Techniques for Techniques for determination of deep level determination of deep level

trap parameters in irradiated trap parameters in irradiated silicon detectorssilicon detectors

AUTHOR: Irena DolencAUTHOR: Irena Dolenc

ADVISOR: prof. dr. Vladimir CindroADVISOR: prof. dr. Vladimir Cindro

MotivationMotivationSilicon detectors usually used as inermost part of tracking system in Silicon detectors usually used as inermost part of tracking system in

particle physics detectors → can receive severe levels of radiation → particle physics detectors → can receive severe levels of radiation → cause of bulk damage which deteriorates detector performancecause of bulk damage which deteriorates detector performance

Radiation damage effects caused by latticRadiation damage effects caused by latticee deformation: deformation: change in effective dopant concentration which effects full depletion voltagechange in effective dopant concentration which effects full depletion voltage increase of leakage currentincrease of leakage currentdeterioration of charge collection efficiency: part of the drifting charge, deterioration of charge collection efficiency: part of the drifting charge,

created by ionizing particle, is temporarly trapped by the defects introduced created by ionizing particle, is temporarly trapped by the defects introduced with irradiationwith irradiation

Damage mechanism: Damage mechanism: irradiation particles knock off irradiation particles knock off SiSi atoms atomsdislocated dislocated SiSi atoms (interstitials), empty lattice sites (vacancies), impurities atoms (interstitials), empty lattice sites (vacancies), impurities

(eg. (eg. OO, , CC) form defect complexes → introduction of energy levels (traps) in ) form defect complexes → introduction of energy levels (traps) in the band gapthe band gap

For the developement of more radiation hard For the developement of more radiation hard SiSi detectors the detectors the knowledge of defect kinetics and of correlation between knowledge of defect kinetics and of correlation between microscopic defects and macroscopic properties of the detector microscopic defects and macroscopic properties of the detector is neededis needed

The main tools for characterization of deep level defects: The main tools for characterization of deep level defects: DDeep eep LLevel evel TTransient ransient SSpectroscopypectroscopy (DLTS) (DLTS) TThermally hermally SStimulated timulated CCurrenturrent (TSC) technique (TSC) technique

Operation principles of silicon Operation principles of silicon detectorsdetectors

V

N

w

SVC

SwNeQVC eff

eff

dVdQ

0

0)()(CapacitanceCapacitance

Si detector: a diode operated under reverse bias where the Si detector: a diode operated under reverse bias where the depleted region acts like ionization chamber depleted region acts like ionization chamber

Desired detector operation voltage Desired detector operation voltage V V > full depletion > full depletion voltage voltage VVFDFD

nnDppA xNxN ,,

nD

binnp Ne

VVxxxw

,0

0 )(

Usually Usually pp++-n-n junction junction → →

Neutrality of the Neutrality of the system →system →

nDpA NN ,,

If If NND,nD,n comparable with comparable with NNA,nA,n than than NND,nD,n must be replaced with effective must be replaced with effective dopant concentration dopant concentration NNeff eff =│ N=│ ND,nD,n - N - NA,nA,n││effNVVw )(

Shockley-Read-Hall Shockley-Read-Hall statisticsstatistics

Occupation of a defect states wth concentration Occupation of a defect states wth concentration NNt t , energy , energy

level level EEtt and average occupation probability and average occupation probability PPtt

density of occupied defectsdensity of occupied defectsdensity of non occupied defectsdensity of non occupied defects

ttt PNn ttt nNp

rat generationexcesselectron

rate generationexcess hole

)()( nnppt RGRG

dt

dn

Change of a defect occupancy Change of a defect occupancy possible by possible by electron captureelectron capture with rate with rate electron emissionelectron emission with rate with rate hole capturehole capture with rate with rate hole emissionhole emission with rate with rate

Defect parameters (besides Defect parameters (besides EEtt ) that are ) that are determened by DLTS or TSC method:determened by DLTS or TSC method: capture coefficients capture coefficients →→ emission probabilities emission probabilities → → een,pn,p

tnn npcR tnn neG

tpp peG tpp pncR

pnpnpn vc ,,,

The rate of change of the defect The rate of change of the defect ocupancyocupancy

Shockley-Read-Hall statisticsShockley-Read-Hall statisticsThermal equilibriumThermal equilibrium PPtt = Fermi function= Fermi function steady state →steady state → dn dnt t /dt /dt =0=0 no currentno current → no net flow of electrones or → no net flow of electrones or

holes between conduction and valence band holes between conduction and valence band

→ → RRpp=G=Gpp, R, Rnn=G=Gnn

Relations between Relations between een,pn,p and and ccn,pn,p remain valid non-equilibrium conditions remain valid non-equilibrium conditions → defect is fully→ defect is fully

described by described by EEtt and and n,pn,p

TkEE

pip

TkEE

nin

B

it

B

it

cne

cne

exp

exp

Space charge region in steady Space charge region in steady statestate carrier concentration carrier concentration negligiblenegligible: :

p,n ~p,n ~ 0 0 → → capture processes can be capture processes can be neglectedneglected

steady state →steady state → dn dnt t /dt /dt =0=0

extraction of extraction of PPtt →→ E Eii acts acts

like like EEFF in thermal in thermal equilibiriumequilibirium

electron electron trapstraps

hole hole trapstraps

generation generation centerscenters(leakage (leakage current)current)

DLTS: principle of operationDLTS: principle of operationDeep level transient spectroscopyDeep level transient spectroscopy (DLTS): uses capacitance (DLTS): uses capacitance transient signals resulting from relaxation processes following transient signals resulting from relaxation processes following an abrupt change of bias voltage or light applian abrupt change of bias voltage or light applieedd to the sample to the sample being being investigatedinvestigated..1.1. Electron trapElectron trap → located in the upper half of the → located in the upper half of the

band gap:band gap:

effNw

C 1

During the measurement the device must be partially During the measurement the device must be partially depleted!depleted!

DLTS: principle of operationDLTS: principle of operation2.2. Hole Hole traptrap → located in the → located in the lowerlower half of the band half of the band

gap:gap:

DLTS: DLTS: determination of trap determination of trap parametersparametersElectron Electron traptrap of acceptor type: of acceptor type:

trapsoccupied

donorsshalow

)()( tnNtN tDeff

Effective dopant concentration after the Effective dopant concentration after the filling pulse filling pulse

ne

ett

e

tntn

/1

)0()(

time emission

exp

pn

pn

nnppt

eeRR

RGRGdt

dn

:trap electron :SCR in 0~,

)()(

Density of occupied traps after the Density of occupied traps after the filling pulsefilling pulse

D

tR

NN

D

tR N

tnC

N

tnCtC Dt

2

)(1

)(1)( usually

Capacitance after the Capacitance after the filling pulsefilling pulse

1.1. Capacitance after the filling pulseCapacitance after the filling pulse

DLTS: DLTS: determination of trap determination of trap parametersparameters2.2. DLTS spectrumDLTS spectrum ee tt

tntCtCC //21

21)0()()( ee

Low Low TT: emission : emission process to slow to process to slow to

be observedbe observed

High High TT: emission : emission process to fast to be process to fast to be

observedobserved

Peak observed at Peak observed at TTmaxmax where emission time where emission time satisfiessatisfies

)/ln()(

21

21max

tt

ttTe

Several temperature scans with Several temperature scans with

different different ttW W = = tt22--tt11 →→ Arrhenius plot Arrhenius plot::

3.3. Extraction of Extraction of parametersparameters

maxmax2

max /1 versus)(ln TTT e

)ln()(ln 2n

B

tCe A

Tk

EETT

Connection between Connection between eenn

and and nn

Nt extracted form DLTS peak since nt(0) Nt

Et extracted from slope of Arrhenius plot

n from the intercept of Arrhenius plot with ordinate

DLTS: DLTS: example of a DLTS example of a DLTS spectrumspectrum

TSC: principles of operationTSC: principles of operation♦ DLTSDLTS: : observing the change in depth of SCR, due to emission of observing the change in depth of SCR, due to emission of

trapped charge, by trapped charge, by measuring capacitance transientmeasuring capacitance transient♦ Thermally Stimulated CurrentThermally Stimulated Current technique (TSC): observing release of technique (TSC): observing release of

trapped charge directly, by trapped charge directly, by measuring the current due to emission of measuring the current due to emission of trapped carrierstrapped carriers

Measurement process:Measurement process:

1.1. Cooling: Cooling: Sample is coold to a low Sample is coold to a low T. T. Cooling under Cooling under reverse bias → traps are not filled with carriersreverse bias → traps are not filled with carriers

2.2. Filling:Filling: − Switching to zero bias Switching to zero bias → filling with → filling with

electronselectrons− Switching to forward bias → electrone and Switching to forward bias → electrone and

hole injectionhole injection− Illumination with short-Illumination with short-λλ laser of laser of nn-side -side

(filling only with electrons) or (filling only with electrons) or pp++-side (filling -side (filling only with holes)only with holes)

3.3. Recording:Recording: Heating under reverse bias with constant Heating under reverse bias with constant heating rate. At some heating rate. At some TT emission probability is emission probability is no longer negligible → trapped charge is rapidly no longer negligible → trapped charge is rapidly emitted and swept out of SCR → peaks in emitted and swept out of SCR → peaks in current signalcurrent signal

TSC: example of TSC spectrumTSC: example of TSC spectrum

TSC: determination of trap TSC: determination of trap parametersparameters

tntt tdtentn

0)(exp)0()(

pn

pn

nnppt

eeRR

RGRGdt

dn

:trap electron :SCR in 0~,

)()(

Density of occupied traps during the Density of occupied traps during the heatingheating

)(

0 defects

0 2

)()()()(

twtptn

TSC dxtpetnte

SetI

Current due to emitted trapCurrent due to emitted trapped chargeped charge during the during the heatingheating

− eenn>>>>eepp

− During whole TSC scan device fully depleted During whole TSC scan device fully depleted → → ww((tt)=detector thickness )=detector thickness D D

)()(2

)( 0 tnteSDe

tI tnTSC

Electron trapElectron trap

Determination of Determination of NNtt

−From the area under From the area under TSC peak TSC peak

−From the peak heightFrom the peak height

)0()( 021/2

1t

dtdTT

T TSC DnedTTIQ )0(max, tTSC nI

TSC: determination of trap TSC: determination of trap parametersparameters1.1.Variable heating Variable heating

methodmethodPosition of the TSC peak at Position of the TSC peak at TTmaxmax which which

satisfiessatisfies

nBB Ak

ETk

ET

lnln

max

4max 1

→ ΔΔE=E=EEC C ––EEtt → extracted from → extracted from the slope of the plotthe slope of the plot

→ σσnn → extracted from the → extracted from the intercept with the ordinateintercept with the ordinate

By repeating TSC temperature scans By repeating TSC temperature scans with with

different heating rate different heating rate ββ, a plot , a plot is obtainedis obtained

max

4max 1 versusln T

T

2.2.Delayed heating Delayed heating methodmethodSeveral TSC scans Several TSC scans performedperformed from starting from starting point point TT00 with different delay times with different delay times ττd d between between end of the filling pulse and start of the heating end of the filling pulse and start of the heating → → plotplot ln(ln(IITSC,mTSC,maxax)) versus versus ττd d

IITSCTSC((tt) ) nntt(0)(0) exp(- exp(- eenn((TT00) ) ττd d )) →→ TSC peak TSC peak decreases with increasing decreases with increasing ττdd

from the slope of a from the slope of a plot plot emission emission

probability probability eenn((TT00) is ) is obtainedobtained

Repeating procedure at different Repeating procedure at different TT00 → →

Arrhenius plotArrhenius plotConnection between Connection between eenn and and nn::

002

0 /1 versus)(ln TTT e )ln()(ln 2

nB

e ATk

ETT

→ ΔΔE=E=EEC C ––EEtt → → extracted from the extracted from the slope of the plotslope of the plot

→ σσnn → extracted from → extracted from the intercept with the intercept with the ordinatethe ordinate3.3. DeconvolutionDeconvolution method method::

numerical fit to the whole numerical fit to the whole spectrumspectrum

SummarySummary

TSC and DLTS TSC and DLTS →→ techniques for determination of deep level techniques for determination of deep level traps parameters, based on observing reversely biased detector traps parameters, based on observing reversely biased detector response to applied light or an abrupt change of biased voltage response to applied light or an abrupt change of biased voltage (filling of traps with holes and/or electrons)(filling of traps with holes and/or electrons)

Difference between DLTS and TSCDifference between DLTS and TSCDLTS method:DLTS method: Capacitance transient after the filling process is Capacitance transient after the filling process is

measuredmeasured Capacitance transient caused by the change of the width Capacitance transient caused by the change of the width

of SCR due to emission of carriers that were trapped of SCR due to emission of carriers that were trapped during the fillingduring the filling

During the measurement device must be biased with the During the measurement device must be biased with the voltage lower than full depletion voltagevoltage lower than full depletion voltage

TSC method:TSC method: Sample is cooled to a low temperature before filling Sample is cooled to a low temperature before filling Emission of trapped carriers observed directly by Emission of trapped carriers observed directly by

measuring the current while heating the sample measuring the current while heating the sample Device is fully biased during the measurementDevice is fully biased during the measurement