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Nano Res
1
Graphene-GaN Schottky diodes
Seongjun Kim1, Tae Hoon Seo2, Eun-Kyung Suh1, Keun Man Song3, Myung Jong Kim2, and Hyunsoo Kim1()
Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-014-0624-7
http://www.thenanoresearch.com on November 4, 2014
© Tsinghua University Press 2014
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Nano Research
DOI 10.1007/s12274-014-0624-7
1
Graphene-GaN Schottky
TABLE OF CONTENTS (TOC)
Graphene-GaN Schottky diodes
Seongjun Kim1, Tae Hoon Seo2, Eun-Kyung Suh1, Keun
Man Song3, Myung Jong Kim2 and Hyunsoo Kim1,*
1. Chonbuk National University, Republic of Korea
2. Korea Institute of Science and Technology, Republic of
Korea
3. Korea Advanced Nano Fab Center, Republic of Korea
The electrical characteristics of graphene-GaN Schottky diodes with
excellent rectifying behavior are presented, i.e., the Schottky barrier
height of 0.90 eV and 1.240.13 eV as determined by thermionic
emission and barrier inhomogeneity model.
Provide the authors’ website if possible.
Author 1, http://psdl.jbnu.ac.kr/
2
Graphene-GaN Schottky diodes
Seongjun Kim1, Tae Hoon Seo2, Eun-Kyung Suh1, Keun Man Song3, Myung Jong Kim2, and Hyunsoo Kim1()
1 School of Semiconductor and Chemical Engineering, Semiconductor Physics Research Center, Chonbuk National University, Jeonju
561-756, Republic of Korea 2 Soft Innovative Materials Research Center, Korea Institute of Science and Technology, Jeonbuk 561-905, Republic of Korea 3 Korea Advanced Nano Fab Center, Suwon 443-700, Republic of Korea
Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher)
© Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011
ABSTRACT The electrical characteristics of graphene Schottky contacts formed on undoped GaN semiconductor were
investigated. Excellent rectifying behavior with a rectification ratio of ~107 at 2 V and a low reverse leakage
current of 1.010−8 A/cm2 at −5 V were observed. The Schottky barrier height, as determined by the thermionic
emission model, Richardson plots, and barrier inhomogeneity model, were 0.90, 0.72, and 1.240.13 eV,
respectively. Despite the predicted low barrier height of ~0.4 eV at graphene-GaN interface, the formation of
excellent rectifying characteristics with much larger barrier height is attributed to the presence of a large number
of surface states (1.21013 states/cm2/eV) and the internal spontaneous polarization field of GaN, resulted in a
significant upward surface band bending or a bare surface barrier height as high as of 2.9 eV. Using the S
parameter of 0.48 (measured from the work function dependence of Schottky barrier height) and the mean barrier
height of 1.24 eV, the work function of graphene in Au/graphene/GaN stack could be approximately estimated
to be as low as 3.5 eV. The obtained results indicate that graphene is a promising candidate for use as a Schottky
rectifier in the GaN semiconductors with n-type conductivity.
KEYWORDS Graphene, GaN, Schottky diode, Schottky barrier height, Fermi level pinning
Introduction
Two-dimensional hexagonal carbon array, also known
as graphene, has been extensively studied for
application as an active or passive layer in electronic
and optoelectronic semiconductor devices due to its
outstanding physical and optical characteristics,
including high intrinsic electron mobility, quantum
electronic transport, low optical absorption, and good
chemical and mechanical stability [1−14]. In particular,
Nano Res DOI (automatically inserted by the publisher)
Review Article/Research Article Research Article
Address correspondence to Hyunsoo Kim, [email protected]
3
good electrical conductivity and excellent optical
transparency across the entire spectrum of
wavelengths of graphene made it a promising
candidate for use as a transparent contact in
optoelectronic devices such as solar cells [15−18] and
light-emitting diodes (LEDs) [19−27].
Very recently, a number of studies focusing on the
application of graphene p-contacts in GaN-based
LEDs have been reported by several groups [22−27].
Jo et al. [22] demonstrated the first GaN-based LEDs
fabricated with transparent graphene p-contact.
However, the forward voltage obtained was very
large in the study, which was due to a poor graphene
Ohmic contact to p-GaN associated with the large
work-function mismatch between graphene (G=4.5
[28−30]4.6 eV [31−33]) and the p-GaN (~7.5 eV),
namely, the expected Schottky barrier height (B) was
as high as of ~3.0 eV according to the Schottky-Mott
theory. Accordingly, Chandramohan et al. [23] showed
that the specific contact resistance of graphene contact
formed on heavily Mg-doped p-GaN was as large as
~101 Ωcm2. To overcome this poor p-Ohmic contact, a
hybrid-type contact combining graphene with
indium-tin-oxide islands [24], Au nanoparticles [25],
Ag nanowires [26], and Ag nanocluster [27] were
introduced by our groups.
On the one hand, thermally stable rectifying
behavior was also observed for the graphene contact
formed on n-type GaN by Tongay et al. [33, 34],
suggesting that the graphene on n-GaN be a
promising candidate for the Schottky rectifiers.
Indeed, this finding is strange considering that the B
expected to form at graphene/n-GaN interface is as
low as ~0.4 eV due to the small work-function
difference between graphene and n-GaN, implying
that the graphene contact to n-GaN should produce
poor rectifying behavior. Furthermore, the
experimentally obtained B of 0.74 eV [33, 34] was
even larger than the predicted value. These
inconsistent results indicate that, for the successful
development of graphene integrated GaN-based
semiconductor devices, the electrical characteristics
and carrier transport mechanism of graphene-GaN
contact should be thoroughly investigated. However,
in-depth comprehensive studies on this subject are
still lacking.
In this study, we investigated the electrical
characteristics and carrier transport mechanism of the
graphene contact formed on undoped GaN. For this
purpose, first, we focused on the fabrication of reliable
Schottky diodes by transferring the synthesized
graphene onto the GaN wafer, followed by oxygen
plasma etching to define accurate active region.
Indeed, it is worth noting that, according to the
previous studies [34−36], the active region of the
graphene Schottky contact was too large (e.g.,
10002000 m2) [34] to extract reasonable Schottky
parameters, since current crowding can occur as a
result of the limited conductivity of graphene. More
importantly, the active region of previous studies
could not be clearly defined due to the use of a
window-frame structure, where the transferred
graphene would be lying on the bottom GaN surface
and the top side of window-frame structure
simultaneously, i.e., the boundary of active region
cannot be clearly defined. Meanwhile, our fabrication
method was developed to solve these problems. In
addition, previously [33, 34], the fabricated Schottky
diodes were solely analyzed by the thermionic
emission (TE) model. However, the obtained ideality
factor (n) of 2.9 was much larger than the unity,
indicating that the obtained B of 0.74 eV might be
incorrect due to the use of inadequate conduction
model. In this regard, we attempted to analyze the
Schottky diodes by using TE, activation energy plots,
and barrier inhomogeneity model based on the
measured current-voltage-temperature (I-V-T) data.
Experimental
For this study, 4.5-μm-thick undoped GaN wafers,
which were grown on sapphire substrates by metal-
organic chemical vapor deposition system (MOCVD),
were used. The Hall-effect measurements of GaN
wafer showed an electron concentration (N) of
3.11016 cm-3 and a Hall mobility () of 297 cm2/V-s.
For the fabrication of Schottky diodes, first, the
graphene which was formed on 25-μm-thick Cu foil
by CVD method was transferred to the GaN wafers by
using polymethyl methacrylate (PMMA) sacrificial
4
layer. Note that, prior to the graphene transfer, the
surface of GaN wafers were cleaned with acetone,
isopropyl alcohol, H2SO4:HCl (1:1) solutions, a
buffered oxide etchant, and deionized water. The
detailed synthesis of graphene by CVD method and
transfer techniques can be found elsewhere [24−27, 36]
(also shown in Fig. S1 in the Electronic
Supplementary Materials, ESM). The surface
morphology, physical and optical properties of
graphene transferred to GaN wafers were measured
using an atomic force microscope (AFM), Raman
spectroscopy (excited with a 514 nm-line of an Ar ion
laser), X-ray photoelectron spectroscopy, and UV/VIS
spectrometer.
The Schottky diodes were fabricated by using the
conventional photographic technique and oxygen
plasma etching, as illustrated in Fig. 1. For example,
first, using an e-beam evaporator, a ~10-nm-thick Au
layer was deposited on the graphene for enhanced
current spreading. The circular pattern of the
Schottky contact with a diameter of 100 μm was
defined by conventional photolithographic technique,
followed by wet etching (HCl:HNO3=3:1) for 10 sec to
remove the Au layer, and then oxygen plasma etching
in an inductively-coupled plasma reactive ion etching
system to remove the graphene. Ti/Au (50 nm/50 nm)
Ohmic contacts were then deposited on the exposed
GaN layer. Finally, a Cr/Au (30 nm/250 nm) layer was
deposited on top of the Schottky region for the
formation of a probing contact pad having the
diameter of 50 m (see the inset of Fig. 3). The
Schottky diodes were evaluated using a parameter
analyzer (HP4156A) and a variable temperature probe
station in darkroom conditions. Note that the probe
station was equipped with a vacuum chamber (<
2102 Torr) to suppress the undesirable reaction of
graphene with the ambient atmosphere.
Results and discussion
Figure 2 show the Raman spectra of graphene
transferred to GaN/sapphire substrates. Typical G
band and 2D band peaks of graphene were observed
at the wave number of 1586 and 2682 cm-1, which are
associated with the LO and TO modes of the graphite
honeycomb lattice vibration generating at the Γ point
of the Brillouin zone and two inelastic electron-
phonon scattering events involving opposite
momenta, respectively [37−39]. At around 1350 cm-1,
D band peak was also observed, indicating that the
elastic electron-phonon scattering caused by defects,
wrinkles, and disorders in the graphitic material or
edges of graphene occurred. Consistently, as shown in
the inset of Fig. 2(a), the AFM surface images revealed
the presence of a few wrinkles and white spots. It is
known that the wrinkles form at the atomistic defect
lines (e.g., step edges of Cu terraces) of Cu foils during
growth of graphene by CVD [40]. In addition, the
white spots may originate from the residual chemical
contaminants such as PMMA. Due to these possible
defects or disorder of graphene, D band peak
appeared. However, it is worth noting that, except
these defects, the graphene is quite flat and uniformly
transferred on GaN surface
To further investigate the chemical components,
XPS measurement was also performed for the bare
GaN and graphene-GaN samples, as shown in Fig.
2(b). The survey XPS spectra showed that the
chemical components and their relative peak
intensities of both samples are nearly the same except
C 1s peak, i.e., the C 1s intensity of bare GaN was
negligibly small, while that of graphene-GaN sample
was large. Figure 2(c) shows the C 1s peak of both
samples, where the deconvolution of C 1s exhibited
the presence of C-C, C-O, and C=O bonds. Besides C-
C bond for the graphene, the presence of C-O and
C=O bonds is attributed to PMMA residue [41].
Indeed, the PMMA contaminants may hinder the
formation of sound graphene-GaN junction, as this
will also relate to the electrical characteristics..
Figure 1 Fabrication procedure of graphene-GaN Schottky
diodes.
5
In the Raman spectra, note that the full width at half
maximum (FWHM) of 2D peak was 30.8 cm-1,
indicating that the graphene is a mono layer [42]. In
addition, 2D/G peak intensity ratio was 1.65, which is
also indicative of mono-layer graphene because it
exceeds 1.0 [25]. The evidence of mono-layer
graphene was further confirmed by the excellent
optical transmittance (see Fig. S2 in the ESM). For
example, at the wavelength of 350600 nm, the
integrated transmittance of graphene transferred on
GaN sample relative to bare GaN was as high as 97 %,
which is consistent with the fact that the mono-layer
graphene has 2.3 % optical absorption [3].
Figure 3(a) shows the semi-logarithmic I-V
characteristics of a graphene-GaN Schottky diode at a
measurement temperature of 300 K (under dark
condition); a top-view optical microscopy image of
the diodes is provided in the inset. Excellent rectifying
behavior with a rectification ratio as high as 1.1107 at
2.0 V was observed. In addition, the reverse leakage
current measured at 5 V was as low as 1.0108 A/cm2,
which is even lower than the previous result [34].
Such findings indicate that the graphene/GaN
junction was soundly constructed.
The forward I-V curves should be analyzed using
the appropriate conduction model, which can be
determined depending on the N value of GaN and
temperature (T), i.e., E00/kT<0.5 for TE, 0.5<E00/kT<5 for
thermionic field emission, and E00/kT>5 for field
emission [43, 44]. Here, k is the Boltzmann constant
and E00 is the tunneling parameter, which is given by
E00=qh/4(N/m*s)1/2, where q is the electronic charge, h
is the Planck constant, m* is the effective electron mass,
and εs is the dielectric constant of GaN. The
calculation for our sample showed that E00/kT=0.09,
indicating that the TE model is appropriate to use.
Under the condition that V 3kT/q, the general diode
equation by TE is given by [45]:
1exp0
nkT
qVII (1)
where I0 is the saturation current. Here, I0 is given by:
kT
qTAAI Bexp2**
0 (2)
where A is the Schottky contact area and A** is the
Richardson constant (26.4 A/cm2K2) [46]. Over a
voltage range of 0.10.5 V, the theoretical fits of the
forward I-V curves using Eqs. (1) and (2) revealed that
I0=9.81014 A, n=1.32, and B=0.90 eV. It is noted that
the obtained n value is much smaller than the
previously reported value of 2.4, indicating that the
Figure 2 (a) Raman spectra of graphene transferred to
GaN/sapphire substrates. The inset shows 20×20 μm2 AFM image
of graphene. (b) Survey XPS spectra and (b) C1s peaks of bare
GaN and graphene-GaN samples.
6
Schottky parameters obtained by the I-V method are
much more accurate and reliable than those in
previous works [33, 34].
However, the obtained n value is larger than the
unity, which may be attributed to GaN surface states
originating from native crystal defects [47, 48],
interfacial layers [49, 50], barrier inhomogeneities [43,
44], residual chemical contaminants [34], and to the
effect of series resistances (Rs) [51]. Indeed, the Rs has
a significant influence on the I-V curve in the high
voltage range exceeding 0.6 V, revealing a saturated
behavior. According to the Norde plots as shown in
the Fig. S3 of the ESM, the Rs was estimated to be as
high as 7600 K, while the reference Schottky diode
fabricated with thick Ni contact exhibited the Rs of
26.1 K. Despite the use of the same wafers and
process except Schottky contact, the extremely large
Rs value of graphene-GaN diode is attributed to the
low electrical conductivity of graphene, as will be
further discussed in details (with and without
illumination). The obtained n value of 1.32 is, however,
thought to be unaffected by the Rs since it was
estimated in the low voltage range of 0.10.5 V (note
that the forward I-V curve showed a very evident
conduction regimes below and above the critical
forward voltage of ~0.6 V).
Instead, other origins associated with the GaN
surface states are thought to induce the large n value.
Specifically, the interface quality of graphene-GaN
could be analyzed using the voltage dependence of n
value, i.e.,
kT
qV
JLogd
dV
kT
qn
exp1
(3)
where V' is the effective applied voltage, i.e., V'=VIRs
(Fig. 3b). In Fig. 3b, note that the n value has a bell-
shaped behavior with its maximum at around 0.08 V,
which is indicative of the presence of surface states, as
reported in the GaAs [52], InP [53], and GaN [54].
Assuming that these surface states are in equilibrium
with the GaN, the surface states density (Ds) can be
estimated according to
Wqn
qtD s
i
is 22
)1(
(4)
where i and ti are the permittivity and thickness of
the interfacial layer and W is the depletion width.
Here, i and ti , which are unknown parameters, were
assumed to be the permittivity of free space and 5 Å ,
so as to compare our obtained Ds value with those of
Cowley and Sze [55, 56], Schmitz et al. [57], and
Arulkumaran et al [58]. Using Eq. (4) and the n(V)
Figure 3 (a) The I-V characteristics of a graphene Schottky diode
fomed on GaN; a top-view optical microscopy image of the
fabricated diodes is shown in the inset. (b) n versus V, and (c) Ds
vs. EC−E plots.
7
values, the bias voltage dependence of Ds, i.e., Ds(V)
could be obtained according to the relation of
ECE=BqV', as shown in Fig. 3c. Note that the Ds(V)
values are in the range of ~1013 states/cm2/eV, which
are consistent with the reported values (~1-21013
states/cm2/eV) for the conventional contacts formed
on GaN [57, 58]. Particularly, the Ds(V) showed a bell-
shaped behavior with its maximum at around ~0.8 eV,
indicating that the deep-level states locates 0.8 eV
below the conduction band (EC) edge. Indeed, this
position is slightly larger than the commonly
observed level of EC0.6 eV associated with nitrogen
antisites (NGa) [54], while it corresponds to the level of
0.73 and 0.89 eV observed in n-GaN grown on SiC [59].
Meanwhile, as suggested in the literature [54], the
deep-level states might originate from the interfacial
oxide generated during the process step, which
should be further investigated. Based on our findings,
the n value larger than unity is primarily due to the
deep-level states of GaN associated with native
defects. As observed in the XPS spectra, the PMMA
contaminant might be also responsible for the large n
value. For example, Pirkle et al. [41] showed that the
effective removal of PMMA residue could increase 2
times higher average mobility of graphene-based field
effect transistors. The influence of PMMA
contaminant on the n value, however, seemed to be
small in the study considering that the n value of
reference Schottky diode fabricated with Ni contact
(no PMMA contaminant) was 1.22 (Fig. S4 of the ESM).
Note that for the measurements of B using the I-V
method, the Richardson constant A** was assumed to
be 26.4 A/cm2K2, which is a theoretical value.
However, the uncertainty in A** may cause errors in
the estimation of B. Therefore, to obtain
experimental values of both A** and B, activation
energy plots were generated using I0 values measured
at various temperatures between 300400 K, as shown
in Fig. 4. The lower inset shows the typical forward I-
V curves of Schottky diodes obtained at 300, 340, and
380 K. The theoretical fits of ln(I0/AT2) vs 1000/T data
using a rewritten form of Eqs. (1) and (2), i.e., so-called
Richardson plots having the expression [45]:
kT
qA
AT
I B
**
2
0 lnln (5)
yielded a ΦB value of 0.72 eV and an experimental A**
value of 1.36102 A/cm2K2. Note that the ΦB value
obtained by the Richardson plots is slightly lower
than the ΦB obtained by the I-V method, but the
experimental A** is much lower than the theoretical
value. Indeed, a large discrepancy between the
experimental and theoretical A** values has frequently
been observed for Schottky contacts in GaN systems
[55, 60]. Such differences are attributed to abnormal
carrier transport at the contact/GaN interface.
Furthermore, the ΦB value obtained by the I-V method
increased with temperature (see the inset of Fig. 4),
which is in contrast to the implicit assumption of the
Richardson plot method that the barrier height should
be independent of temperature [45]. Consequently,
the large n and small A** that were obtained and the
temperature-dependent ΦB value suggest that a more
appropriate model should be used to explain the
carrier transport at the graphene-GaN interface.
Recent studies have shown that one of the most
reasonable ways to explain carrier transport in a
contact-GaN system is to use the barrier
inhomogeneity [43, 44] and/or a thermionic field
emission model [49, 61, 62]. In our studies, the barrier
inhomogeneity model was applied, since a positive
temperature coefficient of ΦB is a distinctive feature of
the presence of inhomogeneous barriers [43, 44, 62,
63], i.e., carriers can flow through the local shallow
Figure 4 Activation energy plots of the Schottky diodes obtained
over a temperature range of 300−400 K. The upper inset shows
the ΦB values vs. temperature, and the lower inset shows the
typical forward I-V curves obtained at 300, 340, and 380 K.
8
barrier at relatively low temperatures (due to the low
thermal energy of carriers), resulting in a reduced
mean barrier height and vice versa. In this model, the
barrier height can be assumed to have a Gaussian
distribution with a mean barrier height (ΦB,m) and a
standard deviation (σ), i.e., [43, 44]:
kT
qTT
2)0()(
2
mB,B
. (6)
From a linear fit of the ΦB(T) data using Eq. (6), the
value of ΦB,m at 0 K could be estimated to be 1.24 eV
with σ=0.13 eV, as shown in Fig. 5. It is noted that the
value of σ is in agreement with the value obtained for
a conventional metal contact formed on GaN [62, 63].
Consequently, the resulting schematic band diagram
of graphene contact to GaN can be drawn as shown in
the inset of Fig. 5.
According to the barrier inhomogeneity model, the
voltage dependence of ΦB can be also estimated based
on the temperature dependence of n value, namely [54,
64, 65],
baB
kT
q
nV
21
1
, (7)
where a and b are the voltage coefficients that are
used in the relations of B,m= B,m0 + aV and σ2=
σ20+bV (here, the subscript “0” denotes a zero bias).
The linear regression fit of the experimental data
using Eq. (7) yielded that a =0.58 and b = 17 mV. The
positive signs of a and b indicate that the mean
barrier height B,m and the standard deviation σ
increase with increasing bias voltages.
Considering that the large fluctuation of the barrier
height (or large value) was reported to be largely
due to the GaN surface states associated with crystal
defects in studies, the carrier transport at the
graphene-GaN interface is also expected to be
influenced by the surface states of GaN. Accordingly,
the ΦB,m of 1.24 eV obtained by using the barrier
inhomogeneity model was found to be much larger
than the ΦB of 0.90 eV obtained by using the I-V
method. This is due to the fact that the ΦB obtained
using the I-V method mainly reflects the lowest
barrier height of local shallow patches, while ΦB,m
reflects the average value of the fluctuating barriers as
shown in the inset of Fig. 5. To verify the obtained ΦB,m
value, we performed capacitancevoltage (C-V)
measurements of the diodes, since the ΦB,m value is
equivalent to the ΦB value obtained using the C-V
method (due to the fact that the lateral potential
fluctuations of the barrier height do not influence the
C-V characteristics) [66]. However, we could not
obtain reliable C-V data for graphene-GaN diodes. In
keeping with this, Tongay et al. [33] also showed that
reliable C-V measurements for graphene-GaN and
graphene-SiC were not available due to the high Rs
values of wide-band-gap semiconductors, while the
C-V measurements gave reasonable information for
graphene-GaAs and graphene-Si. This indicates that,
consistently, the large Rs value of our diodes impede
the reliable measurement of C-V data. This requires
further study.
According to the Schottky-Mott theory [45], i.e.,
B,m=GGaN, where ΦB,m=1.24 eV and GaN is the
electron affinity of GaN (4.1 eV), G is obtained to be
5.34 eV. This estimated G is much larger than the
reported value of 4.5 [28−30] −4.6 eV [31−33]. One
possible explanation for such a discrepancy is the
tuning of G value caused by the doping effect
associated with charge transfer (due to the presence
of an interface dipole layer) between graphene and
metal [28−30] and/or metal contact induced charge
inhomogeneity [67−69]. Song et al. [30] summarized
the practical G value under various metals based on
the C-V analysis of metal-graphene-oxide-
semiconductor capacitors, e.g., G under Pd and Au
was ~4.62 eV, G under Cr/Au bilayer was ~4.3 eV, and Figure 5 Plots of ΦB vs. 1000/T. The inset shows the schematic
band diagram of graphene-GaN interfaces with inhomogeneous
Schottky barriers.
9
G under Ni was ~5.0 eV. Note that, although the G
was shown to change significantly depending on the
contact metals, there are no metals reaching our
estimated G value of 5.34 eV. In addition, for the
graphene under Au, the G difference between the
estimated value (5.34 eV) and the reported one (4.62
eV) is as high as 0.72 eV, indicating that this
explanation in terms of G tuning is somewhat
limited.
Another possible explanation is the pinning of the
surface Fermi level (EF) of GaN that is associated with
surface states, namely, Bardeen model. Indeed, this
model becomes more practical in GaN
semiconductors due to the presence of strong
spontaneous polarization field (Psp) built from the Ga-
terminated surface (Ga-polarity surface) toward the
bulk direction [70−72] as shown in the upper inset of
Fig. 6. For example, under the presence of Psp, the
electron carriers are readily transported to the Ga-face
side and are subsequently trapped at the surface
states, i.e., the surfaces are negatively charged,
inducing a compensating positive charge near the
GaN surface (or the formation of induced electric
dipoles) and hence causing upward surface bending.
Specifically, the degree of surface band bending or the
bare surface barrier height (s) of GaN can be
estimated according to the following equation [56, 73]:
S
qSqq GB
s
1
)( (8)
where S is the S parameter, which is defined as S=
d(B)/d(M). To obtain the S parameter of our samples,
we fabricated Schottky diodes with different work
functions of metals (M) including Pt, Ni, and Cr, and
obtained their B,m values using the barrier
inhomogeneity model as shown in the lower inset of
Fig. 6.
Figure 6 shows the B,m plotted as a function of M,
yielding the S parameter of 0.48. The fact that the S
parameter of our sample is slightly higher than those
reported in the literature (0.385 [57], 0.41 [58]) is
attributed to the better epitaxial quality of GaN film.
Consequently, the bare surface barrier height s could
be estimated to be as high as 2.9 eV, indicating that the
large ΦB,m of graphene is essentially associated with
large upward surface band bending or surface Fermi-
level pinning. Indeed, the Ds could be also estimated
using the obtained S parameter and the Cowley-Sze
model [55], i.e.,
i
is
tSq
SD
2
)1(
,
(9)
yielding that Ds = 1.21013 states/cm2/eV. Note that the
obtained Ds value using S parameter is in good
agreement with reported values (1.6−1.81013
states/cm2/eV) [57, 58] and partly the values obtained
using Eq. (4). Therefore, the large Ds value and the
presence of Psp is expected to pin the surface EF,
causing a deviation from the ideal Schottky-Mott
theory, a significant upward surface band bending
and hence the eventual formation of large barrier
height.
Interestingly, using the B,m vs. M relations with
S=0.48 and ΦB,m=1.24 eV for the graphene-GaN contact,
the G value in the Au/graphene/GaN system could
be inversely estimated to be 3.5 eV (see the
extrapolated line of Fig. 6). This finding is quite
interesting, since we can roughly obtain the G value
in the complicated stack of (Au/Cr)/Au/graphene
/GaN system. However, the obtained G value is
likely to be somewhat under-estimated. This is first
attributed to the limited accuracy of S parameter. For
example, for obtaining more reliable and accurate S
parameter, various Schottky diodes having different
work function of metals, particularly having low
Figure 6 ΦB,m vs. ΦM plots. The upper and lower insets show the
schematic band diagram of bare GaN surface and the ΦB-T
diagram for the Pt, Ni, and Cr Schottky diodes, respectively.
10
work function in the range of 3.54.5 eV, had to be
evaluated. However, we just evaluated three types of
diodes with the large work function in the range of
4.55.6 eV due to the absence of appropriate contact
metals (i.e., due to the near Ohmic behavior by low
work-function contact). Second, the doping effect
associated with the charge transfer at each junction of
GaN-graphene, Au-graphene, and/or Au/Cr/Au-
graphene may tune the G. This is presently under
investigation.
Lastly, it will be instructive to reconsider the
anomalously large Rs value of graphene-GaN diodes.
Figure 7 shows the semi-logarithmic I-V curves of a
graphene-GaN Schottky diode at 300 K, taken with
and without illumination. The illumination source
was the conventional fluorescent lamp (PHILIPS
LIFEMAX TLD 32W/365) placed on the ceiling of the
equipment, i.e., the illumination intensity was
relatively weak and the emission spectrum was broad
(Fig. S5 of the ESM). Note that, despite the weak
illumination intensity, the photocurrent (at reverse
bias voltage) and the open circuit voltage were
observed with illumination, indicating
photoexcitation of carriers and generation of
photocurrent. Indeed, this finding is nearly the same
as that of graphene-Si diodes under reverse bias
condition [35]. However, in our case, the illumination
energy (h, =frequency) was lower than the Eg of
GaN (3.4 eV), i.e., B < h < Eg. This indicates that the
origin of photocurrent is the electrons excited from
the graphene over the barrier into the GaN.
Distinctive feature of our finding, which is different
from the graphene-Si diodes [35], is much more
pronounced photocurrent under forward bias
condition (> 0.6 V), as shown in the inset. Accordingly,
the Norde plot showed a significantly reduced Rs
value of 140 K with illumination (see Fig. S3 of the
ESM). This finding indicates that, first, the
anomalously large Rs value (7600 K) of graphene-
GaN Schottky diodes is primarily associated with the
electrical conductivity of graphene. Second, the
noticeable photocurrent generated under both reverse
and forward bias condition in spite of very weak
illumination intensity provides the new possibility of
graphene-GaN Schottky diodes to use as a functional
photodetectors. Indeed, this is presently under
investigation.
Conclusions
To summarize, the electrical characteristics and
carrier transport mechanism of graphene Schottky
contacts formed on undoped GaN were analyzed.
Unlike the expected poor Schottky contact, the
fabricated Schottky diodes exhibited excellent
rectifying behavior with a rectification ratio of ~107 at
2 V and a low reverse leakage current of 1.010−8
A/cm2 at −5 V. Based on the I-V analysis with TE
conduction model, the B of 0.90 and n of 1.32 could
be obtained. More reasonably, the barrier
inhomogeneity model applied to the I-V-T data
yielded the mean barrier height B,m of 1.24 eV with
the standard deviation of 0.13 eV. The formation of
excellent graphene-GaN Schottky contact is attributed
to the presence of a large number of surface states of
GaN (1.21013 states/cm2/eV) and the internal Psp,
causing a significant upward surface band bending
up to 2.9 eV and hence the formation of large B at the
GaN/graphene interface. Using the obtained barrier
height and S parameter of 0.48, the work function of
graphene G in Au/graphene/GaN stack could be
approximately estimated to be as low as 3.5 eV. Such
a result indicates that graphene is a potential
candidate for use as a Schottky rectifier.
Acknowledgements
Figure 7 The I-V characteristics of graphene-GaN diodes with and
without illumination. The inset shows linear I-V plot.
11
This study was supported in part by a Priority
Research Center Program through the National
Research Foundation of Korea, funded by the
Ministry of Education, Science and Technology of the
Korean government (2011-0027956) and in part by the
Ministry of Education and National Research
Foundation of Korea(NRF) through the Human
Resource Training Project for Regional Innovation
(2013H1B8A2032197).
Electronic Supplementary Material: Supplementary
material (The detailed synthesis of graphene by CVD
method and transfer techniques, and optical
transmittance result) is available in the online version
of this article at http://dx.doi.org/10.1007/s12274-***-
****-*
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14
Electronic Supplementary Material
Graphene-GaN Schottky diodes
Seongjun Kim1, Tae Hoon Seo2, Eun-Kyung Suh1, Keun Man Song3, Myung Jong Kim2, and Hyunsoo Kim1()
1 School of Semiconductor and Chemical Engineering, Semiconductor Physics Research Center, Chonbuk National University, Jeonju
561-756, Republic of Korea 2 Soft Innovative Materials Research Center, Korea Institute of Science and Technology, Jeonbuk 561-905, Republic of Korea 3 Korea Advanced Nano Fab Center, Suwon 443-700, Republic of Korea
Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)
INFORMATION ABOUT ELECTRONIC SUPPLEMENTARY MATERIAL.
————————————
Address correspondence to Hyunsoo Kim, [email protected]
Figure S1 The synthesis of graphene by CVD method and transfer techniques on GaN substrate.
Figure S2 The optical transmittance characteristics of bare GaN and graphene on GaN substrate.
15
Figure S3 Norde function, F(V) versus V of (a) Ni-GaN diode, (b) graphene-GaN diode without illumination, and (c) graphene-
GaN diode with illumination.
16
-6 -4 -2 0 210
-15
10-12
10-9
10-6
10-3
Ni-GaN Schottky diode
I (A
)
V (V)
n = 1.22
B= 1.08 eV
Figure S4 The I-V characteristics of Ni-GaN Schottky diodes.
1.5 2.0 2.5 3.0 3.5 4.0
In
ten
sit
y (
a. u
.)
h (eV)
Figure S5 The luminescence spectrum of the fluorescent lamp.