ppe s/c detector lecturesdr r. bates1 semiconductor detectors an introduction to semiconductor...

PPE S/C detector lectures Dr R. Bates 1

Semiconductor detectors

An introduction to semiconductor detector physics as applied to particle physics


Contents

4 lectures – can’t cover much of a huge field

Introduction Fundamentals of operation The micro-strip detector Radiation hardness issues


Lecture 1 - Introduction

What do we want to do Past, present and near future Why use semiconductor detectors


What we want to do - Just PPE

Track particles without disturbing them Determined position of primary interaction vertex

and secondary decays Superb position resolution

Highly segmented high resolution Large signal

Small amount of energy to crate signal quanta Thin

Close to interaction point Low mass

Minimise multiple scattering Detector Readout Cooling / support


Ages of silicon - the birth

J. Kemmer Fixed target experiment with a planar

diode*

Later strip devices -1980 Larger devices with huge ancillary

components

* J. Kemmer: “Fabrication of a low-noise silicon radiation detector by the planar process”, NIM A169, pp499, 1980


Ages of Silicon - vertex detectors

LEP and SLAC ASIC’s at end of ladders Minimise the mass inside tracking volume Minimise the mass between interaction point and

detectors Minimise the distance between interaction point

and the detectors Enabled heavy flavour physics i.e. short

lived particles


ALEPH


ALPEH – VDET (the upgrade)

2 silicon layers, 40cm long, inner radius 6.3cm, outer radius 11cm 300m Silicon wafers giving thickness of only 0.015X0 S/N r = 28:1; z = 17:1 r = 12m; z = 14m


Ages of silicon - tracking paradigm

CDF/D0 & LHC Emphasis shifted to tracking + vertexing Only possible as increased energy of particles

Cover large area with many silicon layers Detector modules including ASIC’s and

services INSIDE the tracking volume Module size limited by electronic noise due

to fast shaping time of electronics (bunch crossing rate determined)


ATLAS

A monster !


ATLAS barrel

2112 Barrel modules mounted on 4 carbon fibre concentric Barrels, 12 in each row

1976 End-cap modules mounted on 9 disks at each end of the barrel region


What is measured

Measure space points Deduce

Vertex location Decay lengths Impact parameters


Signature of Heavy FlovoursStable particles > 10-6 s c

n 2.66km

658m

Very long lived particles > 10-10 s

, K±, KL0 2.6 x 10-8 7.8m

KS0, E±, 0 2.6 x 10-10 7.9cm

Long lived particles > 10-13 s

± 0.3 x 10-12 91m

Bd0, Bs

0, b 1.2 x 10-12 350m

Short lived particles

0, 0 8.4 x 10-17 0.025m

, 4 x 10-23 10-9m!!


Decay lengths

By measuring the decay length, L, and the momentum, p, the lifetime of the particle can be determined

Need accuracy on both production and decay point

L

Primary vertexSecondary vertex

L = p/m c

E.g. B J/Ks0


Impact parameter (b)

b

beam

b = distance of closest approach of a

reconstructed track to the true interaction point


Impact parameter

Error in impact parameter for 2 precision measurements at R1 and R2 measured in two detector planes:

a=f(R1 & R2) and function of intrinsic resolution of vertex detector

b due to multiple scattering in detector c due to detector alignment and stability

2

2

2 cp

bab


Impact parameter

b = f( vertex layers, distance from main vertex, spatial resolution of each detector, material before precision measurement, alignment, stability )

Requirements for best measurement Close as possible to interaction point Maximum lever arm R2 – R1 Maximum number of space points High spatial resolution Smallest amount of material between interaction point and 1st

layer Good stability and alignment – continuously measured and

correct for 100% detection efficiency Fast readout to reduce pile up in high flux environments

Impact parameter*

Effect of extra mass and

distance from the interaction point

Blue = 5mm

Black = 1mm (baseline)

Green = 0.5 mm

Red = 0.1 mm

GR Width Flux increase(%) to silicon Improvement of the IPres. wrt 1mm(%)

5mm -44 -38.10.9

0.5mm +14.1 +5.8 0.7

0.1mm +27.7 +10.0 0.7

Lower Pt

*Guard Ring Width Impact on d0 Performances and Structure Simulations. A Gouldwell, C Parkes, M Rahman, R Bates, M Wemyss, G Murphy, P Turner and S Biagi. LHCb Note, LHCb-2003-034


Why Silicon

Semiconductor with moderate bandgap (1.12eV) Thermal energy = 1/40eV

Little cooling required Energy to create e/h pair (signal quanta)= 3.6eV

c.f Argon gas = 15eV High carrier yield better stats and lower Poisson stats noise Better energy resolution and high signal no gain stage required


Why silicon High density and atomic number

Higher specific energy loss Thinner detectors Reduced range of secondary particles

Better spatial resolution High carrier mobility Fast!

Less than 30ns to collect entire signal Industrial fabrication techniques Advanced simulation packages

Processing developments Optimisation of geometry Limiting high voltage breakdown Understanding radiation damage


Disadvantages

Cost Area covered Detector material could be cheap – standard Si Most cost in readout channels

Material budget Radiation length can be significant

Effects calorimeters Tracking due to multiple scattering

Radiation damage Replace often or design very well – see lecture 4


Radiation length X0

High-energy electrons predominantly lose energy in matter by bremsstrahlung

High-energy photons by e+e- pair production The characteristic amount of matter traversed for

these related interactions is called the radiation length X0, usually measured in g cm-2.

It is both: the mean distance over which a high-energy electron

loses all but 1=e of its energy by bremsstrahlung the mean free path for pair production by a high-energy

photon A

ZrZZN

X

eA

312

0

183log141


Lecture 2 – lots of details

Simple diode theory Fabrication Energy deposition Signal formation


Detector = p-i-n diode

Near intrinsic bulk Highly doped contacts Apply bias (-ve on p+ contact)

Deplete bulk High electric field

Radiation creates carriers signal quanta

Carriers swept out by field Induce current in external circuit

signal

ND~1012cm-3

n+ contact ND=1018cm-3

p+ contact NA=1018cm-3


Why a diode?

Signal from MIP = 23k e/h pairs for 300m device Intrinsic carrier concentration

ni = 1.5 x 1010cm-3

Si area = 1cm2, thickness=300m 4.5x108 electrons 4 orders > signal

Need to deplete device of free carriers Want large thickness (300m) and low bias

But no current! Use v.v. low doped material p+ rectifying (blocking) contact


p-n junctionp+ n

Dopantconcentration

Space chargedensity

Carrier density

Electric field

Electric potential

(1)

(2)

(3)

(4)

(5)

(6)

(7)


p-n junction

1) take your samples – these are neutral but doped samples: p+ and n-

2) bring together – free carriers moveo two forces drift and diffusiono In stable state

Jdiffusion (concentration density) = Jdrift (e-field)

3) p+ area has higher doping concentration (in this case) than the n region


p-n junction4) Fixed charge region

5) Depleted of free carrierso Called space charge region or depletion regiono Total charge in p side = charge in n sideo Due to different doping levels physical depth of space

charge region larger in n side than p sideo Use n- (near intrinsic) very asymmetric junction

6) Electric field due to fixed charge

7) Potential difference across deviceo Constant in neutral regions.

dx

dVE


Resistivity and mobility

Carrier DRIFT velocity and E-field:

n = 1350cm2V-1s-1 : p = 480cm2V-1s-1

Resistivity

p-type material

n-type material

pnq pn

1

Dp Nq 1

ANq 1

Ev


Depletion width

Depletion Width depends upon Doping Density:

For a given thickness, Full Depletion Voltage is:

W = 300m, ND 5x1012cm-3: Vfd = 100V

AD NNq

VW

112

2

2WqNV D

fd


Reverse Current

Diffusion current From generation at edge of depletion region Negligible for a fully depleted detector

Generation current From generation in the depletion region Reduced by using material pure and defect free

high lifetime Must keep temperature low & controlled

kTTjgen 2

1exp23

Wn

qj igen

02

1

kT

ENNn g

VCi exp2 KTforjgen 82


Capacitance

Capacitance is due to movement of charge in the junction

Fully depleted detector capacitance defined by geometric capacitance

Strip detector more complex Inter-strip capacitance dominates

WVV

Nq

VNN

NNq

dV

dQC D

DA

DA

222


Noise

Depends upon detector capacitance and reverse current

Depends upon electronics design Function of signal shaping time Lower capacitance lower noise Faster electronics noise contribution

from reverse current less significant


Fabrication

Use very pure material High resistivity

Low bias to deplete device Easy of operation, away from breakdown, charge spreading for

better position resolution Low defect concentration

No extra current sources No trapping of charge carriers

Planar fabrication techniques Make p-i-n diode pattern of implants define type of detector (pixel/strip) extra guard rings used to control surface leakage currents metallisation structure effects E-field mag limits max bias


Fabrication stages

Starting material Usually n-

Phosphorous diffusion P doped poly n+ Si

Stages dopants to create p- & n-type regions passivation to end surface dangling bonds and protect

semiconductor surface metallisation to make electrical contact

n- Si


Fabrication stages

Deposit SiO2

Grow thermal oxide on top layer

Photolithography + etching of SiO2 Define eventual

electrode pattern


Fabrication stages

Form p+ implants Boron doping thermal

anneal/Activation

Removal of back SiO2

Al metallisation + patterning to form contacts


Fabrication

Tricks for low leakage currents low temperature processing

simple, cheap marginal activation of implants, can’t use IC

tech gettering

very effective at removal of contaminants complex


Energy Deposition

Charge particles Bethe-Bloch Bragg Peak

Not covered Neutrons Gamma Rays

Rayleigh scattering, Photo-electric effect, Compton scattering, Pair production


At 3 dE/dx minimum independent of absorber (mip)

Electrons mip @ 1 MeV E>50 MeV radiative energy

loss dominatesMomentum transferred to a free electron at rest when a charged particle passes at its

closest distance, d. integrate over all possible values of d

Charge particles- concentrating on electrons


at end of range specific energy loss increases particle slows down deposit even more energy per unit distance

Bragg Peak

Useful when estimating properties of a device

E = 5 MeV in Si:(increasing charge)

R (m)p 220 2516O 4.3

Well defined range


Energy Fluctuation

Electron range of individual particle has large fluctuation

Energy loss can vary greatly - Landau distribution Close collisions (with bound

electrons) rare energy transfer large ejected electron initiates

secondary ionisation Delta rays - large spatial

extent beyond particle track Enhanced cross-section for K-,

L- shells Distance collisions

common M shell electrons - free

electron gas


e/h pair creation

Create electron density oscillation - plasmon requires 17 eV in Si

De-excite almost 100% to electron hole pair creation Hot carriers

thermal scattering optical phonon scattering ionisation scattering (if E > 3/4 eV)

Mean energy to create an e/h pair (W) is 3.6 eV in Si (Eg = 1.12 eV 3 x Eg)

W depends on Eg therefore temperature dependent


Delta rays

a) Proability of ejecting an electron with E T as a function of Tb) Range of electron as a function of energy in silicon


Displacement from -electrons

Estimate the error Assume 20k e/h from track 50keV -electron produced perpendicular to track Range 16m, produces 14k e/h Assume ALL charge created locally 8m from

particle’s track

mkk

mkmk 3.3

1420

814020


Consequences of d-electrons

Centroid displacement Resolution as function of pulse height

0

1

2

3

4

5

6

0 1 2 3 4 5 6

Pulse height (mip)

Res

olu

tio

n (

mic

ron

s)0

10

20

30

40

50

60

70

0 2 4 6 8 10

Displacement (microns)

Pro

bab

ilit

y (%

)


Consequence of -electrons

45º

15 m

E.g. CCD

E.g. Microstrip

45º

300 m

Most probable E loss = 3.6keV10% proby of 5keV pulls track up by 4 m

Most probable E loss = 72keV10% proby of 100keV pulls track up by 87 m


Signal formation

Signal due to the motion of charge carriers inside the detector volume & the carriers crossing the electrode Displacement current due to change in

electrostatics (c.f. Maxwell’s equations)

Material polarised due to charge introduction

Induced current due to motion of the charge carriers

See a signal as soon as carriers move


Signal

Simple diode Signal generated equally from movement through entire

thickness Strip/pixel detector

Almost all signal due to carrier movement near the sense electrode (strips/pixels)

Make sure device is depletedunder strips/pixelsIf not: Signal small Spread over many strips


Lecture 3 – Microstrip detector

Description of device Carrier diffusion

Why is it (sometimes) good Charge sharing

Cap coupling Floating strips

Off line analysis Performance in magnetic field Details

AC coupling Bias resistors Double sides devices


What is a microstrip detector?

p-i-n diode Patterned implants as strips

One or both sides Connect readout electronics to strips Radiation induced signal on a strip due

to passage under/close to strip Determine position from strip hit info


What does it look like?

P+ contact on front of n- bulk Implants covered with thin thermal

oxide (100nm) Forms capacitor ~ 10pF/cm

Al strip on oxide overlapping implant Wirebond to amplifier

Strips surrounded by a continuous p+ ring The guard ring Connected to ground Shields against surface currents

Implants DC connected to bias rail Use polysilicon resistors M Bias rail DC to ground

HV


Resolution

Delta electrons See lecture 2

Diffusion Strip pitch

Capacitive coupling Read all strips Floating strips


Carrier collection

Carriers created around track Φ 1m Drift under E-field

p+ strips on n- bulk p+ -ve bias Holes to p+ strips, electrons to n+ back-plane

Typical bias conditions 100V, W=300m E=3.3kVcm-1

Drift velocity: e= 4.45x106cms-1 & h=1.6x106cm-1

Collection time: e=7ns, h=19ns


Carrier diffusion

Diffuse due to conc. gradient dN/dx Gaussian

Diffusion coefficient:

RMS of the distribution: Since D & tcoll 1/

Width of distribution is the same for e & h As charge created along a strip

Superposition of Gaussian distribution

dxDt

x

DtN

dN

4exp

4

1 2

q

kTD

collDt2


Diffusion

Example for electrons: tcoll = 7ns; T=20oC = 7m

Lower bias wider distribution For given readout pitch

wider distribution more events over >1 strip Find centre of gravity of hits better position

resolution Want to fully deplete detector at low bias

High Resistivity silicon required


Resolution as a f(V)

V<50V charge created in undeleted region lost, higher noise

V>50V reduced drift time and diffusion width less charge sharing

more single strips

0

1

2

3

4

5

0 20 40 60 80 100

Bias (V)

Res

olu

tio

n (

mic

ros)Spatial

resolution as a function of bias

Vfd = 50V


Resolution due to detector design

Strip pitch Very dense Share charge over many strips Reconstruct shape of charge and find centre Signal over too many strips lost signal (low

S/N) BUT

FWHM ~ 10m Limited to strip pitch 20m

Signal on 1 or 2 strips


Two strip events

Track between strips Find position from signal on 2 strips Use centre of gravity or Algorithm takes into account shape of charge

cloud (eta, ) Track mid way Q on both strips

best accuracy Close to one strip

Small signal on far strip Lost in noise


Capacitive coupling Strip detector is a C/R network Cstrip to blackplace = 10x Cinterstrip

Csb || Cis ignore Csb

Fraction of charge on B due to track at A:

ACeff

eff

effACis

ACisB

CBA

B

CBA

B

CC

CK

CCC

CCC

CCC

C

QQQ

QK

2 isAC CCas

smallisK

C

CK

CC

AC

is

iseff

A

B

C

ACC

ACC

ACC

isC

isCsQ


Floating strips

Large Pitch (60m)

Intermediate strip

1/3 tracks on both stripsAssume = 2.2m2/3 on single strips = 40/12 = 11.5mOverall:

= 1/3 x 2.2 + 2/3 x 11.5 = 8.4m

60m

20m

20m 20m 20m Capacitive charge coupling2/3 tracks on both stripsNO noise losses due to cap coupling1/3 tracks on single strips = 2/3 x 2.2 + 1/3 x 20/12

= 3.4m

20m strip pitch =2.2m


Off line analysis

Binary readout No information on the signal size Large pitch and high noise

Get a signal on one strip only

-½ pitch ½ pitch

P(x) <x> = 0

1212

1

)(

)(

21

21

2

21

21

22

Pitch

dxxPx

dxxPxxx


Have signal on each strip Assume linear charge sharing between strips

Centre of Gravity

PHL PHR

P

x

stripsii

stripsiii

PH

xPH

X

RL

R

PHPH

PPHX

Q on 2 strips & x = 0 at left strip

e.g. PHL = 1/3PHR

PP

X4

3

4331

43031


Eta function

Non linear charge sharing due to Gaussian charge cloud shape

PHL PHR

P

x

More signal on RH strip than predicted with uniform charge cloud shape

Non-linear function to determine track position from relative pulseheights on strips


Eta function

Measured tracks as a function of incident particle flux

Measured and predicted particle position


Lorentz force Force on carriers due to magnetic force

Perturbation in drift direction Charge cloud centre drifts from track position Asymmetric charge cloud No charge loss is observed

Can correct for if thickness & B-field known

E H L

vh

ve

B

c

vEqF


Details

Modern detectors have integrated capacitors Thin 100nm oxide on top of implant Metallise over this Readout via second layer

Integrated resistors Realise via polysilicon

Complex Punch through biasing

Not radiation hard Back to back diodes – depleted region has high R


Details

Double sided detectors Both p- and n-side pattern

Surface charge build up on n-side Trapped +ve charge in SiO Attracts electrons in silicon near surface Shorts strips together p+ spray to increase inter-strip resistance


Lecture 4 – Radiation Damage

Effects of radiation Microscopic Macroscopic Annealing

What can we do? Detector Design Material Engineering Cold Operation Thin detectors/Electrode Structure – 3-D device


Effects of Radiation

Long Term Ionisation Effects Trapped charge (holes) in SiO2

interface states at SiO2 - Si interface Can’t use CCD’s in high radiation environment

Displacement Damage in the Si bulk 4 stage process Displacement of Silicon atoms from lattice Formation of long lived point defects & clusters


Displacement Damage

Incoming particle undergoes collision with lattice knocks out atom = Primary knock on atom

PKA moves through the lattice produces vacancy interstitial pairs (Frenkel Pair) PKA slows, reduces mean distance between

collisions clusters formed

Thermal motion 98% lattice defects anneal defect/impurity reactions

Stable defects influence device properties


PKA Clusters formed when

energy of PKA< 5keV Strong mutual

interactions in clusters Defects outside of

cluster diffuse + form impurity related defects (VO, VV, VP)

e & don’t produce clusters


Effects of Defects

Generation Recombination Trapping Compensation

e

h

e e

h h

Leakage Current Charge Collection Effective DopingDensity

eEC

EV


Reverse Current

1011 1012 1013 1014 1015

eq [cm-2]

10-6

10-5

10-4

10-3

10-2

10-1

I /

V

[A/c

m3 ]

n-type FZ - 7 to 25 Kcmn-type FZ - 7 to 25 Kcmn-type FZ - 7 Kcmn-type FZ - 7 Kcmn-type FZ - 4 Kcmn-type FZ - 4 Kcmn-type FZ - 3 Kcmn-type FZ - 3 Kcm

n-type FZ - 780 cmn-type FZ - 780 cmn-type FZ - 410 cmn-type FZ - 410 cmn-type FZ - 130 cmn-type FZ - 130 cmn-type FZ - 110 cmn-type FZ - 110 cmn-type CZ - 140 cmn-type CZ - 140 cm

p-type EPI - 2 and 4 Kcmp-type EPI - 2 and 4 Kcm

p-type EPI - 380 cmp-type EPI - 380 cm

I = Volume Material independent

linked to defect clusters

Annealing material independent

Scales with NIEL Temp dependence

kT

ETTI g

2exp2

= 3.99 0.03 x 10-17Acm-1

after 80minutes annealing at 60C


Effective Doping Density

0 0.5 1 1.5 2eq [1014cm-2]

1

2

3

4

5

|Nef

f| [1

012 c

m-3

]

50

100

150

200

250

300

Vde

p [V

] (

300

m)

1.8 Kcm Wacker 1.8 Kcm Wacker 2.6 Kcm Polovodice2.6 Kcm Polovodice3.1 Kcm Wacker 3.1 Kcm Wacker 4.2 Kcm Topsil 4.2 Kcm Topsil

Neutron irradiationNeutron irradiation Donor removal and

acceptor generation type inversion: n p depletion width

grows from n+ contact

Increase in full depletion voltage V Neff

cNN effeff exp0 = 0.025cm-1 measured after beneficial anneal


Effective Doping Density Short-term beneficial

annealing Long-term reverse

annealing temperature dependent stops below -10C

Neff = Zero

BA

RA BARA BA

Increasing Radiation

1 10 100 1000 10000

annealing time at 60oC [min]

0

2

4

6

8

10

N

eff [

1011

cm-3

]

NY, = gY eq

NC

NC0

gC eqgC eq

NA = ga eqNA = ga eq

0 1 2 3 4 5 6 7 8 9 10

time [years]

200

400

600

800

1000

Vde

p (2

50m

) [V

]

200

400

600

800

1000

standard silicon

oxygenated silicon

operation voltage: 600 V


Signal speed from a detector

Duration of signal = carrier collection time

Speed mobility & field Speed 1/device thickness PROBLEMS

Post irradiation mobility & lifetime reduced lower longer signals and lower Qs

Thick devices have longer signals


Signal with low lifetime material

Lifetime, , packet of charge Q0 decays

In E field charge drifts Time required to drift distance x:

Remaining charge: Drift length, L is a figure of merit.

toeQtQ )(

E

x

v

xt

LxEx eQeQxQ 00)(


Parallel plate detector:

In high quality silicon detectors: 10ms, e = 1350cm2V-1s-1, E = 104Vcm-1

L 104cm (d ~ 10-2cm)

Amorphous silicon, L 10m (short lifetime, low mobility) Diamond, L 100-200m (despite high mobility) CdZnTe, at 1kVcm-1, L 3cm for electrons, 0.1cm for holes

d

Lxd

s dxeQd

dxxQd

Q0

0

0

1)(

1 Lds e

d

LQQ 10

d

L

Q

QdL s

0

:

Induced charge


What can we do?

Detector Design Material Engineering Cold Operation Electrode Structure – 3-D device


Detector Design

n-type readout strips on n-type substrate post type inversion substrate p type

depletion now from strip side high spatial resolution even if not fully depleted

Single Sided Polysilicon resistors W<300m thick limit max depletion V Max strip length 12cm lower cap. noise


Multiguard rings

Enhance high voltage operation

Smoothly decrease electric field at detectors edge

Poly

Guardrings V

strip biasback planebias


Substrate Choice

Minimise interface states Substrate orientation <100> not <111>

Lower capacitive load Independent of ionising radiation

<100> has less dangling surface bonds


Metal Overhang

Used to avoid breakdown performance deterioration after irradiation

Bre

akdo

wn

Vol

tage

(V

)

Strip Width/Pitch

(1) (2)1

2

4m

0.6mp+

p+

n+

SiO2

n

<111> after 4 x 1014 p/cm2


Material Engineering

Do impurities influence characteristics? Leakage current independent of impurities Neff depends upon [O2] and [C]

St = 0.0154

[O] = 0.0044 0.0053

[C] = 0.0437

0

1E+12

2E+12

3E+12

4E+12

5E+12

6E+12

7E+12

8E+12

9E+12

1E+13

0 1E+14 2E+14 3E+14 4E+14 5E+14

Proton fluence (24 GeV/c ) [cm-2]

|Nef

f| [c

m-3]

0

100

200

300

400

500

VF

D fo

r 3

00

m t

hic

k d

etec

tor

[V]

Standard (P51)O-diffusion 24 hours (P52)O-diffusion 48 hours (P54)O-diffusion 72 hours (P56)Carbon-enriched (P503)


O2 works for charged hadrons

Neff unaffected by O2 content for neutrons

Believed that charge particle irradiation produces more isolated V and I

0 0.5 1 1.5 2 2.5 3 3.5eq [1014cm-2]

1

2

3

4

5

6

7

|Nef

f| [1

012 c

m-3

]

100

200

300

400

Vde

p [V

] (

300

m)

neutronsneutrons

neutronsneutronspionspionsprotonsprotons

pionspionsprotonsprotons

oxygen rich FZ

standard FZstandard FZ V + O VOV + VO V2OV2O reverse annealing

High [O] suppresses V2O formation


Charge collection efficiency

Oxygenated Si enhanced due to lower depletion voltage

CCI ~ 5% at 300V

after 3x1014 p/cm2

CCE of MICRON ATLAS prototype strip detectors irradiated with 3 1014 p/cm2


ATLAS operation

0 1 2 3 4 5 6 7 8 9 10

time [years]

200

400

600

800

1000

Vde

p (2

50m

) [V

]

200

400

600

800

1000

standard silicon

oxygenated silicon

operation voltage: 600 V

Damage for ATLAS barrel layer 1

Use lower resistivity Si toincrease lifetime in neutron field

Use oxygenated Si to increase lifetime in charge hadron field


Cold Operation

Know as the “Lazarus effect”

Recovery of heavily irradiated silicon detectors operated at cryogenic temps observed for both

diodes and microstrip detectors


The Lazarus Effect For an undepleted heavily irradiated detector:

Traps are filled traps are neutralized Neff compensation (confirmed by experiment)

B. Dezillie et al., IEEE Transactions on Nuclear Science, 46 (1999) 221

trap

driftt

D

dCCE

exp

2

effNd

12

T = 300 K

e

h

hole trapping

electron de-trapping

hole de-trapping

electron trapping

conduction band

valence band

T = 77 K

conduction band

valence band

e

h

trap filled

e

htrap filled

dD

undepleted region

active region

where


Reverse Bias

Measured at 130K - maximum CCECCE falls with time to a stable value


Cryogenic Results

CCE recovery at cryogenic temperatures CCE is max at T ~ 130 K for all samples CCE decreases with time till it reaches a stable value

Reverse Bias operation

MPV ~5’000 electrons for 300 m thickstandard silicon detectors irradiated with21014 n/cm2 at 250 V reverse bias and T~77 K

very low noise

Forward bias is possible at cryogenic temperatures

No time degradation of CCE in operation with forward bias or in presence of short wavelength light same conditions: MPV ~13’000 electrons


Electrode Structure

Increasing fluence Reducing carrier lifetime Increasing Neff

Higher bias voltage Operation with detector under-depleted

Reduce electrode separation Thinner detector Reduced signal/noise ratio Close packed electrodes through wafer


The 3-D device

Co-axial detector Arrayed together

Micron scale USE Latest MEM

techniques

Pixel device Readout each p+ column

Strip device Connect columns

together


Proposed by S.Parker, Nucl. Instr. And Meth. A 395 pp. 328-343(1997).

Equal detectors thickness

W2D>>W3D

Equal detectors thickness

W2D>>W3D

h+

e-

-ve +veSiO 2

W3D

E

Bulk

h+

e-

+ve

E

p+

n

n+

Operation

Carriers drift total thickness of material

Carriers swept horizontallyTravers short distance between electrodes

W2D

+ve -ve-ve -ve


Advantages

If electrodes are close Low full depletion bias Low collection distances Thickness NOT related to collection

distance No charge spreading Fast charge sweep out


A 3-D device

Form an array of holes Fill them with poly-silicon Add contacts

Can make pixel or strip devices

Bias up and collect charge


Real spectra

At 15VPlateau in Q collectionFully active

Very good energy resolution


3-D Vfd in ATLAS

0 1 2 3 4 5 6 7 8 9 1 0

t i m e [ y e a r s ]

5 0 0

1 0 0 0

1 5 0 0

2 0 0 0

Vde

p (2

00

m)

[V]

5 0 0

1 0 0 0

1 5 0 0

2 0 0 0s t a n d a r d s i l i c o n

o x y g e n a t e d s i l i c o n

opera tion voltag e: 600 V

6 000 e fo r B-layer 6 000 e fo r B-layer

Damage projection for the ATLAS B-layer(3rd RD48 STATUS REPORT CERN LHCC 2000-009, LEB Status Report/RD48, 31 December 1999).

• 3D detector!


Summary Tackle reverse current

Cold operation, -20C Substrate orientation Multiguard rings

Overcome limited carrier lifetime and increasing effective doping density Change material Increase carrier lifetime Reduce electrode spacing


Final Slide

Why? Where? How? A major type A major worry

ppe s/c detector lecturesdr r. bates1 semiconductor detectors an introduction to semiconductor...

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