Detector Physics 2020/2021, Lecture 7

DetectorPhysics2020/2021,Lecture7

Semiconductordetectors

AlessandraTonazzo ([email protected])

Laboratoire APC– Université deParis

13/10/2020 Semiconductordetectors 1

Additionalreferences• NPAC lectures by J. Bolmont, 2019 (à thanks!)

https://npac.lal.in2p3.fr/wp-content/uploads/2019/Cours-S1/Detectors/NPAC_Detector_Physics_Part5_JB_light.pdf

• Lectures on Detectors for CERN Summer Students:

Daniela Bortoletto 2015https://indico.cern.ch/event/243648/attachments/415356/577104/daniela_l4_post.pdf

Werner Riegler 2019https://indico.cern.ch/event/817555/

• M. Krammer and F. Hartmann, Silicon Detectors, EDIT20111https://indico.cern.ch/event/124392/contributions/1339904/attachments/74582/106976/IntroSilicon.pdf

• G. Lutz, Semiconductor radiation detectors: device physics. Springer, Berlin, 1999


Motivation

Semiconductordetectorsprovide• highsegmentation• highdensity=>smallsize


Introducedinthe1960’sforspectroscopy

à dramaticimprovementofenergyresolution

Fromthe1980’s:needtodeterminepositionofprimaryinteractionvertexand/orsecondarydecays

Semiconductorsensors:principle• Fundamental information carriers : e--hole pairs produced along the path of the charged particle

• Detection signal formed by collecting the e--hole pairs (basically a ionization chamber)


Segmentingthesensorelectrodesprovidespositioninformation

H.Spieler,SemiconductorDetectorSystems,ClarendonPressOxofrd

Fasttimeresponse,(<20ns)

Typically:Si,Ge

Semiconductorproperties• Crystals : periodic lattice of atoms• Electron energy levels are smoothed into ”bands” due to reciprocal interaction of atoms close toeach-other

• In semi-conductors, Egap ~ Ethermal : with thermal energy, electrons can cross the band gap andreach the conduction band. A vacancy is created in the valence band : electron-hole pair


Insulators Semi-conductors Conductors

Fermilevel:50%probabilityofbeingoccupied=maximumelectronenergyatT=0K

Semiconductorproperties• Probability for an electron to move from the valence to the conduction band

P 𝑇 = 𝑐𝑇%/'𝑒𝑥𝑝 −𝐸-2𝑘0𝑇

Example : Silicon (tetravalent : 4 nieghbours)

• At very low temperature, all the electrons participate in bonds between the atoms


LowT

Eg

T:absolutetemperaturec:proportionalityconstantcharacteristicofmaterialEg :bandgapenergykB :Boltzmannconstant

Semiconductorproperties• Probability for an electron to move from the valence to the conduction band

P 𝑇 = 𝑐𝑇%/'𝑒𝑥𝑝 −𝐸-2𝑘0𝑇

Example : Silicon (tetravalent : 4 nieghbours)

• At room temperature, some bonds are broken by thermal energyà electron-hole pair creation


Eg

RoomT

T:absolutetemperaturec:proportionalityconstantcharacteristicofmaterialEg :bandgapenergykB :Boltzmannconstant

Intrinsicsemiconductors• In a perfectly pure material, an electron moving to the conduction band leaves a hole in the valenceband. The number of free electrons equals that of free holes : intrinsic carriers

ne=ne=ni• In practice, conduction by intrinsic carriers is difficult to achieve, due to captures on impurities.

Typical values : ni=1.5 1010 cm-3 in Si, 2.4 1013 cm-3 in Ge

• Compare with the signal from a MIP :dE/dx = 3.87 MeV/cmassume detector thickness Δ = 300 μm, area A=1cm2

mean energy to form a e-h pair in Si : w = 3.63 eV

e-h pairs fromMIP = 12/13 456

= 3.210<

e-h pairs from thermal noise = niAΔ = 4.35 108 >> signal

à need to deplete the free charge carriers


dE/dxofprotonsinSi

Dopedsemiconductors• Add impurities with different number of valence electrons

• Create additional levels in the bandgap, to be occupied by charge carriers

• Very small concentrations : 1012-1015 cm-3, vs 1020 cm-3 atoms of Si or Ge


Donorimpurities(n-typedoping) :As,P,Sb5valenceelectrons

Acceptorimpurities(p-typedoping) :Ga,B,In3valenceelectrons

allthee- inexcessinthedonorlevelaretransferredtotheconductionbandbecauseofthermalenergy

theacceptorlevelisfilledwithe- fromthevalencebandbecauseofthermalenergy

p-njunctionObtained by diffusing p-type impurities on one side of a n-type Si bar (”bulk”)

• (free) electrons migrate to the p side and fill the holes• (free) holes migrate to the n side and capture electrons

• creation of a depletion region with no charge carriers

• the n side is now charged +, the p side is charged - :

• creation of an electrical potential (built-in potential)

• this balances the chemical potential : equilibrium


p-njunction:depletiondepthsuppose uniform charge density ρ(x) on both sides

charge conservation NAxp=NDxn

solve Poisson equation1=>13=

= −? 3@

using boundary conditions E(-xp)=E(xn)=0(Exercise1)

à ∆𝑉 = C'@

𝑁E𝑥F' + 𝑁H𝑥I'

à 𝑥F ='@∆>

CJK LMJK/JN

L/'𝑥I =

'@∆>CJN LMJN/JK

L/'

à 𝑑 = 𝑥F + 𝑥I ='@∆>C

JKMJNJKJN

L/'


ρ(x)-xp

xn

ρ=eND

ρ=-eNA

p-njunction:depletiondepth• Typical values in Si P-N junctions :

• NA = 1015 cm-3 in p+ region , ND = 1012 cm-3 in n bulk << NA

• à xp=0.02 μm and xp=23 μm

𝑑 = 𝑥F + 𝑥I ≈ 𝑥I ≈2𝜖∆𝑉𝑒𝑁H

L/'

can be expressed as a function of mobility μ

and resistivity 𝜌 = 𝑒𝑁H𝜇C TL

𝑑 ≈ 2𝜖𝜌𝜇C∆𝑉 L/'


ρ(x)-xp

xn

ρ=eND

ρ=-eNA

Fromthep-njunctiontothedetector• The depth of the depletion region can be increased by applying an external voltage: reverse bias

ΔV -> ΔV+VB in the formula

(VB>>ΔV, so ΔV+VB~VB)

example : d=1mm VB=300V

The voltage V needed to deplete a device of thickness d is called the depletion voltage Vd

• The depleted region is the useful volume for detection

ionizing particles produce e--hole pairs

e- (holes) drift to the n (p) sideà current pulses

proportional to the energy deposit


Semiconductordetectorcharacteristics• Charge collection

the drift velocity of electron/holes is ve = μe E, vh = μh E respectivelyThe mobility μ depends on T and E.

• Linearitythe depleted region has a capacitance C=εA/d

the observed voltage on the electrodes is then 𝑉 = UV= 𝑛 2

6Và linear with the deposited energy

• Energy resolution (Exercise 2)the intrinsic energy resolution depends on the number of carriers E/w and the Fano Factor F~0.12

𝑅 = 2.35𝐹𝑤𝐸

�

(+ contributions from other sources, e.g. electronics)


A,d :detectorareaandthicknessn :collectionefficiencyE:electricfieldw:averageenergyfore-hpaircreation

ChargecollectionstimesO(10ns)à fast!

(parenthesis:theFano Factor)• Assume the formation of charge carriers to be a Poisson process:

• if 𝑁 carriers generated, the statistical fluctuation on that number is 𝑁�

• since 𝑁 is usually large, the detector response will follow a Gaussian distribution with mean = 𝑁 andstandard deviation = 𝑁�

• For a linear detector, where the response E is proportional to 𝑁, E=kN, the resolution is

𝑅 =𝐹𝑊𝐻𝑀𝐸 =

2.35𝜎 𝐸𝐸 =

2.35𝑘 𝑁�

𝑘𝑁 =2.35 𝑁�

𝑁• This is the intrinsic limit from statistical fluctuations. Better values are found from measurements in somedetectors.

• Processes leading to the formation of charge carriers are probably not completely independent: departurefrom Poisson statistics, quantified by the Fano Factor F

𝐹 =𝑜𝑏𝑠𝑒𝑟𝑣𝑒𝑑𝑣𝑎𝑟𝑖𝑎𝑛𝑐𝑒𝑜𝑛𝑁

𝑃𝑜𝑖𝑠𝑠𝑜𝑛𝑝𝑟𝑒𝑑𝑖𝑐𝑡𝑒𝑑𝑣𝑎𝑟𝑖𝑎𝑛𝑐𝑒(= 𝑁)

• so the resolution is 𝑅 = '.%lm J� n�

mJ= 2.35 n

J� = 2.35 n6

2�


inscintillatorsF~1inSi/GeF~0.1

Semiconductordetectorcharacteristics


Semiconductordetectorcharacteristics• Leakage current

Small fluctuating current flowing when VB is applied,although a reverse bias diode is ideally non-conducting.It limits the smallest signal pulse height that can beobserved.Several sources:• Generated current: thermally generated e-h pairs,separated by the electric field and collected at theends of the semiconductor volumeà reduced by using pure and defect-free

materials with high carriers lifetime, operateat low T

• Diffusion current: movement of minority chargecarriers


M.Bomben,HDR

SomepropertiesofSiandGeasdetectorsSilicon(Si) Germanuim (Ge)

Z,A 14,28.1 32,72.6

ρ [g/cm2] 2.33 5.32

Eg (T=300K)[eV] 1.1 0.7

Eg (T=0K)[eV] 1.21 0.785

I0 (T=300K)[eV] 3.62 -

I0 (T=77K)[eV] 3.81 2.96

μe (T=300K)[cm2/Vs] 1350 3900

μh (T=300K)[cm2/Vs] 480 1900


only~1/3ofdepositedenergygoesintoproductionofe-hpairs

• HighZ:goodforγ detection(σ forphotoelectriceffectx60Si)• Needscooling

• LowZ:goodforchargedparticles

Positionmeasurement:Microstrip detectorsBy segmenting the implant, the position of thetraversing particle can be reconstructed in 1dimension


300µm

50-150µm

• Anamplifierisconnectedtoeachstrip.• ACcouplingblocksleakagecurrentfromtheamplifier

Positionmeasurement:Microstrip detectorsBy segmenting the implant, the position of thetraversing particle can be reconstructed in 1dimension

àposition resolution = pitch/sqrt(12)~ O(10-50μm)

Diffusion:• caused by random motion during the drift

• width of charge cloud 𝜎H = 2𝐷𝑡� with 𝐷 = 𝑘𝑡𝜇/𝑒• typical 𝜎H = 8μm after 300μm drift

• can be used to improve position resolution (sharing ofsignal between adjacent strips)


300µm

50-150µm

Double-SidedSiliconDetectors

• Reconstruction of two coordinates with onedetector layer

+ Advantage: minimize multiple scattering

- Disadvantage: ”ghost hits” at high particle multiplicity

The solution: Pixel detectors…


PixelDetectors• Two-dimensional arrays of p-n diodes organised ina matrix

• Each p-n junction is readout by a dedicatedreadout integrated circuit

• Sensor and readout electronics are physicallyseparated and linked by a bump bonding

à Hybrid Pixel Detectors (HPDs)

+ Unambiguous 2D position measurement, pitch ~50μm

+ Smaller leakage currents than DSSD

- Large number of readout channels: complex solutions,large power consumption


Examples

• To measure lifetimes of charmedmesons

• Surface: 24 cm2 (2” wafer) 1200 strip, 20μm pitch. Every 3rd/6th strip connected.Precision 4,5 μm !

• 8 Si detectors (2 in front, 6 behind theTarget)


NA11atCERN,1983

Firstuse ofaposition-sensitiveSidetectorinHEP

Examples


SiliconTrackerDetector(STD):9layersofdouble-sidedSisensors(2300)• Totalinstrumentedsurfacearea:6.45m2 with196kreadoutchannels

• Positionresolution:8.5µm(30µm)inthebending(non-bending)plane

• particlerigiditymeasurement(p/Z)of1.5%at10GeVandamaximumdetectablerigidityabove1TeV forprotons

• ChargeseparationofnucleiuptoZ=26(bydE/dxmeasurement)

AMS-02Large-acceptancespectrometerontheISS,since2011.Toperformprecisemeasurementsofchargedcosmicraysfluxesinawideenergyrange

Examples


CMSThe4mainLHCexperiments(ATLAS,CMS,ALICE,LHCb)haveSitrackers

ATLASPixels:1744modules,80x106channels.Pixelsize50x400μm2 .Resolution14x115μm2

Strips:61m2 ofsilicon ,4088modules,6x106channelsReadout pitch80μm,hitresolution17μm

theworldlargest silicon tracker

~1m2 ofpixelsensors,60x106channels,hitresolution 10x17μm2

200m2 ofstrip sensors (singlesided)11x106readout channels,hitresolution 40-60μm

Examples


Radiationdamage• Inelastic collisions of impinging particles on Si cause displacement of a Si atom from its lattice siteà point defects

• several defects grouped together can form a cluster the probability depends on the particle typeand energy. The amount increases with particle fluence.

àMacroscopic effects (”ageing”):

• increase of leakage current

• change in operation voltage

• reduction of signal amplitude

• à R&D for radiation-hard sensors


SomeotherSidetectors


Photodetectors:AvalanchePhotoDiodes (APD)

Conversion of photons into a charge current :e-h pair creation + avalanche multiplication

+ Gain O(100)+ Fast response times(formation of avalanche <ns)

- Large operation voltage (E > 105 V/cm)- Poor energy resolution (fluctuations in theavalanche), extremely sensitive to voltagechanges


HamamatsuSiAPDS8890-10

Photodetectors:AvalanchePhotoDiodes (APD)


Photodetectors:SiliconPhotoMultipliers (SiPM)Matrices of 100s-1000s APDs operated in Geiger mode

• The signal in each cell is not proportional to energy:on/off

• Count number of hit cellsàmeasure the photon flux

+ Gain ~105-107 (cfr PMTs)

+ Good charge resolution

+ No need for HV

+ Not sensitive to magnetic field

- Dark noise, crosstalk, afterpulses, …


source:Hamamatsu

ExercisesExercise 1

Compute the solution to Poisson’s equation to obtain the depth of the depletion region (slide 11).

Exercise 2

Scintillators, gaseous and silicon detectors can be used to measure the energy lost by a particle. Thekey parameter is the energy necessary to create a scintillation photon (or a e/ion pair, or a e/holepair). Three cases are considered:

(i) Scintillator. 1 photon emitted for 100 eV deposited, 6% of photons reach the PMT, QE = 30%

(ii) Si detector. 3.6 eV are necessary to create a pair e/hole.

(iii) Gaseous detector. 20 eV necessary to create a pair e/ion.

What is the relative precision with which energy losses of (a) 100 keV, (b) 5 MeV, (c) 20 MeV can bemeasured in the three detectors? NB. We assume a Gauss statistics, the error on a count N issqrt(N).


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Detector Physics 2020/2021, Lecture 7