The Fabrication of Vertical Light-Emitting
Diodes using Chemical Lift-Off Process
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藤井 克司
IEEE Photinics Technology Letters
20
3
175-177
2008
http://hdl.handle.net/10097/47503
doi: 10.1109/LPT.2007.912491
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 3, FEBRUARY 1, 2008
175
The Fabrication of Vertical Light-Emitting Diodes
Using Chemical Lift-Off Process
Jun-Seok Ha, S. W. Lee, Hyun-Jae Lee, Hyo-Jong Lee, S. H. Lee, H. Goto, T. Kato, Katsushi Fujii,
M. W. Cho, and T. Yao
Abstract—Vertical light-emitting diodes (LEDs) were successfully fabricated by a chemical lift-off process using a selectively
etchable CrN buffer layer. The novel CrN metallic layer worked
well as a buffer layer for growth of the GaN LED and was etched
out clearly during selective chemical etching. The vertical LED by
chemical lift-off showed very good current–voltage performance
with low series resistance of 0.65 and low operated voltage of
3.11 V at 350 mA. Also, this device could be operated at a much
higher injection forward current (1118 mA at 3.70 V) by thermally conductive metal substrate which enabled the high current
operation with excellent heat dissipation.
Index Terms—Chemical lift-off, CrN buffer, GaN, light-emitting
diodes (LEDs), vertical light-emitting diode (LED).
I. INTRODUCTION
N RECENT years, considerable attention has been focused
on GaN-based high-power light-emitting diodes (LEDs).
The GaN LEDs are rapidly expanding their application into
the extremely high brightness areas such as a back light unit
of a large size screen and a solid-state lighting system substituting a traditional fluorescent lamp and incandescent bulb. For
theses applications, the improvement of light output power is
indispensable. Many groups have been conducting research to
improve the light output of the GaN-based LED achieved by
the technologies of patterned substrates [1], omni-directional
reflectors [2], photonic crystals [3], rough surfaces [4], and
flip-chip technology [5].
Nonetheless, the most essential factor for the high-power
LED is thought to be the capability of high current injection
to the LED device. For high current injection, much research
was conducted for the direction of removing sapphire substrates which have thermal and electrical insulating properties.
Because of these properties, in case of high current injection,
a rise in junction temperature could decrease the luminous
efficacy by 5% for every 10 C and the LED chips could be
destructed by electrostatic discharge (ESD) [6]. Therefore, the
replacement of sapphire should improve heat extraction from
the active region of GaN-based devices, eliminate the ESD
I
Manuscript received September 11, 2007; revised October 16, 2007.
J.-S. Ha, H.-J. Lee, M. W. Cho, and T. Yao are with the Center for Interdisciplinary Research, Tohoku University, Sendai, Miyagi 980-8578, Japan, and also
with the Institute for Materials Research, Tohoku University, Sendai, Miyagi
980-8577, Japan (e-mail: [email protected]).
S. W. Lee, H.-J. Lee, S. H. Lee, and H. Goto are with the Center for Interdisciplinary Research, Tohoku University, Sendai, Miyagi 980-8578, Japan.
T. Kato and K. Fujii are with the Institute for Materials Research, Tohoku
University, Sendai, Miyagi 980-8577, Japan.
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2007.912491
problem associated with the insulating sapphire substrate, and
enable the fabrication of a vertical current flow device with
a top n-contact and a bottom p-contact, thus improving the
uniform current irradiance.
For these concerns, the fabricating method of vertical-type
free-standing GaN LED without sapphire was developed by
using the laser lift-off (LLO) process [7]–[10], which is the
method of separating the sapphire substrate from GaN LED
for the vertical-type light-emitting devices by irradiation of
high energy laser to the interface between sapphire and GaN,
causing decomposition of GaN to liquid gallium and gaseous
nitrogen. Today, various attempts are being conducted with this
LLO process such as usage of Si substrate [11], [12], metallic
substrate [13], application to UV-LED [14], light extraction
method of patterned substrate [15], and photonic crystal [3].
Furthermore, recently, high-performance LLO vertical LEDs
for the mass production were reported [16].
However, the LLO method has some detrimental aspects.
During the irradiation of the laser, the absorbed photon energy
leads to local heating of the layer above the critical sublimation
temperature of gallium, causing the destruction of the GaN.
LED devices could be failed after such high-energy laser
treatment. Moreover, after being exposed to the irradiated
laser, GaN shows bad reverse-bias leakage current property.
It is reported that these degradations were caused by the LLO
processes, which generated the screw dislocations [17]. Besides, after being lifted off by the laser, a metallic gallium
droplet remained on the separated n-type GaN surface. This
decomposed metallic gallium should be removed for the next
processing step [9], [10].
In this research, we propose the chemical lift-off process as a
new method for fabricating vertical LED. The chemical lift-off
means detaching GaN LEDs from the sapphire substrate by a
selective etching process. As the LLO uses a GaN buffer layer
for the separation, the chemical lift-off uses a novel metallic
buffer layer, CrN, which is etched out selectively by optimum
chemical solution. With this chemical lift-off method, we could
make the vertical LED which has a similar device performance
to that of the LLO method.
II. RESULTS AND DISCUSSION
LED structures were grown by metal–organic vapor
phase epitaxy on (0001) sapphire substrates. About the
possibility of a CrN buffer for the growth of the GaN
LED device, it is already reported that the wurtzite GaN
layer, RS-CrN layer, and corundum c-sapphire showed
a good agreement maintaining the epitaxial relationship
CrN
Al O
,
of GaN
CrN
Al O
[18].
GaN
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176
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 3, FEBRUARY 1, 2008
Fig. 1. Schematic diagrams of vertical LED fabrication (a) p-contact and metal
substrate formation, (b) during chemical lift-off, and (c) structure of vertical
LED by chemical lift-off.
Fig. 1 shows the schematic diagrams of chemical lift-off
process. CrN was formed by deposition of chromium with
a radio-frequency (RF) sputtering system and following
nitridation process. The basic LED structure which contains InGaN–GaN multiquantum well was also grown by
low-pressure metal organic chemical vapor deposition except
the first CrN buffer layer. On the top of the LED structure, as a p-ohmic contact, a Ni–Au metal system was used.
Au
by e-beam evapAfter deposition of Ni
orator, respectively, thermal annealing was conducted with
the condition of 5 min, 600 C, and ambient atmosphere by
rapid thermal annealing (RTA). Before sapphire substrate
removing, in order to transfer the LED structure, the metal
substrate was formed on the p-GaN side. After the deposiAu
for seed metal, gold was
tion of Ti
electroplated to the thickness of 50 m as a metal substrate.
The reason why gold was selected for the metal substrate
is because gold has high resistance and selectivity to the
etching solutions for the chemical lift-off. During the chemical etching step, the buffer layer was etched out and the
LED structure separated from the sapphire substrate. Finally, on the separated n-type GaN, n-contact was formed by
Al
Ti
Au
metal scheme.
Ti
The CrN buffer layer was etched-off by a CrN etchant
which was mixed with deionized (DI) water 200 ml,
Ce NH
NO
50 g,
di-ammonium cerium(IV) nitrate
and perchloric acid
HClO 13 ml at 70 C. The etchant
came through the edge of interfaces between GaN and sapphire
substrate and started to etch the CrN buffer layer; as time went
by, it penetrated into the center of the CrN layers and separated
the LED chips and substrate [Fig. 2(a)]. Fig. 2(b) shows the
cross-sectional view during and after detaching GaN from the
sapphire substrate. From these results, we verified that the GaN
layers grown on CrN buffer could be successfully separated by
chemical selective etching.
In case of the LLO process, after laser irradiation, the
metallic Ga was left over on the surface n-GaN. Because these
Ga droplets were detrimental for not only the device performance but also the following process steps, residues should be
removed by wet chemical etchant such as HCl. However, by the
chemical lift-off process, the detached n-GaN remained clean
without residuals on the surface. Fig. 3 shows the surfaces’
status of both detached n-GaN face and sapphire substrate after
chemical lift off. We could find that the surfaces are clean, so
the extra cleaning process for removing surface residues was
not necessary before the next process step. The roughness root
were 12.4 and 1.9 nm for the n-GaN
mean squares
surface and sapphire substrate, respectively. Compared with
Fig. 2. Chemical lift-off (a) the optical microscope image of GaN top surface. The CrN layers shown as dark area were in the procedure of chemical
etching and (b) scanning-electron-microscope cross-sectional image of chemical lifted-off region.
Fig. 3. (a) Chemical lift-off GaN vertical LED; (b) chemical lift-off sapphire
substrate; (c) atomic force microscopy surface images of chemical lift-off GaN;
and (d) that of sapphire substrate.
bare sapphire, the surface roughness of 1.9 nm is practically
the same value.
Fig. 4 shows the results of measurement of surface bending
for before and after chemical lift-off. The black line shows the
GaN surface bending after growth on the sapphire substrate
which indicated about 4 m for the radius of curvature. However,
the radius of curvature of the sapphire surface after the GaN
layer was chemically lifted off by CLO turned back to the value
of about 17 m. This means that the stress which was applied in
the sapphire substrate was released as the GaN layer was chemically lifted off. The red and green line shows the results of the
sapphire backside before and after chemical lift-off. It is considered that these two results of Figs. 3 and 4 could be considered
as an optimistic signal for the recycling of sapphire substrates.
Typical current–voltage ( – ) characteristics of the chemically lifted off vertical LED are shown in Fig. 5. The size of the
m
m. The forward built-in
vertical LED was
voltage of the vertical LED was 2.27 V and the operating voltage
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HA et al.: FABRICATION OF VERTICAL LEDs USING CHEMICAL LIFT-OFF PROCESS
177
metallic substrate. These low operation voltages and series resistances make it possible to acquire high light output efficiency
and operation performance and it is thought that these developments can lead the solid-state lighting to the general lighting
applications.
Fig. 4. Bending measurement of GaN and sapphire surface before and after
chemical lift-off.
Fig. 5. I –V characteristics of chemically lifted off vertical LED.
at 20 mA was the 2.31 V, respectively. Moreover, only 3.11 V
were necessary for the current of 350 mA to flow into the device. It is noted that this result is one of the best – data. The
series resistance of 0.65 was calculated from the slope of –
curve, which is similar to the result reported by Tran et al. [16].
Compared with the conventional LEDs which have the high series resistance on sapphire because of the lateral current path
and current crowding effect on the bottom of n-electrode, the
vertical LEDs show the lower series resistance and operation
voltage due to the vertical current path. In addition to the lower
operation voltage and series resistance, the high current injection is also regarded as the important factor for the high power
light output efficiency. For this vertical LED, 1118 mA could
be injected into the device at 3.7 V. These results represent that
the chemically lifted off vertical LEDs have a great potential to
be the substitutes for the general lighting applications such as a
fluorescent lamp and an incandescent electric lamp.
III. CONCLUSION
A vertical LED was successfully fabricated using a selectively etchable CrN buffer layer by the chemical lift-off method.
The vertical LED device by the chemical lift-off process showed
the lower operation voltage (2.31 V at 20 mA), series resistance
(0.65 ), and much higher injection forward current (1118 mA
at 3.70 V). This is because the sapphire substrate, which is thermally and electrically insulator, was separated from LED structure, and the vertical current path was established through the
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