energy deposition and charge transport in pixellated semiconductor x-ray detectors

2004-07-25Departement of Information Technology and

MediaElectronics Group

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Energy deposition and charge transport in pixellated semiconductor X-ray detectors

Christer Fröjdh, Hans-Erik Nilsson, Börje Norlin

Mid Sweden University, Sundsvall, Sweden



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OUTLINE

• Background– Motivation – Charge deposition – Charge transport

• Energy deposition – Initial profile– After charge transport

• Spectral response– Initial spectrum– After charge transport

• Summary and conclusions



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Background

• Significant charge sharing has been noticed in photon counting X-ray detectors

– Which is the dominant effect causing charge sharing in pixel detectors?

• How large is the initial charge cloud?

• What is the effect of X-ray fluorescence?

• What is the effect of diffusion during charge transport?

• We have simulated charge deposition and charge transport in a number of different detectors



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Background

• Simulated structures– Pixel size 50 x 50 um

– Layer thickness 500 um

– Materials Si, GaAs and CdTe, Cd0.8Zn0.2Te, TlBr, PbI2

– Charge transport simulated for Si and CdTe assuming Si drift parameters

– Charge deposition simulated with MCNP4C

– Charge transport simulated with MEDICI



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Charge deposition

K (keV)

Zn 8.63

Ga 9.25

As 10.54

Br 11.93

Cd 23.17

Te 27.47

I 28.61

Tl 72.86

Pb 74.96

Primary photon

Photoelectron

Secondary photon

Primary photon

Scattered photon

Compton electron

Photoelectric absorption Compton scattering



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Linear attenuation coefficients

1

10

100

1000

10000

0 20 40 60 80 100

keV

cm-1

GaAs

CdTe

CZT

TlBr

PbI2

Silicon



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Charge transportCharge transport

• The charge transport has been extracted using time resolved drift-diffusion simulations in MEDICI

• 3D-effects has been taking into account using cylindrical coordinates

• Ideal semiconductor materials have been assumed (no effect due to trapping etc)

Cathode

Anode

Initial hit



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Simulated pulse width for 300 um Si and a dental source

FWHM (25 micron)



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Energy deposition

• The energy deposition has been simulated counting the energy deposited in a 2 um thick slice of the detector using a monoenergetic point source located at the centre of the slice. A spatial resolution of 2 um was used during the simulation

Top view

Side view

2 x 2 um

500 um



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Energy deposition at 18 keV

Quantum efficiency (at peak):

Si: 0.17

GaAs: 0.77

CdTe: 0.85

Most energy deposited within 15 m

1,00E-07

1,00E-06

1,00E-05

1,00E-04

1,00E-03

1,00E-02

1,00E-01

1,00E+00

-50 -40 -30 -20 -10 0 10 20 30 40 50

Si

GaAs

CdTe

Si drift



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Quantum efficiency (at peak)

Si: 0.01

GaAs: 0.47

CdTe: 0.36

1,00E-07

1,00E-06

1,00E-05

1,00E-04

1,00E-03

1,00E-02

1,00E-01

1,00E+00

-50 -40 -30 -20 -10 0 10 20 30 40 50

Si

GaAs

CdTe

Si drift



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Quantum efficiency (at peak):

Si: 0.004

GaAs: 0.01

CdTe: 0.05

1,00E-07

1,00E-06

1,00E-05

1,00E-04

1,00E-03

1,00E-02

1,00E-01

1,00E+00

-50 -40 -30 -20 -10 0 10 20 30 40 50

Si

GaAs

CdTe

Si drift



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Spectral response

• Pixel size 50 x 50 um

• Layer thickness 500 um

• Uniform illumination with monoenergetic photons

• Response collected from central pixel in an array of at least 20 x 20 pixels

• Energy bin size 1 keV



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Spectral response at 18 keV

Quantum efficiency.

Total Peak

Si 0.44 0.51

GaAs 1.00 0.82

CdTe 1.00 0.96

CZT 1.00 0.95

TlBr 1.00 0.82

PbI2 1.00 0.87

0,000,100,200,300,400,500,600,700,800,901,00

0 5 10 15 20

keV

Rel.

coun

ts

Si

GaAs

CdTe

CZT

TlBr

PbI2



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Quantum efficiency.

Total Peak

Si 0.15 0.10

GaAs 0.98 0.79

CdTe 1.00 0.55

CZT 1.00 0.58

TlBr 1.00 0.83

PbI2 1.00 0.85

0,000,100,200,300,400,500,600,700,800,901,00

0 10 20 30

keV

Rel.

coun

ts

Si

GaAs

CdTe

CZT

TlBr

PbI2



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Quantum efficiency.

Total Peak

Si 0.02 0.001

GaAs 0.18 0.08

CdTe 0.51 0.17

CZT 0.47 0.16

TlBr 0.87 0.19

PbI2 0.77 0.18

0,00

0,05

0,10

0,15

0,20

0,25

0 20 40 60 80 100

keV

Rel.

coun

ts

Si

GaAs

CdTe

CZT

TlBr

PbI2



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Effects of charge sharing

• Spectrum before and after charge transport for Si at 18 and 30 keV and CdTe at 90 keV



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Response from Si at 18 and 30keV

Response from a Si

detector to flood

illumination with

photons at 18keV and

30keV.

The spectrum is

significantly changed

due to charge

sharing.0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

0,45

0,5

0 10 20 30

18keV-abs

30kev-abs

18keV-drift

30keV-drift



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Response from CdTe at 90keV

Response from a

CdTe detector to

flood illumination

with photons at

90keV.

The spectrum is

significantly changed

due to charge

sharing.

0

0,02

0,04

0,06

0,08

0,1

0,12

0,14

0,16

0,18

0 20 40 60 80 100

keV

Rel.

coun

ts 90keV-abs

90keV-drift



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Total detector response

• The result for a large area detector (1 x 1 mm2) has been calculated for 30 keV and 90 keV



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Response at 30 keV, over 1 mm2

Quantum efficiency.

Total Peak

Si 0.15 0.10

GaAs 0.98 0.96

CdTe 1.00 0.88

CZT 1.00 0.90

TlBr 1.00 0.96

PbI2 1.00 0.98

0,000,100,200,300,400,500,600,700,800,901,00

0 10 20 30

keV

Rel.

coun

ts

Si

GaAs

CdTe

CZT

TlBr

PbI2



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Response at 90 keV, over 1 mm2

Quantum efficiency.

Total Peak

Si 0.02 0.001

GaAs 0.18 0.15

CdTe 0.51 0.42

CZT 0.47 0.40

TlBr 0.87 0.52

PbI2 0.77 0.53

0,00

0,10

0,20

0,30

0,40

0,50

0,60

0 20 40 60 80 100

keV

Rel.

coun

ts

Si

GaAs

CdTe

CZT

TlBr

PbI2



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Summary and Conclusions

• We have simulated charge deposition in a number of detector materials and, in some cases, compared the initial signal to the signal collected at the readout electrodes after drift.

– Charge diffusion is causing most of the charge sharing in the detectors

• The initial charge cloud is very narrow, except for fluorescent photons

– X-ray fluorescence in heavy semiconductors degrades the spectral information in the response

• ”Colour X-ray imaging” at higher energies requires methods to correlate related events in several pixels

• In integrating systems the effect is reasonably low since the energy is spread in a large volume

energy deposition and charge transport in pixellated semiconductor x-ray detectors

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