InGaN barriers

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Numerical study on efficiency droop
of blue InGaN light-emitting diodes
Yen-Kuang Kuo*, Jih-Yuan Chang, and Jen-De Chen
Department of Physics, National Changhua University of
Education, Changhua 500, Taiwan
*E-mail: [email protected]
1
Outline
Introduction
Simulation results and discussion
1.
2.
3.
4.
5.
6.
7.
8.
InGaN barriers
P-doped barriers
P-type last barrier
AlGaN barriers
Varied barrier thicknesses
N-type AlGaN layer
Specific designs on electron blocking layer
Thin last barrier
Conclusion
2
Introduction
Normalized
quantum efficiency (%)
100
90
80
70
with droop
without droop
60
50
0
20
40
60
Current (mA)
80
100
In III-N LEDs, the
quantum efficiency
typically reaches its peak
at low current density and
then decreases with further
increasing drive current,
which is commonly
referred to as “efficiency
droop”.
3
Introduction
Poor hole injection efficiency, electron
current leakage, and non-uniform
distribution of carriers in the active region
usually play an important role for efficiency
droop.
In this work, several specific configurations
in the active region and electron blocking
layer (EBL) of blue InGaN LEDs are
investigated numerically with the APSYS
simulation program.
4
Study 1:
InGaN barriers
5
Study 1: InGaN barriers
※ Original LED (fabricated by EpiStar Inc.)
• Active region: five pairs of In0.21Ga0.79N/
GaN (2 nm/15 nm) QWs
• EBL: 20-nm-thick p-Al0.15Ga0.85N
• Concentration: n-51018 cm–3; p-1.21018 cm–3
• Device geometry: 300300 m2
5
(a)
4
Energy (eV)
3
2
enlarged in (b)
Quasi-Fermi level
enlarged in (c)
1
0
n-side
p-side
104.5
104.6
104.7
Distance (m)
4
4
(b)
(c)
3.6
Energy (eV)
Energy (eV)
3.2
3.2
2.4
hole
electron
1.6
wavefunctions
0.8
0
2.8
104.6
104.65
Distance (m)
104.634
Distance (m)
Fig. 1. The energy band diagram of the original
InGaN/GaN structure at 150 mA.
1.The sloped triangular barriers induced by the
polarization effect cause conduction band edge
of barriers to be higher than that of the EBL.
 This results in the insufficient electron blocking
efficiency and thereby serious electron current
leakage.
2.The electron and hole wavefunctions separate
partially.
 This results in the reduction of the radiative
recombination rate and IQE.
(Appl. Phys. Lett., vol. 95, p. 011116, 2009)
6
Study 1: InGaN barriers
5
p-side
(a)
4
n-side
Energy (eV)
3
2
enlarged in (b)
Quasi-Fermi level
enlarged in (c)
1
0
104.5
104.6
104.7
Distance (m)
4
4
(b)
(c)
3.6
Energy (eV)
Energy (eV)
3.2
3.2
2.4
hole
electron
1.6
wavefunctions
0.8
0
2.8
104.6
104.65
Distance (m)
104.634
Distance (m)
1. The effective barrier height between
last barrier and EBL increases
dramatically due to lower conduction
band energy of the InGaN barrier.
 This results in the enhanced electron
blocking efficiency of EBL.
2. Owing to better match of lattice
constants between the InGaN barrier
and InGaN well, the band bending
situation is less severe.
 This results in less QCSE and better
light-emitting efficiency.
Fig. 2. The energy band diagram of the proposed InGaN
LED structure with In0.1Ga0.9N barriers at 150 mA .
7
15
10
(a)
5
0
104.5
104.6
Distance (m)
104.7
Electron
Hole
50
40
30
20
(b)
10
0
104.5
104.6
104.7
Distance (m)
Fig. 3. Carrier concentrations of (a) original and (b)
proposed structures near active region at 150 mA.
100
300
80
250
InGaN/GaN
InGaN/InGaN
60
40
20
(a)
0
0
25 50 75 100 125 150
Current (mA)
Power (mW)
20
60
IQE (%)
25
Electron
Hole
18
30
-3
35
Carrier concentration (10 cm )
18
3
Carrier concentration (10 cm )
Study 1: InGaN barriers
200
150
100
50
0
0
(b)
25 50 75 100 125 150
Current (mA)
Fig. 4. (a) IQE and (b) L-I curve for the two LED
structures under study.
1. In Fig. 3:
 In the proposed InGaN/InGaN structure, more electrons and holes transport
into the active region and contribute to the radiative recombination.
 Both electron and hole distributions in the QWs of the proposed InGaN/InGaN
structure are more uniform than those of the original structure.
2. In Fig. 4:
 The InGaN/InGaN LED has better light-emitting efficiency (especially at high
current) and is almost without efficiency droop.
8
Study 2:
P-doped barriers
9
Study 2: P-doped barriers
InGaN well
p-side
(a)
Original structure
15 nm
GaN barrier
Mg-doping increase
InGaN well
p = 1×1017 cm–3
p = 5×1017 cm–3
(b)
Structure A
GaN barrier
InGaN well
5 nm
5 nm
p = 3×1017 cm–3
p = 3×1017 cm–3
10 nm
p = 3×1017 cm–3
1. The barriers are partially p-doped in
selected regions in order to increase
the hole injection and improve the
carrier distribution across the MQWs.
2. The reason of “partial” p-doping in
“selected regions” is to avoid the
diffusion of Mg into the quantum
well during crystal growth.
(IEEE Photonics Technol. Lett., vol. 22, p. 374, 2009)
(c)
Structure B
GaN barrier
5 nm
5 nm
p = 3×1017 cm–3
10 nm
Fig. 5. Schematic diagrams of the InGaN/GaN active
region for: (a) the original structure, (b) structure A,
and (c) structure B.
10
Study 2: P-doped barriers
Output power (mW)
120
Experimental data
Original structure
Structure A
Structure B
100
80
Fig. 6. (a) L-I curves and (b) IQEs of the blue InGaN LEDs as a
function of the forward current density.
60
40
20
Internal quantum efficiency (%)
0
70
60
50
40
30
20
10
0
0
50
100
150
2
3
Hole concentration (10 /cm )
Current density (A/cm )
18
16
Original structure
Structure A
Structure B
12
@ 22 A/cm
2
8
200
1. When the partially p-doped GaN barriers are
employed, the output powers and IQEs of the
InGaN LEDs are all enhanced sufficiently.
2. This is because that structure A and B possess
higher concentration of holes inside the QWs
due to the improved hole injection efficiency.
3. When the three barriers near the p-layer are pdoped with a gradually increased doping
concentration (structure A), more holes are
confined within the QWs, especially the two
QWs close to the p-layer, which have the most
contribution to the emission power.
4
0
Growth direction
Fig. 7. Hole concentration of the original structure, structure A,
and structure B in the active region at 22 A/cm2 (20 mA).
11
Study 3:
P-type last barrier
12
Study 3: P-type last barrier
p-side
GaN barrier
15 nm
Mg-doped region
(b)
Mg-doped region
(c)
Output power (mW)
(a)
140
Experimental data
Original structure
120
p = 1x10 cm
160
(a)
100
17
3
17
3
18
3
17
3
17
3
18
3
p = 5x10 cm
80
60
p = 1x10 cm
40
p = 1x10 cm /10 nm
20
Internal quantum efficiency (%)
InGaN well
p = 5x10 cm /10 nm
0
80
p = 1x10 cm /10 nm
70
60
Fig. 9. (a) Simulated
L-I curves and the
experimental data and
(b) IQEs of the blue
InGaN LEDs.
50
40
30
20
(b)
10
0
0
25
50
75
100
125
150
Current (mA)
10 nm
Fig. 8. Schematic diagrams of the
InGaN/GaN active regions: (a) the original
structure, (b) structure with p-doping in the
last barrier, and (c) structure with partial pdoping in two-thirds of the last barrier.
The output power and IQE are both enhanced
sufficiently, when the last undoped GaN barrier
is replaced by a p-type GaN barrier.
(IEEE J. Quantum Electron.,
vol. 22, p. 374, 2009)
13
Study 3: P-type last barrier
18
3
Hole conentration (10 /cm )
16
Original strucutre @ 20 mA
Structure A
Structure B
(a)
12
8
4
0
40
@ 150 mA
(b)
3
Hole conentration (10 /cm )
35
18
30
25
20
As indicated in Fig. 10, both
structure A and structure B
possess relatively high hole
concentration inside the QWs
because the hole injection
efficiency is effectively
improved.
15
10
5
0
104.57 104.58 104.59 104.60 104.61 104.62 104.63
Distance (m)
Fig. 10. Hole concentrations of the original structure, structure A,
and structure B in the active regions at (a) 20 mA and (b) 150 mA.
14
Study 3: P-type last barrier
1
@ 20 mA
3.5
3
2.5
2
28
3
Radiative recombination rate (10 /cm -s)
28
3
Radiative recombination rate (10 /cm -s)
4
@ 150 mA
0.8
0.6
0.4
0.2
0
1.5
1
0.5
104.57 104.58 104.59 104.60 104.61 104.62 104.63
Distance (m)
Original structure
Structure A
Structure B
0
104.57 104.58 104.59 104.60 104.61 104.62 104.63
Distance (m)
Fig. 11. Radiative recombination rate of the original
structure, structure A, and structure B in the active region
at 150 mA. The inset shows the radiative recombination
rates at 20 mA.
1. The total radiative
recombination rates inside the
QWs of structure A and B at 20
(150) mA are improved by a
factor of 1.35 (2.15) and 1.33
(2.06), respectively, as
compared to that of original one.
2. Therefore, at high current, the
optical performance of the blue
InGaN LEDs under study can
be markedly enhanced when the
last barrier is fully or partially
p-doped.
15
Study 4:
AlGaN barriers
16
Study 4: AlGaN barriers
1. The use of an AlGaN barrier instead of a GaN barrier can diminish the
polarization charges accumulating in the last-barrier/EBL interface, which
accordingly can relax the band-bending of the EBL. Due to this phenomenon,
the effective potential barrier height for holes in the valence band decreases and
hence the transportation of holes into the active region becomes easier.
2. In the meantime, the ability of electron confinement in the last-barrier/EBL
interface does not degrade under this variation.
3. As a result, it is expected that the efficiency of hole injection can be enhanced
for the structure with AlGaN barriers without the cost of reduction of electron
confinement.
Table 1. Surface Charge Density Between Well
Layer and Barrier Layer
Table 2. Surface Charge Density Between (last)
Barrier Layer and EBL
(Opt. Lett., vol. 35, p. 1368, 2010 )
17
Study 4: AlGaN barriers
enlarged in (e)
4
enlarged in (f)
3
p- quasi-Fermi
side
level
2 quasi-Fermi
level
enlarged in (c)
1
0
(a)
Energy (eV)
enlarged in (d)
(b)
104.58
104.65
104.58
effective
potential
height
(for holes)
0.5
pside
104.65
effective
potential
height
(for holes)
0
(d)
(c)
3.5
3
(e)
104.62
effective
potential
height
(for electrons)
104.65
(f)
104.62
Distance (m)
effective
potential
height
(for electrons)
104.65
1. The effective potential height for holes in
the valence band of the InGaN/AlGaN
structure is lower than that of the
InGaN/GaN one due to slighter polarization
effect in the last-barrier/EBL interface.
 Better hole injection efficiency is
anticipated.
2. Besides, the effective potential height for
the electrons in the conduction band of the
InGaN/AlGaN structure becomes higher
than the other structure, which demonstrates
the enhancement of electron confinement.
Fig. 12. Energy band diagrams of InGaN/GaN and InGaN/AlGaN
structures at 150 mA.
18
-3
300
Ga
0.95
N barrier
250
GaN barrier
150 mA
20
(a)
200
150
4
100
3.5
(b)
50
(b)
80
4.5
IQE (%)
0.05
Power (mW)
18
40
Al
Electron
Hole
60
100
5
(a)
Voltage (V)
Carrier concentration (10 cm )
Study 4: AlGaN barriers
60
40
GaN barrier
Al Ga N barrier
20
0.05
0
104.58
104.61
Distance (m)
104.64
104.58
104.61
Distance (m)
Fig. 13. Carrier concentrations of (a)
InGaN/GaN and (b) InGaN/AlGaN
structures near active region at 150 mA.
104.64
0
0
50
100
Current (mA)
150
0
0
50
0.95
100
150
Current (mA)
Fig. 14. (a) L-I-V (b) IQE performance curves of the
InGaN/GaN and InGaN/AlGaN structures.
1. Ascribing to the enhancement of hole injection and electron confinement, for the
InGaN/AlGaN structure, there are more carriers in the active region
2. Thus, the InGaN/AlGaN structure has higher IQE and output power in the whole
range of current injection under study.
3. The efficiency droop is markedly improved when the traditional GaN barriers are
replaced by AlGaN.
19
Study 5:
Varied barrier
thicknesses
20
Study 5: Varied barrier thicknesses
1. For this study:
The original structure of the blue LED used in the simulation is with equal
barrier thickness of 15 nm.
In structure A, the thicknesses of all the three barriers next to the n-side
layers are 15 nm, which is identical to the barrier thickness of the original
structure. The thickness of the three barriers next to the p-side layer are
reduced to 10 nm.
In structure B, the thicknesses of the barriers from the n-side to p-side
layers decrease linearly from 15 nm to 5 nm with a step of 2 nm.
2. In the two redesigned structures, the thicker barriers located close to the nside layers are used for increasing the electron transport distance while the
thinner barriers located close to the p-side layers are used for reducing the
hole transport distance.
Based on this rationale, it is expected that the electrons leaking out of the
active region will reduce and the holes injecting into the active region will
increase.
(To be published in IEEE Photonics Technol. Lett.)
21
IQE (%)
Output power (mW)
Study 5: Varied barrier thicknesses
160
120
34
1x10
60
6
cm /s
50
(a)
40
1x10
30
1. The output powers of structures A
and B are enhanced greatly as
compared with the original structure
for the both cases with 1×10–34 cm6/s
and 1×10–30 cm6/s Auger coefficients.
6
cm /s
(c)
30
80
20
40
10
0
80
70
60
50
40
30
20
10
0
0
0
50
(b)
(d)
40
30
Original structure
Structure A
Structure B
Experimental data
20
40
60 80 100 120
Current (mA)
20
10
0
0
20
40
60
80 100 120
Current (mA)
Fig. 15. (a) L-I curves and (b) IQEs of the original
structure, structure A, and structure B under an Auger
coefficient of 1×10–34 cm6/s; (c) L-I curves and (d) IQEs
of the original structure, structure A, and structure B
under an Auger coefficient of 1×10–30 cm6/s.
2. However, the efficiency droop for
the original structure under an Auger
coefficient of 1×10–30 cm6/s is 72.9%,
which is much larger than the value
obtained experimentally (in a range
of 23–53%).
Therefore, an Auger coefficient of
1×10–34 cm6/s is utilized for
subsequent investigations on the
optical properties of the blue LEDs
under study.
22
Study 5: Varied barrier thicknesses
20
16
12
Hole
Electron
@ 20 mA
(a)
8
4
104.57 104.58 104.59 104.60 104.61 104.62 104.63
18
3
Carrier concentration (10 /cm )
0
Hole
Electron
20
16
(b)
12
8
4
0
104.57
20
16
12
104.58
104.60
Hole
Electron
@ 20 mA
104.62
(c)
8
4
0
104.57
104.58
104.60
104.61
Distance (m)
Fig. 16. Electron and hole concentrations
of (a) original structure, (b) structure A,
and (c) structure B at 20 mA.
1. Decrease in thickness of barriers
next to the p-side layers is beneficial
for enhancing the injection of holes
into the active region and hole
transportation in the active region,
which would allow more holes to
reach the QWs near the n-side
layers.
2. Among the two redesigned
structures, structure B possesses the
largest amounts of carriers and the
most uniform distribution of carriers
inside the active region.
23
Study 5: Varied barrier thicknesses
1.6
@ 20 mA
@ 120 mA
1.2
(a)
0.4
3
cm /s)
0.8
0
Radiative recombination rate (10
28
104.57 104.58 104.59 104.60 104.61 104.62 104.63
1.6
@ 20 mA
@ 120 mA
1.2
(b)
0.8
0.4
0
104.57
1.6
104.58
104.60
104.62
104.60
104.61
@ 20 mA
@ 120 mA
1.2
(c)
0.8
0.4
0
104.57
104.58
Distance (m)
Fig. 17. Radiative recombination rates of
(a) the original structure, (b) structure A,
and (c) structure B at 20 mA and 120 mA.
1. As the thickness of barriers close to the p-side
layers decreases, more QWs contribute to
radiative recombination due to the marked
improvement in hole injection efficiency.
2. For the original structure, the total radiative
recombination rate is enhanced by a factor of
3.77 when the injection current increases from
20 mA to 120 mA. On the other hand, for
structures A and B, the total radiative
recombination rates are enhanced by a factor of
4.15 and 5.67, respectively, when the injection
current increases from 20 mA to 120 mA.
The improvement is because that more QWs,
especially the three QWs close to the p-side
layers, can contribute to radiative recombination.
24
Study 6:
N-type AlGaN layer
25
250
Power (mW)
200
Experiment
P-AlGaN
N-AlGaN
P-AlGaN
N-AlGaN
20
15
6 mA
10
150
100
5
0
0
4
8
12
16
50
0
0
20
40
60
80
100
120
Current (mA)
Internal quantum efficiency (%)
Study 6: N-type AlGaN layer
100
80
60
40
15 mA
20
0
P-EBL
N-EBL
0
20
40
60
80
100
120
Current (mA)
Fig. 18. L-I curves of the LEDs with P and N-AlGaNs.
Experimental data are shown by green solid dots.
Fig. 19. IQE of the LEDs with P and N-AlGaNs.
1. The output power of the LED with N-AlGaN at 120 mA is improved by
a factor of 2.37 as compared with that with P-AlGaN.
2. The efficiency droop of the LED with N-AlGaN at 120 mA is markedly
improved as compared to that with P-AlGaN.
(IEEE Photonics Technol. Lett., vol. 21, p. 975, 2009)
26
Study 6: N-type AlGaN layer
25
18
3
(10 /cm )
Hole concentration
30
P-AlGaN
N-AlGaN
p-side
20
15
10
0
30
25
3
20
18
(10 /cm )
Electron concentration
@120 mA p-side
5
(a)
(b)
P-AlGaN
N-AlGaN
15
10
5
0
Growth direction
Fig. 20. (a) Hole and (b) electron concentrations of
the LEDs with P-AlGaN and N-AlGaN at 120 mA.
For the N-AlGaN LED:
1. More holes transport to the QWs next
to the n-side layer due to the absence
of P-AlGaN.
2. The amount of electrons in the active
region also increases, despite that the
concentration of electrons in the last
QW is reduced.
 As a result, the radiative
recombination rates are more uniform
in QWs and almost all QWs can
contribute to output power. Thus, the
IQE droop at high current may be
improved due to the enhanced
electron and hole injection efficiencies.
27
Study 6: N-type AlGaN layer
Energy (eV)
4
(a)
3
2
N-AlGaN
Quasi-Fermi level
1
0
Energy (eV)
4
(b)
3
Quasi-Fermi level
2
P-AlGaN
1
0
104.55
104.58
104.61
104.64
104.66
Distance (m)
Fig. 21. Band diagrams and quasi-Fermi levels of
the LEDs with (a) N-AlGaN and (b) P-AlGaN
layers at 120 mA.
For the N-AlGaN LED:
• The electron distribution in the QWs is
more uniform, as indicated in the relative
position of the quasi-Fermi levels.
For the P-AlGaN LED:
• The strong electric field, caused by the
polarization effect, lowers the conduction
band energy in the last barrier.
 This is a negative effect for the
confinement of electrons at high carrier
density.
 The percentages of electron leakage
current for the LEDs with P-AlGaN and
N-AlGaN are 46.1% and 4.5%,
respectively, at 120 mA.
28
Study 7:
Specific designs on
electron blocking layer
29
Study 7: Specific designs on electron
blocking layer
1. There is a side effect of the usage of EBL  the EBL also acts as a potential
barrier for holes in the valence band and impedes the transportation of holes
into the active region.
2. The downward band-bending in the last barrier/EBL interface induced by
electrostatic polarization makes this problem even more serious.
◎ Without polarization
n-side
◎ With polarization
p-side
@ 150 mA
4
n-side
quasi-Fermi level
1
0
2
quasi-Fermi level
1
0
104.6
104.64
Distance (m)
0.5
104.68
enlarged
0
104.64
104.6
104.68
104.64
Distance (m)
0.5
Effective potential height
for holes
Distance (m)
104.56
Energy (eV)
104.56
Energy (eV)
EBL
3
Energy (eV)
Energy (eV)
3
2
p-side
@ 150 mA
4
EBL
104.68
enlarged
Effective potential height
for holes
(Opt. Lett., vol. 35,
p. 3285, 2010)
0
104.64
Distance (m)
104.68
30
Study 7: Specific designs on electron
blocking layer
Main goal of this study: To increase the efficiency of hole injection without losing
the blocking capability for electrons.
 Plan: Reduce the polarization charge density in the last barrier/EBL interface in
order to mitigate downward band-bending of this interface.
 Method: Some specific designs on EBL of blue InGaN LEDs are proposed.
1. Structure A is the
original structure.
2. The structures of
structure B, C, and D are
identical except for the
EBLs.
Fig. 22. Schematic diagrams of the
original structure and the structures
with redesigned EBLs.
31
5
Table 3. Surface Charge Density in the “Last-Barrier/EBL” and
“Front-EBL/Rear-EBL” Interfaces
Electrostatic field (10 V/cm)
Study 7: Specific designs on electron
blocking layer
3
2
A
B
C
D
150 mA
1
0
-1
-2
-3
-4
104.62
EBL
104.64
104.66
104.68
Distance (m)
Fig. 23. Electric fields of various InGaN/GaN
structures near the EBLs at 150 mA.
The original structure (structure A) possesses much stronger
electrostatic field in the last-barrier/EBL interface than the
redesigned structures because of high surface charge density.
32
3.4
3.3
3.2
104.64
EBL
104.66
Distance (m)
(a)
104.68
-0.1
-0.2
-0.3
104.64
EBL
104.66
(b)
104.68
Distance (m)
Fig. 24. Enlarged energy band diagrams near the
EBLs of various InGaN/GaN structures in (a)
conduction band and (b) valence band at 150 mA.
A
B
60
C
D
(a)
50
20
40
30
18
16
14
20
12
104.56
104.6
104.64
10
0
104.56
104.6
Distance (m)
104.64
-3
B
C
D
0
Energy (eV)
Energy (eV)
3.5
A
150 mA
60
50
Electron concentration (10 cm )
0.1
-3
150 mA
18
3.6
valence band
Hole concentration (10 cm )
conduction band
18
Study 7: Specific designs on electron
blocking layer
40
30
20
20
18
16
14
12
10
8
104.56
(b)
104.6
104.64
150 mA
10
0
104.56
104.6
104.64
Distance (m)
Fig. 25. (a) Hole and (b) electron concentrations
in the active regions of various InGaN/GaN
structures at 150 mA. The insets show the
comparison of cases A and D in LOG scale.
1. The situation of downward band-bending at the interfaces near the EBL of all the
three redesigned structures are slighter than that of the original one, which will lead
to the enhancement of hole injection efficiency.
2. The hole concentrations of the redesigned structures increase obviously, which
demonstrates the improvement of hole injection efficiency. Besides, the redesigned
structures also have higher electron concentrations, which indicates that the
capability of electron confinement does not suffer from the modifications of EBL.
33
Study 7: Specific designs on electron
blocking layer
250
100
A
B
C
D
150
80
IQE (%)
Power (mW)
200
100
50
60
40
20
(a)
0
0
(b)
0
25
50
75
100 125 150
Current (mA)
0
25
50
75
100 125 150
Current (mA)
Fig. 26. (a) L-I curves and (b) IQE as a function of current for various
InGaN/GaN structures.
1. The overall performances of the redesigned structures show significant
enhancement as compared with that of the original structure.
2. Structure D seems to be the most superior one among these redesigned
structures due to the reduced efficiency droop and higher light output
power at elevated level of current injection.
34
Study 8:
Thin last barrier
35
3
20
Last barrier = 12 nm
10
5
0
103.10
103.15
103.20
103.25
3
Distance (m)
25
20
(b)
Last barrier = 9 nm
※ Original structure:
• Active region: ten pairs of In0.21Ga0.79N/
GaN (2 nm/12 nm) QWs
• EBL: 10-nm-thick p-Al0.10Ga0.90N
• Concentration: n-41018 cm–3; p-51017 cm–3
• Device geometry: 245560 m2
15
10
5
0
103.10
103.15
103.20
103.25
Distance (m)
12 nm
3
Electron concentration, log (cm )
Fig. 27. Distribution of hole concentrations of the LEDs with (a)
12-nm and (b) 9-nm last barriers at 55 A/cm2.
(a)
15
18
Hole concentration (10 cm )
25
18
Hole concentration (10 cm )
Study 8: Thin last barrier
9 nm
19
18
Electron
leakage
17
16
(IEEE Photonics Technol. Lett., vol. 22, p. 1787, 2010)
15
14
• A thinner last-barrier could be beneficial for increasing the
hole injection efficiency so that holes can inject into more
QWs within the active region.
2.With better hole injection efficiency, electron leakage can
be depressed correspondingly.
3.The radiative recombination and optical power are
enhanced accordingly when thinner last-barrier is adopted.
103.14
103.18
103.23
Distance (m)
103.28
Fig. 28. Distribution of electron concentrations of the LEDs with
12-nm and 9-nm last barriers at 55 A/cm2.
36
2
L.B. = 12 nm
1.5
26
3
12
L.B. = 9 nm
1
0.5
8
0
103.12
103.14
103.16
103.18
103.20
Distance (m)
4
0
103.10
103.15
103.20
103.25
Distance (m)
Fig. 29. Radiative recombination rates of the LEDs
with 12-nm and 9-nm last barriers at 55 A/cm2.
Internal quantum efficiency (%)
16
(10 cm /s)
Radiative recombination rate
Study 8: Thin last barrier
70
60
50
40
30
Last barrier = 12 nm
Last barrier = 9 nm
20
10
0
0
20
40
60
2
80
100
Current density (A/cm )
Fig. 30. IQE as a function of current density of
the LEDs with 12-nm and 9-nm last barriers.
1. The radiative recombination rates in the QWs next to the n-side layers
greatly enhances when the last barrier is 9 nm because more holes can
transport to the QWs.
2. At high current density, the IQE of the LED with 9-nm last barrier is
improved sufficiently due to higher hole injection efficiency and more
uniform hole distribution.
37
Study 8: Thin last barrier
8
600
7
500
Power (a.u.)
400
5
4
300
3
200
Voltage (V)
6
Fig. 31. (experimental data) Optical power
and forward voltage as a function of
current density for InGaN LED with the
last barrier thicknesses of 12 nm and 9.6
nm.
2
100
0
Last barrier = 12 nm 1
Last barrier = 9.6 nm
0
20
40
80
60
0
100
2
Current density (A/cm )
The experimental results show that, when the thickness of the last
barrier decreases from 12 to 9.6 nm, the optical power is markedly
enhanced, especially at high current density.
38
Conclusion
Some specific designs on band structure near the active region,
including the modifications of barrier material or thickness,
the redesigns of the electron blocking layer (EBL), etc., in the
blue InGaN LEDs are investigated numerically with the
APSYS simulation program. Simulation results show that,
with appropriate designs, the efficiency droop may be
effectively improved due to the increase of hole injection
efficiency, enhancement of blocking capability for
electrons, or uniform distribution of carriers in the active
region. The methods proposed in this paper are advantageous
especially for high current injection.
39
Thank you for
your attention !!
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