（公財）日本板硝子材料工学助成会，41（2023）

SiC パワーデバイスにおける通電劣化抑制法の開発
名古屋大学

未来材料・システム研究所

原田俊太

Suppression of Bipolar Degradation of SiC Power Devices
Shunta Harada
Institute of Materials and Systems for Sustainability, Nagoya University
SiC パワーデバイスは、積層欠陥の拡大による通電劣化という問題を抱えている。この問題は、エピタキシャル層内や基板との界面近くで起こる基底面転位から引き起こされる。
本研究では、UV 照明下でプロトン照射を用いた 4 H-SiC エピタキシャル層の積層欠陥拡大の挙動を調査した。その結果、プロトン照射が積層欠陥の拡大を抑制することが X 線
トポグラフィー観察で明らかとなった。また、過剰キャリア寿命の測定により、1 ╳1011 cm - 2 のドーズ量でのプロトン照射が、過剰キャリア寿命の大幅な減少なしに積層欠陥の拡大を抑制することも示された。加えて、高温熱処理を行って過剰キャリア寿命を回復させた後でも、積層欠陥の拡大は抑制された。これらの結果から、プロトンによる転位芯のパッシベーションが UV 照明下での部分転位の運動を阻害することが示唆された。
SiC bipolar degradation, which is caused by the stacking fault expansion from basal planedislocation in SiC epitaxial layer or near the interface between the epitaxial layer and asubstrate, is one of the critical problems toward widespread usage of high-voltage SiC
bipolar devices. In the present study, we have investigated the stacking fault expansion
behavior in 4 H-SiC epitaxial layer with proton irradiation under UV illumination. X-ray
topography observation revealed that the proton irradiation suppressed the stacking fault
expansion. From excess carrier lifetime measurements, the stacking fault expansion in
4 H-SiC epitaxial layer with the proton irradiation at a fluence of 1 × 1011 cm- 2 was suppressed
without evident reduction of the excess carrier lifetime. Furthermore, the stacking fault
expansion was also suppressed even after high-temperature annealing recovering the excess
carrier lifetime. These results implied that the passivation of the dislocation core by protons
hinders the recombination enhanced dislocation glide motion under UV illumination.
1．Introduction
Hexagonal silicon carbide (SiC) having 4 H polytype is promising semiconductor materials
for high-power and high-temperature devices [ 1 – 3 ]. Owing to the recent progress in SiC
device technology, 1 -kV-class SiC Schottky barrier diodes (SBDs) and metal-oxidesemiconductor field effect transistors (MOSFETs) have been already commercialized andused in various kinds of electronic systems such as power supplies for ser vers andworkstations, solar inverters, uninterruptible power supplies, industrial motor drives, airconditioners, fast chargers, elevators, electric vehicles, and railcars [ 4 , 5 ]. On the other− 172 −NSG Found. Mat. Sci. Eng. Rep.

hand, SiC bipolar devices such as p-i-n diodes, insulated gate bipolar transistors (IGBTs) andthyristors have a problem about device degradation, in which forward voltage is increased
due to expansion of single Shockley-type stacking faults ( 1 SSFs) in epitaxial layer under
for ward-bias condition [ 6 – 12 ]. The expansion of 1 SSFs is originated from extended
dislocations having the Burgers vector of 1 / 3 < 11 - 20 > on the ( 0001 ) basal plane which iscalled basal plane dislocations (BPDs). 1 SSF between two partial dislocations are expanding
during the bipolar operation by the glide of the partial dislocation with the driving force of“negative”stacking fault energy due to the electronic energy lowering by the carrier
trapping at the stacking fault [ 13 – 18 ]. Similarly, double Shockley-type stacking faults( 2 SSFs) in heavily nitrogen doped 4 H-SiC are expanding during high temperature annealing[ 19 – 24 ]. These anomalous behaviors of stacking faults in 4 H-SiC is considered to be due tothe relatively low stacking fault energy, which was estimated to be 14 . 7 mJm- 2 for 1 SSF [ 25 ],and large energy gain due to the energy level of a localized state in the stacking fault
estimated to be 0 . 22 and 0 . 59 eV below the conduction band edge of 4 H-SiC for 1 SSF and2 SSF, respectively [ 20 , 21 ].
Since the 1 SSF expansion was reported to be originated from BPDs in the epitaxial layer,
great efforts for the reduction of BPD density in epitaxial layer was made [ 26 – 29 ]. Thanks
to the dislocation conversion from BPD to threading edge dislocations (TEDs) propagating
to the [ 0001 ] direction during epitaxial growth process, the typical BPD density incommercial SiC epitaxial wafers was almost zero (less than 1 cm- 1 ) [ 3 , 5 , 30 ]. However, the1SSF expansion underneath BPD-TED conversion points under high current stress has beenreported [ 31 – 33 ]. To suppress the 1 SSF expansion, proper design of buffer layer, which isthe first thin layer grown on a substrate, is important. Tawara et al have clearly demonstrate
the relationship between injected carrier concentration and 1 SSF expansion and therecombination enhancing buffer layer in p-i-n diodes suppress the 1 SSF expansion[ 33 , 34 ].
However, thick buffer layer is necessar y to suppress the 1 SSF expansion under a highcurrent density resulting in the increase of process cost. Therefore, it is desirable to develop
other strategy to suppress the stacking fault expansion. The other important aspect of the
spontaneous expansion of 1 SSF is decrease in critical resolved stress (CRSS) for the glide of
the partial dislocation called“recombination-enhanced dislocation glide”in 4 H-SiC reported
by many researchers [ 9 , 13 , 35 ]. Thanks to the drastic decrease in the CRSS, the 1 SSFs wasreported to expand even below room temperatures [ 35 ]. Considering the critical resolved
shear stress for basal slips in SiC was estimated to be as large as 5 - 10 GPa at room
temperature without recombination-enhanced dislocation glide [ 36 , 37 ], anomalous decreasein the CRSS for the partial dislocation
Proton irradiation is widely used in semiconductor process for the purpose of doping andcontrol of lifetime including SiC device process [ 38 – 41 ]. Proton irradiation results in theformation of radiational defects as well as hydrogen doping. It was reported that the defects
by proton irradiation introduced Z 1 / 2 deep level and reduced the carrier lifetime [ 42 – 44 ]. In
the present study, we have investigated the stacking fault expansion behavior in SiC epitaxial
layers with proton irradiation. For the investigation of stacking fault behavior, we have used
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（公財）日本板硝子材料工学助成会，41（2023）

the ultraviolet (UV) illumination optically exciting excess carriers to stimulate stacking fault
expansion [ 9 , 45 , 46 ].
2．Experimental procedure
An N-type 4 H-SiC epitaxial layer with athickness of 10 µm, a nitrogen concentrationof 1 . 0 × 1016 cm- 3 and an off-cut angle of 4°
from the ( 0001 ) basal plane grown by thechemical vapor deposition (CVD) on a 3 -inch
SiC wafer (SiCrystal GmbH) was cut by laser
scribing with a size of 5 - 10 mm in length.
The specimens were irradiated at room
temperature with 0 . 6 -MeV and 0 . 95 -MeV
protons at fluences ranging from 1 × 1011 to1 × 1016 cm- 2 . Hydrogen distribution of thespecimen with a fluence of 1 × 1016 cm- 2 was
i n v e s t i g a t e d b y s e c o n d a r y i o n m a s s Figure 1 . Depth profile of hydrogen density forthe specimen with a proton fluence ofspectrometr y (SIMS) as shown in Fig. 1 .
1 × 1016 cm- 2 measured by SIMS [ 50 ].
Maximum hydr ogen densities in thespecimens with 0 . 6 -MeV and 0 . 95 -MeV proton irradiation were about 5 and 10 µm,
respectively. Grazing incidence synchrotron reflection X-ray topography was performed using
a monochromatic X-ray beam (λ= 0 . 15 nm) with g vector of - 1 - 128 or 11 - 28 at BL 8 S 2 in Aichi
Synchrotron Radiation Center and BL 20 B in the Photon Factor y at the High-Energy
Accelerator Research Organization (details of the condition were referred to [ 47 ]). The X-ray
topography observation enable us to identify the Burgers vector of the dislocations [27,48,49].
Firstly, the positions of propagating BPDs into the epitaxial layer were confirmed, and then UV
light was illuminated at these positions. UV light emission diode with a wavelength of 365 nm
was focused to a diameter of 3 mm, and the illuminated intensity was adjusted to 10 Wcm- 2 .
During the UV illumination, the specimens were heated to 373 K and the temperature wasmeasured by a radiation thermometer. Carrier lifetime of the specimens was evaluated by timeresolved photoluminescence (TR-PL) using a bandpass filter with transmission wavelength of
370 - 410 nm corresponding to luminescence from the band edge of 4 H-SiC (～390 nm) as well
as micro photoconductance decay (µ-PCD) [ 12 , 51 ].
3．Results
Figure 2 shows the X-ray topography image taken from the proton unirradiated 4 H-SiC
epitaxial wafer before UV illumination. Most of the linear contrasts corresponding to BPDs was
accompanied by the small circular contrasts corresponding to TEDs at the right end of the
BPDs like“BPD-I ”. On the other hand, some BPDs were not conver ted to TEDs andpropagated to the epitaxial layers like“BPD-II”and“BPD-III”. The propagated BPDs can berecognized by the characteristic contrast of BPD near the surface (on the right side in Fig. 1 ).
− 174 −NSG Found. Mat. Sci. Eng. Rep.

The contrast pattern of BPD was reported
to depend on the depth below the crystal
surface in 4 H-SiC from the comparison of
the ray-tracing simulation and the X-ray
topography obser vation [ 52 ]. A BPD
located shallower than about 5 µm fromthe surface (right side in Fig. 1 ) is imaged
as a white line bordered by black lines andthe BPD located deeper (left side in Fig. 1 )is imaged as a pure black line contrast.
Therefore, it is possible to judge whether
the BPD was converted or not. We havecarefully checked the positions of the

Figure 2 .

X-ray topography image taken from thespecimen without proton irradiation
before UV illumination.“BPD-I”isconverted to TED, and the BPD/TED
conversion is not noticed for“BPD-II”
and“BPD-III”[ 50 ].

Figure 3 .

X-ray topography images taken from thespecimen without proton irradiation
with the different UV illumination time
(a: 0 h, b: 1 h, c: 2 h, d: 4 h) with a power
density of 10 Wcm- 2 at 373 K [ 50 ].

propagated BPDs and illuminated UV lightat the positions.
Evolution of 1 SSF expansion in the
proton unir radiated epitaxial wafer
during UV illumination was shown in
Fig. 3 . In this X-ray topography geometry,
SFs never have the contrast and thesurrounding partial dislocations have theblack and white contrast depending on
the directions [ 53 ]. By UV illumination,
the line contrast was appeared and thesurrounding area corresponding to the
SF expanding as the UV illumination timeincreases. After the fur ther UV
illumination with power densities of 5 ,2 . 5 and 1 Wcm - 2 for 1 , 2 and 12 h, wehave also observed the expansion of SF.
The current SF expansion behavior wasconsistent with that reported by Tanaka
et al. [ 46 ].
Figure 4 shows the X-ray topography
images taken from the specimens with
0 . 6 -MeV proton irradiation before andafter the UV illumination for 120 h. Even
after UV illumination for 120 h, apparent
SF expansion was not noticed except forthe specimen with proton irradiation at
fluence of 1 × 1011 cm - 2 , in which slight− 175 −

（公財）日本板硝子材料工学助成会，41（2023）

expansion of SF was observed at theposition indicated by the yellow
arrow in Fig. 4 (f). The expansion of
the SF is limited at the left side of the
BPD indicating that the expansion
took place at the bottom side of theepitaxial layer and the proton
irradiation suppress the SF
expansion at the top side of theepitaxial layer.
The lifetime measured by TR-PL
and µ-PCD for the specimen of 0 . 95 MeV proton ir radiation with thedif ferent fluences are shown in
Fig. 5 . Although absolute values oflifetime dif fer depending on themeasurement method (the lifetime
measured by TR-PL was always
larger than that by µ-PCD), the sametendency in which the lifetime isalmost unchanged by the proton afluence of 1 × 1011 cm- 2 and decreases
with increasing proton fluences wasobserved in both TR-PL and µ-PCD
measurements. Note that the
lifetime had a distribution in the

Figure 4 .

(a)-(f) X-ray topography images taken fromthe specimen with proton irradiation fluence
ranging from 1 × 1011 to 1 × 1013 cm-2 before andafter UV illumination for 120 h and (g)schematic illustration of the shape of SF
shown in (f) [ 50 ].

Figure 5 .

Carrier lifetime depending on proton fluences
measured by TR-PL as well as µ-PCD [ 50 ].

epitaxial wafer resulting in large
deviation of the values of lifetime forthe non-irradiated specimens andthe lifetime tends to be small near
the edge of the wafer, where thepropagation of BPDs was frequently
observed.

4．Discussion
4.1．Driving force of SF expansion by UV illumination
Excess carrier concentration in epitaxial layer (∆n) by UV illumination in steady state wasestimated by the following equation for low excess carrier concentration assuming that
surface and interface recombination is negligible [ 54 ];(1)− 176 −NSG Found. Mat. Sci. Eng. Rep.

where τepi is the bulk carrier lifetime in the epitaxial layer and G is the generation rate of
the excess carrier in epitaxial layer calculated from the photon flux and absorption [ 54 ]. The
value of τepi is not identical to the measured values of carrier lifetime by µ-PCD and
TR-PL[ 55 ]. Here the typical value for as-grown n-type SiC epitaxial layer ( 0 . 1 - 1 µs) was
adopted to the estimation. The excess carrier concentration was estimated to be 2 × 1014 - 2 ×
1015 cm - 3 , which led to the electronic energy gain (Δ) around 0 . 5～3 mJ/m 2 from thecalculation result of Ref. 17 .
On the other hand, the driving force of the SF expansion (negative stacking fault energyγ) by UV illumination was possible to estimate from the curvature radius (R) of pinned
par tial dislocations under equilibrium of forces acting on dislocation by the following
equation[ 24 ];(2)
where b, Kedge, Kscrew, θ, r 0 and r 1 stands for the Burgers vector of dislocation, the energy
factors of edge and screw components, the angle between the line vector and the Burgers
vector, and inner and outer cut-off radius. Judging from the shapes of the vertexes in Fig. 3 (d),the value of R was ranging from 5 to 50 µm corresponding to the values of γ ranging from - 0 . 6
to - 6 mJ/m 2 . SF energy without UV illumination (γ0 ) was estimated by the following
relationship;(3)
γ=γ0−Δ
From the obtained values of Δ and γ, the value of γ0 was evaluated as+ 2～- 5 mJ/m 2 .
Considering the SF expansion took place with even lower UV illumination as tabulated in Table
1 , it was expected that the SF energy without UV illumination near room temperature wasmuch lower than the assumed value of γ0 ( 5～20 mJ/m 2 ) based on SF energy evaluated by
the dissociation width of partial dislocations deformed at high temperature[ 17 , 25 ], although
the current estimation was a bit rough.
The shear stress acting on the partial dislocation by the negative stacking fault energy wasestimated to be 3 - 30 MPa, which was much lower than the bulk critical resolved shear stress( 4 . 5 GPa) for the basal slip in 6 H-SiC estimated by microplillar compression test [ 37 ]. This
indicates the occur rence of the drastic decrease in activation energy due to therecombination enhanced dislocation glide by UV illumination as was reported by carrier
injection and electron beam irradiation [ 35 , 56 ].
4.2．Effect of proton irradiation
TR-PL and µ-PCD measurements indicate that the carrier lifetime decreases by the proton
irradiation as was reported by Hazdra et al. [44] They have reported that the carrier lifetime
reduction took place due to the introduction of Z 1 / 2 centers, which are related to the carbon
vacancy. In the present study, 0 . 6 -MeV proton irradiation at fluences ranging from 1 × 1012 to1 × 1013 cm- 2 resulted in the evident reduction of carrier lifetime and the suppression of the SF
expansion, which was in good agreement with the results repor ted by Tawara et al.
demonstrating a short carrier lifetime successfully suppress the SF expansion [ 34 ]. On the− 177 −

（公財）日本板硝子材料工学助成会，41（2023）

other hand, 0 . 6 -MeV proton irradiation at afluence of 1 × 1011 cm- 2 also resulted in the
suppression of the SF expansion although
the carrier lifetime was almost unchangedby the proton ir radiation. This result
implies that the proton irradiation affects
not only on the carrier lifetime but also on
the dislocation motion. The interaction
between dislocations and point defects suchas protons and vacancies would increase

Figure 6 .

the CRSS for the glide of the par tial
dislocation. Kwon et al. have reported thatthe increase in the CRSS with the proton
irradiation by micro-pillar compression test

X-ray topography image taken from theepitaxial layer with a thickness of 5 µm,
a nitrogen concentration of 6 × 1015 cm- 2 ,a 0 . 3 -MeV proton irradiation at afluence of 1 × 1015 cm- 2 after annealing at
1973 K for 1 hour (a) before and (b) after
UV illumination with a power of 10
Wcm- 2 at 373 K for 10 hours [ 50 ].

for 6 H-SiC crystals [ 57 ].
To fur ther confirmed the ef fect of proton irradiation on the dislocation glide, weinvestigated the SF expansion behavior for the specimen with proton irradiation after hightemperature annealing to recover carrier lifetime. Figure 6 shows the X-ray topography
image taken from the epitaxial layer with a thickness of 5 µm, a nitrogen concentration of 6 ×
1015 cm- 2 , a 0 . 3 -MeV proton irradiation at a fluence of 1 × 1015 cm- 2 after annealing at 1973 K
for 1 hour, of which carrier lifetime was confirmed to be unchanged after the annealing byµ-PCD measurements. The propagating BPD in the epitaxial layer was not expanded to SF
even after UV illumination for 10 hours at 373 K as shown in Fig. 6 . These results indicatethat the proton irradiation increase the CRSS for the glide of the partial dislocation under the
UV illumination and suppress the SF expansion. Hydrogen (proton) passivation of surfaces,
point defects and dislocations in semiconductor crystals including silicon and 4 H-SiC was
reported by many researchers [ 58 – 61 ]. If the proton terminates the dangling bonds of thecore of the partial dislocations in epitaxial layer, the mobility of the partial dislocation is
expected to increase as well as the recombination at dislocation is expected to be suppressed
resulting in the suppression of recombination enhanced dislocation glide by UV illumination.
Cur rent results implies that the bipolar degradation in SiC power device would be
suppressed by the proton irradiation, which has a high affinity of semiconductor process.
5．Conclusion
The SF expansion behavior in n-type 4 H-SiC epitaxial layers by UV illumination with thedifferent proton irradiation was investigated by X-ray topography as well as the carrier
lifetime measurements. The results obtained are summarized as follows:( 1 ) The SFs was expanded from the BPDs in 4 H-SiC epitaxial layer without proton
irradiation by UV illumination and the expansion was suppressed by the proton
irradiation at fluences ranging from 1 × 1011 to 1 × 1013 cm- 2 .
( 2 ) From the curvature radius estimated from the X-ray topography observation, the SF− 178 −NSG Found. Mat. Sci. Eng. Rep.

energy without UV illumination at 373 K was evaluated to be+ 2～- 5 mJ/m 2 , which wasmuch smaller than the expected value ( 5～20 mJ/m 2 ) based on SF energy evaluated by
the dissociation width of partial dislocations deformed at high temperature( 3 ) The excess carrier lifetime was evidently reduced by the proton irradiation at fluences
larger than 1 × 1012 cm- 2 from both measurement of TR-PL and µ-PCD.
( 4 ) The SFs expansion in 4H-SiC epitaxial layer with the proton irradiation at a fluence of 1 ×1011 cm- 2 was suppressed without evident reduction of the excess carrier lifetime, which
implies that the recombination enhanced dislocation glide is hindered by the interaction
between the par tial dislocations and the point defects introduced by the proton
irradiation.
( 5 ) The SFs expansion was also suppressed even after high-temperature annealing
recovering the excess carrier lifetime, which support the proton irradiation hinder therecombination enhanced dislocation motion.
Present results implies that the bipolar degradation in SiC power device would be solved by
the proton irradiation, which has a high affinity of semiconductor process.
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