Hongping Zhao
Department of Electrical Engineering and Computer Science, Case Western Reserve University
Cleveland, Ohio, USA [email protected]
SOLAR DURABILITY WORKSHOP Case Western Reserve University, Cleveland, OH
April 9th, 2012
III-Nitride Light-Emitting Diodes for Solid-State Lighting
Impact of Solid State Lighting
NASA Image: Earth’s City Lights
Lighting 22%
US Electricity Consumption
Commercial 59%
Industrial 14%
Residential 27%
Source: Cree “Solid State Lighting Revolution”
LED lighting can reduce electricity needs for lighting by more than 60%!
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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
00.5
11.5
22.5
33.5
44.5
55.5
66.5
3 3.1 3.2 3.3 3.4 3.5 3.6
AlN
GaN
InN
AlGaN
InGaN
Deep UV-LEDs, Lasers, Solar blind detectors
ElectronicsHEMTs
Near UV/visible LEDs,Photovotaics
In-plane Lattice Constant a (Ǻ)
Ener
gy G
ap (e
V)III-Nitride Material System and Applications
III-Nitride Material Applications
III-Nitride Material System Direct band gap Wide spectrum coverage UV up to near infrared 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
White
Red, λ ~ 650-nm
Green, λ ~ 535-nm
Blue, λ ~ 450-nm
1) Multi-chip RGB approach: 1 RED (λ ~ 620 nm) InGaAlP LED 1 GREEN (λ ~ 540 nm) InGaN LED 1 BLUE (λ ~ 455 nm) InGaN LED Pros: theoretically best efficiency; variable color
temp Cons: high cost; efficiency of green LED
InGaN LED
Ce: YAG Phosphor
300 400 500 600 700 800Wavelength (nm)
4000
3000
2000
1000
0
Inte
nsity
2) Single Blue InGaN (λ ~ 450-460 nm) die + Phosphors Pros: relatively low cost, currently highest efficiency Cons: non-trivial control of color temperature
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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
Sapphire Substrate
N-GaN
P-GaN
Transparent Metal InGaN / GaN MQW
Active Region
Ni / Au
Ti / Au
8
Injection Efficiency Fraction of Injected Current that Recombine in the Active Region
Radiative Efficiency Fraction of Injected Current in the Active Region that Recombine Radiatively Generating Photons inside the Semiconductors Extraction Efficiency Fraction of Photon Generated in the Active Region that Exists the Semiconductor Cavity into Free Space
High Efficiency LEDs:
Device Physics of III-Nitride LEDs
extractionRadinjexternal ηηηη ⋅⋅= RadinjIQE ηηη ⋅=
Radη
extractionη
injη
InputElectrical
OutputOpticalPlugWall η
ηη =
9
λ = 463-nm ψe dQW = 30-Å
-3.2
-2.4
-1.6
-0.8
0.0
0.8
1.6
2.4
0 20 40 60 80 100 120 140 160 180 200
z(Å)Ec
& E
hh (e
V)
Without Polarization
Γe-hh = 97.44% Γe-hh = 34.4% Reduce
Large Reduction in Γe-hh due to Polarization Effect
With Polarization λ= 417-nm dQW = 30-Å
ψhh
ψe
ψhh
Non-Polar Vs. Polar In0.2Ga0.8N-GaN QW
Growth Direction
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Approaches for Enhancing the Overlap (Γe_hh)
Nanostructure Engineering (on c-plane GaN) Enhance electron-hole wavefunction overlap (Γe_hh)
Nonpolar InGaN QW1
Remove the polarization field Less mature epitaxy Cost issue
Ec
Ev
Ec
Ev
ψe
ψh
Novel QW / QDConventional QW
• 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
Growth Direction
ψe
ψh
• High electron-hole wavefunctionoverlap (Γe_hh)• Reduces Quantum Confined Stark Effect• Enhances spontaneous emission rate and optical gain• Reduces threshold carrier / current density
1. 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)
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Concept of Staggered InGaN QW Structures
InzGa1-zN
GaN
InxGa1-xN InyGa1-yNInxGa1-xN
InyGa1-yN
(a) ConventionalInzGa1-zN QW
(b) Two-Layer StaggeredInxGa1-xN/InyGa1-yN QW
(c) Three-Layer staggeredInyGa1-yN/InxGa1-xN/InyGa1-yN QW
GaN
Ec
Ev
Ec
Ev
Ec
Ev
GaNGaN GaNGaN
InyGa1-yN
Existence of spontaneous and piezoelectric field
Low electron-hole wave function overlap
Band lineups engineering
Enhance electron-hole wave function overlap
III-V Nitrides
Staggered QW
1. H. Zhao, R. A. Arif, and N. Tansu, IEEE J. Sel. Top. in Quantum Electronics, 15 (4), 1104 (2009). 12
Bottom-Emitting Device Structure good contact low resistivity
Three-Layer Staggered InGaN QWs for active region
Bottom-Emitting InGaN LED Devices
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, and N. Tansu, IET Optoelectronics,vol.3(6),pp. 283-295 (2009).
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Electroluminescence Vs. Injection Current
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
350 400 450 500 550 600 6500
1
2
3
44.5 x 104
Wavelength (nm)
Staggered InGaN QW LED
EL In
tens
ity (a
.u.) I = 200 mA
I = 150 mAI = 100 mAI = 80 mAI = 50 mAI = 30 mA
Area = 510 µ m x 510 µ m
350 400 450 500 550 600 6500
1
2
3
44.5 x 104
Wavelength (nm)
Conventional InGaN QW LED
EL In
tens
ity (a
.u.)
I = 200 mAI = 150 mAI = 100 mAI = 80 mAI = 50 mAI = 30 mA
Area = 510 µ m x 510 µ m
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, and N. Tansu, IET Optoelectronics,vol.3(6),pp. 283-295 (2009).
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Conventional InGaN QW LED Vs. Staggered InGaN QW LED Staggered InGaN QW LED shows improved output power (2.0 - 3.5 times)
0
200
400
600
800
1000
0 20 40 60 80Current Density (A/cm2)
Out
put P
ower
(a.u
.) Room Temp EL
λpeak ~ 520-525 nm
Conventional InGaN LED
3-Layer Staggered InGaN LED
Area = 510 µ m x 510 µ m
In0.28Ga0.72N
In0.21Ga0.79N
Ec
Ev
GaNGaN
3-Layer staggered QW
CW Comparison of Output Power Conventional Vs. Staggered InGaN QW LEDs
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, and N. Tansu, IET Optoelectronics,vol.3(6),pp. 283-295 (2009).
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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
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High Power LED Package
1. Moon-Hwan Chang, Diganta Das, P. V. Varde, Michael Pecht, Microelectronics Reliability, February 2012.
Housing Encapsulant
LEDs
Die
Bond Wires
Die Attach
Lead Frames Metal
heat slug
Solder joints
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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
LED Reliability 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
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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
Failure modes and mechanisms
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
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.
Semiconductor related Die Cracking
Thermal Expansion Coefficient (x10-6/K-1)
GaN 5.6
Si 2.4
Thermal Expansion Coefficient (x10-6/K-1)
GaN 5.6
Sapphire 7.5
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
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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.
Interconnection related Electrical overstress induced bond fatigue and wire bond fatigue
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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).
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)
25
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).
Package-related failure mechanisms encapsulant yellowing
26
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.
Package-related failure mechanisms Phosphor thermal quenching
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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
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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
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
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Thank You !