Modul 10
γ - Ray Irradiation Effects on the Characteristics of New
Material P Type 6H-SiC Ni-Schottky Diodes (Application For
Nuclear Fuel Facilities)
U. Sudjadi１), T. Ohshima２), N. Iwamoto２, ３), S. Hishiki２), and K. Kawano３)
1) Center For Nuclear Fuel Technology, BATAN
Japan Atomic Energy Agency (JAEA), Gunma 370-1292, Japan
3) The University of Electro-Communications, Tokyo, 182-8585, Japan
２)
Keywords: Current Density, On Resistance, Barrier Height, 6H SiC, Schottky Diodes,
γ-ray Irradiation.
Abstract. Effects of γ-ray irradiation on electrical characteristics of new material p
type 6H-SiC Ni-Schottky diodes were investigated. Ni Schottky diodes fabricated on
p type 6H-SiC epi-layer were irradiated with γ-rays at RT. The electrical
characteristics of the diodes were evaluated before and after irradiation. The value of
the on-resistance does not change up to 1 MGy, and the value increases with
increasing absorbed dose above 1MGy. For n factor, no significant increase is
observed below 500 kGy, however, the value increases above 500 kGy. Schottky
Barrier Height (SBH) decreases with increasing absorbed dose. Leakage current
tends to increase due to irradiation.
Abstrak. Efek dari radiasi ɤ pada karakteristik kelistrikan dari Ni-diode Schottky
material baru 6H-SiC tipe p telah diteliti. Ni-diode Schottky difabrikasi pada epilayer 6H-SiC tipe p, di iradiasi dengan sinar ɤ pada temperatur ruang. Karakteristik
kelistrikan dari diode telah dievaluasi sebelum dan setelah iradiasi. Harga dari on
resistance tidak berubah sampai 1 MGy, dan harganya mengalami kenaikan dengan
naiknya penyerapan dosis diatas 1MGy. Untuk n-factor, diamati dibawah 500 kGy
kenaikannya tidak significant, tetapi diamati diatas 500 kGy harganya naik. Schottky
Barrier Height (SBH) turun dengan kenaikan dosis penyerapan. Disebabkan oleh
iradiasi, kebocoran arus kecenderungannya naik.
Introduction.
Silicon carbide (SiC) material is a promising candidate for high power and
high frequency electronic devices because of its excellent thermal and electrical
properties such as stability at high temperature, wide band gap, high breakdown field,
and high thermal conductivity. In addition, since SiC has a strong radiation resistance,
it is also expected to be widely applied in electronic devices used in harsh radiation
environments such as nuclear power plants, nuclear fuel facilities, accelerator
facilities, space and so on [1,2]. For the development of radiation hard devices based
on SiC, it is very important to study radiation response of devices. Gamma-ray
irradiation effect on the characteristics of SiC Schottky diodes have been studied by a
few researchers. M. Bruzzi et al. have irradiated n-type 4H-SiC Schottky diodes with
γ-ray in the dose range 0.1 – 1 Gy. M. Bruzzi et al. have also applied 4H-SiC material
for radiation dosimetry [3, 4]. Nava et al. have investigated the radiation tolerance of
epitaxial n-type 4H-SiC detector for electrons and γ-rays. Nava et al. have irradiated
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Where A*= (m*/m0), m* is effective hole mass, and A* is effective Richardson constant.
In this analysis, we employ for 6H-SiC m*= 0.71 m0, and A* for 6H-SiC is 194.1
(A/cm2)/K2. Figure 3 shows on resistance dependence of the absorbed dose of p-type
SiC Schottky diode samples with 150 μm and 300 μm circular diameters. The value
of the on-resistance does not change up to 1 MGy, and the value increases with
increasing absorbed dose above 1 MGy. Figure 4 shows n-factor (ideality factor)
dependence of the absorbed dose of p-type SiC Schottky diode samples with 150 μm
and 300 μm circular diameters. The n-factor is no significant increases below 500
kGy, however n-factor increases above 500 kGy.
According data in Figures 3 and 4 that on resistance and n-factor does not change up
to 1 MGy and 500 kGy, it is clear that the new material 6H-SiC has a strong radiation
resistance。 It is also clear that the new material 6H-SiC expected to be widely
applied in electronic devices and detector used in harsh radiation environments such
as nuclear power plants, nuclear fuel facilities, accelerator facilities, space and so on.
If comparing with Si as material base for electronic devices. Si material has a low
radiation resistance. Si material has a short life time and not function well for
electronic devices and detector in high radiation area such as nuclear fuel facilities,
nuclear power plants, accelerator facilities, aerospace etc. [8].
Figure 5 shows Schottky barrier height (SBH) dependence of the absorbed dose of ptype SiC Schottky diode samples with 150 μm and 300 μm circular diameters. The
SBH decreases with increasing absorbed dose.
Figure 6 shows current density versus reverse bias of p-type SiC Schottky diode
sample with 250 μm circular diameter. As shown in this figure, the leakage current
tends to increase due to gamma-ray irradiation.
10
Ni,=250m
1
0
0
Ni,=250m
Current Density (A/cm )
Current Density (A/cm )
2
2
8
6
4
0MR
3MR
10MR
69MR
2
0
0.0
0.5
1.0
1.5
2.0
Forward Bias (V)
2.5
3.0
10 -3
0MR
3MR
10MR
69MR
10 -6
10
-9
0.0
0.5
1.0
1.5
2.0
Forward Bias (V)
2.5
3.0
Fig. 1 Current density versus forward bias Fig. 2 Semi-logarithmic plot of current
density (J) versus forward bias (V)
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In the case of C2 = 0, where the density of interface state is ∞ the Fermi level at the
interface is pinned by the surface state at the value Φ0 (neutral level, which is the
position that the Fermi level must assume if the surface is electrically neutral) above
the valence band. The SBH is independent of the metal work function. This situation
of the strongest pinning is the Bardeen limit. When C2 is 1, the situation is called the
Schottky limit, where the density of interface is zero. It means Fermi level is free
from pinning. In the samples, we suggest after irradiated with gamma ray, the SBH
decreases with increasing absorbed dose, where the value of proportional factor C2
increases with increasing absorbed dose, and the density of interface state decreases
with increasing absorbed dose. Therefore SBH decreases with increasing absorbed
dose (see Fig. 5). The increase ideality factor above 500 kGy is indicating an
increase of defect density at the interface with increasing gamma ray irradiation dose
and/or the increase of the ideality factor is due to the lateral in-homogeneity of barrier
height (see Fig. 4) [12]. The ideality factor no significant increase is observed below
500 kGy, this result is not normal behavior, a possible probability this fact is due to
un-sensitivity, of the mean barrier height variation to the field and/or un-sensitivity
the barrier height standard deviation, variation to the field. The electrical properties
change of Schottky diodes after gamma irradiation, are strongly depend on of the
fabrication process and the absorbed dose of gamma ray irradiation. Irradiation of
gamma-ray to the Schottky diodes created Compton and photoelectric effects.
Electrons produced from the Compton and photoelectric effects are the dominant
contributions to the electron flux generated in a metal semiconductor interface of the
p-type 6H-SiC Schottky diode of the samples. Co60 source in cascade have energies of
1,173 and 1.332 MeV, respectively. They are customarily approximated by a single
energy 1.25 MeV [13, 14]. Photons of this energy produce Compton electrons which
provide a main contribution to primary production in metal (Nickel), semiconductor
(6HSiC), and metal semiconductor (Nickel and 6HSiC) interface. These electrons
have influenced to the electrical properties change of the p-type 6H-SiC Schottky
diodes of the samples.
Summary
Ni Schottky diodes fabricated on p-type 6H-SiC epi-layer were irradiated with
gamma-rays, and the electrical characteristics of the diodes were evaluated. The
value of the on-resistance does not change up to 1 MGy, and the value increases with
increasing absorbed dose above 1 MGy. For n factor, no significant increase is
observed below 500 kGy, however, the value increases above 500 kGy. Schottky
barrier height decreases with increasing absorbed dose. Leakage current tends to
increase due to irradiation.
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