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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
3
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
Copyright © 2010 STR Group Ltd. All rights reserved www.str-soft.com
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
Copyright © 2010 STR Group Ltd. All rights reserved www.str-soft.com
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 !
Copyright © 2010 STR Group Ltd. All rights reserved www.str-soft.com
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
Copyright © 2010 STR Group Ltd. All rights reserved www.str-soft.com
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 %
Copyright © 2010 STR Group Ltd. All rights reserved www.str-soft.com
radAdisSRRRRRR +++= IQE = Rrad/R
8 Auger recombination is responsiblefor the IQE rollover
Epi level
Copyright © 2010 STR Group Ltd. All rights reserved www.str-soft.com
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
Copyright © 2010 STR Group Ltd. All rights reserved www.str-soft.com
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
Current density (A/cm2)
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
Current density (A/cm2)
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
Copyright © 2010 STR Group Ltd. All rights reserved www.str-soft.com
Chip level
11 Current density, and IQE distributions in the
active region
Chip level
IQE
J (A/cm2)
Copyright © 2010 STR Group Ltd. All rights reserved www.str-soft.com
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
Current density (A/cm2)
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
Copyright © 2010 STR Group Ltd. All rights reserved www.str-soft.com
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
Copyright © 2010 STR Group Ltd. All rights reserved www.str-soft.com
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
Copyright © 2010 STR Group Ltd. All rights reserved www.str-soft.com
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%)
Copyright © 2010 STR Group Ltd. All rights reserved www.str-soft.com
Chip level
16 General design of LED lamp and
simulation approach
Device level
Copyright © 2010 STR Group Ltd. All rights reserved www.str-soft.com
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
Copyright © 2010 STR Group Ltd. All rights reserved www.str-soft.com
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
Copyright © 2010 STR Group Ltd. All rights reserved www.str-soft.com
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
Copyright © 2010 STR Group Ltd. All rights reserved www.str-soft.com
21
Copyright © 2010 STR Group Ltd. All rights reserved www.str-soft.com
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