The electrical properties of Ni-based ohmic contacts to N-face p-type GaN were investigated. The specific contact resistance of N-face p-GaN exhibits a linear decrease from 1.01 cm 2 to 9 05 × 10 −3 cm 2 for the as-deposited and the annealed Ni/Au contacts, respectively, with increasing annealing temperature. However, the specific contact resistance could be decreased down to 1 03× 10 −4 cm 2 by means of surface treatment using an alcohol-based (NH 4 2 S solution. The depth profile data measured from the intensity of O1s peak in the X-ray photoemission spectra showed that the alcohol-based (NH 4 2 S treatment was effective in removing the surface oxide layer of GaN

Dong Min Lee

E F Schubert

Jae-Kwan Kim

Jae-Sik Yoon

Ji-Myon Lee

Jong-Kyu Kim

Jun Young Kim

Seung Cheol

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Copyright © 2013 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 13, 5715–5718, 2013
Ohmic Contacts to N-Face p-GaN Using Ni/Au for the
Fabrication of Polarization Inverted Light-Emitting Diodes
Seung Cheol Han1 †, Jae-Kwan Kim1, Jun Young Kim1, Dong Min Lee1,
Jae-Sik Yoon2∗, Jong-Kyu Kim3, E. F. Schubert4, and Ji-Myon Lee1∗
1Department of Materials Science and Metallurgical Engineering, Sunchon National University,
Sunchon, Jeonnam 540-742, Korea
2Division of Materials Science, Korea Basic Science Institute, Daejeon 305-333, Korea
3Department of Materials Science and Technology, POSTECH, Pohang 790-784, Korea
4Department for Electrical, Computer, and Systems Engineering, RPI, Troy, NY 12180, USA
The electrical properties of Ni-based ohmic contacts to N-face p-type GaN were investigated. The
specific contact resistance of N-face p-GaN exhibits a linear decrease from 1.01  cm2 to 905×
10−3  cm2 for the as-deposited and the annealed Ni/Au contacts, respectively, with increasing
annealing temperature. However, the specific contact resistance could be decreased down to 103×
10−4  cm2 by means of surface treatment using an alcohol-based (NH42S solution. The depth
profile data measured from the intensity of O1s peak in the X-ray photoemission spectra showed
that the alcohol-based (NH42S treatment was effective in removing the surface oxide layer of GaN.
Keywords: N-Face p-GaN, LED, Surface Treatment, XPS.
1. INTRODUCTION
The group III nitrides including GaN are attractive mate-
rials for optoelectronic devices because of the success in
commercialization of blue light emitting diodes (LEDs).1–5
Generally, nitrides exhibit high spontaneous or piezo-
electric polarization fields that are oriented along the
c direction. This large polarization filed cause spatial
separation of injected electrons and holes that results in
reduced radiative recombination efficiency. To overcome
these polarization effects in LEDs, devices grown on non-
polar or semipolar templates have been developled.
Furthermore, polarization inverted p-side down LEDs
were also realized and showed only 10% efficiency droop
up to 500 A/cm2 by using hydride vapor epitaxy (HVPE).6
However, fabrication of p-down LEDs were known to be
difficult due to vulnerability for dry-etch damage of p-type
GaN.
On the other hand, to obtain a low-resistance ohmic
contact to p-type GaN, a variety of surface treatments of
p-GaN using solutions such as aqua-regia solution,7 and
(NH42Sx,
89 have been proposed. It has been reported
that the surface treatment with a (NH42Sx solution is
especially efficient in removing the native oxide layer
∗Authors to whom correspondence should be addressed.
†Current address: Selcos Co. Suwon, 441-813, Korea.
on the p-type GaN surface, resulting in a low-resistance
ohmic contact to p-type GaN.9 In the area of GaAs
devices, several studies have reported that the alcohol-
based (NH42Sx solution removes the native oxide on the
GaAs surface more efficiently than the normal (NH42Sx
solution due to the low dielectric constant of the alcohol-
based (NH42Sx solution, resulting in a larger improvement
in device characteristics.1011
Zhilyaev et al.12 have reported that the photolumines-
cence intensity of n-type GaN is considerably enhanced
as a result of the surface treatment with an alcoholic sul-
fide solution. Our previous results13 also showed that, for
the case of n-type GaN, the surface treatment with an
alcohol-based (NH42Sx solution removes the insulating
layer on the n-type GaN surface more effectively than the
normal, (NH42Sx solution, leading to more enhanced elec-
trical properties. In this paper, we investigated the effect of
an alcohol-based (NH42Sx solution [t-C4H9OH1(NH42S]
on the characteristics of ohmic contacts to N -face p-type
GaN.
2. EXPERIMENTAL DETAILS
The p-GaN samples were grown on a c-plane sapphire
substrate by using a MOCVD system. After the growth of
the GaN buffer layer and subsequent undoped GaN layer,
J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 8 1533-4880/2013/13/5715/004 doi:10.1166/jnn.2013.7072 5715
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Ohmic Contacts to N -Face p-GaN Using Ni/Au for the Fabrication of Polarization Inverted Light-Emitting Diodes Han et al.
2 um-thick p-GaN:Mg was grown. The GaN layers were
rapid-thermal annealed at 650 C for 1 min in N2 ambient
to activate the Mg dopant. Hall measurements showed that
the carrier concentration was about 1×1017 cm3. Using a
KrF excimer laser (248 nm), a laser lift-off process was
then performed to separate the sapphire substrate from the
GaN layer, followed by HCl:DI (de-ionized) water (= 1:2)
cleaning for 5 min to remove Ga droplets. The undoped
GaN surface was then dry etched to a thickness of 1.7 um
for inductively coupled plasma (ICP). The samples were
first etched in HCl:H2O (1:1) solution for 1 min and then
dipped into alcoholic solution for 5 min at room tem-
perature, which was composed of 90 vol% tert-butanol
(t-C4H9OH) and 10 vol% (NH42S before metal deposi-
tion. After the sulfur treatment, conventional photo resist
mask was used for the fabrication of the circular transfer
length method (CTLM) patterns. The size of the pads was
200×200 um2 and the spacings between the pads were 10,
20, 40, 60, 80, and 100 um. The Ni (20 nm)/Au (80 nm)
films were then deposited by electron-beam evaporation.
Contact resistivity data were measured at room tempera-
ture using a parameter analyzer (HP4145B). To character-
ize the chemical bonding at the interface between the Ni
layer and sulfur-treated p-type GaN, XPS was performed
using the Mg Ka line (1253.6 eV) as an excitation source
in a ultra-high-vacuum system.
3. RESULTS AND DISCUSSION
Figure 1(a) shows the specific contact resistance of the
samples which had been deposited on N -face p-type GaN
as a function of annealing temperatures. Specific contact
resistances for the samples were determined from a plot of
the measured resistance versus the spacings between the
CTLM pads. The specific contact resistance was measured
to be 101× 100  cm2 for the untreated samples due to
the high Schottky barrier height of surface state. As the
0 50 100 150 200 250 300
10–4
10–3
10–2
10–1
100
101
C
on
ta
ct
 r
es
is
tiv
ity
 (
oh
m
 c
m
2 )
 
Annealing Temperature (ºC)
 N-face p-GaN
(a)
10–5
10–4
10–3
10–2
10–1
100
101
C
on
ta
ct
 r
es
is
tiv
ity
 (
oh
m
 c
m
2 )
 
(b)
As-deposited
Treatment
sulfur treated 300 ºC annealed
Fig. 1. Variation of contact resistivity for Ni (20 nm)/Au (80 nm) deposited on N -face p-type GaN as (a) a function of annealing temperatures and
(b) surface treatment.
annealing temperatures were increased to 300 C, the spe-
cific contact resistance was also decreased to the value
of 91×10−3  cm2. However, compared to the value of
reported N -face n-type GaN sample and for the purpose
of fabricating high efficiency LEDs, the specific contact
resistance should be decreased further. We have tried the
surface treatment method in order to quench the specific
contact resistance and results are shown in Figure 1(b).
By treating an alcohol-based (NH42S solution to N -face
p-type GaN, the specific contact resistance was decreased
by four and by two orders of magnitude for untreated and
300 C annealed sample, achieving 103×10−4  cm2 for
the alcohol-based (NH42S treated samples.
To study the mechanisms for the improvement of the
ohmic characteristics, XPS spectra were obtained from
the Ni/sulfur-treated N -face p-type GaN sample. Figure 2
shows the depth-profile data of Ga, Ni, O, C, and N
peak from the Ni/GaN for untreated and alcohol-based
(NH42S-treated samples, respectively.
As shown in Figure 2, the atomic percent of oxide for
untreated sample is about 7% at the interface of Ni and
GaN. However, atomic percent of O peak at the interface
of the samples treated with alcohol-based (NH42S solu-
tion were significantly decreased compared to that of the
untreated sample, indicating that the treatment effectively
remove the oxide layer at the interface.
It was known that the suitable surface treatment can
effectively decrease the Schottky barrier height (SBH).
According to the metal semiconductor band theory1415
the effective SBHs can be influenced by the presence
of native oxide at the Ni/p-GaN interface, which can be
described as,15
qb = qb0+4kT /h2m	1/2

where b0 is the SBH without native oxide. The second
term is associated with the presence of an oxide layer,
where m is the mean tunneling effective mass of the
5716 J. Nanosci. Nanotechnol. 13, 5715–5718, 2013
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Han et al. Ohmic Contacts to N -Face p-GaN Using Ni/Au for the Fabrication of Polarization Inverted Light-Emitting Diodes
Fig. 2. XPS depth profiles of (a) untreated and (b) an alcohol-based
(NH42S-treated N -face p-type GaN.
carriers, 	 the mean tunneling barrier for carrier injec-
tion form a metal to a semiconductor, and 
 the oxide
layer thickness. This indicates that the effective removal
of native oxide due to the surface treatment (Fig. 2) could
make a contribution to the observed reduction of the SBHs
due to the decreased 
. This result shows that the oxide
layer was efficiently removed from the N -face p-GaN sur-
face by the surface treatment with alcohol-based (NH42S
solution. In other words, increased barrier height for hole
injection from metal to p-type GaN by formation of oxide
layer could be reduced by treatment with the alcohol-based
(NH42S solution. Hence, the improved ohmic character-
istics for samples can be attributed to the removal of the
interfacial oxide layer.
Figure 3 shows the Ga2p peak in the Ni/GaN interface
regions of the samples before and after the treatment. It is
shown that the Ga2p core level of the treated sample shifts
toward the lower binding energy side by 0.3 eV, compared
to that of the untreated sample.
The position of surface Fermi level can be determined
from the energy position of the Ga N peaks in the core-
level spectra. The difference between the Ga2p core-level
binding energy and valence band maximum in bulk GaN
is 1116.7 eV. Therefore, the following equation is used to
determine the position of surface Fermi level:16
EF = EGa2p−11167 eV
(a)
Ga 2p
1112 1116 1120 1124
P
ho
to
em
is
si
on
 in
te
ns
ity
 (
ar
b.
U
ni
ts
)
(b)
Binding energy (eV)
Fig. 3. Ga2p core-level spectra of the Ni/N -face p-type GaN interface
for (a) an untreated sample and (b) an alcohol-based (NH42S-treated
sample.
where EF is surface Fermi level position relative to
the valence band maximum and EGa2p core-level binding
energy for Ga N bonds.
This implies that the alcohol-based (NH42S-treated
samples causes the surface Fermi level to shift toward the
valence-band edge, resulting in a reduction in the band
bending in the p-GaN. A previous study17 reported that
sulfur does not bond with nitrogen but bonds with gallium
or occupies nitrogen-related vacancies.
The S ions in the solution would then bond to the Ga
atoms on the N -face GaN sub-surface, producing Ga sul-
fides and this sulfides are soluble in the sulfur solution.18
Hence, the simultaneous process of reaction and dissolu-
tion can leave a very thin sulfide layer on the sulfur-treated
GaN surfaces, leading to the effective removal of surface
oxide, formation of Ga vacancies, and protection from the
formation of the native oxide on exposure to air prior to
metal deposition.
Figure 4 shows the schematic energy band diagram
of Ni/p-GaN ohmic contact to N -face p-type GaN. As
the binding energy of Ga2p peak of surface treated sam-
ple shifted to valence-band, the valence-band maxima of
the GaN also shifted to the Fermi level, resulting in the
decrease of band bending between metal and GaN inter-
face. According to this, the Schottky barrier height should
be decreased as follows;
qp = ETGa2p− ENTGa2p−EV 
where qp is SBH between Ni and p-GaN, and E
T
Ga2p,
ENTGa2P, and Ev is a binding energy of core-level spectrum
J. Nanosci. Nanotechnol. 13, 5715–5718, 2013 5717
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Ohmic Contacts to N -Face p-GaN Using Ni/Au for the Fabrication of Polarization Inverted Light-Emitting Diodes Han et al.
Fig. 4. Schematic energy band diagram of Ni/p-GaN ohmic contact.
for surface treated and non-treated sample, and valence
band maxima, respectively. From these results, the reduced
band-bending renders the decrease of effective SBH result-
ing in the enhanced transfer of holes between metal and
valence band of GaN, indicating that the alcohol based
(NH42S treatment effectively enhance the ohmic contact
properties of Ni/N -face p-type GaN.
4. CONCLUSION
We report on results about electrical properties of Ni-
based ohmic contact on N -face p-type GaN. Contact resis-
tance are decreased as the annealing temperature was
increased. In order to lower a contact resistance further,
alcohol-based (NH42S surface treatment on N -face p-type
GaN were conducted. Compared to the untreated sample,
the specific contact resistance was drastically decreased
by four orders of magnitude. The oxygen depth-profiling
data showed that the alcohol-based (NH42S treatment is
very effective in the removal of the surface oxide layer.
The Ga2p core-level peak for the alcohol-based (NH42S-
treated sample showed a red-shift by 0.3 eV toward the
valence-band edge compared to that for the untreated sam-
ple. The drastic improvement in the ohmic characteris-
tics of the alcohol-based (NH42S-treated sample can be
attributed to the effective removal of the surface oxide and
the shift of the surface Fermi level toward the valence-
band edge, resulting in the lowering of SBH.
Acknowledgment: This work was supported by
the Ministry of Knowledge Economy (MKE) through the
project of Regional Innovation Center (RIC) and by the
Korea Basic Science Institute (KBSI) grant (T32604).
References and Notes
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Received: 23 March 2012. Accepted: 4 May 2012.
5718 J. Nanosci. Nanotechnol. 13, 5715–5718, 2013


Ohmic Contacts to N-Face p-GaN Using Ni/Au for the Fabrication of Polarization Inverted Light-Emitting Diodes

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Ohmic Contacts to N-Face p-GaN Using Ni/Au for the Fabrication of Polarization Inverted Light-Emitting Diodes

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