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nanomaterials
Article
32 ×32 Pixelated High-Power Flip-Chip Blue
Micro-LED-on-HFET Arrays for Submarine
Optical Communication
Tae Kyoung Kim †, Abu Bashar Mohammad Hamidul Islam †, Yu-Jung Cha and Joon Seop Kwak *
Citation: Kim, T.K.; Islam, A.B.M.H.;
Cha, Y.-J.; Kwak, J.S. 32 ×32 Pixelated
High-Power Flip-Chip Blue Micro-
LED-on-HFET Arrays for Submarine
Optical Communication. Nanomaterials
2021,11, 3045. https://doi.org/
10.3390/nano11113045
Academic Editors: Iván Mora-Seró
and Aurora Rizzo
Received: 24 September 2021
Accepted: 9 November 2021
Published: 12 November 2021
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Copyright: © 2021 by the authors.
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This article is an open access article
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Attribution (CC BY) license (https://
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4.0/).
Department of Energy Technology, Korea Institute of Energy Technology, Ujeong-ro, 72, Naju-si 58330, Korea;
tkkim@sunchon.ac.kr (T.K.K.); abmhis@kth.se (A.B.M.H.I.); yjrino@gmail.com (Y.-J.C.)
*Correspondence: jskwak@kentech.ac.kr
† Both authors have the equal contribution in this manuscript.
Abstract:
This work proposes the use of integrated high-power InGaN/GaN multiple-quantum-well
flip-chip blue micro light-emitting diode (
µ
-LED) arrays on an AlGaN/GaN-based heterojunction
field-effect transistor (HFET), also known as a high electron mobility transistor (HEMT), for various
applications: underwater wireless optical communication (UWOC) and smart lighting. Therefore, we
demonstrate high-power
µ
-LED-on-HEMT arrays that consist of 32
×
32 pixelated
µ
-LED arrays and
32
×
32 pixelated HEMT arrays and that are interconnected by a solder bump bonding technique.
Each pixel of the
µ
-LED arrays emits light in the HEMT on-state. The threshold voltage, the off-state
leakage current, and the drain current of the HEMT arrays are
−
4.6 V, <~1.1
×
10
−9
A at gate-to-
source voltage (V
GS
) =
−
10 V, and 21 mA at V
GS
= 4 V, respectively. At 12 mA, the forward voltage
and the light output power (LOP) of µ-LED arrays are ~4.05 V and ~3.5 mW, respectively. The LOP
of the integrated
µ
-LED-on-HEMT arrays increases from 0 to ~4 mW as the V
GS
increases from
−
6
to 4 V at V
DD
= 10 V. Each pixel of the integrated
µ
-LEDs exhibits a modulated high LOP at a peak
wavelength of ~450 nm, showing their potential as candidates for use in UWOC.
Keywords:
micro light-emitting diodes (
µ
-LEDs); high electron mobility transistor (HEMT); flip-chip;
solder bump bonding; µ-LED arrays
1. Introduction
III-nitride compound semiconductors are very promising candidates for use in light-
emitting diodes (LEDs) [
1
] and high electron mobility transistors (HEMTs) [
2
]. LEDs
have been widely used in smart lighting applications such as solid-state lighting, micro
displays, visible light communications, adaptive headlights for vehicles, and photo dy-
namic therapy [
3
–
6
]. Currently, underwater wireless optical communication (UWOC) is
receiving significant attention with regard to the development of underwater navigation,
sensor networks, and real-time underwater digital video streaming [
7
–
9
]. One of the
most important parts of UWOC is the light source. The blue spectrum has a relatively
small water attenuation [
7
,
9
]. High-power blue LEDs are a more suitable light source than
lasers or laser diodes due to their low cost and their being less harmful to human eyes [
7
].
Therefore, high-power LEDs with a driving circuit are required for implementing these
applications, where the driving circuit plays crucial role in improving the performance of
an LED. On the other hand, traditional LED drivers suffer from a low efficiency and high
power consumption due to the increase in parasitic elements caused by bonding wires [
10
].
The issues of parasitic elements can be circumvented by integrating LEDs with on-chip
drivers [
10
–
12
]. AlGaN/GaN-based field-effect transistors (FETs) are GaN-based drivers
that are very suitable for integrating GaN-based LEDs for high-power and high-frequency
switching applications owing to their high breakdown voltage [
13
], wide range of operating
temperatures [
14
], high operating frequency and speed [
15
], low specific on-resistance
(Ron) [4,10,15], and low power loss [16].
Nanomaterials 2021,11, 3045. https://doi.org/10.3390/nano11113045 https://www.mdpi.com/journal/nanomaterials
Nanomaterials 2021,11, 3045 2 of 12
The on-chip interconnection between LED and HEMT can be formed using monolithic
integration [
2
–
5
,
10
,
12
,
13
,
17
–
23
] and the flip-chip bonding technique [
24
,
25
]. Both technolo-
gies have their pros and cons. The on-chip monolithic integration of GaN-based LEDs with
high-power electronic devices was demonstrated using the selective epi removal (SER)
process [
2
,
3
,
20
,
22
], selective epitaxial growth (SEG) process [
5
,
17
,
21
,
23
], and a combination
of the SER and SEG processes [
12
], where the LED-on-HEMT monolithic integration uses
either with-wire [
2
–
4
,
17
,
18
,
20
] or without-wire [
5
,
10
,
12
,
13
,
19
,
21
] connections. The degra-
dation of integration-induced device performances was reduced by the flip-chip bonding
technique [
12
,
24
]. However, additional processes (substrate polishing and thinning) are
required to complete the device fabrication, which results in an increase in the device
fabrication complexity [
25
]. During SER monolithic integration, inductive-coupled-plasma
(ICP) etching damages the HEMT underneath and the p-GaN surface, which seriously
degrades the performances of LED-on-HEMT devices [
4
,
12
,
17
]. Somehow, the proposed
SEG process minimizes the limitations of the SER process [
4
,
17
]; nevertheless, it suffers
from the reliable metal interconnections between the SEG epitaxial layer and the SiO
2
mask [
26
], as well as the epitaxial growth temperature between InGaN/GaN-based MQWs
and AlGaN/GaN-based HEMT [
17
]. Therefore, the use of a combination of SER and SEG
processes was demonstrated to be effective for removing the drawbacks of the SER and
SEG processes, showing the excellent off-stage characteristics of HEMT and minimizing the
number of parasitic components involved [
12
]. Unfortunately, fabricated LED-on-HEMT
devices can only be implemented in low-power applications due to the low light output
power (LOP) at a gate-to-source voltage (V
GS
) of 1 V [
12
]. To address these issues, a flip-chip
solder bump bonding technique that is effective between LED and AlGaN/GaN-based
(HEMT) will provide a better solution for the monolithic integration and improve the
performances of LED-on-HEMT devices.
In this study, integrated
µ
-LED-on-HEMT arrays were created by interconnecting
32 ×32
pixelated flip-chip blue
µ
-LED arrays with 32
×
32 pixelated HEMT arrays; then,
the performance of the
µ
-LED-on-HEMT device was investigated. The drain current,
threshold voltage (V
th
), off-state leakage current, output characteristics, and transfer char-
acteristics of HEMT arrays and the current-voltage (I–V) characteristics, LOP, and peak
wavelength of LED arrays were systematically studied in order to understand the change
in the performance of
µ
-LED-on-HEMT arrays. The
µ
-LED-on-HEMT arrays have a very
low off-state leakage current <10
−9
A at V
GS
=
−
10 V. The LOP is about 2 times higher
than that of the monolithic integrated LED-on-HEMT devices, meaning that these will be
suitable for various novel applications.
2. Device Structure and Measurement Setup
AlGaN/GaN-based heterojunction field-effect transistors (HFETs), or HEMT devices,
with a similar epitaxial structure were grown on Si(111) substrate using metal-organic
chemical vapor deposition (MOCVD). The epitaxial structure consisted of a thin AlN seed
layer, a 450 nm-thick AlGaN/GaN super-lattice buffer layer, a 4.5
µ
m-thick carbon-doped
GaN buffer layer, a 250 nm-thick unintentionally doped GaN channel layer, a 1 nm-thick
AlN barrier layer, a 21 nm-thick Al
0.15
Ga
0.85
N barrier layer, and a 2 nm-thick GaN capping
layer, which were consecutively grown on the Si substrate as described in Figure 1a. After
cleaning the wafer, photolithography was used for patterning the HEMT mesa structure,
then it was selectively etched using ICP etching until a depth of 350 nm from the top was
reached, as shown in Figure 1b. A Ti/Al/Ni/Au (20/80/50/50 nm) metal contact was
deposited on the GaN/Al
0.15
Ga
0.85
N layer by electron-beam evaporation (EBE) to serve as
source and drain electrodes, as described in Figure 1c. Generally, a thin GaN capping layer
is used on the Al
0.15
Ga
0.85
N barrier in order to decrease the ohmic contact resistance. In
order to make ohmic contact, the device was then annealed at 850
◦
C in ambient N
2
for
1 min. A 100 nm-thick SiO
2
gate oxide layer was deposited on the Al
0.15
Ga
0.85
N barrier
by sputtering and then a lift-off process was carried out, as presented in Figure 1d. A
Nanomaterials 2021,11, 3045 3 of 12
Ni/Au (50/100 nm) metal gate was deposited on the SiO
2
/GaN channel and then the drain
electrode layer was exposed by EBE, as described in Figure 1e.
Nanomaterials 2021, 11, 3045 3 of 12
(50/100 nm) metal gate was deposited on the SiO2/GaN channel and then the drain electrode
layer was exposed by EBE, as described in Figure 1e.
Figure 1. (a–h) Schematic fabrication steps of an HEMT device. (i) Basic epitaxial structure of a fabricated μ-LED.
Figure 1. (a–h) Schematic fabrication steps of an HEMT device. (i) Basic epitaxial structure of a fabricated µ-LED.
Nanomaterials 2021,11, 3045 4 of 12
A 1st 4
µ
m-thick organic photosensitive polyimide (PSPI) inter-metal dielectric (IMD)
layer was formed and then patterned as via-holes on each source, drain, and gate electrode
by photolithography. Using an oven furnace, the deposited 1st PSPI layer was cured for
60 min at 320
◦
C in ambient N
2
, followed by performing an O
2
plasma treatment in order
to improve the adhesion properties between PSPI and the deposited metal pad [
27
]. To
form the 1st metal pad on each source and drain of the HEMTs, the via-holes of the 1st
IMD layer were filled with a 1st metal stack of EBE-based Ni/Al (50/100 nm) followed by
a sputtered Ti/Al/Ti/Al (30/500/30/500 nm) layer; then, a lift-off process was performed.
The deposited 1st IMD and metal pad layers are presented in Figure 1f. A gate pad line
on each HEMT device was also formed during this deposition. A 2nd IMD layer was
then formed with a 4
µ
m-thick PSPI on the 1st metal stack. Via-holes were formed on the
2nd IMD layer in order to deposit a 2nd metal stack on the source and drain of each device,
as shown in Figure 1g. The source pad line was then formed by filling the via-holes of the
2nd IMD layer. After that, a deposited 4
µ
m-thick PSPI 3rd IMD layer was patterned as
via-holes on the 2nd metal stack of each drain. A drain metal pad was deposited by filling
the via-holes with a 3rd metal stack, followed by a Ni/Au/Ni/Au (
50/300/50/300 nm
)
drain bonding pad; then, a lift-off process was performed, as described in Figure 1h. The
2nd and 3rd metal stacks were similar to the 1st metal stack. The curing and plasma
treatment conditions used for the 2nd and 3rd IMD layers were also similar to those used
for the 1st IMD layer. The plasma treatment of PSPI is essential for preventing the metal
from peeling off from the PSPI layer [
27
]. At an RF power = 30 W, PSPI plasma treatment
was performed for 30 s in ambient O
2
. The PSPI layer was found to be more suitable
compared to the SiO
2
/SiN
x
layer due to its large dielectric constant, easy pattering by the
standard photolithography process without any ICP etching, low cost made possible by
reducing the number of complex fabrication processes necessary, and improved yield by
minimizing the local stress [
27
,
28
]. The fabricated source, drain, and gate electrodes are
also interactively described in Figure 2using optical microscope and SEM images.
The blue LED wafers were grown on a c-plane patterned sapphire substrate (PSS) by
MOCVD. The epitaxial structure consisted of a 30 nm-thick GaN buffer layer, a 4
µ
m-thick
Si-doped n-GaN layer, a 100 nm-thick InGaN/GaN superlattice layer, a 41.5 nm-thick
five pairs In
0.18
Ga
0.82
N/GaN multiple-quantum-well (MQW) layer, a 40 nm-thick Mg-
doped p-Al
0.2
Ga
0.8
N electron-blocking layer, and finally a 200 nm-thick Mg-doped p-GaN
layer, which were sequentially grown on the PSS. The
µ
-LED arrays with a pixel area of
115 ×115 µm2
were patterned using a photolithography process, then the mesa structure
was etched by ICP in order to expose the n-GaN layer. The ITO treatment process was
then performed on the p-GaN surface, which was described in our previous work [
29
].
A 250 nm-thick p-type Ag reflector was deposited on the ITO treated p-GaN surface.
After that, a Ni/Al/Ni (100/500/100 nm) capping layer was deposited on the Ag/p-GaN
surface; a Cr/Al/Ni (30/500/100 nm) n-type electrode was deposited on the exposed
n-GaN surface; and finally a Ni/Au/Ni/Au (100/600/100/800 nm) capping layer was
deposited on the n-electrode, which served as a bonding pad. The detailed structure and
fabrication processes used for the 32
×
32 pixelated flip-chip
µ
-LED arrays are reported
elsewhere [29,30]. A fabricated µ-LED with its epitaxial structure is shown in Figure 1i.
In a fabricated
µ
-LED-on-HEMT device, each p-electrode bonding pad (see Figure 1i)
of
µ
-LED arrays is connected with each drain electrode (see Figure 1h) of HEMT arrays
using the flip-chip solder bump technique. First, a liquid soldering flux was deposited on
each p-bonding pad of the
µ
-LEDs. A 25
µ
m-radius solder ball was formed on the top of
liquid soldering flux. In order to form a solder bump, the
µ
-LED arrays were placed on
the hotplate at 260
◦
C for 15 s. Next, the sample was added to ethyl alcohol in order to
remove the flux solution from the p-bonding pad. Each solder bump/p-bonding pad of
the
µ
-LED arrays was properly aligned with each drain bonding pad of the HEMT arrays;
then, the
µ
-LED arrays were transferred onto the HEMT arrays. In order to form a proper
electrical connection, the transferred device was placed onto a hotplate at 300
◦
C for 1 m
and 30 s. Figure 3describes the solder bump on each p-bonding pad of the
µ
-LED arrays.
Nanomaterials 2021,11, 3045 5 of 12
The common n-type bonding pad of the 32
×
32 pixeled
µ
-LED arrays was then connected
with the ground (GND).
Nanomaterials 2021, 11, 3045 5 of 12
Figure 2. (a) Top-view optical microscope image of high-power HEMT arrays. High-resolution FE-SEM cross-sectional images
of fabricated HEMT in the (b) AA’ and (c) BB’ directions according to the optical microscope image. Magnified FE-SEM cross-
sectional images of the (d) drain, (e) gate, and (f) source metal pads, as marked by the boxes in Figure (b,c), respectively.
The blue LED wafers were grown on a c-plane patterned sapphire substrate (PSS) by
MOCVD. The epitaxial structure consisted of a 30 nm-thick GaN buffer layer, a 4 μm-thick
Si-doped n-GaN layer, a 100 nm-thick InGaN/GaN superlattice layer, a 41.5 nm-thick five
pairs In0.18Ga0.82N/GaN multiple-quantum-well (MQW) layer, a 40 nm-thick Mg-doped p-
Al0.2Ga0.8N electron-blocking layer, and finally a 200 nm-thick Mg-doped p-GaN layer,
which were sequentially grown on the PSS. The μ-LED arrays with a pixel area of 115 ×
115 μm2 were patterned using a photolithography process, then the mesa structure was
etched by ICP in order to expose the n-GaN layer. The ITO treatment process was then
performed on the p-GaN surface, which was described in our previous work [29]. A 250
nm-thick p-type Ag reflector was deposited on the ITO treated p-GaN surface. After that,
a Ni/Al/Ni (100/500/100 nm) capping layer was deposited on the Ag/p-GaN surface; a
Cr/Al/Ni (30/500/100 nm) n-type electrode was deposited on the exposed n-GaN surface;
and finally a Ni/Au/Ni/Au (100/600/100/800 nm) capping layer was deposited on the n-
electrode, which served as a bonding pad. The detailed structure and fabrication processes
used for the 32 × 32 pixelated flip-chip μ-LED arrays are reported elsewhere [29,30]. A
fabricated μ-LED with its epitaxial structure is shown in Figure 1i.
In a fabricated μ-LED-on-HEMT device, each p-electrode bonding pad (see Figure 1i)
of μ-LED arrays is connected with each drain electrode (see Figure 1h) of HEMT arrays
Figure 2.
(
a
) Top-view optical microscope image of high-power HEMT arrays. High-resolution FE-SEM cross-sectional images
of fabricated HEMT in the (
b
) AA’ and (
c
) BB’ directions according to the optical microscope image. Magnified FE-SEM
cross-sectional images of the (d) drain, (e) gate, and (f) source metal pads, as marked by the boxes in (b,c), respectively.
Nanomaterials 2021, 11, 3045 6 of 12
using the flip-chip solder bump technique. First, a liquid soldering flux was deposited on
each p-bonding pad of the μ-LEDs. A 25 μm-radius solder ball was formed on the top of
liquid soldering flux. In order to form a solder bump, the μ-LED arrays were placed on
the hotplate at 260 °C for 15 s. Next, the sample was added to ethyl alcohol in order to
remove the flux solution from the p-bonding pad. Each solder bump/p-bonding pad of
the μ-LED arrays was properly aligned with each drain bonding pad of the HEMT arrays;
then, the μ-LED arrays were transferred onto the HEMT arrays. In order to form a proper
electrical connection, the transferred device was placed onto a hotplate at 300 °C for 1 m
and 30 s. Figure 3 describes the solder bump on each p-bonding pad of the μ-LED arrays.
The common n-type bonding pad of the 32 × 32 pixeled μ-LED arrays was then connected
with the ground (GND).
Figure 3. Top-view high-resolution FE-SEM cross-sectional images of fabricated μ-LED arrays (a) without and (b) with a
50 μm solder bump on the p-bonding pad. Inset shows without/with solder bump for one pixel.
Keithley 2602B and 4145B (Cleveland, OH, USA) sourcemeters with a voltage sweep rate
of 0.35 V s−1 were used for measuring the current–voltage (I–V) characteristics of the μ-LED
and HEMT arrays, respectively. The electroluminescence (EL) intensity was measured by a Si
p-i-n photodiode. The EL spectra and the peak emission wavelength of the μ-LEDs were rec-
orded by a fiber-optic spectrometer (AvaSpec-2048, Avantes, Apeldoorn, The Netherlands).
The absolute LOP vs. current (L–I) was measured by an integrating sphere system, which was
connected to a spectrometer (CAS 140 CT, Instrument Systems, Munich, Germany). The light
intensity distributions of the μ-LEDs were measured using a Metrolux beam monitor 8304.
The cross-sectional and surface morphology images of the fabricated HEMT and μ-LED ar-
rays were taken by an ultrahigh-resolution (HR) Schottky field-emission SEM (JEOL JSM-
7610F Plus, Pleasanton, CA, USA).
3. Results and Discussion
Figure 4a shows an optical microscope image of 32 × 32 pixelated HEMT arrays with
drain and source bonding pads that are connected to the p-electrodes of μ-LED arrays and
a supply voltage (VDD), respectively. The gate width, the source-to-gate, and the gate-to-
drain distances of the fabricated HEMTs were 5, 5, and 12 μm, respectively. For a drain-
to-source voltage (VDS) = 10 V, the transfer characteristics of the fabricated AlGaN/GaN-
based HEMT arrays as a function of the gate-to-source voltage (VGS) are explained in Fig-
ure 4b. It was observed that the transfer characteristics of different pixels were fairly sim-
ilar. The Vth at which the drain current (ID) started to conduct was about −4.6 V for the
fabricated HEMT arrays. Figure 4c shows the output characteristics as a function of the
VDS. At VDS = 10 V (saturation region), the ID increased from 33.3 μA to 20.9 mA with the
increase in VGS from −6 to 4 V. The ID characteristics were also similar for different pixel
locations of HEMT arrays, as described in Figure 4d. Figure 4e reveals the measured off-
Figure 3.
Top-view high-resolution FE-SEM cross-sectional images of fabricated
µ
-LED arrays (
a
) without and (
b
) with a
50 µm solder bump on the p-bonding pad. Inset shows without/with solder bump for one pixel.
Nanomaterials 2021,11, 3045 6 of 12
Keithley 2602B and 4145B (Cleveland, OH, USA) sourcemeters with a voltage sweep
rate of 0.35 V s
−1
were used for measuring the current-voltage (I–V) characteristics of
the
µ
-LED and HEMT arrays, respectively. The electroluminescence (EL) intensity was
measured by a Si p-i-n photodiode. The EL spectra and the peak emission wavelength of the
µ
-LEDs were recorded by a fiber-optic spectrometer (AvaSpec-2048, Avantes, Apeldoorn,
The Netherlands). The absolute LOP vs. current (L–I) was measured by an integrating
sphere system, which was connected to a spectrometer (CAS 140 CT, Instrument Systems,
Munich, Germany). The light intensity distributions of the
µ
-LEDs were measured using a
Metrolux beam monitor 8304. The cross-sectional and surface morphology images of the
fabricated HEMT and
µ
-LED arrays were taken by an ultrahigh-resolution (HR) Schottky
field-emission SEM (JEOL JSM-7610F Plus, Pleasanton, CA, USA).
3. Results and Discussion
Figure 4a shows an optical microscope image of 32
×
32 pixelated HEMT arrays
with drain and source bonding pads that are connected to the p-electrodes of
µ
-LED
arrays and a supply voltage (V
DD
), respectively. The gate width, the source-to-gate, and
the gate-to-drain distances of the fabricated HEMTs were 5, 5, and 12
µ
m, respectively.
For a drain-to-source voltage (V
DS
) = 10 V, the transfer characteristics of the fabricated
AlGaN/GaN-based HEMT arrays as a function of the gate-to-source voltage (V
GS
) are
explained in Figure 4b. It was observed that the transfer characteristics of different pixels
were fairly similar. The V
th
at which the drain current (I
D
) started to conduct was about
−
4.6 V for the fabricated HEMT arrays. Figure 4c shows the output characteristics as a
function of the V
DS
. At V
DS
= 10 V (saturation region), the I
D
increased from 33.3
µ
A
to 20.9 mA with the increase in V
GS
from
−
6 to 4 V. The I
D
characteristics were also
similar for different pixel locations of HEMT arrays, as described in Figure 4d. Figure 4e
reveals the measured off-state leakage current for different pixel locations, which was about
1.1 ×10−9A
at V
GS
=
−
10 V and V
DS
= 10 V. For different pixels, the measured I
D
was
about 21 mA at V
GS
= 4 V and V
DS
= 10 V, as shown in Figure 4f. From the experimental
results, we found that each pixel of the HEMT arrays had similar electrical characteristics.
Figure 5a shows the optical microscope image of fabricated 32
×
32 pixelated
µ
-LED
arrays with p- and n-type bonding pads that are connected with drain and GND bonding
pads, respectively, using flip-chip bonding technology. The I–Vcharacteristics of different
pixel positions are shown in Figure 5b; they are similar for each pixel of the
µ
-LED arrays.
The forward voltage was ~2.9 V at 0.25 mA for each pixel of the
µ
-LEDs. The LOP and
the EL spectra at 12 mA for various pixel positions of the
µ
-LED arrays are shown in
Figure 5c,d
, respectively; they were also similar for each pixel, similar to the I–Vcurve. At
12 mA, the forward voltages of different pixel locations are described in Figure 5e; they
were about 4.05 V for the
µ
-LED arrays. Figure 5f shows the LOP characteristics for various
pixel positions at 12 m; these positions were also similar (~3.5 mW) to the I–Vand EL
spectra curves. From the experimental results, it can clearly be observed that each pixel of
the fabricated µ-LED arrays had a similar optoelectronic performance.
Figure 6reveals the optoelectronic performances of the integrated 32
×
32 pixelated
µ
-LED-on-HEMT arrays. An optical microscope image of the integrated
µ
-LED-on-HEMT
arrays with different electrodes is depicted in Figure 6a. The LOP and forward current of
the µ-LED-on-HEMT device as a function of VDD is described in Figure 6b.
Nanomaterials 2021,11, 3045 7 of 12
Nanomaterials 2021, 11, 3045 7 of 12
state leakage current for different pixel locations, which was about 1.1 × 10−9 A at VGS = −10
V and VDS = 10 V. For different pixels, the measured ID was about 21 mA at VGS = 4 V and
VDS = 10 V, as shown in Figure 4f. From the experimental results, we found that each pixel
of the HEMT arrays had similar electrical characteristics.
Figure 4. (a) Optical microscope image of 32 × 32 pixelated HEMT arrays. (b) Transfer characteristics of HEMT arrays for
different pixel positions at VDS = 10 V. (c) Output characteristics for pixel location (16,16). (d) ID characteristics of HEMT
arrays for different pixel locations at VGS = 4 V. (e) Off-state leakage current for different pixel locations at VGS = −10 V and
VDS = 10 V. (f) Drain current for different pixel locations at VGS = 4 V and VDS = 10 V.
Figure 5a shows the optical microscope image of fabricated 32 × 32 pixelated μ-LED
arrays with p- and n-type bonding pads that are connected with drain and GND bonding
pads, respectively, using flip-chip bonding technology. The I–V characteristics of different
pixel positions are shown in Figure 5b; they are similar for each pixel of the μ-LED arrays.
The forward voltage was ~2.9 V at 0.25 mA for each pixel of the μ-LEDs. The LOP and the
Figure 4. (a) Optical microscope image of 32 ×32 pixelated HEMT arrays. (b) Transfer characteristics of HEMT arrays for
different pixel positions at V
DS
= 10 V. (
c
) Output characteristics for pixel location (16,16). (
d
)I
D
characteristics of HEMT
arrays for different pixel locations at V
GS
= 4 V. (
e
) Off-state leakage current for different pixel locations at V
GS
=
−
10 V and
VDS = 10 V. (f) Drain current for different pixel locations at VGS = 4 V and VDS = 10 V.
Nanomaterials 2021,11, 3045 8 of 12
Nanomaterials 2021, 11, 3045 8 of 12
EL spectra at 12 mA for various pixel positions of the μ-LED arrays are shown in Figure
5c,d, respectively; they were also similar for each pixel, similar to the I–V curve. At 12 mA,
the forward voltages of different pixel locations are described in Figure 5e; they were
about 4.05 V for the μ-LED arrays. Figure 5f shows the LOP characteristics for various
pixel positions at 12 m; these positions were also similar (~3.5 mW) to the I–V and EL
spectra curves. From the experimental results, it can clearly be observed that each pixel of
the fabricated μ-LED arrays had a similar optoelectronic performance.
Figure 5. (a) Optical microscope image of 32 × 32 pixelated μ-LED arrays. (b) I–V characteristics, (c)
LOP characteristics, and (d) EL spectra of the fabricated μ-LED arrays for various pixel positions.
(e) Forward voltage and (f) LOP characteristics at 12 mA for various pixel positions of the fabricated
μ-LED arrays.
Figure 6 reveals the optoelectronic performances of the integrated 32 × 32 pixelated
μ-LED-on-HEMT arrays. An optical microscope image of the integrated μ-LED-on-HEMT
arrays with different electrodes is depicted in Figure 6a. The LOP and forward current of
the μ-LED-on-HEMT device as a function of VDD is described in Figure 6b.
Reflective
p -type
electrode
32 pixel
32 pixel
p -type
bonding pad
n -type
bonding pad
(1,1)
(32,32)
(a)
01234
0
2
4
6
8
10
12
(20,20)
(24,24)
(28,28)
(32,32)
Current (mA)
Voltage (V)
Pixel position
(1,1)
(4,4)
(8,8)
(12,12)
(16,16)
(b)
0246810 12
0
1
2
3
4
(20,20)
(24,24)
(28,28)
(32,32)
LOP (a.u.)
Current (mA)
Pixel position
(1,1)
(4,4)
(8,8)
(12,12)
(16,16)
(c)
350 400 450 500 550
0.00
0.05
0.10
0.15
0.20
EL Intensity (a.u.)
Wavelength (nm)
Pixel position
(1,1)
(4,4)
(8,8)
(12,12)
(16,16)
(20,20)
(24,24)
(28,28)
(32,32)
(d)
0 4 8 12 16 20 24 28 32
3.6
3.8
4.0
4.2
4.4
Voltage (V)
Pixel location
At 12 mA
(e)
0 4 8 12 16 20 24 28 32
0
1
2
3
4
LOP (a.u.)
Pixel location
At 12 mA
(f)
Figure 5.
(
a
) Optical microscope image of 32
×
32 pixelated
µ
-LED arrays. (
b
)I–V characteristics, (
c
) LOP characteristics,
and (
d
) EL spectra of the fabricated
µ
-LED arrays for various pixel positions. (
e
) Forward voltage and (
f
) LOP characteristics
at 12 mA for various pixel positions of the fabricated µ-LED arrays.
Nanomaterials 2021,11, 3045 9 of 12
Nanomaterials 2021, 11, 3045 9 of 12
Figure 6. (a) Optical microscope image of the 32 × 32 pixelated μ-LED-on-HEMT arrays. (b) LOP and
forward current characteristics of the μ-LED-on-HEMT arrays at different VGS. (c) EL spectra of fabricated
μ-LED-on-HEMT arrays at different VGS. The inset shows the light intensity distribution at VGS = 4 V.
Both the LOP and forward voltage increased with an increase in VGS. At VDD = 10 V, the
LOP and the forward voltage increased from 0 to 3.7 mW and 0 to 11.8 mA, respectively, as
the VGS increased from −6 to 4 V. At VGS = −6 V, there was no LOP from the integrated μ-LED
arrays with HEMT arrays. The Vth of the HEMT arrays was about −4.6 V (see Figure 4); there-
fore, the integrated μ-LED devices could be operated at only ≥−4.5 V. In order to turn on the
Figure 6.
(
a
) Optical microscope image of the 32
×
32 pixelated
µ
-LED-on-HEMT arrays. (
b
) LOP
and forward current characteristics of the
µ
-LED-on-HEMT arrays at different V
GS
. (
c
) EL spectra of
fabricated
µ
-LED-on-HEMT arrays at different V
GS
. The inset shows the light intensity distribution
at VGS = 4 V.
Nanomaterials 2021,11, 3045 10 of 12
Both the LOP and forward voltage increased with an increase in V
GS
. At V
DD
= 10 V,
the LOP and the forward voltage increased from 0 to 3.7 mW and 0 to 11.8 mA, respectively,
as the V
GS
increased from
−
6 to 4 V. At V
GS
=
−
6 V, there was no LOP from the integrated
µ
-LED arrays with HEMT arrays. The V
th
of the HEMT arrays was about
−
4.6 V (see
Figure 4); therefore, the integrated
µ
-LED devices could be operated at only
≥−
4.5 V. In
order to turn on the integrated
µ
-LEDs, the V
DD
should be greater than or equal to the
turn-on voltage of the
µ
-LEDs (see Figure 5). It is worth noting that the LOP at V
GS
=4V
increased eight times compared to the V
GS
=
−
2 V (0.5 mW), as the V
DD
was kept constant
at 10 V. In addition, the on-state resistance decreased with an increase in V
GS
from
−
4 to
4 V due to the decrease in 2DEG resistance (this can be seen from the slope of the forward
voltage characteristics) [24,31].
The EL spectra of the integrated
µ
-LEDs at different V
G
sare shown in Figure 6c. The
increased V
GS
caused the forward current to increase, which resulted in an increase in the EL
intensity. In addition, there was no shift in the peak emission wavelength with an increase in
the V
GS
. The integrated
µ
-LEDs had peak emission wavelengths of 450 nm. The inset shows
the uniform light intensity distribution over the chip size at a forward current = 11.8 mA,
V
GS
= 4 V, and V
DD
= 10 V. The experimentally obtained optoelectronic performances of the
integrated
µ
-LED-on-HEMT devices were comparable with the monolithically integrated
LEDs with higher-power devices [
3
,
12
,
17
]. In addition, the LOP of the
µ
-LED-on-HEMT
device was 3.5 mW at V
GS
= 4 V, which was two times higher than the reported LOP
(1.5 mW) of the
µ
-LED source for UWOC [
8
]. This demonstrates that the integrated
GaN-based
µ
-LED-on-HEMT device may be a promising candidate for UWOC and other
lighting applications.
4. Conclusions
In summary, we successfully fabricated
µ
-LED-on-HEMT arrays and then investigated
their optoelectronic properties. Firstly, we separately fabricated the Al
0.15
Ga
0.85
N/GaN-
based 32
×
32 pixelated HEMT arrays on a Si wafer and the 32
×
32 pixelated flip-chip blue
µ
-LED arrays on PSS. Then, each p-type bonding pad of the
µ
-LED arrays was connected
with each drain pad of the HEMT arrays by the flip-chip solder bump technique. The
experimental results show that the electrical and optoelectronic performances of each pixel
of the fabricated HEMT and
µ
-LED arrays were fairly similar. Thus, uniform optoelectronic
performances were also obtained for each pixel of the integrated
µ
-LED-on-HEMT arrays.
In order to turn on each pixel of the
µ
-LED-on-HEMT arrays, the V
DD
and the V
GS
voltages
should be
≥
3 and
≥−
4.5 V, respectively. The V
th
of the fabricated HEMT arrays was about
−
4.6 V; at V
GS
= 4 V, the LOP increased eight times compared to the V
GS
=
−
4 V when the
V
DD
was kept constant at 10 V. The optoelectronic performances of the
µ
-LED-on-HEMT
device were comparable with those of other monolithically integrated LED-on-HEMT
devices as well as the
µ
-LED light source of UWOC. Therefore, we anticipate that the
on-chip interconnected
µ
-LED-on-HEMT arrays will be potential candidates for use in
various novel applications.
Author Contributions:
Conceptualization, T.K.K. and A.B.M.H.I.; formal analysis, T.K.K., Y.-J.C.
and A.B.M.H.I.; investigation, T.K.K. and A.B.M.H.I.; resources, T.K.K.; data curation, Y.-J.C. and
A.B.M.H.I.; validation, T.K.K. and Y.-J.C.; visualization, T.K.K. and A.B.M.H.I.; supervision, J.S.K.;
project administration, T.K.K. and J.S.K.; funding acquisition, J.S.K.; writing—original draft prepa-
ration, A.B.M.H.I.; writing—review and editing, A.B.M.H.I. and J.S.K. All authors have read and
agreed to the published version of the manuscript.
Funding:
This research was supported by the KENTECH research (grant no. KRG2021-01-002),
the Nano-Material Technology Development Program through the National Research Foundation
of Korea (NRF) funded by Ministry of Science and ICT (grant no. 2021M3D1A2048623), the Na-
tional Research Foundation of Korea grant funded by the Korean government (MSIT) (grant no.
2020M3H4A1A02084899), and the Korea Evaluation Institute of Industrial Technology, (grant no.
20004946, “Development of light source and frontplane technology for ultra-high resolution pixels
over 50k nits”).
Nanomaterials 2021,11, 3045 11 of 12
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement:
The data are available upon reasonable request from the correspond-
ing author.
Conflicts of Interest: The authors declare no conflict of interest.
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