IEEE SENSORS JOURNAL, VOL. 10, NO. 4, APRIL 2010799
III-Nitride Schottky Rectifiers With an
AlGaN/GaN/AlGaN/GaN Quadruple Layer
and Their Applications to UV Detection
P. C. Chang, K. H. Lee, S. J. Chang, Member, IEEE, Y. K. Su, Fellow, IEEE, T. C. Lin, and S. L. Wu
Abstract—III-nitride Schottky rectifiers (SRs) with (i.e., SR_A)
and without (i.e., SR_B) the AlGaN/GaN/AlGaN/GaN quadruple
layer were both fabricated. It was found that we could achieve a
current than that in SR_B. It was also found that lower on-state
resistance was due to the larger Schottky barrier height and larger
breakdown voltage was due to reduced defect densities in SR_A.
The SRs with a quadruple layer was suitable for applications to
low noise or/and ultraviolet (UV) detection as well.
Index Terms—Quadruple layer, Schottky rectifiers.
superior physical and chemical properties, such as higher
electric field strength, stronger bond polarity, and higher
electron saturation drift velocity, they are quite suitable for
high power device applications and for electronic devices
capable of operating in high temperature environments –.
High-performance GaN Schottky rectifiers (SRs) have been
commonly developed on foreign substrate, such as sapphire.
Although important progress has been made in the past, the
applications of such devices are limited by the high dislocation
) in heteroepitaxial GaN layer .
In particular, the smallness of reverse-bias leakage current
of the SR is a major concern in such devices from the view-
points of precise depletion edge control, operation stability
and reliability, power consumption, and noise performance .
II-NITRIDE wide band gap materials have recently at-
tracted a lot of interest for applications. Due to their
Manuscript received July 13, 2009; accepted October 04, 2009. This work
was supported in part by the National Science Council under Contract NSC
97-2221-E-168-051-MY3. Current version published March 10, 2010. The as-
sociate editor coordinating the review of this paper and approving it for publi-
cation was Dr. M. Abedin.
P. C. Chang and T. C. Lin are with the Department of Electrical Engi-
neering, Kun Shan University, Yung-Kang, 71003, Taiwan, China (e-mail:
K. H. Lee and S. J. Chang are with the Institute of Microelectronics
and Department of Electrical Engineering, Center for Micro/Nano Sci-
ence and Technology, Advanced Optoelectronic Technology Center, Na-
tional Cheng Kung University, Tainan, 70101, Taiwan, R.O.C. (e-mail:
Y. K. Su is with the Department of Electrical Engineering, Kun Shan
University, Yung-Kang, 71003, Taiwan, China. He is also with the Institute
of Microelectronics and Department of Electrical Engineering, Center for
Micro/Nano Science and Technology, Advanced Optoelectronic Technology
Center, National Cheng Kung University, Tainan 701, Taiwan, R.O.C. (e-mail:
S. L. Wu is with the Department of Electronic Engineering, Cheng Shiu Uni-
versity, Kaohsiung, 833, Taiwan, R.O.C. (e-mail: firstname.lastname@example.org).
Digital Object Identifier 10.1109/JSEN.2009.2034626
The reverse leakage current in SR is generally far higher than
expected from thermionic emission –, most likely due
to defect states around the contact periphery . Several tech-
niques have been employed for threading dislocations (TDs)
reduction, for example, epitaxial lateral overgrowth (ELO)
–, short-period strained-layer superlattice –,
TiN nanoporous network –, and
–. Among them, ELO is the most promising way to
reduce the density of dislocations in III-nitride materials. In
this study, we adopt an
quadruple layer structure to achieve the same goal of defect re-
duction as ELO technique. Compared to the conventional ELO,
the use of an
layer in situ prepared nanometer
scale mosaic structure has the advantage of maskless, one-step
processing, and possible contamination associated with the
ex situ lithography process in traditional ELO methods can
be eliminated. The effect of this
dislocation reduction of the GaN epitaxial layer has been
reported by our previous work . In addition, by using the
layer one can take advantage of a larger
band gap to achieve higher critical electric fields (i.e., break-
down voltage) than in GaN alone. III-Nitride SRs with and
without an AlGaN/GaN/AlGaN/GaN quadruple layer were
both fabricated. Physical, optical, and electrical properties of
the fabricated SRs will also be discussed.
layer on the
The samples used in this study were all grown by metalor-
ganic chemical vapor deposition (MOCVD) on 2-inch (0001)
sapphire substrates. After substrates annealing at 1120 C in
, they were first coated with a GaN buffer layer. Then at
a temperature of 1050 C GaN approximately 1.75
was grown. Next, the supply of Group III alkyls was stopped
and the substrate temperature was lowered down to 600 C,
an layer of 0.1-
depositing, the supply of the Group III alkyls was stopped
again and the substrate was heated to 1050 C.
(20 nm) and GaN (0.25 ) layers were subsequently grown.
layer structure was established, as shown in Fig. 1. For compar-
ison, samples without such quadruple layer were also prepared
layer and keeping the total thickness
of GaN layer at 2
Nitride-based SRs were then fabricated based on these two
structures. After mesa etching, we deposited Ti (12 nm)/Al
(100 nm) onto the samples to serve as the ohmic contact before
undergoing an 8 min thermal annealing conducted at 650 C for
-thick was deposited. After
1530-437X/$26.00 © 2010 IEEE
800 IEEE SENSORS JOURNAL, VOL. 10, NO. 4, APRIL 2010
Fig. 1. Device diagram for SRs with a AlGaN/GaN/AlGaN/GaN quadruple
Fig. 2. FE-SEM images of (a) SR_A and (b) SR_B.
ohmic contact alloying. Ni (40 nm)/Au (100 nm) were subse-
quently deposited to serve as the Schottky contact. We kept the
diameter of the fabricated SRs with circular Schottky barrier
contact at 400
. Here, we defined SRs with a quadruple
layer as SR_A, while SRs without the quadruple layer as SR_B.
Room temperature current-voltage ( - ) characteristics of the
fabricated SRs were then measured by an HP-4156 semicon-
ductor parameter analyzer. The noise characteristics were also
measured using a low-noise current preamplifier equipped with
a fast Fourier transform spectrum analyzer.
III. RESULTS AND DISCUSSION
Fig. 2 exhibits the field emission scanning electron mi-
croscopy (FE-SEM) surface morphologies of both samples.
Fig. 1(b) clearly shows that dark surface pits can be observed in
SR_B. In contrast, surface pits were almost invisible in SR_A,
as shown in Fig. 1(a). Due to the large lattice mismatched
Fig. 3. Room temperature ?-? characteristics of the two fabricated SRs. Inset
between the sapphire substrate and GaN film, the island–island
coalescence results in pure edge TDs in the form of low angle
asymmetric tilt boundaries, whereas the TDs with mixed char-
at the boundary of the islands lead to propagate throughout the
whole film and weave dislocation networks. The surface pits
generally represent the surface termination of TDs –.
Therefore, an apparent reduction of surface pits found in SR_A
could be attributed to the stop of dislocation threading and
defect generating as a result of using a quadruple layer. This
result agrees well with the previous reports by others , .
Fig. 3 shows dark -
characteristics of the fabricated SRs
measured at room temperatures. It can be seen clearly that dark
currents measured from SR_A were smaller than those mea-
sured from SR_B. With a
room temperature dark leakage currents of SR_A and SR_B
words, the leakage current of SR_A was more than five orders
magnitude smaller than that of SR_B. Dislocations have been
revealed to be leakage paths in III-nitride-based devices .
The low leakage current achieved here should be attributed to
the low defect density in SR_A with a quadruple layer. Break-
down behaviors of the fabricated devices were also measured,
as shown in the inset of Fig. 3. Here, we defined the breakdown
) as the voltage when the leakage current reached 1
mA. Using this definition, it was found that
for SR_A and SR_B, respectively. The smaller
served from the SR_B could be attributed to the presence of de-
fects (dislocations, nanopipes) leading to premature breakdown
that defect density can be suppressed by using the quadruple
applied bias, it was found that
, respectively. In other
constant (i.e., 8.62
ature in unit of
current density (usually taken to be 100
the effective Richardson constant (assuming 32
is the ideality factor (dimensionless),is Boltzmann’s
), is the absolute temper-
is the forward
,is the electron charge,
CHANG et al.: III-NITRIDE SCHOTTKY RECTIFIERS WITH AN ALGAN/GAN/ALGAN/GAN QUADRUPLE LAYER801
Fig. 4. Measured noise power spectra of (a) SR_A and (b) SR_B.
the auxiliary functions proposed by Cibils et al. , the value
and can be determined. It was found that the values of
were 3.20 and 3.95 for SR_A and SR_B, respectively. It was
also found that the values of
and 0.73 eV, respectively. The current transport mechanism in
our devices may not be the thermionic emission due to the large
values for both devices. Karmalkar et al.  have reported
that trap-assisted tunneling dominates below the temperature of
500 K and thermionic field emission dominates at higher tem-
perature for AlGaN/GaN heterostructure. Therefore, the higher
values for both SRs might be attributed to the dominance of
the trap-assisted tunneling. The relatively smaller
SR_A might be ascribed to the reduced crystal defect density,
which led to a smaller probability for the occurrence of trap-as-
sisted tunneling. Knowing the values of
could be extracted by (1). It was found that the
values were 16.70 and 19.54
oretical value of
That is, a low on-resistance was achieved in our devices, sug-
gesting the crystal qualities of our samples are good. However,
value in SR_A was smaller than that in SR_B. It might be
explained by the fact that the dominant factor in
Thus, although the
in SR_B was larger than that in SR_A,
in SR_A was larger than that in SR_B to a larger ex-
tent, leading to smaller
in SR_A. According to the above-
mentioned results, we can conclude that larger
), is the barrier height in unit of eV,
is the on-state resistance in unit of. By using
of SR_A and SR_B were 0.93
and , the value
, respectively. The the-
Fig. 5. Measured noise power density at 100 Hz as a function of dark current
for (a) SR_A and (b) SR_B.
and dark leakage current for SR_A could be attributed to better
crystal qualities and fewer defects. Therefore, we have success-
fully improvedthe crystal qualities of III-nitride epitaxial layers
and obtained the better rectifying properties in related devices
by using a quadruple layer.
Fig. 4(a) and (b) show the measured noise power densities
of both SRs. The noise curves of both devices obey the noise
model proposed by Hooge et al.  with a fitting parameter
. Notice that the measured low frequency noise of both SRs
could be regarded as a superposition of the
( ) noise. It was found that
noise is dominant in SR_B. Fig. 5(a) and (b)
show the noise power density as a function of dark current
measured at 100 Hz for both devices. The noise power density
can be written as
( ) and
noise is dominant in
From the curves shown in Fig. 5, the extracted values of
both equal to unity. Knowing these values, we can determine
and SR_B, respectively. Morrison et al.  has proposed that
is approximately proportional to the TD density. Therefore,
it indicates that we can reduce the effective TD density in the
III-nitride epitaxial layer by using the quadruple layer. With a
smaller TD density, we can thus minimize the effects of carrier
mobility fluctuation and thus achieve SRs with smaller noise.
802IEEE SENSORS JOURNAL, VOL. 10, NO. 4, APRIL 2010
Fig. 6. Transmission spectra measured from the two samples.
and (b) SR_B.
Our devices are suitably applied to the ultraviolet (UV) de-
tection as well. UV sensors or detectors are useful to the daily
and pollution detecting, and UV meter. Since the leakage cur-
rent problem has been solved by using SRs with a quadruple
layer, our devices could give a high sensitivity because of im-
proved photo-to-dark current ratio and UV-to-visible rejection
ratio due to the lower dark current. Fig. 6 shows transmission
spectra measured from the two samples. The sharp transition
edges occurring at around 360 nm (i.e., bandgap of GaN) also
samples. Hence, a 250W Xe lamp dispersed by a monochro-
mator was used as the light source in order to investigate the de-
scribes the noise performance of the device is the noise equiva-
power required to produce a signal-to-noise ratio (
unity in a bandwidth of 1 Hz and is a measure of the minimum
detectable signal. Also, the NEP can then be expressed as 
is the wavelength of incident light,
is the rms photocurrent,
to background radiation,
including the diode shunt resistance, the series resistance, and
is the Planck’s constant,is the velocity of light,
is the quantum effi-
is the photocurrent due
is the noise equivalent resistance
the load resistance. Fig. 7 shows the NEP as a function of
reverse bias for both samples under
It was found that the NEP was almost bias-independent in
the measurement bias range. It was also found that NEP was
and for SR_A and SR_B, respectively.
Notice that the NEP is inversely proportional to the detection
capability , . Thus, these values indicate that we can
achieve a lower noise level and a larger detection capability by
using an AlGaN/GaN/AlGaN/GaN quadruple layer.
III-nitrideIn summary,SRs with an AlGaN/GaN/
AlGaN/GaN quadruple layer were fabricated. It was found
that the reduced defect densities and improved crystal quality
could be achieved by incorporating the quadruple layer. It was
also found that we could achieve a lower leakage current, lower
on-state resistance, larger breakdown voltage, and lower noise
level by inserting the quadruple layer into the SRs. We also
show that our devices can be used as UV sensors or detectors.
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P. C. Chang received the B.S. degree from the
Department of Electronic Engineering, Chung Yuan
Christian University, Jhongli, Taiwan, in 1988, and
the M.S. degree from the Institute of Electronic
Engineering, National Taiwan University of Science
and Technology, Taipei, Taiwan, in 1993, and the
Ph.D. degree from the Institute of Microelectronics
and the Department of Electrical Engineering,
National Cheng Kung University, Tainan, Taiwan, in
Currently, he is an Associate Professor with the
Department of Electrical Engineering, Kun Shan University, Tainan. His re-
search concerns the fabrication of III-V-based optoelectrical devices.
K. H. Lee was born in Taipei, Taiwan, in 1983. He
received the B.S. and M.S. degrees in electrical en-
gineering from the National Cheng Kung University
NCKU), Tainan, Taiwan, in 2006 and 2007, respec-
tively. He is currently working towards the Ph.D. de-
gree in nitride-based compound semiconductor and
optoelectronic devices at the Institute of Microelec-
S. J. Chang (M’06) was born in Taipei, Taiwan, on
January 17, 1961. He received the B.S.E.E. degree
from the National Cheng Kung University (NCKU),
Tainan, Taiwan, in 1983, the M.S.E.E. degree from
the State University of New York, Stony Brook, in
1985, and the Ph.D. degree in electrical engineering
from the University of California, Los Angeles, in
He was a Research Scientist with NTT Basic
Research Laboratories, Musashino, Japan, from
1989 to 1992. He became an Associate Professor
with the Department of Engineeringn, NCKU, in 1992 and was promoted to
Full Professor in 1998. Currently, he also serves as the Director of the Institute
of Microelectronics, Deputy Director ofthe Center for Micro/Nano Science and
Technology, and Deputy Director of the Advanced Optoelectronic Technology
Center, 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 Research Center for Advanced Science and Technology, University of
Tokyo, Japan, from July 1999 to February 2000, a Visiting Scholar with the
Institute of Microstructural Science, National Research Council, Canada,
from August 2001 to September 2001, a Visiting Scholar with the Institute of
Physics, Stuttgart University, Germany, from August 2002 to September 2002,
and a Visiting Scholar with the Faculty of Engineering, Waseda University,
Japan, from July 2005 to September 2005. He is also an Honorary Professor
at the Changchun University of Science and Technology, China. His current
research interests include semiconductor physics, optoelectronic devices, and
Prof. Chang received Outstanding Research Awards from the National Sci-
ence Council, Taiwan, in 2004.
804 IEEE SENSORS JOURNAL, VOL. 10, NO. 4, APRIL 2010 Download full-text
Y. K. Su (F’07) was born in Kaohsiung, Taiwan,
on August 23, 1948. He received the B.S. and Ph.D.
degrees in electrical engineering from the 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&T Bell Lab-
oratories 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. Now, he is a Professor in the
Department of Electrical Engineering and Dean of Academic Affair at NCKU.
He is presently in charge of the Advanced Optoelectronic Technology Center
(AOTC), NCKU, and is also the President of Kun-Shan University, Tainan,
Taiwan. He has published over 300 papers in the area of thin-film materials
and devices and optoelectronic devices. His research activities have been in
compound semiconductors, integrated optics, and microwave 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 (NSC), R.O.C., during 1986–1995. He also received the
Best Teaching Professor Fellowship from the Ministry of Education, R.O.C., in
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). In 2001,
he was awarded as Chair Professor by NCKU for his distinguished academic
achievements. In 2007, he received the Award of Science Profession Medals by
NSC, Taiwan, and was elected as the IEEE Fellow for his outstanding contribu-
tion to optoelectronics and nanophotonics research and education.
T. C. Lin received the B.S. degree from the Depart-
ment of Materials Science, Feng Chia University,
Taichung, Taiwan, in 1985, and the M.S. degree and
the Ph.D. degree from the Department of Materials
Science and Engineering, National Cheng Kung
University, Tainan, Taiwan, in 1987 and 1997,
Currently, he is an Associate Professor with the
Department of Electrical Engineering, Kun Shan
University, Tainan. His research concerns the fab-
rication of transparent conductive oxides and their
S. L. Wu received the M.S. and Ph.D. degrees from
the Department of Electrical Engineering, National
Cheng Kung University, Tainan, Taiwan, in 1988,
Currently, he is a Professor and the Director with
the Department of Electronic Engineering, Cheng
Shiu University, Kaohsiung, Taiwan. His research
areas include the implementation of strained-SiGe,
strained-Si devices to enhance the CMOS perfor-
mance and noise characteristics in various devices