e-Journal of Surface Science and Nanotechnology
e-J. Surf. Sci. Nanotech. Vol. 12 (2014) 396-399

23 August 2014

Conference - ACSIN-12 & ICSPM21 -Evaluation of Scintillation Properties of GaN∗
Takayuki Yanagida† and Yutaka Fujimoto
Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu, Kitakyushu 808-0196, Japan

Masanori Koshimizu
School of Engineering, Tohoku University, 6-6 Aramaki, Aoba, Sendai 980-8579, Japan(Received 14 December 2013; Accepted 6 June 2014; Published 23 August 2014)
Optical and scintillation properties of GaN crystalline film were investigated. It showed 30-50% in-line transmittance at wavelength longer than 400 nm and intense emission appeared at 365 nm under 213 nm excitationin photoluminescence (PL) spectrum. When PL decay time was investigated under 280 and 200 nm excitation,
luminescence decay time resulted few ns. In X-ray induced radioluminescence spectra, intense emission appeared
at 365, 420, and 550 nm. The former one was ascribed to the exciton emission and the latter two would be due todefects related emission. Scintillation decay time of GaN was quite fast as few ns which was consistent with PL
results and no slow component was observed. [DOI: 10.1380/ejssnt.2014.396]
Keywords: Synchrotron radiation photoelectron spectroscopy; Visible/ultraviolet absorption spectroscopy; Luminescence;
Photoluminescence; Gallium nitrideI.

INTRODUCTIONGaN is well known direct bandgap semiconductor thatis used as a blue light emitting device (LED). Up to now,
most studies about GaN have been devoted to electroor photo-luminescence for LED applications. However,there is one possibility for another application, scintillation device. Scintillators are phosphors which convert
high energy (keV MeV) ionizing radiation to thousandsof visible photons immediately [1] and are widely used incommon applications, such as medical imaging (PET and
X-ray CT) [2], security systems in ports and airports [3],well-logging [4], astrophysics [5], and particle physics [6].
Direct transition semiconductor materials are potentiallygood scintillators because a fast decay time can be expected. One of good examples is ZnO. In ZnO, sub-nanoseconds fast scintillation decay time due to a free excitonand medium level of scintillation light yield were observed[7]. Thus, similar scintillation properties can be expected
in GaN.
GaN was rarely studied for radiation detectors and scintillation properties were reported few times [8-11]. In
those reports, GaN was doped with Si or combined withconverter materials such as LiF for thermal neutron detection. Almost all previous studies were devoted to investigate other kinds of luminescence, such as photoluminescence (PL) or cathodeluminescence. Very recently
after our submission of this work, proton excited scintillation was reported [12] and the main aim of that study
was not to grasp scintillation detector properties. Therefore, it means that scintillation responses of GaN are still
unclear and there remains a room for study. In most scintillation detectors, majority is bulk materials with few cm
to m to detect high energy photons (X-ray or γ-ray) or
neutrons eﬃciently. However, ﬁlm scintillators are suited

FIG. 1. Appearance of GaN sample, product of Optostar Ltd.

to charged particle (e.g., α-ray) detectors since high energy photons or neutrons become noise for charged particle detection and insensitivity of ﬁlm scintillators against
ionizing radiations with high penetration power are ideal.
For this purpose, we [13] and some other groups [14-15]
examined ZnO ﬁlm scintillators for α-ray detectors.
In the present work, we investigated scintillation characteristics of GaN ﬁlm scintillator for the ﬁrst time. The
sample was prepared by Optostar Ltd. and had a size of
5 × 5 × 0.3 mm3 . Though in some other ﬁelds like semiconductor ﬁlm means a sub-micron thickness, scintillationﬁeld recognizes several hundreds micron thick materials
as ﬁlm scintillators so that we call the sample as ﬁlm inthis work. Figure 1 represents an appearance of GaN. In
this sample, wide surface areas were polished and oxygen impurity level was investigated by the manufacturer
and it was around 1019 cm−3 . The origin of dark color
was blamed for this oxygen impurities. In scintillators,
dopant concentrations of famous materials are 1-10 mol%and the oxygen impurity level of the present GaN sample
was smaller in several orders.

∗ This paper was presented at the 12th International Conferenceon Atomically Controlled Surfaces, Interfaces and Nanostructures (ACSIN-12) in conjunction with the 21st International Colloquium on Scanning Probe Microscopy (ICSPM21), Tsukuba
International Congress Center, Tsukuba, Japan, 4-8 November,2013.
† Corresponding author: yanagida@lsse.kyutech.ac.jp

II.

EXPERIMENTAL

In-line transmittance was evaluated by using JASCO
V670 spectrometer from 190 to 2700 nm with 1 nm step.
PL spectrum under 213 nm excitation was recorded withc 2014 The Surface Science Society of Japan (http://www.sssj.org/ejssnt)
ISSN 1348-0391 ⃝396e-Journal of Surface Science and Nanotechnology

Volume 12 (2014)

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FIG. 2. Transmittance and PL spectra under 213 nm excitation of GaN. The inset shows the expanding image of PL.

JASCO FP8600 spectroﬂuorometer. PL decay time proﬁles were investigated under 280 nm excitation which was
the shortest wavelength of the instrument monitoring at
365 and 420 nm by using Hamamatsu Quantaurus-τ andunder 200 nm excitation monitoring to 420 nm at Synchrotron facility (UVSOR BL-7B). The detailed explanation about Synchrotron experiments can be shown in ourpast report [16].
X-ray induced radioluminescence spectra was evaluatedby using our original setup [17-18]. The excitation source
was an X-ray generator supplied with 80 kV bias voltage
and 2.5 mA tube current. Scintillation photons were fed
into a monochromater via optical ﬁber and into CCD. Radioluminescence was evaluated by a reﬂection-type measurement as same as general PL measurements to avoid
self-absorption of the sample. X-ray was irradiated at
an angle of 45◦ to the sample and readout ﬁber was also
put at an angle of 45◦ to the sample. Scintillation decay time proﬁles were evaluated by using pulse X-ray
equipped streak camera system for scintillation characterization which was also developed by us [19-20] and becamea product of Hamamatsu. The mean and maximum end
point energy of X-ray were 20 and 30 keV, respectively.
In scintillation spectrum and decay time evaluations, undoped ZnO was also evaluated for comparison. Undoped
ZnO was a product of Tokyo Denpa and had a size of
10 × 10 × 0.5 mm3 . All experiments were conducted atroom temperature.
RESULTS AND DISCUSSIONS

Figure 2 demonstrates in-line transmittance and PL
spectra of GaN. In-line transmittance was 30-50% at
wavelength longer than 400 nm and reached to 0% around1500 nm. In PL spectrum, one sharp line at 365 nm due toband-edge (exciton) emission was observed. This exciton
emission was reported many times in PL and cathodeluminescence evaluations [21-22]. Compared with scintillation described later, broad lines around 420 and 550 nm
were also observed. The origin of 420 emission was mainly

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420 nm50

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FIG. 3. PL decay time profiles of GaN under 365 and 420 nm
excitation.

ascribed to Ga vacancy (VGa ) [23] and the most plausible explanation about 550 nm emission was unexpectedcontamination of very low content of Zn or Si [24]. In
PL spectrum, exciton emission became quite intense under 213 nm excitation. Such a basic study about PL wasinvestigated so many times (e.g., [23]) so that we did not
discuss PL further.
In Fig. 3, PL decay time proﬁles using Hamamatsu
Quantaurus-τ are shown. The instrumental response wasdeconvoluted and deduced decay times were 4.5 and 7.7
ns for 365 and 420 nm emission, respectively. Comparedwith ZnO, primary decay time was slightly slower but
defect originated emission (420 nm) was faster than thedefect emission of ZnO [25]. When 550 nm emission wasinvestigated, signiﬁcant diﬀerence with 420 nm emission
was not observed. At least in PL, no slow emission wasobserved in GaN.
To investigate PL decay time under higher energy (200nm) excitation, we used Synchrotron radiation (UVSOR).
Figure 4 exhibits decay time proﬁle of 420 nm emission
under 200 nm excitation. As shown in the ﬁgure, main
decay time resulted sub-ns which was signiﬁcantly fasterthan 280 nm excitation. Sometimes excitation bandsof defects emission appears around vacuum ultra violet
wavelength in insulator materials [26]. By 200 nm excitation, exciton emission at 365 nm was not eﬃciently
excited in our sample and the surface condition wouldaﬀect such a PL evaluation.
Scintillation emission spectrum under X-ray irradiationis shown in Fig. 5. Although emission lines appeared
around 365, 420, and 550 nm similar to PL one, intensitiesof each line diﬀered. Exciton emission at 365 nm weakened on a comparison with PL spectrum partially due toa self-absorption in ultra violet wavelength of the sample
since X-rays were absorbed not only at the surface. In this
X-ray energy region, the X-ray energy is absorbed mainlyby photoelectric eﬀects. The penetration depth of several
tens keV X-rays to GaN is at least several µm and thispenetration depth is larger than that of PL. Though scintillation could be much suﬀered by self-absorption thanPL, the clear detection of exciton emission suggested that
GaN would be potentially available for scintillation detectors. In Fig. 5, scintillation spectrum of undoped ZnO

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Yanagida, et al.

Volume 12 (2014)
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FIG. 5. X-ray induced radioluminescence spectra of GaN and
ZnO. Bias voltage and tube current of X-ray generator were
80 kV and 2.5 mA, respectively.

was compared. In ZnO, free exciton emission at 380 nm
and defect emission at 550 nm were observed. As same as
GaN, exciton emission was not so intense in scintillation.
Figure 6 depicts scintillation decay time proﬁle of GaN.
The present data was not resolved in wavelength assuming an actual scintillation detectors so that all emissions(365, 420, and 550 nm) were included. The time proﬁles
was well reproduced by a double exponential assumption.
Deduced decay times were 1.1 ns (88%) and 6.5 ns (12%).
The impressive point was that no slow component wasobserved in µs order observation. Scintillation decay time
proﬁle of undoped ZnO is also presented in Fig. 6. Obviously, very slow component due to defect was detected
as well as fast luminescence. Fast and slow components
resulted 3.3 ns and 7.7 µs, respectively. As clearly seen in
the ﬁgure, most part of scintillation in ZnO was slow emission. In ZnO based scintillators, slow µs component due
to oxygen vacancy at visible wavelength [27] sometimes
causes a degradation of timing resolution of detectors. It
is an advantage of GaN that does not have any slower
decay component when compared with the other famoussemiconductor scintillator ZnO.

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FIG. 6. X-ray induced scintillation decay time profiles of GaN
and ZnO. Insets of both panels show expanding image. The
mean energy of X-ray was around 20 keV.

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CONCLUSIONS

Optical and scintillation properties of GaN crystal were
investigated. In-line transmittance showed 30-50% transmittance from visible to near infrared wavelengths. In
PL and X-ray induced radioluminescence spectra, emission lines appeared at 365, 420, and 550 nm. The originof 365 nm line was ascribed to exciton. Origins of 420
and 550 nm emissions were Ga vacancy and very low content of unexpected Zr/Si impurities. PL and scintillation
decay times were quite fast and no slow component wasobserved. The advantage of GaN scintillator was a fast
timing property when compared with other conventional
semiconductor scintillator, ZnO.

ACKNOWLEDGMENTS

This work was mainly supported by a Grant in Aid
for Scientiﬁc Research (A)-26249147 from the Ministry of
Education, Culture, Sports, Science and Technology of
the Japanese government (MEXT) and partially by JST
A-step. Partial assistance from Nippon Sheet Glass Foundation for Materials Science and Engineering, Tokuyama
Science foundation, Iketani Science and Technology Foundation, Hitachi Metals Materials Science Foundation,
Mazda Foundation, JFE 21st century Foundation, and
The Asahi Glass Foundation, the Cooperative Research
Project of Research Institute of Electronics, Shizuoka University are also gratefully acknowledged.

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