Synchrotron studies of carrier physics in a CZT ring...

VG 1 - 5th NDIP conference - Aix-les-Bains - 15 − 20 June 2008 - Roland den Hartog

Synchrotron studies of carrier physics in a CZT ring detector

R. den Hartog, A. Owens,European Space Agency, ESTEC / SCI-PAT, Noordwijk, The Netherlands

A.G. Kozorezov, J.K. WigmoreDepartment of Physics, Lancaster University, United Kingdom

V. Gostilo, V. KondratjevBruker Baltic, Riga, Latvia

A. Webb, E. WelterHASYLAB at DESY, Hamburg, Germany

LANCASTERU N I V E R S I T YDepartment of Physics


Ring detectorby Apotovsky et al. for Digirad, 1997

5 mm

1 mm

Vcathode

small anode: low capacitive noise

max. ΔV tested: ~150 VVGR « Vcat : drift modeVcat « VGR : hemispherical mode

X-rays


PerformanceInteresting features:• Low noise, no hole collection

• Wide dynamic range from at least6 to 662 keV

• Excellent energy resolutionup to 0.73% FWHM at 662 keV

• Charge collection adaptable to detector shape and material


Synchrotron characterization

HASYLAB X-1 synchrotron beamline at DESY, Hamburg:

• 10.5 − 100 keV monochromatic light

• MOSTAT intensity stabilizer

• ΔE < 1 eV at 10 keV

• ΔE < 20 eV at 100 keV

• Beamsize < 50 x 50 μm2

Spatial response strongly non-uniform


Model for charge transporta. Solve electrostatic problem:

Dirichlet problem for electrostatic potential Φ(r,L) in cylindrical coordinates- renormalize to Vcat = 0, so that potential differences are limited to top surface- potential only specified on surface electrodes, not in between

b. Integrate electron and hole trajectories in potential, until top or bottom surface is reached

c. Electrons are detected once they traverse near field of anode

[ ]∫∫ −+=Φ⇒

⎪⎪⎩

⎪⎪⎨

⎧

≥

≤≤≤≤

≤

===Φ

==Φ

=∂Φ∂

+∂

Φ∂+

∂Φ∂

∞ GRr

GRGR

GRGR

R

R

aanode

VxVtxxJdxtrJtzttdtzVzr

rrVrrrVrrrV

rrV

rVLzr

zrrrrz

00

00

22212

12111

2

2

2

2

)()()(sinhsinh),(

::::

)(),(

0)0,(

01

Owens et al., JAP 102 (2007)

Kozorezov et al., IEEE NSS/MIC 2007


Model for charge transporthemispherical mode

drift mode

dead zone

detected

undetected

undetected

electrons

holes

detection only in near field


Model vs synchrotron scans

Vcat=−46.2 V Vcat=−89 V Vcat=−100 V Vcat=−119 V

Model provides at least a qualitative explanation for complex patterns in drift mode

nice !

E = 20 keV


Model correctly predicts:• widening of central spot with E• widening of ring with E• shrinking of ring at highest E• absense of central response at lowest E(near field of anode not shown)


Vcat=−100 V

Extent and width of ring structure not fully explained

10 keVλ = 14 μm

20 keVλ = 84 μm

50 keVλ = 164 μm

100 keVλ = 1.05 mm


Collection of e- seems to take place across R1: • ring sizes are confirmed by bondwire shadows• anode near field extends out to first ring• extension into first ring conflicts with data for

Vcat= −119 V

Bond wires

0.5 mm Ø

ring structure to scale

50 μm Ø beam0.1 mm step size

ra = 0.24 μmra = 0.14 μm

V cat

= −1

00 V

V cat

= −1

19 V



Advantage of monochromatic light

E = 10 keV E = 20 keV E = 50 keV E = 100 keV

low detection threshold

high detection threshold

Secondary ring + sausage feature show up above low rejection threshold:• ring forbidden by topology of field lines in cylindrical model• sausage breaks cylindrical symmetry → causes ring feature?


Differential spectrometry

Te inclusions identified in CZT sample using IR microscopy by Bolotnikov et al. IEEE NSS/MIC 2007

Dislocation

Dislocation

Sub-grain boundary

Bolotnikov et al. (2007):• Te inclusions found along grain boundary and dislocations• no effect on charge transport for Te inclusions ≤ 3 μm and

densities ≤ 3x105 cm−3, i.e. fractional enhancement of 3.4x10−5

Hypothesis:• features due to Te inclusions along grain boundary• Te density enhancement affects charge transport

Is Te excess density at level where it can be detected by cross-edge differential spectrometry?


Differential spectrometryMethod:• 8 scans, at two E on either side of Cd and Te edge

(resp. 26710 and 31814 eV)• Use ratios rmeas(E) = ( ∑A-∑B) / ∑C to avoid normalization

and systematic effects• Make a simultaneous fit with a model with 4 parm.:

x1 : depth under surface of featurex2 : bottom of featureδ : enhancement of Te fraction over 45%η : differential detection efficiency for photons landing

in A or B, compared to photons landing in C

AC

B

x1

x2

Cd0.45Zn0.1Te0.45 → λ0(E)

Cd0.45-δ Zn0.1Te0.45+δ → λ1(E,δ)

⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛ −−−⎟⎟

⎠

⎞⎜⎜⎝

⎛−=

),(exp1

)(exp)(

1

12

0

1

δλλη

Exx

ExErfit


Differential spectrometry

# free parms.: x1 [μm] x2 [μm] η δ χ2

3 43.1 86.4 0.743 = 0.0 10754 45.1 83.2 0.779 0.0011 262

• Fit with Te enhancement better than without, while other parms are consistent• Fitted enhancement level ~30x higher than levels found by Bolotnikov et al.• Method appears at least sensitive to Te enhancements at 0.1% level


Conclusions• Application of synchrotron radiation to detector characterisation

extends well beyond FWHM measurements

• High-resolution surface scans appear a powerful tool for charge transport diagnostics, in particular in combination with model

• Cross-edge differential spectrometry is capable of in-situ characterization of defects in detector: 3-D shape, composition 0.1% enhancement of Te demonstrated

• Next steps:Application to TlBr, optimization for effective volumeSimulation of line shapes, optimization of resolution

Date post:	09-Mar-2021
Category:	Documents
Upload:	others
View:	3 times
Download:	0 times

Synchrotron studies of carrier physics in a CZT ring...

Documents