PbIPbI22 as a direct as a direct semiconductor for use in semiconductor for use in radiation imaging radiation imaging detectorsdetectors

Glenn C. Tyrrell and Jonathan P. CreaseyGlenn C. Tyrrell and Jonathan P. Creasey

Applied Scintillation Technologies LtdFluorescent & Scintillation Products for Industry, Science & Medicine

OverviewOverview

Motivation Motivation Why PbIWhy PbI22?? Material ProcessingMaterial Processing Optical SpectroscopyOptical Spectroscopy Electrical CharacteristicsElectrical Characteristics PerspectivesPerspectives ConclusionConclusion

MotivationMotivation

Inexpensive ($10-20/sq.in) deposition on an a-Inexpensive ($10-20/sq.in) deposition on an a-Si active matrix flat panel imager (AMFPI)Si active matrix flat panel imager (AMFPI)

Target market – Medical x-ray fluoroscopy Target market – Medical x-ray fluoroscopy (visualisation on catheter and stent based (visualisation on catheter and stent based procedures in primarily in thoracic areas)procedures in primarily in thoracic areas)

Direct detection reduces AMFPI costs by Direct detection reduces AMFPI costs by eliminating the photodiode component and eliminating the photodiode component and significantly cutting extensive processing significantly cutting extensive processing steps (e.g. GE/NIST)steps (e.g. GE/NIST)

To reduce dark current value to <10 nA/cmTo reduce dark current value to <10 nA/cm22

To reduce image lag for the high frame rate To reduce image lag for the high frame rate (30 fps) fluoroscopy applications (30 fps) fluoroscopy applications

Why PbIWhy PbI22??

layered semiconductor, anisotropiclayered semiconductor, anisotropic hexagonal close packed (HCP)hexagonal close packed (HCP) av. absorption coeff. 57 cmav. absorption coeff. 57 cm-1-1

k edges (88; 33.1)k edges (88; 33.1) density, 6.2g/cmdensity, 6.2g/cm33

band gap, 2.55eV - band gap, 2.55eV - indicates that devices should operate a indicates that devices should operate a low leakage currents at high temperature. low leakage currents at high temperature.

carrier mobilitiescarrier mobilities– electrons electrons 8 cm8 cm22/V s/V s– holes holes 2 cm2 cm22/V s/V s

– electrons electrons 1010-5 -5 cmcm22/V /V – holes holes 2.102.10-6 -6 cmcm22/V /V

conversion efficiency conversion efficiency approx.240 eapprox.240 e--/keV/keV

conversion efficiency x5 CsI:Tl, Gd2O2S:Tbx10 a-Se

(~5 eV/e-)

Who has investigated lead Who has investigated lead iodideiodide

......and for what application ......and for what application?? Nuclear spectrometers Nuclear spectrometers Radiation Monitoring Devices, Inc (RMD) Watertown, MA, US Radiation Monitoring Devices, Inc (RMD) Watertown, MA, US - K.Shah et al (1990’s - K.Shah et al (1990’s

- )- ) Tohoku University, JapanTohoku University, Japan - T.Shoji et al (1990’s - )- T.Shoji et al (1990’s - ) Hebrew University of Jerusalem,Hebrew University of Jerusalem, - M. Roth et al (1980’s - )- M. Roth et al (1980’s - ) Fisk University, Nashville, TN, USFisk University, Nashville, TN, US - A. Burger (1980’s - )- A. Burger (1980’s - ) University of Bari, ItalyUniversity of Bari, Italy - C. Manfredotti (1977)- C. Manfredotti (1977) SiemensAG, Erlangen, GermanySiemensAG, Erlangen, Germany - S. Roth and W.R. Willig - S. Roth and W.R. Willig

(1971)(1971)

Advanced solid state batteries Advanced solid state batteries (Ionic Conductivity)(Ionic Conductivity) Sandia National Labs, Albuquerque, NM, US Sandia National Labs, Albuquerque, NM, US - G.A. Samara(1970’s/80’s)- G.A. Samara(1970’s/80’s) University of Illinois, Urbana-Champaign, IL, USUniversity of Illinois, Urbana-Champaign, IL, US - J. Oberschmidt (1970’s/80’s)- J. Oberschmidt (1970’s/80’s)

Image recording and high resolution photographyImage recording and high resolution photography University of Bristol, UKUniversity of Bristol, UK - A.J. Forty et al (1950’s-1960’s)- A.J. Forty et al (1950’s-1960’s)

A process for manufacture of A process for manufacture of PbIPbI22 thick films thick films

Purification (zone refining)• initial high quality feedstock• high level of purification (ppb)

Thick film deposition• semiconductor cleanliness• control of polycrystallinity• control of stoichiometry

Compression• even compression, contact method • what effects does it have on structure and polytype formation (XRD). Spectroscopy

RAW MATERIAL

ZONE REFINING

THERMAL EVAPORATION COMPRESSION

Zone refiningZone refining

RF Coils

Graphite susceptor

Porous alumina crucible

Quartz ampoule

Glass envelope

PbI2 ingotMolten

zone

vacuum

Ar• RF heating enables larger diameter quartz ampoules to be used; therefore larger batches of purified PbI2

• Encapsulating chamber allows vacuum or inert gas environment; therefore can be used for removing volatile components prior to zoning when ampoule is not enclosed• Evolving design of graphite susceptors to optimise zoning process.

Deposition systemDeposition system

ROTARY

CRYOPUMP

EXHAUST FILTER

FORELINE TRAP

PIRANI

ROUGHING VALVE

FORELINE VALVE

THROTTLE VALVE

PIRANI

CHAMBER

ROUGHING LINE

VENT

GATE VALVE

SCHEMATIC OF PUMPING SYSTEM

ELBOW

FLEXIBLE HOSE

FLEXIBLE HOSE

FLEXIBLE HOSE

FLEXIBLE HOSE

OPTIONAL

MASS SPECTROMETER

• Large area deposition system for PbI2 on amorphous silicon flat panels• box system w24” x h30” x d30”• front door, allows easy access and maintenance of source, shields, substrate mounting, etc.• allows large panel deposition• cryopump for clean pumping• mass spectrometer for process control and quality/reproducibility monitoring• side mounted pumping for ease of access to source heat, feedthroughs, substrate.• provision for additional gas lines, iodine compensation, annealing, etc.• provision for high pressure analysis with differentially pumped mass analyser•

Surface morphology of lead Surface morphology of lead iodide filmsiodide films

compressed

as deposited

Film thickness variable100 – 500 m

CompressionIncreases densityReduce voidsIncreases microcrystallite

contact

What effect does compression have on the optical and electrical characteristics of the layer?

Low temperature (10K) Low temperature (10K) photoluminescencephotoluminescence

wavelength (nm)

480 490 500 510 520 530 540 550 560

ph

oto

lum

ine

sc

en

ce

in

ten

sit

y (

a.u

)

0

1

2

3

4

5

6

7

T = 12K

360nm cw excitation

Phonon replica

Green edge

Exciton

Photoluminescence spectra of high quality lead iodide (zone refined) I

T = 10K

Photon wavelength (nm)

490 495 500 505 510 515 520 525 530 535

PL

Inte

nsi

ty (

a.u

)

0

2

4

6

8

10

12

14

550 600 650

4

6

8

10

12

14

Photoluminescence spectra of high quality lead iodide (zone refined) II

Deep trap region

Emission Wavelength (nm)

480 500 520 540 560 580 600 620 640 660

PL

In

ten

sit

y (

a.u

)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Increasing compression inducesdeep level traps

Low temperature photoluminescence of compressed PbI2

Electrical measurementsElectrical measurements

V Source:Keithley Instruments 237

Quasistatic-CV:Keithley Instruments 595 Adapters

Test Station:Hewlett-Packard S1160

PC control andacquisition

Network

CV:Keithley Instruments 590

• Dark current• I-t• I-V• C-V

Dark current Dark current measurementsmeasurements

I (A)

Time (s)

Time (s)

Dielectric interlayer (Parylene)100 pA/cm2

Post-processed PbI21 nA/cm2

Semiconductor Semiconductor contactscontacts

graphite PbI2 breakdownfrom underside of contact

Material choice dictates contact stability, e.g. Ag promotes rapid failure; Au, Te and colloidal graphite

Other failure modesOther failure modes

• Device drive conditions (V/cm-1)• Operational temperatures• Other material impurities

EDX spectra of filmsEDX spectra of films

High K Low K

Contact stability in conditions with minimal potassium contamination

PerpectivesPerpectives

Project ceased due to reorganisation of key Project ceased due to reorganisation of key account partneraccount partner

Further work is required to optimise Further work is required to optimise contact technology and drive contact technology and drive characteristicscharacteristics

IP for process in progessIP for process in progess Key account partner being sought for Key account partner being sought for

taking project development to the next taking project development to the next stagestage

……any offers???any offers???

ConclusionsConclusions

Cost effective large area deposition achieved

Compression proven to be ineffective for high quality imaging layers

excessive deep level trapping resulting in image lag

Dark current targets achieved and exceeded

Optimum drive conditions and ultimate performance not yet established

AcknowledgementsAcknowledgements

Dr Bhaswar Baral – Dr Bhaswar Baral – materials growthmaterials growth

Dr Xuefeng Liu – Dr Xuefeng Liu – electrical characterisationelectrical characterisation

Dr Derek Day – (Dr Derek Day – (formerly of Varian Medical Systems, Sunnyvale, formerly of Varian Medical Systems, Sunnyvale, CA)CA)

analysis of electrical data analysis of electrical data Terry Brown -Terry Brown -(Metal Crystal and Oxides Ltd, (Metal Crystal and Oxides Ltd, Harston, Cambridge, UKHarston, Cambridge, UK))

for advising on, and supplying, RF zone refining for advising on, and supplying, RF zone refining systemsystem

Applied Scintillation Technologies Ltd

Fluorescent & Scintillation Products for Industry, Science & Medicine

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