advantages of blue ingan light- emitting diodes with slightly-doped step-like electron-blocking...
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Advantages of Blue InGaN Light-Emitting Diodes with Slightly-Doped Step-Like Electron-Blocking Layer
Tsun-Hsin WangTsun-Hsin WangPh.D. Candidate, Department of Physics, Ph.D. Candidate, Department of Physics, National Changhua University of EducationNational Changhua University of Education
Advisor: Prof. Yen-Kuang KuoAdvisor: Prof. Yen-Kuang Kuo
2
OutlineIntroduction and Motivation
Device Structure
Simulation Results
Conclusion
Reference
Tsun-Hsin Wang/BLL/NCUE
3
Introduction S. Pimputkar, J. S. Speck, S. P. DenBaars, and S.
Nakamura, Nat. Photonics 3, 180 (2009). More than one-fifth of US electricity is used to power artificial
lighting. Light-emitting diodes (LEDs) based on group III/nitride
semiconductors are bringing about a revolution in energy-efficient lighting.
Tsun-Hsin Wang/BLL/NCUE
4
Introduction E. F. Schubert and J. K. Kim, Science 308, 5276 (2005). Energy savings and environmental benefits Spectral power distribution Spatial distribution Color temperature Temporal modulation Polarization properties
Spontaneous polarization
=>Asymmetric wurtzite Piezoelectric polarization
=>Lattice mismatch
Tsun-Hsin Wang/BLL/NCUE
5
Motivation
Tsun-Hsin Wang/BLL/NCUE
Development of InGaN LEDsGaN-InGaN-GaN barriers
InGaN-AlGaN-InGaN barriers
Slightly-doped step-like electron blocking layer (EBL)
Shallow first well
Kuo et al., Appl. Phys. Lett. 99, 091107 (2011).
Kuo et al., Appl. Phys. Lett. 100, 031112 (2012).
Wang et al., IEEE Photonics Technol. Lett. (2012).
Kuo et al., IEEE Photonics Technol. Lett. 24, 1506 (2012).
Ene
rgy
(eV
)
quasi-Fermi level
p-side
GaN barriers (original)@ 300 mA
6
Device Structure
Tsun-Hsin Wang/BLL/NCUE
sapphire
i-GaN
n-GaN
n-GaN
i-InGaN/GaN
p-AlGaN
p-contact
n-contact
p-GaN
Kuo et al., Appl. Phys. Lett. 95, 011116 (2009).
Ene
rgy
(eV
)
quasi-Fermi level
p-side
GaN barriers (original)@ 300 mA
0
10
20
30
40
50
60
70
0 60 120 180 240 300
Current (mA)
IQE
(%
)
7Tsun-Hsin Wang/BLL/NCUE
sapphire
i-GaN
n-GaN
n-GaN
i-InGaN/GaN
p-AlGaN
p-contact
n-contact
p-GaN
Ene
rgy
(eV
)
quasi-Fermi level
p-side
GaN barriers (original)@ 300 mA
0
10
20
30
40
50
60
70
0 60 120 180 240 300
Current (mA)
IQE
(%
)
Device Structure
8Tsun-Hsin Wang/BLL/NCUE
sapphire
i-GaN
n-GaN
n-GaN
i-InGaN/GaN
p-AlGaN
p-contact
n-contact
p-GaN
Ene
rgy
(eV
)
quasi-Fermi level
p-side
GaN barriers (original)@ 300 mA
0
10
20
30
40
50
60
70
0 60 120 180 240 300
Current (mA)
IQE
(%
)
Device Structure
9
Device Structure Drawbacks of polarization electric field:
– Serious tilting of energy band – Severe leakage current of electrons – Insufficient injection efficiency of holes – Nonradiative Auger recombination induced by
non-uniform distribution of carriers
=> Efficiency droop!
Tsun-Hsin Wang/BLL/NCUE
10Tsun-Hsin Wang/BLL/NCUE
Device Structure
sapphire
i-GaN
n-GaN
n-GaN
i-InGaN/GaN
p-AlGaN
p-contact
n-contact
p-GaN compositiondoping (1018 cm–3)
conventional EBL (original)
Al0.15Ga0.85N 1.2
slightly-doped EBL Al0.15Ga0.85N 0.6
slightly-doped step-like EBL
Al0.15Ga0.85N
Al0.075Ga0.925N
GaN
Al0.075Ga0.925N
Al0.15Ga0.85N
0.6
Impact ionization– Hole concentration is conventionally 1% of dopant concentration.
11Tsun-Hsin Wang/BLL/NCUE
Simulation Results
1
2
3
4
Ene
rgy
(eV
)
quasi-Fermi level
(a) conventional EBL (original)@ 300 mA
p side
1
2
3
4
Ene
rgy
(eV
)
quasi-Fermi level
(b) slightly-doped EBL @ 300 mA
p side
1
2
3
4
104.64 104.65 104.66 104.67
Ene
rgy
(eV
)
quasi-Fermi level
(c) slightly-doped step-like EBL@ 300 mA
Distance (m)
p side
Effective potential height– Conduction band:
electron leakage current
– Valence band: hole injection efficiency
12Tsun-Hsin Wang/BLL/NCUE
Simulation Results
1
2
3
4
Ene
rgy
(eV
)
quasi-Fermi level
(a) conventional EBL (original)@ 300 mA
p side
1
2
3
4
Ene
rgy
(eV
)
quasi-Fermi level
(b) slightly-doped EBL @ 300 mA
p side
1
2
3
4
104.64 104.65 104.66 104.67
Ene
rgy
(eV
)
quasi-Fermi level
(c) slightly-doped step-like EBL@ 300 mA
Distance (m)
p side
Effective potential height– Conduction band:
electron leakage current
– Valence band: hole injection efficiency
13Tsun-Hsin Wang/BLL/NCUE
Simulation Results
1
2
3
4
Ene
rgy
(eV
)
quasi-Fermi level
(a) conventional EBL (original)@ 300 mA
p side
1
2
3
4
Ene
rgy
(eV
)
quasi-Fermi level
(b) slightly-doped EBL @ 300 mA
p side
1
2
3
4
104.64 104.65 104.66 104.67
Ene
rgy
(eV
)
quasi-Fermi level
(c) slightly-doped step-like EBL@ 300 mA
Distance (m)
p side
Last barrier– Two dimensional
electron gas (2DEG)
14Tsun-Hsin Wang/BLL/NCUE
Simulation Results
0
400
800
1200conventional EBL (original)slightly-doped EBLslightly-doped step-like EBL
104.55 104.60 104.65
@ 300 mA
Ele
ctro
n cu
rren
t den
sity
(A
/cm
2 )
Electron leakage current
Distance (m)
15Tsun-Hsin Wang/BLL/NCUE
Simulation Results
0
5
10
15
20
25
30 (a) conventional EBL (original)@ 300 mA
ElectronHole
Car
rier
con
cent
ratio
n (l
og)
(cm
3)
0
5
10
15
20
25
30 (b) slightly-doped EBL@ 300 mA
ElectronHole
Car
rier
con
cent
ratio
n (l
og)
(cm
3)
0
5
10
15
20
25
30 (c) slightly-doped step-like EBL@ 300 mA
ElectronHole
104.55 104.60 104.65
Car
rier
con
cent
ratio
n (l
og)
(cm
3)
Distance (m)
0.0
0.5
1.0
1.5
(a) conventional EBL (original)@ 300 mA
Rad
iati
ve r
ecom
bina
tion
rat
e
(1028
cm
3s1
)
0.0
0.5
1.0
1.5
(b) slightly-doped EBL@ 300 mA
Rad
iati
ve r
ecom
bina
tion
rat
e
(1028
cm
3s1
)
0.0
0.5
1.0
1.5
(c) slightly-doped step-like EBL@ 300 mA
104.55 104.60 104.65
Rad
iati
ve r
ecom
bina
tion
rat
e
(1028
cm
3s1
)
Distance (m)
16
Conclusion The advantages of blue InGaN LED
with slight-doped step-like EBL are studied numerically.
According to the simulation results, the LED has enhanced carrier concentrations in the QWs due to appropriately modified energy band diagrams which are favorable for the injection of holes without the price of confinement of electrons.
Tsun-Hsin Wang/BLL/NCUE
17
Reference
1. T.-H. Wang and Y.-K. Kuo, IEEE Photonics Technol. Lett. accepted (2012).
2. Y.-K. Kuo, T.-H. Wang, J.-Y. Chang, and J.-D. Chen, IEEE Photonics Technol. Lett. 24, 1506 (2012).
3. Y.-K. Kuo and T.-H. Wang, IEEE J. Quantum Electron. 48, 946 (2012).
4. Y.-K. Kuo, T.-H. Wang, and J.-Y. Chang, Appl. Phys. Lett. 100, 031112 (2012).
5. Y.-K. Kuo, T.-H. Wang, J.-Y. Chang, and M.-C. Tsai, Appl. Phys. Lett. 99, 091107 (2011).
Tsun-Hsin Wang/BLL/NCUE
18
Acknowledgement: This work was supported by the National Science Council under grant NSC-99-2119-M-018-002-MY3.
Thank you for your attention!
Tsun-Hsin Wang/BLL/NCUE
19
Q & A – Physical modelsPoisson equation: ∇2V=−ρ /ε, where ρ: volume charge density, ε: dielectric constant.Continuity equation: J+∂ρ/∂t=0, where J: ∇current density, t: time.Complex wave equation: ∇2W+k2(ε−β2)W=0, where W: optical wave function, k: wave vector, β: real eigen-value.Rate equation: ∂S/∂t=c(g−α)/n, where c: speed of light, n: refractive index, g: gain, α: loss, S: photon number.Gain equation: g=α+[ln(1/R1R2)]2L, where R: reflectance of mirrors, L: cavity length.
Tsun-Hsin Wang/BLL/NCUEAPSYS by Crosslight Software Inc.
20
Q & A – Physical models
Equations ParametersPoisson equation: V, n, p, S, W, gContinuity equation: V, n, pComplex wave equation: n, p, S, W, gRate equation: n, p, W, lambda, gGain equation: n, p, lambda, g
V: potential, n and p: electron and hole concentration, S: photon number, W: optical field intensity, lambda: wavelength, g: gain.
Tsun-Hsin Wang/BLL/NCUEAPSYS by Crosslight Software Inc.
Polarization
1
2
( )
( ) (1 ) ( ) (1 ) ( )
0.042 0.034 (1 ) 0.038 (1 )[ / ]
sp x x
sp sp sp
P In Ga N
x P InN x P GaN x x B InGaN
x x x x C m
1 1 1( ) ( ) ( )total x x sp x x pz x xP In Ga N P In Ga N P In Ga N
1
2
( )
( ) (1 ) ( ) (1 ) ( )
0.090 0.034 (1 ) 0.021 (1 )[ / ]
sp x x
sp sp sp
P Al Ga N
x P AlN x P GaN x x B AlGaN
x x x x C m
Vurgaftman et al., J. Appl. Phys. 94, 3675 (2003).
1 1 1( ) ( ) ( )total x x sp x x pz x xP Al Ga N P Al Ga N P Al Ga N
21Tsun-Hsin Wang/BLL/NCUE
Q & A – Parameters
Polarization
0xx yy
a a
a
13
33
2zz xx
C
C
1( ) ( ) (1 ) ( ) ( )pz x x pz pz pzP Al Ga N x P AlN x P GaN x P AlN 2 2( ) 1.27 7.56 [ / ]pzP InN C m
2 2( ) 1.81 7.89 [ / ]pzP AlN C m
1( ) ( ) (1 ) ( ) ( )pz x x pz pz pzP In Ga N x P InN x P GaN x P InN
Wu, J. Appl. Phys. 106, 011101 (2009).
2 2( ) 0.918 9.541 [ / ]pzP GaN C m
22Tsun-Hsin Wang/BLL/NCUE
Q & A – Parameters
Energy band gap
Wu, J. Appl. Phys. 106, 011101 (2009).
2 20.91( , ) ( ,0) 3.5 [ ]
830GaN
g gGaN
T TE GaN T E GaN eV
T T
2 20.41( , ) ( ,0) 0.69 [ ]
454InN
g gInN
T TE InN T E InN eV
T T
2 21.8
( , ) ( ,0) 6.3 [ ]1462
AlNg g
AlN
T TE AlN T E AlN eV
T T
23Tsun-Hsin Wang/BLL/NCUE
Q & A – Parameters
Energy band gap
1( )
( ) (1 ) ( ) (1 ) ( )
0.69 3.5 (1 ) 1.4 (1 )[ , 0 ]
g x x
g g g
E In Ga N
x E InN x E GaN x x B InGaN
x x x x eV T K
1( )
( ) (1 ) ( ) (1 ) ( )
6.3 3.5 (1 ) 0.6 (1 )[ , 0 ]
g x x
g g g
E Al Ga N
x E AlN x E GaN x x B AlGaN
x x x x eV T K
Wu, J. Appl. Phys. 106, 011101 (2009).
24Tsun-Hsin Wang/BLL/NCUE
Q & A – Parameters
Mobility
2
2
( ) 2
:
[ / ]
( ) 10[ / ]
h
h
InGaN cm V s
Ho
AlGaN cm V s
le
Kuo et al., IEEE J. Quantum Electron. 46, 1214 (2010).
max minmin
2
1.3717
2
0.2917
( )1 ( )
298( , ) 386 [ / ]
1 ( )1.0 10
174( , ) 132 [ / ]
1 ( )1.0
:
10
e
ref
e
e
NN
N
InGaN N cm V sN
AlGaN N cm V sN
Electron
25Tsun-Hsin Wang/BLL/NCUE
Q & A – Parameters
Recombination rate
Kuo et al., IEEE J. Quantum Electron. 46, 1214 (2010).
2 3
2
2 3
3
2 3
Recombination=Radiative+Nonradiative
Nonradiative=Shockley-Read-Hall(SRH)+Auger
1SRH Radiative Auger
SRH
Radiative
Auger
A n
A n B n C n
B n
A n B n C n
C n
A n B n C n
7
11 3
34 6
3.3 10 [1/ ]
2 10 [ / ]
1 10 [ / ]
A s
B cm s
C cm s
26Tsun-Hsin Wang/BLL/NCUE
Q & A – Parameters
Efficiency droop
max min
max
100%IQE IQE
Efficiency droopIQE
27Tsun-Hsin Wang/BLL/NCUE
Q & A – Parameters