Presentation Mark Ramm 3

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November 2, 2010

STR Group

Modeling as a powerful

Approach to Design and

Optimization of advanced

Nitride-based LEDs

2Modeling Solutions for Crystal Growth and Devices

1984: Start of the simulation and modeling activities at Ioffe Institute,

St. Petersburg, Russia;

1993-1996: Group for modeling of crystal growth and deposition at

University of Erlangen-Nuernberg, Germany;

Today:

STR Group:

- HQ in St.Petersburg, Russia

- STR, Inc., Richmond VA, USA

- STR GmbH, Erlangen, Germany

More than 40 scientists and software engineers, local representatives in

China, Korea, Taiwan and Japan.

Copyright © 2010 STR Group Ltd. All rights reserved www.str-soft.com

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Software & consulting services :

- Modeling of crystal growth from the melts and solutions: CGSim

- Modeling of polysilicon deposition by Siemens process: PolySim

- Modeling of bulk crystal growth of SiC, AlN, GaN: Virtual Reactor

- Modeling of deposition and epitaxy of compound semiconductors: CVDSim

- Modeling of optoelectronic and electronic devices: SimuLED

Customer base:

• More than 100 companies and research institutes/universities worldwide

• Top LED, LD and solar cell manufacturers

• Top sapphire, GaAs, GaP, GaN, AlN and SiC substrate manufacturers

• Top MOCVD reactor manufacturers


Modeling Solutions for Crystal Growth and Devices

4

http://en.wikipedia.org/wiki/Light-emitting_diode

Color Wavelength (nm) Materials

infrared λλλλ > 760 GaAs, AlGaAs

red 610 < λλλλ < 760 AlGaAs, GaAsP, AlInGaP

orange 590 < λλλλ < 610 GaAsP, AlInGaP

yellow 570 < λλλλ < 590 AlInGaP

green 500 < λλλλ < 570 AlGaP, InGaN, CdZnO

blue 450 < λλλλ < 500 ZnSe, InGaN, CdZnO

violet 400 < λλλλ < 450 InGaN,CdZnO

ultraviolet λλλλ < 400 GaN, AlInGaN, AlGaN, ZnO, MgZnO

, InGaN

Light-emitting diodes family


5 Challenges in design and optimization of

advanced light-emitting diodes

� Coupled electrical, thermal, and optical

problems; very non-linear governing

equations

� Complex multi-scale 3D geometry of

state-of-the-art LEDs and LDs

� Non-ordinary physical properties of

novel III-nitride

Huge simulation time and demanded computer resources

make straightforward approach ineffective for practical

engineering of advanced light-emitting devices !


6 Hybrid approach and interrelations between

SimuLED modules for LED design

Epi level

Chip level

Device level

Development & optimization of LED structures

Development & optimization of

LED chips

Development & optimization of

LED lams, arrays, etc.

SimuLAMP

SiLENSe

SpeCLEDRATRO

Sim

uLED


7Factors affecting the Internal Quantum EfficiencyEpi level

104

105

106

107

108

109

1010

1011

0.0

0.2

0.4

0.6

0.8

1.0

n0= 1×10

17 cm

-3

n-GaN

∆∆∆∆n = 5×1016

cm-3

∆∆∆∆n = 5×1017

cm-3

∆∆∆∆n = 5×1018

cm-3

Lig

ht

em

iss

ion

eff

icie

ncy

Dislocation density (cm-2)

{ }

PL

in

ten

sit

y (

arb

.un

its)

dislocation

density

operation

temperature

MQW barrier doping

structure design

10-4

10-3

10-2

10-1

100

101

102

103

10-2

10-1

100

10-3

10-2

10-1

5x1017

cm-3

1x1018

cm-3

3x1018

cm-3

5x1018

cm-3

Nd= 10

8 cm

-2

Inte

rna

l e

mis

sio

n e

ffic

ien

cy

Current density (A/cm2)

Ex

tern

al e

ffic

ien

cy

experiment (b026)

with ηηηηext

= 13 %


radAdisSRRRRRR +++= IQE = Rrad/R

8 Auger recombination is responsiblefor the IQE rollover

Epi level


Two companies

reported on the

importance of

the Auger

recombination

at ICNS-2007

New LED structure

has been suggested

by Lumileds

9 New structure design suppressed Auger recombination

Epi level


SiLENSe™ simulation

results, accounting for Auger

recombination

Measurements reported by

Phillips Lumileds in: N. F.

Gardner, et al., APL 91 (2007)

243506

1 10 100 10000.0

0.1

0.2

0.3

0.4

Inte

rna

l q

ua

ntu

m e

ffic

ien

cy


Nd = 5x10

8 cm

-2

λλλλ = 430 nm

six 3 nm QWs

13 nm active

layer

1 10 100 10000.0

0.1

0.2

0.3

0.4

Inte

rna

l q

ua

ntu

m e

ffic

ien

cy


Nd = 5x10

8 cm

-2

λλλλ = 430 nm

six 3 nm QWs

13 nm active

layer

Conventional heterostructure Modifiedheterostructure

10

Basic design of 815×875 µm2 blue LED die

n-pad

n-electrodes

n-contact layer

textured surface

active

region

p-contact layer

n-contact

layer

highly reflective p-electrode

Micrograph of the die by MuAnalysis, Inc., 2008

Γ-shaped

V. Härle et al., Proc. SPIE 4996 (2003) 133 / phys. stat. solidi (a) 201 (2004) 2736


Chip level

11 Current density, and IQE distributions in the

active region

Chip level

IQE

J (A/cm2)


local IQE reduction in high-current density

area

10-3

10-2

10-1

100

101

102

103

0.0

0.1

0.2

0.3

0.4

0.5

0.6

T = 25°C

Inte

rnal q

ua

ntu

m e

ffic

ien

cy


efficiency droop at high current density caused

by Auger recombination

12Light extraction from the LED

probability of light extraction falls down under and next to n-electrode

n-electrode

Current I = 700 mA

EP (%)

Light generated under n-pad is not practically extracted

from the die because of incomplete multiple reflection

from metallic electrode


Chip level

13 Dependence of light extraction

efficiency on current

0 200 400 600 800 1000 1200

57

60

63

66

69

72

75 n-contact layer:

3 µµµµm, 5x1018

cm-3

3 µµµµm, 5x1019

cm-3

6 µµµµm, 5x1019

cm-3

To

tal L

EE

(%

)

Forward current (mA)

strong dependence of LEE on forward

current

variation of n-contact layer parameters

affects weakly the current crowding and,

hence, the LEE at ~700-800 mA

alternative approaches are required

Approach 1: insertion of an insulating layer under the n-pad to avoid parasitic current

flow in this region

Approach 2: use of narrower Г-shaped electrodes with reduced spacing between them


Chip level

14 Current spreading in LED dice of

modified designs

J (A/cm2)J (A/cm2)

Total current through the diode I = 700 mA

parasitic current flow under the n-pad is partly suppressed

reduction of the current density contrast in the active region

both approaches are found to work well


Chip level

15 Assessment of performance improvement due to

variation of LED die design

0 200 400 600 800 1000 120055

60

65

70

75

80

85

conventional

with blocking layer under pad

refined electrode

BL & refined electrode

To

tal L

EE

(%

)

Forward current (mA)

0 200 400 600 800 10000

100

200

300

400

500

600

700

conventional

BL under pad

refined electrode

BL + refined

electrodeTo

p o

utp

ut

po

wer

(mW

)Forward current (mA)

3.1 3.2 3.3 3.4 3.5 3.60

200

400

600

800

1000 conventional

BL under pad

refined electrode

BL + refined

electrode

Cu

rren

t (m

A)

Forward voltage (V)

0 200 400 600 800 1000

20

25

30

35

40 conventional

BL under pad

refined electrode

BL + refined

electrode

WP

E (

%)

Current (mA)

Performance improvements at the current of 700 mA:

• LEE �� from 60 to 70%

• Vf remains the same• optical power �� from

530 to 635 mW (by~20%)

• WPE �� from 23 to 28%(by ~22%)


Chip level

16 General design of LED lamp and

simulation approach

Device level


Problems to be solved with modeling

� Thermal design of LED lamps and arrays with account of heat release in both LED chip(s) and encapsulants (via light absorption)

� Optimization of light conversion and analysis of color performance of white LED lamps and arrays

� Design and optimization of multi-chip packages, including RGB lamps and multi-pixel arrays

� Optimization of diode interconnection and circuits in the multi-chip lamps with account of thermal coupling of individual devices

� Analysis of lamp operation under DC, AC and pulse power supply, including transient effects

SimuLAMP is the Software for Optical and

Thermal Design and Optimization of LED

Lamps and Arrayshttp://www.str-soft.com/products/SimuLED/SimuLAMP/

17 General design of LED lamp and

simulation approach

Device level


Characteristics of blue chip are considered as temperature

dependent

Phosphor characteristics are considered as temperature dependent

1.5 2.0 2.5 3.0 3.5 4.00

200

400

600

800

1000 250 K

298 K

340 K

380 K

420 K

460 K

Cu

rre

nt

(mA

)

Forward bias (V)0 200 400 600 800 1000

0

200

400

600

800

1000

1200

1400 simulation

by SpeCLED

250 K

298 K

340 K

380 K

420 K

460 K

Ou

tpu

t p

ow

er

(mW

)

Current (mA)

420 440 460 480 500 5200.0

0.2

0.4

0.6

0.8

1.0

1.2

250 K

460 K

No

rmalized

em

issio

n in

ten

sit

y

Wavelength (nm)

200 300 400 500 600

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Qu

an

tum

eff

icie

nc

y

Temperature (oC)

T1 = 520 K

T2 = 70 K

YAG:Ce3+

approximation

0 100 200 300 400 500

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

Wavele

ng

th s

hif

t (n

m)

Temperature (K)

shift of emission spectrum with temperature

Micrograph of the K2 Luxeon lamp by

MuAnalysis, Inc., 2008

heatsink

domeinternal

lens

LED die

phosphor

Scheme of LED lamp, used in modeling

18 Color characteristics under dimming by current

variation and under ambient temperature variation

300 K

450 K

color distribution in

the far-field zone

250 300 350 400 450

60

62

64

66

68

70

72

74

I = 700 mA

Co

lor

ren

deri

ng

in

dex

Ambient temperature (K)

250 300 350 400 4502000

4000

6000

8000

10000

12000

CC

T (

K)

Ambient temperature (K)

I = 700 mA


Device level

0 200 400 600 800 10002000

4000

6000

8000

10000

12000

Ta = 300 K

CC

T (

K)

Current (mA)

0 200 400 600 800 1000

60

62

64

66

68

70

72

74

Co

lor

ren

deri

ng

in

dex

Current (mA)

Ta = 300 K

19 Why CRI behavior is so different under current

or temperature variation?

Temperature variation shifts the chromatic coordinates along theblack-body radiation locus, thus increasing CCT

Сharacter of movement of chromatic coordinates under changing the LED operation conditions explains its effect on white light characteristics

Device level

Сurrent dimming moves the chromatic coordinates away from the black-body radiation locusCopyright © 2010 STR Group Ltd. All rights reserved www.str-soft.com

20What we need device modeling for ?

� Better understanding of complex coupled electrical, optical, and thermal processes occurring in advanced LEDs with account of specific features of wide-bandgap materials

� Saving man-power, time, and money for optimization of available and development of new light-emitting devices; partial substitution of trial-and-error experimental approach at the R&D stage; support of experimental activity

� Testing of new technical solutions that require considerable modification of available fabrication technology; analysis of prospective/promising trends in device fabrication

� Education and training. Detailed info on s/w capabilities including Code description, GUI manual, Technical papers published by STR research team are available upon request

� Generation of intellectual properties; support for patent applications


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