High-energy electron irradiation of different silicon materials

S. Dittongo1,2, L. Bosisio1,2, M. Ciacchi1, D. Contarato3, G. D’Auria4, E. Fretwurst5,

G. Lindström5

High-Energy Electron Irradiation of Different Silicon Materials

1 Università di Trieste, Dipartimento di Fisica, Italy2 INFN, Sezione di Trieste, Italy3 DESY, Hamburg, Germany4Sincrotrone Trieste S.C.p.A., Trieste, Italy5 Institut für Experimentalphysik, Universität Hamburg, Germany

Selenia Dittongo 2nd SIRAD workshop – Legnaro, April 1-2, 2004 2

Overview• Introduction: why high-energy electrons?• Irradiation:

– irradiated devices – experimental conditions

• Experimental results:– Effective dopant concentration– Reverse leakage current, damage constant α– Isothermal annealing effects– Charge collection efficiency

• Conclusions


Why high-energy electrons?• In last years, much effort devoted to study

– the radiation hardness of silicon detectors against different particle types (charged hadrons, neutrons and γ rays)

– the improvements achievable by using different silicon substrates

• By contrast, very few contributions devoted to damage induced by high-energy (GeV) electrons

• In a previous irradiation of high-resistivity standard and oxygenated FZ devices with 900 MeV electrons, up to φ~5x1014

e/cm2, bulk type inversion has been observed. No significant effect of oxygen diffusion up to this fluence

• New experiment to reach higher fluences and to extend these investigations on a wider range of substrate materials (standard and oxygenated float-zone, Czochralski and epitaxial silicon)


Tested devicesp+/n-/n+ diodes fabricated on different silicon substrates (thickness ~300µm), provided with a 100 µm wide guard ring, surronded by floating rings

FZ and DOFZ devices by ITC-irst (Trento, Italy)• fabricated on Topsil (111) and (100) substrates, resistivity~10-20 kΩ⋅cm• DOFZ: 12 hour oxidation @ 1150oC + 36 hour diffusion in N2@ 1150oC,

[O]~1-3x1017 cm-3

FZ and DOFZ devices by CiS (Erfurt, Germany)• fabricated on Wacker (111) substrates, resistivity~3-4 kΩ⋅cm• DOFZ: oxygen diffusion performed in N2 environment for 72 hours @1150oC,

[O] ~ 1.2x1017 cm-3

CZ devices by CiS• fabricated on Sumitomo (100) substrates, resistivity~1.2 kΩ⋅cm

EPI devices by CiS• processed on a 50 µm thick epitaxial layer (resistivity~50 Ω⋅cm) grown by ITME

(Warszawa, Poland) on a 300 µm thick low resistivity (0.01 Ω⋅cm) Czochralski (111) substrate


Experimental conditionsIrradiations

• irradiations performed with the 900 MeV electron beam of the LINAC injector at Elettra (Trieste, Italy)

• fluence measured by a toroidal coil coaxial with beam

• devices kept unbiased during irradiation, at room temperature (~25oC)

(2.09±0.004±0.08)x10158(1.37±0.01±0.06)x10157(7.82±0.03±0.31)x10146

(4.04±0.01±0.16)x10145(1.25±0.01±0.05)x10144(7.24±0.04±0.29)x10133

(2.31±0.01±0.09)x10132(1.74±0.06±0.07)x10121

Fluence (e/cm2)stepMeasurements• irradiated devices electrically caracterized by

reverse I-V and C-V measurements, performed first ~1 day after irradiation and again after 8minutes @ 80oC

• C-V measurements @ 10 kHz • currents normalized to 20oC

• isothermal annealing cycles up to a few 10000 min @ 80oC on the devices irradiated at thetwo highest fluences


Effective dopant concentration: FZ & DOFZ

post-inversion slope (β values)

– FZ (IRST) ~ 1.0x10-3 cm-1

– FZ (CiS) ~ 1.3x10-3 cm-1

– DOFZ (IRST) ~ 0.6x10-3 cm-1

– DOFZ (CiS) ~ 0.5x10-3 cm-1

• lower β for DOFZ devices• differences between IRST and CiS diodes

probably due to different starting materials and oxygenation procedures

FZ & DOFZ by CIS

-2.5E+12-2.0E+12-1.5E+12-1.0E+12-5.0E+110.0E+005.0E+111.0E+121.5E+12

0.0E+00 5.0E+14 1.0E+15 1.5E+15 2.0E+15Fluence (e/cm2)

Nef

f (1

/cm

3 )

FZ, after irrad

FZ, 8min@80°C

DOFZ, after irrad.

DOFZ, 8min@80°C

FZ & DOFZ by IRST

-2.2E+12

-1.7E+12

-1.2E+12

-7.0E+11

-2.0E+11

3.0E+11

8.0E+11

0.0E+00 5.0E+14 1.0E+15 1.5E+15 2.0E+15

Fluence (e/cm2)

Nef

f (1

/cm

3 )

DOFZ(111)-IRSTFZ(111)-IRSTFZ(100)-IRSTDOFZ(111)-IRST-8min@80°CFZ(111)-IRST-8min@80°CFZ(100)-IRST-8min@80°C

type inversion occurs at:• φ ~ 2.5x1014 e/cm2 for ITC-irst devices• φ ~ 5x1014 e/cm2 for CiS devices

(higher initial doping)


• samples used for various irradiations have non negligibly differing values of Neff => to obtain a meaningful dependence on fluence, an “equalization” procedure has been applied:

Neff_equal=[Neff(after) – Neff(before)] + <Neff>(pre-irrad)

• the initial trend appears to be ~ linear with fluence (slope~-1.0x10-3 cm-1); a simple extrapolation of this trend would lead to type inversion at φ~4.5x1015 e/cm2 (within reach of our setup => try next!)

Effective dopant concentration: CZ

type inversion not observed (but the pre-irradiation dopant concentration is higher than for FZ substrates...)

1.5E+12

2.5E+12

3.5E+12

4.5E+12

0.0E+00 5.0E+14 1.0E+15 1.5E+15 2.0E+15

Fluence (e/cm2)

Nef

f_eq

ual (

1/cm

3 )

after irrad 8min@80°C


Effective dopant concentration: EPI

6.E+13

7.E+13

8.E+13

0.0E+00 5.0E+14 1.0E+15 1.5E+15 2.0E+15Fluence (e/cm2)

Nef

f_eq

ual (

1/cm

3 )

EPI1-after irradEPI1-8min@80°CEPI2-after irradEPI2-8min@80C

• same “equalization” procedure as for CZ

• type inversion not observed:– the pre-irradiation dopant concentration

is even higher than for CZ substrates...– the oxygen concentration is higher than

for DOFZ (under investigation)

• the observed variations of Neff are small and comparable with the uncertainty coming from depletion voltage measurements => difficult to extract a meaningful dependence on the fluence


Leakage current & constant damage αthe increase in leakage current does not depend on substrate material(as already observed after hadron irradiations)

after 8 minutes @ 80°Cα = 9.06x10-19 A/cm

hardness factor kexp = α(900 MeV e-)/α(1 MeV n) = 2.3x10-2

at equal NIEL, high energy electrons are ~3.5 times less effective than 1 MeVneutrons in degrading the carriergeneration lifetime of substrate material

ktheo = NIEL(900 MeV e-)/NIEL(1 MeV n) = 8.1x10-2

ktheo/ kexp = 3.5

the NIEL scaling hypothesis is not adequatewhen comparing electrons with hadrons

Leakage current after 8 min @ 80°C

0

0.0005

0.001

0.0015

0.002

0.0025

0.0E+00 5.0E+14 1.0E+15 1.5E+15 2.0E+15

Fluence (e/cm2)

Jle

ak (

A/c

m3 )


,⎟⎟⎠

⎞⎜⎜⎝

⎛⋅−+⎟⎟

⎠

⎞⎜⎜⎝

⎛−⋅==

00

II

eq

leak tlntexpΦ

(t)J(t)τ

βατ

αα

• The evolution of the leakage current vs. annealing time is proportional to the time evolution of the damage constant α:

Annealing of leakage current

(2.43±0.15)x10-18 A/cmβ

(3.85±0.11)x10-17 A/cmα0

(23.94±4.59) minτI

(1.06±0.11)x10-17 A/cmαI

αI, τI, α0 and β are free parameters, τ0=1min

Initial short-term annealing Long-term annealing

eleq ΦΦ ⋅= κ

0.0

1.0

2.0

3.0

4.0

5.0

6.0

1 10 100 1000 10000 100000Annealing time (min)

α(1

0-17

A/c

m)

EPI 16, 2.1E15 e/cm2EPI 56, 1.4E15 e/cm2CZ 37, 2.1E15 e/cm2CZ 25, 1.4E15 e/cm2FZ 27, 2.1E15 e/cm2FZ 42, 1.4E15 e/cm2DOFZ 11, 2.1E15 e/cm2DOFZ 56, 1.4E15 e/cm2Fit curve


Annealing of Neff: FZ and DOFZ• FZ and DOFZ diodes (inverted) reach a minimum in the effective acceptor

concentration after ~10 minutes (beneficial annealing), followed by an increase (reverse annealing)

• Higher effect in FZ devices, more pronunced for CiS devices• Measurements performed after 24 hours @ RT (for CiS devices) show

bistable damage effect in FZ but not in DOFZ devices

0

1

2

3

4

5


|Nef

f| (1

012/c

m3 )

FZ 27, 2.1E15 e/cm2FZ 42, 1.4E15 e/cm2DOFZ 11, 2.1E15 e/cm2DOFZ 56, 1.4E15 e/cm2after 24 hours @ RT

0

1

2

3

4

5


|Nef

f| (1

012/c

m3 )

FZ(111), 2.1E15 e/cm2FZ(100), 2.1E15 e/cm2DOFZ(111), 2.1E15 e/cm2FZ(111), 1.4E15 e/cm2FZ(100), 1.4E15 e/cm2DOFZ(111), 1.4E15 e/cm2

FZ & DOFZ by IRST FZ & DOFZ by CiS


Annealing of Neff: CZ and EPI• EPI devices (non-inverted) show an increase of effective donor

concentration with time; a decreasig trend starts at very long annealing times. Variations of the order of a few %

• CZ devices (non-inverted) show an oscillating behaviour, observed also after hadron irradiation. Possible reasons under investigation...

0.94

0.96

0.98

1.00

1.02

1.04

1.06


N eff

/Nef

f(p

re-a

nn)

EPI 16, 2.1E15 e/cm2EPI 56, 1.4E15 e/cm2EPI 33, 2.1E15 e/cm2EPI 61, 1.4E15 e/cm2

0.50

0.70

0.90

1.10

1.30

1.50


N eff

/Nef

f(p

re-a

nn)

CZ37, 2.1E15 e/cm2

CZ25, 1.4E14 e/cm2

EPI by CiS CZ by CiS


• measured with the Transient Current Technique (TCT) on samples annealed for 8 minutes @ 80oC (all devices by CiS)

• charge injection from a collimated source of α particles (244Cm)• bias voltage≥150 V for EPI devices, ≥300 V for CZ, FZ and DOFZ devices• CCE defined as the ratio between charge induced in irradiated device and

charge induced in non-irradiated device

Charge collection efficiency

0.94

0.96

0.98

1

1.02

1.04

0 5E+14 1E+15 1.5E+15

Fluence (e/cm2)

CCE

EPICZFZDOFZ

• The decrease of CCE at the highest fluences is of 1-3%, more pronounced for FZ and DOFZ devices


Conclusions• Leakage current: no difference is observed among different

materials, as already known after hadron irradiation

• Neff:• a slightly beneficial effect of oxygen diffusion in FZ

substrates is observed

• CZ: a naive extrapolation of the observed linear trend suggeststhat type inversion would be reached at ~4.5x1015 e/cm2; we plan to reach these fluences in future irradiations

• EPI: substrate type inversion is not even approched up to 2.1x1015 e/cm2; the effect is still too small to allow a quantitative assessment

• CCE: very small reduction (1-3%) observed at the highest fluence


α: comparison with other particles• Use asymptotic value of displacement cross section for 200 MeV electrons

[G.P.Summers et al., IEEE Trans.Nucl.Sci. 40(6) (1993), 1372-1378] (no higher-energy values available)

NIEL(900 MeV e)/NIEL(1 Mev n) = 8.106x10-2

1 MeV n equiv.measured

4.0x10-17(*)1 MeV n1.12x10-179.06x10-19900 MeV e-1.9x10-184.5x10-201.8 MeV e-

α(A/cm)particles

531280

n)MeV NIEL(1 / e)MeV NIEL(900n)MeV (1eMeV 900

../)(

==αα

(*) M.Moll et al., NIM, vol. A426, pp.87-93, 1999

High-energy electron irradiation of different silicon materials

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