Investigation of the Impact of Polarization and Auger Recombinationon the Wurtzite and Zincblende GaN-Based Green LEDs
Yi-Chia Tsai
Ph.D. Student
Department of Electrical and Computer Engineering
University of Illinois at Urbana-Champaign, Illinois, USA
Innovative COmpound semiconductoR LABoratory (ICORLAB)
PI: Prof. Can Bayram,
Assistant Professor
Department of Electrical and Computer Engineering
University of Illinois at Urbana-Champaign, IL, USA
EMAIL: [email protected] WEBPAGE: icorlab.ece.Illinois.edu
Motivations
1. Why we need efficient green LEDs?
– To generate natural white light via color mixing
– To make semiconductor green lasers
– To save power
2. Green gap
– Phosphor-coated blue LED → Down-conversion → Energy loss
– Phosphide-based materials → Indirect bandgap → Low rad. rate
– Nitride-based green LEDs → Several factors might involve
2
• Possible mechanisms
– Shockley-Read-Hall recombination
– Auger recombination
– Polarization
– Thermal effect
– Electron leakage
• Solution: Zincblende GaN
– Zero polarization
– Smaller Auger coefficient
Issues of nitride-based green LEDs
3
Prohibitively impossible to be distinguished in
experiments → Simulations are paramount
1. Poisson Equation:
2. Schrödinger Equation:
3. Drift-Diffusion Equations:
4. Current Continuity Equations:
Quantum-Corrected DD Model
4
( ) ( )0 ,D A pP q p n N N n + = − − + − +
22
*2i i i ie E
m − + = ( )
22 i F i
i
n f E E = −
.
1
1
,n
n SRH Rad Au
p
ger
p
SRH Rad Auger
R R
RJ R
J Rq
Rq
−
= + +
+ = +
,
,p p p
n n nJ E qkT
J
n
q E qk
q
T
n
p p
+
−
=
=
Device Structure – 5QWs
5
p-GaN (NA = 1025 m-3) 0.1 µm
p-GaN (NA = 3 x 1023 m-3) 20 nm
In0.3Ga0.7N (5 nm)/i-GaN (5 nm) x 5 QWs
n-GaN (ND = 5 x 1024 m-3) 0.5 µm
n-GaN (ND = 5 x 1024 m-3) 2.5 µm
200 µm 100 µm
50 µm
70 µm
1. Short p-region
2. Fixed barrier thickness: 5 nm
3. In0.3Ga0.7N is used for QWs
– For 550 nm wavelength
Exemplification of Band Diagrams
6
0.00 0.02 0.04 0.06 0.08 0.10-1
0
1
2
3
4
5
6
Operated under V = 5V
Ba
nd P
ositio
n (
eV
)
Distance (m)
Hexagonal GaN
Cubic GaN
n-contact p-contact
e-
e-
h+
h+
Green Gap AnalysisIdentify the major mechanisms
7
❑ Single QW
❑ Thickness: 2.5 nm
❑ Varying In mole fraction
Spontaneous Emission
8
0.3 0.4 0.5 0.6 0.70
5
10
15
20
25
30
35
40
45
50
Tota
l S
p. R
ate
(10
20 s
-1eV
-1m
-1)
Wavelength (m)
Bias = 5V
In = 0.1
In = 0.2
In = 0.3
In = 0.4
In = 0.5
# of QW = 1
Thickness = 2.5 nm
❑ The increase of In mole fraction
– Deeper QW
– Increases wavelength emission
– Decreases spontaneous emission rate
– Why?
Recombination Processes and Leakage
9
0.1 0.2 0.3 0.4 0.5
0.0
0.2
0.4
0.6
0.8
1.0
Pe
ak R
eco
mbin
atio
n R
ate
s (
10
28 c
m-3
s-1
)
In mole fraction
Current density = 100 A/m
SRH
Radiative
Auger
1E-7
1E-6
1E-5
1E-4
0.001
0.01
0.1
1
10
100
Electron Leakage
Ele
ctr
on
Le
aka
ge
(A
-cm
-2)
# of QW = 1
Thickness = 2.5 nm
❑ In mole fraction = 0.1
– Shallow QW
– Strong electron leakage
❑ In mole fraction > 0.2
– Higher electron density
• Auger recomb. increases
– Stronger QCSE
• Radiative recomb. drops
Impact of Auger Coefficient
10
0 20 40 60 80 100 120 140 160 180 200
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
IQE
Current Density (A/m)
×102
×101
×1
×10-1
×10-2
×10-3
# of QW = 1
Thickness = 2.5 nm
❑ Default: 2.96 × 10−30 cm-6s-1
❑ Reduced by an order
– Double IQE
– Gain an extra 40% efficiency
❑ Reduced by more than two orders
– Droop-free performance
❑ Material engineering is necessary.
Impact of Polarization
11
0 20 40 60 80 100 120 140 160 180 2000.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
IQE
Current Density (A/m)
In = 0.1 (polar) In = 0.1 (non-polar)
In = 0.2 (polar) In = 0.2 (non-polar)
In = 0.3 (polar) In = 0.3 (non-polar)
In = 0.4 (polar) In = 0.4 (non-polar)
In = 0.5 (polar) In = 0.5 (non-polar)
# of QW = 1
Thickness = 2.5 nm
❑ Low carrier injection
– Triangular QW is formed
– Better carrier localization
– Only works for narrow QW
❑ Impact of polarization is stronger for
– Thick QW
– High carrier injection
❑ Zero polarization leads to a better IQE.
Thermal Effect – 250K to 500K
12
0 20 40 60 80 100 120 140 160 180 2000.1
0.2
0.3
0.4
IQE
Current Density (A/m)
# of QW = 1
Thickness = 2.5 nm
T = 250K
T = 300K
T = 350K
T = 400K
T = 450K
T = 500K
250 300 350 400 450 500-0.05
0.000.10
0.15
0.20
0.25
0.90
0.95
1.00
1.05
# of QW = 1
Thickness = 2.5 nm
Peak R
ecom
bin
ation R
ate
s (
10
28 c
m-3
s-1
)
Temperature (K)
Current density = 100 A/m
SRH
Radiative
Auger
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
Electron Leakage
Ele
ctr
on L
eakage (
A-c
m-2
)
(a)
(b)
Optimization of Wurtzite GaN-based Green LEDs
13
❑ # of QW: 1 - 5
❑ Thickness: 1 - 5 nm
❑ In mole fraction: 0.3
Wall-Plug Efficiency
14
20.0
12.8
18.216.4
9.20
14.6
11.0
12.8
424
542
542
519
495
471
447
1 2 3 4 5
1
2
3
4
5
Thic
kn
ess o
f Q
W (
nm
)
Numbers of QW
7.40
9.20
11.0
12.8
14.6
16.4
18.2
20.0
21.8
Wall-Plug Efficiency (%)
Operated under I = 100 A/m❑ Green emission (𝜆 > 500 𝑛𝑚)
– Thickness > 3 nm
❑ Issue of thick QW
– Polarization
– Low radiative rate
❑ Issue of multiple QWs
– Higher resistance
❑ Optimal efficiency
– Under 100 A/m carrier injection
– 12.8%
Optimization of Zincblende GaN-based Green LEDs
15
❑ # of QW: 1 - 5
❑ Thickness: 1 - 11 nm
❑ In mole fraction: 0.3
Wall-Plug Efficiency
16
47.8
44.7
41.0
37.3
33.629.9
26.222.5
538
536
531
527
519
495
471447
1 2 3 4 5
1
2
4
7
11
Thic
kn
ess o
f Q
W (
nm
)
Numbers of QW
18.8
22.5
26.2
29.9
33.6
37.3
41.0
44.7
48.4
Wall-Plug Efficiency (%)
Operated under I = 100 A/m
❑ Green emission (𝜆 > 500 𝑛𝑚)
– Thickness > 3 nm
❑ Thick QW
– Higher electron concentration
– Higher radiative recombination
❑ Multiple QWs
– Higher resistance
❑ Optimal efficiency
– Under a constant current of 100 A/m
– 47.8% (3.7 times higher than 12.8%)
Conclusion
1. Auger recombination and polarization are the key to
bridge green gap.
2. The highest wall-plug efficiency of wurtzite GaN-based
green LEDs under 100 A/m carrier injection is 12.8%.
3. Yet, the highest wall-plug efficiency of zincblende GaN-
based green LEDs under the same condition is 47.8%
even though it has the same Auger coefficient.
17
Thank you very much for your attention
18
Q&A2 mins
19