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zhang.3789@osu.edu; rajan@ece.osu.edu 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
zhang.3789@osu.edu; rajan@ece.osu.edu 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
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 3
• 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
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 4
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.
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 5
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
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 6
P-GaN
P-AlGaN
P-AlGaN/AlGaN SL
MQW N-AlGaN
P-type contact
Conventional UV LEDs
Why we need tunnel-injected 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
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 7
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
Why we need tunnel-injected 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
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 8
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).
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 9
Overview of the tunnel junction technology
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)
Polarization engineered tunnel junctions at OSU
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
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).
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 10
Overview of the tunnel junction technology
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)
Polarization engineered tunnel junctions at OSU
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
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).
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 11
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
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 12
N-AlGaN
P-AlGaN
Tunnel Junction
MQW N-AlGaN
N-type contact
TJ-UV LEDs
Challenge: p-AlGaN compensation
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)
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 13
Challenge: p-AlGaN compensation
0 50 100 150 200-4-3-2-1012345
Ener
gy (e
V)
Thickness (nm)
Proper p-n junction
Without compensation
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 14
Challenge: p-AlGaN 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
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 15
Challenge: p-AlGaN 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?
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 16
Outline: Tunnel-injected UV LED
• Motivation
• Tunnel-injected UV LEDs enable analysis of hole doping
• CV measurement to probed compensation impurities
• Acceptor free UV LEDs
• High efficiency UV LEDs
• Summary
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 17
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
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 18
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.
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 19
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
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 20
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)
Depletion Width (nm)
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
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 21
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)
Depletion Width (nm)
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
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 22
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)
Depletion Width (nm)
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
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 23
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)
Depletion Width (nm)
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
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 24
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)
Depletion Width (nm)
23 24 251019
1020
1021
N eff (c
m-3)
Width (nm)
A B
C
C: [Mg] = 3x1019 cm-3
A
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 25
Origin of the compensation impurities
Donor-type compensating impurity density is 1.2x1019 cm-3 -- O/C/ native defects?
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 26
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?
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 27
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
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 28
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.
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 29
EL characteristics
• Single peak emission at 325 nm for all the samples.
• 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
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 30
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.
• 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.
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 31
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 ↑
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 32
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
Current Density (A/cm2)
EQE (
%)
EL characteristics
0 200 400
0.0
0.5
1.0
1.5
Current Density (A/cm2)
WPE
(%)
0 200 400 600 8001E-4
1E-3
0.01
0.1
1
WPE
(%)
Current Density (A/cm2)
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.
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 33
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)
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 34
• 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
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 35
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)
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 36
Summary
• C-V measurement is used to extract compensating impurity density of 1.2x1019 cm-3 in the p-AlGaN layer.
• 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)
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 37
Summary
• Acceptor free UV LED emitting at 325 nm achieved using polarization engineering.
• C-V measurement is used to extract compensating impurity density of 1.2x1019 cm-3 in the p-AlGaN layer.
• 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)
zhang.3789@osu.edu; rajan@ece.osu.edu Tunnel-injected UV LEDs 38
Summary
• Acceptor free UV LED emitting at 325 nm achieved using polarization engineering.
• C-V measurement is used to extract compensating impurity density of 1.2x1019 cm-3 in the p-AlGaN layer.
• Using graded p-AlGaN, obtained record on-wafer EQE=3.37%, WPE=1.62% at 325 nm.
• 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)
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