Investigation of the Impact of Polarization and Auger Recombinationon the Wurtzite and Zincblende GaN-Based Green LEDs

Yi-Chia Tsai

Ph.D. Student

Department of Electrical and Computer Engineering

University of Illinois at Urbana-Champaign, Illinois, USA

Innovative COmpound semiconductoR LABoratory (ICORLAB)

PI: Prof. Can Bayram,

Assistant Professor

Department of Electrical and Computer Engineering

University of Illinois at Urbana-Champaign, IL, USA

EMAIL: cbayram@Illinois.edu WEBPAGE: icorlab.ece.Illinois.edu

Motivations

1. Why we need efficient green LEDs?

– To generate natural white light via color mixing

– To make semiconductor green lasers

– To save power

2. Green gap

– Phosphor-coated blue LED → Down-conversion → Energy loss

– Phosphide-based materials → Indirect bandgap → Low rad. rate

– Nitride-based green LEDs → Several factors might involve

2

• Possible mechanisms

– Shockley-Read-Hall recombination

– Auger recombination

– Polarization

– Thermal effect

– Electron leakage

• Solution: Zincblende GaN

– Zero polarization

– Smaller Auger coefficient

Issues of nitride-based green LEDs

3

Prohibitively impossible to be distinguished in

experiments → Simulations are paramount

1. Poisson Equation:

2. Schrödinger Equation:

3. Drift-Diffusion Equations:

4. Current Continuity Equations:

Quantum-Corrected DD Model

4

( ) ( )0   ,D A pP q p n N N n + = − − + − +

22

*2i i i ie E

m − + = ( )

22 i F i

i

n f E E = −

.

1

1

,n

n SRH Rad Au

p

ger

p

SRH Rad Auger

R R

RJ R

J Rq

Rq

−

= + +

+ = +

,

,p p p

n n nJ E qkT

J

n

q E qk

q

T

n

p p

+

−

=

=

Device Structure – 5QWs

5

p-GaN (NA = 1025 m-3) 0.1 µm

p-GaN (NA = 3 x 1023 m-3) 20 nm

In0.3Ga0.7N (5 nm)/i-GaN (5 nm) x 5 QWs

n-GaN (ND = 5 x 1024 m-3) 0.5 µm

n-GaN (ND = 5 x 1024 m-3) 2.5 µm

200 µm 100 µm

50 µm

70 µm

1. Short p-region

2. Fixed barrier thickness: 5 nm

3. In0.3Ga0.7N is used for QWs

– For 550 nm wavelength

Exemplification of Band Diagrams

6

0.00 0.02 0.04 0.06 0.08 0.10-1

0

1

2

3

4

5

6

Operated under V = 5V

Ba

nd P

ositio

n (

eV

)

Distance (m)

Hexagonal GaN

Cubic GaN

n-contact p-contact

e-

e-

h+

h+

Green Gap AnalysisIdentify the major mechanisms

7

❑ Single QW

❑ Thickness: 2.5 nm

❑ Varying In mole fraction

Spontaneous Emission

8

0.3 0.4 0.5 0.6 0.70

5

10

15

20

25

30

35

40

45

50

Tota

l S

p. R

ate

(10

20 s

-1eV

-1m

-1)

Wavelength (m)

Bias = 5V

In = 0.1

In = 0.2

In = 0.3

In = 0.4

In = 0.5

# of QW = 1

Thickness = 2.5 nm

❑ The increase of In mole fraction

– Deeper QW

– Increases wavelength emission

– Decreases spontaneous emission rate

– Why?

Recombination Processes and Leakage

9

0.1 0.2 0.3 0.4 0.5

0.0

0.2

0.4

0.6

0.8

1.0

Pe

ak R

eco

mbin

atio

n R

ate

s (

10

28 c

m-3

s-1

)

In mole fraction

Current density = 100 A/m

SRH

Radiative

Auger

1E-7

1E-6

1E-5

1E-4

0.001

0.01

0.1

1

10

100

Electron Leakage

Ele

ctr

on

Le

aka

ge

(A

-cm

-2)

# of QW = 1

Thickness = 2.5 nm

❑ In mole fraction = 0.1

– Shallow QW

– Strong electron leakage

❑ In mole fraction > 0.2

– Higher electron density

• Auger recomb. increases

– Stronger QCSE

• Radiative recomb. drops

Impact of Auger Coefficient

10

0 20 40 60 80 100 120 140 160 180 200

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

IQE

Current Density (A/m)

×102

×101

×1

×10-1

×10-2

×10-3

# of QW = 1

Thickness = 2.5 nm

❑ Default: 2.96 × 10−30 cm-6s-1

❑ Reduced by an order

– Double IQE

– Gain an extra 40% efficiency

❑ Reduced by more than two orders

– Droop-free performance

❑ Material engineering is necessary.

Impact of Polarization

11

0 20 40 60 80 100 120 140 160 180 2000.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

IQE

Current Density (A/m)

In = 0.1 (polar) In = 0.1 (non-polar)

In = 0.2 (polar) In = 0.2 (non-polar)

In = 0.3 (polar) In = 0.3 (non-polar)

In = 0.4 (polar) In = 0.4 (non-polar)

In = 0.5 (polar) In = 0.5 (non-polar)

# of QW = 1

Thickness = 2.5 nm

❑ Low carrier injection

– Triangular QW is formed

– Better carrier localization

– Only works for narrow QW

❑ Impact of polarization is stronger for

– Thick QW

– High carrier injection

❑ Zero polarization leads to a better IQE.

Thermal Effect – 250K to 500K

12

0 20 40 60 80 100 120 140 160 180 2000.1

0.2

0.3

0.4

IQE

Current Density (A/m)

# of QW = 1

Thickness = 2.5 nm

T = 250K

T = 300K

T = 350K

T = 400K

T = 450K

T = 500K

250 300 350 400 450 500-0.05

0.000.10

0.15

0.20

0.25

0.90

0.95

1.00

1.05

# of QW = 1

Thickness = 2.5 nm

Peak R

ecom

bin

ation R

ate

s (

10

28 c

m-3

s-1

)

Temperature (K)

Current density = 100 A/m

SRH

Radiative

Auger

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

Electron Leakage

Ele

ctr

on L

eakage (

A-c

m-2

)

(a)

(b)

Optimization of Wurtzite GaN-based Green LEDs

13

❑ # of QW: 1 - 5

❑ Thickness: 1 - 5 nm

❑ In mole fraction: 0.3

Wall-Plug Efficiency

14

20.0

12.8

18.216.4

9.20

14.6

11.0

12.8

424

542

542

519

495

471

447

1 2 3 4 5

1

2

3

4

5

Thic

kn

ess o

f Q

W (

nm

)

Numbers of QW

7.40

9.20

11.0

12.8

14.6

16.4

18.2

20.0

21.8

Wall-Plug Efficiency (%)

Operated under I = 100 A/m❑ Green emission (𝜆 > 500 𝑛𝑚)

– Thickness > 3 nm

❑ Issue of thick QW

– Polarization

– Low radiative rate

❑ Issue of multiple QWs

– Higher resistance

❑ Optimal efficiency

– Under 100 A/m carrier injection

– 12.8%

Optimization of Zincblende GaN-based Green LEDs

15

❑ # of QW: 1 - 5

❑ Thickness: 1 - 11 nm

❑ In mole fraction: 0.3

Wall-Plug Efficiency

16

47.8

44.7

41.0

37.3

33.629.9

26.222.5

538

536

531

527

519

495

471447

1 2 3 4 5

1

2

4

7

11

Thic

kn

ess o

f Q

W (

nm

)

Numbers of QW

18.8

22.5

26.2

29.9

33.6

37.3

41.0

44.7

48.4

Wall-Plug Efficiency (%)

Operated under I = 100 A/m

❑ Green emission (𝜆 > 500 𝑛𝑚)

– Thickness > 3 nm

❑ Thick QW

– Higher electron concentration

– Higher radiative recombination

❑ Multiple QWs

– Higher resistance

❑ Optimal efficiency

– Under a constant current of 100 A/m

– 47.8% (3.7 times higher than 12.8%)

Conclusion

1. Auger recombination and polarization are the key to

bridge green gap.

2. The highest wall-plug efficiency of wurtzite GaN-based

green LEDs under 100 A/m carrier injection is 12.8%.

3. Yet, the highest wall-plug efficiency of zincblende GaN-

based green LEDs under the same condition is 47.8%

even though it has the same Auger coefficient.

17

Thank you very much for your attention

18

Q&A2 mins

19