Effects of metallization on TlBr single crystals for detector applications

PSD-7, Liverpool, GB, 2005

Effects of metallization on TlBr single crystals for detector applications

V. Kozlova, M. Leskeläa, M. Vehkamäkia and H. Sipiläb

a Department of Chemistry, University of Helsinki, Finlandb Oxford Instruments Analytical Oy, Espoo, Finland


TlBr properties

• high atomic numbers Z: 81+35 => stopping power

• density (7.56 g/cm3) => compact device

• bandgap (2.68 eV) => room temperature

• inter-pixel resistance ~500 GΩ (gap 100μm, 50V)

=> 2D-array detector*

• optical transparency: 440nm – 50µm => scintillator?

*Owens et al., Nucl. Instr. and Meth. A 531, 18 (2004)


Material problems

• Purity

• Crystal quality

• Manufacturing process

• TlBr – toxic compound

(Knoop hardness of 12 kg/mm2)

Na Pb Nb Sb Al Co S Ca V Si P Pd Pt Cr Cu Fe Mg Zn Sum:0.1

1

10

100

Improving during re-crystallistaion and annealing

S/SR D/DR D/DA

Co

nce

ntr

atio

n, p

pb

Elements

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

0

200

400

600

800

1000

SourceSet67

13(1) 2(1)2=39.134

Hydrothermal Annealing, 150oC

An04Set55

Inte

nsity

, cn

ts

scan angle


Metallization methods

Chemical: photographic process, Pd-dielectrics => possible water inclusion => X

Glue: Ag- and graphite-paste=> crater formation, if poor crystal quality properly treated samples: 2.5 kV/cm

Physical: vapour (PVD), electron beam and sputtering deposition methods =>

Melt: metal melting on TlBr surface =>


Interaction at the interface

“Adhesion”:physical deposition

“Diffusion” physical or melt methods

“Compound” melt deposition orphysical + annealing

-20 -15 -10 -5 0

0

2

4

6

8

10

Depth across interface, A.U.

Me

con

cen

trat

ion

, A.U

.

ADHESION DIFFUSION COMPOUND


Metal – Tl halide interface parameters

“Adhesion”

“Diffusion”

“Compound”

Metal mp, oC Tensile strength*

Stability* Electro-negativity

Cu 1083 24 Atm 1.9

Ag 962 27 Atm 1.9

Au 1064 27 Atm 2.4

Zn 419 13 1.6

Cd 321 36 1.7

Al 660 Split. Bad 1.5

In 157 18 Good 1.7

Tl 304 1.8

Sn 232 6 1.8

Ti 1660 18 Good 1.5

Cr 1857 14 Good 1.6

Fe 1535 23 Good Atm 1.8

Co 1495 22 Good 1.8

Ni 1453 22 Atm 1.8

Mn 1244 X Bad 1.5

Mg 649 X Bad 1.2

Abr.: Atm. - unstable in atmosphere,

Split. – SplittingX – Chemical reaction

*Me-thickness – 0.5 μm units - MN/m2 [6, 8]

TlBr (mp = 460oC)


Results: Vacuum deposition

Electron beam evaporation: thickness 25 and 40 nm Ti, Cr and Al

Ti and Cr: good electrical contacts, stable for ~year in a laboratory environment

Al: stable during all measurements for several days

Fe and Ni – direct contact with TlBr molten => No reaction, if pure metals and complete degassing of system=> Ni-boat was used for the melting of TlBr powder

Output: - Ti, Cr, Fe and Ni elements were inert to TlBr - Ti and Cr could be used as reliable electrodes- Al ?


Results: Al/TlBr interface

TlBr was deposed on a clean Al surface:(surface temperature control)

• t < 100oC => fine films, R% spectra=> stable for ~year, XRD

• t > ~150oC => Tl-drops on Al=> amorph. phase + Tl

peaks

Al(solid) + 3TlBr(gas) => 3Tl(drop) + AlBr3(gas)

Output: no heating, low current

100 200 300 400 500 600 700 800 900 1000 1100

0

3

6

9

12

15

Al4v1cNor Al3v1cNor Al2v1cNor Al1v1cm90

R,

%

, nm

15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

0

50

100

150

200

250

310210

200

111220

211

110

100 (311)

220

200

111

TlBr Al

Inte

nsity

, cnt

s

2


Results: Tl/, In/,Sn/TlBr interfaces

Tl, In and Sn were shortly melted at ~310 oC: ( N2 atmosphere)

• Sn: no reaction, no underlayer

• In: Yellow underlayer compound”pressed drop” on surface

• Tl: Brown underlayer compoundeasily peeled off from TlBr

+ New process + annealing 20h=> active reaction

Output: Tl - aggressive with TlBr

TlBr

metal drop

underlayerIn


Results: In/,Sn/TlBr interfaces

In and Sn were shortly melted at ~250 oC: (opposite side)

• Sn, In: no reaction, no underlayer (In-case)

• Sn: I-V characteristics of Sn-TlBr-Sn were asymmetrical for both electrodes

=> higher temperature for the intermediate compound

Output: Sn electrode was reliable at our conditions.

TlBr

In

In


Results: In/TlBr interface

• In: I-V curves of In-TlBr-Layer-In => rectifier effect was observed

Output: low crystal quality, inner boundaries still limit the use of TlBr detector

-120 -100 -80 -60 -40 -20 0 20 40 60-6.0n

-3.0n

0.0

3.0n

6.0n

9.0n

12.0n

15.0n

18.0n

SL04: Signal to OldDrop of In.

iPPM iMMP

Cur

rent

, nA

Bias Voltage, V

(+)

(-)• High currentconnections:

Breakdown during further I-V & CVat ~200 V/cm

Compare: working field 500 V/cm,Owens et al., Nucl. Instr. and Meth. A 531, 18

(2004)


Conclusions

• Ti, Cr, Fe and Ni elements were inert to TlBr and Ti and Cr could be used as reliable electrodes.

• Al contact can be used for tests at low current without heating.

• Tl is reacting aggressively with TlBr.

• Sn electrode was reliable at our conditions.

• In was shown to form intermediate layer compound.

• In/TlBr interface possesses rectifier properties; however, the crystal quality currently limits the use of this interface.


Acknowledgements

• Crystal growth: I.S. Lisitsky and M. Kuznetsov (GIREDMET, Russia)

• Finnish Technology Agency TEKES


END

• Thank You!

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Effects of metallization on TlBr single crystals for detector applications