MODUL 8
Forward and Reverse Current Characteristics of P-Type 6H-SiC
Schottky Diodes with SiO2 Ramp Profile after Gamma Ray
Irradiated up to 113.1 kGy
U. Sudjadi1), T. Ohshima1), N. Iwamoto1, 2), S. Hishiki1), and K. Kawano2)
1)
Japan Atomic Energy Agency (JAEA), Gunma 370-1292, Japan
2)
The University of Electro-Communications, Tokyo, 182-8585, Japan
Keywords: Forward-reverse currents, 6H SiC, Schottky diodes, SiO2 Termination,
γ-ray irradiation
Abstract
Forward current density, reverse current, leakage current of p-type 6H-SiC
Schottky diodes with a perpendicular edge (900) SiO2 termination after gamma-rays
irradiated up to 113.1 kGy (13 Mrad) at RT were investigated. A perpendicular edge
termination based on oxide ramp profile around the Schottky contact is used on Al
Schottky rectifier fabricated on a 10 μm p-type 6H-SiC epi-layer on p-type 6H-SiC
substrate (3.50 off, Si face), Na: 5.9 x 1015/cm2 . The electrical characteristics of the
diodes are evaluated before and after irradiation. The results have shown that the
current density in the forward bias no significant change is observed below 113.1 kGy.
The leakage current is observed also no significant change below 113.1 kGy.
Introduction
The SiC Schottky diode which uses the oxide ramp, have been studied by
several researcher. G. Brezeanu et al. have simulated 6H-SiC Schottky structure
which uses the oxide ramp etching technique in order to attenuate edge effect. They
have used a MEDICI simulation software program. The MEDICI simulated structure
has a 8x1016 cm -3 doping n-6HSiC epilayer on n+ (2x1018 cm-3) same type substrate.
Ti has used as Schottky metal. A MEDICI simulation has reported to determine the
parameters of oxide ramp for an uniform current density and volume breakdown in a
given diode structure. The 6H-SiC oxide ramp profile Schottky diode with
8x1016 cm-3 epilayer doping has around 300 V volume breakdown for 5 deg.
Maximum ramp and 1μm minimum oxide thickness [1]. G. Brezeanu et al. have
reported also about them experimental research works and compared with simulation
results. A simple termination of the planar Schottky barrier structure was
experimented and successfully tested for Ni/6H-SiC power diodes. The technique is
based on oxide etching under small angles around the Schottky contact window.
The MEDICI simulations showed that for smaller than 50 angles and oxide
thickness over 1 μm a parallel plan electric field vectors and an ideal volume
breakdown are achieved. The experimentally checked on a Ni/6H-SiC Schottky
barrier diode with about 40 ramp oxide profile and a 1.1 μm oxide thickness. Reverse
characteristics showed near-ideal parallel plane breakdown at a voltage of 800 V and
very low leakage current after vacuum annealing of Schottky contact at 900 0 C for 2
min. A remarkable weak reverse current voltage dependence for VR > 100 V has
obtained [1, 2]. Q. Zhang et al. have reported, that the forward current density has
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decrease or increase below 13 MR. Fig. 7 shows the leakage current versus absorbed
dose. The leakage currents are no significant change below 13 MR.
Figs. 8-9 show the forward current (JF – VF) of the P-type 6H-SiC Schottky
Barrier diodes , Φ = 250 μm, with oxide ramp profile, before and after gamma-ray
irradiated at 0 MR, 1 MR, 4 MR, 5 MR, 6 MR, 7 MR, 8 MR, 9 MR, and 10 MR,
respectively. We can see that no difference between un-irradiated sample and
irradiated sample up to 13 MR. Figs. 10-11 show the reverse characteristics of the Ptype 6H-SiC BSD, Φ = 250 μm, with oxide ramp profile, before and after gamma-ray
irradiated at 0 MR, 1 MR, 4 MR, 5 MR, 6 MR, 7 MR, 8 MR, 9 MR, and 10 MR,
respectively. We can see also that no difference between un-irradiated sample and
irradiated sample up to 10 MR. Fig. 12 shows the leakage current versus absorbed
dose. The leakage currents are no significant change below 10 MR.
The forward current density versus forward bias of P-type 6H-SiC diodes, Φ =
200 μm, with oxide ramp profile, before and after gamma-ray irradiated at 1 MR, 3
MR, 6 MR, 7 MR, 8 MR, 9 MR, 10 MR, 11 MR, and 12 MR, respectively shown in
Figures13-15. The forward current density before and after irradiated up to 12
MR shows no significant change. Figs. 16-18 show the reverse characteristics of the
P-type 6H-SiC BSD, Φ = 200 μm, with oxide ramp profile, before and after irradiated
up to 12 MR. The reverse current before and after irradiated up to 12 MR shows also
no significant change. Fig. 19 shows the leakage current versus absorbed dose. The
leakage currents are also no significant change below 12 MR.
Figs. 20-22 show the forward current (JF – VF) of the P-type 6H-SiC Schottky
Barrier diodes , Φ = 150 μm, with oxide ramp profile, before and after gamma-ray
irradiated at 1 MR, 5 MR, 6 MR, 7 MR, 8 MR, 10 MR, and 11 MR, respectively. We
can see that no difference between un-irradiated sample and irradiated sample up to
11 MR. Figs. 24-25 show the reverse characteristics of the P-type 6H-SiC BSD, Φ =
150 μm, with oxide ramp profile, before and after gamma-ray irradiated at 1 MR, 5
MR, 6 MR, 7 MR, 8 MR,10 MR, and 11 MR, respectively. We can see also that no
difference between un-irradiated sample and irradiated sample up to 11 MR. Fig. 26
shows the leakage current versus absorbed. The leakage current shows no significant
change below 11 MR.
Comparing with the Ni-Schottky diodes without SiO2 ramp profile [9, 10] the
data show, that the electrical properties of Ni-Schottky diodes without SiO2 ramp
profile are change after the samples irradiated up to 13 MR. However, the samples
Al-Schottky diodes with SiO2 ramp profile are no change after irradiated up to 13 MR.
These facts shows that the Al-Schottky diodes with SiO2 ramp profile are
stronger toward gamma radiation comparing to the Ni-Schottky without SiO2 ramp
profile.
The electrical properties change of Schottky diodes after gamma-ray
irradiation, are strongly depend on of the fabrication process and the absorbed dose of
gamma-ray radiation [11].
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3
10 1
10
Forward current [A/cm2]
-1
10
-3
10
-5
10
-7
10
-9
10
-11
10
-13
10
10
10MR
11MR
12MR
13MR
BT7-300mic
-15
0
2
4
6
8
Forward bias [V]
10
Fig. 3 Forward current density versus forward bias (10MR, 11MR, 12MR, and
13 MR)
10
10
-5
-7
0MR
3MR
5MR
BT7-300mic
Current [A]
-9
10
10
10
10
-11
-13
-15
0
10 20 30 40
Reverse bias [V]
50
Fig. 4 Current versus reverse bias (0MR, 3MR, and 5MR)
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