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Juozas V.Vaitkus, RD50 3rd Workshop, 2003, CERN

GaN for ionizing radiation detectors

J. Vaitkus1,W. Cunningham2, E.Gaubas1, V.Kazukauskas1,

M. Rahman2, P.J. Sellin3, K.Smith2, A.Zukauskas1

1Institute of Materials Science and Applied Research, Vilnius University2Department of Physics, University of Glasgow

3Radiation Imaging Group,Department of Physics, University of Surrey


Introduction

An initial characterisation has been made of epitaxial GaNdetectors - a material of potential interest for X-ray imaging and charged particle detectionSI epitaxial GaN of 2 µm thickness has been studied with alpha particles:⇒ good charge collection efficiency ; a start of 0.45 mm high resisitivity GaN investigations.CV, luminescence and other analysis used to characterise the initial and irradiated samples


Photon Detection Efficiency for GaN

Photon energy (keV)50 100 150 200 250

Det

ectio

n E

ffici

ency

(%)

1

10

100

AlN

GaAsGaN

CdTeSi

Photon detection efficiencyfor 500 µm thick materials

Atomic numbers of 31 and 7 provide good photoelectric absorptioncross-sections:

Modelled using ‘XCOM’


Growth of epitaxial GaNThin epitaxial GaN layers are grown by metal organic chemical

vapour deposition (MOCVD) - University of Tokushima, JapanGrowth process uses sapphire as a substrate, with an n-type

conducting GaN buffer layerHigh purity semi-insulating (SI) GaN layer is grown on top of the

buffer layerTypical SI GaN layer thickness is 2.5 µmMobility and carrier density can be optimised by varying growth

temperature

sapphire substrate

Schottky contacts

high resistivity doped GaN (2.5µm)

n-type undoped GaN (<1µm)

T. Wang et al, Applied Physics Letters 76 (2000) 2220-2222


IV characteristicsIV data on epitaxial device shows good diode characteristics from Schottky

contact. Reverse-bias current ~10-12 A at -15V (J = 6x10-11 A/cm2)

Volts (V)

-15 -10 -5 0 5 10 15

Cur

rent

(A)

1e-13

1e-12

1e-11

1e-10

1e-9

non-irradiatedXray irradiatedneutron irradiated

Some degradation in IV behaviour is observed in irradiated devices


Bragg curves in GaNAlpha particle range is much greater than 2 µm active thickness of

GaN layer:

Bragg Curves for GaN

Distance (um)

0 2 4 6 8 10 12 14 16 18

Ene

rgy

Loss

dE

/dx

(keV

/um

)

0

100

200

300

400

500

600

5.49 MeV alphas

2 MeV alphas

Energy deposited by alpha particles in 2 mm of GaN estimated from Bragg curves (SRIM):

2 MeV alpha: 949 keV

5.49 MeV alpha: 553 keV


Alpha particle spectra

V1/2

0 1 2 3 4 5 6

Ene

rgy

in s

ilico

n (k

eV)

100

200

300

400

500

600

Coefficients:c = 137.1540198413 keVm = 70.3582289899 keV/ch

Room temperature alpha particle spectra:

⇒ systematic increase in measured energy as a function of bias voltage, as field strength increases

⇒ no evidence for charge trapping in this thin material

Plotting alpha peak centroid vs V1/2:

⇒extrapolate to deposited energy in 2 µm of 553 keV

⇒ shows full charge extraction from layer at V=35V


CV analysis of GaNThe nett impurity concentration ND is calculated from room temperature CV

measurements using

For 1.5 mm diameter contact pads, ND = 1x1013 cm-3 - good quality material

Volts (V)

4 6 8 10 12 14 16 18 20 22

1/C

2 (F-2

)

2.0e+21

2.1e+21

2.2e+21

2.3e+21

2.4e+21

2.5e+21

2.6e+21

2.7e+21

grad = 3.67 x1019 F-2V-1

N = 4.3 x 1012 cm-3

dVCdAqN

RD )1(

222

0εε=


Alpha particle response2 devices were irradiated to investigate radiation hardness - with

600 MRad X-rays and 5x1014 cm-2 neutrons:

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80

100

200

300

400

0 V 2 V 4 V 6 V 10 V 14 V 22 V 28 V

max CCE 92% (0.51 MeV)

Cou

nts,

rel.u

.

Energy, MeV

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

0

250

500

0 V 1 V 4 V 8 V 14 V 29 V Gauss 0.51 eV

92 % CCE (0.51 MeV)

Cou

nts

(arb

. uni

ts)

Energy (MeV)

0.0 0.1 0.2 0.3 0.4 0.5 0.6

0

500

1000

0 V 1 V 2 V 4 V 9 V 18 V 28 V 30 V

max CCE 77% (0.434 MeV)

Cou

nts

(arb

. uni

ts)

Energy (MeV)

Nonirradiated SI-GaN

SI-GaN irradiated by neutrons, >100 keV, up to the fluence of 5·1014cm-2.

SI-GaN Irradiated by X-rays, 10 keV, 600 Mrad


Alpha particle response of irradiated devices

Comparison of GaAs and GaN irradiated by neutrons:

0 1 2 3 4 5 60

10

20

30

40

50

60

70

80

90

100

C.C

.E.,

%

Fluence, 1014/ cm2

GaAs GaN

GaAs from: Bates, R.L., Da Via C., D'Auria S., O'Shea V., Raine C., Smith K.M. and RD8 collaboration. The effects of radiation on gallium arsenide radiation detectors. Nuclear Instruments and Methods in Physics Research Section A, 21997; 395:54-59


2 devices were irradiated to investigate radiation hardness - with 600 MRad X-rays and 5x1014 cm-2 neutrons:

0 10 20 30 40 501

2

4

68

10

20

40

60

GaN1030 τexc= 30ps// λex=350 nm

neutron irradiated 5*1014 n/cm-2

X-ray irradiated 600 Mrad non-irradiated

I CP

C (

a.u.

)

t (ms)

Photoconductive Decay measurements


Ion Beam Induced Charge (IBIC) measurementsSurrey facility

Ion Beam Induced Charge (IBIC) data provides:

- high spatial resolution imaging of charge signal uniformity- single event detection

(~1 kHz event rate onsample)

- true bulk measurement

Ion beam is incident orthogonal to one of the electrodes:- charge drift through the SI GaNlayer to the conducting buffer layer- alpha beam cannot pass through silver DAG


Alpha IBIC spectra from GaNSRIM estimate of energy loss in 2 µm layer = 950 keVSaturation of peak centroid at V=25V

Channels

0 50 100 150 200

Cou

nts

1e+1

1e+2

1e+3

1e+4

1e+5

25V

20V

IBIC pulse height spectrum from 2 MeV alpha particles


Photoconductivity decay measurementsPhotoconductive decay time constants have been measured in the

3 samplesNeutron-irradiated sample shows higher intensity slow components

0 5 10 15 20 25 30

101

Asymptotic CPC decays GaN1030 τexc= 30ps// λex=350 nmneutron irradiated 5*1014 n/cm-2

neutron irradiated 5*1014 n/cm-2

X-ray irradiated 600 Mrad non-irradiated

I CPC

(a.

u.)

t (ms)

0 2 4 6 8 10 12 14

1

10

0 2 4 6

0,1

1

10

0 2 4 6

1

10

I CP

C (a

.u.)

tα=0.7 (msα)

as-grown X-ray irradiated 600 Mrad 5*1014 neutron irradiation

t0.3


Photoluminescence spectra measurements

2.0 2.5 3.0 3.50.01

0.1

1

10

100

PL

inte

nsity

(a.u

.)

Photon energy (eV)

Iex=0.04 a.u. =1

2.0 2.5 3.0 3.50

10

20

30

40

50

60

70

GaN cw excitation 3,81 eV as-grown 600 Mrad X-ray irradiated 5*1014 cm-2 neutron irradiated

PL

inte

nsity

(a.

u.)

Photon energy (eV)


Thermally stimulated conductivity measurements, SI-GaN

0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.010 0.011

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

-30V 30V -10V 10V

TSC

(A)

1/T (1/K)

neutron irradiated SI-GaN

0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.010 0.0111E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

-10V 10V -30V 30V

TSC

(A)

1/T (1/K)

X-rays irradiated SI-GaN

0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.010 0.011

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

-10V 10V -30V 30V

Dar

k cu

rren

t, A

1/T (1/K)

X-rays irradiated SI-GaAs


TSC and I-V measurements, HR-GaN

0.004 0.006 0.008

10-3

TSC Dark current

I, A

1/T (1/K)

23 meV

42 meV

-4 -2 0 2 4

-8

-6

-4

-2

0

2

4

6

8

Dark Illuminated

Room T

I, m

AU, V


TSC and I-V measurements, HR-GaN

0 20 40

5.0x101

1.0x102

1.5x102

2.0x102

I CPC

(a.

u.)

t (µs)

0.28 µs 84 µs

0 5 10 15101

102

103

UC

PC (

a.u.

)

t (ms)

0.5µs 0.6 µs 6.2 µs 6.3 µs

2.0 2.5 3.0 3.5

100

101

102

103

UV PL band

Exc-LO

Exc

Band-to-band

Blue PL band

Yellow PL band bulk GaN H103_1

hνexc= 3.81 eV

20mW HeCd CW laser

T = 300 K

PL

Inte

nsity

(ar

b. u

nits

)

Photon energy (eV)

Iex=I0 Iex= 6*I0 Iex= 10*I0


Conclusions

We have carried out a preliminary characterisation of threeepitaxial SI-GaN detectors:High purity SI GaN detector of 2 µm thickness shows good alpha particle response.

Gold Schottky contacts show good diode behaviour - reverse bias leakage current is ~10-12 A at -15V (J = 6x10-11 A/cm2)

CV analysis gives a carrier concentration of 1x1013 cm-3

initial IBIC imaging shows excellent material uniformity and ~100% CCE in very thin layers

Luminescence, lifetime and TSC shows the defect types

further measurements are required with thicker epitaxial layers, whilst maintaining ~ 1013 cm-3 carrier concentration. What will show the thick one? – Next workshop!

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