Jaap Velthuis (University of Bristol)1 Silicon detectors in HEP Introduction Semi-conductor physics...

Jaap Velthuis (University of Bristol) 1

Silicon detectors in HEP

• Introduction• Semi-conductor physics• Real Si detectors• Radiation damage in Si• Radiation hard sensors• Novel devices/State-of-

the-Art In case of any questions:

[email protected]

Bluffing your way intoparticle physics detectors

mailto:[email protected]

mailto:[email protected]


Introduction• Particle Physics is more than hunting

for Higgs and CP violation• Need to make very advanced detector

systems • Forefront of

– Engineering (stiff light weight support structures, cooling, tunnel building)

– High speed and radiation hard electronics– Computing (web, grid, online)– Accelerators (e.g. cancer therapy,

diffraction)– Imaging sensors (e.g. nth generation light

source, medical imaging)


Introduction• Why semi-conductor

devices• P-N junction• Particle traversing

matter– Scattering– Signal generation

• Summary• Baseline detector


Why semi-conductor devices


Standard experiment

http://mocoloco.com/art/upload/2007/12/purple_onion/chumley_onion.jpg


The “Onion” peeled…• Fundamental parameters:

– Charge– Momentum– Decay products– Life time– Decay vertex– Mass – Spin– Energy

• Need very precise tracking close to primary vertex. Then follow track to calorimeter and measure energy.

Very precise

tracking

tracking

Electro-

magnetic

calorimeter

hadroniccalorim

eter

Muon

chamber

OutwardTrack density drops


Tracking• Track described by 5 parameters• Modern tracking uses “Kalman

Filter”– Start with “proto” track– Add new point– Update 2 – Decide to in- or exclude point based on

2

• Modern Vertexing– Use tracks with errors– Add them to vertex– Calculate 2 etc

• So to do good tracking and vertexing, need detectors with small error and little “deflection”

R-

Pri

mary

vert

ex

Secon

dary

vert

ex


Wire chambers

• Traditionally tracking in wire chambers


Wire chambers

• Problem in wire chambers:– Wires long– Many hits per wire for wires close to primary vertex

(high occupancy)– Leads to ambiguities in track fitting

• Solution: very short wires! solid state


Charged particle traversing matter

• Energy loss described by Bethe-Bloch equation:

Z

C

I

Wvm

A

ZzcrmN

dx

dE eeeav 22

2ln4 2

2max

22

2

222

Some constant

Atomic number/mass absorber

Electric charge incident particle

Mean excitation energy

Maximum kinetic energy which can beimparted to a free electron in a single collision

2

2

222

max

21

2

M

m

M

m

cmW

ee

e

Z

C

I

Wvm

A

ZzcrmN

dx

dE eeeav 22

2ln4 2

2max

22

2

222


Charged…matter• dE/dx different for different particles due to

different M and • Is used to identify different particles


Charged … matterRelevant for detectors

dE/d

x [M

eV c

m2 /

g]

• Energy loss wildly varying function,– MINIMUM IONIZING PARTICLE (4)


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.

• Signal proportional to energy loss

x

ex

xL2

1exp)(0

is most probable value


Multiple scattering• During passage through matter Coulomb

scattering on nucleideviation from original track

• Deflection distribution Gaussian with width 0

• More dense material, more scattering, shorter X0

000 /ln038.01/6.13

XxXxzcp

MeV


Detector trade off

• Thick detectors (in X0) lots of energy lost lots of signal generated

• But loads of scattering bad for tracking

• Loads of -electrons more signal but not right direction


Silicon trivia• Silicon was discovered by Jöns

Jacob Berzelius in 1824• Name from “silicis” (Latin for flint)• With 25.7% second most abundant

element in earth’s crust• First crystalline silicon produced by

Deville in 1824


Why solid state detectors

• Small band gap – low energy required for e-h pair (3.6 eV in

Si ~30 eV gas)– Many e-h pairs per unit length (80/m in

Si)

• High density – Large energy loss per unit length

• Can make thin detectors with high signal

– Small range for -electrons • Very good spatial resolution


Why solid state detectors

• Electron and hole mobility very high– Fast charge collection (~10 ns)

• Excellent rigidity – Self-supporting structures

• Possibility of creating fixed space charge by doping


Intrinsic semi-conductors

• Single non-interacting atom has set of well-defined energy levels

• When forming crystals, levels undergo minor shifts resulting in bands

• Probability for e- to occupy state given by Fermi-Dirac function

kTEE

EFFexp1

1)(


Intrinsic semi-conductors

• Density of states

• Density of free electrons n given by product Fn(E) and density of states

kinkinkinkin dEEh

mdEEN 2/1

2/3

2

24)(

kT

EE

VkT

EE

CkT

EEn

VFFCFC

eNpeNeh

kTmn

2/3

2

22


Intrinsic semi-conductors• Typically for intrinsic Si carrier density at

300K ~1010 cm-3 suppose strip 20m wide, 10 cm long, sensor 0.3 mm thick S/N=410-

3

• Re-writing concentrations:

– Concentrations highly dependent on T and material

• So three solutions:– Use high EG material– Cool device down (ni at 77K ~10-20)– Remove mobile carriers

kT

ENNpnn G

VCi 2exp


“Trick”: doping

• By introducing atoms with different number of valence e- can change number of free carriers– E.g. P, As: 5 valence e-; donor (n-type)– E.g. Al, B: 3 valence e-; acceptor (p-type)– Activation energies

~0.04eV<<EG=1.12eV

Intrinsic N-type P-type


PN-junction• Holes in p-type

recombine with e- in n-type, creating zone without mobile carriers (depletion)

• Depleted silicon ideal for detector. Same signal, but no background!

• Note:– Holes move towards p-type– Electrons move towards n-

type


PN-junction• Can express as function Vbias:

Vjunc

– Depletion width

– Cjunc

2ln

2

)(

i

DAbi

pnnDbiasbijunc

n

NN

q

kTV with

xxxqN

VVV

biasbi

DA

DAd VV

NqN

NNx

2

biasbiDA

DA

VVNN

NqNAC

1

2


PN-junction• By biasing detector, the depletion width

can be extended over entire thickness of detector (full depletion).

• Important: PN junction itself is located at interface between p-strips and n-bulk. Depletion region grows from PN junction towards n-type bias contact.

• Typical values for full depletion 10-100 V before irradiation.

biasbi

DA

DAd VV

NqN

NNx

2


Depletion voltage

• Bias voltage very important:– Creating large depletion zone

•Signal proportional depletion thickness•Depletion zone also reduces

background

– Isolating strips from each other– Separating e-h pairs

• Depletion voltage obtained from C-V curve


C-V curve• Re-write C(Vbias)

relation

• Plotting 1/C2 vs Vbias yields:– depletion voltage– Doping

concentration (for asymmetric doping)

biasbi

DA

DA

biasbiDA

DA VVNqN

NN

ACVVNN

NqNAC

2111

2 22

Depletion voltage


I-V curves• Measure I-V to check long

term stability of sensors and maximum Vbias

• Note current NOT zero (leakage current)

• If Vbias too large, get high currents (breakdown)– Zener breakdown

• Tunnelling from occupied state in p side valence band to n side conduction band

– Avalanche breakdown• Carriers from leakage current

get so much kinetic energy that due to collisions new free carriers are generated

IV Scan

020040060080010001200140016001800

0 100 200 300 400 500Bias voltage (V)

Lea

kag

e cu

rren

t at

20C

(n

A)


Signal generation• Lost energy converted into free carriers• Energy needed to generate 1 e-h pair in Si is 3.6 eV

• Results in 8900 e-h pairs per 100 m Si for a MIP• Charge cloud Gaussian with 10m• E-h pairs might recombine, need (strong) field to prevent

this signal loss

RamankinGpair rEEEE 2

Carriers carrykinetic energy3/5EG

Energy transferredto lattice r10 ERaman0.165 eV Taken from

http://britneyspears.ac


Baseline detector• Need many diodes (here p-strips to

n-bulk)• Need reverse bias to

– Deplete entire sensor– Separate e-h

• Need to readout signals from p-strips

• Design issues:– Thick large signal– Thin less scattering– Thin lower depletion voltage– Short strips less ambiguities– Strips close very precise

measurement impact position– Strips far apart less electronics

hence less expensive

Occupancy: fraction of strips that has been hit


Real detectors• Real sensors have

much more features:– Backplane contacts– Guard rings– Bias resistors– P-strips– Al readout strips– Coupling capacitors– …

• Typical scale:– Sensors 6x6 cm– Pitch ~100 m– 512 Al strips


Charge collection• Determined by

– Spatial distribution of generated charge

– Field strength• Accelerates carriers in field

direction• Determines time charge is

moving• Separation of e-h pairs

– “Horizontal” movement through diffusion

– Hall effect


Charge collection• If pitch > charge cloud all charge

collected on 1 strip

• In this case analog signal value not importantchose digital or binary readout

• To do better need to share charge over more strips need pitch20m for 300 m thick sensor

• Problem: connecting all strips to readout channel yields too many strips

12

11

0

21

0

22 dxxxdxxx


Summary• Semiconductor detectors are used close

to primary vertex to – Limit occupancy and reduce ambiguities– Give very precise space point

• 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


Summary (II)

• Need trick to remove free charge carriers– Use high band gap semiconductor– Cool to cryogenic temperatures– Build p-n junction and deplete detector

• 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)1 Silicon detectors in HEP Introduction Semi-conductor physics...

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