1

Space charge sign inversion Space charge sign inversion and electric field reconstruction and electric field reconstruction

in 24 GeV proton irradiated in 24 GeV proton irradiated MCZ Si pMCZ Si p++-n(TD)-n-n(TD)-n++ detectors detectors

processed via thermal donor introductionprocessed via thermal donor introduction

Z. LiBrookhaven National Laboratory, Upton, NY, USA

E. Verbitskaya, V. Eremin, Ioffe Physico-Technical Institute of Russian Academy of Sciences

St. Petersburg, RussiaJ. Härkönen

Helsinki Institute of Physics, CERN/PH, Geneva, Switzerland

10 RD50 Collaboration WorkshopVilnius, Lithuania, June 4-6, 2007

*This research was supported by the U.S. Department of Energy: Contract No. DE-AC02 -98CH10886

2

OutlineOutline

1. Background: specific features/advantages of p-type Si for development of radiation-hard detectors

2. Experimental: samples; measurement technique

3. Experimental results on TCT pulses after 24 GeV/c proton irradiation: SCSI in p+-n(TD)-n+ detectors

4. Approach for simulation of detector Double Peak response and E(x) profile reconstruction with a consideration of electric field in the “neutral” base

5. New method for E(x) reconstruction

6. Results on E(x) reconstruction in protons irradiated p+-n(TD)-n+ detectors

7. Comparison of E(x) profile reconstructed from TCT data and E(x) profile simulated with a consideration of carrier trapping

Conclusions

Z. Li et al., 10th RD50 Workshop, Vilnius, Lithuania, June 4-6, 2007

3

Specific features and advantages of p-type Si for development of radiation-hard detectors

Recently development of Si detectors based on the wafers from p-type Magnetic Czochralski (MCZ) silicon became an attractive way for radiation hardness improvement [1]. The main advantage is that the major signal arises from electron collection which leads to efficient detector operation [2], and the detector is non-inverting after irradiation. Along with this, a single sided process can be used to fabricate n+-p-p+ detectors that facilitate device fabrication.

1. J. Härkönen et al., Proton irradiation results of p+-n-n+ Cz-Si detectors processed on p-type boron-doped substrates with thermal donor-induced space charge sign inversion, NIM A 552 (2005) 43-48.2.G. Casse et al., NIM A 518 (2004) 340.

4

Experimental

Samples:

p-type MCZ Si, as-processed p+-p-n+ structure (processed at HIP)

Treatment:

annealing at 430 C, Thermal Donor (TD) introduction

Conversion to p+-n(TD)-n+

Irradiation: 24 GeV/c protons at CERN PS (Thanks to Maurice!) 7 fluences (81012 p/cm2 to 51014 p/cm2)

Experimental technique: TCT (measured at BNL and PTI)

V. Eremin, N. Strokan, E. Verbitskaya and Z. Li, NIM A 372 (1996) 388-298

5

SCSI in MCZ p+-n(TD)-n+ detectors

SC + SC – (SCSI)

100 V160 V

200 V300 V

Laser front

180 V

200 V

250 V

300 V

Laser back

8x1012 p/cm2

Laser front

100 V

120 V

160 V200 V

2x1013 p/cm2

100 V

120 V160 V

200 V

Laser back

Laser front

200 V

230 V

250 V

300 V

Laser back

10 V

50 V

100 V

150 V

180 V

6x1013 p/cm2

SC – (SCSI)

6

SCSI in MCZ p+-n(TD)-n+ detectors

DP, SC - (SCSI) 2.14x1014 p/cm2

Laser front

258 V268 V278 V

288 V

Laser front

220 V230 V240 V

250 V

Laser front

308 V

328 V

347 V

367 V

0 V

9 V18 V

27 V

Laser back Laser back

37 V56 V

94 V142 V210 V

Laser back

230 V

268 V308 V

347 V

367 V

7

SCSI in MCZ p+-n(TD)-n+ detectors

DJ, SC - (SCSI), symmetrical TCT pulses 4.2x1014 p/cm2

Laser front Laser front

18 V

43 V

69 V87 V

105 V

132 V

160 V178 V

Laser back Laser back

18 V

43 V

69 V87 V

105 V

132 V

160 V178 V

8

Vfd vs. Fp dependence

# Fp (cm-2) SC V_fd (Q_V) 47 8.00E+12 positive 20049 2.00E+13 negative 4515 6.00E+13 negative 25059 1.48E+14 DP to (-) 30069 2.14E+14 DP to (-) 34026 4.18E+14 DJ48 5.05E+14 DJ

n320, p+-n(TD)-n+, Vfd vs. Fp

0

50

100

150

200

250

300

350

400

0.0E+00 5.0E+13 1.0E+14 1.5E+14 2.0E+14 2.5E+14

Fluence (p/cm2)

Fu

ll d

eple

tio

n v

olt

age

(V)

9

Approach for simulation of detector Double Peak response and E(x) profile reconstruction

with a consideration of electric field in the “neutral” base

Transient current:

Ntr = f(F)

effto ed

EQti /

p+ n+

E1

Eb

E2

W1 Wb W2

h

trthtr Nv

1

Three regions of heavily irradiated detector structure are considered Reverse current flow creates potential difference and electric field in

the neutral base

1) E. Verbitskaya et al. NIM A 557 (2006) 528;2) E. Verbitskaya, pres. RESMDD’6, NIM A (in press)Initiated by PTI, developed in:

10

New method for DP E(x) profile reconstruction

Developed at BNL

Method:• based on three region model;• describes induced current pulse arisen from carrier drift

• initial extraction of charge loss due to trapping “corrected” pulse response

t(F) – known from reference data (e.g. H.W. Kraner et al, NIM A326 (1993) 350-356,

and G. Kramberger et al., NIM A481 (2002) 297)

• fitting of the “corrected” current pulse response best matches the “corrected” current pulse response in: 1) the heights of the two current pulse peaks; 2) the shape and position of the minimum; 3) the time interval between the two current pulse peaks.

tdro t

d

vQti /exp)(

d

vQFttiti dro

neqtcorrected )(/exp)()(

sdr vxE

xEv

/)(1

)(

11

Results on E(x) reconstruction in proton irradiated p+-n(TD)-n+ detectors: proof of SCSI

E(x) reconstruction is made for DP pulses

It gives the final proof of the sign of space charge (Neff)

Laser front (p+), e

Laser front

258 V268 V278 V

288 V

Laser front

308 V

328 V

347 V

367 V

Responses to be simulated

N320-69Fp = 2.14·1014 cm-2

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.00E+00 1.00E-08 2.00E-08 3.00E-08 4.00E-08 5.00E-08

time (s)

sig

nal

(V

)

raw datatrapping extractedsimulation

p+ n+

E1

Eb

E2

W1 Wb W2

h

p+ n+

E1

Eb

E2

W1 Wb W2

h

E1 E2

Eb

W1 W2Wb

p+n+

12

Procedure of E(x) reconstruction

Variable parameters: E1, E2, Eb

W1, Wb Edx = V

d = W1 + Wb + W2

n320-69, Fp = 2.14 cm-2, V = 258 V

-0.0005

0

0.0005

0.001

0.0015

0.002

0.0E+00 1.0E-08 2.0E-08 3.0E-08 4.0E-08 5.0E-08 6.0E-08

time (s)

sig

na

l (V

)

raw data

baselineextractedtrappingextracted

Laser p+ side, e

t0

0.0E+00

5.0E-04

1.0E-03

1.5E-03

2.0E-03

0.0E+00 5.0E-09 1.0E-08 1.5E-08 2.0E-08 2.5E-08

time (s)

Sig

nal

(A

)

experiment, trappingextractedsimulation

258 VLaser p+ side, e

13

Fits of current pulse response at increasing bias voltage

21

3

Fits may be done: - starting from pronounced Double Peak pulse shape - up to its conversion to Single Peak Shape

t = 25 ns

N320-69Fp = 2.14·1014 cm-2

0.0E+00

5.0E-04

1.0E-03

1.5E-03

2.0E-03

2.5E-03

3.0E-03

3.5E-03

0.0E+00 5.0E-09 1.0E-08 1.5E-08 2.0E-08 2.5E-08

time (s)

Sig

nal

(A

)

experiment, trappingextractedsimulation

288 V

Laser p+ side, e

0.0E+00

1.0E-03

2.0E-03

3.0E-03

4.0E-03

5.0E-03

0.0E+00 5.0E-09 1.0E-08 1.5E-08 2.0E-08 2.5E-08

time (s)

Sig

nal

(A

)

experiment, trappingextracted

simulation

308 V

Laser p+ side, e

0.0E+00

5.0E-04

1.0E-03

1.5E-03

2.0E-03

0.0E+00 5.0E-09 1.0E-08 1.5E-08 2.0E-08 2.5E-08

time (s)

Sig

na

l (A

)

experiment, trappingextractedsimulation

258 VLaser p+ side, e

14

Evolution of E(x) profile at increasing bias voltage

At lower V maximal E is near the n+ contact, and W2 2W1

This asymmetric distribution becomes enhanced at higher V.

Finally, positively charged region is immersed by a region with negative charge.

Fp = 2.141014 cm-2

p+

E(x) profile reconstructed from current pulse, n320-69

0

5000

10000

15000

20000

25000

0 0.005 0.01 0.015 0.02 0.025 0.03

x (cm)

E (

V/c

m)

258 V

288 V

308 V

328 V

p+

n+

15

Simulation of DP E(x) profile with a consideration of carrier trapping to midgap energy levels:

E(x) and Neff(x) vs. V

Parameters:

introduction rate of generation centers mj

introduction rate of midgap deep levels, DA and DD concentration ratio k = NDA/NDD

bias voltage V temperature T detector thickness d initial resistivity (shallow donor concentration No)

midgap DLs: DD: Ev + 0.48 eV simulation is based on

DA: Ec – 0.52 eV Shockley-Read-Hall statistics

E. Verbitskayapres. RESMDD’6, NIM A (in press)

16

Comparison between E(x) profile reconstructed from current pulse response and E(x) profile simulation considering

thermal carrier trapping to midgap DDs and DAs

1

3

2

Boundary condition for simulation: electric field at the p+ and n+ contacts equals to the electric field obtained from E(x) reconstructed from TCT pulses

Good agreement between two profiles

0

5000

10000

15000

20000

25000

0 0.005 0.01 0.015 0.02 0.025 0.03x (cm)

E (

V/c

m)

reconstruction

trapping model

258 V

p+ n+

0

5000

10000

15000

20000

25000

0 0.005 0.01 0.015 0.02 0.025 0.03x (cm)

E (

V/c

m)

reconstruction

trapping model288 V

0

5000

10000

15000

20000

25000

0 0.005 0.01 0.015 0.02 0.025 0.03

x (cm)

E (

V/c

m)

reconstruction

trapping mode.

308 V

p+

n+

p+

n+

p+

n+

17

E(x) and Neff(x) simulated considering free carrier trapping to midgap DDs and DAs

n320-69

At V 300 V negatively charged region W2

extends over the total detector thickness

Good agreement between E(x)

E(x) profile reconstructed from current pulse

0

5000

10000

15000

20000

25000

0 0.005 0.01 0.015 0.02 0.025 0.03

x (cm)

E (

V/c

m)

258 V

288 V

308 V

328 V

p+ n+

E(x) related to trapping

0

5000

10000

15000

20000

25000

0 0.005 0.01 0.015 0.02 0.025 0.03

x (cm)

E (

V/c

m)

258 V

288 V

308 V

328 V

p+ n+

Neff related to trapping

-1E+13

-8E+12

-6E+12

-4E+12

-2E+12

0

2E+12

4E+12

6E+12

0 0.005 0.01 0.015 0.02 0.025 0.03

x (cm)

Ne

ff (

cm

-3)

258 V288 V308 V328 V

p+ n+

Neff

p+

n+

p+

n+

p+

n+

18

Conclusions

SCSI does occur in 24 GeV/c proton-irradiated p+n(TD)-n+ MCZ detectors!

– similar to n-type FZ and DOFZ Si

This finding reverts us to the “puzzle” of space charge sign in 24 GeV proton irradiated n-type MCZ Si detectors –

whether there is a dependence on the sample pre-history? New method of E(x) reconstruction in heavily irradiated Si detectors allows to get nice agreement between experimental “corrected” response independent on fluence, and a simulated response arisen from the carrier drift in detector bulk with three regions of electric field profile

DP

Original n-type MCZ (p+-n-n+), 24 GeV p as-irradiated, 1x1015 p/cm2, V = 360V

Reconstructed E-profile

p+ n+

SCSI?

Laser front (p+), e

19

Acknowledgements

This work was supported in part by:

• U.S. Department of Energy: contract No: DE-AC02-98ch10886,• INTAS-CERN project # 03-52-5744,• Grant of the President of RF SSc-5920.2006.2.

Thank you for your attention!

This work within the frame works of CERN RD39 and RD50 Collaborations.