Reduction of Forward Bias Degradationin 4H-SiC PiN Diodes
Fabricated on 4H-SiC Bonded Substrates
N. Hatta1),2) *, S. Ishikawa1), K. Ozono1), K. Masumoto1), K. Yagi2),
M. Kobayashi2), S. Kurihara3), S. Harada1), and K. Kojima1)
1) Advanced Power Electronics Research Center, National Institute ofAdvanced Industrial Science and Technology (AIST)
2) SICOXS Corporation
3) Phenitec Semiconductor Co., Ltd1

Acknowledgements

This work has been implemented under a joint research project ofTsukuba Power Electronics Constellations (TPEC).
The co-authors, S. Ishikawa and K. Ozono are assigned from Phenitec Semiconductor Co., Ltd.

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Outline Introduction- What is a 4H-SiC bonded substrate?
- Forward bias degradation in 4H-SiC bipolar devices
 Experimental- PiN diode employed in this study- Experimental procedure Results and Discussion- Forward current stress test for both bonded and conventional bulk substrates- Photoluminescence (PL) imaging- Investigation of number of the bar-shaped Shockley-type stacking faults (SSFs)
after applying high forward current stress- Prediction of a mechanism of basal-plane-dislocation(BPD) reduction in
4H-SiC bonded substrates with referring some prior studies.
 Summary3What is a 4H-SiC bonded substrate?
 Structure• A stacked substrate with two different SiC
polytype using direct wafer bonding technologies

 Features• No unstable interlayer at the bonded interface→Enable to apply various high temperature process of SiC

4H-SiC bonded substrate(SiCkrest®)• Extremely thin (the order of submicrons) monocrystalline 4H-SiC layer→ Minimize the volume of high-quality monocrystalline 4H-SiC portion4

What is a 4H-SiC bonded substrate?
 Benefits in characteristics• Its unique structure is expected to bring somebenefits that are not possible with aconventional 4H-SiC bulk substrate.

 Outcomes (Reported in Japan, 2020)• Reduction of on-state resistance in 4H-SiC PiN diodes• Backside ohmic contact formation requiring no thermal
annealing process

4H-SiC bonded substrate(SiCkrest®)

 Outcomes (Report in Switzerland, 2022)
・”Reduction of Forward Bias Degradation in 4H-SiC PiN Diodes”
will be demonstrated.
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Forward bias degradation in 4H-SiC bipolar devices
 Phenomenon• Forward voltage (Vf) increases withforward current stress through the pn junction.

 Cause• Due to the expansion of Shockley-type
stacking faults (SSFs) from basal-planedislocation (BPD) induced by electron-hole pairrecombination

The electrical and optical observations of PiN diodes
fabricated on both the substrates were performed.
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Experimental(Experimental procedure) Flow chart
Forward current stress conditions• Temperature: 175 ℃• Forward current density:
Forward current
stress test
on PiN diodes

1st 100 A/cm2
2nd 500 A/cm2
3rd 1000 A/cm2
4th 1500 A/cm2

for 600 sec (DC)for 300 sec (Pulsed)

Definition of forward voltage shift (ΔVf)

Photoluminescence(PL)imaging

ΔVf：
Forward voltage shift
from initial to stressedat the forward currentof 60 A

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Results and Discussion(Forward current stress tests and PL imaging)
Changes in the ΔVf caused by forward current stress for both the substrates

Increasing steeply
at 1000 A/cm2 or more

Remaining low
even at 1500 A/cm29

Results and Discussion(Forward current stress tests and PL imaging)
Plot of correlation between ΔVf and total area of SSFs

ManyTriangularBar-shapedSSFsTriangular
SSFs

Bar-shapedExpansion(a) ΔVf=83 mV(b) ΔVf=178.6 mV

Few SSFs(c) ΔVf=20.1 mV

The ΔVf reflects the total area of the expanded SSFs,
which depend on the forward current stress in the bulk substrate.
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Results and Discussion(Investigation of number of the bar-shaped SSFs )Investigation of the number of bar-shaped SSFs within
the electrically stressed diodes with high forward current density

Some bar-shaped SSFs observed
from almost all the investigated diodes

No bar-shaped SSFs observed
from all the investigated diodes
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Results and Discussion(Forward current stress tests and PL imaging)
Difference in forward bias degradation for both the substrates

Under high-forward current density (1000 A/cm2 or more),- Bulk substrate: Some bar-shaped SSFs expanded.
- Bonded substrate: No bar-shaped SSFs expanded.
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Results and Discussion(Prediction of a mechanism of BPD reduction in 4H-SiC bonded substrates)

1. Origin of the bar-shaped SSFs

Origin (main): BPDs converted into TEDs
in the early stage of epitaxial growth
Under high-forward current density (1000 A/cm2 or more),- Bulk substrate: Some BPDs
bar-shaped
SSFs expanded.
contributed
to bar-shaped SSF expansion.
- Bonded substrate: No BPDs
bar-shaped
SSFs expanded.
contributed
to bar-shaped SSF expansion.
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Results and Discussion(Prediction of a mechanism of BPD reduction in 4H-SiC bonded substrates)

2. Contribution of forward current density
S. Hayashi et al., Appl. Phys. Express 12, 051007 (2019)depth ≥ 550nm@1000 A/cm2or more
Notice:BPD-TED conversion point is locatedbelow the epilayer/substrate interface.
Underhigh-forwardcurrentdensity(1000 A/cm2 or more),At the submicron
depthbelow
the epilayer/substrateinterface,- Bulk substrate: Some BPDs
bar-shaped
SSFs expanded.
contributed
to bar-shaped SSF expansion.
- Bonded substrate: No BPDs
bar-shaped
SSFs expanded.
contributed
to bar-shaped SSF expansion.
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Results and Discussion(Prediction of a mechanism of BPD reduction in 4H-SiC bonded substrates)

3. The reason that a BPD-TED conversion point can move to
below the epilayer/substrate interface
Threading Edge dislocation Glide (TED glide)

Molecular dynamics simulation ofthe BPD-TED conversion of perfect screw dislocation
Y. Tamura et al., Jpn. J. Appl. Phys. 58, 081005 (2019)15

Results and Discussion(Prediction of a mechanism of BPD reduction in 4H-SiC bonded substrates)

Always BPD existing

No BPD existing
4H-SiC bonded substrates are highly advantageous
for reducing forward bias degradation in 4H-SiC bipolar devices,
especially under high-forward current stress.
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Summary
The advantage in reducing forward bias degradation ofPiN diodes using a 4H-SiC bonded substrate is demonstrated.
 Forward current stress teston the PiN diodes
 PL imaging
We found the distinct difference inthe bar-shaped SSF expansion athigh forward current density(1000 A/cm2 or more).
 Prediction with referring someprevious studies
The mechanism of BPD reduction
associated with a TED glide waspredicted in the 4H-SiC bondedsubstrate.
17About sample used in this study

URL https://www.sicoxs.com
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