sub 300 nm wavelength iii-nitride tunnel-injected ... 300 nm wavelength iii-nitride tunnel-injected...

[email protected]; [email protected] Sub 300 nm Wavelength III-Nitride Tunnel-Injected UV LEDs

Sub 300 nm Wavelength III-Nitride Tunnel-Injected Ultraviolet LEDs

Yuewei Zhang, Sriram Krishnamoorthy, Fatih Akyol, Sadia Monika Siddharth Rajan

ECE, The Ohio State University

Andrew Allerman, Michael Moseley, Andrew Armstrong Sandia National Labs

Jared Johnson, Jinwoo Hwang MSE, The Ohio State University

Funding: NSF EECS-1408416

mailto:[email protected]


[email protected]; [email protected] Sub 300 nm Wavelength III-Nitride Tunnel-Injected UV LEDs 2

Outline: Tunneling injected UV LED

• Motivation

• Polarization engineered III-Nitride tunnel junctions

• Tunneling junction for hole injection into UV LEDs.

• Electrical characteristics

• Optical characteristics

• Sub-300 nm emission

• Summary





• Motivation






• Summary




Motivation

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




Motivation

• UV lighting market is increasing.

• UV LEDs are replacing the traditional UV lamps.

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

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




Challenges for III-Nitride UV LEDs

Thermal Injection

Rass, Jens, et al. SPIE OPTO, 2015.

• Lack of high quality substrates • Poor hole injection • Poor light extraction • Poor p-type contact

Challenges:





Thermal Injection



Solved by growth optimization Challenges:






Thermal Injection


Solved by growth optimization

Caused by high acceptor activation energy in (Al)GaN

Challenges:






Thermal Injection


Solved by growth optimization

Caused by high acceptor activation energy in (Al)GaN

Challenges:

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




P-contact and light extraction

Trade-off between ƞinjection & ƞLEE

Current designs

p AlGaN

p GaN

MQW

n AlGaN

LED

p AlGaN

p AlGaN/AlGaN SL

MQW

n AlGaN

LED

Absorption loss

Electrical loss

Increased absorption losses

Increased voltage drop




P-contact and light extraction

Current designs Thin p AlGaN

Tunnel Junction

MQW

n AlGaN

n AlGaN

LED

p AlGaN

p GaN

MQW

n AlGaN

LED

p AlGaN

p AlGaN/AlGaN SL

MQW

n AlGaN

LED

Absorption loss

Electrical loss

TJ-UV LED

Tunneling injection




Non-equilibrium injection

Current designs Thin p AlGaN

Tunnel Junction

MQW

n AlGaN

n AlGaN

LED

p AlGaN

p GaN

MQW

n AlGaN

LED

p AlGaN

p AlGaN/AlGaN SL

MQW

n AlGaN

LED

Absorption loss

Electrical loss

TJ-UV LED

VLED e-

h+

e- Ec

Ev

• Replace p-type contact using tunneling contact.

• Non-equilibrium injection.

• Reduced light absorption loss • Better contacts.

Tunneling injection




0 20 40 60 80 100

1E15

1E16

1E17

1E18

Electrons Holes

Carri

er Co

ncen

tratio

n (cm

-3)

Depth (nm)

Electron and hole injection imbalance

Ea=0 meV

Ƞinj = Electrons injected into active region Total electrons

= Holes injected into active regionTotal electrons

~ 𝐽𝐽p

𝐽𝐽n

Va

N- Al0.3Ga0.7N P-Al0.3Ga0.7N

• For ideal junction, equal amount of e/ h are supplied to active region.

• Ƞinj = 1

Jp

Jn




Electron and hole injection imbalance

• For PN junction, hole current is much lower than electron current.

• Low injection efficiency.

Va


Ea=0 meV

Real case: 𝐽𝐽n < 𝐽𝐽p ⇒ Ƞinj << 1

Jp

Jn

Electron blocking layer is used to increase Ƞinj



~ 𝐽𝐽p

𝐽𝐽n

0 20 40 60 80 100

1E15

1E16

1E17

1E18

Electrons Holes

Carri

er Co

ncen

tratio

n (cm

-3)

Depth (nm)

Ea=0.22 meV




UV LED – real junction

0 20 40 60 80 100

1E15

1E16

1E17

1E18

Electrons Holes

Carri

er Co

ncen

tratio

n (cm

-3)

Depth (nm)

Ea=0.22 meV • For PN junction, hole current is much lower than electron current.

• Low injection efficiency. • Injection efficiency decreases

with increasing bandgap.

Va


Ea=0 meV

Real case: 𝐽𝐽n < 𝐽𝐽p ⇒ Ƞinj << 1

Jp

Jn

Electron blocking layer is used to increase Ƞinj



~ 𝐽𝐽p

𝐽𝐽n




Tunneling injection into UV LEDs

Ƞinj ~ 𝐽𝐽p

𝐽𝐽n ~ 𝐽𝐽tunnel

𝐽𝐽n

Jp = Jtunnel

Jn

Va

Thin p AlGaN

Tunnel Junction

MQW

n AlGaN

n AlGaN

LED

TJ-UV LED

e-

h+

e-

• Tunneling injection enables high hole current.

• Increased injection efficiency. • Injection efficiency not sensitive

to the increasing bandgap.




Tunneling injection into UV LEDs

• Tunneling injection enables high hole current.

• Increased injection efficiency. • Injection efficiency not sensitive

to the increasing bandgap.

Ƞinj ~ 𝐽𝐽p

𝐽𝐽n ~ 𝐽𝐽tunnel

𝐽𝐽n

Jp = Jtunnel

Jn

Va

Thin p AlGaN

Tunnel Junction

MQW

n AlGaN

n AlGaN

LED

TJ-UV LED

e-

h+

e-

Required TJ characteristics Voltage drop across TJ should be low On-resistance should be minimal Optical absorption should be minimal




What WPE can we achieve for UV/DUV LEDs? Conventional UV LED




What WPE can we achieve for UV/DUV LEDs? Conventional UV LED

Tunneling injected UV LED





• Motivation






• Summary




Polarization engineering for tunnel junctions

Standard p+/n+ TJ

• Large Eg wide depletion region • Doping limitations • Large energy barrier for tunneling

Low tunneling current density




Polarization engineering for tunnel junctions

n+ (Al)GaN p+ (Al)GaN

+σ

-σ

InG

aN

Standard p+/n+ TJ

• Large Eg wide depletion region • Doping limitations • Large energy barrier for tunneling

Low tunneling current density

Polarization TJ

• High density polarization sheet charge

depletion width greatly reduced. • Tunnel barrier reduced due to InGaN.

AlN barrier TJ: Previous Work M. J. Grundmann, PhD Dissertation (UCSB) J. Simon et.al., PRL 103, 026801 (2009) (Notre Dame)




Overview of the tunnel junction technology

1 2 3 4 510-8

10-6

10-4

10-2

100

102

GaAs

GaSb

/InAs

InP

GaN

AlGa

As/In

AlGa

P

TJ

resis

tance

(Ω cm

2 )

Bandgap (eV)

GaN/AlN GaN/AlN

GdN/GaN

InGaN/GaN

Nano Lett., 13, 2570 (2013)

APL 102, 113503 (2013)

Resistance down to 10-4 Ohm cm2 achieved for GaN tunnel junctions.




Overview of the tunnel junction technology

1 2 3 4 510-8

10-6

10-4

10-2

100

102

GaAs

GaSb

/InAs

InP

GaN

AlGa

As/In

AlGa

P

TJ

resis

tance

(Ω cm

2 )

Bandgap (eV)

GaN/AlN GaN/AlN

?

GdN/GaN

InGaN/GaN

Resistance down to 10-4 Ohm cm2 achieved for GaN tunnel junctions.

What would happen when we go to wider bandgap (AlGaN)?

Nano Lett., 13, 2570 (2013)

APL 102, 113503 (2013)




Modeling: tunneling current

• Self-consistent Schrodinger Poisson solution • WKB approximation for tunneling probability

calculation.

( )n p n nT p n z wkbJ q f f v T dEρ ρ= −∫∫

N- Al0.55Ga0.45N P-Al0.55Ga0.45N

In0.

2Ga 0.

8N

-4

-2

0

2

4

Ener

gy (e

V)

Depth (nm)

φn

φp




-4

-2

0

2

4

Ener

gy (e

V)

Depth (nm)

φn

φp

Modeling: tunneling current

1 10 100 10001E-4

1E-3

0.01

Resis

tanc

e (Oh

m cm

2 )

Current Density (A/cm2)

0 1 2 30

1k

2k

Curre

nt D

ensit

y (A/

cm2 )

Voltage (V)

• Resistance reaches 7E-4 Ohm cm2. • High current density could be achieved

with low voltage drop.

N- Al0.55Ga0.45N P-Al0.55Ga0.45N

In0.

2Ga 0.

8N

• Self-consistent Schrodinger Poisson solution • WKB approximation for tunneling probability

calculation.

( )n p n nT p n z wkbJ q f f v T dEρ ρ= −∫∫




0.2 0.4 0.6 0.8

10-6

10-5

10-4

10-3

TJ

Res

istan

ce (Ω

cm

2 )

InGaN composition

MODEL

Beyond the GaN bandgap: Design of AlGaN TJs

• Low resistance TJ could be created for high composition AlGaN.

• Hole injection could be achieved through high bandgap AlGaN TJs.

1 2 3 4 510-8

10-6

10-4

10-2

100

102

GaAs

GaSb

/InAs

InP

GaN

AlGa

As/In

AlGa

P

TJ

resis

tance

(Ω cm

2 )

Bandgap (eV)

GaN/AlN GaN/AlN

GdN/GaN

InGaN/GaN

70%

50%

30%

20%

10%

AlxGa1-xN ?





• Motivation






• Summary




MBE-grown TJ-UV LED

TJ

n-AlGaN top contact

Active region

• TJ as a tunneling contact to p-AlGaN • Enables extraction from top surface no need for flip chip bonding • Low spreading resistance in n-AlGaN reduced metal coverage

+c

QWs

TJ

-4 -2 0 2 4

150

100

50

0

Dept

h (nm

)

Energy (eV)N-Al0.3Ga0.7N on

Sapphire

50 nm n-Al0.3Ga0.7N [Si] =3 X 1018 cm-3

4 nm In0.25Ga0.75N

50 nm p-Al0.3Ga0.7N [Mg] = 2X 1019 cm-3

100 nm n-Al0.3Ga0.7N [Si] = 5 X 1019 cm-3

300 nm n-Al0.3Ga0.7N [Si] =1.2 X 1019 cm-3

12 nm p type Al0.46Ga0.54N

15 nm p+ -Al0.3Ga0.7N [Mg] = 5 X 1019 cm-3

15 nm n+ AlGaN [Si] = 1 X 1020 cm-3

QWs

50nm




MBE-grown TJ-UV LED

TJ

n-AlGaN top contact

• TJ as a tunneling contact to p-AlGaN • Enables extraction from top surface no need for flip chip bonding • Low spreading resistance in n-AlGaN reduced metal coverage

50nm

5nm

• Flat and sharp interfaces • Embedded p-AlGaN layer MBE is a better technique

for TJ-UV LED growth

Active region




TJ-UV LED – optical characteristics

280 300 320 340 360 380 400 4200

1x104

2x104

3x104

4x104

5x104In

tens

ity (a

.u.)

Wavelength (nm)

RT, CW 0.1mA to 20mA

50µm device

0 5 10 15 200.0

0.1

0.2

0.3

0.4

0.5

0.6

Powe

r (m

W)

Current (mA)

0 5 10 15 200.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6W

PE (%

)

EQE (

%)

Current (mA)0.2

0.4

0.6

0.8

1.0

1.2

1.4 • Single peak emission at 327 nm • Peak EQE and WPE are 1.5% and

1.08%, respectively. • At 120 A/cm2, voltage is 5.9 V,

power is 6 W/cm2. • Proof of efficient hole injection

through tunneling.

Y. Zhang, Appl. Phys. Lett. 106, 141103 (2015)

On-wafer measurement




TJ-UV LED – electrical characteristics

-6 -4 -2 0 2 4 60

1k

2k Full metal coverage L shape metal

Curre

nt D

ensit

y (A/

cm2 )

Voltage (V)

-6 -4 -2 0 2 4 61n100n10µ1m

100m101k

Curre

nt D

ensit

y (A/

cm2 )

Voltage (V)1 10 100 100010-4

10-3

10-2

10-1

100

Experiment Simulation

Resis

tanc

e (Ω

cm2 )


• Lowest TJ resistance of 5.6 x 10-4 Ohm cm2 is obtained for Al0.3Ga0.7N TJ

Forward Resistance = Rseries + RTJ + Rc

Voltage @ J=20 A/cm2

Voltage @ J=2 kA/cm2

R @ 2 kA/cm2

(50um*50um)

4.8 V 7.47 V 7.5E-04 Ohm cm2

1.9E-04 ~ 1E-06 Ohm cm2




TJ-UV LED – electrical characteristics

-6 -4 -2 0 2 4 60

1k

2k Full metal coverage L shape metal

Curre

nt D

ensit

y (A/

cm2 )

Voltage (V)

-6 -4 -2 0 2 4 61n100n10µ1m

100m101k

Curre

nt D

ensit

y (A/

cm2 )

Voltage (V)1 10 100 100010-4

10-3

10-2

10-1

100

Experiment Simulation

Resis

tanc

e (Ω

cm2 )


• Lowest TJ resistance of 5.6 x 10-4 Ohm cm2 is obtained for Al0.3Ga0.7N TJ

• Polarization engineered TJ enables orders of magnitude lower resistance.




TJ-UV LED – Sub-300 nm emission

+c

QWs

TJ

Al0.78Ga0.22N on Sapphire

50 nm n-Al0.55Ga0.45N [Si] =3 X 1018 cm-3

4 nm In0.2Ga0.8N

20 nm p-Al0.55Ga0.45N [Mg] = 2X 1019 cm-3

300 nm n-Al0.55Ga0.45N [Si] = 5 X 1019 cm-3

600 nm graded to n-Al0.55Ga0.45N

[Si] =1.2 X 1019 cm-3


15 nm p+ -AlGaN [Mg] = 5 X 1019 cm-3

15 nm n+ AlGaN [Si] = 1 X 1020 cm-3

QWs

030

6090

120

-4 -2 0 2 4

Energy (eV) Depth (nm

)

-6 -4 -2 0 2 4 6 8 10 120

200

400

600

800

1000

1200

Curre

nt D

ensit

y (A/

cm2 )

Voltage (V)

-6 -4 -2 0 2 4 6 8 10 121E-3

0.01

0.1

1

10

100

1000

Curre

nt D

ensit

y (A

/cm

2 )

Voltage (V)

1 10 100 10001E-4

1E-3

0.01

0.1

1

Resis

tanc

e (Oh

m cm

2 )


• Voltage @ 20A/cm2 is 7.1 V. • Resistance @ 1kA/cm2 is

1.6E-3 Ohm cm2





+c

QWs

TJ

Al0.78Ga0.22N on Sapphire

50 nm n-Al0.55Ga0.45N [Si] =3 X 1018 cm-3

4 nm In0.2Ga0.8N

20 nm p-Al0.55Ga0.45N [Mg] = 2X 1019 cm-3

300 nm n-Al0.55Ga0.45N [Si] = 5 X 1019 cm-3

600 nm graded to n-Al0.55Ga0.45N

[Si] =1.2 X 1019 cm-3


15 nm p+ -AlGaN [Mg] = 5 X 1019 cm-3

15 nm n+ AlGaN [Si] = 1 X 1020 cm-3

QWs

030

6090

120

-4 -2 0 2 4

Energy (eV) Depth (nm

)

-6 -4 -2 0 2 4 6 8 10 120

200

400

600

800

1000

1200

Curre

nt D

ensit

y (A/

cm2 )

Voltage (V)

-6 -4 -2 0 2 4 6 8 10 121E-3

0.01

0.1

1

10

100

1000

Curre

nt D

ensit

y (A

/cm

2 )

Voltage (V)

• Voltage @ 20A/cm2 is 7.1 V. • Resistance @ 1kA/cm2 is

1.6E-3 Ohm cm2 1 2 3 4 510-8

10-6

10-4

10-2

100

102

Al0.

55Ga

0.45

N

Al0.

3Ga 0.

7N

GaAs

GaSb

/InAs

InP

GaN

AlGa

As/In

AlGa

P

TJ re

sistan

ce (Ω

cm2 )

Bandgap (eV)

This work




0 200 400 6000.0

0.1

0.2

0.3

0.4

EQE (

%)

Current (A/cm2)

0 200 400 6000.00

0.02

0.04

0.06

0.08

0.10

0.12

Powe

r (m

W)

Current (A/cm2)


• Single peak emission at 295 nm. • Peak EQE is 0.4%. EQE curve

indicates high non-radiative recombination in active region.

• Al0.55Ga0.45N/ In0.2Ga0.8N TJ is demonstrated for the first time.

250 300 350 4000

1x104

2x104

3x104

4x104In

tens

ity

Wavelength (nm)

0.3 mA to 6 mA

30um device

On-wafer measurement





• Motivation






• Summary




Summary

First report of tunneling hole injection through wide band gap Al0.55Ga0.45N

tunnel Junctions with Eg ~ 4.7 eV.

• Single peak emission at 295 nm

• Tunneling injection gives EQE of 0.4% (on-wafer)

• Lowest TJ resistance of 1.6E-3 Ohm cm2

Tunnel Junctions are promising for high efficiency UV/ DUV LEDs

1 2 3 4 510-8

10-6

10-4

10-2

100

102

Al0.

55Ga

0.45

N

Al0.

3Ga 0.

7N

GaAs

GaSb

/InAs

InP

GaN

AlGa

As/In

AlGa

P

TJ re

sistan

ce (Ω

cm2 )

Bandgap (eV)

This work

250 300 350 4000.0

5.0x1031.0x1041.5x1042.0x1042.5x1043.0x1043.5x1044.0x1044.5x104

Inte

nsity

Wavelength (nm)

0.3 mA to 6 mA

30um device

-6 -4 -2 0 2 4 6 8 10 120

200

400

600

800

1000

1200

Curre

nt D

ensit

y (A/

cm2 )

Voltage (V)

-6 -4 -2 0 2 4 6 8 10 121E-3

0.01

0.1

1

10

100

1000

Curre

nt D

ensit

y (A

/cm

2 )

Voltage (V)



[email protected]; [email protected] Sub 300 nm Wavelength III-Nitride Tunnel-Injected UV LEDs

UV Tunnel Junction LEDs

Backup slides



[email protected]; [email protected] Sub 300 nm Wavelength III-Nitride Tunnel-Injected UV LEDs 40 40

Absorption losses due to TJ

n AlGaN

p AlGaN

Hole injected back into the active region “recycled” Absorbed/injected back/ emitted recursively

𝐼𝐼𝐼𝐼=𝐼𝐼0exp (−𝛼𝛼t)

1−Loss= T + T(1-T)R + T(1-T)2R2+ T(1-T)3R3 + … + T(1-T)NRN

Absorption loss = 2% ,assuming IQE – 50%

• absorption coefficient (α) of 1×105 cm-1 • 3.9% photons absorbed in one pass • 0.039*IQE is emitted again, and absorbed




Tunable wavelength

41

260 280 300 320 340 360 380 400 420

Inte

nsity

Wavelength (nm)

20% AlGaN QW

10% AlGaN QW

0 2 4 6 8 1012.012.513.013.514.014.515.015.516.0

10% AlGaN QW 20% AlGaN QW

FWHM

Current (mA)




Output power with time

42

-1 0 1 2 3 4 5 6 7 8 945

46

47

48

49

50

51

52

Powe

r (uW

atts)

Time (hr)0 1 2 3 4 5 6 7 8

96

97

98

99

100

101

102

103

Powe

r (uW

atts)

Time (hr)

1 mA 40A/cm2

2 mA 80A/cm2

• Power increases by about 6% and 4% with time for 1mA and 2mA, respectively. Power decreases by 4% for 4mA.

0 1 2 3 4 5 6 7 8 9141142143144145146147148149150151

Powe

r (uW

atts)

Time (hr)

4 mA 160A/cm2




What WPE can we achieve for UV/DUV LEDs?

Input Power 100%

Output Power

ƞinj ƞIQE

Non-equilibrium injection

• Crystal quality (TDD)

• Active region

Minimal absorption (similar to visible LEDs)

< 80% < 70% ~ 45%

ƞLEE < 80% Thin p AlGaN

Tunnel Junction

MQW n AlGaN

n AlGaN

LED

TJ-UV LED

p AlGaN

p GaN

MQW

n AlGaN

LED

p AlGaN

p AlGaN/AlGaN SL

MQW

n AlGaN

LED

Absorption loss

Electrical loss

Input Power 100%

Output Power

ƞinj ƞIQE ƞLEE

• Low hole density

• P-contact

• Crystal quality (TDD)

• Active region

• Absorption loss

• Reflection

< 50% < 70% < 25%

< 8 %

M. Shatalov, et al. APEX 5 082101 (201

Highest value < 5.5%




• First report of Al0.3Ga0.7 N interband Tunnel Junctions (TJ) for hole injection in UV LEDs

• Low TJ resistance of 5.6 x 10-4 ohm cm2

• 327 nm LEDs with 0.58 mW at 20 mA (on-wafer)

• Peak EQE – 1.5%, Peak WPE – 1.08%

• Stable output power of 6 W/cm2 @ 120 A/cm2 @ 5.9 V

Key Results




Backup slides

• Absorption losses.. • Calculation details… • Exact quantum well design.. • Tunable wavelength.. • Stability/ reliability of output power! • All previous tunnel junction work! • Latest MOCVD Work!



sub 300 nm wavelength iii-nitride tunnel-injected ... 300 nm wavelength iii-nitride tunnel-injected...

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