MoS2 nanosheet phototransistors with thickness- modulated optical ...
Supporting Information
MoS2 nanosheet phototransistors with thicknessmodulated optical energy gap Hee Sung Lee,† Sung-Wook Min,† Youn-Gyung Chang, † Park Min Kyu, ‡ Taewook Nam,# Hyungjun Kim,# Jae Hoon Kim,† Sunmin Ryu‡ and Seongil Im*,† †
Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, Korea
‡
Department of Applied Chemistry, Kyung Hee University, Yongin, Gyeonggi 446-701,
Korea #
School of Electrical and Electronic Engineering, Yonsei University, Seoul, 120-749, Korea
*Tel.: 82-2-2123-2842, fax: 82-2-392-1592, e-mail: [email protected]
Materials and methods Exfoliation and characterization of MoS2 We prepared single- and a few-layer MoS2 nanosheets from bulk MoS2 (SPI supplies, natural molybdenite) on 285 nm SiO2 on p++-doped silicon substrate by using a standard micromechanical exfoliation scotch tape method. Our MoS2 nanosheet flakes were as sizable as ~10 µm in one side, so that a long 5 µm channel was possible in our device (W/L ratio was ~1). We have previously found that the total layer number of MoS2 nanosheet can be conjectured by optical microscope observation while it is then clearly identified by Raman spectroscopy.
Top-gate MoS2-based FET fabrication We made source (S) and drain (D) electrodes using photo-lithography and lift-off processes. First, we coated the lift-off layer (LOL 2000: Micro Chem) and photo-resist (PR) layers by spin casting. Then the substrate was exposed to ultraviolet (UV) light with photomask for our S/D electrode pattern. After developing patterns, we first deposited a 50 nmthick Au for S/D electrodes by DC sputtering system. The lift-off process was done with acetone and LOL remover. As a high-κ dielectric, 50 nm-thick Al2O3 layers were deposited by atomic layer deposition (ALD; ForAll Co., Ozone100A) at a temperature of 150 ℃. Finally, we made top-gate pattern with 50 nm-thick ITO through the photo lithography, DC sputtering and lift-off processes.
Electrical and Photoelectric Characterization All current-voltage (I–V) and photoelectric properties of our MoS2 transistors were measured using a semiconductor parameter analyzer (HP 4155C, Agilent Technologies) and
the photo-electric probing system that consists of a light source of 500 W Hg(Xe) arc lamp, a gratings monochromator covering the spectral range of 254 ∼ 1000 nm, and an optical fiber (core diameter of 200 µ m), delivering individual monochromatic light with an average illumination intensity of 0.1 mW cm-2.
Figure S1. Optical microscope images of exfoliated (a) double- and (b) triple-layer MoS2 flakes. Source/Drain/Gate-patterned images of our top-gate transistors with (c) double- and (d) triple-layer MoS2 nanosheets. For the channels of (c) and (d) transistors, we used the MoS2 flakes of (a) and (b), respectively.
Figure S2. Photo-induced transfer curves obtained from transparent top-gate nanosheet transistors with (a) single-layer, (b) double-layer, and (c) triple-layer MoS2, on which more than 50 serial wavelength photons were applied ranging from low 1.2 eV to high 2 eV in energy. Based on the series of photo-induced transfer characteristics, we measured the series of photo-induced threshold voltage shift ∆Vth(ε) and plotted ∆Qeff (ε) [= Cox∙∆Vth(ε)] with respect to the photon energy, ε. Quite abrupt voltage shift ∆Vth was observed at the onset of the energy band gap, due to the band-to-band light absorption. Experimental details are provided in Fig. S3.
Figure S3. (a) Photo-electric probing system to measure the optical energy gap (Eg) of MoS2-layer channel of our top-gate transistors. Hg(Xe) arc lamp, optical filters, gratings monochromator, and optical fiber (200µm diameter) were used to deliver the monochromatic light with an average intensity of 0.1 mW cm-2. (b) Band diagram of single-, double-, and triple-layer MoS2 under a gate bias for charge accumulation. (c) Illustration of Vth shift under the photons of different energies over Eg.
Figure S4. The voltage transfer curves of the double-layer MoS2-based resistive-type inverter with serially connected 22 MΩ resistor. The respective supply voltages (VDD) of 0.5, 1, 2, and 3 V were provided. The inset curves show the voltage gain values.
Figure S5. (a) Photo-induced voltage transfer curves of a resistive-type photo-inverter with single-layer MoS2 transistor and serially connected 22 MΩ resistor. (b) Voltage dynamics of the resistive-type photo-inverter under monochromatic green and UV lights.
Acknowledgement The authors acknowledge the financial support from NRF (NRL program, No. 20120000126), BK21 Project. H. S. Lee acknowledges the tuition support from the LOTTE fellowship. J. H. Kim acknowledges the financial support from NRF (program No. 20110000990).
MoS2 nanosheet phototransistors with thicknessmodulated optical energy gap Hee Sung Lee,† Sung-Wook Min,† Youn-Gyung Chang, † Park Min Kyu, ‡ Taewook Nam,# Hyungjun Kim,# Jae Hoon Kim,† Sunmin Ryu‡ and Seongil Im*,† †
Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, Korea
‡
Department of Applied Chemistry, Kyung Hee University, Yongin, Gyeonggi 446-701,
Korea #
School of Electrical and Electronic Engineering, Yonsei University, Seoul, 120-749, Korea
*Tel.: 82-2-2123-2842, fax: 82-2-392-1592, e-mail: [email protected]
Materials and methods Exfoliation and characterization of MoS2 We prepared single- and a few-layer MoS2 nanosheets from bulk MoS2 (SPI supplies, natural molybdenite) on 285 nm SiO2 on p++-doped silicon substrate by using a standard micromechanical exfoliation scotch tape method. Our MoS2 nanosheet flakes were as sizable as ~10 µm in one side, so that a long 5 µm channel was possible in our device (W/L ratio was ~1). We have previously found that the total layer number of MoS2 nanosheet can be conjectured by optical microscope observation while it is then clearly identified by Raman spectroscopy.
Top-gate MoS2-based FET fabrication We made source (S) and drain (D) electrodes using photo-lithography and lift-off processes. First, we coated the lift-off layer (LOL 2000: Micro Chem) and photo-resist (PR) layers by spin casting. Then the substrate was exposed to ultraviolet (UV) light with photomask for our S/D electrode pattern. After developing patterns, we first deposited a 50 nmthick Au for S/D electrodes by DC sputtering system. The lift-off process was done with acetone and LOL remover. As a high-κ dielectric, 50 nm-thick Al2O3 layers were deposited by atomic layer deposition (ALD; ForAll Co., Ozone100A) at a temperature of 150 ℃. Finally, we made top-gate pattern with 50 nm-thick ITO through the photo lithography, DC sputtering and lift-off processes.
Electrical and Photoelectric Characterization All current-voltage (I–V) and photoelectric properties of our MoS2 transistors were measured using a semiconductor parameter analyzer (HP 4155C, Agilent Technologies) and
the photo-electric probing system that consists of a light source of 500 W Hg(Xe) arc lamp, a gratings monochromator covering the spectral range of 254 ∼ 1000 nm, and an optical fiber (core diameter of 200 µ m), delivering individual monochromatic light with an average illumination intensity of 0.1 mW cm-2.
Figure S1. Optical microscope images of exfoliated (a) double- and (b) triple-layer MoS2 flakes. Source/Drain/Gate-patterned images of our top-gate transistors with (c) double- and (d) triple-layer MoS2 nanosheets. For the channels of (c) and (d) transistors, we used the MoS2 flakes of (a) and (b), respectively.
Figure S2. Photo-induced transfer curves obtained from transparent top-gate nanosheet transistors with (a) single-layer, (b) double-layer, and (c) triple-layer MoS2, on which more than 50 serial wavelength photons were applied ranging from low 1.2 eV to high 2 eV in energy. Based on the series of photo-induced transfer characteristics, we measured the series of photo-induced threshold voltage shift ∆Vth(ε) and plotted ∆Qeff (ε) [= Cox∙∆Vth(ε)] with respect to the photon energy, ε. Quite abrupt voltage shift ∆Vth was observed at the onset of the energy band gap, due to the band-to-band light absorption. Experimental details are provided in Fig. S3.
Figure S3. (a) Photo-electric probing system to measure the optical energy gap (Eg) of MoS2-layer channel of our top-gate transistors. Hg(Xe) arc lamp, optical filters, gratings monochromator, and optical fiber (200µm diameter) were used to deliver the monochromatic light with an average intensity of 0.1 mW cm-2. (b) Band diagram of single-, double-, and triple-layer MoS2 under a gate bias for charge accumulation. (c) Illustration of Vth shift under the photons of different energies over Eg.
Figure S4. The voltage transfer curves of the double-layer MoS2-based resistive-type inverter with serially connected 22 MΩ resistor. The respective supply voltages (VDD) of 0.5, 1, 2, and 3 V were provided. The inset curves show the voltage gain values.
Figure S5. (a) Photo-induced voltage transfer curves of a resistive-type photo-inverter with single-layer MoS2 transistor and serially connected 22 MΩ resistor. (b) Voltage dynamics of the resistive-type photo-inverter under monochromatic green and UV lights.
Acknowledgement The authors acknowledge the financial support from NRF (NRL program, No. 20120000126), BK21 Project. H. S. Lee acknowledges the tuition support from the LOTTE fellowship. J. H. Kim acknowledges the financial support from NRF (program No. 20110000990).
