Irradiation Effects of High Energy Proton Beams on MoS2 Field Effect ...
Supporting Information
Irradiation Effects of High Energy Proton Beams on MoS2 Field Effect Transistors
Tae-Young Kim,† Kyungjun Cho,† Woanseo Park,† Juhun Park, † Younggul Song,† Seunghun Hong,†,‡ Woong-Ki Hong,§ and Takhee Lee†,*
†
Department of Physics and Astronomy, and Institute of Applied Physics, Seoul National University,
Seoul 151-747, Korea; ‡Department of Biophysics and Chemical Biology, Seoul National University, Seoul 151-747, Korea; §Jeonju Center, Korea Basic Science Institute, Jeonju, Jeollabuk-do 561-180, Korea
* Corresponding Author. E-mail: [email protected]
1
Table of Contents 1.
Device fabrication
2.
Representative electrical characteristics of a proton-irradiated MoS2 FET device with a fluence of 1013 cm-2
3.
Contour plots of transconductance of the devices before and after proton irradiation
4.
Raman spectra of MoS2 flakes
5.
Stopping and Range of Ions in Matter (SRIM) analysis
6.
The IG-VG leakage plot of MoS2 FET
References
2
1. Device fabrication Figure S1 illustrates the device fabrication processes for MoS2 FET devices. First, prepare a highly doped p-type Si wafer piece with 270-nm-thick SiO2 layer. The MoS2 flakes were transferred onto silicon substrate by the mechanical exfoliation using a scotch tape from a bulk MoS2 crystal (purchased from SPI Supplies).S1 Using an optical microscope, candidate MoS2 flakes with a few layers thickness were found and fabricated into FET devices. The thickness of MoS2 flakes was measured using an atomic force microscope (AFM) (Park Systems, NX10). The typical thickness of selected MoS2 flakes was determined as 2‒8 nm, which corresponds to 3‒12 MoS2 layers (the thickness of single MoS2 layer is 0.65 nm).S2 Then, we spin-coated a double-layer resist as the electron-beam resist; first methyl methacrylate (MMA) (8.5) MAA (9% concentration in ethyl lactate) was spin-coated on the samples at 4,000 rpm for 50 sec and then the samples were baked on a hotplate at 180 °C for 90 sec. Next, poly methyl methacryllate (PMMA) 950K (5% concentration in anisole) was spin-coated on MMA-coated samples at 4,000 rpm for 50 sec, followed by a bake on the hotplate at 180 °C for 90 sec. Then, we designed the source and drain electrode patterns using an electron beam lithography (JEOL, JSM-6510) and development process with methyl isobutyl ketone: isopropyl alcohol (MIBK:IPA) (1:3) solution for 50 sec. Finally, we deposited Au(100 nm)/Ti (10 nm) as the source and drain electrodes using an electron beam evaporator.
3
Figure S1. Schematic illlustration of device fabrication processs.
4
2. Representative electrical characteristics of a proton-irradiated MoS2 FET device with a fluence of 1013 cm-2 Figure S2(a)-(b) show the representative electrical characteristics of a MoS2 FET device that was irradiated with a proton beam with a fluence of 1013 cm-2. Figure S2(a) is the output characteristics (source-drain current versus source-drain voltage, IDS‒VDS) measured for the MoS2 FET at gate voltage (VG) ranging from -30 to 30 V in increment of 10 V before and after the proton beam irradiation with the fluence of 1013 cm-2 which corresponds to the irradiation time of 200 sec. Figure S2(b) shows the transfer characteristics (source-drain current versus gate voltage, IDS‒VG) measured for the same device at a fixed source-drain voltage (VDS) of 0.5 V before and after proton irradiation with the same proton beam condition used in Figure S2(a). The current values in both output and transfer curves decreased after proton irradiation as compared to those measured prior to proton irradiation. The amount of decreased current is not as much as that of the proton-irradiated MoS2 FET device with a fluence of 1014 cm-2 (see Figure 2(c) and 2(d) of the main manuscript).
5
Before irradiation
IDS (A)
30 20 10
After irradiation (Φ = 1013 cm-2)
VG = 30 V
VG = -30 V
VG = 20 V
VG = -20 V
VG = 10 V
VG = -10 V
VG =
VG =
0V
(b) 0.25
0V
VG = -10 V
VG = 10 V
VG = -20 V
VG = 20 V
VG = -30 V
VG = 30 V
Before irradiation After irradiation Both @ VDS = 0.5V
0.20
IDS (A)
(a)
0.15 0.10 0.05 0.00
0 0
1
2 3 VDS (V)
4
5 0
1
2 3 VDS (V)
4
5
-30
-20
-10
0 10 VG (V)
20
30
Figure S2. Representative electrical characteristics of a MoS2 FET device before and after proton irradiation with a beam fluence of 1013 cm-2 (corresponding to an irradiation time of 200 sec). (a) IDS‒VDS curves measured for different gate voltages before (left) and after (right) proton irradiation. (b) IDS‒VG curves measured at a fixed VDS = 0.5 V before (open circles) and after (filled circles) proton irradiation.
6
3.
Contour plots of transconductance of the devices before and after proton
irradiation Figures S3(a)-3(c) show the contour plots of the transconductance (gm = dIDS/dVG), which were obtained from the IDS-VG curves measured for VDS in the range of 0.1-0.5 V, as a function of VG and VDS before and after proton radiation under the three different fluence conditions of 1012, 1013, and 1014 cm-2. The transconductance increased with VDS both before and after proton irradiation. In Figure S3(a) (1012 cm-2 fluence), the transconductance was approximately constant before and after proton irradiation. However, in Figure S3(b) (1013 cm-2 fluence case), the transconductance following proton bream irradiation decreased noticeably compared with the before-proton-irradiation case. This effect was even more pronounced under higher fluence conditions, as demonstrated in Figure S3(c) (1014 cm-2 fluence case); i.e., the transconductance following proton bream irradiation decreased more significantly under higher fluence conditions. Here, we found that the transconductance depended significantly on the proton irradiation effect and the gate voltage.
7
before and after protonn Figure S3. Contour pllots of transsconductancce (gm = dIDS D /dVG) of the devices b 1 cm-2, (b)11013 cm-2, an nd (c) 1014 cm c -2. irradiation with fluencces of (a)1012
8
4. Raman spectra of MoS2 flakes We measured Raman spectra of three samples (a newly prepared sample #1, and two old samples #2 and #3 that were irradiated with proton beam) of MoS2 films with another Raman spectrometer (XperRam 200, Nanobase) that used a laser (wavelength of 532 nm) as a source. The results are shown in Figure S4. In the data of Figure S4, we didn’t observe a noticeable shift in the Raman peaks. And, when we compare the data of Figure S4 with those in Figure 3(e) in the
Intensity (arb. unit.)
manuscript, we could see very similar peak positions among these data.
350
Sample #1 Sample #2 Sample #3
375 400 425 -1 Raman Shift (cm )
450
Figure S4. Raman spectra of three samples of MoS2 films. Sample #1 is a newly prepared sample. Samples #2 and #3 are old samples that were irradiated with a high proton beam fluence condition (Φ = 1014 cm-2) about four months ago.
9
5.
Stopping and Range of Ions in Matter (SRIM) analysis To understand the behavior of protons, we performed simulations using Stopping and Range of
Ions in Matter (SRIM) software.S3,S4 The structure of our MoS2 FET devices is MoS2 (2‒8 nm)/SiO2(270 nm)/Si (500 m). We found the protons with 10 nA current and 10 MeV energy deposited most of their energy near 700 m from the top surface (Figure S4) while some amount of energy can also be transferred to the SiO2 dielectric layer. This process creates electron-hole pairs
Energy trasfer (Arb. unit)
Energy transfer (Arb. unit)
and eventually induces charge traps in the oxide layer or at the interfaces.
0
~700 μm
0
100
200
300
Depth (nm)
200
400
400 600 Depth (m)
800
Figure S5. Energy loss depth profiles of irradiated protons calculated using SRIM. The inset image is a zoomed plot for the energy loss depth profile in the range of 0‒400 nm from the top surface.
10
6.
The IG-VG leakage plot of MoS2 FET We measured the gate leakage currents of MoS2 FETs. The result is shown in Figure S6. The
leakage current was found small enough as compared to the signal current range of A (see Figure 2 in the manuscript).
5 4
IG nA)
3 2
VDS = 0.1 V VDS = 0.2 V VDS = 0.3 V VDS = 0.4 V VDS = 0.5 V
1 0 -1
-20
-10
0 VG (V)
10
20
Figure S6. The leakage current of a MoS2 FET.
11
References S1.
Ghatak, S.; Pal, A. N.; Ghosh, A. Nature of Electronic States in Atomically Thin MoS2 Field-Effect Transistors. ACS Nano 2011, 5, 7707-7712.
S2.
Frindt, R. Single Crystals of MoS2 Several Molecular Layers Thick. J. Appl. Phys. 1966, 37, 1928-1929.
S3.
Ziegler, J. F.; Ziegler, M. D.; Biersack, J. P. SRIM-The Stopping and Range of Ions in Matter (2010) Nucl. Instrum. Methods Phys. Res., Sect. B 2010, 268, 1818-1823.
S4.
Ziegler, J. F.; Biersack, J. P. The Stopping and Range of Ions in Matter. Springer: 1985.
12
Irradiation Effects of High Energy Proton Beams on MoS2 Field Effect Transistors
Tae-Young Kim,† Kyungjun Cho,† Woanseo Park,† Juhun Park, † Younggul Song,† Seunghun Hong,†,‡ Woong-Ki Hong,§ and Takhee Lee†,*
†
Department of Physics and Astronomy, and Institute of Applied Physics, Seoul National University,
Seoul 151-747, Korea; ‡Department of Biophysics and Chemical Biology, Seoul National University, Seoul 151-747, Korea; §Jeonju Center, Korea Basic Science Institute, Jeonju, Jeollabuk-do 561-180, Korea
* Corresponding Author. E-mail: [email protected]
1
Table of Contents 1.
Device fabrication
2.
Representative electrical characteristics of a proton-irradiated MoS2 FET device with a fluence of 1013 cm-2
3.
Contour plots of transconductance of the devices before and after proton irradiation
4.
Raman spectra of MoS2 flakes
5.
Stopping and Range of Ions in Matter (SRIM) analysis
6.
The IG-VG leakage plot of MoS2 FET
References
2
1. Device fabrication Figure S1 illustrates the device fabrication processes for MoS2 FET devices. First, prepare a highly doped p-type Si wafer piece with 270-nm-thick SiO2 layer. The MoS2 flakes were transferred onto silicon substrate by the mechanical exfoliation using a scotch tape from a bulk MoS2 crystal (purchased from SPI Supplies).S1 Using an optical microscope, candidate MoS2 flakes with a few layers thickness were found and fabricated into FET devices. The thickness of MoS2 flakes was measured using an atomic force microscope (AFM) (Park Systems, NX10). The typical thickness of selected MoS2 flakes was determined as 2‒8 nm, which corresponds to 3‒12 MoS2 layers (the thickness of single MoS2 layer is 0.65 nm).S2 Then, we spin-coated a double-layer resist as the electron-beam resist; first methyl methacrylate (MMA) (8.5) MAA (9% concentration in ethyl lactate) was spin-coated on the samples at 4,000 rpm for 50 sec and then the samples were baked on a hotplate at 180 °C for 90 sec. Next, poly methyl methacryllate (PMMA) 950K (5% concentration in anisole) was spin-coated on MMA-coated samples at 4,000 rpm for 50 sec, followed by a bake on the hotplate at 180 °C for 90 sec. Then, we designed the source and drain electrode patterns using an electron beam lithography (JEOL, JSM-6510) and development process with methyl isobutyl ketone: isopropyl alcohol (MIBK:IPA) (1:3) solution for 50 sec. Finally, we deposited Au(100 nm)/Ti (10 nm) as the source and drain electrodes using an electron beam evaporator.
3
Figure S1. Schematic illlustration of device fabrication processs.
4
2. Representative electrical characteristics of a proton-irradiated MoS2 FET device with a fluence of 1013 cm-2 Figure S2(a)-(b) show the representative electrical characteristics of a MoS2 FET device that was irradiated with a proton beam with a fluence of 1013 cm-2. Figure S2(a) is the output characteristics (source-drain current versus source-drain voltage, IDS‒VDS) measured for the MoS2 FET at gate voltage (VG) ranging from -30 to 30 V in increment of 10 V before and after the proton beam irradiation with the fluence of 1013 cm-2 which corresponds to the irradiation time of 200 sec. Figure S2(b) shows the transfer characteristics (source-drain current versus gate voltage, IDS‒VG) measured for the same device at a fixed source-drain voltage (VDS) of 0.5 V before and after proton irradiation with the same proton beam condition used in Figure S2(a). The current values in both output and transfer curves decreased after proton irradiation as compared to those measured prior to proton irradiation. The amount of decreased current is not as much as that of the proton-irradiated MoS2 FET device with a fluence of 1014 cm-2 (see Figure 2(c) and 2(d) of the main manuscript).
5
Before irradiation
IDS (A)
30 20 10
After irradiation (Φ = 1013 cm-2)
VG = 30 V
VG = -30 V
VG = 20 V
VG = -20 V
VG = 10 V
VG = -10 V
VG =
VG =
0V
(b) 0.25
0V
VG = -10 V
VG = 10 V
VG = -20 V
VG = 20 V
VG = -30 V
VG = 30 V
Before irradiation After irradiation Both @ VDS = 0.5V
0.20
IDS (A)
(a)
0.15 0.10 0.05 0.00
0 0
1
2 3 VDS (V)
4
5 0
1
2 3 VDS (V)
4
5
-30
-20
-10
0 10 VG (V)
20
30
Figure S2. Representative electrical characteristics of a MoS2 FET device before and after proton irradiation with a beam fluence of 1013 cm-2 (corresponding to an irradiation time of 200 sec). (a) IDS‒VDS curves measured for different gate voltages before (left) and after (right) proton irradiation. (b) IDS‒VG curves measured at a fixed VDS = 0.5 V before (open circles) and after (filled circles) proton irradiation.
6
3.
Contour plots of transconductance of the devices before and after proton
irradiation Figures S3(a)-3(c) show the contour plots of the transconductance (gm = dIDS/dVG), which were obtained from the IDS-VG curves measured for VDS in the range of 0.1-0.5 V, as a function of VG and VDS before and after proton radiation under the three different fluence conditions of 1012, 1013, and 1014 cm-2. The transconductance increased with VDS both before and after proton irradiation. In Figure S3(a) (1012 cm-2 fluence), the transconductance was approximately constant before and after proton irradiation. However, in Figure S3(b) (1013 cm-2 fluence case), the transconductance following proton bream irradiation decreased noticeably compared with the before-proton-irradiation case. This effect was even more pronounced under higher fluence conditions, as demonstrated in Figure S3(c) (1014 cm-2 fluence case); i.e., the transconductance following proton bream irradiation decreased more significantly under higher fluence conditions. Here, we found that the transconductance depended significantly on the proton irradiation effect and the gate voltage.
7
before and after protonn Figure S3. Contour pllots of transsconductancce (gm = dIDS D /dVG) of the devices b 1 cm-2, (b)11013 cm-2, an nd (c) 1014 cm c -2. irradiation with fluencces of (a)1012
8
4. Raman spectra of MoS2 flakes We measured Raman spectra of three samples (a newly prepared sample #1, and two old samples #2 and #3 that were irradiated with proton beam) of MoS2 films with another Raman spectrometer (XperRam 200, Nanobase) that used a laser (wavelength of 532 nm) as a source. The results are shown in Figure S4. In the data of Figure S4, we didn’t observe a noticeable shift in the Raman peaks. And, when we compare the data of Figure S4 with those in Figure 3(e) in the
Intensity (arb. unit.)
manuscript, we could see very similar peak positions among these data.
350
Sample #1 Sample #2 Sample #3
375 400 425 -1 Raman Shift (cm )
450
Figure S4. Raman spectra of three samples of MoS2 films. Sample #1 is a newly prepared sample. Samples #2 and #3 are old samples that were irradiated with a high proton beam fluence condition (Φ = 1014 cm-2) about four months ago.
9
5.
Stopping and Range of Ions in Matter (SRIM) analysis To understand the behavior of protons, we performed simulations using Stopping and Range of
Ions in Matter (SRIM) software.S3,S4 The structure of our MoS2 FET devices is MoS2 (2‒8 nm)/SiO2(270 nm)/Si (500 m). We found the protons with 10 nA current and 10 MeV energy deposited most of their energy near 700 m from the top surface (Figure S4) while some amount of energy can also be transferred to the SiO2 dielectric layer. This process creates electron-hole pairs
Energy trasfer (Arb. unit)
Energy transfer (Arb. unit)
and eventually induces charge traps in the oxide layer or at the interfaces.
0
~700 μm
0
100
200
300
Depth (nm)
200
400
400 600 Depth (m)
800
Figure S5. Energy loss depth profiles of irradiated protons calculated using SRIM. The inset image is a zoomed plot for the energy loss depth profile in the range of 0‒400 nm from the top surface.
10
6.
The IG-VG leakage plot of MoS2 FET We measured the gate leakage currents of MoS2 FETs. The result is shown in Figure S6. The
leakage current was found small enough as compared to the signal current range of A (see Figure 2 in the manuscript).
5 4
IG nA)
3 2
VDS = 0.1 V VDS = 0.2 V VDS = 0.3 V VDS = 0.4 V VDS = 0.5 V
1 0 -1
-20
-10
0 VG (V)
10
20
Figure S6. The leakage current of a MoS2 FET.
11
References S1.
Ghatak, S.; Pal, A. N.; Ghosh, A. Nature of Electronic States in Atomically Thin MoS2 Field-Effect Transistors. ACS Nano 2011, 5, 7707-7712.
S2.
Frindt, R. Single Crystals of MoS2 Several Molecular Layers Thick. J. Appl. Phys. 1966, 37, 1928-1929.
S3.
Ziegler, J. F.; Ziegler, M. D.; Biersack, J. P. SRIM-The Stopping and Range of Ions in Matter (2010) Nucl. Instrum. Methods Phys. Res., Sect. B 2010, 268, 1818-1823.
S4.
Ziegler, J. F.; Biersack, J. P. The Stopping and Range of Ions in Matter. Springer: 1985.
12
