Title: Selective AuCl3 Doping of Graphene for Reducing
Contact Resistance of Graphene Devices
Authors: Dong-Chul Choi, Minwoo Kim, Young Jae Song,
Sajjad Hussain, Woo-Seok Song, Ki-Seok An, Jongwan Jung
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Please cite this article as: Dong-Chul Choi, Minwoo Kim, Young Jae Song, Sajjad
Hussain, Woo-Seok Song, Ki-Seok An, Jongwan Jung, Selective AuCl3 Doping of
Graphene for Reducing Contact Resistance of Graphene Devices, Applied Surface
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Selective AuCl3 Doping of Graphene for Reducing Contact
Resistance of Graphene Devices
Dong-Chul Choi a,b, Minwoo Kimc, Young Jae Song c,d* , Sajjad Hussain a,b, Woo-Seok Songe,
Ki-Seok Ane, Jongwan Jung a,b*
Graphene Research Institute, Sejong University, Seoul 143-747, Korea
Faculty of Nanotechnology & Advanced Materials Engineering, Sejong University, Seoul
SKKU Advanced Institute of Nano Technology (SAINT), Sungkyunkwan University
(SKKU), Suwon 16419, Korea
Department of Physics, Sungkyunkwan University (SKKU), Suwon 16419, Korea
Thin Film Materials Research Center, Korea Research Institute of Chemical Technology,
Daejon 305-600, Korea
*To whom correspondence should be addressed. E-mail: (Y.J.S) email@example.com, E-mail:
5mM 5 days
Graphene was doped selectively using AuCl3 solution only to metal-graphene contact
With 10 mM-AuCl3 at 80 oC, a low contact resistivity was obtained.
The stability of the contact resistivity in atmospheric environment was evaluated.
The increase of the contact resistivity due to de-doping was much lower than sheet
resistance due to covering metal.
Low contact resistance between metal-graphene contacts remains a well-known challenge for
building high-performance two dimensional materials devices. In this study, CVD-grown
graphene film was doped via AuCl3 solution selectively only to metal (Ti/Au) contact area to
reduce the contact resistances without compromising the channel properties of graphene.
With 10 mM-AuCl3 doping, doped graphene exhibited low contact resistivity of ~897 Ω mm,
which is lower than that (~1774 Ω mm ) of the raw graphene devices. The stability of the
contact resistivity in atmospheric environment was evaluated. The contact resistivity
increased by 13 % after 60 days in an air environment, while the sheet resistance of doped
graphene increased by 50 % after 30 days. The improved stability of the contact resistivity of
AuCl3-doped graphene could be attributed to the fact that the surface of doped-graphene is
covered by Ti/Au electrode and the metal prevents the diffusion of AuCl3.
Keywords: Graphene; Contact resistivity; AuCl3; Stability
Graphene, an atomically thin semi-metal with sp2–bonded carbon atoms arranged in a
honeycomb lattice, is at center stage of intense research in the field of fundamental physics as
well as in low-cost flexible transparent electronics, photovoltaics or microelectronics
devices[1-3]. For electronic devices application, the metal to graphene contact resistance is
the big obstacle hindering the further progress of high-performance graphene-based devices
with ultrafast carrier transport properties. Contact resistance is a major limiting factor for the
on state current of nanoscale graphene field effect transistor. To date, several studies for
reducing contact resistances of graphene to metal have been reported. Post annealing [4-6]
and cleaning of exposed graphene surface are common approaches for reducing contact
resistance, since the graphene surface is most likely contaminated during the fabrication
process. Ultraviolet/ozone treatment [7, 8], plasma treatment [4, 9-11], and atomic force
microscopy cleaning for source/drain contact regions prior to metallization [12, 13] have
been reported. It was observed that contact resistance of two‐dimensional MoS2 thin-film was
significantly reduced after laser annealing and the field‐effect mobility was increased from
24.84 cm2V‐1s‐1 to 44.84 cm2V‐1s‐1. These approaches however could damage the
graphene. Thermal annealing also affects the graphene, and heat can damage to the
components or sensitive parts where thermal effects should be avoided. Chan et al. also found
that annealing did not significantly affect the contact resistance of their device . It was
observed in some cases that the annealing process increases the contact resistance instead, as
a result of the structural disorders along the plasma-etched graphene edges. Another effective
approach is to dope graphene. Most of the doping experiments have been performed using
electrochemical or electrostatic techniques because these methods provide an easy way to
control the Fermi energy of graphene. Chemical doping of graphene using AuCl3 has been
used for p-type doping [16-21]. A work function of graphene could be controlled by dipping
graphene films into AuCl3 solution. The AuCl3 doping method has been applied to increase
the conductivity of transparent graphene films [16-21]. Contrary to those several reports in
which graphene doping using AuCl3 to increase the conductivity of films, the effect of
organic doping such as AuCl3 to graphene-metal contact resistivity has not been explored yet.
Here in this work, AuCl3 doping was applied selectively to source/drain contact area of
chemical vapor deposition (CVD)-grown graphene to reduce the contact resistances without
compromising the channel properties of graphene. It was observed that AuCl3 doping
between graphene and metal pad significantly improved contact resistance. We also found
that the stability of contact resistance is much higher than that of the sheet resistance of
AuCl3-doped graphene in the atmospheric environment because the contact area is covered
with metal pads while the graphene channel is directly exposed to air.
First, highly crystalline graphene was grown on a Cu foil by rapid thermal CVD, and the
CVD-grown graphene was transferred to 300 nm-SiO2/Si substrate. FeCl3 etchant was used
for etching of the Cu foil and poly (methyl methacrylate) (PMMA) was used as a supporting
film for the transfer process. The active areas were opened by photolithography, and a thin
Cu film was evaporated using an evaporator, in which the Cu film (~20 nm) was used for a
shadow mask for etching graphene. After lift-off, the exposed graphene area was etched out
by O2 plasma using a reactive ion etcher (RIE). The Cu film was then etched out by FeCl3
etchant. Source and drain pad areas for graphene transistors were opened by photolithography
again. AuCl3 solution was dropped and spin-coated with 2000 rpm for 1 min on the open
contact area, while the channel region of graphene was protected by photoresist. Even though
nitromethane is the most common solvent for AuCl3, water was used instead for solvent since
we noticed that nitromethane remains after lift-off process. The samples were heated to
50 °C-140 °C on a hot plate for dopant activation. Then, Ti/Au (8/50 nm) was evaporated
using a thermal evaporator for source and drain metal and lifted off. The overall fabrication
scheme for device is illustrated in Fig. 1.
Results and Discussion
The Raman spectra of the graphene before and after doping using AuCl3 solution are shown
in Fig. 2. First, the CVD-grown graphene film was identified as a single layer graphene based
on I2D/IG (>2) and transmittance data (~97.3% @ 550 nm) on a glass. The G and 2D peaks of
the pristine graphene (non-treated graphene with AuCl3) are observed at 1583.4 cm−1 and
2677.9 cm−1, respectively, as shown in Fig. 2a and Fig. 2b. On the other hand, the G peaks of
the doped graphene using different AuCl3 concentration are found to be at 1586.7, 1589.5,
1590.8 and 1593.1 cm−1, respectively for AuCl3 concentration of 2.5, 5, 10, and 15 mM,
respectively. While 2D peak shifted upward to 2683.5, 2683.9, 2688.7, and 2691.5 cm−1,
respectively for 2.5, 5, 10, and 15 mM of AuCl3, respectively. The upward shift of G and 2D
peak positions is a clear indication of p-type doping of graphene [16, 22, 23]. The ratio of
ID/IG was changed to 0.28 after AuCl3 doping, respectively, as compared to the 0.02 ratio for
the pristine CVD-grown graphene, most likely because the AuCl3 doping increased the
disorder of the graphene basal plane . On the contrary, Raman spectra were not
changed noticeably on a graphene channel (Fig. 2c) since the graphene channel was
protected by photoresist during AuCl3 doping process as shown in Fig. 1.
X-ray photon spectroscopy (XPS) was analysed for non-treated and AuCl3-doped graphene.
The C 1s peak, corresponding to sp2 C – sp2 C bond at ~284.9 eV was not changed with heat
treatment up to 140 °C. In Fig. 3b, the Au4f core level was resolved to AuCl3 and Au0,
respectively. These results demonstrate a successful doping of Au or Cl on the graphene layer.
Fig. 3c and Fig. 3d show that Au0 4f
peak position shifted monotonically downward with
increasing annealing temperature (87.3 eV for 50 °C to 86.9 eV at 140 °C), while Cl 2p
peak position was not changed, indicating an increase in the size of Au0 clusters [17, 25].
This could be attributed to the fact that Au
ions were reduced and agglomerated into Au0
cluster. Fig. 3e shows that atomic composition of Cl and Au3+ ions presents a slight peak at
80 °C and decreases slightly beyond 80 °C. The decrease of Au3+ ions above 80 °C indicates
that Au3+ ions were reduced to Au0 due to transformation of the unstable Au3+ in AuCl4- to a
stable Au0 cluster by Cl desorption during heat treatment [17, 25].
Scanning Kelvin probe microscopy (SKPM) was employed to analyze the surface potential of
graphene sheets. A contact potential difference (CPD) was measured with modulations of
amplitude of ~0.3-0.5 V and a frequency of 17 kHz in a Pt- coated AFM probe. Fig. 4a and
Fig. 4b show topography and surface potential mapping image of non-treated and AuCl3doped graphene as functions of molar concentration (annealing temperature was set to 50 °C).
As mole concentration of AuCl3 increases, the Au-cluster size increases. Work functions of
graphene sheets increased from ~4.65 eV to ~4.79 eV with increasing the AuCl3
concentration from 0 mM to 10 mM as shown in Fig. 4c. And the work function saturates in
AuCl3 doping above 10 mM. In this work, work function of raw graphene was 4.65 eV which
was a quite higher than the reported value of 4.5 eV for ideal graphene[24, 25]. It is believed
that organic residues still remain on the film or substrate during transfer process. The residual
contaminants and oxygen trapped at the interface of graphene and substrate make graphene pdoped. Fig. 4d shows the effects of annealing temperatures on work function of graphene.
With increasing the temperature, work function of doped graphene also increases. However,
when the annealing temperature exceeds 80° C, the work function of the doped graphene
becomes saturated and decreases again. It is believed that beyond 80° C, Au3+ ions were
transformed to Au0, consistent with the previous XPS results. The topography and surface
potential mapping image of as-doped graphene and as-annealed graphene at different
temperatures are shown in Fig. S2.
Contact resistance between the doped graphene and metal (Ti/Au) was measured by using
transmission line method (TLM). The TLM was designed with contact distances from 5 to 30
mm, contact width (W) of 20 mm, and contact length (d) of 100 mm. Fig. 5a shows a
schematic image of the designed TLM devices, and typical images of the fabricated devices
are shown in Fig. 5b and Fig. 5c. Current and resistance values between the TLM pads were
measured using a parameter analyzer, B1500A (Agilent Co.). Fig. 6 shows the measured
resistance and extracted contact resistance values for the fabricated graphene samples. With
respect to naming of the contact resistance, Rc refers to the contact resistance (measured in Ω),
the specific contact resistivity (or specific contact resistance) is denoted as ρcspecific (in Ω mm2)
and the contact resistivity ρc= Rc∙𝑊 (in Ω μm). The contact resistance was calculated based
on the y-intercept of a linear data fit of the total resistance (RT). The sheet resistance of
graphene channel (Rsh) was extracted from the slope of a linear data fit. It can be noted that
Rsh (the slope of the curves) was not much changed for all samples because only the source
and drain contact area were doped. In TLM, the equation between RT and channel length L
provides an estimate of ρcspecific through the transfer length LT. The intersection of the RT
curve for RT=0 (Lx), gives 2 LT for Rsk =Rsh , Rsh being the sheet resistance of the graphene
channel and Rsk the sheet resistance of the graphene under the contact . For a precise
extraction of LT, one should take into account the difference of the sheet resistance in the
channel and underneath the metal contact . In our experiment, the graphene beneath the
contact was doped by AuCl3, whereas, the graphene channel was not. Besides, it has been
reported that Rsh under the contact is strongly dependent on the deposition process .
Comparison of transfer length LT with the actual contact length d indicates whether the
current flow is crowded to the edge of the contact or flows into the whole contact area.
Because the contact length d (100 mm) of the TLM device is much larger than the intersection
of the RT curve for RT=0, current crowding is so high that the contact resistivity is described
effectively as the contact resistivity rc =Rc∙W (in Ω mm). The contact resistivity was ~1774 Ω
mm for the raw graphene, and it was decreased to ~978, ~897, and ~909 Ω mm, for 5 mM, 10
mM, and 15 mM AuCl3-doping, respectively (Fig. 7a). The smallest contact resistivity (~897
Ω mm) value was obtained from the doped graphene with 10 mM-AuCl3 at 80 °C. Please note
that the contact resistivity difference at 10 mM and 15 mM is very small, and this result is
consistent with the previous surface potential mapping data (Fig. 4c). Fig. 7b shows the
contact resistivity change at different annealing temperatures. The contact resistivity of doped
graphene exhibited the lowest value (~897 Ω mm) at 80 °C and slight change at different
annealing temperatures (Fig. S3). The choice of contact metal is critical to reduce the
graphene contact resistance. For Ti/Au metal, the contact resistivity varies largely by many
orders of magnitude from ~103 to ~106 Ω mm . The obtained specific contact resistivity
values are positioned in the low range value compared with the previous results from Ti/Au
metal/graphene contact (103 to ~106 Ω mm , 600-1000 Ω mm , 800 Ω mm , 568
Ω mm , 7500 Ω mm , 2000 Ω mm ). We also investigated the stability of contact
resistivity over time. The sheet resistance of doped graphene and contact resistivity was
compared. Fig. 8 shows the sheet resistance of doped graphene and contact resistivity of
doped graphene with Ti/Au electrode over time. The sheet resistance of AuCl3-doped
graphene exposed in an air environment was increased by 15 % after 5 days and by 50 %
after 30 days. On the contrary, the contact resistivity of doped graphene increased by 13 %
after 60 days. The improved stability of the contact resistivity of graphene-metal could be
attributed to the fact that the surface of doped graphene is covered by Ti/Au electrode, and
the metal electrode obstructs the diffusion of AuCl3, while the graphene channel is directly
exposed to the air. These results show that chemical doping by using AuCl3 solution is an
effective and easily applicable approach for reducing contact resistances of graphene.
In summary, we report a treatment to improve the graphene-metal contacts through doping by
AuCl3 and annealing at various temperatures. With 10 mM-AuCl3 doping, and annealing at
80 °C, the doped graphene devices exhibit a contact resistivity of ~897 Ω mm which is half
of the raw-graphene. We also noticed that the contact resistivity has much higher stability
than the sheet resistance of graphene channel exposed to an air environment. The improved
stability of the contact resistivity of AuCl3-doped graphene could be attributed to the fact that
the surface of doped graphene is covered by Ti/Au electrode and the metal prevents the
diffusion of AuCl3. These results show that chemical doping by using AuCl3 solution is an
effective and easily applicable approach for reducing contact resistances of graphene.
This research was supported by the Basic Science Research Program through the National
Research Foundation of Korea (NRF), funded by the Ministry of Education (2010-0020207,
2016R1D1A1B01015047, 2015R1A1A1A05027585, 2011-0030046, 2014M3C1A3053024,
2015M3A7B4050455), and by Nano·Material Technology Development Program through the
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Fig. 1. Schematic of the AuCl3 doping process for contact area.
shift (cm (nm)
Fig. 2. (a) Raman spectra of graphene doped with different AlCl3 concentration. (b) Shifts of
the G and 2D peaks as functions of doping concentration. (c) Raman spectra of graphene
channel protected by photoresist before and after AuCl3 treatment.
Fig. 3. XPS spectra of doped graphene for various annealing temperatures from 50 °C to 140
C. XPS spectra of (a) carbon, (b) gold and (c) chloride, (d) shifts of Cl 2p3/2 and Au0 4f5/2 (e)
atomic percent of Cl and Au3+
Work Function (eV)
Work Function (eV)
Fig. 4. (a) Topography and (b) surface potential mapping image of doped graphene as
functions of AuCl3 doping concentration. (c) Work functions of doped graphene at different
doping concentrations and (d) at different annealing temperatures.
Fig. 5. (a) TLM design. (b) Optical image of the fabricated TLM device. (c) SEM image of
the fabricated TLM device (The designed contact width and contact distance is 20 mm and 25
mm, respectively. The measured values are very close to the designed values.)
Equation y = a +
Adj. R-Sq 0.9980
rc = 1773.84 Ω·㎛
Lx =2LT = 5.94 um
Interce 89.727 18.68918
rc = 897.26 Ω·㎛
Lx = 2.88 um
rc = 978.80 Ω·㎛
Lx = 3.13 um
Adj. R-Sq 0.9942
Channel length (μm)
rc = 909.56 Ω·㎛
Lx = 2.90 um
Adj. R-Sq 0.9952
Adj. R-Sq 0.9984
Interce 177.38 11.41318
Fig. 6 (a)-(d) Measured resistance values and extracted contact resistivity versus contact
separations of 5 to 30 mm are plotted. (a) Raw graphene, (b)-(d) doped graphene with
different AuCl3 concentrations (5 mM, 10 mM, 15 mM). The contact width was 20 mm.
110 degree 140 degree
Fig. 7 (a) Contact resistivity of graphene-electrode as functions of doping concentration (80
C annealing) (b) The contact resistivity of graphene-electrode as functions of annealing
temperature (10 mM of AuCl3)
Fig. 8 (a) Sheet resistance of AuCl3 as-doped graphene after 5 days and 30 days, (b) Contact
resistivity of graphene-metal measured immediately and after 60 days.