History and Recent Progress of LED

1

Hao-chung (Henry) Kuo

Associate VP and Distinguished Professor

Department of Photonics and Institute of Electro-optical Engineering National Chiao-Tung University (NCTU)

History and Recent Progress of LED

IEEE senior member IET Fellow SPIE Fellow and OSA Fellow

The Nobel Prize in Physics 2014 lecture

2

Prof. Hao-chung Kuo CV

08/02- now, Distinguished Professor, National Chiao-Tung University, Associate Dean, office of International office;

Associate Director, Photonics Center, NCTU 02/10-08/12 TD director, TSMC

12/05-02/10 Consultant,

Epistar Corporation

02/2002-now Advisor, ITRI,

06/2009-12/2009 Technical Director and Visiting Professor,

LED and Solar cell program, Nano and advanced material Institute, HKUST 06/2005-12/2005, Senior Manager and Visiting Professor, HK ASTRI

2000-2002 Engineer Manager (Epitaxy), LuxNet Corp, Fremont, CA.

1999-2000 Senior R&D Scientist, Agilent Technologies, CA 1993-1995 Bell Lab Lucent /AT&T

IEEE senior member IET Fellow SPIE Fellow and OSA Fellow

1990 NTU Physics (under Prof. YF Chen special project on Semiconductor)

1995 MS in Rutgers University (NJ) 1998 UIUC Ph.D. in ECE/Applied Physics

3

4

5

LED Chip

• Determines raw

brightness and efficacy

Phosphor system

• Determines color point and color point stability

Package

• Protects the chip and phosphor

• Helps with light and heat extraction

• Primary in determining LED lifetime

LED Technology

LED Chip

Package Phosphor LED Chip

Package Phosphor

6

…A Brief History of Lighting

1879 Edison Light

Bulb

U.S. 223,898

1901 Fluorescent

Tube 1919 Sodium Vapor Lamp

1970s First Red

LED

~1990

“High Brightness”

Red, Orange, Yellow, & Green LEDs

1995

“High Brightness”

Blue, Green LEDs 2000

White LED Lamp demonstrates Incandescent

Efficacy (17 lm/W) 2005

White LED Lamp demonstrates

Fluorescent Efficacy (70 lm/W)

2009

Production White LED Lamp Exceeds 100 lm/W

• Current lighting technology is over 120 years old

• LEDs began as just indicators, but are now poised to become the most efficient light source ever created

Calculators and Indicators

Monochrome signs

Full Color Signs

Solid State Lighting

7

1907 H. Round

8

No Efficient Blue LED till 1990

9

10

TG and Nichia start patent war

Nobel Prize cite paper

11

Brief History of LEDs

• 1955 – RCA reports IR emission using GaAs

• 1961 – TI gets patent for IR LED

• 1962 – GE develops first visible LED (Holonyak)

• 1968 – Monsanto develops first commercially available LEDs for HP35 calculator

• 1970’s - GaP-based red, green and yellow

• 1980’s – AlGaAs/AlInGaP red and amber LEDs

• 1990’s – InGaN LEDs and YaG phosphor – Nakamura,

Akasaki,Amano

• 2000’s – White LEDs for SSL

12

2007 at UIUC microelectronic lab

My advisor Prof. Greg Stillman for C-doped HBT And my Advisor’s Advisor

Prof. Nick Holonyak Jr -Father of visible LED, O-VCSEL

Special thanks to

2012 at UIUC LED 50th

2003 at UIUC coffee hour

13

Prof. Holonyak Invention

Prof. John Bardeen

14

*

*

Prof. John Bardeen

*

*

15

2014 the Nobel Prize in Physics Awarded

16

2014 the Nobel Prize in Physics Awarded to Isamu Akasaki

Brief Bio:

Isamu Akasaki (赤崎 勇) was born in Kagoshima, Japan. Dr. Akasaki graduated from Kyoto University in 1952, and obtained a Ph.D degree in Electronics from Nagoya University in 1964. He started working on GaN- based blue LEDs in the late 1960s. Step by step, he improved the quality of GaN crystals and device structures at Matsushita Research Institute Tokyo,Inc.(MRIT), where he decided to adopt metalorganic vapour phase epitaxy (MOVPE) as the preferred growth method for GaN.

Important Contribution:

In 1981, he started afresh growth of GaN by MOVPE at Nagoya University, and in 1985 he and his group succeeded in growing high-quality GaN on sapphire substrate by pioneering the low- temperature (LT) buffer layer technology.

This high-quality GaN enabled them to discover p-type GaN by doping with magnesium (Mg) and subsequent activation by electron irradiation (1989), to produce the first GaN p-n junction blue/UV LED (1989), and to achieve conductivity control of n-type GaN (1990) and related alloys (1991) by doping with silicon (Si), enabling the use of heterostructures and multiple quantum wells in the design of more efficient p-n junction light emitting structures. He also verified quantum size effect (1991) and quantum confined Stark effect (1997) in nitride system, and in 2000 showed theoretically the orientation dependence of piezoelectric field and the existence of non-/semi- polar GaN crystals, which have triggered today’s world-wide efforts to grow those crystals for application to more efficient light emitters.

17

2014 the Nobel Prize in Physics Awarded to Hiroshi Amano

Brief Bio:

Hiroshi Amano (天野 浩) was born in Hamamatsu, Japan. He received his BE, ME and DE degree in 1983, 1985 and 1989, respectively, from Nagoya University. He joined Professor Isamu Akasaki's group in 1982 as an undergraduate student. Since then, he has been doing research on the growth, characterization and device applications of group III nitride semiconductors. He is the first one who demonstrated growing p-typr GaN and fabrication a p-n junction GaN LED in the world.

Important Contribution:

In 1985, he developed low-temperature deposited buffer layers for the growth of group III nitride semiconductor films on a sapphire substrate, which led to the realization of group-III-nitride semiconductor based light-emitting diodes and laser diodes. In 1989, he succeeded in growing p- type GaN and fabricating a p-n-junction GaN-based UV/blue light-emitting diode for the first time in the world.

18

2014 the Nobel Prize in Physics Awarded to Shuji Nakamura

Brief Bio:

Shuji Nakamura (中村 修二) was born in Ikata, Japan. He graduated from the University of Tokushima in 1977 with a BS degree in electronic engineering, and obtained an MS degree in 1979. It was while working for Nichia that Nakamura invented the first high brightness gallium nitride (GaN) LED whose brilliant blue light, when partially converted to yellow by a phosphor coating, is the key to white LED lighting, which went into production in 1993. He was awarded a Ph.D degree from the University of Tokushima in 1994 and took a position as a professor of engineering at the University of California, Santa Barbara.

Important Contribution:

Previously, J. I. Pankove and co-workers at RCA put in considerable effort, but did not manage to make a marketable GaN LED in the 1960s. The principal problem was the difficulty of making strongly p-type GaN. Nakamura and his co-workers worked out the physics and pointed out the culprit was hydrogen, which passivated acceptors in GaN. He managed to develop a thermal annealing method and obtained controlled conductivity of p-GaN. He invented two-flow MOCVD growth method for InGaN, and hence to obtain high brightness blue/UV LED in 1993.

He also demonstrate pulse emission of InGaN/GaN blue laser diode at room temperature, opening a way to obtain blue ray emission head for optical communication. His most important contribution of high brightness blue LED leads him to be called as the “father of blue LED”.

19

Related News

https://www.youtube.com/watch?v=J-oBvPYx1NQ https://www.youtube.com/watch?v=iMNTLDfqCvU https://www.youtube.com/watch?v=9in3hZreYts

Photo with Prof. Nakamura

Prof. HC Kuo

20

2015 year of Light

The Nobel Prize in Physics 2014

Isamu Akasaki, Hiroshi Amano and Shuji Nakamura

"for the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources“

The Nobel Prize in Physics 2010

Andre Geim and Konstantin Novoselov

"for groundbreaking experiments regarding the two- dimensional material graphene“

The Nobel Prize in Physics 2009 Charles Kuen Kao

"for groundbreaking achievements concerning the transmission of light in fibers for optical communication"

Willard S. Boyle and George E. Smith

"for the invention of an imaging semiconductor circuit - the

CCD sensor"

21

Semiconductor laser/laser Physics

The Nobel Prize in Physics 2005 Roy J. Glauber

"for his contribution to the quantum theory of optical coherence"

John L. Hall and Theodor W. Hänsch

"for their contributions to the development of laser- based precision spectroscopy, including the optical frequency comb technique“

The Nobel Prize in Physics 2000

"for basic work on information and communication technology"

Zhores I. Alferov and Herbert Kroemer

"for developing semiconductor heterostructures used in high-speed- and opto-electronics"

Jack S. Kilby

"for his part in the invention of the integrated circuit"

22 The Photoelectric Effect

Using his theory of quanta, Einstein explained the photoelectric effect.

He showed that when quanta of light energy strikes atoms in the metal, the quanta force the atoms to release electrons.

Einstein’s work helped justify the quantum theory. The photoelectric cell resulted from Einstein’s work. This device made possible sound motion pictures, television and many other inventions. Einstein received the 1921 Nobel Prize in physics for his paper on quanta.

The work of Planck and Einstein quickly established the Quantum Theory, not only in light but also in many forms of energy. The quantum physics was born.

The Photoelectric Effect

Photon

23

LED Benefits (Diode)

• Lower cost of ownership - Energy savings

- Maintenance savings

• Reliability/Ruggedness

• Safety - Low voltage & Low heat generation

• Small and light - Flexible for styling, unique spaces

• Dimmable, flashable, and instant turn on

• Excellent for distributed light

• Excellent control of light directionality

- Minimize light pollution

24

One teaspoon of mercury can contaminate a 20

acre lake. .

Effects of Mercury on the Environment

* www.lightbulbrecycling.com

Forever.

Each year, an estimated 600 million fluorescent lamps are disposed of in

U.S. landfills amounting to 30,000 pounds of mercury waste.

The mercury from one fluorescent

bulb can pollute 6,000 gallons of

water beyond safe drinking levels.

25

LED Applications

• Traffic light

• Mobile phone, notebook-BLU (100%) LED TV BLU (100% by 2014) flash lamp

• Outdoor full color display (100%)

• Automotive lighting-break light, daytime running lamps, turn signal, etc.

• SSL ~15-20 % lm/$?

26

Current LED Lighting Applications

• Lumens

• LPW

• Lumens/$

27

4K2K LCD with LED

Slim <1cm

28

Smart Lighting -everywhere

29 0

50 100 150 200

2004 2006 2008 2010 2012 2014 2016 2018 2020

Year

Eff ica cy (lm/W )

Laboratory Projection - Cool White

Commercial Product Projection - Cool White Commercial Product Projection - Warm White Laboratory - Cool White

Commercial Product - Cool White Commercial Product - Warm White Maximum Efficacy - Warm White Maximum Efficacy - Cool White

DOE Roadmap 150 lm/W (lab >200 lm/W)

US Department of Energy 2009 Multi-Year Plan for SSL

Cree cool white production Cree warm white production

30

Haitz’s Law

31

32

Outline

 Challenges for Lighting Applications

LED LEE efficiency

 Physical mechanisms-efficiency droop

 How to eliminate droop at c-plane LED with strong QCSE

Graded-composition electron blocking layer (AlGaN)

 Efficiency droop in c-plane and m-plane GaN LED

 Semipolar {10-11} InGaN/GaN Nanopyramid LED

Conclusion

33

20 YEARS OF WORK ON LED

EPITAXIAL LAYER

ELECTRON-HOLE RECOMBINATION

LIGHT GENERATION ACROSS JUNCTION LIGHT EXTRACTION

PACKAGING

34

8

LED Efficiency Improvement (Blue+Phosphor)

LER (Lm/W)

Sources: DOE SSL MYPP(2013)

IQE (droop) ,Vf, chip LEE, Package

…

35

Droop – Key for SSL

Press release (Epistar) > 200 lm/W on COG

36

Outline

 Challenges for Lighting Applications

LED LEE efficiency (H-die, HV LED)

 Physical mechanisms-efficiency droop

 How to eliminate droop at c-plane LED with strong QCSE

Graded-composition electron blocking layer (AlGaN)

 Efficiency droop in c-plane and m-plane GaN LED

 Semipolar {10-11} InGaN/GaN Nanopyramid LED

37

SiO

/TiO

DBR back reflector (NCKU)

Side-wall undercut etching Nano-patterned substrate or PSS

(NCTU/HKUST)

Nano surface roughness (NCKU, NCTU)

SiO

current blocking layer underneath

LEE in H-die (ITO) improve reach >85%

Nanotechnology transfer to Epistar (low cost)

Electrochemical and Solid-State Letters, 10 (6) H175-H177 (2007)

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL.

29, NO. 7 (2011)

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 8 (2009) APPLIED PHYSICS LETTERS 93, 081108 (2008)

Nanotechnology, 16, 1844–1848 (2005)

(NCKU) (NCKU)

Current spreading

OPTICS EXPRESS, Vol. 20, No. 5, (2012)

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 18, (2009)

NUS

38

Vertical LED with nanocone (NCTU/NCKU)

Laser lift-off technique

J. Appl. Phys., Vol. 95, No. 8, 15, 2004

Electrode Patterns Design(NCTU)

Jpn. J. of Appl. Phys., Vol. 44, No. 11, pp. 7910 –7912 , 2005

Current blocking layer

Hwan Hee Jeong et. al., ESSL, 13 7 H237, 2010

Photonic crystal structure

APPLIED PHYSICS LETTERS 94, 123106 (2009)

ZnO Nanorod Arrays

Electrochemical and Solid-State Letters, 11 4 H84-H87, 2008

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 11, 2009

Semiled : Spin off from NCTU

NCTU/ITRI

NCTU/UCB

39

Evolution of Chip (NCTU,NCKU)

 Al Mirror + TiO2/SiO2 DBR Backside Reflector

 Mesh ITO p-Contact and Nanopillars

 Phosphoric Acid Etched Undercut Sidewalls

 Laser-induced periodic structure

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 29, NO. 7 (2011)

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 18, (2009)

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 8, (2009)

OPTICS EXPRESS, Vol. 20, No. 5, (2012)

 Thin GaN LEE (Cree, Lumiled, Semileds) also >85%

 Novel thin GaN Target：LEE > 90%

Semileds : technology transfer from NCTU

40

What Causes Efficiency Droop ?

• Simple answer: We don’t know yet

• Several competing theories/explanations

1) Electron overflow at high current densities due to inadequate electrical confinement layers (RPI, GIT)

2) Electron overflow due to polarization fields in the MQW region (Rensselaer Ploytechnic Institute)

3) Auger recombination due to high carrier density (Lumileds, UCSB) Defects assist Auger, Auger electron (UCSB)

4) Poor hole transport in MQW (Virginia Commonwealth Univ.)

5) 3D roughnesss (NTU, UCSB)

41

InGaN Active Regions: “MQW” ?

• Improve carrier distribution within the MQW region

top QW

n-GaN

n-GaN

p-GaN

p-GaN

• Fake MQWs

• Light generated only top QWs !

• Electron overflow is high

• Real MQWs

• Light generated in all QWs

• Electron overflow is reduced

• Vf reduced significantly

?

Electron / Hole mobility are not match

QCSE due to polarization mismatch

42

Outline

 Challenges for Lighting Applications

LED LEE efficiency (Thin GaN, H-die, flip chip HV)

 Physical mechanisms-efficiency droop

 How to eliminate droop at c-plane LED with strong QCSE

Graded-composition electron blocking layer (AlGaN)

 Efficiency droop in c-plane and m-plane GaN LED

 Semipolar {10-11} InGaN/GaN Nanopyramid LED

43

c-plane sapphire (0001) or m-plane free-standing GaN semi-polar (1-101) GaN on Si

n-GaN

GaN LT Buffer or AlN buffer

pre-strain insertion layer InGaN/GaN MQW

p-AlGaN p-GaN

1 2 3, 5

 Substrate

 non-polar m-plane substrate

 (1-101) semi-polar GaN on Si for QCSE control

 Insertion layer between MQW and n-GaN

 super lattice insertion layer for strain reduction

 MQW design

 graded-thickness MQW (GQW)

 graded-composition MQB (GQB) for hole transport

EBL design

 graded-composition EBL (GEBL) for hole injection

 Quaternary barrier

 InAlGaN quaternary barriers

4

Semiconductor Today, Applied Physics Letters, 2010, 97, 181101 Semiconductor Today, Applied Physics Letters, 2011, 99, 171106

Semiconductor Today, SPIE newsroom Applied Physics Letters, 2010, 98, 261103

Compound Semiconductor, Applied Physics Letters, 2011, 98, 211107 Applied Physics Letters, 2010, 97, 251114

Applied Physics Express, 2011, 4, 012105 Applied Physics Letters, 2010, 96, 231101

Development of Low Droop LED (NCTU, NCKU)

44

 Efficiency Droop in LED reported by Compound Semiconductor、Semiconductor Today及SPIE Newsroom

 Invited talk in Photonic West 2011 Photonic West and ICMOVPE 2012

45

EBL p-GaN MQWs

Graded band gap

n-side p-side

Energy band

2. Hole injection Improvement by Graded-composition EBL (GEBL)

• In conventional LED (black line), the valance band of EBL slopes upward from the n-GaN side toward p-GaN side.

Retarding the holes to transport across the triangular barrier.

• If the composition of Al in EBL increases from the n-GaN side toward p-GaN side, the band-gap broadens gradually.

 The barrier in valance band could be level down and even overturn.

The slope of conduction band could be enhanced.

46 -1

0 1 2 3 4

(a)

Al₀GaN - Al_0.15Ga_0.85N

Energy (eV)

Energy band

Fermi level

(b)

Al0GaN - Al

0.25Ga

0.75N

(c)

Al0GaN - Al

0.35Ga

0.65N

Different composition for GEBL (Simulation with Crosslight)

@ 100 A/cm

△E

between the last GaN barrier and the EBL is diminished in all three LEDs with GEBL.

 Better electron confinement and hole injection could be expected using GEBL

15% 25% 35%

47

0 50 100 150

0.2 0.4 0.6 0.8 1.0

Normalized Efficiency (a.u.)

Current Density (A/cm²)

GEBL

Characteristics of LED with GEBL

• Peak efficiency occurs at ~5 A/cm

• At 100 A/cm

=> Efficiency droop ~ 45%

C-plane GaN LED C-plane GaN with GEBL LED

• Peak efficiency occurs at ~17 A/cm

• At 100 A/cm

=> Efficiency droop ~23%

48

12 13 14 15 16 17 18

19

(a)

Hole concentration(cm-3 )(log)

Conventional GEBL

12 13 14 15 16 17 18

19

(b)

Electron concentration(cm-3 )(log)

Conventional GEBL

Carrier Distribution LED with GEBL (Simulation)

 Injected holes uniformly distribute along the EBL region compared to conventional LED, and hole concentration in MQWs is significantly increased as expected- better hole transport.

 Electron overflow is more effectively reduced than conventional EBL

49

0.00 0.02 0.04 0.06 0.08 0.10

80 90 100 110 120 130 140 150 160 170 180 190

Effi ci ency (l m/ W)

Current (A)

Vf- Flip Chip HV 45.6V Vf- HV 47V

At 0.02A

150 lm 150lm/W old structure 157 lm 168 lm/W with SL EBL 162 lm 177 lm/W with SL EBL

•Ref Face up 45x45 with SL EBL 150 lm/W

HV Flip Chip LED with Graded SL EBL (Epistar)

1W input

50

First electrically pump VCSEL Structure -@ RT

CW UV to Blue VCSEL@ RT

Appl. Phys. Lett. 97, 071114 (2010) Appl. Phys. Lett. V92, 141102(2008) CLEO post deadline (2010)

IEEE JSTQE invited paper

AlGaN/GaN DBR - Japan Patent AlN/GaN DBR (2004)

Optical pumping GaN VCSEL (2006)

77K Electric pumping GaN VCSEL (2008)

RT Electric pumping GaN VCSEL (2010)

51

Reported by Prof. Nakamura

52

VCSEL with GEBL

53

0 0.2 0.4 0.6 0.8 1

0 5 10 15 20

Experimental data Simulation data

Power (a.u.)

Current Density (kA/cm²)

0 2 4 6 8 10 12

0 5 10 15 20

Original structure GEBL structure

Current Density (kA/cm²)

Power (mW)

GaN VCSEL with GEBL barrier

Better Jth (12kA/cm2->9.8kA/cm2) and SE

Laser Physics Letter (2014)

54

400 500 600 700 800

0 100 200 300 400

Intensity(a.u.)

Wavelength(nm)

0 5 10 15 20 25 30

0 2 4 6 8 10 12 14 16

18 I-V

L-I

Current (kA/cm2)

Vol tag e (Vo lt)

0 2 4 6

WP E (%)

WPE ~ 6%@ 25kA/cm2

Low Droop due to – low Auger

55

56

Outline

 Challenges for Lighting Applications

LED LEE efficiency (Thin GaN, H-die, flip chip HV)

 Physical mechanisms-efficiency droop

 How to eliminate droop at c-plane LED with strong QCSE

Graded-composition electron blocking layer (AlGaN)

 Efficiency droop in c-plane and m-plane GaN LED

 Semipolar {10-11} InGaN/GaN Nanopyramid LED

57

58

Non-polar/Semipolar GaN LEDs (UCSB)

• LED grown on semipolar GaN (10-1-3)

• Reduced polarization fields and QCSE in active region

• Clearly reduces blue shift (direct consequence of polarity reduction)

• “Droop” is still present but improved a lot

59

Sample Structure

6 pair InGaN/GaN 3nm/12nm

Kyma SC Ling and HC Kuo

Applied Physics Letters, 2010, 96, 231101

60

Current-Output Power-Efficiency (460nm)

• Peak efficiency occurs at ~5 A/cm

• At 100 A/cm

=> Efficiency droop ~ 45%

C-plane GaN LED M-plane GaN LED

• Peak efficiency occurs at ~23 A/cm

• At 100 A/cm

=> Efficiency droop ~13%

61

Band Diagram and Current Overflow

• QCSE and band-bending are induced by polarization field in C-plane InGaN/GaN and create triangular energy barrier in active region, which favors electron overflow .

• Polarization field is eliminated by using m-plane GaN, and electron overflow is

significantly reduced.

62

Electron and Hole Distributions

• Polarization field induces non-uniform hole distribution within MQWs.

• Electrons accumulate at the interface between last GaN barrier and AlGaN EBL.

• Polarization-free GaN LED has relatively uniform hole distribution due to the

elimination of triangular barrier in band diagram.

63

Experimental and Simulated Results

• Simulation results show good agreement with experiments.

• Strongly inherent polarization fields are responsible for the significant efficiency droop of c-plane LEDs.

• m-plane LED exhibits the efficiency retention at high current injection as a

result of the absence of polarization fields.

64

M-plane LED by Panasonic

SC Ling and HC Kuo

Applied Physics Letters, 2010, 96, 231101

65

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340

0 20 40 60 80

EQE (%)

Current (A/cm²)

GaN on Sapphire GaN on GaN

NCTU data Chip size 10*23 mil

Peak EQE 60% EQE >57% at 200A/cm2 (QW 3nm->6nm/8nm)

NCTU

NCTU M-plane NCTU peak EQE 75% (IQE 88%, LEE 85%), 66% 35A/cm2

M-plane MQWs

EBL

3nm/12nm

66

Semi-polar (1-101) InGaN/GaN multiple quantum wells grown on patterned silicon substrates (with ITRI/Nagoya Univ.)

Si [111] [0001]

Stripe mask pattern;

Photo-lithography and KOH etching

Selective growth of GaN on (111)Si facets

ELO and Coalescence

(1-101) GaN

(001)Si substrate (001) Si

SEM images

5µm 1µm

(001) Si substrate

(001) Si substrate

<1-101> GaN

<0001> GaN

[1-101]

SiO₂

GaN

High surface quality of the semipolar GaN film

67

XTEM Semi-polar (1-101) InGaN/GaN

The threading dislocations generated near AlN buffer layer/Si interface turn to the perpendicular direction of (0001).

Low TDDs (10E8 1/cm2) at the top of (1-101) as the growth proceeding.

68

Result-II

The EL emission peak wavelength of semi-LEDs is slightly blue-shifted (about 1.7nm) at 60mA/cm

 EL spectra shows negligible wavelength shift due to less QCSE in semipolar LEDs

3 InGaN/GaN QW 3nm/7.5nm

69

Band Diagram and Current Overflow

Semi-polar GaN reduces polarization field in LED structure.

Electron leakage current is significantly reduced in semi-polar GaN based LED.

70

Simulation result-I

 Simulation results show good agreement with experiments.

 Internal field dominates the droop behavior in our simulation.

 The efficiency droop improved from 42% to 10%

c-plane Semi-polar

Screening effect 50% 20%

Auger coefficient (cm⁶s^-1)

6*10^-30 7*10^-30

Hole mobility lower higher

71

Summary (I)

Several methods for reduction of efficiency droop were proposed.

 Non-polar and semi-polar GaN substrate to reduce QCSE and overflow

 GEBL structure to enhance hole transportation and reduce electron overflow

 HV LED combine with GBEL design and Red HV LED – 170lm/W 2700K CRI 90 was achieved

 M –plane LED with wider well (6nm, 6 QW) peak EQE 60%, 57% 200A/cm2

only 5% drop (cost)

72

Sandia lab (G.T. Wang)

NTU (C. C. Yang) & Epistar, Bottom-up MOCVD NCHU (Gwo)

NCTU (H. C. Kuo) & Epistar

Sophia Univ. (Kishino , Japan) SAG MBE

Nanophysique (French) Bottom-up MOCVD

Braunschweig U. of Tech.

(Waag, Germany) & Osram

Samsung LED (Korea) SAG MOCVD

USC (Dapus) Glo AB (Heersee, Sweden)

SAG MOCVD McGill Univ. (Canada)

MBE

Harvard univ.

(Charles M. Lieber)

Research Activity of GaN nanostructures

73

• Efficiency of LED : η

= η

× η

 Light extraction efficiency (LEE)

 Internal quantum efficiency (IQE)

Superlattices (SLs)

Nanorod LED

How to Improve the Efficiency of LEDs ?

Nitride-based nanorod LEDs attract a lot of attentions in the last few years.

Patterned template

Nonpolar / Semipolar substrate

Surface roughness

Reflector (DBR, metal etc.)

Improving methods:

[1] J. P. Zhang et. al., Appl. Phys. Lett. 80, 19 (2002)

[2] Arpan Chakraborty et. al., Appl. Phys. Lett. 85, 22 (2004) [3] Y. J. Lee et.al., IEEE Photon. Technol. Lett. 18, 1152 (2006) [4] T. Fujii et. al., Appl. Phys. Lett. 84, 6 (2004)

[5] Jong Kyu Kim et. al., Appl. Phys. Lett. 84, 22 (20040 [6] From Web of Knowledge, retrieved 06/2014

nanorod LEDs

Defect density QCSE

02/26

74

[7] S. Li et al, J. Appl. Phys., 111, 071101 (2012) [11] Y. H. Ra et al, J. Mater. Chem. C, 2, 2692–2701 (2014) [8] H. Y. Ryu, Nanoscale Research Letters, 9:58 (2014) [12] S. Nakamura, MRS bulletin, vol. 34, p. 101-107, (2009).

[9] M. Y. Ke et al, IEEE J. Quantum Electron, 15, 4 (2009) [13] James S. Speck, Solid State Lighting, UCSB

[10] S. Noda et al, Nature news & views, 3, (2009) [14] S. D. Hersee et al, J. Appl. Phys. 85, 6492 (1999)

Advantages of Nanorod LEDs

[11]

2D Template

c-axis

3D nano structure template

 Growth of non/semi-polar MQWs

 Larger area of active layers

2D 3D

2 2

rod film

A rh h

A r r



  

 Higher light extraction efficiency

[13]

[11]

[8]

 Strain energy relaxation

[14]

Crystal quality IQE QCSE

03/26

75

•GaN Nanorods produce zero dislocation, non-polar/semipolar facets on which to grow LED active regions.

•The creation of non-polar /semi-polar planes on conventional orientation substrates accesses the advantages of non-

polar/semipolar orientations without the cost of bulk substrates.

•3D active regions may further reduce the efficiency droop associated high current operation.

•Nanostructures can be grown on Si or other low cost substrates to further reduce the cost.

Summary of Nanostructure advantages

[1] S.P. Chang, H.C. Kuo et al, Appl. Phys. Lett. 100, 261103 (2012).

[2] S.P. Chang, H.C. Kuo et al, OPTICS EXPRESS, Vol. 20, No. 11, 12457 (2012).

[3] S.P. Chang, H.C. Kuo et al, Appl. Phys. Lett. 100, 061106 (2012) [4] S.P. Chang, H.C. Kuo et al, OPTICS EXPRESS (2013)

76

Sample Preparation

{10-11}

Nano imprint lithography

PQC pattern designed by Southampton

2um

Dry etching 750nm 350nm

ITO: 180 nm

Nanorod passivation by SiO₂

GaN regrowth

MQW: In_0.3Ga_0.7N/GaN (3 nm/12nm) × 10 pairs

p-GaN

p-GaN SiO₂ GaN C-sapphire

ITO MQWs

77

Uniform and large area ordering are achievable while using our novel nano-pyramidal structure.

Nano-pyramid LED researches in NCTU

78

Cross section TEM image and CL of MQWs

MQW InGaN well GaN barrier

Tg(^oC) 830 910

Rg(A/s) 0.1 0.2

Nano-pyramid 3 12

Nano-pyramid MQWs

 wavelength is longer at apex region –

In composition is higher at apex of pyramid QCSE is higher at apex.

 Uniform composition along semipolar {10-11} plane

450 500 550 600 650 700

10 mA 20 mA 30 mA 40 mA 50 mA 60 mA 70 mA 90 mA 120 mA 150 mA

Intensity (a.u.)

Wavelength (nm)

PQC

79

LED device performance

The turn on voltage is about 2.5V@20mA, which is very close to the band gap of active layers.

The out put power linearly increases with injecting current even to the high injection level 160A/cm

.

[1] H.C. Kuo et al, Appl. Phys. Lett. 101, 233104 (2012) 0

10 20 30 40 50

Power(a.u.)

Current density(A/cm²)

Power

0 20 40 60 80 100 120 140 160 180

0 1 2 3 4 5 6

Voltage (V)

80

0 20 40 60 80 100 120 140 160 180

0 1 2 3 4 5 6 7 8

EQE (a.u.)

J (A/cm²)

EQE

500 510 520 530 540 550 560 570 580 590 600

Peak Wavlength (nm)

Peak Wavlength(nm)

Efficiency Droop properties

There is an intersection between efficiency and emission wavelength.

inject at APEX then spread out along semipolar {10-11} plane

A stable and very low efficiency droop green emitter can be obtained after 40 A/cm

.

EQEMax~15%

81

FEM simulation

40 A/cm

120 A/cm

80 A/cm

200 A/cm

[13]

82

Emission Energy vs. Tg

 There is a high linear relationship between the

emission energy and growth temperature on semipolar facet, the high In content for LEDs are available for

various the growth temperature

83

Exploring nanopyramid approach to longer- wavelength nitride LEDs

Bridging the amber-green gap and white LEDs

Mike Cooke reports on recent reports of various techniques to create light-emitting

diodes that could fill the chasm, possibly leading to whiter LEDs.

84

Nanowire LED commercialization/Industry efforts

National Chiao Tung University

85

400 450 500 550 600 650 700 750 800

EL intensity (a.u.)

Wavelength (nm)

10 mA 20 mA 30 mA 40 mA 50 mA 60 mA 70 mA 80 mA 90 mA 100 mA 110 mA 120 mA 130 mA 140 mA 150 mA

low current high current

Coreshell LED by NCTU/Yale U

SiN

SiO

ITO p-GaN

MQWs n-GaN p-contact

120 A/cm²

Graphene GaN on Si or Sapphire Flexible substrate

Graphene

86

 Tip-free NR LED  Tip NR LED

g= [11-20] g= [11-20]

[11-20] non-polar [1-100]

[0002] semi-polar [1-102]

43.261°

Reduced-semipolar MQWs

Nonpolar MQWs

Nonpolar MQWs Semipolar MQWs

No In cluster

In cluster

 Tip-free NR was fabricated and MQWs are formed both on semi-polar and non-polar plane.

 The area of semi-polar MQWs could be decreased by SiNx passivation.

 The formation of In clustering was prevented.

Transmission Electron Microscope (TEM) Measurement

87

350 400 450 500 550 600 650 700 750

15 mA 25 mA 30 mA 35 mA 40 mA 45 mA 50 mA 60 mA 70 mA 80 mA 90 mA 100 mA 120 mA 150 mA

EL intensity (a.u.)

Wavelength (nm)

0 1 2 3 4 5 6

0.00 0.05 0.10 0.15 0.20

Current (A)

Voltage (V)

Tip-free Nanorod LED Nano-pyramid LED

Nanorod embedded LED

523nm 486nm

509nm

0 20 40 60 80 100 120 140 160 400

450 500 550 600 650 700 750

Polar NR LED

Tip-free NR LED

Semi-polar Nano-pyramid LED

Wavelength (nm)

Current (mA)

This work (2014)

S. P. Chang et al, Opt. Exp.(2013) Y. J. Hong et al, Adv. Mater. (2011)

0 20 40 60 80 100 120 140 160 -100

0 100 200 300 400 500

Semi-polar Nano-pyramid LED

Non-polar Tip-free NR LED NR embedded LED

FWHM (nm)

Current (mA)

This work (2014)

S. P. Chang et al, Opt. Exp.(2013) Y. J. Hong et al, Adv. Mater. (2011)

180nm

114nm

57nm 160nm

60nm 61nm

Blue shift FWHM

 The smaller blue shift of EL peak of tip-free NR LED can be attributed to the

improvement of In clustering on the top of nanorods and elimination of semi-polar MQWs.

Benchmark & Summary

88

Results and Discussion

0 30 60 90 120 150 180

0.0 0.2 0.4 0.6 0.8 1.0

Tip-free DOP= 54.5%

Tip DOP= 18.8%

Normalizied Intensity (a.u.)

Polarization angle (degree)

Degree of Polarization (DOP)

Linearly polarized PL measurement

 DOP of tip-free and tip NR LEDs are 54.4% and 18.8%, respectively.

 The higher DOP was introduced by larger area of non-polar MQWs for tip-free NR LEDs.

16/26

 Tip-free NR LED  Tip NR LED

Reduced-semipolar MQWs

Nonpolar MQWs Nonpolar MQWs Semipolar MQWs

non-polar

Tip-free NR LED

Reduced-semipolar MQWs

Non-polar MQWs

89

InN nanostructure-for Solar cell or THz emission

CLEO postdeadline paper 2014

90

Next killing application :

Google glass/Apple/wearable

LED on graphene/ITO

microdisplay

LuxVue Inc. Acquired by Apply May, 2014

91

Nanoring LED/QD LED

92

Inkjet printing of QDs on micro-LED

93

Conclusions

 LEE and current spreading improvement a lot for past few year (academic and industrial effort)

 The improvment of the IQE and Droop –EBL design and material, Non-polar or semi-polar– good for LED and Laser

Green LED on c-plane sapphire still need to be improvement

 The III-nitride nanopyramid LED should be a promising solution

for full color emitter (fill green gap) and droop improvement

94

Semiconductor Laser Lab

members (NCTU/NCKU) 4 professors 2 postdoc 30 students

S.J. Chang, Wei-Chih Lai (NCKU)

T.C. Lu, S.C. Wang , G. C. Chi, C.C. Lin, C. Y. Chang (NCTU)

95

Future work 180 lm/W->250 lm/W

Wide Bandgap Group: High Power GaN LED Prof. S.C. Wang, H.C. Kuo

2008/11/10

95

96

• 東京工業大學Prof. Iga及Koyama , UC Berkley Connie Chang 合作 – Photonics crystal and VCSEL design

• 史丹佛大學 Prof. Yamamoto, Prof. Fan合作 – Microcavity polariton laser

• HKUST – prof. KM Lau , Xiamen U – Prof. B.P. Zhang

• 耶魯大學Prof. Hui Cao, Prof. Han合作– Growth of non-polar GaN

• 壬色列理工學院Prof. Shawn-Yu Lin合作– Photonics crystal design, EF Shubert –LED Droop

• 日本京都大學Prof. Noda合作– Photonics crystal surface-emitting lasers

• 英國南安普敦大學(University of Southampton)- Dr. Martin D. B. Charlton合作- Cu(In,Ga)Se₂ Solar Cells

International Collaborations

Domestic Collaborations

• 中央研究院應用科學中心程育人教授(Ph.D. Stanford)合作– Cavity quantum electro-dynamics (CQED)

• NDL 謝嘉明 博士 - High efficiency Solar cell

• 交大電子所張俊彥教授–High efficiency UV LED

• 工研院電光所– High-quality GaN substrate; 工研院綠能所 - High efficiency Solar cell

Thnaks for your support

Industrial Collaborations

180 × 50 - displaysearch

SAS TSMC

97

SCI paper and conference 2009-2013

Journal Paper x 90

• ACS Nano x 2

• Advanced Material x 2

• Advanced Energy Material x 1

• Advanced Functional Material x 1

• Nano Letters x 2

• Solar Energy Materials and Solar x 5

• Journal of The Electrochemical Society x 2

• IEEE J. Select. Topics Quatum Electron x 4

• IEEE Electron Device Lett. x 3

• Appl. Phys. Lett. x 12

• Journal of Applied Physics x 3

• Optics Express x 10

• IEEE J. Select. Topics Quatum Electron. x 4

• JOURNAL OF LIGHTWAVE TECHNOLOGY x 2

• IEEE PHOTONICS TECHNOLOGY LETTERS x 12

• NANOTECHNOLOGY x 3

• APPLIED PHYSICS EXPRESS x 4

• JAPANESE JOURNAL OF APPLIED PHYSICS x 10

Conference Paper x 50

•SPIE Proceedings x 10

• 4 invited talk

•The Conference on Lasers and Electro-Optics (CLEO) x 20

• Two CLEO post deadline

• one invited talk

• IEEE International NanoElectronics Conference x 2

•Conference on Lasers and Electro- Optics (CLEO/PR) x 10

Thanks for your attendtion

98 E-Gun

Photonics -Equipments for LEDs and VCSEL

MBE

RIE

奈米製程 磊晶 x2

IQE system

光電特性及熱效應量測

Ti: sapphire laser

J T Meas. system

LCS-100

CIE and Power Meas. Nanorobotics Manipulator