Charge collection close to the si si0 2 interface of silicon strip sensors
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Charge collection close to the Si-Si0 2 interface of silicon strip sensors. Thomas Pöhlsen, Eckhart Fretwurst , Robert Klanner , Sergej Schuwalow , J ö rn Schwandt, Jiaguo Zhang University of Hamburg. O verview. Introduction Charge collection close to the Si-Si0 2 interface

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Charge collection close to the Si-Si0 2 interface of silicon strip sensors

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Charge collection close to the si si0 2 interface of silicon strip sensors

Charge collectionclosetothe Si-Si02interfaceofsiliconstripsensors

Thomas Pöhlsen, Eckhart Fretwurst, Robert Klanner, Sergej Schuwalow, Jörn Schwandt, JiaguoZhang

University of Hamburg


O verview

Overview

Introduction

Charge collectionclosetothe Si-Si02interface

Weighting potential

Time resolvedsignals

Integrated signals

Results: Charge losses vs. humidityandbiashistory

Conclusions

Outlook

Charge collection close to the Si-Si02 interface of silicon strip sensors


Motivation why surface studies

Motivation – whysurfacestudies?

Surfaceeffects:

Relevant forsensorstability (breakdown, stabilityofdarkcurrent, etc.)

Charge carrierlosses

Humidityfoundtoinfluencetheelectricfield in sensor

Electricfieldattheinterface ?

(surfacecharges, surface potential, oxidecharges, etc. => boundaryconditions ?)

humidity

H20, H+ OH-, dirt

passivation

aluminium

aluminium

n type Si

p+ implant

p+ implant

Si02

Si02

Si02

Charge collection close to the Si-Si02 interface of silicon strip sensors

Seite 3


Sensors and irradiation

Sensors andirradiation

Irradiation:

Non-irradiated

Irradiated(1 MGyx-rays, 12 keV)

surface damage only

fixedoxidecharge: Nox = ~ 2 1012cm-2

surfacecurrent: Isurf= ~ 6 µA cm-2

Atmosphereduringmeasurement:

Humid (> 50% humidity)

Dry (nitrogen, < 5% humidity)

T = ~24 °C (roomtemperature)

passivation

al

al

n type

p+

p+

* + 2 µm Al overhang

Si02

Si02

Si02

Charge collection close to the Si-Si02 interface of silicon strip sensors


Measurement procedure red laser tct

Measurement procedure (redlaser TCT)

Redlaser light (front illumination,  = 660 nm, penetrationdepth ~ 3 µm)

Sub ns-pulses (FWHM 100 ps, 1 kHz, 30 000 to 500 000 eh-pairs)

Focus:  = 3 µm (+ tails)

Readout: 2 strips + 1 rearcontact

MiteqAM-1309 currentamplifiers

Tektronixoscilloscope, 2.5 GHz bandwidth

Neighbourstrips on ground (via 50 W)

Charge Q calculatedoffline:

Q = ∫I(t) dt

Currentsignal I(t)

laser

al

al

n type

p+

p+

Si02

Si02

Si02

Charge collection close to the Si-Si02 interface of silicon strip sensors


Accumulation layer and electric field simulation

Accumulationlayerandelectricfield (simulation)

Electronaccumulationlayer

1 MGyirradiation (surfacedamage)

Nox = 2 1012 cm-2 , Isurf = 6.4 µA cm-2

Electronaccumulationlayerpresent

electrons

leaving

2 1012/cm2

200 V

  • Electronlosses !

b.c.:  = 0

  • Influencestheweighting potential w, j

  • Calculatew, junderbias:

readoutstrip j: 0 V

otherstrips: 0 V

rearside: 200 V

read out strip j: 1 V

otherstrips: 0 V

rearside: 200 V

w,j=

also see Hamel, Julien NIMA 597(2008), 207

Charge collection close to the Si-Si02 interface of silicon strip sensors


Weighting potential simulation

Weighting potential (simulation)

readout j

Electronaccumulation

1 MGyirradiation (surfacedamage)

Nox = 2 1012 cm-2 , Isurf = 6.4 µA cm-2

Electronaccumulationlayerpresent

electrons

leaving

2 1012/cm2

200 V

2 1012/cm2

200 V

( µm )

  • Influencestheweighting potential w, j

  • Calculatew, junderbias:

readoutstrip j: 0 V

otherstrips: 0 V

rearside: 200 V

read out strip j: 1 V

otherstrips: 0 V

rearside: 200 V

w,j=

also see Hamel, Julien NIMA 597(2008), 207

Charge collection close to the Si-Si02 interface of silicon strip sensors


Weighting potential and induced current

Weighting potential andinducedcurrent

Charge carriers (q)

driftin theelectricfield: vdr = µ E

Inducedcurrent: Ij = q Ew,j· vdr ,

Collectedcharge : Qj = ∫Ijdt

12 µm

∫ I dt ~ 70 000 e

nolosses

Ew, j = w, j

readout j

38µm

laser

12µm

laser

Weighting potential

38 µm

h

h

nolosses

e

e

∫ I dt ~ 0

Charge collection close to the Si-Si02 interface of silicon strip sensors


Weighting potential and induced current1

Weighting potential andinducedcurrent

Charge carriers (q)

driftin theelectricfield: vdr = µ E

Inducedcurrent: Ij = q Ew,j· vdr ,

Collectedcharge : Qj = ∫Ijdt

12 µm

electronlosses (~97 %)

nolosses

Ew, j = w, j

readout j

38µm

laser

12µm

laser

Weighting potential

38 µm

h

h

e

e

nolosses

electronlosses (~97 %)

Charge collection close to the Si-Si02 interface of silicon strip sensors


Weighting potential and induced current2

Weighting potential andinducedcurrent

12 µm

Charge carriers (q)

driftin theelectricfield: vdr = µ E

Inducedcurrent: Ij = q Ew,j· vdr ,

Collectedcharge : Qj = ∫Ijdt

electronlosses (~97 %)

Ew, j = w, j

nolosses

Weighting potential, rear

h

e

38 µm

electronlosses (~97 %)

nolosses

readout j

Charge collection close to the Si-Si02 interface of silicon strip sensors


Collected charge vs laser position

Collectedcharge vs. laserposition

NL

Rear

Assumptions:

w = constataccumulationlayer, linear else

Holes: collectedatcloseststrip

Light profile: gaussianwiths=2 µm

L

R

Model

Data

0 Gy, humid

0 Gy, driedat500 V

laser

1 MGy, driedat 0 V

Strip L

strip R

laserposition [µm]

+ hole diffusion

Fit results

1 MGydriedat 500Vhumid

electr. 1k 35k33k

holes 29k 7k31k

acclayer 38 µm30 µm-

Charge collection close to the Si-Si02 interface of silicon strip sensors


Collected charge vs laser position1

Collectedcharge vs. laserposition

NL

Rear

Assumptions:

w = constataccumulationlayer, linear else

Holes: collectedatcloseststrip

Light profile: gaussianwiths=2 µm

L

R

Data

Model

laser

Strip L

strip R

laserposition [µm]

readout

Fit results

1 MGydriedat 500Vhumid

electr. 1k 35k33k

holes 29k 7k31k

acclayer 38 µm30 µm-

Charge collection close to the Si-Si02 interface of silicon strip sensors


Collected charge vs laser position2

Collectedcharge vs. laserposition

NL

Rear

Assumptions:

w = constataccumulationlayer, linear else

Holes: collectedatcloseststrip

Light profile: gaussianwiths=2 µm

L

R

Data

Model

laser

Strip L

strip R

laserposition [µm]

Readout: rearcontact

Fit results

1 MGydriedat 500Vhumid

electr. 1k 35k33k

holes 29k 7k31k

acclayer 38 µm30 µm-

Charge collection close to the Si-Si02 interface of silicon strip sensors


Results on humidity and bias history

Results on humidityandbiashistory

humid: steadystate* reached after < 5 min

dry: steadystate* reached after >> 1 hour (hoursordays)

( time constantsdepend on manyparameters )

same steadystatefor all humiditiesandbiashistories!

* steadystate in respecttochargelossbehavior

Charge collection close to the Si-Si02 interface of silicon strip sensors


Results on humidity and bias history1

Results on humidityandbiashistory

humid: steadystate* reached after < 5 min

dry: steadystate* reached after >> 1 hour (hoursordays)

( time constantsdepend on manyparameters )

same steadystatefor all humiditiesandbiashistories!

Time dependent surface charges ?

Danglingbonds ?

* steadystate in respecttochargelossbehavior

Charge collection close to the Si-Si02 interface of silicon strip sensors


Summary and conclusions

Summary andconclusions

Charge collection close to the Si-Si02 interface was investigated in TCT setup

and described succesfully by model.

Significantlossesofelectronsand / orholesobserved.

Charge losses depend on applied voltage, humidity, bias history andirradiation.

Charge collection close to the Si-Si02 interface of silicon strip sensors


Outlook s aturation of electron losses

Outlook: Saturation ofelectronlosses

burstmodeoperation (20 shots)

20 shots, seperatedby 12.5 ns

1 mslater: next 20 shots

electronlossesdissapear*

* forlatershots

  • methodtoestimatethe maximal amountofelectronlosses in thegap

  • andpotentiallydeptrapping time

Charge collection close to the Si-Si02 interface of silicon strip sensors


Charge collection close to the si si0 2 interface of silicon strip sensors

Charge collection close to the Si-Si02 interface of silicon strip sensors


Saturation of electron losses

Saturation ofelectronlosses

burstmodeoperation (20 shots)

20 shots, seperatedby 12.5 ns

1 mslater: next 20 shots

electrons

holes

Charge collection close to the Si-Si02 interface of silicon strip sensors


Collected charge for carrier losses

Collectedchargeforcarrierlosses

Fullchargecollection:

Collection: holesatstrip L, electronsatrearside

QL = #holes·qo = 3 qo

Qrear= - 3 qo

QR,NL,NR = 0

Charge losses(not collectedat end ofintegration time):

Qind,j = ± q · w, j ( final position)

QL < 3

|Qrear| < 3

QR,NL,NR > 0 for hole losses

< 0 forelectronlosses

Laser

electron

hole

Charge collection close to the Si-Si02 interface of silicon strip sensors


Collected charge for carrier losses1

Collectedchargeforcarrierlosses

Fullchargecollection:

Collection: holesatstrip L, electronsatrearside

QL = #holes·qo = 3 qo

Qrear= - 3 qo

QR,NL,NR = 0

Charge losses(not collectedat end ofintegration time):

Qind,j = ± q · w, j ( final position)

QL < 3

|Qrear| < 3

QR,NL,NR > 0 for hole losses

< 0 forelectronlosses

Qind

0

3

0

-3

Laser

electron

hole

Charge collection close to the Si-Si02 interface of silicon strip sensors


Collected charge for carrier losses2

Collectedchargeforcarrierlosses

Fullchargecollection:

Collection: holesatstrip L, electronsatrearside

QL = #holes·qo = 3 qo

Qrear= - 3 qo

QR,NL,NR = 0

Charge losses(not collectedat end ofintegration time):

Qind,j = ± q · w, j ( final position)

QL < 3

|Qrear| < 3

QR,NL,NR> 0 for hole losses

< 0 forelectronlosses

Qind

0.05

2.4

0.3

-2.9

Laser

electron

hole

Charge collection close to the Si-Si02 interface of silicon strip sensors


Collected charge for carrier losses3

Collectedchargeforcarrierlosses

Fullchargecollection:

Collection: holesatstrip L, electronsatrearside

QL = #holes·qo = 3 qo

Qrear= - 3 qo

QR,NL,NR = 0

Charge losses(not collectedat end ofintegration time):

Qind,j = ± q · w, j ( final position)

QL < 3

|Qrear| < 3

QR,NL,NR > 0 for hole losses

< 0 forelectronlosses

Qind *

-0.05

2.6

-0.3

-2.1

Laser

electron

hole

Charge collection close to the Si-Si02 interface of silicon strip sensors


Boundary conditions

Boundaryconditions

~ humid ?

boundaryconditions:

constant potential: f = 0 V (Dirichlet)

zeroelectricfieldcomponent: Ey = 0 (Neumann)

~ ifdriedat 0 V ?

FelDirichletb.c.

Fel Neumann b.c.

+

-

-

+

Charge collection close to the Si-Si02 interface of silicon strip sensors


Charge collection close to the si si0 2 interface of silicon strip sensors

Charge collection close to the Si-Si02 interface of silicon strip sensors


Time dependence after 1 mgy

Time dependence after 1 MGy

Charge collection close to the Si-Si02 interface of silicon strip sensors


200 v 1 mgy dried at 0 v

200 V, 1 MGy, driedat 0 V

Charge collection close to the Si-Si02 interface of silicon strip sensors


Messablauf f r elektronenverluste

Messablauf für Elektronenverluste

n-Typ

Messablauf:

Sensor getrocknet bei 0 V

→ 200 V

Was passiert im Detektor?

0 V : Oxidladungen kompensiert durch freie Ladungsträger

200 V : Oxidladungen unzureichend kompensiert

freie

Ladungsträger

(Elektronen)

+

+

+

-

-

-

-

freie

Ladungsträger

(Elektronen)

Charge collection close to the Si-Si02 interface of silicon strip sensors


Messablauf f r elektronenverluste1

Messablauf für Elektronenverluste

Messablauf:

Sensor getrocknet bei 0 V

→ 200 V

Was passiert im Detektor?

0 V : Oxidladungen kompensiert durch freie Ladungsträger

200 V : Oxidladungen unzureichend kompensiert

Charge collection close to the Si-Si02 interface of silicon strip sensors


Bersicht der ladungsverluste

Übersicht der Ladungsverluste

nonirradiated

after 1 MGyphotons

driedat 500 V

Hole losses

driedat 500 V

humid, steadystate

6 h

humid

Electronlosses

Electronlosses

driedat 0 V

dried0 V

Charge collection close to the Si-Si02 interface of silicon strip sensors


Measured signal compared to calculation

Measuredsignalcomparedtocalculation

Free parameters:

numberofelectrons

numberofholes

diffusionofholes:sdiff

stripposition

accumulationlayerwidth

Fixed parameters:

light profiles1=3µm

+ tailss2=9µm

fN=0.35, fNN =0.05, frear =0.06

stripwidth = 12 µm

Charge collection close to the Si-Si02 interface of silicon strip sensors


Measured signal compared to calculation1

Measuredsignalcomparedtocalculation

measurementand fit

Free parameters:

numberofelectrons

numberofholes

diffusionofholes:sdiff

stripposition

accumulationlayerwidth

Fixed parameters:

light profiles1=3µm

+ tailss2=9µm

fN=0.35, fNN =0.05, frear =0.06

stripwidth = 12 µm

read out strip

1 MGydriedat 500Vhumid

electr. 1k 35k33k

holes 29k 7k31k

acclayer 38 µm30 µm-

rearside

Charge collection close to the Si-Si02 interface of silicon strip sensors


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