Rapid thermal annealed InGaN/GaN flip-chip LEDs

Abstract

Nitride-based flip-chip (FC) light-emitting diodes (LEDs) emitting at 465 nm with Ni transparent ohmic contact layers and Ag reflective mirrors were fabricated. With an incident light wavelength of 465 nm, it was found that transmittance of normalized 300°C rapid thermal annealed (RTA) Ni(2.5 nm) was 93% while normalized reflectance of 300°C RTA Ni(2.5 nm)/Ag(200 nm) was 92%. It was also found that 300°C RTA Ni(2.5 nm) formed good ohmic contact on n⁺ short-period-superlattice structure with specific contact resistance of 7.8×10^-4 Ω·cm². With 20-mA current injection, it was found that forward voltage and output power were 3.15 V and 16.2 mW for FC LED with 300°C RTA Ni(2.5 nm)/Ag(200 nm). Furthermore, it was found that reliabilities of FC LEDs were good.

Full text

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32 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 53, NO. 1, JANUARY 2006
Rapid Thermal Annealed InGaN/GaN
Flip-Chip LEDs
W. S. Chen, S. C. Shei, S. J. Chang, Y. K. Su, Senior Member, IEEE, W. C. Lai, C. H. Kuo, Y. C. Lin, C. S. Chang,
T. K. Ko, Y. P. Hsu, and C. F. Shen
Abstract—Nitride-based flip-chip (FC) light-emitting diodes
(LEDs) emitting at 465 nm with Ni transparent ohmic con-
tact layers and Ag reflective mirrors were fabricated. With an
incident light wavelength of 465 nm, it was found that trans-
mittance of normalized 300
C rapid thermal annealed (RTA)
Ni(2.5 nm) was 93% while normalized reflectance of 300 C RTA
Ni(2.5 nm)/Ag(200 nm) was 92%. It was also found that 300 C
RTA Ni(2.5 nm) formed good ohmic contact on n?short-pe-
riod-superlattice structure with specific contact resistance of
? ???
?? cm?. With 20-mA current injection, it was found
that forward voltage and output power were 3.15 V and 16.2 mW
for FC LED with 300 C RTA Ni(2.5 nm)/Ag(200 nm). Further-
more, it was found that reliabilities of FC LEDs were good.
Index Terms—Flip-chip (FC), GaN, light-emitting diodes
(LEDs), rapid thermal anneal (RTA), reflective mirrors, trans-
parent ohmic contact.
I. INTRODUCTION
R
pound materials exhibit wurtzite crystal structure, direct and
wide band gap, high thermal conductivity and high saturation
velocity. At room temperature, energy bandgap of AlInGaN
varies from 0.8 to 6.2 eV depending on its composition. These
properties make nitride-based materials attractive for short
wavelength emitters [1]–[4]. Indeed, nitride-based blue and
green light-emitting diodes (LEDs) are already extensively
used in traffic light lamps and full color displays [5]–[7].
Nitride-based LEDs are also potentially useful for solid-state
lighting and backlight of liquid crystal display panels. For
lighting applications, incandescent lamps and fluorescent
lamps are the two most commonly used light sources in our
daily life. However, incandescent lamps consume large power
ECENTLY, gallium nitride (GaN) and its related ternary
compounds have attracted much attention. These com-
Manuscript received July 12, 2005; revised October 7, 2005. This work was
supportedin partbytheNationalScienceCouncilunderGrantNSC90-2215-E-
008-043,GrantNSC90-2112-M-008-046,andGrantNSC93-212XM-006-006,
in part by the Center for Micro/Nano Technology Research, National Cheng-
Kung University, under projects from the Ministry of Education. The review of
this paper was arranged by Editor L. Lunardi.
W. S. Chen, Y. K. Su, Y. C. Lin, T. K. Ko, Y. P. Hsu, and C. F. Shen are with
the Institute of Microelectronics and the Department of Electrical Engineering,
National Cheng-Kung University, Tainan 70101, Taiwan, R.O.C.
S. C. Shei and C. S. Chang are with the South Epitaxy Corporation, Hsinshi
744, Taiwan, R.O.C.
S. J. Chang is with the Institute of Microelectronics and the Department
of Electrical Engineering, National Cheng-Kung University, Tainan 70101,
Taiwan, R.O.C. (e-mail: changsj@mail.ncku.edu.tw).
W. C. Lai is with the Institute of Electro-Optical Science and Engineering,
National Cheng-Kung University, Tainan 70101, Taiwan, R.O.C.
C.H.KuoiswiththeInstituteofOpticalScience,NationalCentralUniversity
Chung-Li 320, Taiwan, R.O.C.
Digital Object Identifier 10.1109/TED.2005.860760
with short lifetime. Fluorescent lamps contain environmentally
hazard mercury. Thus, much attention has been focused on
generating white light with nitride-based LEDs. The most
commonly used method to achieve white LEDs is to combine a
phosphor wavelength converter with a GaN blue LED chip. The
blue light emitted from the GaN LED chip is absorbed by the
phosphor and re-emitted as long-wavelength phosphorescence.
Thus, white light can be generated by the combination of the
two emission bands. However, output power of the current
white LEDs is still low. In other words, we need to improve the
output intensity of nitride-based blue LED chips before we can
realize feasible solid-state lighting [8].
Conventional nitride-based blue LEDs use semitransparent
Ni/Au on p-GaN as the upper contacts. However, transmittance
of Ni/Au is only around 55%–75% [9], [10]. Recently, we
demonstrated nitride-based blue LEDs with n
short period superlattice (SPS) tunneling contact layers and
indium–tin–oxide (ITO) upper contacts [11], [12]. Compared
with conventional nitride-based blue LEDs with Ni/Au upper
contacts, it was found that we could achieve a much larger elec-
troluminescence (EL) intensity by using ITO upper contacts
due to the transparent nature of ITO. However, a significant
amount of photons emitted from the ITO upper contact layers
are still obscured by the bonding pads of the devices. As a
result, external quantum efficiency and output power of the
packaged LEDs will both become smaller. One possible way to
solvethisproblemistouseflip-chip(FC)technology[13]–[20].
By using FC technology, we should be able to achieve a larger
output power since no bonding pads or wires exist on top of
the devices so that photons could be emitted freely from the
substrates. Also, we should be able to further improve the
output power of FC LEDs by depositing a reflector layer at
the bottom of the devices so that down emitting photons could
be reflected upward. However, it is difficult to find a reflector
material with good adhesion and small contact resistance. One
possible way to solve this problem is to insert a transparent
ohmic contact layer, as shown in Fig. 1. It should be noted
that Fig. 1 is just a schematic diagram of FC LED. The actual
thickness of sapphire substrate should be much larger than that
of the epitaxial layers. In this paper, we fabricated FC LEDs
with Ni transparent ohmic contact layers and Ag reflective
mirrors. The characteristics of the fabricated FC LEDs will also
be reported and discussed extensively.
InGaN/GaN
II. EXPERIMENTS
Samples used in this investigation were all grown by
metal–organic chemical vapor deposition (MOCVD) on c-face
0018-9383/$20.00 © 2005 IEEE
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CHEN et al.: RAPID THERMAL ANNEALED InGaN/GaN FLIP-CHIP LEDs 33
Fig. 1.
reflective mirror.
Schematic diagram of the FC LED with transparent ohmic contact and
2-in sapphire (Al O ) (0001) substrates. Details of the growth
can be found elsewhere [21]–[23]. In order to investigate con-
tact properties of the transparent ohmic contact layers, we first
deposited a 30-nm-thick low-temperature GaN nucleation layer
followed by a 2- m-thick Mg-doped GaN epitaxial layer and a
Si-doped InGaN/GaN SPS structure. The n
tact structure consists of four pairs of n
(5
/5). The samples were then annealed at 750 C in N for
20 min to activate Mg in the p-type layers. We subsequently
evaporated Ni with various thicknesses onto the samples at
100 C followed by the evaporation of Ag reflective mirrors.
Rapid thermal annealing (RTA) was then performed at various
temperatures for 35 s to improve electrical properties of the
contacts. Circular transmission line model (CTLM) was then
used to determine specific contact resistances of the deposited
contacts. Standard photolithography process was used to define
circular patterns. The outer circular contact had a contact diam-
eter of 500
m while the gap between inner and outer contact
was 60 m. Opticalproperties of thecontacts are also important
for practical device applications. After RTA, we used a Hitachi
U3010 spectrophotometer to measure the transmittance of Ni
and reflectance of Ni/Ag.
Nitride-based FC LEDs were also prepared. The LED struc-
ture consists of a 50-nm-thick GaN nucleation layer grown
at 550 C, a 3- m-thick Si-doped n-GaN buffer layer grown
at 1050
C, an unintentionally doped InGaN/GaN multiple
quantum well (MQW) active region grown at 770
50-nm-thick Mg-doped p-Al
layer grown at 1050
C, a 0.25- m-thick Mg-doped p-GaN
contact layer grown at 1050 C and a Si-doped n
SPS structure. The InGaN/GaN MQW active region consists
of five periods of 3-nm-thick In
7-nm-thick GaN barrier layers. After annealing to activate Mg
in the p-type layers, we partially etched surfaces of the LED
samples until the n-type GaN layers were exposed. Ti/Al/Ti/Au
contacts were subsequently deposited onto the exposed n-type
GaN layers to serve as the n-type electrodes. SiO
then deposited over all wafers by plasma-enhanced chemical
vapor deposition (PECVD). Photolithography and HF solu-
tion etching were subsequently performed to define the P/N
pad pattern for bump electroplating. It should be noted that
step coverage of the SiO films should be well controlled to
cover the entire chips. Sn/Au(15
electroplated onto the samples while P/N bumps were defined
SPS tunnel con-
GaInN/GaN
C, a
GaN electron blocking
InGaN/GaN
Ga N well layers and
layers were
m/5m) layers were then
Fig. 2. Normalized transmission spectra of the deposited Ni.
by lift-off. The processed wafers were then lapped down to
about 110
m. We then used scribe and break to fabricate the
280
m360m FC LEDs. For comparison, nonFC (NFC)
ITO and NFC Ni/Au LEDs were also fabricated with exactly
the same epitaxial structure and the same chip size. These chips
were then encapsulated with epoxy and packaged into LED
lamps. It should be noted that FC LEDs were soldered onto
Si sub-mounts prior to packaging. Current–voltage (I–V) mea-
surements of the fabricated LED lamps were then performed
at room temperature. EL characteristics of these lamps were
also evaluated by injecting different amount of dc currents into
these LEDs. The output powers were then measured using the
molded LEDs with integrated sphere detector from the top of
the devices. Reliability tests of these LED lamps were also
performed by injecting 30-mA dc current into these devices at
room temperature.
III. RESULTS AND DISCUSSION
In order to determine the transmittance of Ni and reflectance
of Ni/Ag, we also deposited these metals onto glass substrates.
Fig. 2 shows normalized transmission spectra of the deposited
Ni. These transmission spectra were normalized with respect
to the transmission spectrum of the glass substrate. In other
words, these reflectivities shown in Fig. 2 are respect to glass
substrates. It can be seen that the deposited Ni films became
more transparent after RTA. Such a result is probably due to the
formation of NiO. With an incident light wavelength of 465 nm,
it was found that transmittances of 250 C RTA Ni(2.5 nm),
300 C RTA Ni(2.5 nm) and 350 C RTA Ni(2.5 nm) were
87%, 93%, and 96%, respectively. It should be noted that there
might exist some errors of a few percent for the measured
transmittance and reflectance due to the thickness variations
of the glass substrates and the deposited metal layers. Fig. 3
shows I–V characteristics of Ni deposited on n
It was found that the as-deposited Ni(1 nm) and 350 C RTA
Ni(2.5 nm) both form Schottky contact on n
In other words, although both of them were highly transparent,
they were not usable electrically as the contact material. On the
other hand, it was found that 300 C RTA Ni(2.5 nm), 250 C
RTA Ni(2.5 nm), as-deposited Ni(2.5 nm) and as-deposited
Ni(5nm)couldallformohmiccontactonn SPSstructurewith
specificcontactresistancesof
SPS structure.
SPS structure.
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34 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 53, NO. 1, JANUARY 2006
Fig. 3.
I–V characteristics of Ni deposited on nSPS structure.
Fig. 4.Normalized reflectances of as-deposited and RTA Ni/Ag.
and
reflectances of as-deposited and RTA Ni/Ag. In this figure,
the reflection spectra were normalized with respect to the
reflection spectrum of the glass substrate. With an incident
wavelength of 465 nm, it was found that reflectances of as-de-
posited Ni(2.5 nm)/Ag(200 nm), Ni(2.5 nm)/Ag(400 nm), and
Ni(2.5 nm)/Ag(50 nm) were 81%, 82%, and 70%, respectively.
Such a result suggests 200-nm-thick Ag should be enough to
serve as good reflective mirrors. For Ni(2.5 nm)/Ag(200 nm),
it was further found that reflectances measured at 465 nm
were 85%, 92%, and 86% after the RTA process performed
at 350
C, 300C, and 250
normalized transmittances of Ni and normalized reflectances
of Ag measured at 465 nm. Although transmittance of Ni RTA
at 300 C was slightly smaller than that of Ni RTA at 350 C,
reflectance of Ag RTA at 300 C was much larger than that of
Ag RTA at 350 C probably due to the oxidation of Ag film at
350 C. In other words, although Ni became more transparent
after 350 C RTA, we still achieved the largest reflectance from
300 C RTA Ni(2.5 nm)/Ag(200 nm), as shown in Fig. 4.
We also measured the performances of FC LEDs with
Ni(2.5 nm) transparent ohmic contact and Ag(200 nm) re-
flective mirrors annealed at 350 C (i.e., LEDI), 300 C (i.e.,
LEDII), 250 C (i.e., LEDIII) and without RTA (i.e., LEDIV).
Fig. 5 depicts measured I–V characteristics of these FC LEDs.
For comparison, I–V characteristics of NFC LEDs with ITO
cm , respectively. Fig. 4 shows normalized
C, respectively. Table I lists
TABLE I
NORMALIZED TRANSMISSION AND REFLECTION SPECTRA OF THE
DEPOSITED Ni AND Ag METAL
Fig. 5. Measured I–V characteristics of the fabricated LEDs.
(i.e., LEDV) and Ni/Au (i.e., LEDVI) upper contacts were also
shown in the same figure. It was found that forward voltages
of FC LEDs were all higher than that of the LED with the
Ni/Au p-contact. Among the four FC LEDs, it was found that
forward voltage of LEDII RTA at 300 C was the smallest. As
shown in Fig. 3, this should be attributed to the small specific
contact resistance of Ni annealed at 300
current injection, it was found that forward voltages were 3.70,
3.15, 3.24, 3.33, 3.24, and 3.03 V for LEDI, LEDII, LEDIII,
LEDIV, LEDV, and LEDVI, respectively. Fig. 6 shows room
temperature EL spectra of LEDII, LEDV, and LEDVI. It can be
seen that EL peak positions of these three LEDs all occurred
at around 465 nm. However, it was found that EL intensity of
FC LEDII was much larger than those of NFC LEDV and NFC
LEDVI.
Fig. 7 shows intensity–current (L–I) characteristics of the six
fabricated LEDs. It can be seen clearly that output powers of
the four FC LEDs were larger than those of the two NFC LEDs.
This can be attributed to the fact that no bonding pads or wires
exist on top of the FC LEDs. It should also be noted that re-
fractive indexes of sapphire substrate and GaN are 1.7 and 2.5,
respectively. Thus, photons generated in the LEDs should be
abletofindtheescapeconeeasierfromthesapphire/airinterface
C. With 20 mA
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CHEN et al.: RAPID THERMAL ANNEALED InGaN/GaN FLIP-CHIP LEDs35
Fig. 6.Room temperature EL spectra of LEDII, LEDV, and LEDVI.
Fig. 7.
L–I characteristics of the six fabricated LEDs.
(i.e., FCLEDs), as compared to theGaN/air interface(i.e., NFC
LEDs) [24]–[26]. In other words, the smaller refractive index of
sapphire substrate could also enhance the LED output intensity.
Among the four FC LEDs, it was found that LEDII exhibits the
largest output power. This can again be attributed to the highly
reflective nature of the 300 C RTA Ni(2.5 nm)/Ag(200 nm)
mirror, as shown in Fig. 4. With 20 mA current injection, it was
found that output powers were 13.0, 16.2, 14.1, 10.8, 9.1, and
5.6 mW for LEDI, LEDII, LEDIII, LEDIV, LEDV, and LEDVI,
respectively. Fig. 8 shows life tests of relative luminous inten-
sities measured from the fabricated LEDs, normalized to their
respective initial readings. During life test, all six LEDs were
driven by 30-mA current injection at room temperature. After
1000 h, it was found that luminous intensities of FC LEDI,
LEDII, LEDIII, and LEDIV decreased by 20.7%, 4.1%, 12.4%,
and 14.6%, respectively. With small specific contact resistance
and small forward voltage, we thus achieved much smaller EL
degradation from LEDII 300 C RTA Ni(2.5 nm)/Ag(200 nm)
mirror. On the other hand, luminous intensities of NFC LEDV
and LEDVI decreased by 28% and 19%, respectively, during
the same period [27]–[29]. Compared with conventional NFC
LEDs, thermal path of FC LEDs was much shorter. Thus, most
heat generated in LED chips could flow easily to the heat sink
for FC LEDs. As a result, we could also achieve much more re-
liable LEDs using the FC technology.
Fig. 8.
LEDs, normalized to their respective initial readings.
Life tests of relative luminous intensities measured from the fabricated
IV. CONCLUSION
Nitride-based FC LEDs emitting at 465 nm with Ni trans-
parent ohmic contact layers and Ag reflective mirrors were
fabricated. With an incident light wavelength of 465 nm, it
was found that transmittance of normalized 300 C RTA Ni
(2.5 nm) was 93% while normalized reflectance of 300
RTA Ni(2.5 nm)/Ag(200 nm) was 92%. It was also found that
300 C RTA Ni(2.5 nm) formed good ohmic contact on n SPS
structure with specific contact resistance of
With 20-mA currentinjection, it was found thatforward voltage
and output power were 3.15 V and 16.2 mW for FC LED with
300
C RTA Ni(2.5 nm)/Ag(200 nm). Furthermore, it was
found that reliabilities of FC LEDs were good.
C
cm .
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SPS upper contact,” IEEE
C-grown
W. S. Chen was born in Chia-Yi, Taiwan, R.O.C.,
in 1977. He received the B.S. degree in electrical en-
gineering from the National Cheng-Kung University
(NCKU), Tainan, Taiwan, R.O.C. in 2001, and the
M.S. degree from the Institute of Microelectronics,
NCKU, in 2003. Currently, he is pursuing the Ph.D.
degree in nitride based optoelectric devices in the
Institute of Microelectronics, NCKU.
S. C. Shei received the B.S., M.S., and Ph.D.
degrees from the National Cheng-Kung University,
Tainan, Taiwan, R.O.C., in 1988, 1990 and 1995,
respectively.
His major field of research is focused on III-V
optoelectrical semiconductors. Currently, he is the
research and development Associate Vice President
in the South Epitaxy Corporation, Hsinchi, Taiwan,
R.O.C.
S. J. Chang was born in Taipei, Taiwan, R.O.C. on
January 17, 1961. He received the B.S.E.E. degree
from National Cheng-Kung University (NCKU),
Tainan, Taiwan, R.O.C., the M.S.E.E. degree from
the State University of New York, Stony Brook,
and the Ph.D.E.E. degree from the University of
California, Los Angeles, in 1983, 1985, and 1989,
respectively.
He was a Research Scientist with NTT Basic Re-
search Laboratories, Musashino, Japan, from 1989
to 1992. He became an Associate Professor, Depart-
ment of Electrical Engineering, NCKU in 1992 and was promoted to full Pro-
fessor in 1998. Currently, he also serves as the Director of Semiconductor Re-
search Center, NCKU. He was a Royal Society Visiting Scholar, University of
Wales, Swansea, U.K. from January 1999 to March 1999, a Visiting Scholar,
Research Center for Advanced Science and Technology, University of Tokyo,
Tokyo, Japan, from July 1999 to February 2000, a Visiting Scholar, Institute
of Microstructural Science, National Research Council, Canada, from August
2001 to September 2001, a Visiting Scholar, Institute of Physics, Stuttgart Uni-
versity,Stuttgart,Germany,fromAugust2002toSeptember2002andaVisiting
Scholar, Waseda University, Waseda, Japan from July 2004 to September 2004.
He is also an Honorary Professor, Changchun University of Science and Tech-
nology,China.Hiscurrentresearchinterestsincludesemiconductorphysicsand
optoelectronic devices.
Dr.ChangreceivedtheoutstandingresearchawardfromtheNationalScience
Council, Taiwan, R.O.C., in 2004.
Y. K. Su (S’77–M’84–SM’91) was born in Kaoh-
siung, Taiwan, R.O.C., on August 23, 1948. He
received the B.S. and Ph.D. degrees in electrical
engineering from National Cheng-Kung University
(NCKU), Tainan, Taiwan.
From 1979 to 1983, he was with the Department
of Electrical Engineering, NCKU, as an Associate
Professor and was engaged in research on compound
semiconductors and optoelectronic materials. In
1983, he was promoted to Full Professor in the
Department of Electrical Engineering. From 1979
to 1980 and 1986 to 1987, he was on leave, working at the University of
Southern California, Los Angeles, and at AT&T Bell Laboratories, Murray
Hill, NJ, as a Visiting Scholar. He was also a Visiting Professor at Stuttgart
University, Germany, in 1993. In 1991, he became an Adjunct Professor at
the State University of New York, Binghamton. He is now a Professor in the
Department of Electrical Engineering at NCKU and Director General of the
Department of Engineering and Applied Science, National Science Council
of Taiwan. His research activities have been in compound semiconductors,
integrated optics, and microwave devices. He has published over 200 papers in
the area of thin-film materials and devices and optoelectronic devices.
Dr. Su is a member of SPIE, the Materials Research Society, and Phi Tau
Phi. He received the Outstanding Research Professor Fellowship from the Na-
tional Science Council, during 1986–1992 and 1994–1995. He also received the
Best Teaching Professor Fellowship from the Ministry of Education, R.O.C., in
1992.In1995,hereceivedtheExcellentEngineeringProfessorFellowshipfrom
the Chinese Engineering Association. In 1996 and 1998, he received the Award
from the Chinese Electrical Engineering Association. In 1998, he also received
the Academy Member of Asia-Pacific Academy of Materials (APAM).
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