JOURNAL OF APPLIED PHYSICS
VOLUME 92, NUMBER 5
1 SEPTEMBER 2002
Improvement in light-output efficiency of InGaNÕGaN multiple-quantum well
light-emitting diodes by current blocking layer
Chul Huh, Ji-Myon Lee,a) Dong-Joon Kim,b) and Seong-Ju Parkc)
Nanophotonic Semiconductor Laboratory, Department of Materials Science and Engineering,
Kwangju Institute of Science and Technology, Kwangju 500-712, Korea
共Received 17 January 2002; accepted for publication 5 June 2002兲
The fabrication and characterization of an InGaN/GaN multiple-quantum well 共MQW兲
light-emitting diode 共LED兲 with a SiO2 current blocking layer inserted beneath the p-pad electrode
is described. The light-output power and external quantum efficiency for the InGaN/GaN MQW
LED chip with a current blocking layer were significantly increased compared to those for the
conventional InGaN/GaN MQW LED chip. The increase in the light-output power can be attributed
to the injection of additional current into the light-emitting quantum well layer of the LED by the
SiO2 current blocking layer and a reduction in parasitic optical absorption in the p-pad electrode.
© 2002 American Institute of Physics. 关DOI: 10.1063/1.1497467兴
I. INTRODUCTION
eration and degradation in the reliability of LEDs due to the
high-electrical carrier density in a localized area of the device. Therefore, to increase the light output from GaN-based
LEDs, a more efficient light extraction through the chip,
packaging, and device design will be required. In this article,
we report on an investigation of the fabrication and characterization of InGaN/GaN multiple-quantum well 共MQW兲
LEDs with a current blocking layer 共CBL兲 inserted beneath
the p-pad electrode by means of an insulating SiO2 layer.
The results show that the light-output power of an InGaN/
GaN MQW LED chip with a CBL is considerably enhanced
compared to that of an InGaN/GaN MQW LED chip with a
conventional structure.
GaN-based wide band-gap semiconductors have recently
attracted considerable interest, in terms of applications for
optoelectronic devices, which operate in the blue, green, and
ultraviolet 共UV兲 wavelength regions, as well as for electronic
devices
operating
at
high-temperature/high-power
conditions.1,2 As the result of the recent developments of
GaN-based light emitting diodes 共LEDs兲 with increased
brightness, applications including displays, traffic signals,
exterior automotive lighting, and backlights for cell phones
have become possible. In particular, an interest has developed concerning the use of white LEDs for general illumination using high-brightness LEDs. However, even though
the brightness of visible/UV LEDs based on GaN-based materials continues to increase, the light output is still low compared to conventional light sources in high-flux lighting systems and it is necessary to further improve the light-output
efficiency from LEDs.3
A general requirement of GaN-based LEDs is that both
n- and p-type electrodes are formed on the same side of the
epitaxial layer due to the insulating properties of the sapphire
substrates. The p-type electrode, which is fabricated on the
top surface of a mesa structure, is composed of a thin transparent layer of metal and a thick metal layer for wire bonding. In such a structure, the p electrode is situated in the
middle of the light path, which extends from the active region of the LED structure to the top side. Hence, some loss
of light is inevitable as a result of photon absorption near the
p-pad electrode. Another problem arising from the lateral
structure of GaN-based LEDs involves current crowding
around the p-pad electrode, depending on the electrical properties of the n-GaN layer and the light-transmitting layer.4
This current crowding adversely affects the uniformity and
stability of the far-field emission. In addition, current crowding may also cause a decrease in the efficiency of light gen-
II. EXPERIMENTAL DETAILS
The device samples were grown on c-plane sapphire
substrates using a metal-organic vapor deposition system
共Emcore D125™兲 with a rotating-disk reactor. The
multilayer structures consisted of a GaN nucleation layer, a
Si-doped n-GaN layer (2⫻1018 cm⫺3 ), five periods of
InGaN/GaN MQW active layers, and a Mg-doped p-GaN
capping layer. The samples were then thermally annealed at
950 °C for 60 s in a nitrogen atmosphere using a rapid thermal annealing process in order to activate the Mg acceptors.
This process produced a room-temperature p-layer concentration of 2⫻1017 cm⫺3 .
As shown in Fig. 1共a兲, an InGaN/GaN MQW LED with
an insulating SiO2 CBL inserted beneath the p-pad electrode
was fabricated using standard photolithographic patterning
and inductively coupled plasma dry etching. To insert the
CBL, the surface region of the p-GaN layer beneath the
p-pad electrode was partially etched by means of an inductively coupled CH4 /Cl2 /H2 /Ar plasma until the n-GaN layer
was exposed. An insulating SiO2 layer was then grown using
a plasma-enhanced chemical-vapor deposition system. A
Ti/Al layer 共30/80 nm兲 was deposited on the n-GaN layer by
e-beam evaporation, to serve as the n electrode, and a Pt film
共8 nm兲 as a light-transmitting layer was then deposited onto
the p–GaN layer. Finally, a Ni/Au 共30/80 nm兲 film was de-
a兲
Present address: Basic Research Laboratory, Electronics and Telecommunications Research Institute, Daejeon 305-350, Korea.
b兲
Present address: Samsung Electro-Mechanics, Suwon 442-743, Korea.
c兲
Electronic mail: sjpark@kjist.ac.kr
0021-8979/2002/92(5)/2248/3/$19.00
2248
© 2002 American Institute of Physics
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J. Appl. Phys., Vol. 92, No. 5, 1 September 2002
Huh et al.
2249
FIG. 3. Room-temperature EL spectra of the InGaN/GaN MQW LED chip
with a current blocking layer as a function of the forward current.
FIG. 1. 共a兲 Schematic diagram showing the structure of the InGaN/GaN
MQW LED chip and the position of the CBL in the LED structure. 共b兲
Possible lateral current paths in the LED structure.
posited on the light-transmitting layer, to serve as the p-pad
electrode. All metal contacts were annealed at 500 °C for 30
s under an N2 ambient to obtain low-resistance ohmic contacts. Except for the additional fabrication process for the
CBL, the process for the fabrication of a LED chip with a
CBL is almost identical to that used for a conventional LED
chip. Conventional InGaN/GaN MQW LED chips with the
same device area (300⫻300 ␮ m2 ) were also fabricated using the same wafer for comparison studies. Figure 1共b兲
shows the possible current paths in the conventional MQW
LEDs.
III. RESULTS AND DISCUSSION
Figure 2 shows the power efficiencies 共output power/
input power兲 for the InGaN/GaN MQW LED chip with a
CBL and a conventional InGaN/GaN MQW LED chip. The
power efficiency was calculated from the measured light output power and I – V curve. The power efficiency for LEDs
with a CBL is higher than that for LEDs without a CBL and
FIG. 2. Power efficiencies 共output power/input power兲 for the InGaN/GaN
MQW LED chip with a CBL and a conventional LED chip.
increases with increasing input power, as shown in Fig. 2.
From I – V curves of two LEDs 共not shown兲, the forward
voltage at 20 mA for the InGaN/GaN MQW LED chip with
a CBL was slightly increased by 0.1 V compared to that of a
conventional InGaN/GaN MQW LED chip. Acceptable electrical properties are also exemplified by the low-reverse current which is less than 10 ␮A at ⫺10 V for both cases. The
slight increase in forward voltage can be attributed to the
reduction in the total area of the p-type metal contact between the transparent Pt layer and the p-GaN as the result of
the presence of an insulating SiO2 layer, which is used as a
CBL. The area of the CBL which is inserted beneath the
p-pad electrode is about 30% of the total p-type GaN area of
the InGaN/GaN MQW LED fabricated in this study. Jin
et al.5 also reported that a slight increase in the forward voltage at 20 mA can be attributed to the reduction in the total
contact area between the p-type GaN and the metal in the
case of microdisk LEDs.
Figure 3 shows the room-temperature electroluminescence spectra of the InGaN/GaN MQW LED chip with a
CBL as a function of the forward current. The peak wavelength and the full width at half maximum of the emission
spectra are 470 and 20 nm at 20 mA, respectively. When the
forward current is increased from 20 to 80 mA, the peak
wavelength of the LED chip with a CBL slightly shifts to
466 nm. It is generally known that the shift of wavelength
can be attributed to the In composition fluctuation in the
InGaN well layer due to the natural phase separation of InGaN during growth or the band-filling effect with the increment in the current injection. Here, the slight shift toward the
short wavelength in emission spectra is caused by the bandfilling effect in InGaN/GaN MQW due to the increase in the
injection current. This is also in good agreement with our
previous result.6
Figure 4 shows the light-output power 共P兲 and external
quantum efficiency ( ␩ ext) calculated from the measured
light-output power of bare LED chips with and without a
CBL as a function of forward current. The light-output
power was measured from the topside of the bare LED wafer
chips with an area of 300 ␮ m⫻300 ␮ m using a calibrated Si
photodiode 共Model 818-UV兲 connected to an optical power
meter 共Model 1835-C兲. As shown in Fig. 4, the light-output
power of the InGaN/GaN MQW LED chip with a CBL at 20
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2250
Huh et al.
J. Appl. Phys., Vol. 92, No. 5, 1 September 2002
FIG. 4. Light-output power 共P兲 and external quantum efficiency ( ␩ ext) for
an InGaN/GaN MQW LED chip with a CBL and a conventional LED chip
measured through the light-transmitting layer on an unpackaged chip as a
function of forward current.
mA was increased by 62% compared to that of the conventional InGaN/GaN MQW LED chip, although the lightemitting active area was reduced due to the CBL region inserted into the LED structure, as shown in Fig. 1共a兲. These
results show that the current blocking layer inserted into the
LED structure can significantly improve the light-output efficiency of a conventional LED chip.
GaN-based LEDs grown on an insulating sapphire substrate should have a lateral current path because two electrodes are formed on the same side of the substrate,7 as
shown in Fig. 1共b兲. Perfectly uniform current spreading
across the active area of the LED can be achieved only when
the total voltage drop across path A is same as the voltage
drop across path B. However, the current will flow efficiently
in only one of the two current paths, depending on the electrical properties of current paths A and B. If the lighttransmitting metal layer is relatively thick and thus becomes
a perfect current spreader, the resistivity of the n-type GaN
layer would be expected to significantly affect the current
spreading. Otherwise, the resistivity of the light-transmitting
layer would be expected to play a crucial role in the current
flow. It has been reported that when the electron concentration in an n-type GaN layer is below 1018 cm⫺3 and the
conductivity of the n-type GaN layer is, therefore, insufficient to meet the current uniformity conditions, the current
will flow via the shortest path B through the lighttransmitting metal layer, resulting in current crowding near
the n-pad electrode.4 The electron concentration of the n-type
GaN layer in our InGaN/GaN MQW LED was about 2
⫻1018 cm⫺3 and the resistivity, as determined by Hall measurement was about 10 m⍀ cm. The thickness of the lightemitting Pt layer used in this study was 8 nm. It is generally
accepted that the resistivity of a metal film is inversely proportional to the thickness of the metal and the resistivity of a
thin metal film with a thickness below 10 nm is about 1000
times higher than that of a bulk film.8 Because the resistivity
of a Pt bulk film is about 10.8 ␮⍀ cm,9 the resistivity of a Pt
thin film with a thickness of 8 nm will therefore be above 20
m⍀ cm. Under these conditions, the current from the p-pad
electrode will flow more easily through path A than path B as
shown in Fig. 1共b兲 due to less resistance to the current flow.
Therefore, the current would be crowded near the p-pad electrode in this case. This current crowding near the p-pad electrode can result in low light-output efficiency due to both
inefficient current injection into the active area in the LED
structure and limited recombination near the p-pad electrode.
It has been reported that the increase in the current injection
into the active area of the LED structure through a lighttransmitting metal layer plays an important role in improving
LED performance.10,11 Hence, the increase in the light-output
power of the InGaN/GaN MQW LED chip with a CBL can
be attributed to the increase in current injection into the entire active area of the LED through path B 共light-transmitting
layer兲 as shown in Fig. 1共b兲. In addition, for the case of the
InGaN/GaN MQW LED chip with a CBL, because the current was dominantly injected into the middle area of LED
through the light-transmitting layer 共through path B兲 as discussed above, the parasitic optical absorption12 can be reduced due to the reduction in light absorption at the thick
p-pad electrode. This results in a further improvement in the
light output of the LED chip with a CBL compared to that of
a conventional LED chip.
IV. CONCLUSION
We report here on the fabrication of an InGaN/GaN
MQW LED chip having a CBL inserted beneath the p-pad
electrode and its characterization. The light-output power of
the InGaN/GaN MQW LED chip with a CBL was found to
be greatly increased compared to that of a conventional LED
chip structure due to the increase in current injection into the
active layer of the LED structure via the light-transmitting Pt
layer and a reduced parasitic optical absorption at the p-pad
electrode.
ACKNOWLEDGMENT
This work was partially supported by a grant from the
National Research Laboratory Program on Nanophotonic
Semiconductors in Korea.
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