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Review of using gallium nitride for ionizing radiation detectionJinghui Wang, Padhraic Mulligan, Leonard Brillson, and Lei R. Cao Citation: Applied Physics Reviews 2, 031102 (2015); doi: 10.1063/1.4929913 View online: http://dx.doi.org/10.1063/1.4929913 View Table of Contents: http://scitation.aip.org/content/aip/journal/apr2/2/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Radiation hardness of three-dimensional polycrystalline diamond detectors Appl. Phys. Lett. 106, 193509 (2015); 10.1063/1.4921116 Neutron detection using boron gallium nitride semiconductor material APL Mat. 2, 032106 (2014); 10.1063/1.4868176 Narrow-band radiation sensing in the terahertz and microwave bands using the radiation-inducedmagnetoresistance oscillations Appl. Phys. Lett. 92, 102107 (2008); 10.1063/1.2896614 Efficiency of composite boron nitride neutron detectors in comparison with helium-3 detectors Appl. Phys. Lett. 90, 124101 (2007); 10.1063/1.2713869 Fundamental limits to detection of low-energy ions using silicon solid-state detectors Appl. Phys. Lett. 84, 3552 (2004); 10.1063/1.1719272
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APPLIED PHYSICS REVIEWS—FOCUSED REVIEW
Review of using gallium nitride for ionizing radiation detection
Jinghui Wang,1,2 Padhraic Mulligan,1 Leonard Brillson,3,4 and Lei R. Cao1,a)
1Nuclear Engineering Program, Department of Mechanical and Aerospace Engineering,The Ohio State University, Columbus, Ohio 43210, USA2Department of Radiology, Stanford University, Stanford, California 94305, USA3Department of Electrical and Computer Engineering, The Ohio State University, Columbus, Ohio 43210, USA4Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA
(Received 12 July 2015; accepted 12 August 2015; published online 3 September 2015)
With the largest band gap energy of all commercial semiconductors, GaN has found wide
application in the making of optoelectronic devices. It has also been used for photodetection such as
solar blind imaging as well as ultraviolet and even X-ray detection. Unsurprisingly, the appreciable
advantages of GaN over Si, amorphous silicon (a-Si:H), SiC, amorphous SiC (a-SiC), and GaAs, par-
ticularly for its radiation hardness, have drawn prompt attention from the physics, astronomy, and nu-
clear science and engineering communities alike, where semiconductors have traditionally been used
for nuclear particle detection. Several investigations have established the usefulness of GaN for alpha
detection, suggesting that when properly doped or coated with neutron sensitive materials, GaN
could be turned into a neutron detection device. Work in this area is still early in its development,
but GaN-based devices have already been shown to detect alpha particles, ultraviolet light, X-rays,
electrons, and neutrons. Furthermore, the nuclear reaction presented by 14N(n,p)14C and various
other threshold reactions indicates that GaN is intrinsically sensitive to neutrons. This review
summarizes the state-of-the-art development of GaN detectors for detecting directly and indirectly
ionizing radiation. Particular emphasis is given to GaN’s radiation hardness under high-radiation
fields. VC 2015 Author(s). All article content, except where otherwise noted, is licensed under aCreative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4929913]
TABLE OF CONTENTS
I. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
II. GAN MATERIAL PROPERTIES . . . . . . . . . . . . . . 2
A. Basic parameters . . . . . . . . . . . . . . . . . . . . . . . . 2
B. GaN growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
III. RADIATION DETECTION . . . . . . . . . . . . . . . . . . 4
A. Alpha particle response . . . . . . . . . . . . . . . . . . 4
1. Double-Schottky contact structure . . . . . . 4
2. Mesa structure . . . . . . . . . . . . . . . . . . . . . . . 4
3. Sandwich structure . . . . . . . . . . . . . . . . . . . 5
B. X-ray detection . . . . . . . . . . . . . . . . . . . . . . . . . 5
C. Betavoltaic application . . . . . . . . . . . . . . . . . . . 6
D. Neutron detection . . . . . . . . . . . . . . . . . . . . . . . 6
E. Intrinsic neutron sensitivity . . . . . . . . . . . . . . . 7
IV. HARSH-ENVIRONMENT PERFORMANCE . . . 8
A. Neutron irradiation damage . . . . . . . . . . . . . . . 8
B. High temperature performance . . . . . . . . . . . . 9
V. CONCLUSION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
I. INTRODUCTION
Gallium nitride (GaN) semiconductors are now
commonly found in optoelectronic and high-power devices,
e.g., light-emitting diodes (LEDs),1,2 lasers,3 and high elec-
tron mobility transistors (HEMTs).4 GaN can also be used
for detecting ionizing radiation under extreme radiation
conditions due to its properties such as a wide band-gap
(3.39 eV), large displacement energy (theoretical values
averaging 109 6 2 eV for N and 45 eV for Ga),5 and high
thermal stability (melting point: 2500 �C).6 Compared to
narrower band-gap semiconductors such as silicon, GaN can
operate at higher temperatures; while a comparison with
other wide band-gap semiconductors, such as silicon carbide,
demonstrates GaN’s higher electron mobility7 and potential
for better carrier transport properties. In addition, the high
Z-value and density of GaN makes it a suitable material for
X-ray detection in medical imaging. The first group of
reports showing GaN as an alpha particle detector used devi-
ces in a double Schottky structure, fabricated from a
2–2.5 lm thick epitaxial GaN layer, grown via metal organic
chemical vapor deposition (MOCVD)8–10 on a sapphire
substrate. Based on these studies, a review article in 2006
compared the use of a few wide band-gap semiconductor
materials in very high radiation environments, to be used in
the next generation of high-energy physics experiments at
the Large Hadron Collider (LHC).11 The article concluded
that GaN is a very promising candidate for use in such
experiments, despite it still being a relatively immature
semiconductor material. Subsequent studies have furthera)Email: [email protected]
1931-9401/2015/2(3)/031102/12 VC Author(s) 20152, 031102-1
APPLIED PHYSICS REVIEWS 2, 031102 (2015)
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demonstrated that GaN-based sensors can detect a-par-
ticles12–14 and X-rays,15–17 indicating the growing potential
use of GaN for ionizing radiation detection. Furthermore, a
study investigating alternative materials for neutron detec-
tion, driven by the shortage of 3He gas, has considered the
use of GaN for neutron sensors in harsh environments.18
More recently, the growth methods for GaN are shifting
from foreign substrate epitaxial to free standing type, bulk
GaN with a thickness of several hundred micrometers. Based
on these different types of materials, various GaN sensor
structures have been fabricated for ionizing radiation detec-
tors. For a-particle detectors, the lateral double-Schottky
contact (DSC),8–10 mesa,6,12,13,19 sandwich,14,20 and p-i-n
structures21 have been tested. For X-ray detectors, Schottky
Metal-Semiconductor-Metal (MSM),15 Schottky diode,16,17
and p-i-n structures22 have all been reported. For electron
detection, the applications for making betavoltaic energy
converters,23,24 using pn,25 p-i-n,23,26–28 and Schottky type29
structures have now been tested. Thermal neutron detection
using a 6Li converter in a sandwich structure has been
recently reported.20 The nuclear reaction presented by14N(n,p)14C (1.8 b for neutron at 0.025 eV) and a various
other threshold reactions producing charged particles suggest
that GaN is intrinsically sensitive to neutrons, including fast
neutron, e.g., at 1 MeV or above. However, the commerciali-
zation of GaN detectors for radiation detection is still
impeded by the lack of high-quality materials. The various
defects present in GaN, including point defects, extended
defects, and surface defects form scattering centers, recombi-
nation and trapping centers limit the quality of the material.
In addition, the growth of p-type GaN is still under develop-
ment due to the lack of a suitable dopant. In this review, we
discuss the properties of various GaN device structures and
their constituent materials to understand the use of GaN for
detecting ionizing radiation at a fundamental level. In addi-
tion, device performance in a high radiation field and high
temperature environment is also summarized. This under-
standing will facilitate the effective application of GaN in
fabricating ionizing radiation detectors and the potential
applications in extreme radiation conditions such as those
found in nuclear power reactors, accelerators, and fusion
reactors, which require radiation-hard devices.
II. GaN MATERIAL PROPERTIES
A. Basic parameters
Despite the limited number of devices reported for radi-
ation detection, GaN holds several advantages over other
semiconductors for high temperature and high radiation field
applications. Compared to the widely used Si and Ge detec-
tors, both of which are limited to either room or liquid nitro-
gen cooled temperatures, respectively, GaN is characterized
by a much wider band-gap, making it capable of working in
environments well above room temperature. Shortcomings
in other wide band-gap semiconductors such as short carrier
lifetimes (10 ns) in GaAs due to the dominant EL2 native
deep-level defect,30 the large number of deep-level defects31
in AlN, and the high cost of diamond32 limits their
implementation as radiation detectors. Compared to SiC that
has an in-direct band-gap, GaN has a higher mobility and
thus better electrical properties. For instance, GaN can form
a high mobility 2D electron gas by the polarity effect for
field effect transistor applications. GaN may also be more
radiation hard due to its higher ionic bond strength, large
crystal density, and fewer polytypes.33,34 GaN also has a
higher Z-value and should thus be more suitable for X- and
c-ray detection. Although there are other candidates with
high Z-value suitable for radiation detection, for example
HgI2, the issues of difficulty in growing large scale crystals
and controlling material quality have hindered their further
applications.35 It is indeed the progress in the photonics and
electronics areas that drives the advancement of GaN,
mainly from materials synthesize, which in turn benefits its
applications in the relatively small market represented by the
radiation detections. In addition, GaN is a superior material
for optoelectronic applications, since it has a direct band-gap
and can alloy with Al and In, representing a tunable band-
gap value of 1.9 (InN) to 6.2 eV (AlN).36 Note that the band
gap of InN is still under debate, while a new value of
�0.7 eV is recently more in consensus theoretically37 and
experimentally.38 A comparison of the properties of these
semiconductors can be found in Table I.
B. GaN growth
Growth methods and defects formed in GaN have been
previously reviewed in various publications.47–52 A brief
review is given here with the focus on GaN’s properties that
are most relevant to radiation detection, though these proper-
ties are also important for other applications. Due to the lack
of a native substrate, GaN is mostly grown on foreign
substrates, such as AlN,53 Si,54 sapphire,55 and SiC.56 The
mismatch between the two layers results in a high density of
threading dislocations in the GaN epilayer, which includes
pure edge, pure screw, and mixed dislocations. These dislo-
cations have a significant effect on the device behavior.
They behave as non-radiative recombination centers with
energy levels in the forbidden gap and thus form trapping
centers, act as charged scattering centers,57 and provide a
leakage current pathway.58,59 Recent research has found that
pure screw components, which are solely responsible for the
leakage paths, are uncharged, while edge dislocations behave
as negatively charged scatterers because the associated traps
are filled with electrons.60 The edge dislocation has a repul-
sive potential around its line, which will not deteriorate the
device’s performance in which electrons transport parallel to
the edge. The screw dislocations, however, are a major con-
cern in terms of device performance.57
For epitaxial GaN, the threading dislocation density can
be as high as 108–1010 cm�2 when GaN is grown directly
on a foreign substrate.47 By introducing buffer layers61 and
using the epitaxial laterally overgrown (ELOG) tech-
nique,50,62 the density can be lowered down to 106 cm�2
(Ref. 50) and 105 cm�2 (Ref. 63), respectively. When grow-
ing bulk material, such as in hydride vapor phase epitaxy
(HVPE), dislocations can be controlled by increasing the
thickness of the material, resulting in interactions between
031102-2 Wang et al. Appl. Phys. Rev. 2, 031102 (2015)
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dislocations which leads to a decrease in the dislocation den-
sity near the top surface.64 Research shows that by removing
the top layer of the HVPE substrate, a high-quality bulk GaN
(several hundred micrometers thick) with a dislocation den-
sity of �106 cm�2 can be produced,65 such as those produced
by one of GaN wafer suppliers.66
In addition to dislocations, another key growth factor
that could dominate the device performance is the doping
level. Undoped GaN shows n-type properties due to the re-
sidual shallow donors such as oxygen in MOCVD-grown
GaN13 and silicon in HVPE-grown GaN67 (shallow donors
are defined as impurities that ionize at room temperature,
which corresponds to an activation energy of 100 meV).
Oxygen donors result in a background free-carrier concen-
tration between 1015 and 1017 cm�3 with an activation
energy of 30–33 meV (Ref. 68) that is below the conduction
band minimum (CBM). GaN can be intentionally doped
with Si to form an n-type material through either as-growth
or post-growth implantation. For Si-doped GaN, activation
energy levels of 12–17 meV,69 30–60 meV,70 18.1 meV,
and 273.9 meV (Ref. 71) have all been reported. Most of
these levels can be ionized at room temperature to donate
an electron to the material.33
For p-type doping, Mg provides a hole by occupying a
Ga site.72 However, the hole population is limited to within
1018 cm�3 in p-type GaN:Mg, and the resistivity is always
greater than 104 X cm (Ref. 73) due to (a) the large thermal
activation energy of Mg in GaN (120–250 meV) resulting
in low activation efficiencies of 1%–2%;74 (b) the con-
sumption of Mg by hydrogen passivation, i.e., the forma-
tion of the electrically inactive neutral complex (Mg-H)0
during the growth or high-temperature annealing pro-
cess;75 (c) the hole compensation by oxygen impurities
causing a high-resistivity, semi-insulating (SI) material;76
and (d) the consumption of Mg by self-compensation, i.e.,
the formation of a deep donor with a nitrogen vacancy,
MgGaVN.77,78
Besides n- and p-type doping, semi-insulating GaN has
also been successfully produced. Fe ions can be introduced
to compensate the residual donors to obtain SI GaN, result-
ing in SI substrates with a high resistivity of �1010 X/�.79
Fe forms the charge transfer deep levels FeGa3þ/2þ when it
occupies the Ga lattice sites,80 and the charged FeGa3þ state
can transform to FeGa2þ by capturing an electron and thus
compensate the residual donors. The energy level that repre-
sents the energy required to emit electrons captured from
the donor by the Fe acceptor is between 0.34 and 0.87 eV
below the conduction band edge, so the compensation
should be thermally stable at room temperature.81 SI
GaN:Fe shows good crystal quality and the strain-free incor-
poration of Fe.82 However, the Fe doping pins the Fermi
level to approximately 0.5–0.6 eV below the CBM.83 In
addition to Fe, Mg ions can also be used to compensate for
the residual donors, and moreover, Mg can improve the
crystal quality. Research has shown that crystal strain and
point defects are largely eliminated by the substitution of
Ga by Mg atoms.84 For example, SI GaN:Mg produced by
one of GaN wafer suppliers has a low dislocation density of
�104 cm�2.85 Although the compensated semi-insulating
GaN may exhibit a good crystal quality, it is not suitable for
making radiation detectors due to the short carrier lifetime
caused by the high-density of impurities.
TABLE I. Material properties (mechanical, electrical, thermal) of major semiconductors for radiation detection at 300 K.11,39 Note: lh: light hole; hh: heavy
hole; tr: transverse; l: longitude; hz: heavy hole at kz direction, hx: heavy hole at kx direction, lz: light hole at kz direction, lx: light hole at kx direction. GaN
has the largest breakdown voltage among all the commercial semiconductors, its electron mobility exceeds all but GaAs, Ge, and diamond, its thermal conduc-
tivity exceeds all but AlN and diamond.
Property AlN Diamond GaN 4H-SiC CdTe GaAs Si Ge
Crystal structure Wurtzite Diamond Wurtzite Wurtzite Zinc blende Zinc blende Diamond Diamond
Average atomic number 10 12 19 10 50 31.5 14 32
Bandgap (eV) 6.2 5.5 3.39 3.23 1.44 1.424 1.12 0.661
Density (g/cm3) 3.23 3.515 6.15 3.211 5.85 5.32 2.33 5.32
e-h pair creation energy (eV) 15.3 12 8.9 7.8 4.43 4.2 3.62 2.96
Effective electron masses (m0) 0.4 1.40 (l, at 85 K) 0.2 0.29 (l) 0.11 0.063 0.98 (l) 1.6 (l)
0.36 (tr, at 85 K) 0.42 (tr) 0.19 (tr) 0.08 (tr)
Effective hole mass (m0) 3.53 (hz) 0.8 0.4
10.42 (hx) 2.12 (hh, at 1.2 K) 1.75 (l) 0.51 (hh) 0.49 (hh) 0.33 (hh)
3.53 (lz) 0.70 (lh, at 1.2 K) 0.66 (tr) 0.082 (lh) 0.16 (lh) 0.043 (lh)
0.24 (lx)
Electron mobility (cm2/(V�s)) 300 1800–2200 1000 800–1000 1100 �8500 1450 �3900
Hole mobility (cm2/(V�s)) 14 1200–1600 30 50–150 100 �400 450 �1900
Breakdown field (106 V/cm) 1.2–1.8 1–10 �5 3–5 — 0.4 0.3 0.1
Saturated electron drift velocity
(107 cm/s)
1.4 2.7 3 (Ref. 40) 2.0 1.3 (Ref. 41) 1.2 1.0 6.5
Threshold displacement energy (eV) 43 35 (Ref. 42) Ga: 18, N: 22
(Ref. 43)
C: 20, Si: 35
(Ref. 44)
Te: 7.8, Cd: 8.9
(Refs. 45 and 46)
10 13–20 25
Melting point (�C) 3000 4373 (at 125 kbar) 2500 2857 (at 35 atm) 1092 1240 1412 937
Thermal conductivity (W/cm �C) 2.85 6–20 1.3 3.7 0.06 0.55 1.3 0.58
Thermal expansion coefficient
(10�6/ �C)
5.27 0.8 5.59 3.7 5.9 5.73 2.6 5.9
031102-3 Wang et al. Appl. Phys. Rev. 2, 031102 (2015)
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III. RADIATION DETECTION
A. Alpha particle response
1. Double-Schottky contact structure
GaN-based a-particle detection was first realized by
Vaiktus et al.9 using a DSC structure shown in Fig. 1(a). The
active layer in this device was a 2-lm-thick epitaxial GaN
(epi-GaN) layer grown by MOCVD on a sapphire substrate.
Under the epi-GaN is a 2-lm-thick, highly doped, n-type
GaN buffer layer that is employed to minimize the disloca-
tion density. Two Au Schottky contacts are deposited on the
top of the epi-GaN to complete the detector. Alpha particle
detection was performed using 5.48 MeV a-particles emitted
from an 241Am source, and the spectra under different bias
voltages before breakdown (29 V) is shown in Fig. 2.9 One
can see that as the bias voltage increases to 27 V, any further
increase in the voltage does not change the peak channel
number, which indicates that the device is fully depleted. A
fit of the pulse peak with a Gaussian distribution gives a
Charge collection efficiency (CCE) of �92%. A better fit
can be obtained with two Gaussian functions, and Vaitkus
et al.9 suggested that the double-peak structure is related to
the complicated drift of the charge carriers in a layer contain-
ing different drift and trapping barriers. This two Gaussian
fit to alpha spectrum has also been reported in devices
utilizing a p-i-n structures.21 Subsequent research10 verified
the electric path for this double-Schottky structure: electric
field lines through the epilayer are perpendicular to each
Schottky contact, connected by the highly doped buffer
layer.
The double-Schottky contact structure has three main
advantages:86 (a) The fabrication process is simple in that it
requires only one masking step; (b) the structure can be used
without any optimized Ohmic contact and is thus usually
employed to study and optimize Schottky contacts; and (c)
since the thickness of epi-layer is usually less than the range
of the a-particles, both electrons and holes contribute to the
detection signal and significant trapping of charge carriers is
not expected.10 On the other hand, the structure suffers from
several drawbacks: (a) The thin epilayer (<10 lm) results in
only partial energy deposition; (b) multiple Gaussian func-
tions are needed to fit the a spectrum; and (c) parasitic capac-
itance and resistance may exist due to the long distance
between the two metal contacts.
2. Mesa structure
The mesa structure (mesa-1) is based on thin-film GaN
as shown in Fig. 1(b).6,12 An Ohmic contact on the highly
doped buffer layer is realized by etching through the
epi-GaN layer. Polyakov et al.12 deposited 1 mm diameter
Ni Schottky contacts on mesa structures to study three types
of undoped GaN with carrier concentrations of more than
1015 cm�3: 3-lm GaN grown by MOCVD, 3-lm GaN grown
by molecular-beam epitaxy (MBE), and 12-lm GaN grown
by MOCVD using the ELOG technique. For all three sam-
ples, deep-level transient spectroscopy (DLTS) analysis has
revealed two electron traps with activation energies of 0.25
and 0.6 eV, respectively, and that the MBE sample had the
highest concentration. Due to these traps, the CCE of the
MBE device is lower than that of the MOCVD and ELOG
devices, both of which are close to 100%. Additionally, all
three CCEs are higher than those reported based on semi-
insulating materials, which again is a result of the low
trapping impurity concentration.
The other mesa structure (mesa-2) shown in Fig. 1(c) is
that reported by Lu et al.13 In this structure, the etching does
not reach the buffer layer, and the Ohmic contact is realized
on the same epilayer. The epi-GaN is a 3-lm layer of
undoped GaN (carrier concentration of �4� 1016 cm�3)
FIG. 1. Structures of a-particle GaN
detectors: (a) Double-Schottky contact
structure, (b) and (c) mesa structure,
and (d) sandwich structure.
FIG. 2. Alpha particle pulse height spectra from the double-Schottky contact
structure a-particle detector. Reproduced with permission from Vaitkus
et al., Nucl. Instrum. Methods Phys. Res., Sect. A 509, 60–64 (2003).
Copyright 2003 Elsevier.
031102-4 Wang et al. Appl. Phys. Rev. 2, 031102 (2015)
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grown by MOCVD on a sapphire substrate. The Schottky
contact is realized by depositing a circular Ni/Au contact
with a diameter of 940 lm, and the Ohmic contact employs
the Ti/Al/Ni/Au metal scheme, achieved by etching away
2-lm of the epi-GaN. Using this mesa structure, Lu et al.13
found that the a-particle pulse height spectra have one dis-
tinct peak that can be fitted precisely by a Gaussian.
Compared with lateral devices, i.e., the double-Schottky
contact structure, mesa geometry detectors exhibit superior
performance, e.g., a lower noise level and fewer dislocation
mobility fluctuations,87 which are related to the better trans-
port properties of carriers flowing vertically. For these two
mesa structures, mesa-1 may suffer from an over-etching
problem, since the applied buffer layer is usually thin (less
than 2 lm), while mesa-2 needs a high temperature annealing
process to form the Ohmic contact. Moreover, for both struc-
tures, the distance between the Schottky and Ohmic contacts
should be kept small (less than several tens of micrometers)
in order to minimize the parasitic capacitance and resistance.
If this is overlooked, the device will have a high on-
resistance (as seen in the current–voltage curve), and the
capacitance–voltage data may no longer be reliable.
3. Sandwich structure
The sandwich structure a-particle detector (Fig. 1(d))
fabricated on bulk GaN was reported recently by Lee et al.14
It consists of a �500–lm-thick n-type free-standing GaN
purchased from Kyma (grown by HVPE), which is charac-
terized by a very low carrier concentration of
�1013–1014 cm�3 in the upper �30-lm (the concentration in
the rest of the GaN is �1016 cm�3). The Schottky contact is
made on the Ga side by a circular Ni contact with a diameter
of 1 mm. DLTS and ODLTS (DLTS measurements with op-
tical injection) measurements show that the low carrier con-
centration is due to compensation by unidentified acceptor
centers with an activation energy of 0.2 eV and by the major
H5 hole traps with an activation energy of 1.2 eV. However,
owing to these H5 traps, the CCE of the device is limited,
and a voltage of 120 V is necessary to overcome the trapping
and obtain 100% charge collection.
Sandwich structure has also been fabricated on normally
grown bulk GaN (Kyma) by Mulligan et al.20 The 450 lm
wafer had an unintentionally n-type doping (impurity Si) of
1.68� 1016 cm�3. Due to the relatively high doping concen-
tration, the depletion region is only around 2 lm under a
reverse bias of less than 20 V. Experiment results showed
that the charge collection efficiency within this thin depletion
region is almost 100%. Further simulation88 indicated that
the carrier gain due to impact ionization in the depletion
region and the diffusion components from the undepleted
region is significantly small compared with the drift carriers
in the depletion region, and the carrier loss due to hole trap-
ping is negligible.
Compared with thin-film-based structures, the bulk-
sandwich structure has a potential advantage of offering a
large depletion region and thus a full a-particle energy depo-
sition. Furthermore, it has superior carrier transport proper-
ties and, in particular, suffers less of a current crowding
problem.88,89 At the same time, the structure maintains fabri-
cation simplicity. We may therefore conclude that the sand-
wich structure is a preferred choice for a-particle detection.
B. X-ray detection
X-ray detection with thin-film GaN is difficult due to the
low absorption coefficient shown in Fig. 3. Both theoretical
calculations and experimental measurements indicate that
the absorption is large only for photon energies between 10
and 20 keV.15 At higher energies such as 40 keV, the coeffi-
cient is about 50 cm�1, which corresponds to a 1/e attenua-
tion length of 200 lm. Thus for high-energy X-rays, a
multilayer GaN film or bulk structure is needed to absorb a
significant number of incoming photons.
Nevertheless, GaN-based X-ray detection has been dem-
onstrated by Duboz et al. using both the Schottky MSM15
and Schottky diode structures.16,17 The MSM structure was
fabricated on 10-lm-thick undoped GaN grown by MOCVD
and employed a Pt/Au Schottky contact with the finger width
varying from 2 to 20 lm and the spacing varying from 2
to 10 lm. When the X-ray source was switched off, the
signal showed a fast decrease in less than 1 s followed by a
long exponential transient with a time constant of 40 s.
The Schottky diode detector was fabricated on 20-lm
homogeneous-growth, undoped GaN with a Pt/Au Schottky
contact area that varied between 1 and 2 mm2. Fig. 4 shows
the transient characteristics after the X-ray source was
switched on and off at two different power levels.16
X-ray detection has also been performed with a p-i-n
structure,22 but despite the different structures employed, a
fast transient increase or decrease (of less than 1 s) followed
by a slow exponential increase or decay is seen in all the
measured photocurrents. According to Duboz et al.,15 this
photocurrent characteristic can be explained in terms of two
currents: photovoltaic and photoconductive. The photovoltaic
current generated in the depletion region has a fast response,
while the photoconductive current created by the X-ray
activated carrier traps has a slow response. Recently, Lu’s
group decoupled the photoconductive current into several
FIG. 3. Experimental (dots) and theoretical (line) absorption coefficient of
GaN as a function of the photon energy. Reproduced with permission from
Appl. Phys. Lett. 92, 263501 (2008). Copyright 2008 AIP Publishing LLC.
031102-5 Wang et al. Appl. Phys. Rev. 2, 031102 (2015)
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components related to electrons trapped in different energy
levels characterized by their own lifetime constants.22,90 To
decrease or eliminate the slow transient component, the only
solution is to fabricate high-quality materials.
C. Betavoltaic application
A biased semiconductor detector could easily be per-
ceived as a voltaic device when no bias is applied. Whether
it is alpha-voltaic, beta-voltaic, gamma-voltaic, or even
neutron-voltaic, the only difference is which source causes
ionization. Due to its superior properties already mentioned,
GaN is suitable to make radiation-voltaic batteries for aero-
nautic, marine, and medical applications that require a long
life time and high reliability power supply.23 Currently, GaN
has been investigated for beta particle response in its applica-
tion of making beta-voltaic micro-batteries.
Theoretical calculations from a conceptual design24,91
utilized two GaN pn junctions sandwiched with a 63Ni radio-
isotope source (with a half-life of 100 yr). The results indi-
cated a maximum short circuit current of 1.1 lA/cm2,
maximum open circuit voltage of 2.7 V, and ideal efficiency
of 25%. The efficiency obtained is higher than those pres-
ently available from thermoelectric converters made of
Silicon, which is around 15%. Similar results were recently
obtained by another calculation using the same structure and
a 147 Pm source.92
Two groups have tested their GaN-based beta-voltaic
devices simultaneously. Chen’s group studied both pn and
p-i-n structures.25–27 In one of their p-i-n devices, using a63Ni source with activity of 2 mCi, an open-circuit voltage of
1.62 V, short-circuit current density of 16 nA/cm2, filling
factor of 55%, and energy conversion efficiency of 1.13%
were obtained.26 Another group lead by Lu studied both
Schottky and p-i-n structures,23,28,29 reporting a p-i-n device
with an open circuit voltage of 1.07 V, short circuit current
of 0.554 nA, and a filling factor of 24.7% using a 147 Pm
source.23
The performance of GaN-based beta-voltaic devices is
still far from its theoretical calculated values, and further
improvement largely relies on the availability of high quality,
thick GaN. High purity GaN would result in a wide active
region with low recombination and trapping effects,25,29
increasing the energy conversion efficiency. In addition, the
structure of the device should be optimized by using a thin
electrode layer to minimize the dead layer and backscattering
for electrons,28 and a thin passivation layer to decrease the
leakage current.93 Admitting that the development of GaN
beta-voltaic is still at its early infancy, GaN is highly promis-
ing as a potential candidate for long-life nuclear micro-
batteries used as power supplies for microelectrochemical
system devices.
D. Neutron detection
For neutron detection using a semiconductor device,
neutron sensitive converters are typically employed to pro-
duce charged particles for subsequent electron-hole produc-
tion. Commonly used neutron converters are 10B, 6Li, and157Gd,94 and the converter can either be coated directly onto
the surface of the GaN as a thin-film or be incorporated by
doping,95,96 as illustrated in Fig. 5. While these commonly
applied neutron convertors have high cross-sections in the
thermal energy region (�2200 m/s), the detection of fast
neutron will involve bulky moderation materials or ineffi-
cient reaction process (e.g., proton recoil). For thin-film-
coated detectors, the fabrication process is relatively simple,
but the intrinsic neutron detection efficiency is limited by the
fact that only one of the two charged particles generated
from the neutron capture reaction can enter the active region
FIG. 4. Photocurrent transient measured for two incident X-ray powers of
P¼ 10 and P¼ 40 (corresponding to currents of 10 and 40 mA on the X-ray
tube, respectively). The applied reverse voltage is �10 V. Reproduced with
permission from J. Appl. Phys. 105, 114512 (2009). Copyright 2009 AIP
Publishing LLC.
FIG. 5. (a) Thin-film-coated and (b)
solid-form semiconductor thermal
neutron detectors.
031102-6 Wang et al. Appl. Phys. Rev. 2, 031102 (2015)
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of the device. Another fundamental limitation on detection
efficiency is the conflict between the relatively long neutron
mean free path and the short range of the charged particles in
these convertor materials. In solid-state detectors, the
conversion atoms may be introduced by high-temperature
diffusion, ion implantation, or in-grow processes, during
which defects will likely be introduced and degrade the over-
all performance of the detector. Both configurations shall be
explored to study the behavior of GaN-based neutron detec-
tors. In addition, c-ray discrimination is always a valid con-
cern with solid-state neutron detection devices.97 Although
the thin-film device is relatively gamma blind, the use of
high-Z elements as the convertor, such as Gd, may generate
a considerable number of X-rays via c-ray activation. These
phenomena, however, boost the c-ray detection efficiency
when Gd is combined with GaN or other semiconducting
materials.98,99
A rare-earth-doped GaN thin film might lead to signifi-
cant improvements in device performance in sensor applica-
tions; GaN is potentially neutron sensitive when doped with
Gd.100,101 Melton et al.102 found that the carrier concentra-
tion in Gd-doped GaN increases as a result of the Gd doping.
Cao and Myers employed a 500-nm superlattice doping
structure grown by MBE to maintain the quality of the
material.18 Although the device showed a large leakage
current and low breakdown voltage, which indicated a large
dislocation density, increasing the gap between Gd clusters
may improve the charge collection efficiency, possibly lead-
ing to a working superlattice device. Melton et al.103 also
fabricated a GaN-based neutron scintillator by coating
Si-doped GaN (n-type doping of �5� 1018 cm�3) with 6LiF
and Gd converters. When electron–hole pairs are created in
GaN by secondary charged particles, they recombine to
produce scintillation photons, which are then detected by
photo-sensors. The Si-doped sample was chosen because its
near-band-edge recombination intensity is an order of mag-
nitude higher than undoped samples.104
A thermal neutron induced spectrum in GaN was first
realized by Cao.105 A sandwich structure Schottky diode with
a diameter of 1 mm was fabricated on special growth n-type
GaN grown by Kyma. A 6LiF:ZnS thin film (0.3 mm) was
used as the neutron-conversion material placed in front of the
detector. The set up was then placed directly in the 3-cm neu-
tron beam with a nuclear reactor operating at a power of
250 kW. The spectrum of the neutron response is given in Fig.
6. The two charged particles emitted from 6Li capture, i.e., 3H
at 2727 keV and 4He at 2055 keV were clearly discernible in
the spectrum. However, because the depletion depth of the de-
tector was 1.6 lm, much less than the range of these particles
in GaN, only a fraction of the initial energy of the particles
was deposited into the depletion region. Because the stopping
power of 4He is greater than that of 3H, 4He deposits more
energy in the depletion region than 3H.
E. Intrinsic neutron sensitivity
GaN is uniquely qualified for intrinsic neutron detection
due to a 584 keV monoenergetic proton emission following
neutron capture in 14N. Detection of this proton serves as an
indication of a neutron interaction with the detector.
Although the cross section of 14N is significantly smaller
than other isotopes typically employed as converter material
in neutron detectors, this apparent shortcoming could be
quite advantageous in high flux environments. Detection effi-
ciency can be sacrificed in neutron rich environments by
choosing conversion materials with a smaller cross section.
Materials with high neutron cross sections can be depleted
quickly in such environments, while materials with a smaller
cross section result in less material depletion during a given
time interval and thus, a longer operational lifetime. The 1.8
b cross section of 14N at thermal energy could be sufficient
for high flux neutron detection, while avoiding the acute
depletion seen with other conversion materials (Fig. 7). On
the other hand, 14N’s sensitivity to fast neutron is in the
same order of magnitude with that of 10B because of its
reduced difference in neutron capture cross-section at, for
example, 1 MeV (0.03 b for 14N versus 0.22 b for 10B), and
also taking into account 14N’s much larger natural abundance
ratio (99.6% for 14N versus 20% for 10B). Transmutation of
the semiconductor material, leading to unwanted doping and
changes in the detector’s characteristics, should also be con-
sidered in high neutron flux environments. Although the
cross sections of 69Ga and 71Ga (60% and 40% atomic abun-
dance, respectively) are somewhat larger than other semicon-
ductor materials suited for high flux environments (namely,
SiC), Ga holds a cross section similar to N, and will therefore
FIG. 6. Observed spectrum due to charged particle emissions following 6Li
neutron capture and 14N neutron capture (a), and experimental setup (b).
Reproduced with permission from L. Cao, Battelle Energy Alliance, LLC
Project No. 11–3004, 2015, pp. 1–44. Copyright U.S. Department of
Energy, Nuclear Engineering University Program 2015.
031102-7 Wang et al. Appl. Phys. Rev. 2, 031102 (2015)
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deplete concurrently with N. A high flux neutron detector of
GaN would therefore suitably detect neutrons, while delay-
ing the effects of converter material depletion and neutron
transmutation doping in the semiconductor. Work has been
completed to test the efficacy of a GaN based neutron detec-
tion by placing a sample of bulk GaN in a high flux neutron
beam and measuring the proton emission spectrum in vac-
uum. Results shown in Fig. 8 indicated a sufficient number
of pulses were detected for GaN to be used as a neutron con-
version material, even though a very small solid angle
(�0.001 Sr.) was used in the experiment.106
The energy spectrum of proton emissions from GaN
was measured with a Si detector, detecting those protons
produced inside GaN by nuclear reactions and escaping the
materials to reach a Si detector a few cm away. The spectrum
indicates that proton particles induced by thermal neutrons
can effectively produce electron-hole pairs within GaN if it
were to be fabricated into a rectifying junction. The difference
in spectrum width can be explained by the different thick-
nesses of two samples measured. The detectable depth limit of
the 14N in GaN is 4.13 lm from the surface, based on the
Stopping and Range of Ions in Matter (SRIM) calculation of
the projected range of the 584 keV proton in the GaN material.
In Fig. 8, the epi-layer of GaN in epi-layer GaN sample is
�2 lm, and Ammono is much thicker than proton range, thus
an extended peak into the background region.
IV. HARSH-ENVIRONMENT PERFORMANCE
A. Neutron irradiation damage
The radiation damage to GaN materials and devices
under various radiation species, such as proton, neutron,
gamma-ray, and electrons has been reviewed in other refer-
ences;107,108 here we only provide a concise discussion in
neutron irradiation effects. Both fast and thermal neutrons
damage the GaN lattice. Fast neutrons cause damage by
elastic collisions and the thermal neutrons by nucleus recoil
during neutron activation of Ga and N atoms.109 For fast
neutron irradiation, Nordlund et al.5 did a simulation study
by employing molecular dynamics method, they found that
although GaN has low threshold displacement energies
(22 6 1 eV for Ga and 25 6 1 eV for N), the average values
are relatively high (45 6 1 eV for Ga and 109 6 2 eV for N).
The average values for displacement are higher than the
threshold is due to the fact that in some collisions, energies
are transferred via other mechanisms instead of displace-
ment, such as generation of phonons. A 1 MeV fast neutron
can transfer up to 55 keV to a primary knock-on atom (PKA)
of Ga or 230 keV to a PKA of N; these energies exceed the
displacement energies and will thus lead to cascade colli-
sions.109 A study in 2006 (Ref. 110) showed that fast neutron
irradiation with a fluence of 6.7� 1018 cm2 produces approx-
imately 1900 and 7200 displaced atoms per PKA of Ga and
N, respectively.
Thermal neutrons can produce indirect atomic displace-
ments as a result of radioactive capture (n,c) reactions. The
main thermal reactions in GaN are listed in Table II.111,112
Among these reactions, the energy of recoil atoms generated
by the first two reactions is sufficient to cause approximately
2� 105 displacements according to a SRIM simulation. The
third reaction releases most of its energy to c-rays, and the
effect of the last reaction is negligible due to the extremely
small cross section (2.42� 10�5 b). Note that isotopes of
FIG. 7. Neutron capture cross section plot of primary constituent isotopes in
GaN and SiC (a) and charged particle emission cross section plot (b) in com-
monly used neutron converter materials. 14N has a sufficiently small cross
section to delay the effects of material depletion in high flux environments.
FIG. 8. Energy spectrum of protons emitted following 14N neutron capture
in two types of GaN wafers.106
031102-8 Wang et al. Appl. Phys. Rev. 2, 031102 (2015)
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70Ga, 72Ga, and 16N will also undergo beta decay; however,
these beta-particles and c-rays will not cause serious damage
to the GaN lattice, since most of their energy will be lost to
interactions with outer orbital electrons.
Neutron irradiation may change the lattice constant and
induce strain inside GaN. Research by Marques et al.109 and
Lorenz et al.113 indicates that under neutron irradiation, Ga
atoms are preferentially displaced along the c-axis, but the in-
plane lattice parameter does not change. For instance, for a
neutron fluence of 8� 1019 cm�2, the lattice constant c
increases by 0.38% and the lattice constant a nearly
unchanged.114 Neutron irradiation can also alter the resistivity
or even cause type-reversion of the material. Compensation
schemes have been identified for both p-type115 and n-type116
GaN when the neutron fluence exceeds 1016 cm�2. Wang
et al.116 suggested that the neutron-irradiation-induced struc-
ture defects GeGa give rise to carrier trap centers that are
responsible for the observed reduction in the carrier concen-
tration in irradiated n-type GaN. Due to these induced
defects, trapping centers are created, and the Fermi level is
pinned within the energy levels of these defects. For example,
Polyakov et al.117 found that under a high neutron fluence of
�1018 cm�2, a deep electron trap with an activation energy of
0.75 eV was introduced in both n- and p-type GaN, and the
Fermi level was pinned to near Ec-0.85 eV. For Mg-doped
p-type GaN, the Fermi level pinning near Ec-(0.8–0.9) eV
has also been reported.115 For Fe-compensated semi-insulat-
ing GaN, both N vacancies and deep level defects were
observed by DRCLS (depth-resolved cathodoluminescence
spectroscopy) analysis after fast and thermal combined
neutrons irradiation with fluences from 1014 to 1016 n/cm2
(Ref. 118). For low neutron fluences on the order of
�1011 cm�2, the damage can be repaired by a self-annealing
process at room temperature for several days.119,120 For me-
dium (�1014 cm�2) and high fluences (�1019 cm�2), most of
the lattice damage can be repaired under a high temperature
annealing process at around 1000 �C; however, a certain
amount of optical and electrical damage remains.113
In a general sense, neutron irradiation will degrade the
device performance. For instance, Grant et al.6,19 found that
neutron irradiation of Schottky diode detectors fabricated on
2- and 12-lm thin-film semi-insulating GaN produced a
non-linear increase in the leakage current as the neutron
fluence was increased from 1014 to1016 cm�2, and the CCE
of the 12-lm device under a fluence of 1016 cm�2 decreased
from 53% to 20%. They suggested that the degradation is
mainly due to an increased number of recombination/trap-
ping centers. Similar results were reported by Mulligan
et al.121 after studying the degradation of the electrical
properties of a Schottky diode device, a turning point at a
total neutron fluence of 1015 n/cm2 was discovered.
However, there may be an optimal fluence for which the de-
vice performance is enhanced. One study by Wang122,123
shows that after neutron irradiation with a fluence of
1� 1013 cm�2, the Au/GaN Schottky barrier photodetectors
showed superior current�voltage characteristics, which was
attributed mainly to the effective repression of the deep elec-
tron traps Et.
The existence of the threshold neutron fluence on the
performance of GaN Schottky diodes was further validated
by Lin,124 which found that fast neutrons with fluences
�1015 n/cm2 increases deep level defects densities, while
thermal neutron with fluences �1015 n/cm2 anneal these
defects and improve the GaN crystalline quality. Their fur-
ther studies125 revealed that both fast and thermal neutrons
introduce recombination centers in GaN, forming an insulat-
ing phase between GaN and metal contact, raising the
Schottky contact ideality factor, increasing the Ohmic con-
tact sheet resistance, and lowering the current transport for
both contacts. Above the threshold fluence, thermal neutrons
can introduce thermal spikes and elevated temperatures that
drive GaN out diffusion and reaction with metal contacts.
Fig. 9 shows the current-voltage characteristics of the
Schottky diodes under different neutron fluences, from
which the Schottky barrier height and ideality factor were
derived and analyzed.
B. High temperature performance
Both Ohmic and Schottky contacts require a thermal
annealing process to improve the quality of the contact.
After annealing at a certain temperature, the resistance of the
Ohmic contact can be reduced, and the leakage of the
Schottky contact can be decreased. However, annealing at
temperatures higher than the optimum value degrades the
contacts.
In terms of Ohmic contacts, Dobos et al.126 showed that
at an annealing temperature of 700 �C, there was a lateral
TABLE II. Thermal-neutron-induced reactions in GaN.111,112
Reactions
Main c-ray energy
(main transition probability) (MeV)
Recoil atom
energy (MeV)
69Gaðn; cÞ70Ga 1.039 (65%) 6.62271Gaðn; cÞ72Ga 0.834 (96%) 5.69014Nðn; cÞ15N 10.82 (50%) 0.01415Nðn; cÞ16N Negligible due to low cross section (2.42� 10�5 b)
FIG. 9. The log(jcurrentj) vs. voltage characteristics for the neutron-
irradiated GaN Schottky diodes for voltage from �5 V to 3 V. Reproduced
with permission from J. Appl. Phys. 115, 123705 (2014). Copyright 2014
AIP Publishing LLC.
031102-9 Wang et al. Appl. Phys. Rev. 2, 031102 (2015)
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diffusion of Al for the Al-only-based contact, and the conti-
nuity of the Ti layer was broken for the Ti/Al contact. After
annealing at a temperature of 900 �C, the two contacts no
longer exhibited linear behavior. For the Ti/Al/Ni/Au
contact, Fan et al.127 reported an optimum annealing temper-
ature of 750 �C, and the multi-metal layer scheme has been
widely used to form Ohmic contact on GaN.128–132
In Schottky contacts, the degradation varies according
to the metal employed. For Pd, Pt, Au, and Ni, degradation
starts after exposure to temperatures as low as 300, 400, 575,
and 600 �C, respectively.33 Kim et al.133 showed that W con-
tacts degrade at annealing temperatures greater than 500 �C,
and the metal silicide WSix shows a stable Schottky barrier
height of �0.5 eV at temperatures above 600 �C. Thus, the
W-based Schottky contact is currently the most promising
contact that can sustain temperatures of up to 600 �C. The
thermal stability of the Schottky contact is also related to the
metal scheme employed. Monroy et al.134 studied Pt- and
Ni-based Schottky contacts and demonstrated the smooth
decay of the barrier height in both Ni/Au and Pt/Ti/Au
diodes as the temperature increased, and a decrease of 20%
after annealing at 500 �C. This is in contrast to the Pt/Au
contact, which suffers from barrier height degradation at
300 �C. They attributed this difference to the thin Ti layer,
which may behave either as a diffusion barrier or as a
wetting layer to prevent the Pt from deforming during
the annealing process. High-temperature resistant Schottky
contacts are essential for harsh-environment applications.
Fig. 10 summarizes the behavior of GaN and related devices
at different temperatures.70,71,120,126,135
The long-term stability of Schottky diodes has also been
studied. Research by Luther et al.120 showed that Pt
Schottky contacts can maintain at 400 �C for 500 h without
degradation. O’Mahony et al.119 studied Ni-based Schottky
contacts and found that under a forward current of 1.3 A/cm2
or a reverse bias of �3.5 V while stored at 300 �C in an N2
environment for 466 h, the diodes exhibit a drift of less than
10% in both the ideality factor and barrier height. Despite
these cases, more research is still needed to understand the
long-term operating performance.
V. CONCLUSION
GaN is one promising semiconducting material for ion-
izing radiation detection, particularly in hash environments.
The defects present in GaN, such as the dislocations and
unintentional doping, still presents a main challenge in terms
of improving device-level performance. In spite of the
defects, prototype detectors fabricated on GaN have shown
response to alpha particles, electrons, X-rays, and neutrons.
Their special capability in operating in high radiation fields
and elevated temperature conditions are promising for many
applications. The nitrogen neutron caption reaction also
makes GaN an interesting material for intrinsic neutron
detection in high flux environments. Future progress in crys-
tal growth to produce a low carrier concentration region
comparable to the ranges of various radiations in GaN, and
fabrication techniques to minimize the leakage current and
parasitic capacitance are still required to expand GaN’s
application as a mature radiation detection material.
ACKNOWLEDGMENTS
We acknowledge the support of this work by U.S.
Department of Energy Office of Nuclear Energy’s Nuclear
Energy University Programs and Department of Defense,
Defense Threat Reduction Agency [Grant No. HDTRA1-11-
1-0013]. The authors also gratefully acknowledge National
Science Foundation, Grant No. DMR-1305193 (Charles
Ying and Haiyan Wang) for support of this work.
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