Supplementary information
Schottky Barrier Contrasts in Single and Bi-layer Graphene
Contacts for MoS2 Field Effect Transistors
Hyewon Du,1 Taekwang Kim,1 Somyeong Shin,1 Dahye Kim,1
Hakseong Kim,2 Ji Ho Sung,3,4 Myoung Jae Lee,3 David H. Seo,5 Sang
Wook Lee,2 Moon-Ho Jo3,4 and Sunae Seo1*
1Department
2Divison
of Physics, Sejong University, Seoul, 143-747, Korea
of Quantum Phases & Devices, Department of Physics, Konkuk University, Seoul, 143-701,
Korea
3Center for Artificial Low-Dimensional Electronic Systems, Institute for Basic Science (IBS), 77
Cheongam-Ro, Pohang 790-784, Korea
4Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), 77
Cheongam-Ro, Pohang 790-784, Korea
5Samsung
Electronics Company, Limited, System LSI Division, TD Team, Gyunggi 446-711, Korea
E-
mail: sunaeseo@sejong.ac.kr
S1. Experimental methods
FIG. S1. Schematic illustration of the mechanical transfer process of MoS 2/Graphene heterostructure
device.
We prepared MoS2 and graphene flakes on SiO2 (300 nm)/Si substrate using mechanical exfoliation method. For
stacking MoS2 and graphene, a mechanical transfer process was used as illustrated in Fig. S1. MoS2 was
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transferred onto the prepared graphene to prevent folding or tear of graphene during fabrication process. After the
formation of heterojunction, source-drain electrodes of Au (100 nm)/Ti (10 nm) were constructed by E-beam
lithography, followed by a Lift-off process.
20k
4
A1g
E2g
10k
1M1G
4M1G
4M2G
I2D/IG
Intensity (counts)
S2. Layer thickness analyzations by Raman and AFM
(a) MoS2
(b) 2D
5
3
2
5
4
3
2
1
0
2D
1M1G
G
4M1G
1000 2000 3000
4M2G
1
0
0
380
400
420
-1
2600
Raman shift (cm )
2650
2700
2750
-1
Raman shift (cm )
(c)
(d)
FIG. S2. Raman spectra of (a) MoS2 and (b) enlarged 2D mode of graphene. The AFM images of MoS2
flake in 1M-SG (c) and 4M-SG (d).
Fig. S2(a) shows frequency difference between single (1M) and 4-layer MoS2 (4M). The frequency differences
in 1M and 4M are 18.9 cm-1 and 20.7 cm-1.
We could confirm graphene thickness by the normalized intensity of 2D peak by G peak intensity with an
intensity ratio of I2D/IG ~5.3 (~3.6) on 1M-SG (4M-SG) as shown in Fig. S2(b). In bi-layer graphene (4M-BG), it
consists of 4 components; 2D1B, 2D1A, 2D2A and 2D2B.
For the direct thickness analyzation of 1M (1M-SG) and 4M (4M-SG), we measured thickness using Atomic
force microscope (AFM). As shown in inset data of Fig. S2(c-d), we obtained MoS2 thicknesses used in 1M-SG
and 4M-SG are 0.7nm and 2.7 nm, respectively. (Single-layer MoS2 has 0.65nm thick.)
2
S3. Raman spectra in MoS2/Graphene junction
3
2
4M2G
1
A1g
0
4M1G
400
420
-1
Raman shift (cm )
4M1G
4
1
1M1G
2
0
380
4M2G
8
6
2
4M1G
(c) 2D mode
Counts (a.u)
4M2G
E2g 1M1G
10
(b) G mode
Counts (a.u)
Counts (a.u)
3
(a) MoS2
1M1G
0
2600
1560 1580 1600
-1
Raman shift (cm )
2700
2800
Raman shift (cm-1)
FIG. S3. Raman spectra of (a) MoS2 (b) Graphene (c, d) enlarged G, 2D mode of graphene. The soid lines
and dotted lines represent only MoS2 or only graphene area and MoS2/graphene overraid area.
Fig. S3(a) shows Raman shift of MoS2 modes for MoS2-only (solid line) and MoS2/graphene (dotted line) region.
The two first-order MoS2 Raman active modes exist at 385 ±5 cm-1 (E2g) from the in-plane electron-electron
interaction and 405 ±5 cm-1 (A1g) from the out-of-plane van der Waals (VDW) interaction. The blue shift of A1g
peak was considered as p-doping (less n-doping) or interlayer VDW interaction between MoS2 and graphene. The
raised up-shift of A1g peak could be originated from increased charge transfer from MoS2 to graphene or from the
increased interlayer VDW interaction.
For intrinsic graphene, two Raman active modes are in-plane vibration ones, G (E2g) at 1584 cm-1 and 2D (A1g) at
2690 cm-1 on SiO2/Si substrate. We could confirm graphene thickness by the normalized intensity of 2D peak by
G peak intensity with an intensity ratio of I2D/IG ~5.3 (~3.6) on 1M-SG (4M-SG) as shown in Fig. S3(b).
To compare the details of Raman spectra of MoS2/graphene with graphene-only area, both G and 2D mode are
enlarged and plotted at Figs. S3(c) and S3(d) with y-axis offset for clear view.
Raman spectra of graphene covered with MoS2 can be illustrated using charge carrier doping or compressive
stress. G peak is sensitive to carrier doping. It stiffens and narrows with carrier concentration regardless of hole
and electron. Although 2D peak softens for hole and stiffens for electron doping but its intensity reduction is
dominant with carrier concentration. On the other hand, the compressive stress can also generate blue shift of the
G and 2D mode but induce larger 2D shift than G.
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S4. Photocurrent
FIG. S4. (a) Schematic illustration of the experimental setup for photocurrent imaging with a diffractionlimited resolution. Photocurrent mapping images in (b) 1M-SG (c) 4M-SG (d) 4M-BG. The red, white and
blue line denote the edge of Au/Ti, MoS2 flake and graphene flake, respectively.
We measured Photocurrent at zero bias with 4 W low incident power and a 532 nm wavelength laser in ambient
air. Fig. S4(a) shows a schematic illustration of the experimental setup for photocurrent imaging. A focused beam
was scanned over a device while the electrical signal from the device is measured as a function of the position of
laser spot to construct a spatial image with a diffraction-limited resolution. The representative Photocurrent
images are plotted in Figs. S4(b)-S4(d). The overlaid lines, based on reflectance and optical microscopy image,
display the boundary of the materials at the Photocurrent images.
With illumination along the contacts, photocurrent shows opposite polarity of the signal. Photo-generated
electrons flow into MoS2 from contact. The opposite signs of the PC signal at the source/drain contact can be
explained by Photovoltaic effect (PVE) at each Schottky barrier or Photothermoelectric effect (PTE). The zero
bias data are not sufficient to discuss the photocurrent mechanism by PVE and PTE but at least it indicates possible
formation of Schottky barrier.
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