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