http://dx.doi.org/10.5573/JSTS.2015.15.1.114
JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.15, NO.1, FEBRUARY, 2015
Effects of Current Spreading in GaN-based Lightemitting Diodes Using ITO Spreading Pad
Jang Hyun Kim1, Garam Kim1, Euyhwan Park1, Dong Hoon Kang2, and Byung-Gook Park1
Abstract— In conventional LEDs, a mesa-structure is
usually used and it causes the current to be
overcrowded in a specific region. We propose a novel
structure of GaN-based LED to overcome this
problem. In order to distribute the current in an
active region, a spreading pad is inserted at the p-type
region in the GaN based LED device. The inserted
spreading pad helps the current flow because it is
more conductive than the p-type GaN layer. By
performing electrical and optical simulations, the
effects of the spreading pad insertion are confirmed.
The results of electrical simulation show that the
current spreads more uniformly and more radiative
recombination is produced as well. Moreover, from
the optical simulation, it is revealed that the ITO is
less absorptive material than p-GaN if the condition
of specific wavelength sources is satisfied.
Considering all of the results, we can conclude that
the luminescent power is enhanced by the spreading
pad.
Index Terms—Spreading pad, GaN LED, current
spreading
I. INTRODUCTION
Recently, the GaN-based LED has received much
attention for its high efficiency and long life expectancy
Manuscript received Jul. 17, 2014; accepted Dec. 29, 2014
1
Inter-University Semiconductor Research Center and Department of
Electrical Engineering and Computer Science, Seoul National
University, Seoul 151-744, Republic of Korea
2
Samsung Electronics Co. Ltd., Yongin 446-711, Republic of Korea
E-mail : bgpark@snu.ac.kr
[1-4]. Moreover, it has been improved in aspects of
material quality and extraction efficiency to become a
general light source [5, 6]. Typical LED devices usually
take a shape of the mesa-structure [7-9]. The reason is
that the mesa structure has advantages in that contacts are
readily implementable, based on the epi-grown p-n
junction [10, 11]. The performance of LED devices with
a mesa structure depends on lateral carrier injection.
However, the major problem of using the lateral carrier
injection is that it can lead to a non-uniform current
spreading in the GaN based LEDs [12, 13]. Non-uniform
current spreading can significantly degrade the
performance of GaN-based LED in that the current
becomes overcrowded in a localized region of the device.
This problem is caused by the difculties in
manufacturing a heavily doped p-type material [14-16].
Since non-uniformity occurs due to the lateral resistance
of p-type layer, modification of physical thickness might
not be the right solution. Many researches have tried to
solve this problem by using spreading layers [17-19].
Until now, indium-tin-oxide (ITO) has been used broadly
as a current spreading layer for its transparency and low
resistance [20-22]. Although the ITO in the LED device
makes current spread widely toward the active region,
however, the non-uniformity of current spreading has not
been solved completely [23, 24]. The reason is that as the
ITO layer becomes thicker, the problem of non-uniform
current in the LED can be improved. Meanwhile,
emitting light through the ITO layer gets also lowered.
To be specific, in case of operating the device with high
bias, the current, injected into the typical LED, can be
overcrowded in the region near n-type contact due to the
difference of the resistance between current spreading
layer and p-type GaN layer. Therefore, for the purpose of
JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.15, NO.1, FEBRUARY, 2015
0.1 mm
1.0 mm
0
2
4
6
8
10
12
Spreading pad location [mm]
Fig. 1. Schematic cross view of GaN-based LED device using
spreading pad. The spreading pad consist the ITO same with
spreading layer.
0.5
Total current density [a.u.]
II. DEVICE STRUCTURE
Fig. 1 shows the schematic cross-view of the GaNbased LED device in which spreading pad is inserted.
The used LED device is the conventional one with mesa
structure and the undoped layer is used as a substrate.
The cross-section area of the undoped GaN layer is 10 x
1 μm115. On the undoped GaN layer, the n-type GaN
layer is set and its depth is 0.5 μm. On the n-type GaN
layer, the active layer whose cross-section area is 9 x
0.001 μm2. This active layer consists of InGaN, which
radiates blue light through electron-hole pair
recombination. The next step is to form n-type Al contact
on top right of n-type and p-type layers respectively.
Finally, the p-type GaN layer is set on the active layer
and its depth is 0.1 μm. Doping concentration of the ptype GaN and n-type GaN are 1 x 1017 cm-3 and 2 x 1018
cm-3, respectively.
In order to spread current, the ITO layer is put on the
p-type GaN layer and its depth is 0.05 μm. On top left of
it, Al is deposited to form the contact as a p-type contact.
The cross-section area of Al contact metal is the same as
0.5 μm x 0.5 μm respectively. After completing the
conventional LED setting, a spreading pad which
consists of ITO is inserted within the p-type GaN layer.
In the simulation process, the dimension of spreading pad
is varied for confirming how current-flow changes in
each case. Contact resistivity between ITO and p-GaN is
50 mΩ/cm2 and the conductivity of ITO is 2.4 Ω•μm.
Active layer (1 nm)
1
p-type GaN
InGaN
InGaN
n-type GaN
0.5 mmlocation
Pad location
0.4
4 mm 5 mm
6 mm
7 mm
1.7
1.6
8 mm
0.3
w/o Pad
w/ Pad
7 mm
6 mm
5 mm
8 mm
4 mm
0.2
1.5
1.4
1.3
1.2
1.1
0.1
1.0
0.0
0.9
0
1
2
3
4
5
6
7
8
Radiative Recomb. Rate [a.u.]
improving the uniformity of current spreading, the
structure of the LED needs to be redesigned.
In this paper, we propose the insertion of a spreading
pad in order to increase uniformity of current in the GaNbased LED device. To confirm the effects of inserting
spreading pad, the electrical and optical TCAD
simulations were performed. In addition, optical
simulation was performed to find out how the generated
light in the proposed structure influences on external
quantum efficiency. From the simulation, it was
confirmed that the generated light in the active region
can be emitted through the inserted spreading pad when
it is with specific wavelengths.
115
9
Data measured position in active layer [mm]
Fig. 2. Total current density and radiative recombination rate
depending on the active position. The spreading pad is inserted
at the center of the LED device (location @ 5 μm). The width
and depth of spreading pad are 0.5 μm, 90 nm respectively.
III. ELECTRICAL DEVICE SIMULATION
The purpose of performing electrical device simulation
is to confirm the electrical effects occurring on the active
layer. SILVACOTM ATLAS is used as a simulation tool.
At first, the TCAD electrical simulation is performed by
inserting spreading pad. The active layer is a single layer
with 1 nm In0.2Ga0.8N. The recombination parameters are
set as follows: Shockly-Read-Hall (SRH) coefficient is
5.0 x 106 s-1 and the radiative recombination coefficient
is 1.0 x 10-11 cm-3s-1. Then auger recombination
coefficient is 2.0 x 10-31 cm6s-1
First, spreading pad is inserted into p-type layer,
specifically at the positions of 5~8 μm, in order to
identify the changes of currents and radiative
recombination. Fig. 2 indicates the current density and
radiative recombination rate when the spreading pad is
0
1.04
2
4
6
8
Spreading pad depth
20 nm
40 nm
60 nm
80 nm
90 nm
95 nm
105 nm
1.03
1.02
1.01
1.00
0.20
0.99
0.00
0.98
0
1
2
3
4
5
6
7
Spreading
pad location [mm]
S
8
9
(b)
Luminescent Power (a.u.)
inserted at the center of the p-type layer. The point on the
gragh (Fig. 1), the left edge of the pad, represents the
pad’s position in Fig. 2. As spread pad’s location
becomes increased, it is shifted toward the n-type contact.
The width and depth of the spreading pad is 0.5 μm and
90 nm respectively. In the active layer, the impacts of
inserting the pad appear as the increase of total current,
which is the sum of electron current and hole current and
radiative recombination rate. From this total current and
radiative recombination results, it can predict changes of
optical power by inserting spreading pad. From the
simulation, the enhancement of current spreading is
observed and also the number of radiative recombination
is increased as well. Thus, improved current spreading
can be verified in the proposed structure. First, the
enhancement of current spreading happens due to the
differences in resistance. To be specific, the pad is made
of ITO and its resistance is less than that of p-GaN.
Therefore, in the proposed structure, more current can be
supplied through the pad to the active region below the
pad. Second, the increased number of radiative
recombination is due to the increased current in the
active region.
Furthermore, to maximize the effect of current
spreading in the active layer, the physical dimension of
pad is adjusted. The effect of current spreading pad
depends on the physical dimensions since the physical
dimension of the spreading pad can affect the current
flow in the active region. Therefore, the width, depth and
the location of spreading pad needs to be optimized as
well.
The results of the simulation by varying the location
and depth of the spreading pad are illustrated in Fig. 3(a).
With the fixed width of the pad, 0.5 μm, the deeper the
location of spreading pad is, the more the luminescent
power increases. As illustrated in Fig. 3(a), once the pad
is inserted over the p-type GaN, however, the
luminescent power sharply decreases since the holes are
injected directly in the n-type region. The reason is that
deeper location of spreading pad enhances carrier
injection in the active region. The simulation is
performed to observe how the luminescent power
changes as the location of spreading pad is varied. At the
position of 5.5 μm, the highest luminescent power is
obtained. If the spreading pad is located less than 5.5 μm
away from the p-type contact, the luminescent power
Luminescent Power Enhancement Ratio@ 0.2A
JANG HYUN KIM et al : EFFECTS OF CURRENT SPREADING IN GAN-BASED LIGHT-EMITTING DIODES USING ITO …
0.01 A
0.1 A
-1
Luminescent Power Enhancement Ratio (a. u.)
116
15
12
9
0
1
2
3
4
5
6
7
Spreading pad location [mm]
8
9
(c)
Pad width
0.1 mm
0.3 mm
0.5 mm
1.0 mm
1.5 mm
3.0 mm
6
3
0
0.00
0.05
0.10
0.15
0.20
0.25
Applied Current [A]
Fig. 3. Luminescent power versus spreading pad location with
various (a) depth (at 0.2 A current), (b) current, (c)
Luminescent power versus applied current with various width,
(d) Illustration of the current crowding in the wide spreading
pad.
JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.15, NO.1, FEBRUARY, 2015
current doesn’t flow uniformly and that results in the
decrease of radiative recombination.
IV. OPTICAL DEVICE SIMULATION
The ITO and GaN materials mentioned above have
been assumed to be fully transparent and the reflection
and absorption is ignored in the previous electric
simulation. In other words, all the results are obtained
assuming that the external quantum efficiency is close to
1. However, the ITO and GaN materials are somewhat
absorptive in the visible region of the optical spectrum[25, 26]. In addition, the light derived from the active
layer can be reflected depending on the structure.
Therefore, the amount of externally emitted light cannot
be accurately calculated by the electronic simulation
alone. Moreover, the inserted spreading pad can have
some effects on the reflection or absorption of the light.
Consequently, performing optical simulation is required.
We chose the COMSOL as a tool for optical simulation.
Considering the shape of the active layer in the proposed
structure, we insert the optical source with the same shape
(Fig. 4). Then, the refractive indexes of GaN and ITO are
set. The refractive index is composed of a real part and an
imaginary part [27]. The real part is related to the refraction
between bordered materials. The imaginary part reveals the
optical extinction [28] (Eq. (1)).
c = c ¢ + j c ¢¢
(1)
where c ¢ represents the refractive coefficient and χ``
represents the extinction coefficient. The refractive index
is shown in Fig. 5 [29-31]. After optical light source with
power 1 W is inserted, the width and depth of pad is
1500
1250
Height [nm]
shows just a little enhancement compared with the
conventional LED structure since the resistance under the
p-type contact is too high to reduce by adapting a
spreading pad. Meanwhile, as the location moves more
than 5.5 μm away from the p-type contact, meaning that
the spreading pad is located near the n-type contact, the
luminescent power drops dramatically. This is because
the spreading pad located closer to the n-type contact
makes the current to be crowded. This can explain the
reason why the current flow spreads less uniformly in the
active region.
In addition, to confirm the properties of different LED
operating current, both high and low current need to be
considered (Fig. 3(b)). At low current level, the
luminescent power increases as the pad moves closer to
the n-type contact. This phenomenon occurs as more
current is concentrated in the active layer near the n-type
contact. Injecting the low current is not appropriate for
maximizing luminescent power in the active layer. Thus
the closer the spreading pad locates to the n-type contact,
the more the current crowded and the efficiency increases.
At high current level, however, as the spreading pad
moves close to the n-type contact, the total luminescent
power decreases due to the current crowding effect. In
this case, the current crowding effect increases the carrier
concentration to such a high level that Auger
recombination dominates. Then, most of the carriers
recombine non-radiatively by Auger recombination
mechanism, resulting in the reduction of the luminescent
power. From the results, it is confirmed that the location
of the spreading pad, should be adjusted by operating
current.
Next, we characterized the effects of the pad width. As
shown in Fig. 3(c), the highest luminescent power is
observed when the width of spreading pad is 1.5 μm.
When the width becomes larger than 1.5 μm, the
luminescent power decreases due to the current crowding
as the current increases. Consequently, the best efficiency
of luminescent power can be obtained at the point of 1.5
μm, considering the current crowding and lowered
improvement of resistance. As the width of spreading
layer becomes narrower, the current is pushed to the
narrow region. If the width becomes too large, however,
the current redirection effect of the pad would be
compromised, since the current crowding can occur
within the width of the pad (Fig. 3(d)). Thus, the whole
117
1000
750
500
250
0
1
2
3
4
5
Location [mm]
6
7
8
9
Fig. 4. Illustration of simulated results. In the simulation,
spreading pad is inserted with uniform light source. And the
reflective indexes of GaN and ITO are set.
118
JANG HYUN KIM et al : EFFECTS OF CURRENT SPREADING IN GAN-BASED LIGHT-EMITTING DIODES USING ITO …
100
4.0
3.5
Extinction coefficient
Refractive coefficient
ITO Refec.
GaN Refec.
ITO Extinc.
GaN Extinc.
3.0
2.5
10-1
2.0
1.5
1.0
0.5
0.0
300
400
500
600
700
800
Wavelength [nm]
10-2
900 1000
Fig. 5. Refractive and extinction coefficients as a function of
wavelength.
layer, the amount of luminescent power increases. In
addition, as the width of spreading pad becomes wider, the
luminescent power increases linearly. Since the spreading
pad absorbs the light less than p-type GaN layer, the
spreading pad can help the luminescent power increase.
To identify more effects of extinction coefficient, 350
nm light source and its reflective index are utilized in the
simulation (Fig. 6(b)). According to the simulation
results, the overall luminescent power increases when
using 450 nm light source compared with 350 nm light
source. As shown in Fig. 5, the difference of extinction
coefficient between ITO and GaN causes this happen.
The difference becomes much bigger when using 350 nm
than using 450nm of light source. The refraction and
extinction coefficient depend on the wavelength for GaN
and ITO.
To summarize, inserting spreading pad can lead to
increase of the luminescent power. Such increase of
optical power, however, is observable only with the
specific wavelengths. The wavefunction of the generated
light from the active layer becomes
U ( z ) = A exp(- j c z )
(2)
where z is the distance from the active layer and A is
amplitude. The optical intensity is the absolute square of
its wavefunction.
Optical intensity = U ( z )
Fig. 6. Luminescent power enhancement compared with
conventional GaN-based LED. The active layer is set as a (a)
450 nm, (b) 350 nm light source.
varied. The extracted luminescent power is measured
over the top of the LED. As shown in Fig. 6(a), more
than 40 nm of depth does not lead to decrease of
luminescent power with 450 nm light source.
Another simulation is performed when the depth of
spreading pad is 80 nm. The luminescent power increases.
These observations certify that as we insert the spreading
pad more deeply, more light can be emitted externally. In
other words, as the spreading pad gets closer to the active
2
(3)
It was shown in Eqs. (2) and (3), the intensity of the
generated light is exponentially decreased with thickness
through the media and extinction coefficient [32].
Therefore, in the wavelength from 250 nm to 450 nm, the
extinction coefficient of ITO becomes smaller than that
of GaN and the penetration of light through the spreading
pad can be enhanced. Consequently, the spreading pad
would be advantageous in the LED whose active layer
radiates the light at 250 nm ~ 450 nm. Additionally, we
tried to divide the spreading pad into half and used both
in the simulation, intending to extend the boundary lines
and find out whether their reflection effects are
influenced or not. As shown in Figs. 6(a) and (b), effects
of the reflectance by boundary can be ignored.
In summary, in order to manufacture LED devices
with spreading pad, we need to consider both the
extinction coefficient of spreading layer and the
luminescent power on the active region. In the GaN-
JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.15, NO.1, FEBRUARY, 2015
based LED with ITO-pad, the luminescent power is
mainly influenced by the difference of each extinction
coefficient. In addition, the proposed structure using ITO
spreading layer has advantages in LEDs with 250 ~ 450
nm wavelength.
V. CONCLUSION
A novel structure in the GaN-based LED with a
spreading pad is proposed in order to overcome the
current crowding problem in the conventional LED. The
inserted spreading pad helps the current to be distributed
better because it has less resistance than the p-type GaN
layer. In order to confirm the effects of the spreading pad
insertion, electrical and optical simulations are performed
for internal and external efficiency calculation. The
results of electrical simulation show that the current
spreads more uniformly and more radiative
recombination is produced. From the optical simulation,
ITO is revealed as less absorptive material than p-GaN
with the specific wavelength sources. As a result,
luminescent power is enhanced by the spreading pad.
ACKNOWLEDGMENTS
This work is supported by Samsung Electronics, the
Future Semiconductor Device Technology Development
Program (10044842) funded By MOTIE (Ministry of
Trade, Industry & Energy), KSRC(Korea Semiconductor
Research Consortium) and the Brain Korea 21 Plus
Project in 2014.
REFERENCES
[1]
[2]
[3]
S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T.
Yamada, T. Matsushita, Y. Sugimoto, and H. Kiyoku,
“High-Power, Long-Lifetime InGaN Multi-QuantumWell-Structure Laser Diodes,” Jpn. J. Appl. Phys, vol.
36, L1059-L1061, 1997.
S. Nakamura, M. Senoh, N. Iwasa, and S. Nagahama,
“High-Brightness InGaN Blue, Green and Yellow
Light-Emitting Diodes with Quantum Well
Structures,” Jpn. J. Appl. Phys. vol. 34, pp. L797L799, 1995.
N. F. Gardner, G. O. Müller, Y. C. Shen, G. Chen, S.
Watanabe, W. Götz, and M. R. Krames, “Blue-
119
emitting InGaN–GaN double-heterostructure lightemitting diodes reaching maximum quantum
efficiency above 200A/cm2,” Appl. Phys. Lett. Vol. 91,
pp. 243506, 2007.
[4] M. R. Krames, O. B. Shchekin, R. Mueller-Mach, G.
O. Mueller, L. Zhou, G. Harbers, and M. George
Craford, “Status and Future of High-Power LightEmitting Diodes for Solid-State Lighting,” IEEE J.
Disp. Technol. vol. 3, pp. 160, 2007.
[5] Y. H. Ra, R. Navamathavan, J. H. Park and C. R. Lee,
“High-Quality Uniaxial InxGa1–xN/GaN Multiple
Quantum Well (MQW) Nanowires (NWs) on Si(111)
Grown by Metal-Organic Chemical Vapor Deposition
(MOCVD) and Light-Emitting Diode (LED)
Fabrication,” ACS Appl. Mater. Interfaces vol. 5, pp.
2111-2117, 2013.
[6] H. M. Kim, Y. H. Cho, H. Lee, S. I. Kim, S. R. Ryu,
D. Y. Kim, T. W. Kang, K. S. Chung, “HighBrightness Light Emitting Diodes Using DislocationFree Indium Gallium Nitride/Gallium Nitride
Multiquantum-Well Nanorod Arrays,” ACS Nano Lett.
vol.6, pp.1059-1062, 2004.
[7] M. Ali, O. Svensk, L. Riuttanen, M. Kruse, S.
Suihkonen, A. E. Romanov, P. T. Torma, M. Sopanen,
H. Lipsanen, M. A. Odnoblyudov and V. E. Bougrov,
“Enhancement of near-UV GaN LED light extraction
efficiency by GaN/sapphire template patterning,”
Semicond. Sci. Technol. vol. 27, pp. 082002, 2012.
[8] Y. S. Park, H. G. Lee, C.-M. Yang, D.-S. Kim, J.-H.
Bae, S. Cho, J.-H. Lee and I. M. Kang, “Contactgeometry engineering in a circular light-emitting
diode (LED) for improved electrical and optical
performances,” Semicond. Sci. Technol., vol.28,
pp.015006, 2013.
[9] Z. Li, J. Waldron, T. Detchprohm, C. Wetzel, R. F.
Karlicek, Jr., and T. P. Chow, “Monolithic integration
of light-emitting diodes and power metal-oxidesemiconductor
channel
high-electron-mobility
transistors for light-emitting power integrated circuits
in GaN on sapphire substrate,” Appl. Phys. Lett.
vol.102, pp.192107, 2013.
[10] X. A. Cao and S. D. Arthur, “High-power and reliable
operation of vertical light-emitting diodes on bulk
GaN,” Appl. Phys. Lett. vol.85, pp.3971, 2004.
[11] J.-S. Ha, S. W. Lee, H.-J. Lee, H.-J. Lee, S. H. Lee, H.
Goto, T. Kato, K. Fujii, M. W. Cho, and T. Yao, “The
Fabrication of Vertical Light-Emitting Diodes Using
120
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
JANG HYUN KIM et al : EFFECTS OF CURRENT SPREADING IN GAN-BASED LIGHT-EMITTING DIODES USING ITO …
Chemical Lift-Off Process,” IEEE Photon. Technol.
Lett. vol. 20, pp.175-177, 2008.
I. Eliashevich, Y. Li, A. Osinsky, C. A. Tran, M. G.
Brown, R. F. Karlicek, Jr., “Part of the SPIE
Conference on Light-Emitting Diodes: Research,
Manufacturing and Applications III,” San Jose,
California, Proc. SPIE 3621, 28, 1999.
X.A. Cao, E.B. Stokes, P. Sandvik, N. Taskar, J.
Kretchmer and D. Walker, “Optimization of current
spreading metal layer for GaN/InGaN-based light
emitting diodes,” Solid-State Electronics vol.46,
pp.1235-1239, 2002.
S.-H. Han, C.-Y. Cho, S.-J. Lee, T.-Y. Park, T.-H.
Kim, S. H. Park, S.W. Kang, J. W. Kim, Y. C. Kim,
and S.-J. Park, “Effect of Mg doping in the barrier of
InGaN/GaN multiple quantum well on optical power
of light-emitting diodes,” Appl. Phys. Lett. vol.96,
pp.051113, 2010.
M. Zhang, P. Bhattacharya, W. Guo, and A. Banerjee,
“InGaN/GaN self-organized quantum dot green light
emitting diodes with reduced efficiency droop,” Appl.
Phys. Lett. vol.96, pp.132103, 2010.
S. Brochen, J. Brault, S. Chenot, A. Dussaigne, M.
Leroux, and B. Damilano, “Dependence of the Mgrelated acceptor ionization energy with the acceptor
concentration in p-type GaN layers grown by
molecular beam epitaxy,” Appl. Phys. Lett. vol.103,
pp.032102, 2013.
M. V. Bogdanov, K. A. Bulashevich, O. V. Khokhlev,
I. Yu. Evstratov, M. S. Ramm, and S. Yu. Karpov,
“Effect of ITO spreading layer on performance of blue
light-emitting diodes,” Phys. Status Solidi C, vol.7,
pp.2127-2129, 2010.
S.-C. Hsu, D.-S. Wuu, C.-Y. Lee, J.-Y. Su, and R.-H.
Horng, “High-Efficiency 1-mm 2 AlGaInP LEDs
Sandwiched by ITO Omni-Directional Reflector and
Current-Spreading Layer,” IEEE Photon. Technol.
Lett., vol.19, pp.492-494, 2007.
J. K. Sheu, G. C. Chi, and M. J. Jou, “Enhanced
output power in an InGaN&ndash,GaN multiquantum well light-emitting diode with asymmetric
wells,” IEEE Photon. Technol. Lett., vol. 13, pp.1164
-1166, 2001.
D. V. Morgan, I. M. Al-O and Y. H. Aliyu, “Indium
tin oxide spreading layers for AlGaInP visible LEDs,”
Semicond. Sci. Technol., Vol.15, pp.67-72, 2000.
T. H. Seo, K. J. Lee, T. S. Oh, Y. S. Lee, H. Jeong, A.
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
H. Park, H. Kim, Y. R. Choi, E.-K. Suh, T. V. Cuong,
V. H. Pham, J. S. Chung, and E. J. Kim, “Graphene
network on indium tin oxide nanodot nodes for
transparent and current spreading electrode in
InGaN/GaN light emitting diode,” Appl. Phys. Lett.,
vol.98, pp.251114, 2011.
A. Porch, D. V. Morgan, R. M. Perks, M. O. Jones,
and P. P. Edwards, “Electromagnetic absorption in
transparent conducting films,” Appl. Phys. Lett.,
vol.95, pp.4734, 2004.
H. Kim, J.-M. Lee, C. H., S.-W. Kim, D.-J. Kim, S.-J.
Park and H. Hwang, “Modeling of a GaN-based lightemitting diode for uniform current spreading,” Appl.
Phys. Lett., vol.77, pp.1903, 2000.
Hyunsoo Kim, Seong-Ju Park, and Hyunsang Hwang,
“Effects of current spreading on the performance of
GaN-based light-emitting diodes,” IEEE Trans.
Electron Device, vol.48, pp.1065-1069, 2001.
R. Dingle, D. D. Sell, S. E. Stokowski, and M.
Ilegems, “Absorption, Reflectance, and Luminescence
of GaN Epitaxial Layers,” Phys. Rev. B, vol.4,
pp.1211-1218, 1971.
Ray-Hua Horng, Dong-Sing Wuu, Yi-Chung Lien,
and Wen-How Lan, “Low-resistance and hightransparency Ni/indium tin oxide ohmic contacts to ptype GaN,” Appl. Phys. Lett., vol.79, pp.2925, 2001.
Mandy M. Y. Leung, Aleksandra B. Djurii, and E.
Herbert Li, “Refractive index of InGaN/GaN quantum
well,” J. Appl. Phys., vol.84, pp.6312, 1998.
K.-S. Lee and M. A. El-Sayed, “Dependence of the
Enhanced Optical Scattering Efficiency Relative to
That of Absorption for Gold Metal Nanorods on
Aspect Ratio, Size, End-Cap Shape, and Medium
Refractive Index,” J. Phys. Chem. B, vol.109,
pp.20331-20338, 2005.
E. Ejder, “Refractive index of GaN,” phys. stat. sol.
(a), vol.6, pp.445-448, 1971.
Y. Yang, X.W. Sun, B.J. Chen, C.X. Xu, T.P. Chen,
C.Q. Sun, B.K. Tay, Z. Sun, “Refractive indices of
textured indium tin oxide and zinc oxide thin films,”
Thin Solid Films, 510, pp.95-101, 2006.
R.A. Synowicki, “Spectroscopic ellipsometry
characterization of indium tin oxide film
microstructure and optical constants,” Thin Solid
Films, vol.313, pp.394-397, 1998.
M. E. A. saleh, M. C. Teich, Fundermental of
photonics, (Wiley, New York, 2007) 2nd ed., p. 43.
JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.15, NO.1, FEBRUARY, 2015
Jang Hyun Kim was born in Seoul,
Korea, in 1985. He received the B.S
degree from Electrical Engineering
Department in KAIST, Daejeon,
Korea in 209 and M.S. degrees from
Seoul National University (SNU) in
2011, Seoul, Korea. He received Best
Paper Award from SNU in 2011. He is currently working
toward the Ph. D. degree in the Department of Electrical
Engineering, Seoul National University, Seoul, Korea.
He has been a Teaching Assistant of Ion Implantation at
the Inter-University Semiconductor Research Center,
SNU, since 2010. His resent interesting is Tunneling FET.
Garam Kim was born in Jeonju,
Korea, in 1985. He received the B.S.
and Ph.D. degrees in electrical
engineering from Seoul National
University (SNU), Seoul, Korea, in
2008 and 2014, respectively. His current
research interests include the design,
fabrication, measurement, characterization, and modeling of
GaN-based LEDs, nanoscale CMOS/CMOS compatible
devices, neuromorphic systems, and capacitorless 1-transistor
DRAM. Mr. Kim is a student member of the Institute of
Electronics and Engineers of Korea (IEEK).
Euyhwan Park received the B.S
degrees in 2010 from Seoul National
University (SNU). He is currently
working toward the Ph.D. degree in
Electrical Engineering at Seoul
National University (SNU), Seoul,
Korea. His current research interests
include the ultra-low-power multi-channel transistors,
tunnel FET, GaN based LEDs. Mr. Park is a Student
Member of the Institute of Electrical and Electronics
Engineers (IEEE) and the Institute of Electronics
Engineers of Korea (IEEK)
Donghoon Kang was born in Yeosu,
korea in 1974. He received the B.S
and M.S degree in Material Science
and Engineering from Korea
University (KRU), Soul, Korea in
1998 and 2002 respectively. From
121
2002 to 2008 he had worked with the Samsung Electromechanism in charge of Laser Diode development. Since
2009, he has participated in LED development Business
of Samsung Electronics. His major responsibility is
electrical and optical characterization of LED. Mr. Kang
had lead the project for “Characterization and Analysis of
LED” as a form of cooperative Research between SNU
and Samsung Electronics from 2011 to 2013.
Byung-Gook Park received his B.S.
and M.S. degrees in Electronics
Engineering from Seoul National
University (SNU) in 1982 and 1984,
respectively, and his Ph. D. degree in
Electrical Engineering from Stanford
University in 1990. From 190 to 1993,
he worked at the AT&T Bell Laboratories, where he
contributed to the development of 0.1 micron CMOS and
its characterization. From 1993 to 1994, he was with Texas
Instruments, developing 0.25 micron CMOS. In 1994, he
joined SNU as an assistant professor in the Department of
Electrical Engineering and Computer Science, where he is
currently a professor. In 2002, he worked at Stanford
University as a visiting professor, on his sabbatical leave
from SNU. He led the Inter-university Semiconductor
Research Center (ISRC) at SNU as the director from June
208 to 2010. His current research interests include the
design and fabrication of nanoscale CMOS, flash memories,
silicon quantum devices and organic thin film transistors.
He has authored and co-authored over 80 research papers in
journals and conferences, and currently holds 53 Korean
and 2 U.S. patents. He has served as a committee member
on
several
international
conferences,
including
Microprocesses and Nanotechnology, IEEE International
Electron Devices Meeting, International Conference on
Solid State Devices and Materials, and IEEE Silicon
Nanoelectronics Workshop (technical program chair in
2005, general chair in 2007). He is currently serving as an
executive director of Institute of Electronics Engineers of
Korea (IEEK) and the board member of IEEE Seoul
Section. He received “Best Teacher” Award from SoEE in
1997, Doyeon Award for Creative Research from ISRC in
203, Educational Award from College of Engineering, SNU,
in 206, and Haedong Research Award from IEEK in 2008.