Carrier recombination and emission lifetimes in heavily irradiated pad–detectors and their impact on operational characteristics of pin

diodes

E.Gaubas1, T.Čeponis1, R.Grigonis2, A.Uleckas1, and J.Vaitkus1

1Vilnius University, Institute of Applied Research, Vilnius, Lithuania (VU)2Vilnius University, Laser Research Center, Vilnius, Lithuania (VU)

Outline

Carrier capture (MW-PCD/E) and emission (I-V) lifetime variations

Barrier capacitance variations with fluence (BELIV)

Time and spectral resolved priming of BELIV transients

Summary

Recombination lifetime during and after irradiation

During protons irradiation/ protons pre-irradiated (MW-PCD) - wafer samples

After irradiation - wafer and diode samplesirradiated by neutrons and protons

100

101

102

10-2

10-1

100

experiment

pre-irradiated >1012

cm-2

6 MeV protons fixed 1.5 nA

simulated

Ncl,0

=1012

cm-2

R (s

)

Exposure time (s)

1013

1014

1015

1016

100

101

102

103non-irradiated

Fluence (cm-2)

Neutron irradiated FZ diodes n-epi diodes p-epi diodes

(ns

)

10-2 10-1 100 101 10210-2

10-1

100

101

102

=1

=1/3

=2

Exposure time under irradiation by protons (s)

R

(s

)

MCZ Si wafer8 MeV protons at 285K :

fixed 4nA varying current by steps

10-1 100 101 10210-2

10-1

100

101

102

103

experiment varying current

0.2 -4 by steps fixed 4 nA

simulated

R,cluster,

c=0.33, n-fixed, 0=10-18 cm2

R (s

)

Exposure time (eqv. 4 nA) (s)

- diode samples with applied field

n<n0

n<ni<<n0

10 1000

2

4

6

ICDC MW-PCT-E R

ICD

C, M

W-P

CT

-E (s

)

texposure

(s)

recombination prevails

recombination competes with multi-trapping effect

1012 1013 1014 1015 101610-1

100

101

102

103

104

Okmetic MCZ<100> 1kOhm·cm 300 m MWR, CIS 8556- 14 wafers 1 - 63 passivated CIS 8556- 14 wafers measured by DG technique CIS 8556-01 diodes neutron irradiated

R (

ns)

Fluence (n/cm2)

M=0/en0

Impact of recombination within ENRwith R M might be crucial (if n0~1/) on functionality of junction

Carrier capture-recombination-generation lifetimes (simple S-R-H approach)

ni/ND

20 10 0 10 201

10

10015

1

taug x( )

taur x 0.01( )

taur x 0.001( )

taur x 0.0001( )

2020 x(EDL-Ei)/kT

Gen

erat

ion

lif

etim

e

Rec

om

bin

atio

n li

feti

me

re

c, g

en

]cosh(2

1[1

kT

EE

n

n

NvR

n iDLei

DLrec

DL

iDL

igen NvkTEE

Gn

)cosh(2

/

at nvn pvp

Single-type prevailing traps

of M=NDL<<n0

n-Si

Photo-injection URT

ne pe > ni2

ne · pe < ni2ne · pe - ni

2 =0

n-Si

Photo-injection URT

ne pe > ni2

ne · pe < ni2ne · pe - ni

2 =0

Without applied E field

With applied dc E field

Eqv.R>0

G<0

S-R-H is limited by conditions:i) M<<n0. then traps are filled by n0 without change in n0;

ii) single type centers dominateiii) traps do not interactiv) charge on traps can be ignored relatively to dopants one

Thus, validity of S-R-H conditions should be estimated in applications

J.S. Blakemore, in: Semiconductor Statistics, Ch. 8, Pergamon Press, (1962)

A.Rose. Concepts in photoconductivity and allied problems. Interscience Publishers, John Wiley & Sons, New York-London, 1963.

Traps with barriers, multitrappingmulti-charge/multi-valency defects

Traps with exp distributed levels Redistribution of carriers via interaction of traps Photo-, thermo- quenching effects

EC

EVx

n0

M

Carrier recombination lifetimes (for M>>n0) Single-species (type) traps

1

0

1

000

1

00000

11

1)(

n

N

N

nMnp

N

nMNnPP

cM

cM

cMcMpvMn

p

1

0

1

000

1

00000

11

1)(

p

P

P

pMnp

P

pMPpNn

vM

vM

vMvMncMp

n

M0, n 0, S-R-H

11100

0

p

PM

M

n

NM

e

Mm

vMcMkT

FM

00

00

00

00 pn

Pp

pn

Nn vMn

cMpnp

M, M>>n0, S-R-H invalid

000

01

1 pp

cMpp mn

N

MmMp

P

nn

n

vMnn

1

)(

11 0

000

MMv nrec

11

C

MC

C

MC

gen NkT

EE

NvkT

EE

)exp()exp(

Although relaxation to equilibrium/steady-stateis kept by M=pM+nM

n p

J.S. Blakemore, in: Semiconductor Statistics, Ch. 8, Pergamon Press, (1962)

cE

cefhf dEEgEPEEPNr

c

c

E

ce

E

ce

n

dEEgEP

dEEgEPE

Capture coefficient <n> should be used

instead of v within rigorous analysis

fhfn EPNnr EC

EVx

Only shallow dopants of proper densityare able to support fast

operation (w(U)) of a diode ~1/M <1/em

to ensure maj. carriers on level are in equilibrium with band

Carrier recombination=capture lifetimes (for M>>n0)

Single-species (type) traps

100 150 200 250 3000

100

200

300

400

DLS

(m

V)

T (K)

Electron irradiation

Cz Si (doped 1014 cm-3)

1015 e-/cm2

1016 e-/cm2

Only deep slow levelscan be filled

and observed as carrier emitters

0.0 0.1 0.2 0.3 0.4 0.5 0.610-13

10-11

10-9

10-7

10-5

10-3

10-1

R

SC

cp

M

em, cp

, S

C, R

, M

(s)

E (eV)

ND=1012 (cm-3)

em

cp

, NT=8x1011 cm-3

cp

, NT=1014 cm-3

SC

R, N

T=8x1011 cm-3

M

em

NT=8x1011 cm-3

NT=1014 cm-3

dX Xinvalid

Space chargegeneration current

Interaction of several type centers appearsdue to carrier redistribution through bands

Carrier recombination lifetimes for Mi,s>n0 multi-valency(i)/multi-species(s) centers

Interaction of the whole system of centers appears due to carrier redistribution through bands, by inter-center recombination (capture-emission) and via charging /configurational transforms of defects

System neutrality is supported by free and localized charges/fields.Relaxation is long and complicated. It is similar to the random-walk processes in disordered materials.

0 2000 4000 6000 8000

10-1

100

2

1

1 CPC excited with 355 nm light pulse of 30 ps2 MWA excited with 355 nm light pulse of 30 ps

t (s)

Nor

mal

ized

pho

tore

spon

se U

CP

C, M

WA

/Um

ax

0 2 4 6100

101

experimental decayin stretched-exponent time scale

UC

PC

(ar

b.un

its)

t=0.7 (ms)

Relaxation is similar to multi-exponential in any narrow display segmentDifferent techniques may give different lifetime values

A single lifetime parameter se can be extracted only when stretched-exponent time scale is employed

The stretched-exponent model is widely used UCPC =U0exp[–(t/se)]

S.Havlin and D.Ben-Avraham, Advances in physics 51, 187 (2002).L.Pavesi, J. Appl. Phys. 80, 216 (1996).

E. Gaubas, S. Juršenas, S. Miasojedovas, J. Vaitkus, and A. ŽukauskasJOURNAL OF APPLIED PHYSICS VOLUME 96, NUMBER 8 15 OCTOBER 2004

Carrier generation/emission lifetime (for M>>n0)

1013

1014

1015

1016

10-6

10-5

10-4

10-3

IL|UR=100V

IL|UR=200V

1/IL|UR=100V

I L (A

)

(n/cm2)

103

104

105

106

g~1/

I L (A

-1)

-200 -150 -100 -50 0 5010-13

10-11

10-9

10-7

10-5

10-3

I

(A)

U (V)

=1013 cm-2

=1014 cm-2

=1016 cm-2

b

Qualitative emission lifetime dependence on fluencecan be estimated from I-V

1012 1013 1014 1015 101610-1

100

101

102

103

104

Okmetic MCZ<100> 1kOhm·cm 300 m MWR, CIS 8556- 14 wafers 1 - 63 passivated CIS 8556- 14 wafers measured by DG technique CIS 8556-01 diodes neutron irradiated

R (

ns)

Fluence (n/cm2)

Nearly linear reduction of generation lifetimewith enhancement of fluence is similar to that of

of recombination lifetime characteristic

Examined MW-PCT characteristics imply prevailing of intricate system of defects and reduction of majority carriers. The recombination capture lifetimes become shorter than dielectric relaxation time.

Carrier emission lifetime decrease (increase of leakage current – at UR in I-V), follows capture lifetime reduction (increase of serial resistance R~1/n0 - at UF in I-V), and both manifest a close to a linear decrease with enhancement of fluence

Items to clarify: Whether diode/detector is functional under heavy irradiations? What is a system of defects and levels, which governs extraction of carriers (UR) and state of material? Which models are acceptable for prediction of charactreristics?

The Barrier Evaluation by Linearly Increasing Voltage (BELIV) transient technique has been employed to clarify, how a reduction of carrier capture lifetime and emission affects junction and material

GLIVpin

diode

RL=50 oscilloscope

GLIVGLIVpin

diode

RL=50 oscilloscope

2/30

)1(

21

)()(

bi

bib

bbC

U

At

U

At

ACU

CUC

t

U

dt

dqti

BELIV technique

Abrupt junction

Reverse bias

3/4,0,

],

1[

],3

21[

LgUbi

AtLgUbi

At

ACi LgbLgC

Linearly grade junction

2/10

2/30 )1()1(

)1(

21

)()()()(big

iTBk

eAt

diff

bi

bibgdiffCR U

AtSwenei

U

At

U

At

ACtitititi

])0()0(2

3)

4

)0(()0(

4

)0([

)0(2

gcc

gC

g

bie ii

ii

i

Ai

Ut

te

dxxt

xitit

RCC

RCCM

0]

)(exp[)(

1)(

dt

tdUCtUn

d

Se

dt

dUC

ndeS

dt

dU

d

S

dt

dp

dt

dndeSti C

geomCnC

geomtr

CFD

)()(

22)(

2|)(| 0

00

Variations of BELIV transients with temperature and priming by steady-state IRBI

as well as dc UF

Short carrier capture lifetime reduces n0ND and increases a serial resistance of ENR. Supply of majority carriers from rear electrode by dc UF priming (due to shrinkage of depletionw width, forward current) restores a junction.

Combined priming of BELIV transientsby temperature reducing (increased e) and by IR illumination

(n0) leads to restore of a junction

Reduction of temperature increases (e) and decreases space charge generation current, however, Cb0Cg at UC<0.3 V

Priming by IR illumination increases n0 and barrier capacitance observed in BELIV transientsrestores a junction but enhances leakage current

when fast carrier capture/emission is present

Barrier capacitance as a function of fluence extracted at 300 K

1012 1013 1014 1015 1016

101

102

103

Cgeom

C

b0 (

pF)

Fluence (n/cm2)

Cb0

: BELIV, PL

=1.4 sNeutron irradiated diodes

reverse bias forward bias

50 100 150 200 250101

102

Neutron irradiated diodesf=100 kHz, U

ac=1V

Cp, =1012 cm-2

Cs, 1012 cm-2

Cp, 1014 cm-2

Cs, 1014 cm-2

Cp, 1016 cm-2

Cs, 1016 cm-2

C (

pF)

UR (V)

0 50 100 150 U (V)10

100

C (pF)

MCz Si diodai, T=300KU

ac=20 mV, f=100 kHz

=1012 cm

-2:

Cs Cp

=1013 cm

-2:

Cs Cp

=1014 cm

-2:

Cs Cp

=1016 cm

-2:

Cs Cp

C-V’s as a function of fluence at 100 kHz and 300 Kdisplacement in barrier capacitance is controlled by (LRC) measurements of phase shift for the ac test signal at fixed frequency in routine C-V

Applicability of LRC measured C-V is doubtful

for diodes irradiated with > 1E13 n/cm2

Barrier partially recovers by n0 priming with IR, dc UF and combined priming with temperature (emission lifetime) decreasing only in diodes irradiated with fluence of <1014 n/cm2. Short carrier capture and emission times determine low barrier capacitance (capability to to collect charge (transient) at fixed voltage) and large space charge generation (leakage) current in heavily irradiated diodes. The space charge generation current prevails in heavily irradiated diodes over barrier charging (displacement, which is controlled by measurements of phase shift for the ac test signal in routine C-V), therefore applicability of C-V technique is doubtful for control of heavily irradiated detectors. Carrier capture and emission lifetimes are short, and barrier capacitance decreases to geometrical its value at low (U< Ubi) applied voltage of the diodes with enhancement of fluence. Operation of a diode is similar to that of capacitor.

Items to clarify: Whether diode/detector of the present design is functional after heavy irradiations?- doubtful What is the system of defects and levels, which governs extraction of carriers (UR) and state of material?- material becomes similar to insulator. Additonal issues: - if there are filled levels those compensate material; - how rapidly these levels are able to response to external voltage changes Which models are acceptable for prediction of characteristics?

The BELIV technique with spectrally resolved fs pulsed IR (1.1 – 10 µm) biasinghas been employed to clarify what is a system of levels and if these levels are filled

EC

EV

EC

EV

Variations of BELIV transients by pulsed IR of varied spectrum

mono-polar photoconductivity

change of charge state- capacitance

OPO DFG fs pulse

To approve principlesstructure with known technological defects

For qualitative understanding(within depletion approximation):

Cb~(ND=n0)1/2~1/w(n0)

Cb(t) ~1/M=(en0|tr)/0

(using extended depletion approximation)

OPO DFG wavelength/quantum for which appears suppression of traps, i.e. recover of BELIV transient of Cb,indicates a system of filled single type deep levels

0 20 40 60

0.0

0.1

0.2

UB

ELI

V (

a.u.

)

t (s)

Si junctionpulsed bias illumination OPO-DFG

pulse= 100 fs, rep rate 1kHz

ex

= 4 m, Epulse = 30 J

IR limit h= 0.31eV

Carrier capture rate

)]()([22

22,

0

0

ENREEe

e

Une CC

drM

for transient processes

Variations of BELIV transients by pulsed IR of varied spectrumin neutron irradiated detectors

The shallowestE1 h 0.3 eV

E3 h 0.41eV

E4 h 0.51eV

E2 h 0.36 eV

Variations of BELIV transients by pulsed IR of varied spectrumin neutron irradiated detectors

0 5 10 15 20

0.00

0.03

0.06

0.09

0.12

MCz Si diode irradiated 1014 n/cm2

ULIV

= 12V

ex

= 1.15m 1.3 1.5 2.4 2.5 2.6

BE

LIV

sig

nal (

a.u.

)

t (s)

The most shallow filled Ered threshold =h 0.5 eV

Edeepest -h 1.08eV

0 3 6 9

0.00

0.03

0.06

0.09

0.12

MCz Si diode irradiated 1016 n/cm2

ULIV

= 12V

ex

= 1.2m 1.5 2.4 2.6 3.0

BE

LIV

sig

nal (

a.u.

)t (s)

The most shallow filled, or a half of two step generation

Ered threhold =h 0.52 eV

Edeepest -h 1.08eV

For 1E14 n/cm2 and h> 0.83 eV possiblepartial recovering of a barrier, while for h≤ 0.5 eV space charge generation current prevails (rapidcapture/emission processes

For 1E16 n/cm2 only space charge generation current increases (extremely rapidcapture/emission processes) while barrier capacitanceis close to Cgeom

A clear structure of deep levels is absent in heavily irradiated diodes >1014 n/cm2 while carriers are generated by inter-band excitation.

Items to clarify: Whether diode/detector of the present design is functional after heavy irradiations?- doubtful What is the system of defects and levels, which governs extraction of carriers (UR) and state of material?- material becomes similar to insulatorAdditonal issues: - if there are filled levels those compensate material; - no, high density of various species levels is more probable those are only weakly filled by small n0

- how rapidly these levels are able to response to external voltage changes - fast capture of excess carrier and fast space charge generation current response

But system relaxes to equilibrium state very slowly – as estimated from I-V point-by-point measurements at T<150 K

Which models are acceptable for prediction of characteristics?

The disordered material models-?

Summary

Examined MW-PCT characteristics imply prevailing of intricate system of defects and reduction of majority carriers. The recombination capture lifetimes become shorter than dielectric relaxation time. Carrier emission lifetime (increase of leakage current – at UR in I-V), follows capture lifetime (increase of serial resistance R~n0 - at UF in I-V)

Barrier capacitance decreases to geometrical value of the diodes with enhancement of fluence. Carrier capture and emission lifetimes are short. The pointed system of deep levels can be revealed only in diodes irradiated with fluence of <1014 n/cm2.

A clear structure of deep levels is absent in heavily irradiated diodes >1014 n/cm2 while carriers can be generated by inter-band excitation.

Thanks to G.Kramberger for neutron irradiations.E.Tuominen, J.Harkonen and J.Raisanen are appreciated for samples (substrates and pin diodes) as well as for proton irradiations.

Thank You for attention!

)(1

02

2

xdx

d

xy

yy

dzzzdzzyx

)()(

1)()(

0

)()( xnNex d

kT

xenN

e

dx

xdd

)(exp

)(0

02

2

kT

xeN

e

kT

e

kTxN

exE dd

)(exp)(

2)(

0

2

)(

0

1 )(xx

x

dEdxd

ekTx /)( assumption

)(2

)(0

2 xeN

xE d

2

0

)(2

)( xxeN

x dd

2

02 dd x

eNV

)()(

0

xxeN

dx

dxE d

d

Depletion aproximation for material

containing only dopants:

only numerical integration

n

p

d

m

x

x

x

x

dzzdzz 0)()(

n

p

x

x

dxxxV )(1

)(0

2

2

1exp1)(

D

dd L

xxeNx

2

2

1exp

D

dd L

xxN

Limitations:for steady-state

2

0

2

)(2

exp)( xxkT

NeNxn d

dd

e

kT

L

xV

D

d

2

2

12

1

20

d

D Ne

kTL

Utrtr

M 22

2

tr

for transients

gives depletion approximation

P.Blood and J.W.Orton. The electrical characterization of semiconductors: majority carriers and electron states, (Academic Press, London –San Diego-New York, 1992).

Transient currents in depletion region

in deep traps containing material

x

x

c dt

dnxxedx

dt

dnexJ

2

))(()()( 2

dt

dpxWe

dt

dnxexJ c )()()(

)()()(),(00

tNe

tnNNxWe

txE ctt

dt

dNe

dt

dnxWe

t

D ct

x

)(

WtNW

tnNNe

V ctt )(2

))((2

0

dt

dnW

dt

dN tc

2

dt

dnWxe

t

D t

2

dt

dp

dt

dneW

dt

dn

dt

dpWx

dt

dpxW

dt

dnxetJ

22)()(

trTest harmonic signalUac<kT/e, to evaluate Cb,, by control of phase shift avoiding xd>tr , i.e.desirable regime EC(x)-EC(xd)<kT.

P.Blood and J.W.Orton. The electrical characterization of semiconductors: majority carriers and electron states, (Academic Press, London –San Diego-New York, 1992).