III-Nitride Light-Emitting Diodes for
Solid-State Lighting
Hongping Zhao
Department of Electrical Engineering and Computer Science,
Case Western Reserve University
Cleveland, Ohio, USA
Hongping.zhao@case.edu
SOLAR DURABILITY WORKSHOP
Case Western Reserve University, Cleveland, OH
April 9th, 2012
Impact of Solid State Lighting
Lighting
22%
Commercial 59%
NASA Image: Earth’s City Lights
Industrial
14%
Residential 27%
US Electricity
Consumption
Source: Cree “Solid State Lighting Revolution”
LED lighting can reduce electricity
needs for lighting by more than 60%!
2
Outline
 Introduction on Current State-of-the-Art III-Nitride LEDs
 Enhancement of LED total external quantum efficiency
 Nanostructure Engineering for Enhancing LED Radiative Efficiency
 Staggered InGaN QW
 Linear-Shaped Staggered InGaN QW
 Type-II InGaN-GaNAs QW
 Strain-compensated InGaN-AlGaN QW
 InGaN-delta-InN QW
 Effect of Current Injection on Efficiency Droop in InGaN QW LEDs
 InGaN-AlInN QW-barrier structure to suppress efficiency droop
 Enhancement of LED Light Extraction Efficiency
 III-nitride microspheres
 LED Reliability and Tests
 Failure modes and mechanisms in LEDs
 Summary
3
Outline
 Introduction on Current State-of-the-Art III-Nitride LEDs
 Enhancement of LED total external quantum efficiency
 Nanostructure Engineering for Enhancing LED Radiative Efficiency
 Staggered InGaN QW
 Linear-Shaped Staggered InGaN QW
 Type-II InGaN-GaNAs QW
 Strain-compensated InGaN-AlGaN QW
 InGaN-delta-InN QW
 Effect of Current Injection on Efficiency Droop in InGaN QW LEDs
 InGaN-AlInN QW-barrier structure to suppress efficiency droop
 Enhancement of LED Light Extraction Efficiency
 III-nitride microspheres
 LED Reliability and Tests
 Failure modes and mechanisms in LEDs
 Summary
4
Energy Gap (eV)
III-Nitride Material System and Applications
6.5
6
5.5
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
AlN
Deep UV-LEDs, Lasers,
Solar blind detectors
AlGaN
Electronics
HEMTs
Near UV/visible LEDs,
Photovotaics
GaN
III-Nitride Material System
InGaN
InN
Direct band gap
Wide spectrum coverage
3
3.1
3.2
3.3
3.4
3.5
3.6
 UV up to near infrared
In-plane Lattice Constant a (Ǻ)
III-Nitride Material Applications
High-Al AlGaN  Deep UV-LEDs, Lasers, Solar blind detectors
Low-Al AlGaN  High power transistors for high power electronics
InGaN  Near UV / Visible LEDs, Lasers, Photovoltaic
InN  Photovoltaic, Terahertz generation
5
White LEDs
Red, λ ~ 650-nm
1) Multi-chip RGB approach:
1 RED (λ ~ 620 nm) InGaAlP LED
1 GREEN (λ ~ 540 nm) InGaN LED
White
Green, λ ~ 535-nm
1 BLUE (λ ~ 455 nm) InGaN LED
 Pros: theoretically best efficiency; variable color
temp
Blue, λ ~ 450-nm
InGaN LED
 Cons: high cost; efficiency of green LED
4000
Intensity
Ce: YAG Phosphor
2) Single Blue InGaN (λ ~ 450-460 nm) die + Phosphors
3000
 Pros: relatively low cost, currently highest efficiency
2000
 Cons: non-trivial control of color temperature
1000
0
300
400
500
600
Wavelength (nm)
700
800
6
Outline
 Introduction on Current State-of-the-Art III-Nitride LEDs
 Enhancement of LED total external quantum efficiency
 Nanostructure Engineering for Enhancing LED Radiative Efficiency
 Staggered InGaN QW
 Linear-Shaped Staggered InGaN QW
 Type-II InGaN-GaNAs QW
 Strain-compensated InGaN-AlGaN QW
 InGaN-delta-InN QW
 Effect of Current Injection on Efficiency Droop in InGaN QW LEDs
 InGaN-AlInN QW-barrier structure to suppress efficiency droop
 Enhancement of LED Light Extraction Efficiency
 III-nitride microspheres
 LED Reliability and Tests
 Failure modes and mechanisms in LEDs
 Summary
7
III-Nitride Light-Emitting Diodes
Ni / Au
Transparent
Metal
InGaN / GaN MQW
Active Region
P-GaN
Ti / Au
N-GaN
Sapphire Substrate
8
Device Physics of III-Nitride LEDs
High Efficiency LEDs:
η external = ηinj ⋅η Rad ⋅η extraction
η IQE = ηinj ⋅η Rad
Injection Efficiency η inj
 Fraction of Injected Current that Recombine in the Active Region
Radiative Efficiency η Rad
 Fraction of Injected Current in the Active Region that Recombine
Radiatively Generating Photons inside the Semiconductors
Extraction Efficiency η extraction
 Fraction of Photon Generated in the Active Region that Exists the
Semiconductor Cavity into Free Space
η Wall Plug
ηOptical Output
=
η Electrical Input
9
Non-Polar Vs. Polar In0.2Ga0.8N-GaN QW
Without Polarization
dQW = 30-Å
ψe
ψhh
λ = 463-nm
dQW = 30-Å
1.6
Ec & Ehh (eV)
λ= 417-nm
With Polarization
2.4
ψe
0.8
0.0
-0.8
ψhh
-1.6
-2.4
-3.2
0
20
40
60
80
100
120
140
160
180
200
z(Å)
Growth Direction
Γe-hh = 97.44%
Reduce
Γe-hh = 34.4%
Large Reduction in Γe-hh due to Polarization Effect
10
Approaches for Enhancing the Overlap (Γe_hh)
Nonpolar InGaN QW1
Remove the polarization field
Less mature epitaxy
1.
Cost issue
T. Koyama, T. Onuma, H. Masui, A. Chakraborty,
B. A. Haskell, S. Keller, U. K. Mishra, J. S. Speck,
S. Nakamura, S. P. DenBaars, T. Sota, and S. F.
Chichibu, Appl. Phys. Lett. 89, 091906 (2006)
Nanostructure Engineering (on c-plane GaN)
Enhance electron-hole wavefunction overlap (Γe_hh)
Conventional QW
Novel QW / QD
Ec
ψe
ψe
Ec
Growth Direction
Ev
•
•
•
•
ψh
Low electron-hole wavefunction overlap (Γe_hh)
Large built-in Quantum Confined Stark Effect
Low spontaneous emission rate and optical gain
Large threshold carrier / current density
ψh
•
•
•
•
Ev
High electron-hole wavefunction overlap (Γe_hh)
Reduces Quantum Confined Stark Effect
Enhances spontaneous emission rate and optical gain
11
Reduces threshold carrier / current density
Concept of Staggered InGaN QW Structures
InzGa1-zN
GaN
GaN
InxGa1-xN
InyGa1-yN
InxGa1-xN InyGa1-yN InyGa1-yN
GaN
GaN
GaN
Ec
Ec
Ec
Ev
Ev
Ev
(a) Conventional
InzGa1-zN QW
III-V Nitrides
Staggered QW
GaN
(b) Two-Layer Staggered (c) Three-Layer staggered
InxGa1-xN/InyGa1-yN QW InyGa1-yN/InxGa1-xN
/InyGa1-yN QW
Existence of spontaneous
and piezoelectric field
Band lineups
engineering
Low electron-hole wave
function overlap
Enhance electron-hole
wave function overlap
1. H. Zhao, R. A. Arif, and N. Tansu, IEEE J. Sel. Top. in Quantum Electronics, 15 (4), 1104 (2009).
12
Bottom-Emitting InGaN LED Devices
Bottom-Emitting Device Structure
 good contact  low resistivity
Three-Layer Staggered InGaN QWs for active region
1. H. Zhao, G. Liu, X. H. Li, G. S. Huang, J. D. Poplawsky, S. Tafon Penn, V. Dierolf, and N. Tansu, Appl. Phys.
Lett., 95, 061104 (2009).
2. (Invited Paper) H. Zhao, G. Liu, X. H. Li, R. A. Arif, G. S. Huang, J. D. Poplawsky, S. Tafon Penn, V. Dierolf,
13
and N. Tansu, IET Optoelectronics,vol.3(6),pp. 283-295 (2009).
Electroluminescence Vs. Injection Current
4
4
EL Intensity (a.u.)
4
4.5
Conventional
InGaN QW LED
Area = 510 µ m x 510 µ m
3
2
I = 200 mA
I = 150 mA
I = 100 mA
I = 80 mA
I = 50 mA
I = 30 mA
1
0
350
400
x 10
Staggered
InGaN QW LED
4
EL Intensity (a.u.)
4.5
x 10
450
500
550
Wavelength (nm)
600
650
Area = 510 µ m x 510 µ m
3
2
I = 200 mA
I = 150 mA
I = 100 mA
I = 80 mA
I = 50 mA
I = 30 mA
1
0
350
400
450
500
550
Wavelength (nm)
600
650
Conventional InGaN QW LED Vs. Staggered InGaN QW LED
 Staggered InGaN QW LED shows improved peak EL intensity
 Staggered InGaN QW LED shows broader EL spectrum  Larger FWHM
 Both LEDs show blue-shift as injection current increases
1. H. Zhao, G. Liu, X. H. Li, G. S. Huang, J. D. Poplawsky, S. Tafon Penn, V. Dierolf, and N. Tansu, Appl. Phys.
Lett., 95, 061104 (2009).
2. (Invited Paper) H. Zhao, G. Liu, X. H. Li, R. A. Arif, G. S. Huang, J. D. Poplawsky, S. Tafon Penn, V. Dierolf,
14
and N. Tansu, IET Optoelectronics,vol.3(6),pp. 283-295 (2009).
CW Comparison of Output Power
Conventional Vs. Staggered InGaN QW LEDs
1000
In0.21Ga0.79N
GaN
Output Power (a.u.)
800
GaN
λ peak ~ 520-525 nm
Area = 510 µ m x 510 µ m
Room Temp EL
Ec
600
3-Layer staggered QW
400
3-Layer Staggered
InGaN LED
Ev
In0.28Ga0.72N
200
Conventional InGaN LED
0
0
20
40
Current Density
60
(A/cm2)
80
Conventional InGaN QW LED Vs. Staggered InGaN QW LED
 Staggered InGaN QW LED shows improved output power (2.0 - 3.5 times)
1. H. Zhao, G. Liu, X. H. Li, G. S. Huang, J. D. Poplawsky, S. Tafon Penn, V. Dierolf, and N. Tansu, Appl. Phys.
Lett., 95, 061104 (2009).
2. (Invited Paper) H. Zhao, G. Liu, X. H. Li, R. A. Arif, G. S. Huang, J. D. Poplawsky, S. Tafon Penn, V. Dierolf,
15
and N. Tansu, IET Optoelectronics,vol.3(6),pp. 283-295 (2009).
Outline
 Introduction on Current State-of-the-Art III-Nitride LEDs
 Enhancement of LED total external quantum efficiency
 Nanostructure Engineering for Enhancing LED Radiative Efficiency
 Staggered InGaN QW
 Linear-Shaped Staggered InGaN QW
 Type-II InGaN-GaNAs QW
 Strain-compensated InGaN-AlGaN QW
 InGaN-delta-InN QW
 Effect of Current Injection on Efficiency Droop in InGaN QW LEDs
 InGaN-AlInN QW-barrier structure to suppress efficiency droop
 Enhancement of LED Light Extraction Efficiency
 III-nitride microspheres
 LED Reliability and Tests
 Failure modes and mechanisms in LEDs
 Summary
16
High Power LED Package
Encapsulant
Housing
Die
Attach
Die
LEDs
Lead
Frames
Metal
heat slug
Solder
joints
Bond
Wires
1. Moon-Hwan Chang, Diganta Das, P. V. Varde, Michael Pecht, Microelectronics Reliability, February 2012.
17
LED Reliability Test
 Operating life tests
 Applying electrical power loads at various operating environment
temperatures to LEDs
 room temperature test
 high temperature test
 low temperature test
 wet/high temperature test
 temperature humidity cycle test
 on/off test
 Environmental tests
 Non-operating life tests
 reflow soldering test
 thermal shock test
 temperature cycle test
 moisture resistance cyclic test
 high / low temperature storage test
 temperature humidity storage test
 vibration test
 electro-static discharge test
1. Moon-Hwan Chang, Diganta Das, P. V. Varde, Michael Pecht, Microelectronics Reliability, February 2012.
18
LED Lifetime Estimation
 The Alliance for Solid-State Illumination Systems and Technologies (ASSIST)
 50% light output degradation (L50) for display industry
 70% light output degradation (L70) for lighting industry
 Accelerated test approach
 measuring the light output of samples at each test readout time
 estimating LED life under the accelerated test conditions (using functional
curve fitting)
 calculating an acceleration factor (AF)
 predicting lifetime by using the AF multiplied by the lifetime of the test condition
19
Failure modes and mechanisms
 Semiconductor related




Defect and dislocation generation and movement
Die cracking
Dopant diffusion
Electromigration
 Interconnection related
 Electrical overstress-induced bond fatigue
 Electrostatic discharge
 Electrical contact metallurgical interdiffusion
 Package related






Carbonization of the encapsulant
Delamination
Encapsulant yellowing
Lens cracking
Phosphor thermal quenching
Solder joint fatigue
1. Moon-Hwan Chang, Diganta Das, P. V. Varde, Michael Pecht, Microelectronics Reliability, February 2012.
20
Semiconductor related
Defect and dislocation generation and movement
 Light output degradation due to nonradiative recombination
 Crystal defects are mainly generated in
 contacts
 active region
 Lead to reduction of non-equilibrium electron hole pairs lifetime
increase of multi-phonon emission under high drive current
1. Sugiura L. Dislocation motion in GaN light-emitting devices and its effect on device lifetime. J Appl Phys;
81:1633–8, 1997
21
Semiconductor related
Die Cracking
Thermal Expansion Coefficient (x10-6/K-1)
Thermal Expansion Coefficient (x10-6/K-1)
GaN
5.6
GaN
5.6
Si
2.4
Sapphire 7.5
 Lattice mismatch between epitaxial layer and substrate
 Different thermal expansion coefficient
 Extreme thermal shock can break LED dies
1. Moon-Hwan Chang, Diganta Das, P. V. Varde, Michael Pecht, Microelectronics Reliability, February 2012.
22
Semiconductor related
Dopant Diffusion & Electromigration
Dopant Diffusion
 InGaN QW LEDs use Mg as p-type dopant
 Mg diffuses into QWs during the growth
 Mg acts as nonradiative recombination center  reduction of IQE
Electromigration
 Movement of metal atoms in the electrical contact to the LED die surface
 Causes short circuit
 Metal diffuses toward inner region from p-contact across the junction
 create spikes along the direction of current flow
23
Interconnection related
Electrical overstress induced bond fatigue and wire bond fatigue
 Thermal mechanical stress
 Mismatch of thermal expansion coefficients between wire bond and chip
 Repetitive, high-magnitude thermal cycles lead to rapid failure
 The reliability of the joint varies with
 bond wire length and loop height
 bond wire material
1. Moon-Hwan Chang, Diganta Das, P. V. Varde, Michael Pecht, Microelectronics Reliability, February 2012.
24
Package-related failure mechanisms
Carbonization of the encapsulant
Optical micrograph of LED after
plastic removal (the darkened
areas over the p-contact area
are damaged plastic which could
not be removed)
 Carbonization of plastic encapsulation material leads to Joule heating
 Carbonization of encapsulant  Reduction of insulation resistance 
Initiate a thermal runaway process  Carbonization of encapsulant
1. D. L. Barton, M. Osinski, P. Perlin, P. G. Eliseev, Jinhyun Lee,Microelectronics Reliability 39, 1219-1227 (1999). 25
Package-related failure mechanisms
encapsulant yellowing
 Transparent epoxy resin yellowing due to
 Prolonged exposure to short wavelength emission (blue/UV radiation)
 Excessive junction temperature
 The presence of phosphors
 Leads to photodegradation
 The degradation and associated yellowing increases exponentially with
exposure energy
1. D. L. Barton, M. Osinski, P. Perlin, P. G. Eliseev, Jinhyun Lee,Microelectronics Reliability 39, 1219-1227 (1999). 26
Package-related failure mechanisms
Phosphor thermal quenching
 Efficiency of phosphor degrades when temperature rises
 Reduction of light output at high temperature
 Change of color temperature
1. Moon-Hwan Chang, Diganta Das, P. V. Varde, Michael Pecht, Microelectronics Reliability, February 2012.
27
Outline
 Introduction on Current State-of-the-Art III-Nitride LEDs
 Enhancement of LED total external quantum efficiency
 Nanostructure Engineering for Enhancing LED Radiative Efficiency
 Staggered InGaN QW
 Linear-Shaped Staggered InGaN QW
 Type-II InGaN-GaNAs QW
 Strain-compensated InGaN-AlGaN QW
 InGaN-delta-InN QW
 Effect of Current Injection on Efficiency Droop in InGaN QW LEDs
 InGaN-AlInN QW-barrier structure to suppress efficiency droop
 Enhancement of LED Light Extraction Efficiency
 III-nitride microspheres
 LED Reliability and Tests
 Failure modes and mechanisms in LEDs
 Summary
28
Summary
 Enhancement of External Quantum Efficiency of LEDs
 Novel Nitride based Quantum Well (QW) LEDs with Enhanced
radiative efficiency
 Staggered InGaN QW
 Linearly-Shaped Staggered InGaN QW
 Type-II InGaN-GaNAs QW
 Strain-Compensated InGaN-AlGaN QW
 InGaN-delta-InN QW
 Enhanced overlap with wavelength extension
 Surface Plasmon (SP) Based Nitride LEDs
 SP Dispersion Engineering via Double-Metallic Layers
 Efficiency droop and current injection efficiency in nitride LEDs
 Novel QW-barrier designs to suppress efficiency-droop
 Enhancement of LED light extraction efficiency
 III-nitride microspheres
 LED Reliability and Tests
 LED lifetime measurement and estimation
 Failure modes and mechanisms in LEDs
29
Acknowledgments
 Case Nanophotonics Group
 PI: Dr. Hongping Zhao
 Graduate Students:
Peng Zhao
Xuechen Jiao
 Collaborators
 Lehigh: Dr. N. Tansu (PhD advisor), Dr. R. Arif, Dr. Y. Ee, Dr. G. Huang, Dr.
M. Jamil, Dr. H. Tong, G. Liu, X. Li, J. Zhang
 CWRU: R. French (Material Science), K. Kash (Physics), W. Lambrecht
(Physics), M. Sankaran (Chemical Engineering)
 Ferro Corporation
 Rambus
 Sanan
Thank You !
30