Engineering of Hole Transport in Tunneling Injected UV -A LEDs · Tunnel-injected UV LEDs . [email protected]; [email protected] • Motivation • Tunnel-injected UV LEDs enable

[email protected]; [email protected] Tunnel-injected UV LEDs

Engineering of Hole Transport in Tunneling Injected UV-A LEDs

Yuewei Zhang, Sriram Krishnamoorthy, Fatih Akyol, Zane Jamal-Eddine Siddharth Rajan

ECE, The Ohio State University

Andrew Allerman, Michael Moseley, Andrew Armstrong Sandia National Labs

Funding: NSF EECS-1408416

mailto:[email protected]


[email protected]; [email protected] Tunnel-injected UV LEDs 2

• Motivation

• Tunnel-injected UV LEDs enable engineering of hole doping

• CV measurement to probed compensation impurities

• Acceptor free UV LEDs

• High efficiency UV LEDs

• Summary

Outline: Tunnel-injected UV LED




• Motivation





• Summary





Motivation

• UV lighting market is increasing.

• UV LEDs are replacing the traditional UV lamps.

UV C UV B UV A 400 nm 315 nm 280 nm 100 nm

UV curing

Printing

Sensing

Phototherapy

Medical imaging

Protein analysis

Drug discovery Sterilization

Sensing

Disinfection

DNA sequencing

Y. Muramoto, Semicond. Sci. Technol. 29 (2014) 084004.




Why we need tunnel-injected UV LEDs

• Dramatic decrease of WPE for shorter wavelengths.

• WPE < 6% for state-of-the-art UV LEDs

200 250 300 350 400 4501E-3

0.01

0.1

1

10

100

EQE WPE

Effic

iency

(%)

Wavelength (nm)

UVC

UVBUVA




P-GaN

P-AlGaN

P-AlGaN/AlGaN SL

MQW N-AlGaN

P-type contact

Conventional UV LEDs


Na=1 x 1019 cm-3 GaN: 140 meV, Na-=7 x 1017 cm-3 AlN: 630 meV, Na-=6 x 1013 cm-3

Absorption loss

High resistance




P-GaN

P-AlGaN

P-AlGaN/AlGaN SL

MQW N-AlGaN

P-type contact N-AlGaN

P-AlGaN

Tunnel Junction

MQW N-AlGaN

N-type contact

Low resistance

Transparent

Conventional UV LEDs TJ-UV LEDs


Na=1 x 1019 cm-3 GaN: 140 meV, Na-=7 x 1017 cm-3 AlN: 630 meV, Na-=6 x 1013 cm-3

Absorption loss

High resistance Efficient hole injection

• Reduced light absorption loss • Enhanced injection efficiency

VLED e-

h+

e- Ec

Ev




Overview of the tunnel junction technology

1 2 3 4 510-8

10-6

10-4

10-2

100

102

GaA

s

GaS

b/In

As InP

GaN

AlG

aAs/

InAl

GaP

TJ

resis

tanc

e (Ω

cm2 )

Bandgap (eV)

Polarization engineered tunnel junctions at OSU

GaN TJs: UCSB, JT Leonard, APL 107 (9), 091105 (2015) Meijo/ Nagoya, M Kaga, JJAP 52 (8S), 08JH06 (2013) EPFL, M Malinverni, APL 107, 051107, (2015). OSU, S. Krishnamoorthy, Nano Lett., 13, 2570 (2013) OSU, S. Krishnamoorthy, APL 102, 113503 (2013) OSU, F. Akyol, APL 108 (13), 131103 (2016).





1 2 3 4 510-8

10-6

10-4

10-2

100

102

Al0.

3Ga 0.

7N

GaA

s

GaS

b/In

As InP

GaN

AlG

aAs/

InAl

GaP

TJ

resis

tanc

e (Ω

cm2 )

Bandgap (eV)


UV TJs Y. Zhang, APL, 106 (14), 141103 (2015). Y. Zhang, 73rd DRC, 69 (2015). Y. Zhang, APEX 9 (5), 052102 (2016). Y. Zhang, APL 109 (12), 121102

280 300 320 340 360 380 400 4200

1x104

2x104

3x104

4x104

Smooth Rough

Inte

nsity

Wavelength (nm)

200 400 600 800101

102

103

104

Inte

nsity

Wavelength (nm)

326 nm emission

2014






1 2 3 4 510-8

10-6

10-4

10-2

100

102

Al0.

55G

a 0.45

N

Al0.

3Ga 0.

7N

GaA

s

GaS

b/In

As InP

GaN

AlG

aAs/

InAl

GaP

TJ

resis

tanc

e (Ω

cm2 )

Bandgap (eV)


2014

2015

250 300 350 4000

1x104

2x104

3x104

4x104

Inten

sity

Wavelength (nm)

0.3 mA to 6 mA

30um device

292 nm emission

UV TJs Y. Zhang, APL, 106 (14), 141103 (2015). Y. Zhang, 73rd DRC, 69 (2015). Y. Zhang, APEX 9 (5), 052102 (2016). Y. Zhang, APL 109 (12), 121102





Challenge: p-AlGaN compensation

N-AlGaN

P-AlGaN

Tunnel Junction

MQW N-AlGaN

N-type contact

TJ-UV LEDs

Buried p-AlGaN layer • MBE is better than MOCVD




N-AlGaN

P-AlGaN

Tunnel Junction

MQW N-AlGaN

N-type contact

TJ-UV LEDs


Buried p-AlGaN layer • MBE is better than MOCVD

500 550 600 650 700

1E19

1E20

1E21

Mg c

once

ntra

tion (

cm-3)

Growth temperature (°C)

MBE growth: • High growth temperature is preferred

for AlGaN • Mg incorporation reduces with

increasing temperature • Compensation shows up at low Mg

doping level Compensation

Oxygen/ Carbon/ Native defects – vacancies/ dislocations APL 94, 091903 (2009) PRB 65, 155212 (2002)





0 50 100 150 200-4-3-2-1012345

Ener

gy (e

V)

Thickness (nm)

Proper p-n junction

Without compensation





Compensation of the acceptors • Increased depletion of the p-AlGaN layer • Increased tunneling barrier • Reduced hole injection

0 50 100 150 200-4-3-2-1012345

Ener

gy (e

V)

Thickness (nm)0 50 100 150 200

-4-3-2-1012345

Ener

gy (e

V)

Thickness (nm)

Proper p-n junction

Without compensation With compensation





Compensation of the acceptors • Increased depletion of the p-AlGaN layer • Increased tunneling barrier • Reduced hole injection

0 50 100 150 200-4-3-2-1012345

Ener

gy (e

V)

Thickness (nm)0 50 100 150 200

-4-3-2-1012345

Ener

gy (e

V)

Thickness (nm)

Proper p-n junction

Without compensation With compensation

How to determine the compensation impurity density?





• Motivation

• Tunnel-injected UV LEDs enable analysis of hole doping




• Summary




Tunnel-injected UV LED structure

Sample Mg doping (cm-3)

A 0

B 7 x 1018

C 3 x 1019

Epitaxial Structures

MBE growth Sharp interfaces Sharp doping profile

N

Tunnel junction

P

N

Al0.3Ga0.7N Template

50 nm n-Al0.3Ga0.7N

4 nm In0.25Ga0.75N

150 nm n+ Al0.3Ga0.7N

400 nm n+ Al0.3Ga0.7N

1.5 nm AlN

15 nm graded n++ AlGaN

QW ×3

20 nm p-AlGaN

Al0.3Ga0.7N Al0.75Ga0.25N

+c

QWs




0 10 20 30 40 50 60 70 80 90-5-4-3-2-1012345

Ener

gy (e

V)

Thickness (nm)

0

NA-

+ρ3D

-σ

+σ

-ρ3D Depletion charges

ND+

𝐹 𝐹

n+ n++

InGaN

p-grading QWs n

+ρ’3D

-ρ3D

Tunnel-injected UV LED structure

𝑁𝑁𝐴𝐴∗ = 𝑁𝑁𝐴𝐴 − 𝑁𝑁𝑖𝑖𝑖𝑖𝑖𝑖

Polarization grading => Formation of p-n diode even without Mg doping.

NA* = 0

P-AlGaN grading: • Higher barrier to block overflowing

electrons • Provides high density polarization

charge 20 nm grading from Al0.75Ga0.25N to Al0.3Ga0.7N => ρπ = 1.5 x 1019 cm-3

J Simon, Science 327 (5961), 60-64.




C-V results

A: 20 nm, [Mg] = 0

• A shows nearly constant capacitance Modulation of 2D carriers

• Depletion width is 24 nm. Matches p-AlGaN layer thickness Full depletion of the whole p-AlGaN

layer -5 -4 -3 -2 -1 0

2.0

2.5

3.0

3.5

4.0

Capa

citan

ce (1

0-7 F/

cm2 )

Voltage (V)

A

20 25 30 35 40 45 50

3x1018

1019

1020

N eff (c

m-3)

Depletion Width (nm)

23 24 251019

1020

1021

N eff (c

m-3)

Width (nm)

A




C-V results

A: 20 nm, [Mg] = 0

• A shows nearly constant capacitance Modulation of 2D carriers

• Depletion width is 24 nm. Matches p-AlGaN layer thickness Full depletion of the whole p-AlGaN

layer -5 -4 -3 -2 -1 0

2.0

2.5

3.0

3.5

4.0

Capa

citan

ce (1

0-7 F/

cm2 )

Voltage (V)

20 25 30 35 40 45 50

3x1018

1019

1020

N eff (c

m-3)


23 24 251019

1020

1021

N eff (c

m-3)

Width (nm)

A

0 10 20 30 40 50 60 70 80 90-5-4-3-2-1012345

Ener

gy (e

V)

Thickness (nm)

Ceff

NA* < 0

A




C-V results

A: [Mg] = 0 B: [Mg] = 7x1018 cm-3

-5 -4 -3 -2 -1 0

2.0

2.5

3.0

3.5

4.0

Capa

citan

ce (1

0-7 F/

cm2 )

Voltage (V)

B

20 25 30 35 40 45 50

3x1018

1019

1020

N eff (c

m-3)


23 24 251019

1020

1021

N eff (c

m-3)

Width (nm)

A B

0 10 20 30 40 50 60 70 80 90-5-4-3-2-1012345

Ener

gy (e

V)

Thickness (nm)

Ceff

• B shows similar behavior as A Fully compensated.

NA* < 0

A




0 10 20 30 40 50 60 70 80 90-5-4-3-2-1012345

Ener

gy (e

V)

Thickness (nm)

C-V results

-5 -4 -3 -2 -1 0

2.0

2.5

3.0

3.5

4.0

Capa

citan

ce (1

0-7 F/

cm2 )

Voltage (V)

B

20 25 30 35 40 45 50

3x1018

1019

1020

N eff (c

m-3)


23 24 251019

1020

1021

N eff (c

m-3)

Width (nm)

A B

Ceff

• B shows similar behavior as A Fully compensated.

• Full depletion starts at NA*=NA- Nimp= −5x1018 cm-3 from energy band diagrams

Donor-type compensating impurity density is Nimp =1.2x1019 cm-3

NA* < 0

A: [Mg] = 0 B: [Mg] = 7x1018 cm-3

A




C-V results

-5 -4 -3 -2 -1 0

2.0

2.5

3.0

3.5

4.0

Capa

citan

ce (1

0-7 F/

cm2 )

Voltage (V)

B C

20 25 30 35 40 45 50

3x1018

1019

1020

N eff (c

m-3)


23 24 251019

1020

1021

N eff (c

m-3)

Width (nm)

A B

C

C: [Mg] = 3x1019 cm-3

• C shows sharp decrease of the capacitance with increased reverse bias Normal p-n junction behavior

A




C-V results

• C shows sharp decrease of the capacitance with increased reverse bias Normal p-n junction behavior

• Effective doping density reach ~ 3x1018 cm-3 Match the doping density in n-AlGaN Depletion of the n-AlGaN layer

0 10 20 30 40 50 60 70 80 90-5-4-3-2-1012345

Ener

gy (e

V)

Thickness (nm)

CTJ CPN

NA*>> ND

-5 -4 -3 -2 -1 0

2.0

2.5

3.0

3.5

4.0

Capa

citan

ce (1

0-7 F/

cm2 )

Voltage (V)

B C

20 25 30 35 40 45 50

3x1018

1019

1020

N eff (c

m-3)


23 24 251019

1020

1021

N eff (c

m-3)

Width (nm)

A B

C

C: [Mg] = 3x1019 cm-3

A




Origin of the compensation impurities

Donor-type compensating impurity density is 1.2x1019 cm-3 -- O/C/ native defects?




Origin of the compensation impurities

• [C]/ [O] < 3x1017 cm-3 for the AlGaN layers grown under similar growth conditions.

• Much lower than the compensation impurity density 1.2x1019 cm-3

• Indicates high density of native defects (N vacancies, dislocations) • Higher material quality is necessary

Donor-type compensating impurity density is 1.2x1019 cm-3 -- O/C/ native defects?




A: [Mg]=0 B: 7 × 1018 cm-3 C: 3 × 1019 cm-3

I-V characteristics

-3 0 3 6 90

200

400

600

800

1000

Curre

nt D

ensit

y (A/

cm2 )

Voltage (V) 1 10 100 1000

10-3

10-2

10-1

Resis

tanc

e (Oh

m cm

2 )

Current Density (A/cm2)-3 0 3 6 910-8

10-5

10-2

101

104

Curre

nt D

ensit

y (A/

cm2 )

Voltage (V)

• Increasing doping density from 0 to 3x1019 cm-3 • Reduced turn-on voltage

• 6.2 => 5.7 V • Reduced on-resistance

Increasing doping Increasing

doping




300 330 360 390 420

Wavelength (nm)

Inte

nsity

(a.u

.)

A: [Mg] = 0

750 A/cm2

B: 7×1018 cm-3

222 A/cm2

C: 3×1019 cm-3

200 A/cm2

250 ~ 750 A/cm2

56 ~ 667 A/cm2

20 ~ 440 A/cm2

EL characteristics

• Single peak emission at 325 nm for all the samples.




EL characteristics


• A and B show highly non-uniform emission

• InGaN/ AlGaN compositional fluctuations

• Conduction through low tunneling barrier paths

300 330 360 390 420

Wavelength (nm)

Inte

nsity

(a.u

.)

A: [Mg] = 0

750 A/cm2

B: 7×1018 cm-3

222 A/cm2

C: 3×1019 cm-3

200 A/cm2

250 ~ 750 A/cm2

56 ~ 667 A/cm2

20 ~ 440 A/cm2




300 330 360 390 420

Wavelength (nm)

Inte

nsity

(a.u

.)

A: [Mg] = 0

750 A/cm2

B: 7×1018 cm-3

222 A/cm2

C: 3×1019 cm-3

200 A/cm2

250 ~ 750 A/cm2

56 ~ 667 A/cm2

20 ~ 440 A/cm2

EL characteristics


• A and B show highly non-uniform emission

• InGaN/ AlGaN compositional fluctuations

• Conduction through low tunneling barrier paths

Demonstration of Mg free UV LED based on tunneling hole injection.




EL characteristics

0 5 10 150.00.20.40.60.81.01.21.4

Powe

r (m

W)

Current (mA)A

B

C

0 5 10

1E-4

1E-3

0.01

0.1

1

Powe

r (m

W)

Current (mA)

A

B

C

• On-wafer measurement, no integrating sphere. Power values come from direct power reading from the spectrometer coupled with a fiber optic cable.

• Power increases from < 1 uW to above 1.4 mW when acceptor doping density is increased from 0 to 3x1019 cm-3.

1.4 mW @ 12 mA 56 W/cm2 @ 480 A/cm2

NA ↑ NA ↑




0 200 4000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5EQ

E (%)

Current Density (A/cm2)

0 200 400 600 8001E-3

0.01

0.1

1


EQE (

%)

EL characteristics

0 200 400

0.0

0.5

1.0

1.5


WPE

(%)

0 200 400 600 8001E-4

1E-3

0.01

0.1

1

WPE

(%)


A: [Mg]=0 B: 7 × 1018 cm-3

C: 3 × 1019 cm-3

A

B

C

A

B

C NA ↑ NA ↑

• On-wafer EQE of 3% • On-wafer WPE of 1.6%, this is compromised by the voltage drop

across the EBL and TJ layers.

Enhanced interband tunneling injection by overcoming compensation.




Compare to state-of-the-art

OSU on-wafer

EQE and WPE values are comparable to state-of-the-art

On-wafer, no integrating sphere

J Rass, Proc. SPIE 9363, 93631K (2015)




• Motivation





• Summary





Summary

• UV TJs for up to Al0.7Ga0.3N achieved

200 250 300 350 400 4500.0

0.2

0.4

0.6

0.8

1.0

Inte

nsity

(a.u

.)

Wavelength (nm)




Summary

• C-V measurement is used to extract compensating impurity density of 1.2x1019 cm-3 in the p-AlGaN layer.


200 250 300 350 400 4500.0

0.2

0.4

0.6

0.8

1.0

Inte

nsity

(a.u

.)

Wavelength (nm)




Summary

• Acceptor free UV LED emitting at 325 nm achieved using polarization engineering.



200 250 300 350 400 4500.0

0.2

0.4

0.6

0.8

1.0

Inte

nsity

(a.u

.)

Wavelength (nm)




Summary

• Acceptor free UV LED emitting at 325 nm achieved using polarization engineering.


• Using graded p-AlGaN, obtained record on-wafer EQE=3.37%, WPE=1.62% at 325 nm.


200 250 300 350 400 4500.0

0.2

0.4

0.6

0.8

1.0

Inte

nsity

(a.u

.)

Wavelength (nm)



Documents

Engineering of Hole Transport in Tunneling Injected UV -A LEDs · Tunnel-injected UV LEDs . [email protected]; [email protected] • Motivation • Tunnel-injected UV LEDs enable