Review of using gallium nitride for ionizing radiation detection

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, particularly for its radiation hardness, have drawn prompt attention from the physics, astronomy, and nuclear 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. Furthermo...

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 electron mobility transistors (HEMTs). 4GaN 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 mobility 7 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 devices 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][9][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). 11The 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 further demonstrated that GaN-based sensors can detect a-particles [12][13][14] and X-rays, [15][16][17] indicating the growing potential use of GaN for ionizing radiation detection.Furthermore, a study investigating alternative materials for neutron detection, driven by the shortage of 3 He gas, has considered the use of GaN for neutron sensors in harsh environments. 18ore 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 detectors.For a-particle detectors, the lateral double-Schottky contact (DSC), [8][9][10] mesa, 6,12,13,19 sandwich, 14,20 and p-i-n structures 21 have been tested.For X-ray detectors, Schottky Metal-Semiconductor-Metal (MSM), 15 Schottky diode, 16,17 and p-i-n structures 22 have all been reported.For electron detection, the applications for making betavoltaic energy converters, 23,24 using pn, 25 p-i-n, 23,[26][27][28] and Schottky type 29 structures have now been tested.Thermal neutron detection using a 6 Li converter in a sandwich structure has been recently reported. 20The nuclear reaction presented by 14 N(n,p) 14 C (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 commercialization 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, recombination and trapping centers limit the quality of the material.In addition, the growth of p-type GaN is still under development 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 addition, device performance in a high radiation field and high temperature environment is also summarized.This understanding 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.

A. Basic parameters
Despite the limited number of devices reported for radiation detection, GaN holds several advantages over other semiconductors for high temperature and high radiation field applications.Compared to the widely used Si and Ge detectors, both of which are limited to either room or liquid nitrogen 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 defects 31 in AlN, and the high cost of diamond 32 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. 33,34GaN 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 HgI 2 , the issues of difficulty in growing large scale crystals and controlling material quality have hindered their further applications. 35It 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 bandgap value of 1.9 (InN) to 6.2 eV (AlN). 36Note that the band gap of InN is still under debate, while a new value of $0.7 eV is recently more in consensus theoretically 37 and experimentally. 38A comparison of the properties of these semiconductors can be found in Table I.

B. GaN growth
8][49][50][51][52] A brief review is given here with the focus on GaN's properties that are most relevant to radiation detection, though these properties 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. 56The 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 dislocations 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. 58,59Recent 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. 60The edge dislocation has a repulsive 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 concern in terms of device performance. 57or epitaxial GaN, the threading dislocation density can be as high as 10 8 -10 10 cm À2 when GaN is grown directly on a foreign substrate. 47By introducing buffer layers 61 and using the epitaxial laterally overgrown (ELOG) technique, 50,62 the density can be lowered down to 10 6 cm À2 (Ref.50) and 10 5 cm À2 (Ref.63), respectively.When growing 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 dislocations which leads to a decrease in the dislocation density near the top surface. 64Research shows that by removing the top layer of the HVPE substrate, a high-quality bulk GaN (several hundred micrometers thick) with a dislocation density of $10 6 cm À2 can be produced, 65 such as those produced by one of GaN wafer suppliers. 66n 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 residual shallow donors such as oxygen in MOCVD-grown GaN 13 and silicon in HVPE-grown GaN 67 (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 concentration between 10 15 and 10 17 cm À3 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. 33or p-type doping, Mg provides a hole by occupying a Ga site. 72However, the hole population is limited to within 10 18 cm À3 in p-type GaN:Mg, and the resistivity is always greater than 10 4 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 consumption of Mg by hydrogen passivation, i.e., the formation of the electrically inactive neutral complex (Mg-H) 0 during the growth or high-temperature annealing process; 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, Mg Ga V N . 77,78esides 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, resulting in SI substrates with a high resistivity of $10 10 X/ٗ. 79e forms the charge transfer deep levels Fe Ga 3þ/2þ when it occupies the Ga lattice sites, 80 and the charged Fe Ga 3þ state can transform to Fe Ga 2þ by capturing an electron and thus compensate the residual donors.The energy level that represents 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 incorporation 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 $10 4 cm À2 . 85Although 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 k z direction, hx: heavy hole at k x direction, lz: light hole at k z direction, lx: light hole at k x direction.GaN has the largest breakdown voltage among all the commercial semiconductors, its electron mobility exceeds all but GaAs, Ge, and diamond, its thermal conductivity exceeds all but AlN and diamond.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 dislocation 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 241 Am 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 containing 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 research 10 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.

Property
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. 10On the other hand, the structure suffers from several drawbacks: (a) The thin epilayer (<10 lm) results in only partial energy deposition; (b) multiple Gaussian functions are needed to fit the a spectrum; and (c) parasitic capacitance and resistance may exist due to the long distance between the two metal contacts.

Mesa structure
The mesa structure (mesa-1) is based on thin-film GaN as shown in Fig. 1(b). 6,12An 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 10 15 cm À3 : 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 samples, 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 semiinsulating 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 Â 10 16 cm À3 ) 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 distinct 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 transport 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 structures, 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 onresistance (as seen in the current-voltage curve), and the capacitance-voltage data may no longer be reliable.

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 characterized by a very low carrier concentration of $10 13 -10 14 cm À3 in the upper $30-lm (the concentration in the rest of the GaN is $10 16 cm À3 ).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 optical injection) measurements show that the low carrier concentration 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 Â 10 16 cm À3 .Due to the relatively high doping concentration, 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 simulation 88 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 trapping is negligible.
Compared with thin-film-based structures, the bulksandwich structure has a potential advantage of offering a large depletion region and thus a full a-particle energy deposition.Furthermore, it has superior carrier transport properties and, in particular, suffers less of a current crowding problem. 88,89At the same time, the structure maintains fabrication simplicity.We may therefore conclude that the sandwich 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. 15At higher energies such as 40 keV, the coefficient is about 50 cm À1 , which corresponds to a 1/e attenuation 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 demonstrated by Duboz et al. using both the Schottky MSM 15 and Schottky diode structures. 16,17The 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 mm 2 .Fig. 4 shows the transient characteristics after the X-ray source was switched on and off at two different power levels. 16-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 components related to electrons trapped in different energy levels characterized by their own lifetime constants. 22,90To 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 perceived 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 aeronautic, marine, and medical applications that require a long life time and high reliability power supply. 23Currently, GaN has been investigated for beta particle response in its application of making beta-voltaic micro-batteries.
Theoretical calculations from a conceptual design 24,91 utilized two GaN pn junctions sandwiched with a 63 Ni radioisotope source (with a half-life of 100 yr).The results indicated a maximum short circuit current of 1.1 lA/cm 2 , maximum open circuit voltage of 2.7 V, and ideal efficiency of 25%.The efficiency obtained is higher than those presently 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 source. 92wo groups have tested their GaN-based beta-voltaic devices simultaneously.6][27] In one of their p-i-n devices, using a 63 Ni source with activity of 2 mCi, an open-circuit voltage of 1.62 V, short-circuit current density of 16 nA/cm 2 , filling factor of 55%, and energy conversion efficiency of 1.13% were obtained. 26Another 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. 23he 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. 93Admitting that the development of GaN beta-voltaic is still at its early infancy, GaN is highly promising as a potential candidate for long-life nuclear microbatteries 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 produce charged particles for subsequent electron-hole production.Commonly used neutron converters are 10 B, 6 Li, and 157 Gd, 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 inefficient reaction process (e.g., proton recoil).For thin-filmcoated 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 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 overall performance of the detector.Both configurations shall be explored to study the behavior of GaN-based neutron detectors.In addition, c-ray discrimination is always a valid concern with solid-state neutron detection devices. 97Although 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 rare-earth-doped GaN thin film might lead to significant improvements in device performance in sensor applications; GaN is potentially neutron sensitive when doped with Gd. 100,101 Melton et al. 102 found that the carrier concentration 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. 18Although 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 leading 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 Â 10 18 cm À3 ) with 6 LiF 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 magnitude higher than undoped samples. 104 thermal neutron induced spectrum in GaN was first realized by Cao. 105A sandwich structure Schottky diode with a diameter of 1 mm was fabricated on special growth n-type GaN grown by Kyma.A 6 LiF: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 neutron 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 6 Li capture, i.e., 3 H at 2727 keV and 4 He at 2055 keV were clearly discernible in the spectrum.However, because the depletion depth of the detector 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 4 He is greater than that of 3 H, 4 He deposits more energy in the depletion region than 3 H.

E. Intrinsic neutron sensitivity
GaN is uniquely qualified for intrinsic neutron detection due to a 584 keV monoenergetic proton emission following neutron capture in 14 N. Detection of this proton serves as an indication of a neutron interaction with the detector.Although the cross section of 14 N 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 efficiency 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 14 N 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, 14 N's sensitivity to fast neutron is in the same order of magnitude with that of 10 B because of its reduced difference in neutron capture cross-section at, for example, 1 MeV (0.03 b for 14 N versus 0.22 b for 10 B), and also taking into account 14 N's much larger natural abundance ratio (99.6% for 14 N versus 20% for 10 B).Transmutation of the semiconductor material, leading to unwanted doping and changes in the detector's characteristics, should also be considered in high neutron flux environments.Although the cross sections of 69 Ga and 71 Ga (60% and 40% atomic abundance, respectively) are somewhat larger than other semiconductor 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 6 Li neutron capture and 14  deplete concurrently with N. A high flux neutron detector of GaN would therefore suitably detect neutrons, while delaying 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 detection by placing a sample of bulk GaN in a high flux neutron beam and measuring the proton emission spectrum in vacuum.Results shown in Fig. 8 indicated a sufficient number of pulses were detected for GaN to be used as a neutron conversion material, even though a very small solid angle ($0.001Sr.) was used in the experiment. 106he 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 thicknesses of two samples measured.The detectable depth limit of the 14 N 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 references; 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. 109For 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 displacement, 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 collisions. 109A study in 2006 (Ref.110) showed that fast neutron irradiation with a fluence of 6.7 Â 10 18 cm 2 produces approximately 1900 and 7200 displaced atoms per PKA of Ga and N, respectively.
Thermal neutrons can produce indirect atomic displacements as a result of radioactive capture (n,c) reactions.The main thermal reactions in GaN are listed in Table II. 111,112mong these reactions, the energy of recoil atoms generated by the first two reactions is sufficient to cause approximately 2 Â 10 5 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 Â 10 À5 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 commonly 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 14 N neutron capture in two types of GaN wafers. 106a, 72 Ga, and 16 N 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 inplane lattice parameter does not change.For instance, for a neutron fluence of 8 Â 10 19 cm À2 , the lattice constant c increases by 0.38% and the lattice constant a nearly unchanged. 114Neutron irradiation can also alter the resistivity or even cause type-reversion of the material.Compensation schemes have been identified for both p-type 115 and n-type 116 GaN when the neutron fluence exceeds 10 16 cm À2 .Wang et al. 116 suggested that the neutron-irradiation-induced structure defects Ge Ga give rise to carrier trap centers that are responsible for the observed reduction in the carrier concentration 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 $10 18 cm À2 , 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. 115For Fe-compensated semi-insulating 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 10 14 to 10 16 n/cm 2 (Ref.118).For low neutron fluences on the order of $10 11 cm À2 , the damage can be repaired by a self-annealing process at room temperature for several days. 119,120For medium ($10 14 cm À2 ) and high fluences ($10 19 cm À2 ), 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. 113n 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 10 14 to10 16 cm À2 , and the CCE of the 12-lm device under a fluence of 10 16 cm À2 decreased from 53% to 20%.They suggested that the degradation is mainly due to an increased number of recombination/trapping 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 10 15 n/cm 2 was discovered.However, there may be an optimal fluence for which the device performance is enhanced.One study by Wang 122,123 shows that after neutron irradiation with a fluence of 1 Â 10 13 cm À2 , the Au/GaN Schottky barrier photodetectors showed superior currentÀvoltage characteristics, which was attributed mainly to the effective repression of the deep electron traps E t .
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 !10 15 n/cm 2 increases deep level defects densities, while thermal neutron with fluences 10 15 n/cm 2 anneal these defects and improve the GaN crystalline quality.Their further studies 125 revealed that both fast and thermal neutrons introduce recombination centers in GaN, forming an insulating phase between GaN and metal contact, raising the Schottky contact ideality factor, increasing the Ohmic contact 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. 111,112actions Main c-ray energy (main transition probability) (MeV) Recoil atom energy (MeV) 69 Gaðn; cÞ 70   diffusion of Al for the Al-only-based contact, and the continuity 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.][130][131][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. 33Kim et al. 133 showed that W contacts degrade at annealing temperatures greater than 500 C, and the metal silicide WSi x 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. 70,71,120,126,135he 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/cm 2 or a reverse bias of À3.5 V while stored at 300 C in an N 2 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 ionizing 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 crystal 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.

FIG. 4 .
FIG. 4. Photocurrent transient measured for two incident X-ray powers of P ¼ 10 and P ¼ 40 (corresponding to currents of 10 and 40 mA on the X-ray tube, respectively).The applied reverse voltage is À10 V. Reproduced with permission from J. Appl.Phys.105, 114512 (2009).Copyright 2009 AIP Publishing LLC.