New trends in New trends in CdTeCdTe detectors for detectors for X and X and --ray applicationsray applications

Olivier LimousinCEA Saclay / DSM / DAPNIA

Service dAstrophysiqueFrance

New developments in photodetection, Beaune 2002 / Solid state detectors session

SummarySummary

Bulk detectors

Pixel arrays

CdTe as sensitive medium for X and -rays detection

Conclusions : New trends summary

Introduction Increasing demands for new semiconductor detectors for X and -rays (medical, space, nuclear and physics applications)Semiconductors are well suited for compactspectro-imaging devices with a good energy resolution between scintillators and cooledGermanium

Development of integrated front-endelectronics technologies (ASIC)

Progress in technology of producing CdTe and CdZnTe (stability and reproducibility)

CdTe : sensing medium for X and -rays par excellence

High Z (Cd 48, Te 52) well suited forphotelectric effectHigh density (~ 6) well suited for systemcompactness Wide band- gap and High resistivity (109 to 1011 cm) at room temperature Simple detector geometry

High potential for X and gamma raysspectroscopy Energy (keV)

0 100 1000

Prob

abili

ty

0

0,4

1,0

0,2

0,8

0,6

Photoelectric

Compton

CdTe versus other semiconductors

Detection efficiency for 100 keV gamma-ray photon as a functionof detector thickness in CdTe, Si and Ge

density Z Egap Eintrinsic[g cm-3] [eV] [eV/pair] [eV] at 100 ke

2.33 14 1.12 3.6 4505.33 32 0.67 2.9 4005.85 48,52 1.44 4.43 620

52 1.6 4.6 700

Detection efficiency for 5 and 10 mm thick CdTe detectors as afunction of energy

PE only(dashed)

PE + Compton(solid line)

Semi-conductor V

SiGeCdTeCdZnTe 5.81 48,

Data from Takahashi and Watanabe, IEEE TNS , 2001; VOL 48; PART 4; PART 1 , p 950

Two main CdTe families

CdTe:Cl (THM) Cd1-xZnxTe (HPB)0.08 < x < 0.15

gap around 1.5 eV 11010 cmp type crystalsex : 442 mm3

10 nA at 100V, 20C

holes 1 10-4 cm2 V-1 elect. 1 10-3 cm2 V-1

Uniform charge propertiesUp to 50mm waferNo grain boundary in wafers

gap around 1.6 eV = 11011 cmn type crystalsex : 442 mm3

1.5 nA at 100V, 20C

holes 210-5 cm2 V-1 elect. 0.5 to 5 10-3 cm2 V-1

Very good resistivityPossible grain boundaries Bad Yield but detectors up to 1 cm3

Signal induction principle The signal formation is described by the Schockley-Ramo theoremThe signal is induced by charge carrier motion along the electric field lines This motion is seen by capacitive influenceon electrodes depending on their geometry

I(t) = q0 E .EWE Applied field (stationnary regime)EW Weighting field (transient regime)

Lets talk about bulk detectors CdTe:Cl (THM) bulk detectorsEx : ISGRI (Lebrun et al.), Tokamak (Peysson et al.)CdZnTe (HPB) bulk detectorsEx : the PEGASE camera (Mestais et al .)CdTe:Cl Schottky detectorsEx : Takahashi et al.

CdZnTe bulk detectors with capacitiveelectrodes

CdZnTe bulk detectors with other electrodes configurationsEx : Luke et al., Parnham et al. (eV-Products)

Ex. of application : next talk (Lebrun)

Signal induction in a coplanar device

-Ray Photon

e-h+

-100 V

2 mm

t

Q

U0 Q0 E

Signal induction in a coplanar device Schockley-Ramo theorem gives the instantaneous induced current I(t)If the detector is uniform, no space charge

E = V0/LE = 1/LW

The induced charge is proportional to thecharge carrier motion and depends on the penetration depth of the photon

The induced charge dQL at the anode is

dQL = I(t) dt = q0 dx

L

Charge loss and balistic deficit

time

Colle

cted

char

ge

Charge loss (trapping)

15%1 s

10s

timeBalistic deficit

(filtering)

70%~2s

Charge loss and balistic deficit

The collected charge is described bythe hecht relation which take into account physical trapping ie, charge transportproperties (, )

The hole mobility drives the rise-time, ie the balistic deficit in CdTe:Cl

Biparametric diagram

Energy (keV)0 50 100 150

Puls

eri

se-t

ime

(s)

0

4

8

2

6

Energy (keV)0 50 100 150 200

0

5000

Coun

ts

P/V

3

Energy (keV)0 50 100 150

Puls

eri

se-t

ime

(s)

0

4

8

2

6

Energy (keV)0 50 100 150 200

0

8000

Coun

ts

P/V

9

Charge loss correction

ISGRI :In Beaune 1999,ISGRI :In Beaune 1999, we went with thiswe went with this

This time, here we are with ISGRI

with spectacular images

Calibration phase

coded mask apertureshadowgram with ISGRI camera at 511 keV

The eightmodules spectra with 22Na source

and spectra

Rise

-tim

e (

s)

0

4

8

2

6

0 50 100 150Energy (keV)

LT ~12 keV

7,5%

0 20 40 60 80 100 120 140Energy (keV)

Spectral performances of ISGRI

Energy (keV)

CdTe

Res

olut

ion

FWH

M (%

)

10 100 10001

100

10

122 keV

7,5 % (9 keV)

14,4 keV

25 % (3,6 keV)

An example of application in physics In the field of continuous thermonuclear reactions control in a Tokamak (TORE SUPRA)CdTe:Cl allows the design of compact cameras for hard X-ray tomography of the bremsstrahlung emission by electrons in tokamak fusion plamas

Such electrons produce Hard X-raysbetween 20 and 200 keVAnalyse of these electrons providesinformation about current density profiles

Example from Peysson et al., NIMA 458, 2001, p 269

An example of application in physics

Two cameras with 24and 38 CdTe detectors(552 mm3)

Detectors stayed stableeven under high fastneutrons flux and high magnetic field environment

Example from Peysson et al., NIMA 458, 2001, p 269

PEGASE : a CZT camera for medecine Pegase is based onthick bulk CdZnTe crystals

In this configurationhole signal is negligeable

The associated electronics (ASIC)deals with electronpulse rise-time

Example from Mestais et al., NIMA 458, 2001, p 62

PEGASE : electron loss correction

All events in this windoware affected to the 140 keV line of 99mTc source

140 keV line of 99mTc source

70% efficiency at 122 keV in a 6.5 % window

Window selection for the line

Example from Mestais et al., NIMA 458, 2001, p 62

CdTe:Cl with Schottky In contact

The basic idea is to reduce the dark current noise contribution with a Schottky anode contact For thin detectors, it provides very nice spectra, NO BALISTIC DEFICITThe main problem is due to polarization effect. This can be solved by :

- High bias voltage values- Negative temperature down to 40C- Pusing the HV

CdTe:Cl close to Ge

Needs a very low noise preamplifier !This often goes in the wrong direction ifwe must consider power consumption.

220.5 mm3 Schottky CdTe diode, 1400V, -40C

FWHM 830 eV !!

Example from Takahashi et al., NIMA 1999 & IEEE TNS 2001

Modifying weighting potentiel on CZT

The idea is to reduce the influence of the penetration depth in the signal inductionmodifying the weighting potential

Another point is to forget the holes, ie to have a single carrier collection

Then, it gives the opportunity to use thickCZT detectors

- electrode configuration (ex : Parnham et al.,Luke et al.)

- capacitive electrodes (ex : Montemont et al.)

Weighting potentiel in coplanar device

Depth (mm)

cathodeanode

Radi

us (m

m)

Depth (mm)

cathodeanode

Radi

us (m

m)

CAPture geometry, Parnham et al. from eV-Products (USA)

Scheme from Montemont, thesis universit J. Fourier, Grenoble, 2000

eV-Product Design : spectra

CAPture geometry, Parnham et al. from eV-Products (USA)555 mm3 CZT detector

Results with CAPture :

-

CZT coplanar-grid array

Data from Luke et al., NIMA 458, 2001, p 319

Substracting the signals from the two grids removes the hole contribution

Coplanar-grid electrode pattern with edge

compensation

1 cm3 coplanar-grid electrodeCZT coupled to its electronics

A small voltage is applied between the two grids. Electrons are collected on one grid.

Capacitive electrodes CZT

Capacitive electrode geometry, Montemomt et al. from CEA/LETI

Depth (mm)

anode cathode

screenDielectric film

Radi

us (m

m)

Data from Montemont et al., IEEE TNS, 2001; VOL 48; PART 3; PART 1 , p 278

Capacitive electrodes CZT performances446 mm3 Schottky CdTe diode, 400V, 21C

Energy (keV)

NEWTREND !

Performance should not depend on the detector thicness

Bulk detectors in two words

detector FWHM FWHM Thicknesstype [keV] at 122 keV [keV] at 662 keV [mm]

CdTe 5.5 23 2CdTe Schottky 1.5 NA 0.5CZT bulk 6 ? 6 CZT Capture 5 13 5CZT coplanar-grid 9 14 10CZT capacitive electrode 3.6 12 6

Lets talk about pixel arrays

Fine pixel arraysEx : - CdTe Medipix evolution (Manach et al.)

- Infocs (Stahle et al.)

Medium size pixel arraysEx : HEFT (Ramsey, Bolotnikov, Cook et al.)

Small pixel effect in CdTe arrays

Thick CdZnTe pixel arraysEx : Simbol_X

Medipix arrays characteristics European collaboration with CERN The goal is to realize an highly integratedchip (CMOS 0.25 m) for high count rate X and -rays counting imagers with semiconductor detectorsFirst generation (Medipix 1) developpedfor GaAs detectors. Readout of the holesignalNew generation (Medipix 2) developped forelectron collection and allows the use ofCdTe semiconductor

Data from Manach et al., CEA/DRT/LIST and Amendiola et al., NIMA 422, 1999, p 201

Medipix arrays design

Semiconductor detector

Indium bumpinterconnexions

Readout cell55m55m

Medipix2 readout chip (256256 pixels)

Data from Manach et al., CEA/DRT/LIST and Amendiola et al., NIMA 422, 1999, p 201

Infocs CZT pixel arrays

In the field of hard X-rays and -raysastronomyFocal plane for new focussing optics in therange of 10-100 keV with grazing incidence mirrorsThis technic allows a very high spatialresolution

The detector is made of a 26.926.9 mm2CZT crystal, 2 mm thick. It is a 6464 pixels array.

Data from Stahle et al., NIMA 436, 1999, p 138and http://lheawww.gsfc.nasa.gov/docs/balloon/FOCUS.html

Infocs CZT pixel arrays

2.3 keV FWHM

Energy (keV)

Coun

ts

109Cd source spectrum with Infocs CZT detector

Infocs CZT detector assembly

Data from Stahle et al., NIMA 436, 1999, p 138and http://lheawww.gsfc.nasa.gov/docs/balloon/FOCUS.html

HEFT CZT pixel arrays

In the field of hard X-rays and -raysastronomy againFocal plane for the High Energy Focussing TelescopeThe goal of this work is to achieve less than 1 keV at 60 keV (very low noise ASIC)

The detector is made of an 88 pixels array (6.76.72 mm3) with 680650 m pixel size.

Data from Ramsey, Bolotnikov, Cook et al.

HEFT CZT pixel arrays

HEFT CZT detector assembly

Energy (keV)241Am spectra (a) 0.9 keV FWHM at 60 keV, 5C

(b) 1.1 keV FWHM at 60 keV, room temperature

Small pixel effect

These nice results are possible because of the crystal quality, the ASIC performances and also the small pixel effect

Small pixel effect is due to the weighting field distribution close the anode when the pixel size is less than a quarter of the thickness

The nature works fine !

Data from Eskin et al., Hage-Ali et al.

Thick CZT detectors arrays

NEWTREND !

64 pixels CZT arrays, 6 mm thick

Conclusions

Thanks to CdTe detectors, it is now possible to dream of high spectral performances, high spatial resolution and high efficiency simultaneously

The high spectral resolution obliges to think about new geometries and high performance electronicsAmong these new geometries, capacitive electrodes detectors for bulk detectors and thick pixel arrays appear as major new trends