Marko Miku ž University of Ljubljana & Jo ž ef Stefan Institute Ljubljana, Slovenia

Study of Polycrystalline and Single Crystal Diamond

Detectors Irradiated with Neutrons up to 1016 cm-2

Marko MikužUniversity of Ljubljana & Jožef Stefan

InstituteLjubljana, Slovenia

IEEE NSS’07 N44-5

Hawaii, November 1, 2007

Hawaii, November 1, 2007 Irradiation of diamond sensors Marko Mikuž 2

Collaboration

JSI & Univ. Ljubljana, Slovenia M. Mikuž, V. Cindro, I. Dolenc, A. Gorišek,

G. Kramberger, I. Mandić, M. Zavrtanik Ohio State University, USA

H. Kagan, S. Cline, S. Smith University of Toronto, Canada

W. Trischuk

Work performed as part of CERN RD-42 programme


Word of caution

Irradiations and measurements were all performed during the last two months, most of them even in the last two weeks All results strictly preliminary No time yet for a real systematic study 1016 n/cm2 data not available yet – up to

3x1015 n/cm2 scCVD data just starting

But still lots of interesting data available


Aim of study

Diamonds are proposed as sensor material for the innermost tracker layers at sLHC

Fluence benchmark up to 3x1016 particles/cm2 Mosty pions, ~10 % neutrons

Most irradiations use protons from CERN PS Thought to be representative for charged particle

damage NIEL in Si as scaling factor taken for granted NIEL violations observed in Si

Neff in oxygenated Si Trapping p vs. n

NIEL for diamonds much smaller Is NIEL representative of damage ? Si C

W. de Boer et al.arXiv:0705.0171v1


Diamond as sensor material

Property Diamond SiliconBand gap [eV] 5.5 1.12 Low leakage

Breakdown field [V/cm] 107 3x105

Intrinsic resistivity @ R.T. [Ω cm] > 1011 2.3x105

Intrinsic carrier density [cm-3] < 103 1.5x1010

Electron mobility [cm2/Vs] 1900 1350

Hole mobility [cm2/Vs] 2300 480

Saturation velocity [cm/s] 0.9(e)-1.4(h)x 107 0.82x 107

Density [g/cm3] 3.52 2.33

Atomic number - Z 6 14

Dielectric constant - ε 5.7 11.9 Low capacitance

Displacement energy [eV/atom] 43 13-20 Radiation hard

Thermal conductivity [W/m.K] 2000 150 Heat spreader

Energy to create e-h pair [eV] 13 3.61

Radiation length [cm] 12.2 9.36

Spec. Ionization Loss [MeV/cm] 4.69 3.21

Aver. Signal Created / 100 μm [e0] 3602 8892 Low signal

Aver. Signal Created / 0.1 X0 [e0] 4401 8323


Diamond sensor types - pCVD

• Polycrystalline Chemical Vapour Deposition (pCVD)

– Grown in μ-wave reactors on non-diamond substrate

– Exist in Φ = 12 cm wafers, >2 mm thick

– Small grains merging with growth

– Grind off substrate side to improve quality → ~500 μm detectors

– Base-line diamond sensor material

Test dots on 1 cm grid

Surface view of growth side

Side view

All photographs courtesy of Element Six & OSU


Diamond sensor types - scCVD

• Single Crystal Chemical Vapour Deposition (scCVD)– Grown on diamond substrate

– RD-42 has research contract with E6 to develop this material

– Exist in ~ 1 cm2 pieces, max 1.4 cm x 1.4 cm, thickness > 1 mm

– A true single crystal

Size limit - not a real option for sensors at this moment After heavy irradiations expect similar properties to pCVD Clean environment for study of basic material properties


No processing: put electrodes on, apply electric field

Trapping on grain boundaries and in bulk much like in heavily irradiated silicon

Parameterized by Charge Collection Distance, defined by

CCD = average distance e-h pairs move apart Coincides with mean free path in infinite

(t ≫ CCD) detector

Signal from pCVD diamonds

hicknessdetector t -

apart moveh -e distance

t

dddt

dQQ

he

createdcol

μme

36 0

colQ

CCD

CCD measured on recent1.4 mm thick pCVD wafer

mean notmost probable


Radiation Damage - Basics

Charge trapping the only relevant radiation damage effect NIEL scaling questionable a priori

Egap in diamond 5 times larger than in Si Many processes freeze out Typical emission times order of months

Like Si at 300/5 = 60 K – Boltzmann factor Lazarus effect ? Time dependent behaviour

A rich source of effects and (experimental) surprises !

Radiation induced effect

DiamondOperational consequence

SiliconOperational consequence

Leakage currentsmall &

decreasesnone

I/V = αΦ

α ~ 4x10-17 A/cm

Heating

Thermal runaway

Space charge ~ none noneΔNeff ≈ -βΦ

β ~ 0.15 cm-1

Increase of full depletion voltage

Charge trapping YesCharge loss

Polarization

1/τeff = βΦ

β ~ 5-7x10-16 cm2/ns

Charge loss

Polarization

t

thttteff

vPN

)1(1


0

50

100

150

200

250

300

107-2 107-3 107-4 107-5 107-6 107-8 107-9

Diamond

CC

D [m m

]

ccd+

ccd-

Samples and irradiations

7 pCVD and 2 scCVD acquired from Diamond Detectors Ltd. (ex. Element Six)

All 5x5 mm2

Metallization and initial test at OSU CCD between 205 and 250 μm Leakage currents < 10 pA @ 1 kV

1 scCVD ok, 1 exhibits large polarization Suspect surface problems →return to DDL

2 pCVD to PSI pion irradiation in Sep’07 Thanks to Maurice Glaser

200 MeV π+ beam Most representative of LHC Plan for 1 and 2x1015 π/cm2

Obtained 3.2 and 6.1 x1014 π/cm2

Beam problems at PSI

OSU CCD measurements

PS

I ir

radi

atio

n s

ite


Neutron irradiations

TRIGA Mark II research reactor at JSI Irradiation channel in reactor core Flux ~2x1012 neq/cm2 @ full power

Scalable down with power 1016 reached in a good hour !

Dosimetry well established Extensively used by RD-48, 50 Threshold activation Core simulations Standard Si diodes Scales perfectly with time at stable reactor power

Spectrum with two dominant components Thermal – important for activation Fast – exclusive contribution to NIEL in Si

2 samples irradiated to same fluences in log steps One pair: 1014, 1015, 1016 (done, ½ done, to be done) Second: 3x1014, 3x1015

Programme in progress, next step when data understood

Th

ermal

Fast


Sample evaluation

Charge integrating TCT set-up DAQ chain (rate to disk ~ 50 Hz)

ORTEC 142B preamplifier custom made 25 ns shaping amp. Tex 2440 oscilloscope connected

to PC Triggered only by electrons fully

traversing diode ~98% purity assures good

measurements also at low S/N<1 Features

Peltier cooling up to T=-30°C (stable to 0.1°C)

HV (bias) up to 5 kV Full computer control (automatic scans)

Routinely used in Si detector studies Gain calibrated on 241Am photo-peak Absolute charge collection

measurement

Amplifier+shaper

90Sr

water cooled heat sink

cold plate

Peltier cooler scintillator

pad detectorTex2440

thermal insulation


0

50

100

150

200

250

-1500 -1000 -500 0 500 1000 1500

Bias voltage [V]C

CD

[m m

]

neg

pos

First results Non-irradiated samples

CCD of sensors as received agrees within 10 % with OSU data

Signal drops in first hour, then stable on days scale

CCD/V can exhibit polarization effects Proceed to irradiations

1014 and 3x1014 n/cm2

Results initially depressing Large CCD drop observed Signal decreases with time

First pion irradiated sample 3.2x1014 π/cm2

CCD droped by ~20 % Signal increases with time

Are neutrons really that much more damaging than pions ??

Thought of 1015 π/cm2

3x1014 n

3x1014 π

~2h

~1h


Time heals all wounds

“Discovered” pumping 37 MBq 90Sr source too well collimated Pumping slow – takes days to saturate After ~3 days complete CCD recovery for 3x1014 n/cm2

! Need about 10 kRad ionization Automatic in experiment – 10 kRad = 3x 1011 π/cm2

Stable but don’t shine UV on it ! Pumping method

Take 90Sr source out of collimator and put it 4 mm over detector

Pump & check CCD until saturated Move source to collimator Start measurement

Possible qualitative explanation (same as for Lazarus)

Traps get filled by ionization current Cannot trap same carrier any more

Hole and electron trap densities roughly match Saturated space charge compensated

In reality plenty of traps with different properties gt, Et, charge states, capture σ, ….

Happy with results, proceed to 1015, 3x1015 n/cm2

~3 days

Pumping step ~ 20 min

Q-T

CT

sig

nal

[m

V]

~2 hours


At 1015& 3x1015 n/cm2 pumping hits its limits, CCD decreases Residual trapping starts to dominate over trapping on grain

boundaries CCD vs. voltage not saturated @ 1000 V For mean free path in infinite detector expect

With CCD0 initial trapping on grain boundaries, k a damage constant

L107-9 (3e15 n)

0

10

20

30

40

50

60

70

-1500 -1000 -500 0 500 1000 1500

Bias [V]

CC

D [m

m]

L107-9 (3e15n)

Neutrons strike back

3x1015 n

1015 n

kCCDCCD 0

11

Not saturated @

1 kV !


Results summary

Summary of all CCD measurements Neutron curve drawn by hand to match

data according to expected CCD dependence

Parameters used CCD0 = 210 μm k = 3.5x10-18 μm-1cm-2

Definitely too simplistic Material not infinite Errors to be understood

Pion data look overshooting Lack high fluence data to pin down pion

curve Confront with proton data


To do list

Finish irradiations Carefully re-check procedures Check contacts Exchange samples with OSU Add proton data Evaluate scCVD sample, now at 1014

n/cm2

Most apporpriate for basic material study What’s going on in diamond ?

TCT with α particles Large signal enables auto-triggering Can in principle resolve space charge and

trapping TCT of non-irradiated scCVD gives reasonable

data on drift velocity saturation

scCVD TCT

Transient time

Cu

rren

t

Marko Miku ž University of Ljubljana & Jo ž ef Stefan Institute Ljubljana, Slovenia

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