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Smart Wide-Bandgap Omnidirectional Reflector
as an Effective Hole-Injection Electrode for Deep-UV
Light-Emitting Diodes
Tae Ho Lee, Tae Hoon Park, Hee Woong Shin, Noritoshi Maeda, Masafumi Jo,
Hideki Hirayama, Bo-Hyun Kim, and Tae Geun Kim*
medical/biological detection.[1–5] After
the development of GaN-based bluelight-emitting diodes (LEDs), AlGaNbased DUV-LEDs covering the whole
UV spectral range through the control
of the Al content were realized, despite
low efficiencies.[6,7] The key factors to
improve the performance of DUV-LEDs
are the reductions in contents of defects
and threading dislocations of Al(Ga)N,
balance of injected charge carriers at the
active quantum well layer, and efficient
light extraction. They are all reflected in
the optical output power (Pout), external
quantum efficiency (EQE), and wall-plug
efficiency (WPE), defined by the ratio
of the radiometric optical output power
of the LED to the electrical input power.[8]
The contents of defects and dislocations
of Al(Ga)N leading to the low quantum
efficiency have been successfully reduced
by various crystal growth processes.[1] An
electron-blocking layer and highly conductive p- and n-type-doped AlGaN structures
were introduced to increase the charge
balance.[9] In addition, flip-chip (FC) LED structures have been
employed owing to the excellent heat dissipation, large lifetime,
and simple packaging.[10–12]
EQEs in the range of 2.1−20.3%[9,13–20] have been achieved
through additional processing, including photonic crystal,[13]
patterned sapphire substrate,[9,14] lens-like hemispherical
encapsulant,[9] and p-type cladding layers.[16] However, the lack
of an efficient electrode for the contact with the p-type AlGaN
(p-AlGaN) leads to serious problems in DUV-LEDs such as the
very low WPE (0.69−4.5%).[17,21] Recently, Hirayama's group
reported a DUV-LED with a high EQE (>20%) by combining
several techniques such as the use of an AlGaN:Mg contact
layer and Rh mirror electrode to enhance the light extraction. However, the driving voltage was 16 V, higher than that
(9 V) of the conventional DUV-LED.[9] An Mg-doped hexagonal boron nitride was introduced as an alternative to reduce
the p-type resistivity. However, its application was hindered
by the difficulty to achieve a large-scale synthesis.[22] On the
other hand, Ni/Au, commercially used for the Ohmic contact to p-(Al)GaN layers, exhibited a low reflectance at a DUV
wavelength (≈30% at 280 nm).[14,17] Ni (<3 nm)/Al and Pd
Deep-ultraviolet light-emitting diodes (DUV-LEDs) find widespread
applications in various industries and are a focus area in environmental
and life science research. However, the quantum efficiency of DUV-LEDs is
still low because of the unbalanced hole injection, high operation voltage,
and low reflectivity of the p-electrode. In this study, a smart wide-bandgap
omnidirectional reflector (ODR), which simultaneously acts as an effective
hole-injection electrode, is demonstrated for a flip-chip DUV-LED. The
smart ODR is composed of p-type AlGaN (p-AlGaN)/Ni:AlN/Al with a high
reflectivity of 94.2% at 280 nm, in which AlN with a theoretically calculated
thickness of 40 nm is Ni-doped using the pulsed electrical breakdown
(PEBD) method. The smart ODR-mounted AlGaN-multiquantum-well-based
DUV-LED exhibits a remarkable wall-plug efficiency (4.8%) and high external
quantum efficiency (8.5%) at a low operation voltage of 9.75 V owing to
the fused layer of Ni3N and Ga vacancies formed in the interfacial region
of the p-AlGaN and AlN layers, through the mutual diffusion of Ni and Ga
during PEBD. The interface-mediated Ohmic contact leads to an effective
hole injection without reducing the reflectivity, yielding increased optical
output and electric input powers. The proposed smart ODR can provide an
innovative route for the manipulation of DUV light.
1. Introduction
Compact solid-state deep-ultraviolet (DUV) light sources
have attracted considerable attention for various applications
including sterilization, water purification, lithography, and
T. H. Lee, T. H. Park, H. W. Shin, B.-H. Kim, Prof. T. G. Kim
School of Electrical Engineering
Korea University
Seoul 136-701, Republic of Korea
E-mail: tgkim1@korea.ac.kr
H. W. Shin
LED R&D Center
LED Division
LG Innotek Co., Ltd.
Paju 413-901, Republic of Korea
N. Maeda, M. Jo, Dr. H. Hirayama
RIKEN National Institute of Physical and Chemical Research
Wako, Saitama 351-0198, Japan
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adom.201901430.
DOI: 10.1002/adom.201901430
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(<10 nm)/Al hybrid metals exhibited reflectivities of ≈70% at
280 nm.[17,23] However, despite these efforts, most of the metal
schemes proposed for DUV LEDs have shown poor Ohmic
contacts to p-AlGaN; thereby, low hole injection efficiency.[24–27]
According to a theoretical simulation, the insertion of AlGaN
interlayers provided an improved injection/transport of holes,
while the use of a Ni/Al electrode could reduce the optical
absorption.[17] Nevertheless, the low light extraction efficiency
owing to the optical absorption by the p-type metal electrode
should be improved.[13,28] Moreover, the complex structure of
the p-electrode increases the contact resistance and operation
voltage. Accordingly, a p-type electrode with a high reflectivity
and low contact resistance to the p-AlGaN layer is required for
high-performance DUV-LEDs.
An effective approach to extract the light from an FC LED
is to use a triple-layer omnidirectional reflector (ODR).[29] In
this study, we developed a wide-bandgap triple-layer (p-AlGaN/
AlN/Al) ODR structure for FC DUV-LEDs and then employed
the pulsed electrical breakdown (PEBD) method to fabricate
a Ni-doped conductive AlN (Ni:AlN). We used a 3D finitedifference time-domain (FDTD) simulation to optimize the
reflectance of the triple-layer ODR structure in the DUV region.
Based on the triple-layer (p-AlGaN/Ni:AlN/Al) ODR structure,
high-performance FC DUV-LEDs were fabricated and their
optical properties were characterized. Finally, we experimentally
probed the chemical and electrical properties of the triple-layer
ODR structure using various analytical methods to elucidate
the underlying mechanism.
2. Results and Discussion
A conductive triple-layer ODR structure with a wide bandgap
was fabricated as the p-electrode for the FC DUV-LED. The
fabrication is illustrated in Figure S1 in the Supporting Information. The active layer of the DUV-LED consisted of n-AlGaN,
AlGaN multilayer quantum well (MQW), cap barrier, p-AlGaN
multiquantum-barrier (MQB), and p-AlGaN, consecutively
stacked on a sapphire substrate. The emission peak of the
AlGaN MQW was targeted to be at ≈280 nm (see the Experimental Section and the Supporting Information). Figure 1a
shows a cross-sectional scanning transmission electron
microscopy (STEM) image of the active layer, where the wellstacked layers are observed. Before further processing, we
evaluated whether the active layer could emit the targeted DUV
wavelength. Figure 1b,c shows the characteristic photoluminescence (PL) and electroluminescence (EL) of the active layer,
respectively. The PL peaks are observed around 281–284 nm.
The variation in wavelength is related to the excitation power
(Figure 1b). On the other hand, the EL peak (measured at 20 mA
from a quick-check method) is observed at 283 nm independent
of the input current, as expected (Figure 1c). The atomic ratio
Figure 1. Fabrication of the Ni:AlN-ODR-mounted DUV-LED. a) Magnified cross-sectional STEM image showing the MQW, MQB, and p-Al0.6Ga0.4N
contact layers. b) PL spectra of the AlGaN-based DUV-LED wafer measured using a 244 nm KrF excimer laser. c) Typical EL spectra measured for an
AlGaN-based DUV LED wafer using a quick-check method. d) Schematic of the Ni pads (thickness = 30 nm) and PEBD process. e) Current–pulse
bias during the PEBD process. The applied pulse bias is 4 V for 1 µs and 8 V for 500 ns. f) Schematic of the completed 280 nm FC DUV-LED with the
proposed Ni:AlN-ODR.
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(6:4) of Al to Ga at the surface of the p-AlGaN contact layer was
measured by energy-dispersive X-ray spectroscopy (Figure S2,
Supporting Information).[8] After the fabrication of the active
layer, the AlN layer and Ni micropads were sequentially deposited on top of the p-AlGaN layer (Figure 1d). Owing to the
limit of the electric breakdown technique,[30] the micropad was
periodically patterned to obtain dimensions of 10 × 10 µm2.
Using an automatic probing system, a pulse bias was applied
(one by one) on the Ni pad using an automatic contact probing
system (see Video S1, Supporting Information, for details).
Figure 1e shows the current transient response pulse (red solid
line) with respect to the applied voltage pulse (blue solid line)
measured for the AlN film using a pulse pattern generator
(81110A, Agilent) and a 1 GHz four-channel digital oscilloscope (DS6104, Rigol). Before the electrical breakdown on the
AlN layer, no or small tunneling current was observed. However, upon electrical breakdown under a pulse bias of 8 V for
500 ns, the current abruptly increased up to ≈2.5 mA, and
then the conductive state was maintained even at an applied
bias of 4 V. This is attributed to the formation of Ni-based
conductive filaments (CFs) accompanied by a large number of
diffused Ni atoms in the AlN layer, which has been extensively
analyzed by our group.[31] Through the PEBD process, the
Ni-doped conductive AlN layer (Ni:AlN) was fabricated. The last
step of the development of the Ni:AlN-embedded ODR (Ni:AlNODR) structure was the deposition of an Al layer on top of
Ni:AlN. Additionally, a fishbone-like electrode was designed
for a homogeneous current injection on the p-AlGaN layer
(Figure S3, Supporting Information). As the Al content of the
p-AlGaN layer was over 60% (compared to Ga), it seems that
the concentration of charge acceptors was extremely low.[8]
Figure 1f shows a schematic of the 280 nm FC DUV-LED
with the proposed Ni:AlN-ODR structure. We then carried out
further experiments to evaluate the optical and electrical properties of this device.
The reflectivity and optical bandgap of Ni:AlN-ODR are
affected by the thickness of the dielectric material and refractive index contrast unless other structural effects such as
photonic crystal and microlens array are considered.[29] An
optical simulation with the 3D FDTD method was carried out
to elucidate the relation between the thickness of AlN and DUV
reflectivity. Figure 2a presents the simulated 2D color contour map showing the correlation between the thickness and
reflectivity. It demonstrates that the thickness of AlN increased
with an increase in the incident wavelength, thus increasing
the reflectance. When the incident DUV light wavelength was
280–290 nm, the maximum reflectance was achieved by an
≈40 nm thick AlN film. Based on this result, a 40 nm thick AlN
was deposited. The total thickness of Ni:AlN-ODR was 270 nm
(p-AlGaN (80 nm)/Ni:AlN (40 nm)/Al (150 nm)). The simulation structure and process are presented in detail in the Experimental Section and Figure S4 in the Supporting Information.
Figure 2. Theoretical and experimental reflectance values. Simulation results for the maximum reflectance of the AlN/Al reflector. a) Contour plot of
the 3D FDTD-simulated reflectance. b) Comparison of the experimental and simulation reflectance values. The reflectance curves of the Ni/Al and
Ni/Au electrodes are shown for comparison. c) Conceptual cross-sectional view of the triple-layer ODR structure. d) Arithmetically calculated and
FDTD-simulated angle-dependent reflectance curves of Ni:AlN-ODR.
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Figure 2b shows the experimentally measured reflectance
values of Ni:AlN-ODR before and after the PEBD (b-PEBD and
a-PEBD), which are compared with the theoretically simulated
values. The reflectance curve of b-PEBD and simulated curve
are similar, demonstrating the validity of the simulation. Therefore, the 40 nm AlN was successfully introduced in Ni:AlNODR. Notably, although the reflectance of b-PEBD is higher
than the simulated value, that of a-PEBD is slightly lower than
that of b-PEBD. This implies that the PEBD process does not
critically affect the reflectance of the ODR structure. Moreover,
the reflectance of a-PEBD is above 90% in the whole analyzed
wavelength range, which is different from the reflectance
values of the Ni/Al and Ni/Au reflectors. The maximum
reflectivity (94.8% at 280 nm) of Ni:AlN-ODR is sufficient for
a high-performance DUV-LED. Figure 2c illustrates the light
propagation at the interface of the Ni:AlN-ODR structure. For
the ODR structure, the extraction efficiency of the transverse
electric (TE)-polarized DUV light is approximately ten times
higher than that of the transverse-magnetic-polarized DUV
light.[32] Typically, the reflectance of the TE-polarized light as a
function of the incident angle θ1 is described by[29,33,34]
R AlN , TE =
Al
r01 + r12 exp (2iφ )
1 + r01r12 exp (2iφ )
2
(1)
n p cos θ1 − n AlN cos θ 2
,
n p cos θ1 + n AlN cos θ 2
n cos θ 2 − n Al cos θ3
2π
r12 = AlN
, and φ =
n AlN h cos θ 2 .
n AlN cos θ 2 + n Al cos θ3
λ
The refractive indices (ni, i = p-AlGaN, AlN, Al) of the
layers are 2.57, 2.11, and 0.17 + i2.86, respectively,[35] while the
where r01 =
emission wavelength λ is 283 nm. Figure 2d shows arithmetically calculated angle-dependent reflectance curves of Ni:AlNODR and reflector without Ni:AlN, which are compared to the
FDTD-simulated values. Overall, the arithmetically calculated
and FDTD-simulated reflectance curves are similar. However,
the reflectivity of Ni:AlN-ODR is higher than that of the Al
reflector in the whole range of angles due to the ODR effect.
In general, typical metal reflector has a reflection loss at the
interface between the metal and semiconductor because it has
a complex refractive index value with a large extinction coefficient. However, such a loss can be reduced significantly by
inserting a dielectric material such as AlN as a barrier layer
because it has lower absorption loss and refractive index compared to those of p-AlGaN. In addition, when the polarized light
is incident at a Brewster angle, most of the light is transmitted
at the interface of the metal reflector without reflection. However, if the Brewster angle of the light incident on the p-AlGaN/
AlN layer is smaller than the maximum refraction angle in the
ODR structure, most of the light is reflected regardless of the
incident angle of light. Therefore, the reflection loss due to
the Brewster angle can be prevented in the ODR structure.[36]
This result indicates that Ni:AlN-ODR is a promising reflector
for the DUV light regardless of the incident angle.
Further, we evaluate the performance of the Ni:AlN-ODRmounted FC DUV-LED. Figure 3a shows its EL spectra measured at an injected current of 100 mA, which is compared to
those of the DUV-LEDs with Ni/Al and Ni/Au reflectors. For
all three DUV-LEDs, the EL peaks are observed at ≈283 nm,
suggesting that the PEBD process did not affect the MQW of
the active layer. The EL intensity of the Ni:AlN-ODR-mounted
DUV-LED was increased by ≈15% and 85% compared to those
of the DUV-LEDs with Ni/Al and Ni/Au, respectively. The inset
Figure 3. Performances of the AlGaN-based FC DUV-LED. a) EL spectra of the DUV-LEDs with the Ni:AlN/Al, Ni/Al, and Ni/Au electrodes at 100 mA.
Inset: Emission profile of the Ni:AlN-ODR electrode structure. b) I–V characteristics, c) light output power–current (L–I) characteristics, d) EQEs,
e) WPEs, and f) angle dependences of the light output powers of the DUV-LEDs with the Ni:AlN-ODR, Ni/Al, and Ni/Au electrodes.
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shows the emission intensity of the DUV-LED with Ni:AlNODR. Figure 3b shows the current–voltage (I–V) curve, which
reflects the WPE of the DUV-LED. The forward bias at the turnon current (20 mA) is 9.75 V for the Ni:AlN-ODR-mounted
DUV-LED, comparable to those of the DUV-LEDs with Ni/Al
and Ni/Au of 9.22 and 10.89 V, respectively, and remarkably low
compared to the previously reported value.[9] Moreover, the light
output power (L) (Figure 3c) is 30.9 mW at 100 mA, considerably higher than those of the DUV-LEDs with Ni/Al and Ni/Au
of 16.7 and 26.8 mW, respectively. The EQE and WPE are calculated using the measured L–I–V curves (Figure 3d,e). The EQE
is defined as the ratio of the number of photons emitted from
the LED to the number of electrons injected into the device
EQE =
PO /hv PO × λ
=
(%) (2)
I /e
1240 × I
where PO, I, and λ are the optical output power, applied current, and photon wavelength (peak wavelength), respectively.[37]
Figure 3d shows that the maximum EQE of the Ni:AlN-ODR
DUV-LED is 8.49% at 15 mA, considerably higher than those
of the other devices (with Ni/Al and Ni/Au). The EQE of 8.49%
is an outstanding value achieved by the FC DUV-LED without
an additional processing for light extraction. Figure 3e shows
the WPE as a function of the injection current, which can be
calculated as
WPE =
PO
P
= O (%)(3)
Pin I × V
where Pin and V are the input electrical power and diode
voltage, respectively.[38] The reported WPEs of AlGaN-MQWbased DUV-LEDs are lower than 4.5% owing to the high
operation voltages originated from the contact resistances
between the p-electrodes and p-AlGaN. In this study, the
maximum WPE of the Ni:AlN-ODR-mounted DUV-LED is
4.81% at 10 mA, which, to the best of our knowledge, is the
highest WPE achieved without considering other structural
and light extraction effects. Table 1 presents the EL characteristics of the Ni:AlN-ODR-mounted DUV-LED, where the
values are averaged using ten different samples. In addition,
we investigated the angle dependence of the light output power
(far-field intensity) to confirm the effect of the ODR structure
on the directionality of the extracted DUV light. Figure 3f
shows normalized light output power–light emission angle
curves. The light output power of the Ni:AlN-ODR DUV-LED
is higher than those of the reference DUV-LEDs with Ni/Al and
Ni/Au electrodes. This is in good agreement with the simulation results (Figure 2d), demonstrating that Ni:AlN-ODR is a
smart reflector for DUV light.
We demonstrated the high performance of the DUV-LED
with Ni:AlN-ODR as the reflector and p-electrode. To understand the underlying mechanism of the enhancements in
DUV-LED properties, we analyzed Ni:AlN-ODR using various
analytical methods. Figure 4a and Figure S5 in the Supporting
Information show the I–V characteristics, which can be used to
estimate the electrical contact characteristics between Ni:AlN/Al
and p-AlGaN. We already showed that the electrical state of the
AlN thin film was changed from insulating to conducting upon
the PEBD-induced Ni doping (Figure 1e). The I–V curve shows
that the Ni:AlN layer has an Ohmic-like contact to the p-AlGaN
layer, which is an important result of this study. The Ohmic
contact at the interface of Ni:AlN and p-AlGaN is comparable
to that of Ni/Au. However, the reflectance of Ni/Au is considerably lower than that of Ni:AlN-ODR (Figure 2b). In the X-ray
diffraction (XRD) pattern of the surface of p-AlGaN after the
PEBD process (Figure 4b), a new diffraction peak is observed at
2θ = 91.34°, not observed in the XRD pattern of p-AlGaN before
the PEBD process. The new peak is assigned to the (0004) facet
of Ni3N,[39] suggesting that a new structure of Ni3N is formed
between Ni:AlN and p-AlGaN. This is an important clue for the
understanding of the Ohmic contact between the Ni:AlN and
p-AlGaN layers. Figure 4c shows Auger electron spectroscopy
(AES) depth profiles of the Ni:AlN-ODR structure. In the top
panel, Ni atoms are observed throughout the AlN layer after
the PEBD, showing that Ni was diffused into the AlN in the
PEBD process.[31] Furthermore, Ni is detected at the end of AlN
and even on the interface between AlN and p-AlGaN after the
PEBD. This is different from the results in the bottom panel for
the ODR structure before the PEBD, where Ni is not observed
after the e-beam depth profiling for 7.5 min. This is consistent
with the XRD results, supporting the claim that the Ni atoms
diffused into AlN leading to not only the CFs inducing the
Ohmic contact at the interface but also Ni3N structures. Notably,
the Ga peak is also observed for the AlN layer, which implies
that during the PEBD process Ga atoms also diffused into
the AlN layer. This is consistent with the X-ray photoelectron
(XP) spectra of the Ni 2p and Ga 2p core levels (Figure 4d,e,
respectively). As shown in Figure 4d, no peak of Ni is observed
before the PEBD at the interface region, whereas two peaks
at binding energies of 853.1 and 869.9 eV are observed after
the PEBD at the shallow region of p-AlGaN. These two peaks
correspond to Ni 2p3/2 and 2p1/2, respectively, indicating that the
diffused Ni atoms directly contributed to the formation of the
Ni3N structure at the small depths of both AlN and p-AlGaN.[40]
In the XP spectrum of the Ga 2p core level (Figure 4e), the
peak attributed to Ga–N bonds is observed at ≈1116 eV. However, the Ga–N peak gradually blueshifted by ≈0.875 eV upon
the PEBD (Figure 4e), indicating that the Fermi level at the
interface shifted toward the valence band edge of p-AlGaN.
Table 1. Performances of the 280 nm FC DUV-LEDs with the Ni:AlN/Al, Ni/Au, and Ni/Al reflective electrodes. VF: forward voltage, Po: output power,
R: reflectance. All values are averaged using ten different specimens.
Electrode
VF [V] at 20 mA
PO [mW] at 100 mA
Ni:AlN/Al
9.75 ± 0.05
Ni/Au
9.22 ± 0.06
Ni/Al
10.89 ± 0.08
Adv. Optical Mater. 2019, 1901430
EQE [%, max]
WPE [%, max]
30.9 ± 0.3
8.49 ± 0.05
4.81 ± 0.06
94.2 ± 1.1
16.7 ± 0.5
4.32 ± 0.07
2.11 ± 0.09
29.7 ± 1.4
26.8 ± 0.5
7.52 ± 0.06
3.69 ± 0.11
72.4 ± 1.5
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R [%] at 280 nm
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Figure 4. Characteristics of the Ni:AlN-ODR layer. a) I–V curves of the Ni:AlN-ODR, Ni/Al, and Ni/Au electrodes contacting the p-AlGaN contact
layer. b) XRD patterns for the p-AlGaN surface before and after the PEBD process. c) AES depth profiles of the ODR after (top) and before (bottom)
the PEBD process. XP depth profiling spectra of d) Ni 2p and e) Ga 2p before and after the PEBD. All spectra were measured within the yellow region
of (c). Structural models and corresponding energy band diagrams of the f) normal p-AlGaN/AlN and g) Ni-doped p-AlGaN/N:AlN after the PEBD.
The Fermi level shift can lead to a reduction in Schottky
barrier height (SBH) between Ni:AlN and p-AlGaN.[41,42] This
claim is supported by the work function (5.62 and 5.73 eV) of
Ni3N measured by ultraviolet photoelectron spectroscopy and
Kelvin probe systems (Figure S6, Supporting Information). In
addition, the reduction in Ga peak intensity upon the PEBD is
attributed to the out-diffusion of Ga atoms from the p-AlGaN
layer, leaving Ga vacancies at the surface region of p-AlGaN.
As the Ga vacancies act as acceptors, the mutual diffusion of
Ni and Ga at the interface of Ni:AlN and p-AlGaN might contribute to an efficient injection of holes to the active layer. Based
on these results, structural models of the interface between
Ni:AlN and p-AlGaN are shown in Figure 4f,g. Figure 4f shows
an energy band diagram for the normal contact between AlN
Adv. Optical Mater. 2019, 1901430
and p-AlGaN and their crystal structures. Upon the PEBD with
Ni, both Ni and Ga atoms diffused into the shallow interfacial
region of p-AlGaN and AlN, leading to the formation of a fused
interface layer of Ni3N (Figure 4g, left). Accordingly, the SBH
between p-AlGaN and AlN is reduced (Figure 4g, right), which
contributes to a smooth transport of hole carriers to p-AlGaN
via CFs and consequently to a good Ohmic contact.
3. Conclusions
We developed a smart wide-bandgap ODR structure, which
was applied as the p-electrode of the FC DUV-LED. For the
conductive ODR structure, AlN was Ni-doped using the PEBD
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method. The reflectance of the smart ODR structure was over
90% in the whole wavelength range of 250–500 nm. The smart
ODR DUV-LED exhibited remarkably high EQE (8.49%) and
WPE (4.81%) without further structural modification. The high
performance of the DUV-LED, including the high reflectance,
low operation voltage, and Ohmic contact, originated from the
unique structure of Ni:AlN-ODR, where the fused interface of
Ni3N was formed through the mutual diffusion of Ni and Ga
during the PEBD process. The formation of the fused interface
and Ga vacancies led to an efficient hole transport and thus to
high EQE and WPE of the DUV-LED. The proposed smart ODR
structure is a promising reflector and p-electrode for AlGaNMQW-based DUV-LEDs.
3D FDTD Simulation: The 3D FDTD calculation was performed using
the FullWAVE LED simulator (RSoft, Synopsys). A TE-mode 283 nm
DUV-LED lighting source with a full width at half maximum of 10 nm and
single pointing source in the MQW layer were used in the simulation. A
detection monitor was set above the sapphire layers. For convenience,
the reported refractive indices of sapphire, AlN, and AlGaN[43–45] were
used in the simulation.
4. Experimental Section
Acknowledgements
Fabrication of the AlGaN-Based FC DUV-LED: The typical LED
structure consisted of a 3.7 µm thick undoped AlN buffer multilayer,
1.5 µm thick Si-doped (5 × 1018 cm−3) n-type Al0.6Ga0.4N layer, three
pairs of AlGaN-based undoped MQW layers consisting of ≈2 nm thick
AlGaN wells and 10 nm thick AlGaN barriers, 5 nm thick undoped
AlGaN capping barrier, two pairs of Mg-doped AlGaN/AlGaN
MQB (blocking: 10 nm, valley: 5 nm), and 80 nm thick Mg-doped
(1 × 1018 cm−3) Al0.6Ga0.4N p-type contact layer. These structures were
grown using metal–organic chemical vapor deposition on a sapphire
substrate. More information on the epitaxial growth details can
be found in the literature.[8] Subsequently, a 40 nm thick AlN film was
deposited inside the isolated mesa structures onto the p-Al0.6Ga0.4N
contact layer using a radio-frequency (RF) magnetron sputtering
system. The deposited 30 nm thick Ni pad for the PEBD process
had a diameter of 15 µm and gap between dots of 10 µm. An autoprobing contact system (PS-4A2P, Modusystems) was used for the
rapid PEBD process. After the PEBD process, the Ni pad was removed
using a nichrome etchant (Sigma-Aldrich). Finally, the 150 nm thick
Al reflector was deposited using an electron-beam evaporation
system at a deposition rate of 0.1 nm s−1. The chip dimensions were
1100 × 1100 µm2. Standard photolithography and RF sputtering were
then carried out to deposit a 50 nm SiO2 passivation layer for FC
soldering. All FC DUV-LEDs fabricated in this manner were packaged
using Al cup holders and glass/polydimethylsiloxane encapsulants for
a high efficiency and protection. The fabrication of the FC DUV-LED
with the Ni:AlN/Al Ohmic reflector is illustrated in Figure S1 in the
Supporting Information.
Electrical and Optical Characterizations: A Keithley 4200A
semiconductor characterization system was used to measure the
current–voltage curves for the contacts between the reflectors and
p-Al0.6Ga0.4N contact layers, based on transmission line method patterns
with a spacing of 5–25 µm and interval of 5 µm. The optical reflectance
values of the Ni:AlN/Al, Ni/Au, and Ni/Al electrodes deposited on
quartz substrates as a function of the wavelength were measured using
a Lambda 35 UV–vis spectrometer (PerkinElmer). A LED measurement
system (CAS140CT-152, Instrument Systems; spectrometer with an ISP500 integrating sphere) was used to measure the optical and electrical
properties including the L–I–V characteristics, EL intensities, EQE, and
WPE of the 280 nm FC DUV-LED. The angle-dependent light output
power (far-field intensity) was measured using an LED goniometer
facility consisting of a spectrometer and rotating jig. AES (PHI-700,
ULVAC-PHI, Japan) was carried out before and after the PEBD process to
measure the atomic concentrations using Ar-sputtering guns at a rate of
7.9 nm min−1. An XRD (SmartLab, Rigaku, Japan) θ−2θ scan was carried
out using Cu Kα radiation, in the 2θ range of 30°–100° at a scan rate and
speed of 0.01° and 1° min−1, respectively. The voltage and current were
45 kV and 200 mA, respectively. XP spectroscopy was carried out using
a 24.1-W monochromatized Al Kα radiation source (X-tool, ULVAC-PHI,
Japan) at a take-off angle of 45°.
T.H.L. and T.H.P. contributed equally to this work. This study was
supported by the National Research Foundation of Korea, funded by the
Korean government (No. 2016R1A3B1908249).
Adv. Optical Mater. 2019, 1901430
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
deep ultraviolet, light-emitting diodes, omnidirectional reflectors, pulsed
electrical breakdown
Received: August 21, 2019
Revised: October 15, 2019
Published online:
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