1

STR Group

www.str-soft.com

Engineering software package for LEDdesign and optimization

SimuLED ™

2

Prehistory of STR:

1984: Start of the MOCVD modeling activities at Iof fe Institute, St. Petersburg, Russia

1993-1996: Group for modeling of crystal growth and epi taxy at University of Erlangen-Nuernberg, Germany

History of software development

2000: Launch of development of the first specialized software2003: First release of commercial software package 2004: First release of the software for device engin eering

STR Today:

More than 40 scientists and software engineers

Modeling Solutions for Crystal Growth and Devices

3

STR Today

1 – Headquarter and R&D (consulting service and soft ware development)2 – Local offices3 – Local distributors

STR US, Inc.2, Richmond, VA

STR, GmbH2, Erlangen, Germany

SimSciD Corporation 3, Yokohama, Japan

INFOTECH Co.3, Kyunggi-Do, SouthKorea

Paul Materials Co., Ltd. 3, Seoul, South Korea

STR Group, Ltd. 1, St.Petersburg, Russia

Pitotech, Ltd. 3,Chang Hua City, Taiwan

Bulk crystal growth modeling (Si, Ge, SiGe, GaAs, I nP, SiC, AlN, Al 2O3, Optical Crystals)

Epitaxy and deposition modeling (Si, SiGe, SiC, AlGa As, AlGaInP, AlGaInN, high-k oxides)

Modeling of device operation ( LEDs, Laser Diodes, FETs/HEMTs Shottky diodes)

4

Software & consulting services :

- Modeling of crystal growth from the melts and solutio ns: CGSim- Modeling of polysilicon deposition by Siemens process : PolySim- Modeling of bulk crystal growth of SiC, AlN, GaN: ViR- Modeling of epitaxy of compound semiconductors: CVDSim- Modeling of optoelectronic and electronic devices : SimuLED

Customer base:

• More than 100 companies and universities worldwide• Top LED, LD and solar cell manufacturers • Top sapphire, GaAs, GaP, GaN, AlN and SiC wafer manufacturers• Top MOCVD reactor manufacturers

STR activity in software developmentand consulting service

5

SimuLED™

Presentation Layout1. Simulation as a tool for LED design and optimizati on. Why

SimuLED™ ?

2. Basic tools of SimuLED™ package and what questions can be answered using modeling

3. Operation of modern light emitters

A. Current crowding effect on light extraction efficien cy of Thin-Film LEDs

B. Role of ITO-spreading layer on performance of blue LE Ds

C. Modeling of multi-pixel LED array operation

4. SimuLED™ extension: SimuLAMP™ software for Optical and Thermal Management of LED Lamps

5. Conclusion

Main application areas of III-nitride LEDs and LDs

After the first demonstration of high-brightness blue light-emitting diodes (LEDs) in the early 90s. III-nitride semiconductors have soon become the basic materials for visible and ultraviolet optoelectronics,

Green550-520 nm

Blue470-420 nm

Violet405 nm

UV365-340 nm

UV280-250 nm

� Solid-state lighting by RGB mixing

� Signs

� Trafficlights

� Entertainment

� Solid-state lighting with phosphors

� Signs

� Traffic lights

� Displayback-lighting

� Automotive

� DVD writing and playing

� Optical datastorage

� Curing anddrying of inks,coatings, and adhesives

� Medicine

� False banknotedetection

� HR opticallithography

� Water&airdisinfection

� Food steri-lization

� Medicine

� Chemicalcatalysis

� New generationof DVD systems(~300 nm)

� ProjectionTV

7

What is the subject of new LED development?

Heterostructure- IQE, Injection efficiency, reduction of the efficiency droop

Chip- LEE, EQE and WPE, Efficacy- Turn-on Voltage- Output power, series resistance- Active region temperature

Lamp, white LEDs- Temperature distribution, thermal

resistances- Output light spectrum, light

intensity, color homogeneity- Efficiency: efficacy, WPE- Rendering: CRI and CCT

C.Wetzel and T.Detchprohm, 2005

8

Why LED designing is a challenging problem?

Multidisciplinary- materials science- physics of semiconductors- heat transfer theory- optics

Essentially nonlinear- nonuniform voltage drop and IQE distributions overthe active region

- self-heating in active region- current crowding results in nonuniform light

intensity distribution over the active region

Multidimensional and multiscale- QW thickness is ~2-10nm- chip size is ~300µm

9

How to approach?

Experiment- labour-intensive- time consuming- expensive- some details of particular processes/mechanisms

could not be measured directly or visualized

Modeling/Simulations- presently, modeling is recognized as a powerful

tool for device engineering- fast reply could be obtained in

respond to “what if” questions- relatively inexpensive

+

+ Softwarefor LEDsimulation

10

Why SimuLED ?

• Specific experience in modeling is often necessary to run the software and to extract useful results from computations

• 3D geometry specification is often labour-intensive

• Nonlinear problems tend to have poor convergence

• Huge simulation time and computer resources demanded. Special requests to hardware used for computations

• Non-intuitive user interface might take long time to understand and get started.

• Materials properties for novel materials will still have to be collected

Will any software help?

SimuLED™ is tailored for specific applicationand operates in engineer friendly terms, therefore, bypassing the common issues listed above

Gene

ral pu

rpos

e s

oftw

are

11

Maximum efficiency as the key advantage

SimuLED is optimized for maximum efficiency in terms of user effort

and computer resources. High performance is achieved in several

ways: software employs unique models of the physical

processes, it uses problem specific methods and algorithms to

find the solution, which allows SimuLED to operate v ery fast, as

compared to general purpose software.

For the user convenience, task set-up can be done in terms

customarily used in device engineering. A set of examples provides

selection of ready-to-run tasks that can be used as a starting point

in some cases. Material properties are thoroughly selected and

stored in our database, ready to be used in computations.

!!!

12

SimuLED

Simulationdestination

Software is used as a tool demonstrating physical effects and test cases with simple geometry

Software is used by experts in modeling and users who have long-time experience in device modeling

Powerful fast engineering tool operating with actual devices designed by industry and developed by academia

SimuLED serves as a guidance for epitaxy engineers and LED designers in testing new ideas on device performance improvement

Getting started Long time is necessary to start computations

Statement of the problem is complicated due to difficulties in geometry specification and specification of boundary conditions

The user can start computations in several hours after SimuLED installation

Intuitive User Interface operates in terms normally used by engineers.Layer by layer input of 2D layout for 3D geometry. Selection of predefined options typical for LEDs

General concept to LED simulation

Homogeneous app-roach for simulation of multiphysics and multiscale problem

Problems with uniform resolution of physical processes occurring at various spatial scales.

Hybrid approach accounting for specific features of modern LED design

Accurate resolution of key physical processes at each spatial scales

Computation time

Time consuming simulations Extremely fast simulations

Hardware requirements

Special requirements to hardware SimuLED operates on personal computers

Physical models implemented into the software

The software was developed initially for simulation of GaAsand Si-based devices

Conventional physical models used for a long time in modeling of semiconductor devices

SimuLED was initially developed as a tool for simulation of nitride-based LEDs

Both conventional and unique models of physical effects responsible for operation of modern LEDs

Materials properties

The data have to be collected by the user or there is a lack of data needed for computations

SimuLED is supplied with the database of materials properties and the user can start his computations immediately after the software installation

Hot-line support Quick hot-line support, free software update within the license period

Interpretation of results upon customer request

General purpose software SimuLED™

13

SimuLED

Problem of LED

simulation

Hybrid approach

We reject the idea of “homogeneous approach for simulation of multiphysics and multiscale problem ”applied for the whole LED at all space scales

In SimuLED, we replace the homogeneous model by a set of coupled submodels

For each submodel:• Dominant physics• Appropriate dimensionality• Appropriate computational domain

�

SimuLED submodels are realized as modules:

• Carrier transport through the active region: 1D + bipolar carrier transport � jz(Ub,T), ηint(Ub,T), and λ

• Current spreading in LED die: 3D + unipolar carrier transport + active region replaced by nonlinear resistance � W(x,y)

• Light propagation in the LED die: 3D � EQE, Pout, WPE

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14

Basic tools of SimuLED ™software package

SimuLED™

15

Modules for LED modeling

15

Epi level

Chip level

Device level

Development & optimization of LED structures

Development & optimization of

LED chips

Development & optimization of

LED lamps, arrays, etc.

SimuLAMP

SiLENSe

SpeCLEDRATRO

Data exchange between modules is minimized. Modules can work in standalone mode

16

SiLENSe™: module for designing of LED/LD heterostructure

Parameters computed with SiLENSe™

� Band diagrams

� Carrier concentrations

� Electric field

� Rrad, Rinonrad , IQE

� Carrier fluxes

� Energy levels in QWs

� Emission and gain spectra

SiLENSe is supplied with the editable database of materialsproperties

Epi level

SiLENSe™: module for designing of LED/LD heterostructure

effect of composition fluctuation on the rate of dislocation-mediated recombination is accounted for

radAdisSR RRRRR +++=

Shockley-Read non-radiative recombination via

point defects

Dislocation-mediated non-radiative

recombination (original model)

Auger non-radiative recombination

Radiative recombination of

electrons and holes

light emission

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Use of quantum potential model: transport equations are modified in such a way as toreplace the spatial variation of the band edge profiles, EC-qφ and EV-qφ by thequantum potentials accounting for the wave nature (spatial delocalization) of carriers.

Here fn,p is fraction of the delocalized carriers taking part in the non-radiativerecombination at the dislocation cores

Epi level

18

SiLENSe is used to find out heterostucture design providing

IQE maximum

operation temperature

dislocation density

MQW barrier doping

heterostructuredesign

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= 108 cm-2

In

tern

al e

mis

sion

effi

cien

cy

Current density (A/cm 2)

Ext

erna

l effi

cien

cy

experiment (b026)with ηηηη

ext= 13 %

Inte

rnal

qua

ntum

effi

cien

cy

104 105 106 107 108 109 1010 10110.0

0.2

0.4

0.6

0.8

1.0

n0= 1×1017 cm -3n-GaN

∆∆∆∆n = 5×1016 cm-3

∆∆∆∆n = 5×1017 cm-3

∆∆∆∆n = 5×1018 cm-3

Ligh

t em

issi

on e

ffici

ency

Dislocation density (cm -2)

{ }

PL

inte

nsity

(ar

b.un

its)

Inte

rnal

qua

ntum

effi

cien

cy

SiLENSe™ Application Example: Blue MQW LED heterostructure

Epi level

19

Auger recombination is responsible for the IQE roll over

Two companies reported on the importance of

the Auger recombination at ICNS-2007

New LED structure has been suggested

by Lumileds

Epi level

20

New structure design suppressed Auger recombination

N. F. Gardner et al., Appl. Phys. Lett. 91

(2007) 243506

LED with a wide active region may provide an equal or even higher efficiency then LED with a QW

2 QWs

6 QWs

13 nm active layer

λλλλ = 430 nm

1 10 100 10000.0

0.1

0.2

0.3

0.4

Inte

rnal

qua

ntum

effi

cien

cy

Current density (A/cm 2)

Nd = 5x108 cm -2

λλλλ = 430 nm six 3 nm QWs 13 nm active

layer

no effect of the number of QWs in

the structure on the LED efficiency !

Epi level

Optimization of MQW active region in a

dual-wavelength LED structure

control ofemission spectra

by MQW barrier doping

doping of MQW active region has bee optimized to improve the hole injection in all the wells

Epi level

Suppression of efficiency droop in blue

MQW LED structures

experimental data

an LED structure free of AlGaN EBL has been suggested on the basis of simulations, providing a smaller efficiency droop

simulations

Epi level

LEDs with trapezoidal wells

MQW LED with trapezoidal wells was suggested allowing improvement of efficiency droop at high current densities.

Schematic band diagrams of MQWs of (a) LED with

rectangular-shaped well and (b) LED with trapezoidal-shape well

Simulations show that separation of electron and hole wave functions in the wells was reduced in LED B. Overlaps of electron and heavy

wave functions in the well adjacent to p-GaN in LED A and LED B are 37.2 and 41.6, respectively.

Epi level

InGaN Green LEDs with Gradual QWs

A conventional InGaN QW’s structure was replaced by a gradual InGaN QW’s structure.

The transport efficiency of injected holes was increased, because band bending in the valence band was alleviated, increasing the overlap of electron and hole wave functions as well as the rate of radiative recombination.

Most importantly, the leakage of injected electrons to the p-type region is correspondingly decreased, suppressing the efficiency droop of the LEDs.

Institute of Electro-Optical Science and Technology, National Taiwan Normal University, Taiwan

Epi level

12345

390 405 420 435 450 465012345

EV

Car

rier

conc

entr

atio

n (x

1019

cm

-3)

EC

electrons

a

holes

Distance (nm)

12345

392 400 408 416 424012345

b

electrons

EV

Car

rier

conc

entr

atio

n (x

1019

cm

-3)

EC

holes

Distance (nm)

Use of short-period superlattice for reduction of th e

efficiency droop

A partnership between Ioffe Physico-Technical Institute, Epi-Center and STR-Group has modelled electron and hole distributions in two types of LED: (a) a device with a conventional active region, containing five, 3 nm thick quantum wells sandwiched between 10 nm-thick barriers (b) a device with a short-period superlattice active region comprising 2.5 nm-thick wells and barriers.

Far more uniform hole distribution is possible with a superlattice active region comprising 2.5 nm-thick wells and barriers. Such a structure is far better at combating droop

Presented at ICNS-9. Glasgow, 2011

http://www.compoundsemiconductor.net/csc/features-details.php?cat=news&id=19734032&name=Droop%20draws%20the%20crowds%20at%20ICNS-9

Epi level

Quality Factor (Q) and IQE

as a function of current density

IQE as a function of current density obtained from the data reported for (a) green SQW structure [1], (b) blue LED structures with MQW and SPSL active regions [2], and super-high efficiency blue LEDsoperating in CW (circles) and pulsed (squares) modes [3].

0 50 100 150 2000.0

0.2

0.4

0.6

0.8

1.0

λλλλ = 523 nm Q = 0.78

Inte

rnal

qua

ntum

effi

cien

cy

Current density (A/cm 2)0 20 40 60 80 100

0.0

0.2

0.4

0.6

0.8

1.0

λλλλ = 457 nm Q = 2.26

λλλλ = 449 nm Q = 3.15

MQW-structure SPSL-structure

LEE = 60%

Inte

rnal

qua

ntum

effi

cien

cy

Current density (A/cm 2)0 20 40 60 80 100

0.0

0.2

0.4

0.6

0.8

1.0

Inte

rnal

qua

ntum

effi

cien

cy

Current density (A/cm 2)

LEE = 90-95%λλλλ = 444 nm Q = 18

Analysis shows that one can introduce a quality factor Q increasing with the rate of radiative recombination and decreasing with Auger and Shockley-Read recombination. Q is independent of the current density and can be considered as a figure of merit for LED structures of various designs and emitting light in different spectral ranges.

STR developed express method based on EL measurements and predicting self consistently IQE as a function of current density, light extraction efficiency, and quality factor

Epi level

27

SimuLED hybrid approach: SiLENSe/SpeCLED coupling

2.6 2.8 3.0 3.2 3.4 3.6 3.80

1000

2000

3000

4000

5000

Cur

rent

den

sity

(A

/cm

2 )

Active layer bias (V)

300 K 400 K 500 K

2.6 2.8 3.0 3.2 3.4 3.6 3.80.0

0.1

0.2

0.3

0.4

0.5

Inte

rnal

qua

ntun

effi

cien

cy

Active layer bias (V)

300 K 400 K 500 K

2.6 2.8 3.0 3.2 3.4 3.6 3.80

1000

2000

3000

4000

5000

Cur

rent

den

sity

(A

/cm

2 )

Active layer bias (V)

300 K 400 K 500 K

2.6 2.8 3.0 3.2 3.4 3.6 3.80.0

0.1

0.2

0.3

0.4

0.5

Inte

rnal

qua

ntun

effi

cien

cy

Active layer bias (V)

300 K 400 K 500 K

),,(),,(

),,(2

zyx

zyxjzyxQ

σ=

Replacement of the active region by a non-linear resistance

Epi level - Chip level

28

SimuLED hybrid approach: SpeCLED/RATRO coupling

n-pad

n-electrode

n-contact layer textured surface

active region

p-contact layer

n-contact layer

highly reflective p-

electrode

jz, A/cm2

Non-uniform distribution of light emission from the active region

RATRO

SpeCLED computations

),(),(

int yxq

yxjW z ηω ⋅⋅= h

intη

Chip level

29

SpeCLED™: Current Spreading and Heat Transfer in LED dice

Parameters computed with SpeCLED™

� 3D distributions of the electric

potential, current density, and

temperature in the whole die

� 2D distributions of the p-n junction bias,

current density, IQE, and temperature

in the active region plane

� I-V characteristic

� Series resistance

� EQE and WPE

Chip level

30

RATRO™: 3D ray-tracing analysis of optical characteristics of LED dice

Parameters computed with RATRO™

� Light extraction efficiency

from an LED die

� Far- and near-field emission

patterns

� Light polarization distribution

� Consideration of non-uniform

electroluminescence intensity

distribution in the active region plane

� Various die configurations, including shaped

substrate are supported

newnew

Chip level

31

SpeCLED™: easy geometry specification

So one can focus on specification of 2D geometry !!!

+ + =

However, actually due to specific feature of planar technology used for LED fabrication, the 3D geometry implies a stack of layers including:

• Active region• n- and p-contact layers• Electrodes• Intermediate layers• Pads• Pad-contacts

Each layer is presented as a complicated 2D figure plus layer thickness

State-of-the-art III-nitride LEDs normally utilize c hips having 3D geometries and a rather complex electrode configurations

Chip level

32

Planar chip design typical for LEDs fabricated on sa pphire substrate

A local overheating depends on the current density and die confi-guration

420

390

320 K

340350

370n-electrode pad

n-electrode pad

10100

500

1000

2000 A/cm 2

sapphire substrate

n-padp-pad

semitransparent p-electrode

n-contact layer

etched mesa

heat sink

Current crowding and in-plane temperature non-uniformity

Conventional LEDA = 0.0466 mm 2 ; substrate-down mounting

Current crowding occurring at the electrode edge (I = 80 mA) produces a very non-uniform in-plane EL intensity distribution

Chip level

33

I-V characteristic and output power of blue LED

Account of thermal effects is necessaryto provide adequate predictions

2.5 3.0 3.5 4.0 4.5 5.00

20

40

60

80

100

120

experiment with thermal

effects without

thermal effects

C

urre

nt (

mA

)

Forward voltage (V)

Data: S.S. Mamakin et al., Semiconductors 37 (2003) 1107

Chip level

34

RATRO™ : Advanced physical models implemented into the too l

Original model was developed to describe light interaction with surfaces patterned with regular array of hexagonal or rectangular pyramids or holes

Model of light transmission through semitransparent multilayer metallic electrodes

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Specification of pattern parameter

Assigning parameters of

individual layers …

… the user obtains angle-dependent reflection, transmission, and absorption coefficients of DBR surface

Chip level

35

RATRO™: Near-field and Far-field predicted for planar LED

Lumileds low-power chip

Radiation far-field pattern of planar LED die

Predicted Extraction Efficiency through top surface is 8.8% andcorrelates well with fitting results obtained for Lumileds low-power chip

Output light intensity

n-padp-pad

heat sink

sapphire substrate

semitransparent p-electrode

LED struc-ture

030

60

90

120

150180

210

240

270

300

330

Waveguidingleading to light

extraction through the side walls of

sapphire substrate

Bottom emission occurs largely

through the side walls of sapphire

substrate, providing two-peak angular

dependence

Chip level

Optimization of electrode configuration to suppress

current crowding

improvement of LED performance due to appropriate chip design

Chip level

Optimization of electrode configuration to suppress

current crowding

simulations allowed authors to design electrode configuration for a uniform current injection in a large GaInN LED chip and demonstrated a increase in output

power in an optimized device.

Chip level

Sharp Laboratories of Europe Limited

Electroluminesence from blue LED chip

LED chip design using SpeCLED

http://www.sle.sharp.co.uk/research/advanced_optoelectronics/blue_leds.php

Modeling software such as SpeCLED is used to optimize the LED chip design in order to improve operating voltage, light extraction efficiency and junction temperature. Other tools such as SiLENSe are also available to model band diagrams and to understand fundamental theoretical work, such as the piezoelectric effect in nitride-based devices.

Chip level

Enhancement of light extraction in UV LEDs

A nanopixel LED design with an Al reflector was developed resulting in enhanced light extraction in UV LEDs.

Schematic cross-sectional view of nitride-based nanopixel UV LED with

Pd contacts and Al reflector layer

Simulation of the current injection in the active region for nanopixel AlInGaN LEDs with nanopixel size 1x1 µm2 and nanopixel spacing (a) 4 µm, (b) 2 µm, and (c) 1 µm. The total current is constant. In the graph the injection current density as a function of the position along a line through the center of the nanopixels is shown for the different structures.

Chip level

40

3. Operation of modern light emitters

Application of the full SimuLED™ package

41

A. Analysis of Thin -Film LED operation

42

Basic design of 815×875 µm2blue LED die

n-pad

n-electrode

n-contact layer textured surface

active region

p-contact layer

n-contact layer

highly reflective p-electrode Micrograph of the die by

MuAnalysis

43

Current crowding near/under n-electrode at 700 mA

J, A/cm 2

IQE

10-3 10-2 10-1 100 101 102 1030.0

0.1

0.2

0.3

0.4

0.5

0.6

T = 25°C

Inte

rnal

qua

ntum

effi

cien

cyCurrent density (A/cm 2)

current density distribution in the active region

IQE distribution in the active region

efficiency droop at high current density caused

by Auger recombination

local IQE reduction in high-current density

area

IQE distribution in the active region is essentially non-uniform

44

Emission intensity and probability of light extraction in the active region

I (W/cm 2)

EP (%)

maximum of emission intensity is located under/next to n-electrode, despite the IQE droop with current; light emission under the n-pad does

not contribute at all to the extracted light !!

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

n-electrode

Current I = 700 mA

NB!

45

Characteristics of basic Thin-Film LED die

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

Tot

al L

EE

(%

)

Forward current (mA)3.1 3.2 3.3 3.4 3.5 3.60

200

400

600

800

1000 n-contact layer: 3 µµµµm, 5x1018 cm -3

3 µµµµm, 5x1019 cm -3

6 µµµµm, 5x1019 cm -3

Cur

rent

(m

A)

Forward voltage (V)

0 200 400 600 800 100010

15

20

25

30

35

40

n-contact layer: 3 µµµµm, 5x10 18 cm -3

3 µµµµm, 5x10 19 cm -3

6 µµµµm, 5x10 19 cm -3

WP

E (

%)

Current (mA)0 200 400 600 800 1000

0

100

200

300

400

500

600

700

n-contact layer: 3 µµµµm, 5x10 18 cm -3

3 µµµµm, 5x10 19 cm -3

6 µµµµm, 5x10 19 cm -3Top

out

put p

ower

(m

W)

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 mA

alternative approaches are

required

NB!

46

Two approaches for improvement of Thin-Film LED performance

Approach 1: insertion of a current blocking layer (thin

insulating film) under the n-pad to avoid parasitic current

flowing in this region

Approach 2: use of refined electrode structure with larger number of Г-shaped electrode

branches but with smaller width of each of them

newnew newnew

47

Current spreading in LED dice of improved design

J (A/cm 2)J (A/cm 2)

Total current through the diode I = 700 mA

parasitic current flow under the n-pad is partly suppressed

reduction of the current density

contrast on the die periphery

both approaches are found to work well

Approach 1 Approach 2

48

IQE distributions in the active regions of improved LEDs

IQEIQE

a larger area of the active region emits light with higher

efficiency

increased current density at the die periphery results in a slightly lower

IQE compared to basic design

Total current through the diode I = 700 mA

Approach 1 Approach 2

49

Local probability of light extraction from improved LED dice

EP (%)EP (%)

probability of light extraction is comparable with that of

the basic die design

considerable enlarging of the area with high probability of light extraction

Total current through the diode I = 700 mA

Approach 1 Approach 2

50

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

Tot

al 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

electrodeTop

out

put p

ower

(m

W)

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

Cur

rent

(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%

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51

B. Use of ITO spreading layer for improvement of LED

performance

52

General 300×300 µm2 LED die design utilizing ITO spreading layer

n-pad

p-electrode/pad

n-contact layer

sapphire substrate

ITO spreading

layer

53

Factors affecting the overall LED efficiency

IQE of the MQW LED structure depends on both

current density and temperature

Light extraction efficiency (LEE) computations accounting for

interference in the ITO layer and depends on the ITO layer thickness

10-2 10-1 100 101 102 1030.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

300 K 350 K 400 K 450 K

Inte

rnal

em

issi

on e

ffici

ency

Current density (A/cm 2)0.01 0.1 1 10

12

15

18

21

24

27

Ligh

t ext

ract

ion

effic

ienc

y (%

)

ITO layer thickness ( µµµµm)

CompetitiveCompetitiveAdvantageAdvantage NB!

54

Current crowding in LED dice with various ITO thickness at 20 mA

20 nm 50 nm 100 nm

increasing ITO layer thickness (electrical sheet conductivity) results in

redistribution of the current density but does not avoid the current crowding;

moreover, the higher the sheet conductivity, the stronger becomes current

crowding near the n-electrode edge !!

Current density distribution across the LED active region , A/cm 2

NB!

55

IQE distribution in the LED die at 20 mA

because of high current density, the

region of maximum current localization

has the lowest IQE, which is caused by

non-radiative Auger recombination

20 nm

50 nm

100 nm

region of maximum current

localization

IQE

IQE

IQE

56

Near-field emission intensity distribution at 20 mA

Total LEE is ~1.5-2.0 time higher than in the LED with metallic

p-electrode

n-pad p-pad

ITO layer

sapph

ire su

bstra

teheterostructure

top view bottom view

heter

ostru

cture

back side of sapphire substrate

strong side-wall emission due to

waveguide effect

Top emission – 8.4%Bottom emission – 14.7%Total LEE – 23.1%

ITO thickness 100 nm

NB!

57

Characteristics of basic Thin-Film LED die

Conclusion: optimization of output power and operating voltage require quite different thicknesses of the ITO layer 1 2 3 4 5 6 7 8

0

20

40

60

80

100ITO thickness: 20 nm 50 nm 100 nm 200 nm 500 nm

Cur

rent

(m

A)

Forward voltage (V)0 20 40 60 80 100

0

5

10

15

20

25

ITO thickness: 20 nm 50 nm 100 nm 200 nm 500 nm

Out

put p

ower

(m

W)

Current (mA)

0 20 40 60 80 1000

2

4

6

8

10

12

14

ITO thickness: 20 nm 50 nm 100 nm 200 nm 500 nm

Wal

l plu

g ef

ficie

ncy

(%)

Current (mA)

maximum WPE is attained at the

ITO thickness of ~100-200 nm (at

the electric conductivity of 2000 S/cm)

0 20 40 60 80 100300

320

340

360

380

400ITO thickness: 20 nm 50 nm 100 nm 200 nm 500 nm

Max

imum

tem

part

ure

(K)

Current (mA)

NB!

58

C. Multi-pixel LED array operation. Interdigitatedmulti-pixel array (IMPA)

Design of LED array was suggested by A. Chakraborty

et al. (UCSB), Appl.Phys.Lett 88 (2006) 181120

59

Modeling of multi-pixel LED array operation

Interdigitated multi-pixel array (IMPA) containing a hundred of

30×30 µm2 pixels

semitransparent p-electrode p-pad

n-pad

300×300 µm2 square LED

Comparison of LEDs with IMPA and conventional square chip designs

mesa

60

Series resistance ( Ω)

Theory Experiment

Square 7.2-9.5 8

IMPA 1.2 1

Current Current ––voltage characteristicsvoltage characteristics

Series resistance ( Ω)

Theory Experiment

Square 7.2-9.5 8

IMPA 1.2 1

Series resistance ( Ω)

Theory Experiment

Square 7.2-9.5 8

IMPA 1.2 1

Current Current ––voltage characteristicsvoltage characteristics

Current crowding, active region overheating, and se ries resistance

Much lower active region overheating of IMPA LED an d extremely high temperature uniformity is predicted for the current density (~1%) and temperature (~6%)

Excellent agreement between the predicted and

measured series resistance

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5300

400

500

600

700

800

900

1000

square

IMPA

M

axim

um te

mpe

ratu

re (

K)

Forward current (A)

IMPA LED has a much better heat sink, primarily due

to a larger die area

Series resistance is primarily controlled by the electrode perimeter which is much larger for IMPA LED

61

Light extraction from IMPA LED die

top view bottom view

Weak variation of emission intensity

over the back sapphire substrate

IF = 1000 mA

experimentVery uniform distribution of the

emission power among the pixels

62

Output optical power as a function of forward curre nt

0.0 0.2 0.4 0.6 0.80

5

10

15

20

25

30

35

40

square

IMPA

, experiment , theory

Out

put p

ower

(m

W)

Forward current (A)

ηηηη back

= 6%

0 1 2 3 40

20

40

60

80

100

120

300

400

500

600

700 experiment theory

IMPA

ηηηη back

= 6%

Out

put p

ower

(m

W)

Forward current (A)

activelayer

Tem

pera

ture

(K

)

Overheating of square die does not allow power increase higher than 25 mW, whereas the dependence of power on current for IMPA die is close to linear up to current about 0.8 mA

Due to suppressed current crowding and reduced overheating, the IMPA LED is capable of high current operation without significant droop of the EQE at the currents from 1 to 3 A

CompetitiveCompetitiveAdvantageAdvantage

63

Software for Optical and Thermal Management of LED Lamps

SimuLAMP ™

Device level

64

Main options and effects considered

� Heat transfer in complicated lamp geometry

� Heat release in the LED chip

� Heat release in phosphor due to light absorption

� Light conversion in individual phosphor and phosphor mixtures

� Detailed I-V characteristics and WPE of particular LED chip is

imported from SimuLED

Output

� Temperature distribution, thermal resistance

� Near-field and far-field intensity distribution

� Output light spectrum, color uniformity

� Efficacy, WPE

� CRI, CCT and other characteristics of white color

3D Ray-Tracing Coupled with the Heat Transfer in LE D Lamp

CompetitiveCompetitiveAdvantageAdvantage

CompetitiveCompetitiveAdvantageAdvantage

Device level

65

Input/Output data in SimuLAMP software

SimuLED

SimuLAMPsoftware

Materials database:optical and thermal properties

Phosphor properties- phosphor particle size- particle volume fraction- quantum efficiency as a

function of temperature- phosphor excitation

spectrum- phosphor refraction index

Output

CompetitiveCompetitiveAdvantageAdvantage

U(I,T), WPE(I,T), LED spectrum and radiationpattern

Device level

66

1 - Lens2 - Silicone3 - Crystal4 - Heat Sink5 - Frame6 - Foil7 - Textolite plate8 - Radiator

Typical design of LED lamp specified by SimuLAMP

1

2

3

45

6

6

7

8

Device level

67

Geometry specification and grid generation

Geometry specification

Automatic grid generation

� Structured, unstructured,

and combined grids are

supported

� Mismatched grids are

available

Device level

68

Temperature distribution in the lamp and near-field distribution

Temperature, K

Prediction of T p-n

SimuLAMP allows analysis the effect of - LED design- Materials properties- Heat sink design can be

tested via “what if” scenario

Near-field intensity distribution predicted in various lamp designs

Device level

69

Radiation pattern and Far-field Intensity distribut ion

Radiation pattern predicted in computations

x

x

LED and phosphor spectra

LED and phosphor spectra

Output spectrum

Output spectrum

Probe point

Probe point

Probe tool for analysis of color uniformity

CompetitiveCompetitiveAdvantageAdvantage

Device level

Multi-chip packages and AC LEDs

position of red, blue, and green LEDs in a multi-chip LED (Arima Optoelectronics)

interconnections in an AC LED lamp (Epistar)

simulated current through the lamp

simulated optical power

Device level

DC operation of LED array integrated in a package

Operation of a lamp containing 100 LEDs in the

package

temperature distribution

light intensity distribution

Device level

Conclusion

73

Design of novel light-emitting devices and IP gener ation

Patent of Russian Federation No

2304952 issued by October 27, 2007

Patent of Russian Federation No

2309501 issued by October 27, 2007

Patent of Russian Federation No 2277736 issued by June 10, 2006

Patent of Russian Federation No 2262156 issued by October 10, 2005

Patent of Russian Federation No 2262155 issued by October 10, 2005 for UV LEDs

for UV LEDs

for blue LEDs

for 808 nm high-power laser diode

for 808 nm high-power laser diode

IP generation based on results of modeling by SimuLE D package

Our customers

STR currently provides software and consulting services t o over 40 companies and AcademicInstitutions in USA, Europe, and Asia.

• Technische Universitat Berlin, Institut fur Festkorperp hysik, Germany• Tokyo Institute of Technology, Japan• School of Electrical and Computer Engineering, Georgia In stitute of Technology, GA, USA• Tyndall University, Ireland• Electrical & Engineering Department, University of Dela ware, DE, USA• Department of Electronic Science & Engineering, Kyo to University, Group of Prof. Kawakami• College of Optics and Photonics, University of Central Flo rida, FL, USA • National Cheng Kung University, Taiwan• Department of Materials Science and Engineering, Meijo Un iversity, Nagoya, Japan• Department of Electrical Engineering, The National Centr al University, Jhongli, Taiwan• Youngnam University, Korea• University of Maryland, Department of Electrical an d Computer Engineering • Electrical and Computer Engin. Department and Nano Tech Center, Texas Tech University, USA• Chonbuk University, Korea• UCSB, Solid State Lighting and Energy Center, USA• Pohang University of Science and Technology (POSTEC H), Korea• Advanced Optoelectronic Devices Laboratory, National Ta iwan University• Department of Applied Mathematics and Physics, State Univ ersity, Vladimir, Russia• Interdisplinary Graduate School of Science and Engineerin g, Tokyo Institute of Technology• Semiconductor Device Laboratory, Yamaguchi University, Ube, Japan• Korea Polytechnic University, Siheung City, Korea• Academic Physical Technological University, RAS, St .Petersburg, Russia

Our customers

Customers from research centers and companies. We a re grateful to those of our SimuLED™customers from who permitted us to refer their name s.

• Central Electronic Engineering Research Institute, India• Institute of High Pressure Physics, Polish Academy of Sciences, Warsaw, Poland • Bridgelux, USA• Epi-Center, Russia• Palo Alto Research Center, CA, USA• Korea Advanced Nano Fab Center (KANC)• UV Craftory Co., Ltd.• Industrial Technology Research Institute of Taiwan, Taiw an• Kyocera Co., Japan• Gwangju Institute of Science and Technology, Gwangju, Ko rea• Seoul OptoDevice Company, Korea• Fraunhofer Institut Angewandte Festkörperphysik, Freib urg, Germany• Samsung LED, Korea• Sensor Electronic Technology, Inc., USA• Heesung Electronics, Korea• De Core Nanosemiconductors Ltd., Russia• Sharp Laboratories of Europe Limited, UK• Smart Lighting Engineering Research Center, Renssel aer Polytechnic Institute, Troy NY, USA• Seiwa Electric Mfg. Co., Ltd., Japan• Genesis Photonics Inc., Taiwan• Nippon Telegraph and Telephone Co., Japan• Stanley Electric Co., Ltd., Japan

76

� Consulting service & software support:

simuled-support@str-soft.com

� Information on commercial softwarewww.str-soft.com

Detailed info is available upon request:

• Demo version

• Physical summary

• Code description

• GUI manual

• SimuLED tutorials

Contact information