Advantages of Blue InGaN Light- Emitting Diodes with Slightly-Doped Step-Like Electron-Blocking...

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.


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


5

Motivation


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).


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


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


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


0

10

20

30

40

50

60

70

0 60 120 180 240 300

Current (mA)

IQE

(%

)

Device Structure


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


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!



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.


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


Simulation Results

1

2

3

4

Ene

rgy

(eV

)

quasi-Fermi level


p side

1

2

3

4

Ene

rgy

(eV

)

quasi-Fermi level


p side

1

2

3

4

104.64 104.65 104.66 104.67

Ene

rgy

(eV

)

quasi-Fermi level


Distance (m)

p side

Effective potential height– Conduction band:

electron leakage current

– Valence band: hole injection efficiency


Simulation Results

1

2

3

4

Ene

rgy

(eV

)

quasi-Fermi level


p side

1

2

3

4

Ene

rgy

(eV

)

quasi-Fermi level


p side

1

2

3

4

104.64 104.65 104.66 104.67

Ene

rgy

(eV

)

quasi-Fermi level


Distance (m)

p side

Last barrier– Two dimensional

electron gas (2DEG)


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)


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


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


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.


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).


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!


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


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



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



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).



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



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



Efficiency droop

max min

max

100%IQE IQE

Efficiency droopIQE



Advantages of Blue InGaN Light- Emitting Diodes with Slightly-Doped Step-Like Electron-Blocking...

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