Supplementary Information
Overcoming the Fundamental Light-Extraction Efficiency Limitations of Deep-Ultraviolet LightEmitting Diodes by Utilizing Transverse-Magnetic-Dominant Emission
Dong Yeong Kim1†, Jun Hyuk Park1†, Jong Won Lee1, Sunyong Hwang1,
Seung Jae Oh1, Jungsub Kim2, Cheolsoo Sone2, E. Fred Schubert3 and Jong Kyu Kim1*
1Department
of Materials Science and Engineering, Pohang University of Science and Technology,
Pohang, 790-784, Korea
2LED
3Future
Business, Samsung Electronics, Yongin 446-920, Korea
Chips Constellation, Department of Electrical, Computer, and Systems Engineering,
Rensselaer Polytechnic Institute, Troy, NY 12180 USA
*Correspondence and requests for materials should be addressed to J.K.K. (e-mail: kimjk@postech.ac.kr)
[†] These authors contributed equally to this work.
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Supplementary Information S1. Energy band diagram of DUV LED
The energy band diagram of the 285 nm AlGaN DUV LED is calculated by using the Advanced Physical
Model of Semiconductor Devices (APSYS) simulator. The material and band parameters we used in this
calculation can be found in Refs. 1 and 2. Band parameters for ternary layers were calculated by Vegard’s
law modified by the bowing effect. The energy band shown in the Figure S1 is calculated for an injection
current density of 200 A cm-2.
Figure S1. The energy band structure of the DUV LED at a forward injection current density of 200 A cm-2
Supplementary Information S2. Finite element method (FEM) simulation
In order to visualize the distribution of DUV light (electromagnetic wave) in the reference and SEE DUV
LEDs, finite element frequency domain simulation (FEFD) is performed using commercial finite element
method (FEM) software package, COMSOL multiphysics. In this simulation, a modified Maxwell’s
equation is solved which is called the wave equation and it is given by
∇ × (∇ × 𝑬) − 𝑘02 𝜀𝑟 𝑬 = 0
where 𝑘0 is the wavevector in vacuum, 𝑬 is the electric field vector and 𝜀𝑟 is the relative permittivity.
The FEFD simulation shows the spatial distribution of the intensity of the electromagnetic (EM) wave of
given frequency.
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In the simulation, four incoherent dipole sources composed of two vertical dipoles and two lateral dipoles
are used to reproduce our AlGaN DUV LED with polarization ratio around 0. Figure S2 is the electric
field distribution in the reference and SEE DUV LED. Strong sidewall emission is clearly shown in the
reference DUV LED (See Figure S2a) due to effective extraction of TM-polarized DUV photons through
the exposed sidewall and severe DUV absorption at the p-GaN contact formation layer with an extinction
coefficient of 0.42 for 285 nm wavelength [3]. By employing an Al-mirror between the active mesa stripes,
the electric field propagating to the surface-normal direction is increased as shown in Figure S2b. The
figure shows the validity of concept of SEE DUV LEDs and efficient top-emission for the SEE DUV LED
by employing Al mirrors between narrow mesa stripes.
Figure S2. Electric field distribution in (a) reference and (b) SEE DUV LED calculated by the finite
element method. Four incoherent electric dipole sources are used and they generate DUV light whose
polarization ratio is 0. This clearly shows that effective light extraction through the exposed sidewall of the
active region in both reference and SEE DUV LED and enhanced top emission due to reflection at Al
mirrors located between mesa stripes in the SEE DUV LED.
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Supplementary Information S3. Fabrication process
The 285 nm AlGaN DUV LED wafer is grown by metal-organic vapor phase epitaxy (MOVPE) on 4 cplane sapphire substrate, as shown in Figure S3. AlN and AlGaN/AlN superlattice (SL) layers are grown
on the substrate as a buffer layer. The DUV LED layer sequence consists of a 2 μm-thick n-Al0.55Ga0.45N,
AlGaN/AlGaN multiple-quantum walls (MQWs) composed of 5 pairs of 3nm-thick undoped Al0.4Ga0.6N
quantum wells (QWs) and 10nm-thick undoped Al0.55Ga0.45N quantum barriers (QBs), a 20 nm-thick pAl0.7Ga0.3N electron blocking layer (EBLs), a p-AlGaN layer having the Al composition graded from 65%
to 0%, and a 20 nm-thick p-GaN contact formation layer. The fabrication of the DUV LED called
sidewall-emission-enhanced (SEE) DUV LED follows conventional LED fabrication process: mesa
etching, n-contact and p-contact formation, and pad-metal deposition (reference LED and SEE DUV LED).
For the SEE DUV LEDs, n-GaN regrowth and Al-mirror deposition are additional fabrication steps.
Narrow stripe mesa pattern is defined by photolithography followed by reactive ion etching (RIE). A
mixture of BCl3/Cl2 (3:7) is used as etchant. The RIE inductive coupled plasma (ICP) power and RIE
power are 350 W and 100 W, respectively. The measured etching rate is around 220 nm min-1; to etch
the sample to a depth of 1μm, the etching is performed for 270 s. Then, a 50 nm-thick SiO2 layer is
deposited on the whole sample surface by plasma-enhanced chemical vapor deposition at 300 C. For the
selective-area growth of n-GaN stripes, SiO2 is selectively etched by buffered oxide etchant (BOE) so that
the bare n-AlGaN surfaces are exposed. The etching region is defined by photolithography and the mask
opening has a width of 4 μm. The n-GaN microstructures are re-grown by MOCVD on the exposed nAlGaN surface for 3 min at 1000 C under 200 mbar. TMGa and ammonia are used as MO sources with
a molar V/III ratio of 606. Diluted silane is supplied for n-type doping with a flow of 50sccm. Then, the ncontact is formed on the n-AlGaN surface as well as regrown n-GaN stripe surface. Ti/Al/Ni/Au
(30/120/40/200 nm) is deposited by electron-beam or thermal evaporation, and annealed by rapid thermal
annealing (RTA) at 900 C for 2 min in N2 ambient. The p-contact is also deposited on the p-GaN contact
formation layer by election-beam evaporation. The p-contact consists of 20 nm-thick Ni and 100 nmthick Au and is annealed by RTA at 650 C for 1 min in air ambient. A 150 nm-thick Al layer is deposited
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on the regrown n-GaN stripes covered by n-metal by thermal evaporation. Finally, pad metal composed of
20 nm-thick Ti, and 100nm-thick Au is deposited on the n- and p-type contact. The overall fabrication
process is illustrated in Figure S3.
Fabrication process for the SEE DUV LEDs is manufacturable based on the following reasons. First, the
GaN epitaxial regrowth technique is well-established for the reduction of threading dislocations (Epitaxial
Lateral Overgrowth, ELO or pendeo epitaxy, etc.). Second, GaN regrowth presumably has no effect on
DUV LED epi-quality because it is grown at lower temperature than the AlGaN layer growth. Third, the
fabrication process of the SEE DUV LEDs exactly follows the conventional LED fabrication process
except for the GaN regrowth. Therefore, adding one well-established regrowth step should make the SEE
DUV LED manufacturable.1
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Figure S3. Fabrication process for SEE DUV LEDs composed of (a) MOCVD growth, (b) mesa stripe
formation, (c) SiO2 deposition and patterning, (d) n-GaN microstructure regrowth, (e) contact formation,
and (f) Al mirror deposition.
Supplementary Information S4. Spectrum of a DUV LED as a function of TE- and TM-polarization
In order to investigate the polarization ratio of the DUV from our AlGaN DUV LED, polarization
dependent spectral emission is measured by using a spectrometer, Black C-50, produced by StellarNet Inc.
and a UV-enhanced optical fiber (F1000-UV-VIS-SR) at injection current of 100 mA. In order to avoid
heating effects, the injection current of 100 mA was pulsed at 0.2% duty cycle by using Keithley 4200SCS Parameter Analyzer. Light with TE- and TM-polarization states are divided by a Glan-laser calcite
polarizer. Figure S4 is the measured polarization dependent spectrum.
Polarization ratio of our 285nm-DUV LED is 0.067 according to the definition of polarization ratio given
as
Polarization ratio =
𝐼TE − 𝐼TM
𝐼TM + 𝐼TM
where, ITE and ITM are the intensity of TE- and TM-polarized light, respectively. The obtained polarization
ratio is comparable but a little higher than the value of around -0.1 reported in the literature [4,5]. This can
be attributed to different quantum confined states and strain conditions. The quantum confinement effect
rearranges the relative position of three VBs (HH, LH, and CH bands), and makes the HH band raise [6].
Therefore, TE-polarized light from the quantum confined AlGaN QW is stronger than that from the AlGaN
bulk. In addition, strain of the material also affects its band structure, and thus the polarization states of the
emission [4,7]. Strain of the AlGaN QWs is determined by the lattice mismatch to substrate and QBs that
may vary in different samples. In any case, the 285 nm AlGaN DUV LED used in this study emits
strongly TM-polarized light different from visible LEDs.
The peaks at longer wavelengths (> 300 nm) are parasitic emissions which may originate from transitions
involving deep levels (including acceptors) in the p-AlGaN cladding layer [8,9]. Parasitic emission shows
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strong TM-polarization characteristics, which would be related to the polarization selection rule involving
the transition via deep levels, however, is now well understood at the moment.
Figure S4. Spectrum of the 285 nm AlGaN DUV LED as a function of light polarization direction.
Polarization ratio of our AlGaN DUV LED is 0.067 indicating strong sidewall emission characteristics.
Supplementary Information S5. Various SEE DUV LEDs
AlGaN SEE DUV LEDs with different numbers of stripes (5, 10, 15,
1 mm2 chip areas. Figure S5 shows all the fabricated SEE DUV LEDs.
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, 45) are fabricated on the 1
Figure S5. SEM images of all the fabricated SEE DUV LEDs having different number of stripes from 5 to
45. SEE DUV LED with (a) 5, (b) 10, (c) 15, (d) 20, (e) 25, (f) 30, (g) 35, (h) 40, (i) 45 stripes. The scale
is 200 μm for all of the figures shown above.
Supplementary Information S6. LOP vs. Injection current dependence plot
Figure S6 shows the LOP as a function of injection current for the various SEE DUV LEDs. SEE DUV
LEDs with a larger number of stripes show much higher LOP, indicating that the proposed approach,
sidewall-emission-enhancing concept, works very well, and thus, can be used practically.
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Figure S6. Light output power of various SEE DUV LEDs as a function of injection current.
Supplementary Information S7. Light extraction efficiency of SEE DUV LEDs
Relative LEE values of the SEE DUV LEDs are obtained from the measured relative internal quantum
efficiency (IQE) defined as photocurrent divided by injection current, and the light output power (LOP), by
using the equation
LEE = 𝐶 ×
normalized EQE
normalized LOP/𝐼
=𝐶×
normalized IQE
normalized IQE
The approach is based on two assumptions: First, all the DUV LED samples have the same IQE values at
the same injection current density because the LEDs are fabricated using the same epi-wafer. Second, the
value of the constant, C, is the same for all LED samples since the measurement configuration is the same.
Figure S7 shows normalized IQE of a DUV LED fabricated using the same epi-wafer (as the SEE DUV
LED) as a function of injection current density. Normalized IQE values for the various SEE DUV LEDs
indicated by red dots are extracted from the curve considering the fact that each SEE DUV LED sample
has a different active area depending on the number of stripes.
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Figure S7. Normalized IQE curve of a DUV LED fabricated using the same epi-wafer as the SEE DUV
LEDs. Red dots indicate normalized IQE values of SEE DUV LEDs with different number of stripes, i.e.
different area of active mesa region.
The normalized EQE can be directly obtained from the measured LOP at the same injection current, and
LEE can be estimated from normalized EQE and IQE values by using the above equation. Figure S8
shows normalized IQE, EQE, and LEE of various SEE DUV LEDs.
Figure S8. Normalized EQE, IQE and LEE of the various SEE DUV LEDs.
Figure S9a and b are the relative EQE and normalized EQE of five SEE DUV LEDs with different
number of stripes as a function of current density. All the SEE DUV LEDs show similar normalized EQE
curves indicating that their IQE values are similar since the LEE is independent of injection current density,
which validates our assumptions used in the LEE calculation.
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Figure S9. (a) Relative EQE and (b) normalized EQE of various SEE DUV LEDs.
Supplementary Information S8. Hole transfer length measured by transmission line method
Hole transfer length in the p-type contact is measured by the transmission line method (TLM). IV
characteristics are measured from four circular TLM patterns [10], illustrated in the inset of Figure S10,
formed on the p-GaN contact formation layer; the resistance is calculated from the slope of the measured
IV. Figure S10 is the result of the TLM measurement and it shows resistance as a function of ln(𝑅⁄𝑟)
where R and r is the radius of the outer and inner circular pattern, respectively, as shown in the inset. Using
the linear relation between resistance and ln(𝑅⁄𝑟) described in Ref. 10, the hole transfer length of about
1.5 μm and specific contact resistance of about 0.02 Ω∙cm2 are obtained.
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Figure S10. Resistance as a function of ln(R/r) obtained from circular transmission length method (TLM)
where R is outer radius and r is inner radius of circular pattern as shown in the inset. Measured hole
transfer length and p-contact resistance are about 1.5 µm and 0.02 Ω∙cm-2, respectively.
Supplementary Information S9. Geometry of the regrown n-GaN stripes
The schematic cross-section of the SEE DUV LED is shown in Figure S11. The distance between mesa
stripes is designed to be 13 μm for all the SEE DUV LEDs. This distance represents a sufficient margin
between the active mesa and the regrown n-GaN stripes. The angle of the inclined surface of n-GaN stripes
is calculated from the geometry measured by SEM and an alpha-step profiler, and it is about 62,
consistent with inclined angle of the {11̅01} facet. This facet is typically obtained when the window mask
is oriented along the ⟨112̅0⟩ direction because it is an energetically stable surface and has slow growth
rate [11]. Due to regrown n-GaN stripes, light emitted at angles less than about 45 is reflected upward.
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̅𝟎𝟏} facet is obtained during regrowth
Figure S11. Cross-section geometry of the SEE DUV LED. {𝟏𝟏
process and its inclined surface reflects DUV photons emitted from the sidewall toward the top.
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