Supplementary information
High-performance photocurrent generation from two-dimensional WS2
field effect transistors
Seung Hwan Lee,1,2,a) Daeyeong Lee,1,2,a) Wan Sik Hwang,3,b) Euyheon
Hwang,1 Debdeep Jena,4 and Won Jong Yoo1,2,b)
1Department
of Nano Science and Technology, SKKU Advanced Institute of Nano-Technology (SAINT),
Sungkyunkwan University (SKU), 2066 Seobu-ro, Suwon-si, Gyeonggi-do, 440-746, Korea
2
Samsung-SKKU Graphene Center (SSGC), 2066 Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do, 440-746,
Korea
3Department
of Materials Engineering, Korea Aerospace University, 76 Hanggongdaehang-ro, Deogyang-
gu, Goyang-si, Gyeonggi-do, 412-791, Korea
4Department
of Electrical Engineering, University of Notre Dame, Notre Dame, Indiana, 46556, USA
_____________________________
a)
S. H. Lee and D. Lee contributed equally to this work.
b)
Electronic mails of corresponding authors: whwang@kau.ac.kr (W. S. Hwang); yoowj@skku.edu (W. J. Yoo).
Gate bias and wavelength dependent transfer curve and photocurrent
At negative gate bias region (blue arrow in Fig. S1), the photogain is relatively higher than that of
positive gate bias region (red arrow in Fig. S1) and vice versa for the photocurrent. It reveals clearly by
comparing linear (main panel in Fig. S1) and log scale (inset in Fig. S1) graphs.
40
30
ID (nA)
ID (A)
-10
10
-12
20
10
10
-8
10
-10
10
Photocurrent (nA)
-8
10
(b)
Light off
700 nm
630
530
450
300
Photocurrent (A)
(a)
-12
10
-6
-4
-2
6
3
0
0
-6
9
-4
0
VG (V)
-2
2
0 2
VG (V)
4
4
-6
6
-4
-14
6
10
-6
-4
-2
0
VG (V)
-2
0 2
VG (V)
2
4
4
6
6
FIG S1. The log scale transfer (a) and calculated photocurrent (b) curve depending on incident light
energy. Insets are corresponding linear scale graph. The amount of photocurrent is larger in the positive
gate bias region (red arrow), while on/off ratio is larger in the negative gate bias region (blue arrow).
Measurement was done in VD = 20 mV. The legend in (a) applies to all graph in this figure.
2
Photoresponse measurement system setup
The Fig. S2 shows schematic diagram of the photo-response measurement system used in this study.
The xenon arc lamp (component 1) is used as a light source of the monochromator. The light from the
xenon arc lamp is collimated and focused to the entrance slit of the monochromator (light path from 2 to
4). The light from the entrance slit is collimated again by the collimating mirror (component 5) and
dispersed by the grating (component 6). The dispersed light spectrum is focused by the focusing mirror
(component 7) and the monochromatic light is selected by the out-slit (component 8). The
monochromatic light from the out-slit runs through the optical fiber and it is collimated at the output of
the optical fiber (light path from 9 to 10). Finally, the focused light is illuminated on the sample after the
collimated light run through the optic system of the optical microscope (light path from 11 to sample)
4
3
2
1
9
10
11
5
6
7
VD
VG
8
FIG S2. The schematic diagram of the photo-response measurement system used in this study. 1: Xenon
arc lamp, 2: collimating lens, 3: focusing lens, 4: entrance slit, 5: collimating mirror, 6: grating for
dispersing light, 8: out-slit for monochromatic light selection, 9: optical, 10: collimating lens, and 11:
optic system of the optical microscope.
3
Calculation of photocurrent density and optical power density
The photocurrent density, Jph, is calculated by subtracting the drain current without light illumination
from the drain current with light illumination and dividing it by the area of the channel. The channel
dimensions of width and length are 4 and 1 μm, respectively. The optical power density, Popt, is
calculated by dividing optical power into the area of projected light on the sample. The shape of the
projected light is circle and its diameter is ~58 μm. The optical power is measured where samples are
placed using optical power meter.
4
Comparison of the figures of merit for photodetectors prepared using different TMDCs
The figures of merit are summarized and compared with those measured using other TMDC
phototransistors in Table SI. The applied electrical field and optical power are listed together to enable a
fair comparison. Our WS2 phototransistor shows comparable Iillum/Idark and higher photoresponsivity
compared to the bulk MoS2 phototransistor1, even upon application of a 15-fold smaller drain electrical
field. Furthermore, the performance of our WS2 device is comparable to that of other TMCD
phototransistors, even with a relatively small electrical field and optical power (Table SI).
TABLE SI. Comparison of the figures of merit for photodetectors prepared using different TMDCs.
Electrical bias
Photoresponsivity
(A/W)
Iillum/Idark
ED
EG
/ Popt (W/cm2)
d
(mV/nm)
(mV/nm)
a
0.60
0 (on)
92 μ
~10L WS2
0.02
–22 (off)
0.27
102 – 103 / 0.25
~30L WS2
1
38 (on)
36
a
8.00
–259 (off)
880
30 / 2.4
1L MoS2
0.48
167 (on)
7.5m
1L MoS2
1.25
0 (on)
1.04
24 / 2
3L MoS2
0.31
–158 (off)
0.12
102 – 103 / 0.05
~50La MoS2
a
Obtained from calculations or estimates using the information in the reference.
Device structure
b
EQE
(%)
20ma
8
7k
190ka
2a
242a
24a
Ref
2b
This work
3
4
5c
1
WS2 was synthesized using the chemical vapor deposition method. Other TMDCs were exfoliated from
the bulk crystal.
c
The device in this ref had an interdigitated source and drain structure, whereas the device described in
the other ref had a one-fingered source and drain structure.
d
No (no gating), off (transistor-off gate bias was applied), on (transistor-on gate bias was applied). All
values considered the effective oxide thickness.
5
Photoresponsivity calculation
The photoresponsivity of WS2 device is calculated from the maximum slope of the linearly plotted
photocurrent graph as a function of illuminating light power density (see Fig. S3). The photoresponsivity
from 630 nm wavelength of light is relatively high than those from the other wavelength of light.
2
Photocurrent (mA/cm )
5
4
3
2
1
0
630 nm
0.0
0.1
0.2
2
Light power (W/cm )
Fig. S3. The linearly plotted photocurrent density graph as a function of illuminating light power
density. The blue line is the guild line for the maximum slop of this graph. Measurements were
performed at VD = 20 mV and VG = –2 V.
6
REFERENCES
1
W. Choi, M. Y. Cho, A. Konar, J. H. Lee, G.-B. Cha, S. C. Hong, S. Kim, J. Kim, D. Jena, J. Joo, and
S. Kim, Adv. Mater. 24, 5832 (2012).
2
N. Perea-López, A. L. Elías, A. Berkdemir, A. Castro-Beltran, H. R. Gutiérrez, S. Feng, R. Lv, T.
Hayashi, F. López-Urías, S. Ghosh, B. Muchharla, S. Talapatra, H. Terrones, and M. Terrones, Adv.
Funct. Mater. 23, 5511 (2013).
3
O. Lopez-Sanchez, D. Lembke, M. Kayci, A. Radenovic, and A. Kis, Nat. Nanotechnol. 8, 497 (2013).
4
Z. Yin, H. H. Li, L. Jiang, Y. Shi, Y. Sun, G. Lu, Q. Zhang, X. Chen, and H. Zhang, ACS Nano 6, 74
(2012).
5
D.-S. Tsai, D.-H. Lien, M.-L. Tsai, S. Su, K.-M. Chen, J.-J. Ke, Y.-C. Yu, L.-J. Li, and J.-H. He, IEEE
J. Sel. Top. Quantum Electron. 20, 3800206 (2014).
7