD Thesis][M.Moll PhD Thesis]

I-V

C-V Effective doping concentration, Neff / Vdep

Dark Current & stability of device


I. Macroscopic MeasurementsI. Macroscopic Measurements

1313

TCT Analysis

• Electric field in sensor• Full depletion voltage• Effective trapping time• Charge collection efficiency

M Cz, Φeq = 1x1014 cm-2

-0.05

0.00

0.05

0.10

0.15

0.20

0 5 10 15 20 25 30 35

Time [ns]

curr

ent [

arb.

uni

ts]

100 V80 V60 V50 V40 V



Electric field in a damaged silicon sensor (Φeq.= 1 x1014 cm-2)

14Ajay K. Srivastava Uni.- Hamburg 14

[K. Koch, diploma thesis 2007, modified]

CCE (Charge collection efficiency) – signal to noise ratio & detector efficiency

0 2.1015 4.1015 6.1015 8.1015 1016

Φeq [cm-2]

0.50

0.60

0.70

0.80

0.90

1.00

CCE

EPI-ST, 72 μmEPI-ST, 72 μmEPI-DO, 72 μmEPI-DO, 72 μmFZ, 100 μmFZ, 100 μmFZ, 50 μmFZ, 50 μm

244Cm source244Cm source5.8 MeV α-particles5.8 MeV α-particles

07/09/2008


1515

DLTS TSC

07/09/2008

• Concentrations

• Activation energies

• Capture cross sections

Obtained electrical properties of defects:

Ajay K. Srivastava Uni.- Hamburg

II. Microscopic MeasurementsII. Microscopic Measurements

Φeq.≤ 1012cm-2 Φeq. ≤ 1014 cm-2

1616

Time developments of defects depends on

• Temperature• Impurities


Kinetics of DefectsKinetics of Defects

• I= I0 + VΦeq. α

1717

• Test structure: gated diode with 5 gate rings• Only damage in SiO2 and Si-SiO2 interface important

Circuit for I-V and C-V/G-V measurement of gated diode


Radiation Damage of Si Detectors by XRadiation Damage of Si Detectors by X--RaysRays

1818Ajay K. Srivastava Uni.- Hamburg07/09/2008

• Nox reaches a maximum at 5MGy and then decreases – why?

Experimental Results on Oxide Charge DensityExperimental Results on Oxide Charge Density

7/10/2008 Ajay K. Srivastava Uni.- Hamburg 19

CC--V Characteristics of MOS CapacitorV Characteristics of MOS Capacitor

C HF, inv

Accumulation

Depletion

Inversion

Cox- Oxide related capacitance (F)

C HF, inv- High frequency inversion capacitance (F)

C fb- Flat band capacitance (F)

Vfb= Øms – Q’ss/Cox


ISEISE--TCAD Simulation of CMOS CapacitorTCAD Simulation of CMOS Capacitor

2207/09/2008

S./E.= 1.1

Ajay K. Srivastava Uni.- Hamburg

Simulation Result and DiscussionSimulation Result and Discussion

2323Ajay K. Srivastava Uni.- Hamburg07/09/2008

Comparison: Simulation with DataComparison: Simulation with Data

Conclusion: I. Encouraging first result.II. Improve modelling.III. Extend to irradiated sensors.


SummarySummary

1.

High resolution silicon sensors are presently used in all collider experiments

Group has unique expertise and equipment for macroscopic and microscopic radiation damage studies

Detailed simulation for next generation of sensors started

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