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LED China Conference 2016, Shanghai, March 16th, 2016 1 Nikkiso Giken Co.,Ltd
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Cyril Pernot – Manager, Development Department
March 16th, 2016
Improving Deep UV LED Performance LED China Conference 2016, Shanghai
NIKKISO GIKEN Co. Ltd.
Division
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NIKKISO business Fields
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Outline
1- Introduction: Going to shorter wavelength
2- Challenges and Performance of DUV-LED
3- DUV-LED Products Range and Applications
4- Conclusion
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Wide range of Applications in UV
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UV-LEDs are promising UV Sources
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UV-LED Systems have many advantages
Integration
Operation
Environmental Impact
small size
robust
simple driving circuit
low voltage operation
Customizable emission wavelength
Instant On/Off
Modulation of UV output power
Low degradation, long life time
No ozone production
No mercury-filled UV bulbs
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From Blue to UV LED
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Nikkiso in UV-LED Business since 10 years
2006: UV Craftory, Nikkiso’s subsidiary established to develop and manufacture
DUV LED, based on Prof. Akasaki’s and Prof. Amano’s technology 2010: State-of-the-art results for UVC-LED (EQE > 3%)
2012: Starting sales of DUV-LED (mW level TO-can package)
2014: Nikkiso Giken becomes in charge of R&D, production and sales of UV-LED
2015: Nikkiso Hakusan Factory certified ISO9001:2008
Series production of 30 mW high power SMD product
2016: Series production of 50 mW high power SMD product
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DUV-LEDs are based on AlGaN ternary alloy
By changing AlN mole fraction,
possible to tune the emission
wavelength
Difference in lattice parameter => stress, cracks, dislocations
GaN is absorbing wavelength
below 365nm.
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DUV-LEDs have specifics challenges
Package
Sapphire
GaN
n-GaN
Active region
EB
p-GaN
Light output
InGaN-based LED : AlGaN-based DUV-LED :
Package
AlN
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LEE is significantly affecting DUV-LED Performance
※ Best values are greatly depending on the measurement conditions and on the overall performance.
Parameters UV-LED
(InGaN-based)
DUV-LED
(AlGaN-based)
IQE: Internal Quantum Efficiency >80% <80%
LEE: Light extraction Efficiency >80% <25%
EQE: External Quantum Efficiency >80% <15%
EE: Electrical Efficiency >90% <90%
WPE: Wall Plug Efficiency >50% <5%
Max Power /Chip
(Irradiance Working Distance >3mm)
>5 W
( >16 W/cm2)
< 100 mW
(<500 mW/cm2)
Lifetime and reliability > 50000 hours >10000 hours
Cost/ W $ $$$$
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Challenge for p-type doping
H. Amano et al. Proc. Of SPIE Vol. 7216 (2009)
Current DUV-LED devices are using :
・ thin pAlGaN layers for p-clad, and
・ pGaN for contact layer.
・New dopants, short period super lattice, different semiconductors, new
approach are under investigations for improving p-type layers.
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Challenge for n-type doping
・n-type AlN very resistive
・Up to 70% AlGaN, n-type layers with sheet resistance below 60 Ω/□ can be fabricated
1.0E-04
1.0E-03
1.0E-02
1.0E-01
840 860 880 900 920 940 960 980
Co
nta
ct r
esi
tiv
ity
(a
rb.u
nit
)
Anneal temperature (degreeC)
Al:65%
Al:61%
Al:55%
High Al Content
Inazu et al. Autumn Oyobutsuri Conf. 2015
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DUV-LED with low forward voltage
285nm DUV-LED :
=> Electrical efficiency over 90% can be achieved
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High IQE by improving template quality
EQE Limit for
LEE of 10 %
Best EQE results obtained at low current
=> presence of droop at high current
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
300 400 500 600 700
(10-10) XRC FWHM
Ou
tpu
t P
ow
er
(mW
)
280-300 nm LEDs
342 nm LED
342 nm LED on ELO
C. Pernot et al., PSS A 208, 7, 1594-1596(2011)
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Approach Crystal quality Growth Challenge DUV LED
EQE
Availability
AlN on flat
Sapphire
-DD~109cm-2 (DD: Dislocation Density)
Lattice mismatch,
cracks
EQE 6.5%
(>10% for
improved LEE)
Up to 4 inch (demonstrated)
ELO AlN (Epitaxial Lateral
Overgrowth)
-DD<2x108cm-2
Need regrowth, and
thick layer ,
coalescence difficult
(pits)
EQE 3% Difficult for large diameter,
Cost (need process, regrowth,
thick layer growth)
Bulk AlN -DD<105cm-2 UV Transparent
Layer, Low Al%
AlGaN regrowth
EQE 4%
(>7% for
improved LEE)
1 inch available (high cost),
2 inch (R&D stage)
Approaches for AlN Template Fabrication
⇒ Growth of DUV-LED on sapphire remains the competitive approach
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EQE is decreasing at shorter wavelength
Maximum EQE of DUV-LED bare chip as a function of wavelength:
Issues for short wavelength emission:
- Carrier injection efficiency
- Polarization of light
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High internal reflection and absorption
Internal refractive critical angle: αc = na/ns
ns
na
*
Typical LEE for DUV-LED bare chip: 8~10%
n-layer
Active region
p-Layer
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LEE Optimization (1) : Surface Roughening
C Pernot et al. Appl. Phys. Express 3 (2010) 061004
On wafer measurement
・Sapphire substrate with Moth-eye
pattern:
→On wafer Output Power ×1.5
Inoue et al. Appl. Phys. Lett. 106, 131104 (2015)
・ AlN substrate with
Hybrid nano photonic pattern
→Chip Output Power×1.96
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LEE Optimization (2): Reflective Electrodes
Inazu et al. J. J. Appl. Phys. 50 (2011) 122101
・Mesh p-GaN contact layer with high reflective electrodes
Output Power x 1.27 (x1.55 when combining with n-reflective electrodes).
Voltage increase by 0.45V (20mA)
=> Trade-off between LEE improvement and Vf increase
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LEE Improvement (3): Encapsulation
To improve light extraction efficiency, resin for encapsulation is actively researched
Good transparency
Low degradation
Good adherence to the chip
Half-ball shaping for efficient light extraction
Robustness to environment
Refractive index between Air and Sa (AlN)
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Demonstration of over 100 mW at 275 nm
- Optimization of layer design
- Use of a UV-transparent encapsulant
Bare Chip
Chip with encapsulant
Single Chip Performance (R&D level)
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DUV-LED Products and Characteristics
1 JPY coin
(20 mm)
【TO-46 CAN】
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DUV-LED achieves Lifetime over 10000 hours
Temperature and driving current are affecting lifetime of the DUV-LED device
By growing high quality DUV-LED, lifetime over 10000 hours can be achieved.
Output power evolution for 285 nm DUV-LED:
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
10 100 1000 10000
Rel
ativ
e O
utp
ut
Po
wer
Time[h]
Driving Current:100mA (25℃)
00.10.20.30.40.50.60.70.80.9
11.1
10 100 1000 10000
Rel
ativ
e O
utp
ut
Po
wer
Time[h]
Driving Current:350mA (25℃)
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Next Generation DUV LED reaches 50 mW
0
10
20
30
40
50
60
0 100 200 300 400
Ou
tpu
t P
ow
er
(mW
)
Forward Current (mA)
新規開発品
従来品
Ta = 25℃
Output Power x 1.7
Achievement of 50 mW at 350mA driving current
New LED
Standard LED
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Optimization of Chip and Package design
⇒ By inserting reflective surfaces on the back side of the chip, it is possible
to limit absorption, and to improve the light extraction of the device
Highly
Reflective
Electrode
Package
Reflection Reflection
⇒ Improvement in Package Light extraction by using
highly reflecting material and shape
⇒ Increasing the contacting surface area between chip and package:
the thermal resistance 15 K/W => 9 K/W
nAlGaN
MQW
p-Layers
p-electrodes
AlN
Sapphire
n-electrodes
Package
Absorption
Absorption
Standard Device New Device
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UV-LED Applications Map
Disinfection
UVB
UVA
UVC
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Irradiance is much improved by array optimization
Key parameters for high power applications:
LED output Power,
Package/Module Thermal management and
Vf uniformity.
Package and
Module design
Improvement
EQ
E(%
), I
rrad
ian
ce (m
W/c
m2)
EQ
E(%
), I
rrad
ian
ce (
mW
/cm
2)
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Our Product range allows to address various applications
Components Arrays Modules
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UV-LED Market
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• DUV-LED has some clear advantages compared with traditional UV light
source.
• Recently DUV-LED reached performance allowing to address many
applications.
• Foreseeable technology developments will provide higher efficiencies
and further lifetime improvement.
Conclusion
Source: Aquisense Technology, July 2015
50 mW SMD to be mass-produced this year
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Thank you for your attention
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