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Introduction of Master's thesis of JihYuan Chang and Wen-Wei Lin
Speaker:Meng-Lun Tsai
National Changhua University of Education
2015/7/17
National Changhua University of Education
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Electronic Current Overflow and
Inhomogeneous Hole Distribution of the
InGaN Quantum Well Structures
Master's thesis of Jih-Yuan Chang
2015/7/17
National Changhua University of Education
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Outline
Introduction
Electronic Current Overflow of the InGaN
SQW Structures
Inhomogeneous Hole Distribution of the
InGaN Quantum Well Structures
Conclusion
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Introduction
The InGaN materials have important application in visible light-emitting
diodes (LED) and short-wavelength laser diodes. In this work Jih-yuan
investigate the electronic current overflow and the inhomogeneous hole
distribution of the blue InGaN quantum well structures with a LASTIP
(abbreviation of LASer Technology Integrated Program) simulation program.
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Reasons to electron current overflow
There are several causes for the electron
current overflow of III-V Nitrides：
high threshold current
narrow quantum well width
small conduction band-offset
poor hole injection to the active region
These four causes have important influence on
the degree of current overflow.
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Schematic diagram of the preliminary
laser diode structure
The reflectivities of the two end mirrors are 85 % and 90 %
respectively.
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The current distribution and L-I curves for the
preliminary structure
20
10
 = 462 nm
8
15
Laser Power (mW)
2
Current Density (kA/cm)
Overflow Current
Active-Layer Current
10
5
0
0
6
4
2
5
10
15
20
0
0
40
80
2
T ot al Current Densit y (kA/cm)
120
160
200
240
Current (mA)
The laser threshold current is 103.4 mA.
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2
Overflow Current Density (kA/cm )
Current distribution curves at various
p-doping levels
20
p-doping：11017 cm-3
p-doping：31017 cm-3
p-doping：51017 cm-3
p-doping：71017 cm-3
p-doping：11018 cm-3
15
10
5
0
0
5
10
15
20
2
Total Current Density (kA/cm )
The higher the p-doping level, the lower the percentage
of the electronic overflow current.
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L-I curves at various p-doping levels
Laser Power (mW)
20
p-doping：11017 cm-3
p-doping：31017 cm-3
p-doping：51017 cm-3
p-doping：71017 cm-3
p-doping：11018 cm-3
15
10
5
0
0
20
40
60
80
100
120
Current (mA)
The higher the p-doping level, the better the performance
of InGaN laser diode.
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2
Overflow Current Density (kA/cm )
Current distribution curves at various Al mole
fractions
20
Without p-AlGaN Layer
p-AlGaN (Al : 5 %)
p-AlGaN (Al : 10 %)
p-AlGaN (Al : 15 %)
p-AlGaN (Al : 20 %)
p-AlGaN (Al : 25 %)
p-AlGaN (Al : 30 %)
15
10
5
0
0
5
10
15
20
2
Total Current Density (kA/cm )
The higher the Al mole fraction, the lower the percentage
of the electronic overflow current.
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L-I curves at various Al mole fractions
Laser Power (mW)
20
Without p-AlGaN Layer
p-AlGaN (Al : 5 %)
p-AlGaN (Al : 10 %)
p-AlGaN (Al : 15 %)
p-AlGaN (Al : 20 %)
p-AlGaN (Al : 25 %)
p-AlGaN (Al : 30 %)
15
10
5
0
0
20
40
60
80
100
120
Current (mA)
The higher Al mole fraction, the better performance of
InGaN laser diode.
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The current distribution and L-I curves of the
improved structure
20
20
15
Laser Power (mW)
2
Current Density (kA/cm)
Overflow Current
Active-Layer Current
10
5
0
0
5
10
15
20
2
T ot al Current Densit y (kA/cm)
15
10
5
0
0
20
40
60
80
100
120
Current (mA)
The modified structure：  Al mole fraction : 10 %
 p-doping level : 11018 cm-3
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Laser output power as a function of input electric
power for the original and improved structures
Output Power (mW)
30
25
20
Initial Structure
Improved Structure
15
10
5
0
160
240
320
400
480
560
640
720
800
Electric Power (mW)
The improved structure has a better power conversion efficiency.
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Threshold current as a function of temperature
for the original and improved structures
Threshold Current (mA)
240
200
I th  I 0  e
T
T0
160
Initial Structure
（ T0 = 63.40 K ）
120
Improved Structure
（ T0 = 208.60 K ）
80
40
300
310
320
330
340
350
Temperature (K)
The improved structure is more stable, especially for
high-temperature operation.
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L-I curves of the InGaN laser structures of
different quantum well numbers.
20
Double QWs
Single QW
Triple QWs
Laser Power (mW)
15
10
5
0
0
20
40
60
80
Current (mA)
100
0
100 200 300 400 500 600 700 0
Current (mA)
500
1000
1500
2000
Current (mA)
With the increase of the quantum well number, the
performance of the InGaN laser diodes decreases.
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4
4
3
3
Energy (eV)
Energy (eV)
Energy band diagram of the triple-QW structure
2
2
1
1
0
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.19
0.2
Distance (m)
0.21
0.22
0.23
0.24
Distance (m)
The quasi-Fermi level in the valance band is not quite continuous
 inhomogeneity of hole distribution among quantum wells.
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Carrier concentration distribution of the
triple-QW structure
20
-3
Hole Concentration (log) (cm)
-3
Electron Concentration (log) (cm)
20
15
10
5
0
15
10
5
0
0.16
0.18
0.2
0.22
0.24
0.26
0.28
0.16
0.18
Distance (m)
0.2
0.22
0.24
0.26
0.28
Distance (m)
It is obvious that the hole distribution among
quantum wells is quite inhomogeneous.
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6
23
1.2
1
0.8
0.6
0.4
0.2
0
0
8
-3
1.4
Stimulated Recombination Rate (10 cm /s)
28
-3
Radiative Recombination Rate (10 cm /s)
Spontaneous and stimulated emission diagrams
of the triple-QW structure
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
4
2
0
-2
-4
-6
-8
0
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
Distance (m)
Distance (m)
The right quantum well has the most spontaneous emission.
The right quantum well is the only quantum well that possesses
positive stimulated emission.
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2.5
-3
Stimulated Recombination Rate (10 cm /s)
Carrier concentration and stimulated diagrams when
the barriers have a p-doping level of 2.3  1019 cm-3
28
-3
Concentration (log) (cm )
20
15
10
5
Hole Concentration
Electron Concentration
0.18
0.19
0.2
0.21
0.22 0.23
Distance (m)
0.24 0.25
2
1.5
1
0.5
0
0
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
Distance (m)
The distribution of hole concentration is much more homogeneous.
All three quantum wells contribute to stimulated emission.
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L-I curves for various doping concentration of
the barriers
Laser Power (mW)
20
p-doping：2.31019 cm-3
p-doping：2.51019 cm-3
p-doping：2.71019 cm-3
p-doping：3.01019 cm-3
p-doping：3.31019 cm-3
15
10
5
0
0
20
40
60
80
100
120
Current (mA)
When the doping level is at 31019 cm-3, the threshold current is
43.22 mA and the slope efficiency is 25.74%.
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L-I curves for SQW and Triple-QW structures
Laser Power (mW)
20
15
Single QW (without p-barrier doping)
Triple QWs (with p-barrier doping)
10
5
0
0
20
40
60
80
100
Current (mA)
The triple-QW structure has better laser performance.
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Conclusion
It is found that this electronic current overflow is severe in the single
quantum well InGaN laser structure at room temperature, especially
when the p-doping is low. Increasing the p-doping level and using an
AlGaN stopper layer in the p-side can resolve this problem. In addition to
the improvement of laser performance at room temperature, the improved
InGaN laser structure has a higher characteristic temperature and hence is
less sensitive to temperature.
Jih-Yuan have also investigated the deterioration of the laser performance
of the multiple quantum well InGaN lasers caused by the inhomogeneous
distribution of the holes inside the active region. It happens due to the
difficulty for the holes to transport from one quantum to another. JihYuan have proposed to p-dope the barriers between wells to help the
holes to transport and thus help solve the problem of inhomogeneous
hole distribution.
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Theoretical Investigation on Band
Structure of the BAlGaInN
Semiconductor Materials
Master's thesis of Wen-Wei Lin
2015/7/17
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Content
The band-gap energy-gap bowing parameter of the
wurtzite InGaN,AlGaN,AlInN alloys are investigated
numerically with the CASTEP simulation program by
Wen-Wei Lin
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Simulation for the WZ-InGaN
In this simulation , Indium will be constricted between 0
and 0.375.And the lattice constants of the unstrained InGaN
layer depend linearly on the indium composition.
a(x) = 3.501 (x) + 3.162 (1-x)
b(x) = 3.501 (x) + 3.162 (1-x)
c(x) = 5.669 (x) + 5.142 (1-x)
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Band-Gap Energy (eV)
WZ-InxGa1-xN band gap energy
3.6
3.4
3.2
3.0
2.8
2.6
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Indium Composition, x
Eg(x) = x · Eg,InN + (1-x) ·Eg,GaN - b · x · (1-x)
To use this formula to fit the results, and obtain bowing
parameter of 1.210 eV
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Simulation for the WZ-AlGaN
Since AlGaN have large energy band gap , so it is usually
used as barrier of active layer or DBR material.
Since the energy band gap structure of the AlGaN is direct in the whole
range of the aluminum composition, wen-wei study the characteristics of
the AlGaN for the aluminum composition to be between zero and one.
The lattice constants of the unstrained AlGaN layer depend linearly on the
aluminum composition.
a(x) = 3.082 (x) + 3.162 (1-x)
b(x) = 3.082 (x) + 3.162 (1-x)
c(x) = 4.948 (x) + 5.142 (1-x)
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WZ-AlxGa1-xN band gap energy
Band-Gap Energy (eV)
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Aluminum Composition, x
Eg(x) = x · Eg,AlN + (1-x) ·Eg,GaN - b · x · (1-x)
To use this formula to fit the results, and obtain bowing
parameter of 0.353 eV
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Simulation for the WZ-AlInN
Compared to the InGaN and AlGaN alloys, the third ternary nitride
alloy ,AlInN ,is less investigated.This alloys exhibits the largest
variation in band gap and it is a candidate for lattice matched
confinement layers in optical devices.
Wen-wei study the characteristics of the AlInN for the aluminum composition
to be between zero and one.
The lattice constants of the unstrained AlGaN layer depend linearly on the
aluminum composition.
a(x) = 3.082 (x) + 3.501 (1-x)
b(x) = 3.082 (x) + 3.501 (1-x)
c(x) = 4.948 (x) + 5.669 (1-x)
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WZ-AlGaInN band gap energy
Band-Gap Energy (eV)
7.0
6.0
5.0
4.0
InxGa1-xN
AlxGa1-xN
AlxIn1-xN
3.0
2.0
1.0
0
0.2
0.4
0.6
0.8
1
Composition, x
Eg(x) = x · Eg,AlN + (1-x) ·Eg,InN - b · x · (1-x)
To use this formula to fit the results, and obtain bowing
parameter of 3.326 eV
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WZ-AlGaInN
177
AlN
6
207
5
248
4
310
3
GaN
413
2
1
InN
Wavelength (nm)
Band-Gap Energy (eV)
7
620
1240
3.1
3.2
3.3
3.4
3.5
Lattice Constant (Angstrom)
Band-gap energy and corresponding wavelength of the
InGaN,AlGaN, and AlInN as a function of the lattice constant
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