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10 1016j apsusc 2017 07 301

Accepted Manuscript
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
PII:
DOI:
Reference:

S0169-4332(17)32311-5
http://dx.doi.org/doi:10.1016/j.apsusc.2017.07.301
APSUSC 36835

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APSUSC

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6-3-2017
11-7-2017
31-7-2017

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
Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.07.301
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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*

a

Graphene Research Institute, Sejong University, Seoul 143-747, Korea

b

Faculty of Nanotechnology & Advanced Materials Engineering, Sejong University, Seoul

143-747, Korea
c

SKKU Advanced Institute of Nano Technology (SAINT), Sungkyunkwan University

(SKKU), Suwon 16419, Korea
d

Department of Physics, Sungkyunkwan University (SKKU), Suwon 16419, Korea

e



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) yjsong@skku.edu, E-mail:
(J.J) jwjung@sejong.ac.kr

1


Graphical abstract

AuCl3 doping
+heating

600
2000
500

(a)

1400
1200

400

1000

1200
c

300
800

15%
increase

200

50%
increase

400
100

00

rc (Ω·
R㎛)

Rsh (Ω/□)
rc Rc
(W mm)

1600

800
600

(b)

13%
increase

400
200

As-doped
Raw

5mM 5 days
10mM

0

3015mM
days

2

As doped

60 days


Highlights


Graphene was doped selectively using AuCl3 solution only to metal-graphene contact
area.



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.

Abstract
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.

3


Keywords: Graphene; Contact resistivity; AuCl3; Stability

Introduction
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[14]. 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 [15]. It was

4


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.

Experimental details
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
5


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 [24]. On the contrary, Raman spectra were not

6


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

5/2

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

3/2

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

3+

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

7


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

8


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 [26]. 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 [27]. 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 [28].
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 [29]. 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 [29], 600-1000 Ω mm [30], 800 Ω mm [31], 568
Ω mm [32], 7500 Ω mm [33], 2000 Ω mm [34]). We also investigated the stability of contact

9


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.

Conclusion
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.

Acknowledgements

10


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
NRF funded by the Ministry of Science, ICT and Future Planning (2015M3A7B7045194,
2016M3A7B4909942).

References
[1] K.S. Novoselov, A.K. Geim, S. Morozov, D. Jiang, Y. Zhang, S.a. Dubonos, I. Grigorieva,
A. Firsov, Electric field effect in atomically thin carbon films, Science, 306 (2004) 666-669.
[2] F. Schwierz, Graphene transistors, Nat Nanotechnol, 5 (2010) 487-496.
[3] F. Schedin, A. Geim, S. Morozov, E. Hill, P. Blake, M. Katsnelson, K. Novoselov,
Detection of individual gas molecules adsorbed on graphene, Nat Mater, 6 (2007) 652-655.
[4] J.A. Robinson, M. LaBella, M. Zhu, M. Hollander, R. Kasarda, Z. Hughes, K. Trumbull,
R. Cavalero, D. Snyder, Contacting graphene, Applied Physics Letters, 98 (2011) 053103.
[5] O. Balci, C. Kocabas, Rapid thermal annealing of graphene-metal contact, Applied
Physics Letters, 101 (2012) 243105.
[6] C. Malec, B. Elkus, D. Davidović, Vacuum-annealed Cu contacts for graphene electronics,
Solid State Communications, 151 (2011) 1791-1793.
[7] C.W. Chen, F. Ren, G.-C. Chi, S.-C. Hung, Y. Huang, J. Kim, I.I. Kravchenko, S.J.
Pearton, UV ozone treatment for improving contact resistance on graphene, Journal of
Vacuum Science & Technology B, 30 (2012) 060604.
[8] W. Li, Y. Liang, D. Yu, L. Peng, K.P. Pernstich, T. Shen, A.H. Walker, G. Cheng, C.A.
Hacker, C.A. Richter, Ultraviolet/ozone treatment to reduce metal-graphene contact
resistance, Applied Physics Letters, 102 (2013) 183110.
11


[9] M.S. Choi, S.H. Lee, W.J. Yoo, Plasma treatments to improve metal contacts in graphene
field effect transistor, Journal of applied physics, 110 (2011) 073305.
[10] J.T. Smith, A.D. Franklin, D.B. Farmer, C.D. Dimitrakopoulos, Reducing contact
resistance in graphene devices through contact area patterning, ACS nano, 7 (2013) 36613667.
[11] T. Kwon, H. An, Y.-S. Seo, J. Jung, Plasma Treatment to Improve Chemical Vapor
Deposition-Grown Graphene to Metal Electrode Contact, Japanese Journal of Applied
Physics, 51 (2012) 04DN04.
[12] A. Goossens, V. Calado, A. Barreiro, K. Watanabe, T. Taniguchi, L. Vandersypen,
Mechanical cleaning of graphene, Applied Physics Letters, 100 (2012) 073110.
[13] N. Lindvall, A. Kalabukhov, A. Yurgens, Cleaning graphene using atomic force
microscope, Journal of applied physics, 111 (2012) 064904.
[14] H. Kwon, W. Choi, D. Lee, Y. Lee, J. Kwon, B. Yoo, C.P. Grigoropoulos, S. Kim,
Selective and localized laser annealing effect for high-performance flexible multilayer MoS2
thin-film transistors, Nano Research, 7 (2014) 1137-1145.
[15] J. Chan, A. Venugopal, A. Pirkle, S. McDonnell, D. Hinojos, C.W. Magnuson, R.S.
Ruoff, L. Colombo, R.M. Wallace, E.M. Vogel, Reducing extrinsic performance-limiting
factors in graphene grown by chemical vapor deposition, ACS nano, 6 (2012) 3224-3229.
[16] K.K. Kim, A. Reina, Y. Shi, H. Park, L.-J. Li, Y.H. Lee, J. Kong, Enhancing the
conductivity of transparent graphene films via doping, Nanotechnology, 21 (2010) 285205.
[17] D. Hee Shin, J. Min Kim, C. Wook Jang, J. Hwan Kim, S. Kim, S.-H. Choi, Annealing
effects on the characteristics of AuCl3-doped graphene, Journal of Applied Physics, 113
(2013) 064305.

12


[18] Mir Abdullah-Al-Galib, Bo Hou, Tahmeed Shahriad, Sandra Zivanovic, Adarsh D.
Radadia, Stability of few layer graphene films doped with gold (III) chloride, Applied
Surface Science 366 (2016) 78–84.
[19] Y. Shi, K.K. Kim, A. Reina, M. Hofmann, L.-J. Li, J. Kong, Work function engineering
of graphene electrode via chemical doping, ACS nano, 4 (2010) 2689-2694.
[20] F. Gunes, H.-J. Shin, C. Biswas, G.H. Han, E.S. Kim, S.J. Chae, J.-Y. Choi, Y.H. Lee,
Layer-by-layer doping of few-layer graphene film, ACS nano, 4 (2010) 4595-4600.
[21] Hiesang Sohn, Yun Sung Woo, Weonho Shin, Dong-Jin Yun, Taek Lee, Felix Sunjoo
Kim, Jinyoung Hwang, Novel transparent conductor with enhanced conductivity: hybrid of
silver nanowires and dual-doped graphene, Applied Surface Science 419 (2017) 63–69.
[22] K.K. Kim, J.J. Bae, S.M. Kim, H.K. Park, K.H. An, Y.H. Lee, Control of p‐doping on
single‐walled carbon nanotubes with nitronium hexafluoroantimonate in liquid phase, physica
status solidi (b), 246 (2009) 2419-2422.
[23] A.M. Rao, P. Eklund, S. Bandow, A. Thess, R.E. Smalley, Evidence for charge transfer
in doped carbon nanotube bundles from Raman scattering, Nature, 388 (1997) 257-259.
[24] D.H. Shin, J.M. Kim, C.W. Jang, J.H. Kim, S. Kim, S.-H. Choi, Annealing effects on the
characteristics of AuCl3-doped graphene, Journal of applied physics, 113 (2013) 064305.
[25] P. Reinke, J. Howe, S. Eswaramoorthy, E. Thune, Understanding the role of annealing
temperature and ion energy in the growth of Au clusters, Journal of applied physics, 100
(2006) 024303.
[26] G.K.Reeves and H.B.Harrison, Obtaining the specific contact resistance from
transmission line model measurements, IEEE EDL, 3 (1982) 111-113.
[27] Filippo Giubileo, Antonio Di Bartolomeo, The role of contact resistance in graphene
field-effect devices, Progress in Surface Science 92 (2017) 143–175.

13


[28] K. Nagashio, T. Yamashita, J. Fujita, T. Nishimura, K. Kita, A. Toriumi, Impacts of
graphene/SiO2 interaction on FET mobility and Raman spectra in mechanically exfoliated
graphene films, in: IEEE, 2010: p. 23.4.1-23.4.4. doi:10.1109/IEDM.2010.5703421.
[29] K. Nagashio, T. Nishimura, K. Kita, A. Toriumi, Contact resistivity and current flow
path at metal/graphene contact, Applied Physics Letters, 97 (2010) 143514.
[30] S. Russo, M.F. Craciun, M. Yamamoto, A.F. Morpurgo, S. Tarucha, Contact resistance
in graphene-based devices, Physica E: Low-Dimensional Systems and Nanostructures. 42
(2010) 677–679. doi:10.1016/j.physe.2009.11.080.
[31] E. Watanabe, A. Conwill, D. Tsuya, Y. Koide, Low contact resistance metals for graphene
based

devices,

Diamond

and

Related

Materials.

24

(2012)

171–174.

doi:10.1016/j.diamond.2012.01.019.
[32] W. Li, C.A. Hacker, G. Cheng, Y. Liang, B. Tian, A.R. Hight Walker, C.A. Richter, D.J.
Gundlach, X. Liang, L. Peng, Highly reproducible and reliable metal/graphene contact by
ultraviolet-ozone

treatment,

Journal

of

Applied

Physics.

115

(2014)

114304.

doi:10.1063/1.4868897.
[33] B.-C. Huang, M. Zhang, Y. Wang, J. Woo, Contact resistance in top-gated graphene fieldeffect transistors, Applied Physics Letters. 99 (2011) 032107. doi:10.1063/1.3614474
[34] W.J. Liu, M.F. Li, S.H. Xu, Q. Zhang, Y.H. Zhu, K.L. Pey, H.L. Hu, Z.X. Shen, X. Zou, J.L.
Wang, J. Wei, H.L. Zhu, H.Y. Yu, Understanding the contact characteristics in single or multilayer graphene devices: The impact of defects (carbon vacancies) and the asymmetric
transportation behavior, in: IEEE, 2010: p. 23.3.1-23.3.4. doi:10.1109/IEDM.2010.5703420

14


AuCl3 doping
+heating

Fig. 1. Schematic of the AuCl3 doping process for contact area.

15


Intensity
(a.u)
(arb.units)
Intensity

Intensity (arb.units)

(c)
before

after
1400

2100

2800

-1 )
Wavelength
Raman
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.

16


(b)

Intensity (arb.units)

Intensity (arb.units)

Intensity (arb.units)

(a)

(d)

(c)

(e)

Fig. 3. XPS spectra of doped graphene for various annealing temperatures from 50 °C to 140
o

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+

17


Topography

(a)

Surface potential

(b)

R aw

5mM

Work Function (eV)

4.80

(c)

4.76

4.72

4.68

4.64
Raw

5mM

10mM

15mM

Doping level
4.95

10mM

15mM

Work Function (eV)

4.90

(d)

4.85
4.80
4.75
4.70
4.65
As- doped

50℃

80℃

110℃

140℃

Temperature

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.

18


(a)

(b)

(c)
Ti/Au

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.)

19


(a)

1200

(b) 1200

Equation y = a +
Adj. R-Sq 0.9980

800

B

Slope

29.855

0.58613

600
400

rc = 1773.84 Ω·㎛
Lx =2LT = 5.94 um

200

Raw

0
0

(c)

5

10

15

20

25

Equation

(d)

y=a+

Totalresistance
resistance
(Ω)
Total

1000

Value

800

Interce 89.727 18.68918

B

Slope

31.084

0.95979

600
400

rc = 897.26 Ω·㎛
Lx = 2.88 um

200

0

5

10

15

20

Channel length

B

Slope

25

Standard
10.87329

31.220

0.5584

400

rc = 978.80 Ω·㎛
Lx = 3.13 um

200

5mM

0

1200

5
Equation

10

15

30

25

Channel length

30

Adj. R-Sq 0.9942
Value

800

Standard

B

Interce 90.956

B

Slope

31.269

20.67487
1.06176

600
400

0

Channel length (μm)

20

y=a+

rc = 909.56 Ω·㎛
Lx = 2.90 um

200

10mM

0

Interce 97.880

1000

Standard

B

B

600

0

Adj. R-Sq 0.9952

Value

800

30

Channel length

1200

y=a+

Adj. R-Sq 0.9984

1000

Standard

Interce 177.38 11.41318

Total
resistance
(Ω)
resistance
Total

Value
B

Totalresistance
resistance
(Ω)
Total

(Ω)
Total
Totalresistance
resistance

1000

Equation

15mM
0

5

10

15

20

Channel
length
Channel length
(μm)

25

30

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.

20


2000

(a)

(b)

1200

rcR(W
c mm)

rc Rc
(W mm)

1600
1200
800

1000

800

400
0

Raw

5mM

10mM

600

15mM

50 degree

80 degree

110 degree 140 degree

Temperature

Fig. 7 (a) Contact resistivity of graphene-electrode as functions of doping concentration (80
o

C annealing) (b) The contact resistivity of graphene-electrode as functions of annealing

temperature (10 mM of AuCl3)

21


600

(a)

1400

400

1000

300

800

c

15%
increase

200

50%
increase

600

13%
increase

400

100
0

(b)

1200

rc (Ω·
R㎛)

Rsh

(Ω/□)

500

200

As-doped

5 days

0

30 days

As doped

60 days

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.

22



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