Advances & Challenges for AlGaN-based UV-LED technologies
Michael Kneissl Institute of Solid State Physics, TU Berlin, Germany Ferdinand-Braun-Institut gGmbH, Leibniz-Institut für Höchstfrequenztechnik, Berlin, Germany
U.S. DOE 18th Lighting R&D Workshop, February 1st - 4th, 2021
Applications of ultraviolet light emitters
M. Kneissl | Institute of Solid State Physics
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EQE of UV-LEDs: State-of-the-Art M. Kneissl et al., Nature Photonics 13, 233 (2019)
“Deep UV “UVB drop-off” gap”
0.1
AlGaN GaNAlN
0.01
M. Kneissl | Institute of Solid State Physics
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Challenges for deep UV LEDs
p-GaN cap p-AlGaN SPSL
p-AlGaN EBL
(In)AlGaN MQWs
n-AlGaN
AlxGa1-xN transition
AlN base
substrate
M. Kneissl | Institute of Solid State PhysicsPage 4
Ohmic (V), UV-reflective p-contacts (LEE) Low resistance (V), UV-transparent p-layers
Efficient carrier injection (CIE)
High IQE, carrier confinement (CIE),
polarization control (LEE)
Efficient current spreading, n-contact (V) Strain management
Low defect densities (IQE) UV transparency & light extraction (LEE) Heat extraction, high-power (Pmax)
1
10
100
simulation results IQE from PL (our data)
Inte
rnal
Qua
ntum
Effi
cien
cy (%
) Effect of dislocations on the IQE of UV-LEDs
Simulation parameters [3]: AlGaN-MQW LEDs
Ndd = 1/r2
ELO or pss AlN/sapphire
bulk AlN
AlGaN-QW LED = 280 nm, j = 100 A/cm2
From Ref. [1] From Ref. [2]
Pout = 27 mW (@350 mA, flip-chip)
r
Pout= 2.5 mW (@350 mA, flip-chip)
AlN/sapphire
Pout= 73 mW (@350 mA, flip-chip)
1E7 1E8 1E9 1E10 = 280 nm, j = 100 A/cm2
dislocation density (1/cm2) [1] Ban et al., APEX 4, 052101 (2011) No SRH from point defects [2] Mickevicius et al., APL 101, 211902 (2012) Light extraction: extr = 10% [3] Karpov et al., APL 81, 4721 (2002)
M. Kneissl | Institute of Solid State Physics Page 5
sapphire
5.5 µm
TDD ~ 1.5x109 cm-2
TDD of AlN/sapphire templates* AlN/sapphire template technologies
TDD ~ 4.0x109 cm-2
MOVPE AlN
sapphire
1.5 µm
TDD ~ 8.1x108 cm-2
0.9 µm
MOVPE AlN
TDD ~ 8.5x108 cm-2
sapphire
MOVPE AlN 5.5 µm
sputter. AlN
sapphire sapphire
HTA AlN**
MOVPE AlN
sapphire
sputter. AlN
sapphire
HTA AlN**
sapphire sapphire sapphire
*Sylvia Hagedorn et al., phys. stat. sol. (a) 217, 1901022 (2020) **Hideto Miyake et al., Applied Physics Express 9, 025501 (2016)
Page 6 **Hiroyuki Fukuyama, Hideto Miyake et al., Jap. J. of Appl. Phys. 55, 05FL02 (2016)
CL of AlGaN MQWs on different templates
AlGaN MQW heterostructures grown side by side on different AlN/sapphire templates by MOVPE
TDD visualized by CL as their non-radiative recombination causes dark-spots Lowest dark-spot-density (DSD) on HTA MOVPE ELO AlN/sapphire
planar HTA HTA ELO planar AlN/sapphire ELO AlN/sapphire AlN/sapphire AlN/sapphire
DSD: 3.5 × 109 cm-2 DSD: 1.1 × 109 cm-2 DSD: 1.4 × 109 cm-2 DSD: 0.9 × 109 cm-2
N. Susilo et al., Appl. Phys. Lett. 112, 041110 (2018) M. Kneissl | Institute of Solid State Physics
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Effects of TDD on IQE for different templates TDD determined by FWHM of HR-XRD Dark spot density determined by CL TDD determined by XTEM Good agreement between
TDD determined by HR-XRD,
108
109
1010
Thre
adin
g di
sloc
atio
n de
nsity
Dar
k sp
ot d
ensi
ty (c
m -2
)
IQE from
Experiment: IQE = EQE/LEE
Simulation (SiLENSe*)
40 panchromatic CL (DSD), and XTEM
30 Clear correlation between
20 IQE
(%) IQE and TDD
Lowest TDD and highest IQE for MQW on HTA ELO AlN/sapphire
10 *Simulation parameters: j = 13 A/cm², µe = 120cm²/Vs, µh = 6cm²/Vs, TDD based on DSD determined by CL of
0 planar planar HTA ELO HTA ELO
AlN/sapphire AlN/sapphire AlN/sapphire AlN/sapphire
MQWs), Karpov et al. model
M. Kneissl | Institute of Solid State Physics
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Light extraction from UV-LEDs Extraction via substrate Paths of created photons
emis
sion
pow
er (m
W)
60
50
40Poor light extraction efficiencies for UV-LEDs (e.g. flip-chip mounted LED: LEE ~7%) 30
Need for enhanced light extraction 20
Encapsulation with UV-transparent polymers Challenges: UV-absorption, low refractive 10
index, long-term stability
UV-reflective contacts & UV-transparent p-side: 0
Challenges: Ohmic p-contacts, p-AlGaN layer resistance
M. Kneissl | Institute of Solid State Physics
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LI characteristic of a UVC-LED
Flip-chip LED in SMD package Non-reflective contacts = 262 nm
with encapsulant flip-chip LED
0 100 200 300 400
dc current (mA)
DUV-LEDs for in-vivo disinfection
Irradiation system with an array of 118 Light from DUV LEDs (<235 nm) does not DUV-LEDs emitting at 233 nm* penetrate living skin layers
in-vivo disinfection without damage to human skin
In-activation of multidrug resistant bacteria, e.g., MRSA, MSSA
Disinfection of airborne viruses, e.g., SARS-CoV2, influenza
Required DUV dose levels: 2 – 40 mJ/cm2
© FBH/P. Immerz
*M.C. Meinke et al., Management & Krankenhaus 9, 20 (2020)
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15
5
Performance of 233 nm LEDs on sapphire
LEE
RRE
CIE
EQE
𝐄𝐐𝐄 𝐋𝐄𝐄 𝐂𝐈𝐄 𝐑𝐑𝐄
EQE = 0.35% Pout = 1.88mW @100mA
Flip-Chip DUV-LED peak= 233 nm
T = 20°C
2,5
102
2,0
emis
sion
pow
er (m
W)
effic
ienc
y (%
)
10 101
volta
ge (V
)
1,5
100
1,0
10-1
0,5
10-2
0 0,00 20 40 60 80 100
10-3
dc current (mA) 102
10-4
210 220 230 240 250 260 270
emission wavelength (nm)101
Steep drop in EQE for shorter wavelength LEDs 100
I = 100 mA (dc), 20 °C peak = 233 nm
FWHM = 11.5 nm
200 250 300 350 400 450
Degradation in light extraction (LEE), radiative recombination (RRE) & current injection efficiency (CIE)
=> Fundamental physical limitations or engineeringspec
tral p
ower
(µW
/nm
)
challenge?
N. Lobo-Ploch et al., Appl. Phys. Lett. 117, 111102 (2020)wavelength (nm) M. Guttmann et al., Jpn. J. Appl. Phys. 58, SCCB20 (2019)Page 11
10-1
10-2
Summary
Sputtered & high-temperature annealed (HTA) AlN layers on sapphire promising low cost, low TDD template technology for UVC-LEDs
Reduced threading dislocation densities Enhanced IQE, EQE and WPE
Improved lifetimes
Further advances in UVC-LED efficiency will require enhanced light extraction, i.e. UV-reflective contacts, UV-stable encapsulation, …
Pushing the wavelength limits of deep UV-LEDs (<250 nm) 233 nm LEDs with 1.88 mW output power & EQE = 0.35%
Strong decrease in EQE for LEDs wavelength < 250 nm
Drop in in LEE, IQE, and CIE for wavelength < 230 nm Advanced heterostructure designs for improved carrier injection
Acknowledgements
Institute of Solid State Physics, TU Berlin: G. Cardinali, J. Enslin, P. Gupta, M. Guttmann, C. Kuhn, F. Mehnke, F. Nippert, C. Reich, M. Schillig, N. Susilo, S. Wu, L. Sulmoni, T. Wernicke
Ferdinand-Braun-Institute, Berlin: H.K. Cho, J. Glaab, S. Hagedorn, A. Knauer, T. Kolbe, N. Lobo-Ploch, A. Mogilatenko, C. Netzel, J. Rass, J. Ruschel, S. Walde, S. Einfeldt, M. Weyers
Collaborations: L. Cancellara, M. Bickermann, M. Albrecht (Institute for Crystal Growth, Berlin) G. Kusch, R. Martin, C. Trager-Cowan (U. Strathclyde, UK) H. Miyake (Mie University, Japan) M. C. Meinke (Charité – Universitätsmedizin Berlin, Germany) A. Kramer (University of Greifswald Medical School, Germany)
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