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 [email protected]

SOLAR DURABILITY WORKSHOP Case Western Reserve University, Cleveland, OH

April 9th, 2012

III-Nitride Light-Emitting Diodes for Solid-State Lighting

mailto:[email protected]

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

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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

6






Summary

7

III-Nitride Light-Emitting Diodes

Sapphire Substrate

N-GaN

P-GaN

Transparent Metal InGaN / GaN MQW

Active Region

Ni / Au

Ti / Au

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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



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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



15






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


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

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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


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)

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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

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Efficiency of phosphor degrades when temperature rises

Reduction of light output at high temperature

Change of color temperature


Package-related failure mechanisms Phosphor thermal quenching

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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 !

III-Nitride Light-Emitting Diodes for Solid-State Lighting

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