Research Report of Laboratory of Nuclear Seience, Tohoku University Vol. 40 (2007)

31(LNS Experiment : #2581)

Radiation Hardness Test of GaN Diode for Irradiation withHigh Energy Electron Beam
S. Narita1 , Y. Yamaguchi1 , Y. Chiba1 , H. Yuki2 , F. Hinode2 , and J. Kasagi2
1

Department of Electrical and Electronic Engineering, Iwate University, Morioka, 020-8551
2

Laboratory of Nuclear Science, Tohoku University, Sendai, 982-0826

The GaN is expected to be a new material for the particle detector with radiation hardness, as
substitution of Si. We have developed the GaN Schottky diode which is a prototype of the ionizing detector. In this study we performed the test of radiation tolerance for the diode we fabricated, irradiating ahigh energy electron beam. As the result, we found that the performance of the diode was not changed
signiﬁcantly even after irradiating 1016 /cm2 electrons.

§ 1. Introduction
In high-energy collider experiments, the beam luminosity has been increasing, and a semiconductor
detector with radiation hardness is strongly desired in the current and future experiments. Several
semiconductor materials have been investigated for developing new particle detectors as the substitutionof Si [1, 2]. Among them, a wide-gap nitride semiconductor, especially gallium nitride (GaN), is one ofthe possible candidates. Recently, GaN devices have been widely used (e.g., light emitting device) andthe technology to produce high quality substrate has been improved remarkably, so that development of
the particle detector with GaN is now promising [3].
We have been developing the GaN based particle detector and successfully fabricated the GaN
Schottky diode which has high quality in electrical and optical characteristics. Now it enables us torealize the GaN particle detector. In this study, we irradiated the diodes with high energy electrons anddiscussed the radiation hardness of GaN material.

§ 2. Experiment
2.1 GaN diode sample
The Schottky barrier diodes used in this study were fabricated with heterostructure substrate produced by Powdec K.K. growing an epitaxial GaN layer on n-type SiC substrate. Most of the current GaN
devices are based on GaN on Sapphire substrate. For the Schottky diode device using GaN/Sapphire
substrate, both Schottky and Ohmic contacts have to be made on the same surface because Sapphire
is insulator. However, the diode with such a structure is not preferred to use for the particle detector
because the device design is complicated and its sensitive area is limited. On the other hand, the diodewith GaN/SiC substrate can be designed more simply due to the electric conductivity of SiC. The Ohmic
contact can be put on the SiC side. The diode structure is shown in Fig.1.

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Au padSchottky contact

820 μmNi /Auu-GaN
500 μm

n-SiC

120μm

Ohmic contactTi /AuFig.1. GaN diode structure.

The chip size of the diode was 0.5×0.5 mm2 , and the thickness of the GaN and SiC layers were∼900 and ∼280 nm, respectively. The Ti/Au Ohmic contact was deposited on to the SiC and the Schottky Ni/Au electrode was put onto the GaN layer. For the irradiation test, the diode was placed onto
the metal package. The current-voltage (I-V) characteristics of the diodes were measured by the ultrahigh resistance meter (ADVANTEST R8340). The leakage current was well suppressed (∼10 nA/cm2 )and the breakdown voltage was typically above 20V. These performances were comparably good to thepresent GaN diode. We also investigated the photo-conductivity to the UV light, and it showed a good
performance as the photo diode. Actually, the thickness of the sensitive layer was too thin to evaluate
the performance for the charged particle (even for, e.g., α particles), for now.
The I-V characteristics were measured before and after irradiation in order to determine the effectsof the incident electrons.
2.2 Electron beam irradiation
Beam irradiation was carried out at Laboratory of Nuclear Science, Tohoku University (LNS), using
a 150 MeV pulsed electron beam. The beam current was measured by the SEM placed at the downstreamof the target diodes. The conversion factor from the SEM current to the beam current was calibrated
in advance of the experiment, which has about 30% uncertainties. The mean beam current value inthis experiment was found to be 2.5 μA. We assumed that the beam condition was stable during theirradiation, then the current measurement was employed just once in prior to the irradiation. In thisexperiment, we prepared several diodes and irradiated them with electrons with various ﬂuences. During the irradiation, the target diodes were not applied voltage, then the output currents from the diodewere not monitored.
The beam proﬁle was measured by the radio-activity of aluminum foil irradiated with the electron
beam with the same conditions in the irradiation to the diodes. After irradiating the foil, the activity ofthe segmented area of the foil was measured by a Ge detector. The proﬁles along X and Y axes with theﬁtted Gaussian curves are shown in Fig.2. The electron ﬂuences on the diode were obtained considering
the mean current of the beam and these proﬁles. The ﬂuence for each sample is shown in Table 2.2. Theerrors in the list were from the uncertainties of the SEM calibration data.

Relative Intensity

33

1

1

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0-10010Beam X (cm)0-10010Beam Y (cm)Fig.2. Beam proﬁle ﬁtted with the Gaussian. (left:X, right:Y). The ﬁtted curves represent f (x) =1.01e(x+0.98)26.442

for X-axis and f (y) = 1.08e(y−1.24)2
5.902

for Y-axis, respectively.

Table 1. Calculated electron ﬂuences for each sample.
Sample 1
Sample 2
Sample 3
Sample 4

Electron ﬂuence [/cm2 ]2.2 ± 0.7 × 1014
2.2 ± 0.7 × 1015
1.8 ± 0.4 × 1016
3.4 ± 1.0 × 1016

§ 3. Results and Discussion
Figure 3 shows the I-V curves before and after irradiation for each ﬂuence. No signiﬁcant increaseof leakage currents was seen even with the electron ﬂuences above 1016 /cm2 . The break down voltage
was not changed clearly. These results would prove the radiation hardness of the GaN.
The degradation of the semiconductor detector is strongly dependent on the energy loss of particles
irradiated in both non-ionizing (atomic displacement) and ionizing (form of electron-hole pair) processes
in the material. The non-ionizing energy loss of a 150 MeV electron is estimated to be 0.1 keV cm2g−1 [4], and this is smaller than that of other charged particles and a neutron. Besides the type of theparticles, we should consider that the thickness of the diode used in this experiment was just ∼ 1μm. The
particle detectors used practically have much thicker structure, so that non-ionizing and ionizing effects
are supposed to increase in such detectors. Therefore we still need to perform the irradiation test using
heavy charged particles such as protons or α particles to discuss precisely the radiation hardness of theGaN material. Furthermore, we should repeat the test with changing the experimental conditions since
the damage effects depend on various parameters (e.g., with/without applying the voltage, temperature,device structure, and so on).

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before irradiation

after irradiation-2

10-410-610-810-1010Sample-1-15-10-50-15-10-50-10-50-15-10-50-10-50-15-10-50-10-50-15-10-50

2Current density [A/cm ]

-2

10-4
10-610-810-10
10Sample-2-15-210-410-610-8
10-1010Sample-3-15-2

10-4
10-610-810-10
10Sample-4-15

Bias voltage [V]Fig.3. Current-Voltage characteristics for the sample before and after electron beam irradiation.

§ 4. Summary
We have evaluated the tolerance of the GaN material for the radiation damage irradiating withhigh energy electron beam. The performance was not signiﬁcantly changed after irradiating 1016 /cm2electrons. The GaN might be a promising candidate for the material of future particle detectors. We
should keep investigating the performance under various conditions and ensure the advantages.

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Acknowledgment
We acknowledge technical and scientiﬁc staffs of the LNS. This work was supported by Grant-in-Aidfor Young Scientists (B) by the Ministry of Education, Culture, Sports, Science and Technology (MEXT).

References[1]

M. Moll: Nucl. Instrum. Methods Phys. Res., Sect. A 511 (2003) 97.

[2]

J. Grant etal .: Nucl. Instrum. Methods Phys. Res., Sect. A 546 (2005) 213.

[3]

J. Vaitkus etal .:Nucl. Instrum. Methods Phys. Res., Sect. A 509 (2003) 60.

[4]

A. Holmes-Siedle and L. Adams: “Handbook of Radiation Effects” (Oxford University Press, 2001).