S. Dittongo1,2, L. Bosisio1,2, D. Contarato3, G. D’Auria4, E.Fretwurst5, J. Härkönen6, G. Lindström5, E. Tuovinen6

Studies of bulk damage induced in different silicon materials by 900

MeV electron irradiation

1 Dipartimento di Fisica, Universita’ di Trieste, Italy2 INFN, Sezione di Trieste, Italy3 DESY, Hamburg, Germany4Sincrotrone Trieste S.C.p.A., Trieste, Italy5 Institut für Experimentalphysik, Universität Hamburg, Germany6 Helsinki Institute of Physics, Helsinki, Finland

Selenia Dittongo IWORID 2004 – Glasgow, 25-29th July 2

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

– irradiated devices – experimental conditions

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

• Conclusions

Selenia Dittongo IWORID 2004 – Glasgow, 25-29th July 3

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

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

– the improvements achievable by using different silicon substrates

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

• In previous irradiations, we demonstrated the effectiveness of 900 MeV electrons in creating bulk damage

• In this work we extend these investigations to a wider sample of silicon materials (standard and oxygenated float-zone, magnetic and non-magnetic Czochralski, epitaxial silicon) and by reaching very high fluence levels (6x1015 e/cm2)

Selenia Dittongo IWORID 2004 – Glasgow, 25-29th July 4

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

material substrate processed by resistivity

[O]

FZ & DOFZ Wacker (111) n-type

CiS (Erfurt,

Germany)

3-4 kcm 1.2x1017 cm-3

(DOFZ)

FZ & DOFZ Wacker (111) n-type

Helsinki Institute of Physics

1.2 kcm not measured

CZ Sumitomo (100) n-type

CiS 1.2 kcm 8x1017 cm-3

Magnetic CZ Okmetic (100) n-type

Helsinki 1.0 kcm 5-9x1017 cm-3

Magnetic CZ Okmetic (100) p-type

Helsinki 1.8 kcm 5-9x1017 cm-3

EPI ITMECZ (111) n-

type

CiS 50 cm 9x1016 cm-3

Selenia Dittongo IWORID 2004 – Glasgow, 25-29th July 5

Experimental conditionsIrradiations

• 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)

Measurements• devices electrically characterized by reverse I-V and C-V measurements, performed ~1 day after irradiation

• C-V measurements @ 10 kHz

• currents normalized to 20oC

• isothermal annealing cycles up to 16 hours @ 80oC

step Fluence (e/cm2)

1 (1.06±0.01±0.04)x1015

2 (1.97±0.01±0.08)x1015

3 (4.16±0.05±0.17)x1015

4 (4.97±0.05±0.20)x1015

5 (6.11±0.02±0.25)x1015

Selenia Dittongo IWORID 2004 – Glasgow, 25-29th July 6

Photocurrent vs. bias voltage a quick and simple method to check for type

inversion

DOFZ Helsinki - = 2x1015 e/ cm2

before type inversion

0.0E+00

5.0E-07

1.0E-06

1.5E-06

2.0E-06

0 10 20 30 40 50 60 70 80 90 100

V_bias (V)

Phot

ocur

rent

(A

)

Front_ I llumination

Back_I lumination

DOFZ Helsinki - = 4x1015 e/ cm2

af ter type inversion

0.0E+00

5.0E-07

1.0E-06

1.5E-06

2.0E-06

0 10 20 30 40 50 60 70 80 90 100

V_bias (V)

Phot

ocur

rent

(A

)

Front_ I llumination

Back_ I lumination

VTD

VTD

• the diode is illuminated on the front or back side by the (dimmed) microscope light

• the I-V curve in the dark is subtracted from the I-V curve with light, to obtain the photogenerated current vs. bias voltage

• comparing the voltage dependence of the photocurrent for the two situations (front and back illumunation) helps in determining whether the substrate is inverted

Selenia Dittongo IWORID 2004 – Glasgow, 25-29th July 7

Effective dopant concentration: FZ & DOFZ

post-inversion behaviour

• lower Neff variations for DOFZ devices

• Neff(Helsinki devices) ~ 70-80% Neff(CiS devices)

• differences between Helsinki and CiS diodes likely due to different starting materials and oxygenation procedures (CiS: oxygen diffusion performed in N2 environment for 72 hours @1150oC; Helsinki: 8 hour oxidation in O2 @ 1050oC + 60 hour diffusion in N2 @ 1050oC)

type inversion occurs at:

• ~ 5x1014 e/cm2 for CiS devices (from previous irradiations)

• ~ 3x1015 e/cm2 for Helsinki devices (higher initial doping)

-4E+12

-3E+12

-2E+12

-1E+12

0

1E+12

2E+12

3E+12

4E+12

5E+12

0 2E+15 4E+15 6E+15 8E+15

fluence (e/cm2)

Neff

(1/c

m3)

FZ - Helsinki

DOFZ - Helsinki

FZ - CiS

DOFZ - CiS

Selenia Dittongo IWORID 2004 – Glasgow, 25-29th July 8

Effective dopant concentration: CZ & MCZ

n- type CZ & MCZ

0

1E+12

2E+12

3E+12

4E+12

5E+12

6E+12

0 2E+15 4E+15 6E+15 8E+15

fluence (e/cm2)

Neff

(1/c

m3) CZ - CiS

MCZ - Helsinki

-

n-type CZ & MCZ devices:

• Neff decreases, reaches a minimum at ~4x1015 e/cm2 and then

increases again

• type inversion not observed even if the pre-irradiation dopant concentration is comparable with FZ & DOFZ made in Helsinki

• the oxygen concentration is higher than for DOFZ (under investigation)

p-type MCZ devices:

• substrate type inversion is not approched

• very slow rate of decrease in Neff (1.2x10-4 cm-1)

p- type MCZ

0

1E+12

2E+12

3E+12

4E+12

5E+12

0 2E+15 4E+15 6E+15 8E+15

fluence (e/ cm2)

Neff

(1/c

m3)

Selenia Dittongo IWORID 2004 – Glasgow, 25-29th July 9

Effective dopant concentration: EPI

• type inversion not observed:the pre-irradiation dopant concentration is even higher than for CZ substrates...

• the variations of Neff are small (<15%)

• the rate of decrease in Neff is slow(1.5x10-3 cm-1)

0

1E+13

2E+13

3E+13

4E+13

5E+13

6E+13

7E+13

8E+13

0 2E+15 4E+15 6E+15 8E+15

fluence (e/ cm2)N

eff (

1/c

m3)

Selenia Dittongo IWORID 2004 – Glasgow, 25-29th July 10

Leakage current & damage constant

the increase in leakage current does not depend on substrate material(as already observed after hadron irradiations)

after ~1 day @ room temperature 7.9x10-19 A/cm

hardness factor kexp = (900 MeV e-)/(1 MeV n) = 2.0x10-2

at equal NIEL, high energy electrons are ~4.1 times less effective than 1 MeV neutrons in degrading the carrier generation lifetime of substrate material

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

ktheo/ kexp = 4.1

the NIEL scaling hypothesis is not adequatewhen comparing electrons with hadrons

0.E+00

1.E-03

2.E-03

3.E-03

4.E-03

5.E-03

6.E-03

0 2E+15 4E+15 6E+15 8E+15

fluence (e/cm2)

Jle

ak (

A/c

m3)

EPI CZFZ - CiS DOFZ - CiS

MCZn FZ - HelsinkiDOFZ Helsinki MCZp

Selenia Dittongo IWORID 2004 – Glasgow, 25-29th July 11

• The time evolution of the leakage current density for devices made on different substrates and irradiated at different fluences follows a common functional dependence on the annealing time

Annealing of leakage current

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.1 1 10 100 1000

annealing time (min)

Jle

ak

/ J

leak,

0

EPI - 6e15 e/ cm2 CZ - 4e15 e/ cm2CZ - 6e15 e/ cm2 MCZn - 4e15 e/ cm2MCZn - 6e15 e/ cm2 FZ Hels. - 2e14 e/ cm2FZ Hels. - 4e15 e/ cm2 DOFZ Hels - 2e15 e/ cm2DOFZ Hels - 4e15 e/ cm2

Selenia Dittongo IWORID 2004 – Glasgow, 25-29th July 12

= 2x1015 e/cm2 - before type inversion

-400%

-300%

-200%

-100%

0%

100%

0.1 1 10 100 1000annealing time (min)

D N

eff /

Neff

0

FZ

DOFZ

Annealing of Neff: FZ and DOFZ by Helsinki • before type inversion, a maximun in the effective acceptor concentration is reached after ~15

minutes, followed by a decrease– in FZ device, substrate type inversion is observed after 4 hours @ 80oC

• after type inversion, a minimum in the effective acceptor concentration is reached after ~15 (FZ)-60 (DOFZ) minutes (beneficial annealing), followed by an increase (reverse annealing)

• slightly higher effect in FZ devices

= 4x1015 e/cm2 - after type inversion

-60%

-40%

-20%

0%

20%

40%

60%

80%

0.1 1 10 100 1000annealing time (min)

D N

eff /

Neff

0

FZ

DOFZ

type inversion

Selenia Dittongo IWORID 2004 – Glasgow, 25-29th July 13

Annealing of Neff: CZ, MCZn and EPI

CZ and MCZn devices (non-inverted):• show an oscillating behaviour,

observed also after hadron irradiation. Possible reasons under investigation...

• oscillation amplitude is larger at lower fluences and for CZ devices

EPI devices - = 6x1015 e/cm2

-1.0%

0.0%

1.0%

2.0%

3.0%

4.0%

0.1 1 10 100 1000annealing time [min]

D N

eff/

Neff

,0

CZ & MCZn

-60%

-40%

-20%

0%

20%

40%

60%

80%

100%

0.1 1 10 100 1000annealing time (min)

D N

eff /

Neff

,0

CZ - 4e15 e/ cm2

CZ - 6e15 e/ cm2

MCZn - 4e15 e/ cm2

MCZn - 6e15 e/ cm2

EPI devices (non-inverted):• after highest fluence (6x1015 e/cm2),

show an increase of effective donor concentration with time

• variations of the order of a few %

Selenia Dittongo IWORID 2004 – Glasgow, 25-29th July 14

Conclusions• Leakage current: no difference is observed among different

materials, as already known after hadron irradiation

• Neff:

• FZ & DOFZ: a slightly beneficial effect of oxygen diffusion is observed; both show a saturation trend beyond 4x1015 e/cm2

• n-type CZ & MCZ: high fluence irradiations show that the linear trend observed at lower fluences does not continue up to type inversion; instead, Neff goes back up after reaching a minimun at ~4x1015 e/cm2

• EPI & p-type MCZ: substrate type inversion is not even approched up to 6x1015 e/cm2 ; very slow rate of decrease in Neff (1.5x10-3 cm-1 in EPI, 1.2x10-4 cm-1 in p-MCZ)

Selenia Dittongo IWORID 2004 – Glasgow, 25-29th July 15

: 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

particles (A/cm)

measured 1 MeV n equiv.

1.8 MeV e- 4.5x10-20 1.9x10-18

900 MeV e- 9.06x10-19 1.12x10-17

1 MeV n 4.0x10-17(*)

1.424.0

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

n)MeV (1eMeV 900

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