Introduction of Master's thesis of Jih-Yuan Chang and Wen-Wei Lin

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Introduction of Master's thesis of Jih-Yuan Chang
and Wen-Wei Lin
SpeakerMeng-Lun Tsai 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
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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
? 462 nm
The laser threshold current is 103.4 mA.
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Current distribution curves at various p-doping
levels
p-doping1?1017 cm-3 p-doping3?1017
cm-3 p-doping5?1017 cm-3 p-doping7?1017
cm-3 p-doping1?1018 cm-3
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
The higher the p-doping level, the better the
performance of InGaN laser diode.
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Current distribution curves at various Al mole
fractions
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
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
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
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
Initial Structure ( T0 63.40 K )
Improved Structure ( T0 208.60 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.
Double QWs
Triple QWs
Single QW
With the increase of the quantum well number, the
performance of the InGaN laser diodes decreases.
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Energy band diagram of the triple-QW structure
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
It is obvious that the hole distribution among
quantum wells is quite inhomogeneous.
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Spontaneous and stimulated emission diagrams of
the triple-QW structure
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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Carrier concentration and stimulated diagrams
when the barriers have a p-doping level of 2.3 ?
1019 cm-3
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
p-doping2.3?1019 cm-3 p-doping2.5?1019
cm-3 p-doping2.7?1019 cm-3 p-doping3.0?1019
cm-3 p-doping3.3?1019 cm-3
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
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. Jih-Yuan 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
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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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WZ-InxGa1-xN band gap energy
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
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
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