Prospects for Energy Resolving X-Ray Imaging with Compound Semiconductor Pixel Detectors

G. Bertuccio “Prospects for Energy Resolving X-ray Imaging with Compound Semiconductor Pixel Detectors ” 6th International Workshop on Radiation Imaging Detectors - IWORID-2004, July 25-29 , Glasgow (Scotland).

6th International Workshop on Radiation Imaging Detectors IWORID-2004,

July 25-29 , Glasgow (Scotland).

Prospects for Energy Resolving X-Ray Imaging with Compound Semiconductor

Pixel Detectors

Politecnico di MilanoDepartment of Electronics Engineering and Information Science and INFN

Milano - Italy

Giuseppe Bertuccio


Outline

• GaAs and SiC X-Ray detectors


Outline

• GaAs and SiC X-Ray detectors

• The achieved results


Outline• GaAs and SiC X-Ray detectors


• Some related issues :

– Leakage current

– Electronic noise

– Charge transport





– Leakage current







– Leakage current



• Prospects with GaAs and SiC pixel detectors


GaAs Radiation Detectors:

30 years of a fascinating history

• 1971-72 : Eberharth et al. (NIM-94), Kobayashi et al. (NIM-98)

1992 -1998:

• Europe: Aachen, CERN, Freiburg, Glasgow, Imperial

College, Lecce, Leicester, Milano, Modena, Pisa, Sheffield…

• USA : Michigan Univ., Sandia; UCLA, Livermore, Naval,

Berkeley, Wright Univ.


1996 – GaAs single pixel GaAs

Equivalent Noise Energies

@ 20 °C: 532 eV FWHM (53 e- r.m.s.)

@ -30°C: 373 eV FWHM (37 e- r.m.s.)

Equivalent Noise Energies

@ 20 °C: 532 eV FWHM (53 e- r.m.s.)

@ -30°C: 373 eV FWHM (37 e- r.m.s.)

MBE GaAs - 5 m thickp-i-n structure (3·1014 cm-3)

pixel: 170 x 320 m

MBE GaAs - 5 m thickp-i-n structure (3·1014 cm-3)

pixel: 170 x 320 m

IEEE Trans. Nucl. Sci. 44, 1997

Politecnico di Milano – KFA Julich Collaboration

Politecnico di Milano – KFA Julich Collaboration


2002 - Pixel Array

CVD GaAs - 40 m thickp+n structure

Pixel size: 200 m x 200 m

CVD GaAs - 40 m thickp+n structure

Pixel size: 200 m x 200 m

A. Owen, M. Bavdaz, A. Peacock (ESA)

S. Nenomen, H. Anderson (Metorex)

G. Bertuccio, R. Casiraghi, D. Maiocchi (PoliMI)


Epitaxial Layer Characteristics

1.5·1014 cm-3

Equivalent to

20 X Silicon thickness (650 m )

33 m @ 100 V


pA

Junction Leakage Current


pA

Junction Leakage Current

3-12 nA/cm2


200

Ileak Equivalent Noise @ 5 s

30Soft

Cooling

Soft Cooling

Room Temperature

Room Temperature

Can this current (noise) be lowered ?Can this current (noise) be lowered ?


Leakage Current densities

Is the ultimate limit reached for JGaAs ?Is the ultimate limit reached for JGaAs ?


Investigation on

the leakage curent origin

in junctions on GaAs


Leakage current origin in GaAs detectors

Schottky junction GaAs detectors




kT

qV

kT

q

R

R

eeTASI 1

emissionThermionic

B2*

B= 0.91 eVB= 0.91 eV




kT

qV

kT

q

R

R

eeTASI 1

emissionThermionic

B2*

s

mqE

4

loweringBarrier

B0B s

mqE

4

loweringBarrier

B0B

B= 0.91 eVB= 0.91 eV




kT

qV

kT

q

R

R

eeTASI 1

emissionThermionic

B2*

s

mqE

4

loweringBarrier

B0B s

mqE

4

loweringBarrier

B0B

ms

m EqE

4

static loweringBarrier

B0B ms

m EqE

4

static loweringBarrier

B0B

B= 0.91 eVB= 0.91 eV

x 5



p+n junction GaAs detectors

291 K




Thermal Generation

EA = 0.7 eV

G = 1 s

Thermal Generation

EA = 0.7 eV

G = 1 s

291 K




Thermal Generation

EA = 0.7 eV

G = 1s

Thermal Generation

EA = 0.7 eV

G = 1s

Thermal gen. +

Poole Frenkel effect

Thermal gen. +

Poole Frenkel effectx 3

291 K


Leakage current in GaAs: conclusions

• Current densities ≤ 10 nA/cm2 @ 290 K


Leakage current in GaAs: conclusions• Current densities ≤ 10 nA/cm2 @ 290 K

• Schottky junction:

– Barrier height = 0.9 eV

– Significant barrier lowering effects

– Higher barrier, lower doping (smaller Em) required


Leakage current in GaAs: conclusions• Current densities ≤ 10 nA/cm2 @ 290 K

• Schottky junction:

– Barrier height = 0.9 eV

– Significant barrier lowering effects

– Higher barrier, lower doping (smaller Em) required

• p+n junction:

– Generation current ( G=1s ; EA= 0.7 eV )

– Poole-Frenkel effect observed

– Higher purity, lower doping (smaller Em) required


Prototype GaAs pixel array

Test with discrete front-end electronics16 electrons r.m.s. at +20°C

14 electrons r.m.s. at -30°C


Trans. Nucl. Sci. 50, 2003

Room temperature ( 20°C )

242 eV FWHM (24 e - r.m.s.)

Thermoelectric cooling ( -30°C )

163 eV FWHM (16 e - r.m.s.)

GaAs pixel – best resultsWhich are the limits ?

Can we hope higher resolution ?

Which are the limits ?

Can we hope higher resolution ?


Noise Components Analysis20 °C20 °C

GaAs detector connected

Front-end electronics





Series noise





Series noiseCur

rent

noi

se





Series noise

Dielectric noise Curre

nt n

oise


X- Ray Spectroscopy

with epi-GaAs pixels detectors

Results


X- Ray Spectroscopy with GaAs

T = -30°C

ENE = 204 eV FWHM

( 20 e- r.m.s.)

T = -30°C

ENE = 204 eV FWHM

( 20 e- r.m.s.)

465 eV FWHM @ 59.5 keV

(Fano limit: 410 eV FWHM)

465 eV FWHM @ 59.5 keV

(Fano limit: 410 eV FWHM)


Pulser

283 ± 1 eV FWHM

241Am 59.54 keV

501 ± 5 eV FWHM

• No significant charge trapping

• Fano factor : F = 0.124 ± 0.004

• No significant charge trapping

• Fano factor : F = 0.124 ± 0.004

59.5 keV line analysis20 °C20 °C

Is the Charge Collection Efficiency 100 % ?Is the Charge Collection Efficiency 100 % ?


Charge trapping in GaAs detectors

gen

ind

Q

QCCE

gen

ind

Q

QCCE

ph

gen

EQ

phgen

EQ

Accurate knowledge of the electron-hole pair generation energy

required


Generation Energy in GaAs

300 K : = 4.18 ± 0.02 eV300 K : = 4.18 ± 0.02 eV

= 4.55 – 0.00122 T [eV] = 4.55 – 0.00122 T [eV]

Bertuccio, Maiocchi, JAP 92, 2002


Charge Collection Efficiency

CCEW

e

W

e h e h( )1CCEW

e

W

e h e h( )1

e+ h= 1400 m @ 110 V e+ h= 1400 m @ 110 V

CCE ≥ 99 %


Conclusions on tested GaAs pixel• High resolution with epi-GaAs pixels

– 242 eV FWHM @ 20°C

– 163 eV FWHM @ -30°C



– 242 eV FWHM @ 20°C

– 163 eV FWHM @ -30°C

• Main noise component: dielectrics (with discrete electronics)



– 242 eV FWHM @ 20°C

– 163 eV FWHM @ -30°C


• Leakage current density: ≤ 10 nA/cm2 @ 20°C



– 242 eV FWHM @ 20°C

– 163 eV FWHM @ -30°C



• Mean drift length: up to 1.4 mm (CCE>99 % for 30 m)



– 242 eV FWHM @ 20°C

– 163 eV FWHM @ -30°C



• Mean drift length: up to 1.4 mm (CCE>99 % for 30 m)

• Limits: depletion layer depth (detection efficiency)

– residual doping 1014 cm-3

– Epi layer thickness: 40 m


GaAs pixel detectors

• Good Schottky and p+n junction detectors exists !

• Epitaxial thickness >300 m has been grown (Poster: Glasgow-Paris)

Status




• Epitaxial thickness >300 m has been grown (Poster: Glasgow-Paris)

Status

Prospects

• Energy resolution of 180 eV FWHM are reachable at 20°C

• Residual doping ≤1013 cm-3 required

• Integrated spectroscopic-grade front-end electronic required




• Epitaxial thickness >300 m has been grown (Poster : Glasgow-Paris)

Status

Prospects

• Energy resolution of 180 eV FWHM are reachable at 20°C

• Residual doping ≤1013 cm-3 required

• Integrated spectroscopic-grade front-end electronic required


Large Format Detector Readout ASICD. Martin, A. Owens

EUROPEAN SPACE AGENCY, ESA-ESTEC - Noordwjik (NL)

P. Bastia, I. Cappelluti, F. Ferrari, N. Ratti

ALENIA SPAZIO s.p.a. - LABEN - MILANO ( I )

P. Malcovati, F. Borghetti, V. Ferragina

PAVIA UNIVERSITY ( I )

G. Bertuccio, S. Caccia, D. Maiocchi, A. Pullia

POLITECNICO DI MILANO ( I )

A Finmeccanica Company

Readout system &

coordination

Back-end design

Front-end design

Supervision


GaAs 32 x 32 array

1.2 cm

250 m

1.2 cm Design goal E=180 eV @ 5.9 keV

LFDR ASIC for GaAs Pixels


• 0.35 m CMOS (AMS)

• 16 x 16, 32x32, 64x64 pixel channels

• 300 x 300 m2 channel size

• Complete: Preamplifier ADC

• Spectroscopic grade - 500 W/channel (simulated)

• 30 electrons r.m.s. @ 20°C (simulated)

LFDR ASIC (designed – in production)

A Finmeccanica Company


LFDR : ASIC Block diagram

analog cells16x16 - 64x64

cell’s registers

ADC’s

I/O- Interfaces


Silicon Carbide

X-Ray Detectors


Silicon Carbide

X-Ray Detectors

• Introduction on SiC


Silicon Carbide

X-Ray Detectors


• Prototype detectors


Silicon Carbide

X-Ray Detectors



• X-ray spectroscopy at room temperature…and above


Silicon Carbide

X-Ray Detectors



• X-ray spectroscopy at room temperature…and above

• SiC Pixel detectors: a sub-electron electronic noise device


SiC politypes

3C-SiC(EG = 2.2 eV)

4H-SiC(EG = 3.3 eV)

6H-SiC(EG = 3 eV)


SiC propertiesWide Bandgap

EG=3.2 eV

Low thermally generated currents

Room temperature operation

High saturation velocity

vS = 200 m/ns

High frequency/speed

devices

Short transit time

Low trapping probability

High Voltage devices

High Critical Field

EC = 2 MV/cm

High Power devices

High thermal conductivity


SiC High PerformanceElectron Devices…

...a reality


The highest VB diode

Reverse Voltage [kV]

Cur

rent

[ n

A ]

VB = 19 kV

Power MOSFET: 5 kV - 88 m cm2

fT=14.5 GHz, fMAX=38 GHz

IDmax= 400 mA, VDSmax=150V

Gain: 16 dB @ 2 GHz

fT=14.5 GHz, fMAX=38 GHz

IDmax= 400 mA, VDSmax=150V

Gain: 16 dB @ 2 GHz

High Power X-Band MESFET

SiC High Performance Electron Devices


Would SiC be advantageus

for X-Ray Spectroscopy and Imaging ?


SiC Si

Soft X-ray Absorption length

SiCSi

Can SiC compete with Si ? (!)Can SiC compete with Si ? (!)


Electron-hole pairs : Signal !

Q = Eph / Q = Eph /

What about the SiC detector noise ?


Detector noise sources

• Leakage current

• Carriers trapping phenomena

• Dielectric noise

• [Charge collection process (balistic deficit signal

reduction)]


Run of SiC X-Ray Detectors(Technology: Alenia Marconi Systems - Rome)(SiC wafers: CREE Inc., USA)

Schottky contact - Au

n - epilayer 5x1014 - 70 m

n+ buffer 1 m

n+ substrate - 320 m

ohmic contact - Ti/Pt/Au

Array of pixelsArray of pixels


Schottky Barrier Height : b=1.16 eV

I-V Characteristics

Forward bias Reverse bias

!!


Leakage Current DensityState of the art detectors

Si1 nA/cm2

SiC5 pA/cm2

200


Temperature effect: SiC vs. Si

25


Tested devices

• Pixel detector: area=0.03 mm2 ; capacitance: 0.17 pF

• Pad detector : area= 0.3 mm2 ; capacitance : 1.7 pF

Experimental Results on SiC Detectors


SiC detectors at 27 °C ( 300 K )

Energy resolution

415 eV FWHM ( 22 e- r.m.s. )

Energy resolution

415 eV FWHM ( 22 e- r.m.s. )

Energy resolution

315 eV FWHM ( 17 e- r.m.s. )

Energy resolution

315 eV FWHM ( 17 e- r.m.s. )

Pixel : A=0.03 mm2Pixel : A=0.03 mm2Pad : A=0.3 mm2Pad : A=0.3 mm2


SiC pixel at 27 °C ( 300 K )

Energy resolution: 315 eV FWHM ( 17 e- r.m.s. )Energy resolution: 315 eV FWHM ( 17 e- r.m.s. )


Beyond Room Temperature...


Operation beyond room temperature ?

E= 100 kV/cm , T = 27 °C → JR = 14

pA/cm2

T = 100 °C → JR = 0.5

nA/cm2

E= 100 kV/cm , T = 27 °C → JR = 14

pA/cm2

T = 100 °C → JR = 0.5

nA/cm2


R

Termoresistance

SiC Detector

JFETD S

Vout

Temperature

Controller

Vtest

Thermo-controller

Preamplifier

High temperature set-up

Front-end


46 °C ( 319 K - 115 F )

Energy resolution : 330 eV FWHM ( 18 e- r.m.s. )Energy resolution : 330 eV FWHM ( 18 e- r.m.s. )


63 °C ( 336 K - 145 F )



81 °C ( 354 K - 178 F )



100 °C ( 473 K - 212 F )



SiC pixel detector: from 27 °C to 100°C

At high T the resolution is limited by the Front-end, NOT by the detector

At high T the resolution is limited by the Front-end, NOT by the detector


Prototype of SiC Pixel Matrix

• 4 x 4 matrix

• Pixel size: 400 m x 400 m

• 6 x 6 matrix

• Pixel size: 250 m x 250 m


Pixel Forward Bias Characteristic

Saturation current

Is = 3.5·10-18A

Saturation current

Is = 3.5·10-18A

Ideality factor

n = 1.06

Ideality factor

n = 1.06


Measurement of the Pixel Leakage Current

The problem…

SiC Pad detectors : JSiC = 5 – 15 pA/cm2

IREV = 8 fA - 160 fA !!!IREV = 8 fA - 160 fA !!!

Current density of a pixel ?

J = 5 - 100 pA/cm2 , Area = 400 x 400 m2


Instrumention and device noise

± 0.4 fA

Electrometer noise

2 hours

Test-fixture /device

VBIAS=200V; I = 3.16 ± 0.3 fA

VBIAS=0V; I = 0 ± 0.1 fA


Pixel Reverse Bias Characteristic Room Temperature


SiC PixelReverse Bias Characteristic

pA/cm2 @ 27 °CpA/cm2 @ 27 °C

nA/cm2 @ 100 °CnA/cm2 @ 100 °C


Reverse Current Map

Leakage Current @ 27 °C

I = 274 fA : 1 pixel

I = 98 fA : 1 pixel

I = 36 fA : 1 pixel

I < 10 fA : 12 pixel

Leakage Current @ 27 °C

I = 274 fA : 1 pixel

I = 98 fA : 1 pixel

I = 36 fA : 1 pixel

I < 10 fA : 12 pixel

Leakage Current E.N.C. @ 27 °C @ 10s

I = 274 fA : 1 pixel = 5.8 e-

I = 98 fA : 1 pixel = 3.5 e-

I = 36 fA : 1 pixel = 2 e-

I < 10 fA : 12 pixel < 1 e- r.m.s.

Leakage Current E.N.C. @ 27 °C @ 10s

I = 274 fA : 1 pixel = 5.8 e-

I = 98 fA : 1 pixel = 3.5 e-

I = 36 fA : 1 pixel = 2 e-

I < 10 fA : 12 pixel < 1 e- r.m.s.


Summary

• SiC has the same absorption length as Si


Summary


• SiC can give superior resolution at certain T and Areas


Summary



• Pixel and pad detectors have been fabricated and tested


Summary



• Pixel and pad detectors have been fabricated and tested

• Leakage currents: 5 - 14 pA/cm2 @ 27 ° C

500 pA/cm2 @ 100°C


Summary



• Pixel and pad detectors have been fabricated


500 pA/cm2 @ 100°C

• SiC pixel detector: 315 eV FWHM @ 27 ° C

797 eV FWHM @ 100 °C


Summary



• Pixel and pad detectors have been fabricated


500 pA/cm2 @ 100°C

• SiC pixel detector: 315 eV FWHM @ 27 ° C

797 eV FWHM @ 100 °C

• SiC pixel : toward sub-electron (Fano limited) noise


• Thicker epi-SiC layers : t >100 m (now: 50-70 m)

• Ultra-low doping and pure SiC: N 1013cm -3 (now: > 5 1013)

• High purity semi-insulating SiC (?)

• Ultra low noise front-end at room and high temperature

Prospects for SiC detectors:Limits and needed...


Acknowledgements

Prof. Emilio Gatti - Politecnico di Milano

Prof. Claudio Canali - University of Modena

Prof. Filippo Nava – University of Modena

Dr. Antonio Cetronio - Alenia Marconi Systems

Dr. Claudio Lanzieri - Alenia Marconi Systems

Dr. Roberto Casiraghi – ST Microelectronics

Dr. Diego Maiocchi – University of Milano

Dr. Stefano Caccia – Politecnico di Milano

Dr. Enzo Mallardi – Politecnico di Milano

Dr. Marco Mandena – Politecnico di Milano

Documents

Prospects for Energy Resolving X-Ray Imaging with Compound Semiconductor Pixel Detectors