AlGaInP-Based LEDs With a -GaP Window Layer and a Thermally Annealed ITO Contact
ABSTRACT In this paper, indium-tin-oxide (ITO) films were deposited on p-type GaP films with a AuBe-diffused metal layer to form ohmic contacts. Without the AuBe diffused into p-GaP films, the ITO deposited on p-GaP showed a non-ohmic characteristic. After the AuBe diffused, the ITO deposited on p-GaP displayed a linear current-voltage characteristic and the specific contact resistance showed 2.63 × 10-4 ω-cm2. Furthermore, the specific contact resistance could be improved to 1.57 × 10-4 ω-cm2 when the sample post-ITO-deposition annealed at 400°C. The transmittance of ITO film almost was kept at 90% in the wavelength range of 400-700 nm after thermal annealing. These results revealed that the ITO films can be a suitable transparent current spreading layer for the fabrication of AlGalnP-based light-emitting diodes with an AuBe-diffused metal layer. It was also found that the 20 mA forward voltages measured from LEDs with Device A, Device B, Device C and Device D were 1.97, 1.96, 1.95 and 2.66 V and the light output powers were 4.2, 5.7, 6.0 and 6.3 mW, respectively.
- [Show abstract] [Hide abstract]
ABSTRACT: Optically thin AlGaAs/GaAs/AlGaAs double heterostructures, (5000 Å), are floated off their substrates by the epitaxial liftoff technique and mounted on various high reflectivity surfaces. From the absolute photoluminescence intensity, we measure internal and external quantum efficiencies of 99.7% and 72%, respectively. High spontaneous emission quantum efficiency, is important for photon number squeezed light, diode lasers, single‐mode light‐emitting‐diodes, optical interconnects, and solar cells.Applied Physics Letters 02/1993; · 3.52 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: Large size (i.e. 1 mm × 1 mm) high power nitride-based light emitting diodes (LEDs) with Ni/Au and Ni/indium tin oxide (ITO) p-contacts were fabricated. It was found that the 200 mA forward voltage was 3.19 and 3.3 V for the power LEDs with Ni/Au and Ni/ITO p-contacts, respectively. On the other hand, although the 200 mA output power was only 11.59 mW for the power LED with Ni/Au p-contact, the 200 mA output power could reach 16.52 mW for the power LED with Ni/ITO p-contact. Such a significant increase could be attributed to the more transparent nature of Ni/ITO, as compared to Ni/Au. Preliminary life test also indicates that power LEDs with Ni/ITO p-contact are at least as reliable as power LEDs with Ni/Au p-contact.Solid-State Electronics 01/2003; · 1.48 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: Hybrid-type distributed Bragg reflectors (DBRs) with InAlP/GaAs and InAlP/InGaAlP multilayers grown by metalorganic chemical vapor deposition (MOCVD) have been investigated. The structure of the DBRs was designed using a theoretical calculations considering the absorption loss and refractive index of each stacked layer. The wide-band, high-reflectivity characteristics were also experimentally confirmed. Good electrical conductivity through InGaAlP light-emitting diodes (LEDs) with the hybrid-type DBRs was obtained in spite of the many interface needed for the multiple layers of the DBRs. A luminous intensity of 0.8 cd was obtained at 565.7 nm (nearly pure green light).Japanese Journal of Applied Physics 01/1994; 33:6195-6198. · 1.07 Impact Factor
IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 47, NO. 6, JUNE 2011803
AlGaInP-Based LEDs with a p+-GaP Window
Layer and a Thermally Annealed ITO Contact
H. M. Lo, S. C. Shei, X. F. Zeng, Shoou-Jinn Chang, Senior Member, IEEE, and Hsieh-Yen Lin
Abstract—In this paper, indium-tin-oxide (ITO) films were
deposited on p-type GaP films with a AuBe-diffused metal layer to
form ohmic contacts. Without the AuBe diffused into p-GaP films,
the ITO deposited on p-GaP showed a non-ohmic characteristic.
After the AuBe diffused, the ITO deposited on p-GaP displayed
a linear current-voltage characteristic and the specific contact
resistance showed 2.63 × 10−4ω-cm2. Furthermore, the specific
contact resistance could be improved to 1.57 × 10−4ω-cm2
when the sample post-ITO-deposition annealed at 400 °C. The
transmittance of ITO film almost was kept at 90% in the
wavelength range of 400-700 nm after thermal annealing. These
results revealed that the ITO films can be a suitable transparent
current spreading layer for the fabrication of AlGaInP-based
light-emitting diodes with an AuBe-diffused metal layer. It was
also found that the 20 mA forward voltages measured from LEDs
with Device A, Device B, Device C and Device D were 1.97, 1.96,
1.95 and 2.66 V and the light output powers were 4.2, 5.7, 6.0
and 6.3 mW, respectively.
Index Terms—AlGaInP, AuBe-diffused layer, indium-tin-oxide,
light-emitting diode, ohmic contact.
yellow/green spectra regions. Furthermore, AlGaInP can be
precisely lattice matched to GaAs substrates without gener-
ating much misfit dislocation. For these reasons, AlGaInP
has become the most promising material for high brightness
red, yellow and yellow/green light emitting diodes (LEDs).
Indeed, AlGaInP-based LEDs prepared by metalorganic chem-
ical vapor deposition (MOCVD) have already been used
extensively in our daily life. For example, these AlInGaP-
based LEDs are used in traffic light lamps, outdoor full-color
displays and automobile tail lights –. It has been shown
HE (AlxGa1−x)0.5In0.5P quaternary material system has
a direct bandgap transition in between the red and
Manuscript received January 7, 2011; revised February 7, 2011 and February
14, 2011; accepted February 16, 2011. Date of current version May 6, 2011.
This work was supported in part by the Center for Frontier Materials and
Micro/Nano Science and Technology, the Advanced Optoelectronic Technol-
ogy Center, National Cheng Kung University, under project, from the Ministry
of Education, Taiwan, the Ministry of Economic Affairs, under Grant NSC
98-EC-17-A-09020769 and Grant NSC 98-2221-E158-006, and the Bureau of
Energy, Ministry of Economic Affairs of Taiwan, under Contract 98-D0204-6.
H. M. Lo, S.-J. Chang,X. F. Zeng, and
the Department of Electrical Engineering, Center for Micro/Nano Sci-
Institute of Microelectronics, National Cheng Kung University, Tainan
701, Taiwan (e-mail: firstname.lastname@example.org; email@example.com;
S. C. Shei is with the Department of Electrical Engineering, National
University of Tainan, Tainan 700, Taiwan (e-mail: firstname.lastname@example.org).
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JQE.2011.2118744
H.-Y. Lin are with
that internal quantum efficiency of these AlGaInP-based LEDs
has reached near 100% , . However, it is known that
hole concentration is low for the p-type quaternary AlGaInP
epitaxial layers. This will result in severe current crowding for
AlGaInP-based LEDs. With current crowding, most injected
carriers will be confined underneath the thick metal bonding
pad . As a result, a significant amount of light will be
absorbed or reflected by the opaque p-pad electrode. It is also
known that refractive index is high for the quaternary AlGaInP
This will result in a small critical angle [θc
sin−1(nair/nAlGaInP)] for the light generated in the active
region to escape from the LEDs , . These two effects
could both reduce light extraction efficiency of AlGaInP-
based LEDs. One possible method to solve these problems is
to deposit a thick electrically conductive p-GaP window layer
on top of the active LED structure . Using a 45-μm-thick
p-GaP window layer, Huang et al. successfully achieved a
two-fold improvement in the efficiency of AlGaInP-based
LEDs . To grow the thick p-GaN layer, however, one needs
to transfer the samples from the MOCVD system to the HVPE
system, which might result in reduced production yield.
Indium-tin-oxide (ITO) is a hard and chemically inert mate-
rial with a high conductivity and is transparent to photons in
the visible spectrum. With these properties, ITO-coated glass
has been used in liquid crystal display panels. ITO has also
been used as the transparent current spreading material for
GaN-based blue/green LEDs , . Previously, Morgan et
al. compared AlGaInP-based LEDs with ITO current spreading
layer and with p-GaP current spreading layer . They found
that a 70-nm-thick ITO layer could provide the same current
spreading as a much thicker (i.e., 5000 nm) GaP layer. To
use ITO as the current spreading layer for AlGaInP-based
LEDs, it is necessary to insert a p+-GaAs cap layer to protect
the AlGaInP from oxidation and also to form good ohmic
contact –. However, the opaque p+-GaAs cap layer
could result in light absorption and reduced LED output power.
This could be solved by replacing the p+-GaAs cap with a p+-
GaP window layer . However, it is difficult to grow p+-
GaP layer with a high hole concentration by MOCVD .
Indeed, Prof. W-C Liu have proposed the concept of ITO/p-
GaP direct ohmic contact structure. However, the further
study in post-ITO annealing at different temperatures and the
contact mechanisms of ITO/p-GaP related to characteristics of
LEDs have not been investigated. In this study, we report the
conversion of p-GaP layer into p+-GaP layer and study the
post-ITO-deposition annealing temperature effects on the con-
0018−9197/$26.00 © 2011 IEEE
804IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 47, NO. 6, JUNE 2011
Secondary Ion Counts (c/s)
Fig. 1.Measured SIMS profiles of Ga, P, Be and Au after diffusion.
tact mechanisms of ITO/p-GaP. AlGaInP-based LEDs with a
p+-GaP window layer and an ITO contact were also fabricated.
Detailed fabrication process and the electro-optical properties
of the fabricated LEDs will be reported. Furthermore, the
effects of thermal annealing after ITO deposition are also
The AlGaInP-based LEDs, emitting at 630 nm, used in
this study were all grown on 2-inch GaAs substrates by
MOCVD. The epitaxial structure of the LEDs consists of an
n-GaAs buffer layer, an n-GaAs/AlAs distributed Bragg reflec-
tor (DBR) structure, an n-AlGaInP cladding layer, an undoped
AlGaInP/InGaP mutiquantum well (MQW) active region, a
p-AlGaInP cladding layer and an 8-μm-thick p-GaP window
layer. To increase the hole concentration in the window layer,
we subsequently deposited AuBe(80nm)/Au(5nm) onto the
sample surfaces. We then thermally annealed the samples at
450 °C for 15 min in N2ambient to diffuse Au and Be into
samples and to convert the p-GaP layers into p+-GaP layers.
After diffusion, the metal films were removed by immersing
the samples in phosphoric acid and KI solution (H3PO4 :
KI = 2 : 3) for 5 min. Secondary ion mass spectrometry
(SIMS) measurements were then performed to investigate the
distribution profiles of Au and Be after annealing. The Elec-
trochemical capacitance-voltage (ECV) profiling is employed
to determine the hole carrier concentration of p+-GaP layer.
A 320-nm-thick ITO film was subsequently deposited onto
the p+-GaP layers by e-beam evaporation. To further improve
the device performance, the samples were rapid thermally
annealed (RTA) at 300-550 °C for 5 min in N2ambient.
Transmission line model (TLM) was then used to deter-
mine the specific contact resistance between ITO and GaP.
We also deposited ITO films onto glass substrate and per-
formed the same thermal annealing. The optical properties
of the deposited ITO films were then evaluated by a Jasco
V-670 spectrophotometer. For the fabrication of AlGaInP-
based LEDs, AuGe/Au contact was deposited onto the back-
side surface of the sample followed by 380 °C N2annealing
for 10 min to serve as the n-contact. We then used scribe
and break to fabricate the 305 μm × 305 μm (12 mil ×
Fig. 2.Measured ECV hole carrier concentration profile of p+-GaP layer.
12 mil) LED chips. Current-voltage (I-V) characteristics of
the chips were then evaluated by an HP 4156C semiconductor
parameter analyzer. Subsequently, these LED chips were pack-
aged into LED lamps. Room temperature electroluminescence
(EL) characteristics of these fabricated LED lamps were then
evaluated by injecting different amount of DC current into
these devices. The output power was then measured using
the molded LEDs with an integrated sphere detector from top
of the devices. To evaluate reliability of these devices, we
attached the packaged LEDs onto a 7 cm × 7 cm board with
normal metal heat sink. These LEDs were then placed in a
burn-in furnace and injected 30 mA DC current into these
devices at 50 °C.
III. RESULTS AND DISCUSSION
Figure 1 shows measured SIMS profiles of Ga, P, Be and Au
after diffusion. It can be seen clearly that Be was diffused into
GaP with the a diffusion depth of about 200 nm. The diffused
depth of Au was only about 100 nm. The diffused quantity
and depth of Be was very much higher than Au. It has been
reported that Be behaves like an amphoteric dopant in GaP
. It has also been shown that proper thermal annealing
could drive the interstitial Be (i.e., donor) onto acceptor (i.e.,
Ga) sites . We believe the 450 °C, 15 min diffusion could
drive the Be onto acceptor sites to form a p+-GaP layer
near the sample surface. We also measure ECV to determine
the hole carrier concentration of p+-GaP layer as shown in
the Fig. 2. The ECV results indicate that the hole carrier
concentration of the GaP layer increases dramatically from
4.2×1018to 1×1021cm−3after AuBe diffused annealing at
450 °C. The ECV results are consistent with the SIMS data.
In addition, the thickness of the highly doped p+-GaP layer is
about 200 nm due to the AuBe diffused into the GaP surface.
The ECV results are consistent with the SIMS data.
Figure 3 shows I-V characteristics of ITO deposited on
GaP. Without AuBe diffusion, ITO was deposited onto a
p-GaP layer with limited hole concentration. Thus, we
achieved a nonlinear back-to-backSchottky-like behavior. This
should be attributed to the formation of a potential junction
barrier between the n-type ITO film and the p-GaP. In contrast,
linear I-V relationship was observed from the sample with
AuBe diffusion. This should be attributed to the formation
LO et al.: LEDS WITH A p+-GaP WINDOW LAYER AND A THERMALLY ANNEALED ITO CONTACT 805
With Au/Be diffusion
Without Au/Be diffusion
Fig. 3.I-V characteristics of ITO deposited on GaP.
without and with thermal post-ITO-deposition annealing.
Room-temperature I-V characteristics of ITO deposited on p+-GaP
of a thin metallic alloy layer and/or the enhanced tunneling
of carriers at the p+-GaP surface. The effects of thermal
annealing after ITO deposition were also investigated. Figure 4
shows room-temperature I-V characteristics of ITO deposited
on p+-GaP without and with thermal post-ITO-deposition
annealing. Without thermal post-ITO-deposition annealing, it
was found again that we achieved a linear I-V relationship.
Using TLM pads with 10-μm spacing, it was found that the
specific contact resistance was 2.63 × 10−4?-cm2. It was
also found that we could reduce the contact resistance by
properly post-ITO-deposition annealing the samples. Using
the same TLM pads, it was found that the specific contact
resistance decreased to 2.45 × 10−4, 1.57 × 10−4?-cm2as
we post-ITO-deposition annealed the samples at 300 °C and
400 °C, respectively. This should be attributed to the improved
crystalline and carrier mobility for the ITO layer.
As we increased the temperature of post-ITO-deposition
annealing to 450 °C, however, it was found that the specific
contact resistance increased 1.05×10−3?-cm2. As we further
increased the post-ITO-deposition annealing temperature to
500 °C and 550 °C, it was found that the I-V characteristics
became non-ohmic. For ITO deposited on Be-doped p+-GaP,
it has been shown that oxygen diffusion could form an oxygen-
O quantity (a.u)
Annealed at 550 ºC
interface as a function depth.
Normalized SIMS profiles of oxygen measured at the ITO/p-GaP
Specific contact resistance (Ω-cm2)
Fig. 6. Specific contact resistance determined from the temperature dependent
TLM measurements for the samples without post-ITO-deposition annealing,
post-ITO-deposition annealed at 400 °C and 450 °C.
rich layer when annealed above 500 °C . It has also
been shown that Be will react with oxygen to form Be-O
complex , which is detrimental to electrical conduction.
Similar situation should also occur for the ITO deposited
on Be-doped p+-GaAs. Fig. 5 shows the normalized SIMS
profiles of oxygen measured at the ITO/GaP interface as a
function depth. We observe that oxygen diffusion is found
to be very high when the sample is annealed at 550°. This
means that an oxygen-rich interfacial layer is formed within
the semiconductor. SIMS analysis of the above-described
samples can provide information to explain the phenomenon.
Thus, we observed the degraded electrical properties when
the temperature for post-ITO-deposition annealing was too
To understand the transport mechanism across the ITO/
p+-GaP interface, we performed temperature dependent TLM
measurements for the samples without post-ITO-deposition
annealing, post-ITO-deposition annealed at 400 °C and
450 °C. As shown in figure 6, we can thus determine the
specific contact resistances from the temperature dependent
At a metal-semiconductor interface, it is known that car-
riers are transported through thermionic emission, thermionic
field emission or filed emission according to the following
806IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 47, NO. 6, JUNE 2011
N2 5 min
on glass substrate with and without annealing.
Normalized transmission spectra measured from the ITO deposited
300 °C; (c) annealed at 450 °C; and (d) annealed at 550 °C, respectively.
AFM images of ITO thin films: (a) as-deposited; (b) annealed at
(for thermionic emission)(1)
(for thermionic field emission)(2)
where k is the Boltzmann constant, A∗is the Richardson
constant, T is the temperature and ?Bis the Schottky barrier
height. As shown in figure 6, it was found that the specific
contact resistances decreased from 5.3×10−4, 3.1×10−4and
5.2×10−5and 1.63×10−4?-cm2at 327 °C for the samples
without post-ITO-deposition annealing, and with post-ITO-
deposition annealed at 400 °C and 450 °C, respectively. It was
also found that the specific contact resistance for the sample
annealed at 450 °C decreased rapidly as we increased the
temperature. Such a result suggests that carrier transport was
dominated by thermionic emission for this particular sample.
In contrast, measured specific contact resistances were almost
(for field emission)(3)
Device ADevice B
ITO 400 °C annealing
ITO 550 °C annealing
Fig. 9.Schematic diagrams of the fabricated AlGaInP LEDs.
independent of temperature for the sample without annealing
and the sample annealed at 400 °C. This seems to suggest that
carrier transport was dominated by field emission for these
two samples. Figure 7 shows normalized transmission spectra
measured from the ITO deposited on glass substrate with and
without annealing. In this figure, the transmittance of each
film was normalized with respect to the transmission of the
glass substrate. It can be seen that almost all transmittances
measured in this study were larger than 90% in the visible
region. Such a result indicates that these ITO films were all
suitable to serve as the transparent current spreading layer
for the fabricated LEDs. Figure 8 shows the AFM images of
ITO thin films for as-deposited, annealed at 300 °C, 450 °C,
and 550 °C, respectively, and the root-mean-square (RMS)
roughness of the ITO thin films are 20.16, 34.47, 65.82, and
145.8 nm, respectively. From the results, it is clearly observed
that the RMS and grain size of ITO films increased with
increasing the post annealing temperature.
In this study, four AlGaInP-based LEDs were prepared. As
shown in figures 9(a), 9(b), 9(c) and 9(d), Device A was a
conventional AlGaInP-based LED with an 8-μm-thick p-GaP
window layer. Devices B was an AlGaInP-based LED with
an 8-μm-thick p+-GaP window layer and an ITO contact. For
this device, we used the aforementioned method to convert the
p-GaP layer to the p+-GaP layer. The structure of Devices C
and D was identical to that of Device B. However, one extra
RTA process was performed, at 400 °C and 550 °C, after ITO
deposition for Devices C and D, respectively.
Figure 10 shows forward I-V characteristics measured from
the four fabricated LEDs. With 20-mA current injection, it
was found that the forward voltages were 1.97, 1.96, 1.95
and 2.66 V for Device A, Device B, Device C and Device
D, respectively. In other words, the 20 mA forward voltage
observed from Device D was significantly larger than those
observed from the other three devices. As discussed in
figure 4, specific contact resistance of ITO on p+-GaP will
increase when annealed at high temperatures (i.e., ≥ 450 °C).
LO et al.: LEDS WITH A p+-GaP WINDOW LAYER AND A THERMALLY ANNEALED ITO CONTACT807
Fig. 10. Forward I-V characteristics measured from the four fabricated LEDs.
Output power (mW)
Fig. 11. (a) L-I characteristics of the four fabricated LEDs measured at room
temperature. (b) Plot of wall-plug efficiency versus injection current for all
With a large specific contact resistance, we thus observed a
much larger 20 mA forward voltage for Device D. It should
also be noted that we achieved the smallest 20 mA forward
voltage from Device C. This should also be attributed to the
low specific contact resistance for the 400 °C-annealed ITO
Figure 11(a) shows intensity-current (L-I) characteristics
of the four fabricated LEDs measured at room temperature.
Under the same injection current, it was found that we
achieved the highest output power from Device D, followed
Device CDevice D
(c) Device C. (d) Device D.
Near field optical images measured. (a) Device A. (b) Device B.
by Device C and Device B. On the other hand, output power
of Device A was the smallest. With an 8-μm-thick p-GaP
window layer, current crowding could still occur for Device
A without the ITO contact. Thus, we observed a significantly
smaller output power from Device A, as compared to the other
three LEDs. Under 20 mA current injection, it was found
that output powers for Device A, Device B, Device C and
Device D were 4.2, 5.7, 6.0 and 6.3 mW, respectively. In
other words, we could enhance the output power of AlGaInP-
based LEDs by 36%–50% due to the reduction of refractive
index difference by using the ITO contact on p-GaP, since
the refractive index of ITO is the middle of those of the GaP
and air. In addition, we have plotted the wall-plug efficiency
versus injection current for all devices as shown in Fig. 11(b).
It is clearly observed that the 20-mA wall-plug efficiency of
devices B and C (14.5% and 15.4%) are much higher than
devices A and D (11.5% and 11.8%).
Figures 12(a), 12(b), 12(c) and 12(d) show near field optical
images measured from Device A, Device B, Device C and
Device D, respectively, as we injected 20 mA DC current into
these LEDs. Compared with the central region of the chip (i.e.,
p-contact pad), it can be seen clearly that light output intensity
in the chip periphery was much smaller for Device A. This
should again be attributed to the poor current spreading for
Device A so that most of the injected carriers were confined
near the p-contact pad. In contrast, light output intensity was
much more uniform across the whole chip for the other three
LEDs again due to the enhanced current spreading by the
ITO layer. Life tests of EL intensity were also performed. In
Fig. 11(a), Device D (ITO annealing at T = 550 °C) exhibits
the largest output power (6.3 mW) larger than Device C
(6.0 mW, ITO annealing at T = 400 °C) and the others.
The results may be caused by the influence of the annealing
temperature on the surface morphology of ITO thin films.
As shown in Fig. 8, the RMS and grain size of ITO films
increased with increasing the post annealing temperature. In
addition, as the annealing temperature increased, the optical
transmittance in the visible range also slightly increased. In
the case, it may enhance the optical output power of Device D.
Moreover, the increase in annealing temperature may lead to
oxygen-deficient films. In ITO, oxygen deficiency is one of the
reasons for high conductivity. Oxygen deficiencies induce free
808 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 47, NO. 6, JUNE 2011
Normalized EL Intensity (%)
normalized to their respective initial readings.
Time-dependent EL intensities measured from these four LEDs,
electrons to behave as conduction carriers. The phenomenon
will also slightly result in the current crowding observed in
Fig. 12 (d).
Fig 13 shows time-dependent EL intensities measured from
these four LEDs, normalized to their respective initial read-
ings. During life tests, these LEDs were driven by a constant
20 mA current at room temperature. It can be seen clearly that
EL intensity decayed by 25.7% after 1000 hours for Device
A without the ITO contact. With a low LED output power,
a significant amount of the input electrical power will be
transferred into heat for Device A. We thus observed a rapid
decay in EL intensity. Among the three ITO LEDs, it was
found that EL intensity of Device D decayed by 19.9% after
1000 hours. The relatively large EL intensity decay should be
attributed to the degraded p-contact induced by high post-ITO-
annealing temperature. In contrast, EL intensity of Device C
only decayed by 11.5% during the same period. It should be
noted this value was smaller than the 16.4% decay observed
from Device B without the post-ITO-annealing. Such a result
indicates that we could not only simultaneously improve the
electrical and optical properties of AlGaInP-based LEDs but
could also improve their reliability by performing a 400 °C
post-ITO-annealing for 5 min in N2ambient.
In summary, we report the conversion of p-GaP layer into
p+-GaP layer by Be diffusion and the fabrication of AlGaInP-
based LEDs with ITO/p+-GaP double current spreading layer.
It was found that we could reduce the specific contact
resistance between ITO and p+-GaP from 2.63×10−4to
1.57×10−4?-cm2by performing RTA at 400 °C for 5 min
in N2ambient. It was also found that we could also enhance
output power and reliability of AlGaInP-based LEDs by using
the same annealing condition.
The authors would like to thank the light emitting diode
Lighting and Research Center, National Cheng Kung Univer-
sity, Tainan, Taiwan, for the assistance in related measure-
 S.-J. Chang, Y.-K. Su, T. Yang, C.-S. Chang, T.-P. Chen, and K.-H.
Huang, “AlGaInP-sapphire glue bonded light-emitting diodes,” IEEE J.
Quantum Electron., vol. 38, no. 10, pp. 1390–1394, Oct. 2002.
 H. Sugawara, K. Itaya, and G. Hatakoshi, “Hybrid-type InGaAlP/GaAs
distributed Bragg reflectors for InGaAlP light-emitting diodes,” Jpn. J.
Appl. Phys., vol. 33, no. 11, pp. 6195–6198, 1994.
 C. S. Chang, Y. K. Su, S. J. Chang, P. T. Chang, Y. R. Wu, K. H. Huang,
and T. P. Chen, “High-brightness AlGaInP 573-nm light-emitting diode
with a chirped multiquantum barrier,” IEEE J. Quantum Electron., vol.
34, no. 1, pp. 77–83, Jan. 1998.
 G. B. Stringfellow and M. G. Craford, “High brightness light-emitting
diodes,” in Semiconductors and Semimetals, vol. 48. New York: Acad-
 I. Schnitzer, E. Yablonovitch, C. Caneau, and T. J. Gmitter, “Ultrahigh
spontaneous emission quantum efficiency, 99.7% internally and 72%
externally, from AlGaAs/GaAs/AlGaAs double heterostructures,” Appl.
Phys. Lett., vol. 62, no. 2, pp. 131–133, Jan. 1993.
 H. Kim, J. M. Lee, C. Huh, S. W. Kim, D. J. Kim, S. J. Park, and
H. Hwang, “Modeling of a GaN-based light-emitting diode for uniform
current spreading,” Appl. Phys. Lett., vol. 77, no. 12, pp. 1903–1904,
 C. Huh, K. S. Lee, E. J. Kang, and S. J. Park, “Improved light-
output and electrical performance of InGaN-based light-emitting diode
by microroughening of the p-GaN surface,” J. Appl. Phys., vol. 93, no.
11, pp. 9383–9385, Jun. 2003.
 S. J. Chang, L. W. Wu, Y. K. Su, Y. P. Hsu, W. C. Lai, J. M. Tsai,
J. K. Sheu, and C. T. Lee, “Nitride-based LEDs with 800 °C grown
p-AlInGaN-GaN double-cap layers,” IEEE Photon. Technol. Lett., vol.
16, no. 6, pp. 1447–1449, Jun. 2004.
 K. H. Huang, J. G. Yu, C. P. Kuo, R. M. Fletcher, T. D. Osentowski,
L. J. Stinson, M. G. Craford, and A. S. H. Liao, “Twofold efficiency
improvement in high performance AlGaInP light-emitting diodes in the
555–620 nm spectral region using a thick GaP window layer,” Appl.
Phys. Lett., vol. 61, no. 9, pp. 1045–1047, Aug. 1992.
 S. J. Chang, C. S. Chang, Y. K. Su, R. W. Chuang, Y. C. Lin, S. C.
Shei, H. M. Lo, H. Y. Lin, and J. C. Ke, “Highly reliable nitride-based
LEDs with SPS+ITO upper contacts,” IEEE J. Quantum Electron., vol.
39, no. 11, pp. 1439–1443, Nov. 2003.
 Y. C. Lin, S. J. Chang, Y. K. Su, C. S. Chang, S. C. Shei, J. C. Ke,
H. M. Lo, S. C. Chen, and C. W. Kuo, “High power nitride based light
emitting diodes with Ni/ITO p-type contacts,” Solid State Electron., vol.
47, no. 9, pp. 1565–1568, Sep. 2003.
 D. V. Morgan, I. M. Al-Ofi, and Y. H. Aliyu, “Indium tin oxide spreading
layers for AlGaInP visible LEDs,” Semicond. Sci. Technol., vol. 15, no.
1, pp. 67–72, 2000.
 Y. H. Aliyu, D. V. Morgan, H. Thomas, and S. W. Bland, “AlGaInP
LEDs using reactive thermally evaporated transparent conducting indium
tin oxide (ITO),” Electron. Lett., vol. 31, no. 25, pp. 2210–2212, Dec.
 M.-C. Wu, J.-F. Lin, M.-J. Jou, C.-M. Chang, and B.-J. Lee, “High
reliability of AlGaInP LED’s with efficient transparent contacts for
spatially uniform light emission,” IEEE Electron. Dev. Lett., vol. 16,
no. 11, pp. 482–484, Nov. 1995.
 C.-H. Yen, Y.-J. Liu, K.-H. Yu, P.-L. Lin, T.-P. Chen, L.-Y. Chen, T.-
H. Tsai, N.-Y. Huang, C.-Y. Li, and W.-C. Liu, “On an AlGaInP-based
light-emitting diode with an ITO direct ohmic contact structure,” IEEE
Electron. Dev. Lett., vol. 30, no. 4, pp. 359–361, Apr. 2009.
 Y.-J. Liu, C.-H. Yen, K.-H. Yu, P.-L. Lin, L.-Y. Chen, T.-H. Tsai, T.-Y.
Tsai, and W.-C. Liu, “Characteristics of an AlGaInP-based light emitting
diode with an indium-tin-oxide (ITO) direct ohmic contact structure,”
IEEE J. Quantum Electron., vol. 46, no. 2, pp. 246–252, Feb. 2010.
 M. V. Tagare, T. P. Chin, and J. M. Woodall, “Nonalloyed ohmic contacts
to heavily Be-doped GaP and InxGa1−xP,” Appl. Phys. Lett., vol. 68,
no. 24, pp. 3485–3487, Jun. 1996.
 E. Havard, T. Camps, V. Bardinal, L. Salvagnac, C. Armand, C. Fontaine,
and S. Pinaud, “Effect of thermal annealing on the electrical properties
of indium tin oxide (ITO) contact on Be-doped GaAs for optoelectronic
applications,” Semicond. Sci. Technol., vol. 23, no. 3, pp. 035001-1–
 A. E. Neide, S. J. Pearton, W. S. Hobson, and C. R. Abernathy,
“Interaction of Be and O in GaAs,” Appl. Phys. Lett., vol. 54, no. 16,
pp. 1540–1542, Apr. 1989.
LO et al.: LEDS WITH A p+-GaP WINDOW LAYER AND A THERMALLY ANNEALED ITO CONTACT809
H. M. Lo was born in Ping-Dong, Taiwan, in
1978. He received the B.S. degree from the Depart-
ment of Electrical Engineering, Far East University,
Tainan, Taiwan, in 2001, and the M.S. degree from
the Department of Electrical Engineering, National
Cheng Kung University (NCKU), Tainan, in 2007.
He is currently pursuing the Ph.D. degree at the
Institute of Microelectronics, NCKU.
S. C. Shei was born in Nantou, Taiwan, on
November 8, 1964. He received the B.S., M.S.,
and Ph.D. degrees in electrical engineering from
National Cheng Kung University, Tainan, Taiwan,
in 1988, 1990, and 1995, respectively.
He was a Research and Development Associate
Vice President with the South Epitaxy Cooperation,
Tainan Science Based Industrial Park, Tainan, from
2000 to 2006. Currently, he is an Assistant Profes-
sor with the Department of Electrical Engineering,
National University of Tainan, Tainan. His current
research interests include light-emitting diodes, solid-state lighting, photode-
tectors and thin-film copper indium gallium selenide solar cells.
X. F. Zeng was born in Kaohsuing, Taiwan, in 1984.
He received the B.S. degree from the Department
of Electronic Engineering, Kun Shan University,
Tainan, Taiwan, in 2008, and the M.S. degree from
the Department of Electrical Engineering, National
University of Tainan, Tainan, in 2010. He is cur-
rently pursuing the Ph.D. degree at the Institute of
Microelectronics, National Cheng Kung University,
Shoou-Jinn Chang (M’06–SM’10) was born in
Taipei, Taiwan, on January 17, 1961. He received
the B.S.E.E. degree from National Cheng Kung
University (NCKU), Tainan, Taiwan, in 1983, the
M.S.E.E. degree from the State University of New
York at Stony Brook, Stony Brook, in 1985, and the
Ph.D.E.E. degree from the University of California,
Los Angeles, in 1989.
He was a Research Scientist with the NTT
Basic Research Laboratories, Musashino, Japan,
from 1989 to 1992. In 1992, he became an Associate
Professor with the Department of Electrical Engineering, NCKU, and was
promoted to full Professor in 1998. Currently, he serves as the Deputy
Director of the Center for Micro/Nano Science and Technology, the Deputy
Director of the Advanced Optoelectronic Technology Center, and the Director
of the Institute of Microelectronics, NCKU. He was a Royal Society Visiting
Scholar with the University of Wales, Swansea, U.K., from January 1999
to March 1999, a Visiting Scholar with the Research Center for Advanced
Science and Technology, University of Tokyo, Tokyo, Japan, from July 1999
to February 2000, a Visiting Scholar with the Institute of Microstructural
Science, National Research Council, Ottawa, ON, Canada, from August 2001
to September 2001, a Visiting Scholar with the Institute of Physics, Stuttgart
University, Stuttgart, Germany, from August 2002 to September 2002, and a
Visiting Scholar with the Faculty of Engineering, Waseda University, Tokyo,
from July 2005 to September 2005. He is also an Honorary Professor
of Changchun University of Science and Technology, Changchun, China.
His current research interests include semiconductor physics, optoelectronic
devices, and nanotechnology.
Prof. Chang received the Outstanding Research Award from the National
Science Council, Taiwan, in 2004.
Hsieh-Yen Lin received the B.S. degree in electri-
cal engineering from Southern Taiwan University,
Tainan, Taiwan, in 1999. He is currently pursuing
the M.S. degree at the Institute of Microelectronics,
National Cheng Kung University, Tainan.