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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
Juozas V.Vaitkus, RD50 3rd Workshop, 2003, CERN
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
Juozas V.Vaitkus, RD50 3rd Workshop, 2003, CERN
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’
Juozas V.Vaitkus, RD50 3rd Workshop, 2003, CERN
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
Juozas V.Vaitkus, RD50 3rd Workshop, 2003, CERN
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
Juozas V.Vaitkus, RD50 3rd Workshop, 2003, CERN
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
Juozas V.Vaitkus, RD50 3rd Workshop, 2003, CERN
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
Juozas V.Vaitkus, RD50 3rd Workshop, 2003, CERN
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=
Juozas V.Vaitkus, RD50 3rd Workshop, 2003, CERN
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
Juozas V.Vaitkus, RD50 3rd Workshop, 2003, CERN
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
Juozas V.Vaitkus, RD50 3rd Workshop, 2003, CERN
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
Juozas V.Vaitkus, RD50 3rd Workshop, 2003, CERN
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
Juozas V.Vaitkus, RD50 3rd Workshop, 2003, CERN
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
Juozas V.Vaitkus, RD50 3rd Workshop, 2003, CERN
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
Juozas V.Vaitkus, RD50 3rd Workshop, 2003, CERN
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)
Juozas V.Vaitkus, RD50 3rd Workshop, 2003, CERN
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
Juozas V.Vaitkus, RD50 3rd Workshop, 2003, CERN
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
Juozas V.Vaitkus, RD50 3rd Workshop, 2003, CERN
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
Juozas V.Vaitkus, RD50 3rd Workshop, 2003, CERN
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!