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Numerical study on efficiency droop Numerical study on efficiency droop
of blue InGaN light-emitting diodesof 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]
2
Introduction
Simulation results and discussion 1. InGaN barriers
2. P-doped barriers
3. P-type last barrier
4. AlGaN barriers
5. Varied barrier thicknesses
6. N-type AlGaN layer
7. Specific designs on electron blocking layer
8. Thin last barrier
Conclusion
Outline
3
Introduction
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”. 50
60
70
80
90
100
0 20 40 60 80 100
with droopwithout droop
Nor
mal
ized
quan
tum
eff
icie
ncy
(%)
Current (mA)
4
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.
5
Study 1: InGaN barriers
6
Study 1: InGaN barriers
0
1
2
3
4
5
104.5 104.6 104.7
Distance (m)
Ene
rgy
(eV
)
Quasi-Fermi level
n-side p-side
enlarged in (b)
enlarged in (c)
(a)
(c)(b)
2.8
3.2
3.6
4
104.6 104.65
Distance (m)
Ene
rgy
(eV
)
0
0.8
1.6
2.4
3.2
4
104.634
Distance (m)
Ene
rgy
(eV
)
wavefunctions
hole electron
※ 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-51018 cm–3; p-1.21018 cm–3
• Device geometry: 300300 m2
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. Fig. 1. The energy band diagram of the original
InGaN/GaN structure at 150 mA.(Appl. Phys. Lett., vol. 95, p. 011116, 2009)
7
Fig. 2. The energy band diagram of the proposed InGaN LED structure with In0.1Ga0.9N barriers at 150 mA .
Study 1: InGaN barriers
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.
0
1
2
3
4
5
104.5 104.6 104.7
Distance (m)
Ene
rgy
(eV
)
Quasi-Fermi level
n-side
p-side
enlarged in (b)
enlarged in (c)
(a)
(c)(b)
0
0.8
1.6
2.4
3.2
4
104.634
Distance (m)
Ene
rgy
(eV
)
wavefunctions
hole electron
2.8
3.2
3.6
4
104.6 104.65
Distance (m)
Ene
rgy
(eV
)
8
Study 1: InGaN barriers
0
5
10
15
20
25
30
35
104.5 104.6 104.7
ElectronHole
Distance (m)
Car
rier
con
cent
ratio
n (1
018
cm
3)
(a)
0
10
20
30
40
50
60
104.5 104.6 104.7
ElectronHole
Distance (m)
Car
rier
con
cent
ratio
n (1
018
cm
-3)
(b)
0
20
40
60
80
100
0 25 50 75 100 125 150
InGaN/GaNInGaN/InGaN
Current (mA)
IQE
(%
)
(a) (b)0
50
100
150
200
250
300
0 25 50 75 100 125 150
Current (mA)
Pow
er (
mW
)
Fig. 3. Carrier concentrations of (a) original and (b) proposed structures near active region at 150 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 int
o 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.
9
Study 2: P-doped barriers
10
Study 2: P-doped barriers
GaN barrier
(a)
p-sideInGaN well
15 nm
Original structure
GaN barrier
InGaN well
10 nm
p = 5×1017 cm–3p = 1×1017 cm–3
p = 3×1017 cm–3
5 nm 5 nm
Mg-doping increase
(b)
Structure A
GaN barrier
InGaN well
10 nm
p = 3×1017 cm–3p = 3×1017 cm–3
p = 3×1017 cm–3
5 nm 5 nm
(c)
Structure B Fig. 5. Schematic diagrams of the InGaN/GaN active region for: (a) the original structure, (b) structure A, and (c) structure B.
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)
11
Study 2: P-doped barriers
0
20
40
60
80
100
120 Experimental dataOriginal structureStructure AStructure B
Out
put p
ower
(m
W)
0
10
20
30
40
50
60
70
0 50 100 150 200Inte
rnal
qua
ntum
eff
icie
ncy
(%)
Current density (A/cm2)
0
4
8
12
16
Growth direction
Hol
e co
ncen
trat
ion
(1018
/cm
3 )
Original structureStructure AStructure B
@ 22 A/cm2
Fig. 6. (a) L-I curves and (b) IQEs of the blue InGaN LEDs as a function of the forward current density.
Fig. 7. Hole concentration of the original structure, structure A, and structure B in the active region at 22 A/cm2 (20 mA).
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 p-doped 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.
12
Study 3: P-type last barrier
13
Study 3: P-type last barrier
GaN barrier
(a)
(c)
Mg-doped region
p-sideInGaN well
15 nm
10 nm
(b)
Mg-doped region
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 p-doping in two-thirds of the last barrier.
0
20
40
60
80
100
120
140
160 Experimental dataOriginal structure
p = 1x1017cm 3
p = 5x1017cm 3
p = 1x1018cm 3
p = 1x1017cm 3/10 nm
p = 5x1017cm 3/10 nm
p = 1x1018cm 3/10 nm
Out
put p
ower
(m
W)
(a)
0
10
20
30
40
50
60
70
80
0 25 50 75 100 125 150
Current (mA)
Inte
rnal
qua
ntum
eff
icie
ncy
(%)
(b)
The output power and IQE are both enhanced sufficiently, when the last undoped GaN barrier is replaced by a p-type GaN barrier.
Fig. 9. (a) Simulated L-I curves and the experimental data and (b) IQEs of the blue InGaN LEDs.
(IEEE J. Quantum Electron., vol. 22, p. 374, 2009)
14
Study 3: P-type last barrier
0
4
8
12
16
Original strucutreStructure AStructure B
@ 20 mA
Hol
e co
nent
ratio
n (1
018/c
m3 )
(a)
0
5
10
15
20
25
30
35
40
104.57 104.58 104.59 104.60 104.61 104.62 104.63
@ 150 mA
Distance (m)
Hol
e co
nent
ratio
n (1
018/c
m3 )
(b)
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.
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
Study 3: P-type last barrier
0
0.5
1
1.5
2
2.5
3
3.5
4
104.57 104.58 104.59 104.60 104.61 104.62 104.63
Original structureStructure AStructure B
Distance (m)
Rad
iati
ve r
ecom
bina
tion
rate
(10
28/c
m3 -s
)
@ 150 mA
0
0.2
0.4
0.6
0.8
1
104.57 104.58 104.59 104.60 104.61 104.62 104.63
Rad
iativ
e re
com
bina
tion
rate
(10
28/c
m3 -s
)
Distance (m)
@ 20 mA
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.
16
Study 4: AlGaN barriers
17
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 )
18
Study 4: AlGaN barriers
0
1
2
3
4
104.58 104.65
Distance (m)
Ene
rgy
(eV
)
quasi-Fermi level
p-side
enlarged in (c)
enlarged in (e)
(a) (b)
104.62 104.65
effectivepotentialheight
(for electrons)(f)
effectivepotential
height(for holes)
(d)
3
3.5
104.62 104.65
effectivepotentialheight
(for electrons)(e)
0
0.5effectivepotential
height(for holes)
(c)
104.58 104.65
enlarged in (d)
enlarged in (f)
quasi-Fermi level
p-side
Fig. 12. Energy band diagrams of InGaN/GaN and InGaN/AlGaN structures at 150 mA.
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.
19
Study 4: AlGaN barriers
0
50
100
150
200
250
300
3.5
4
4.5
5
0 50 100 150
Pow
er (
mW
) Voltage (V
)
Current (mA)
(a)(b)
0
20
40
60
80
100
0 50 100 150
GaN barrierAl
0.05Ga
0.95N barrier
IQE
(%
)
Current (mA)
Fig. 14. (a) L-I-V (b) IQE performance curves of the InGaN/GaN and InGaN/AlGaN structures.
0
20
40
60 ElectronHole
104.58 104.61 104.64
Car
rier
con
cent
rati
on (
1018
cm
-3)
150 mA
GaN barrier
Distance (m)
(a)
104.58 104.61 104.64
Distance (m)
Al0.05
Ga0.95
N barrier
(b)
Fig. 13. Carrier concentrations of (a) InGaN/GaN and (b) InGaN/AlGaN structures near active region at 150 mA.
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.
20
Study 5: Varied barrier
thicknesses
21
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 n-side 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.)
22
Study 5: Varied barrier thicknesses
0
10
20
30
40
50
60
70
80
0 20 40 60 80 100 120
IQE
(%
)
Current (mA)
0
40
80
120
160
Original structureStructure AStructure BExperimental data
Out
put p
ower
(m
W) 1x10 34cm6/s
(a)
0
10
20
30
40
50
601x10 30cm6/s
0
10
20
30
40
50
0 20 40 60 80 100 120
Current (mA)
(c)
(b) (d)
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.
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.
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.
23
Study 5: Varied barrier thicknesses
0
4
8
12
16
20
104.57 104.58 104.59 104.60 104.61 104.62 104.63
HoleElectron
@ 20 mA
Car
rier
con
cent
ratio
n (1
018
/cm
3 )
(a)
0
4
8
12
16
20
104.57 104.58 104.60 104.62
HoleElectron
(b)
0
4
8
12
16
20
104.57 104.58 104.60 104.61
HoleElectron
Distance (m)
@ 20 mA
(c)
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.
24
Study 5: Varied barrier thicknesses
0
0.4
0.8
1.2
1.6
104.57 104.58 104.59 104.60 104.61 104.62 104.63
@ 20 mA@ 120 mA
Rad
iati
ve r
ecom
bina
tion
rate
(10
28 c
m3
/s)
(a)
0
0.4
0.8
1.2
1.6
104.57 104.58 104.60 104.62
@ 20 mA@ 120 mA
(b)
0
0.4
0.8
1.2
1.6
104.57 104.58 104.60 104.61
@ 20 mA@ 120 mA
Distance (m)
(c)
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.
25
Study 6: N-type AlGaN layer
26
Study 6: N-type AlGaN layer
0
20
40
60
80
100
0 20 40 60 80 100 120
P-EBLN-EBL
Inte
rnal
qua
ntum
eff
icie
ncy
(%)
Current (mA)
15 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.
0
50
100
150
200
250
0 20 40 60 80 100 120
ExperimentP-AlGaNN-AlGaN
Current (mA)
Pow
er (
mW
)
0
5
10
15
20
0 4 8 12 16
P-AlGaNN-AlGaN
6 mA
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)
27
Study 6: N-type AlGaN layer
0
5
10
15
20
25
30
P-AlGaNN-AlGaN
Hol
e co
ncen
trat
ion
(1018
/cm
3 )
(a)
p-side@120 mA
0
5
10
15
20
25
30P-AlGaNN-AlGaN
Growth direction
Ele
ctro
n co
ncen
trat
ion
(1018
/cm
3 )
(b)
p-side
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.
Fig. 20. (a) Hole and (b) electron concentrations of the LEDs with P-AlGaN and N-AlGaN at 120 mA.
28
0
1
2
3
4
Ene
rgy
(eV
)
(a)
Quasi-Fermi levelN-AlGaN
0
1
2
3
4
104.55 104.58 104.61 104.64 104.66
Distance (m)
Ene
rgy
(eV
)
Quasi-Fermi levelP-AlGaN
(b)
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 m
ore uniform, as indicated in the relative position of the quasi-Fermi levels.
For the P-AlGaN LED:• The strong electric field, caused by the po
larization 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.
Study 6: N-type AlGaN layer
29
Study 7: Specific designs on
electron blocking layer
30
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.
0
1
2
3
4
104.56 104.6 104.64 104.68
Distance (m)
Ene
rgy
(eV
)
n-side p-side
quasi-Fermi level
@ 150 mA
EBL
0
0.5
104.64 104.68
Distance (m)
Ene
rgy
(eV
)
Effective potential heightfor holes
◎ Without polarization
0
1
2
3
4
104.56 104.6 104.64 104.68
Distance (m)
Ene
rgy
(eV
)
n-side p-side
quasi-Fermi level
EBL
@ 150 mA
0
0.5
104.64 104.68
Distance (m)
Ene
rgy
(eV
)
Effective potential heightfor holes
enlarged enlarged
◎ With polarization
(Opt. Lett., vol. 35, p. 3285, 2010)
31
Study 7: Specific designs on electron blocking layer
Main goal of this study: Main goal of this study: To increase the efficiency of hole injection without losing the blocking capability for electrons.
PlanPlan:: Reduce the polarization charge density in the last barrier/EBL interface in order to mitigate downward band-bending of this interface.
Method:Method: Some specific designs on EBL of blue InGaN LEDs are proposed.
Fig. 22. Schematic diagrams of the original structure and the structures with redesigned EBLs.
1. Structure A is the original structure.
2. The structures of structure B, C, and D are identical except for the EBLs.
32
Study 7: Specific designs on electron blocking layer
Table 3. Surface Charge Density in the “Last-Barrier/EBL” and “Front-EBL/Rear-EBL” Interfaces
-4
-3
-2
-1
0
1
2
3
104.62 104.64 104.66 104.68
ABCD
Distance (m)
Ele
ctro
stat
ic f
ield
(10
5 V/c
m)
150 mA
EBL
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.
33
Study 7: Specific designs on electron blocking layer
3.2
3.3
3.4
3.5
3.6
104.64 104.66 104.68
Distance (m)
Ene
rgy
(eV
)
EBL
conduction band
150 mA
-0.3
-0.2
-0.1
0
0.1
104.64 104.66 104.68
ABCD
Distance (m)
Ene
rgy
(eV
)
valence band
EBL
150 mA
(a) (b)0
10
20
30
40
50
60
104.56 104.6 104.64
AB
CD
Distance (m)
Hol
e co
ncen
trat
ion
(10
18 c
m-3
)
12
14
16
18
20
104.56 104.6 104.64
0
10
20
30
40
50
60
104.56 104.6 104.64
Distance (m)
Ele
ctro
n co
ncen
trat
ion
(10
18 c
m-3
)
150 mA
8
10
12
14
16
18
20
104.56 104.6 104.64
(a) (b)
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.
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.
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.
34
Study 7: Specific designs on electron blocking layer
0
50
100
150
200
250
0 25 50 75 100 125 150
ABCD
Current (mA)
Pow
er (
mW
)
0
20
40
60
80
100
0 25 50 75 100 125 150
Current (mA)
IQE
(%
)
(a) (b)
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.
35
Study 8: Thin last barrier
36
Study 8: Thin last barrier
(IEEE Photonics Technol. Lett., vol. 22, p. 1787, 2010)
0
5
10
15
20
25
103.10 103.15 103.20 103.25
Last barrier = 12 nm
Hol
e co
ncen
trat
ion
(1018
cm
3)
Distance (m)
(a)
0
5
10
15
20
25
103.10 103.15 103.20 103.25
Last barrier = 9 nm
Hol
e co
ncen
trat
ion
(1018
cm
3)
Distance (m)
(b)
14
15
16
17
18
19
103.14 103.18 103.23 103.28
12 nm 9 nm
Ele
ctro
n co
ncen
trat
ion,
log
(cm
3)
Distance (m)
Electron leakage
Fig. 27. Distribution of hole concentrations of the LEDs with (a) 12-nm and (b) 9-nm last barriers at 55 A/cm2.
Fig. 28. Distribution of electron concentrations of the LEDs with 12-nm and 9-nm last barriers at 55 A/cm2.
※ 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-41018 cm–3; p-51017 cm–3
• Device geometry: 245560 m2
• 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.
37
Study 8: Thin last barrier
0
4
8
12
16
103.10 103.15 103.20 103.25
L.B. = 12 nm
L.B. = 9 nm
Rad
iati
ve r
ecom
bina
tion
rate
(1026
cm3
/s)
Distance (m)
0
0.5
1
1.5
2
103.12 103.14 103.16 103.18 103.20Distance (m)
0
10
20
30
40
50
60
70
0 20 40 60 80 100
Last barrier = 12 nmLast barrier = 9 nm
Inte
rnal
qua
ntum
eff
icie
ncy
(%)
Current density (A/cm2)
Fig. 30. IQE as a function of current density of the LEDs with 12-nm and 9-nm last barriers.
Fig. 29. Radiative recombination rates of the LEDs with 12-nm and 9-nm last barriers at 55 A/cm2.
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.
38
Study 8: Thin last barrier
0
100
200
300
400
500
600
0
1
2
3
4
5
6
7
8
0 20 40 60 80 100
Last barrier = 12 nmLast barrier = 9.6 nm
Current density (A/cm2)
Pow
er (
a.u.
)
Vol
tage
(V
)
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.
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.
39
Some specific designs on band structure near the active region,
including the modifications of barrier material or thickness, th
e redesigns of the electron blocking layer (EBL), etc., in the bl
ue InGaN LEDs are investigated numerically with the APSY
S simulation program. Simulation results show that, with ap
propriate designs, the efficiency droop may be effectively i
mproved due to the increase of hole injection efficiency, en
hancement of blocking capability for electrons, or uniform
distribution of carriers in the active region. The methods pr
oposed in this paper are advantageous especially for high curr
ent injection.
Conclusion
Thank you for your attention !!
