Supporting Information - Pubs.acs

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Supporting InformationInfrared Detectable MoS2 Phototransistor and Its Application toArtificial Multi-Level Optic-Neural SynapseSeung-Geun Kim†, Seung-Hwan Kim‡, June Park§, Gwang-Sik Kim‡, Jae-Hyeun Park‡, KrishnaC. Saraswat , Jiyoung Kim , and Hyun-Yong Yu†, ‡,*†Departmentof Semiconductor Systems Engineering, Korea University, 145, Anam-ro,Seongbuk-gu, Seoul, 02841, Korea‡School of Electrical Engineering, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul, 02841,Korea§Department of Nano Semiconductor Engineering, Korea University, 145, Anam-ro, Seongbukgu, Seoul, 02841, Korea Department of Electrical Engineering, Stanford University, Stanford, CA, 94305, USA Department of Materials Science and Engineering, The University of Texas at Dallas,Richardson, Texas, 75080, USA* Address correspondence to [email protected]

Figure S1. AFM image and height profile of the MoS2 flake. Figure S1 shows AFM imageand height profile of the MoS2 flake used in this work. The thickness of the MoS2 flake wasmeasured as approximately 10.87 nm, which corresponds to approximately 15 layers of MoS2,because the thickness of one layer was 0.6–0.7 nm.Figure S2. Raman spectra of the exfoliated MoS2 flake. Two different Raman peaks wereobserved near 381.3 and 406.6 cm-1 under excitation by a 532 nm line, as shown in Figure S2.For comparison with a previous report, the MoS2 flake used in this work had 5 layers.2

Figure S3. a) Optical characterization of the Ge-gated MoS2 phototransistor under visiblelight (λ 520 and 655 nm) and the energy band diagrams for b) the vertical direction andc) the horizontal direction. In contrast to the optical characteristics under infrared light, the IDincreased by a factor of 1000 at VG -0.5 V and VD -0.5 V under visible light, as shown inFigure S3a. This is explained by the band diagrams. When the visible light was incident onto thedevice, electron–hole pairs were generated in both the Ge gate and the MoS2 channel, as shownin Figure S3b. The electron–hole pairs generated in the Ge gate obstructed the currentconduction through the MoS2 channel by modulating the electrostatic potential of the channel atthe same gate bias. However, the electron–hole pairs generated in the MoS2 channel induced alarge photocurrent, increasing the drain current. Although the two mechanisms yielded oppositeresults in the current-conduction characteristics of the phototransistor, a large increase in ID wasobserved, because the induced photocurrent at the MoS2 channel was the dominant factor, asshown in Figure S3c.Figure S4. a) Electrical transfer curves of the Ge-gated MoS2 phototransistor with 50 and100 nm-thick SiO2. b) An amount of threshold voltage shifting versus gate oxide thickness.An average amount of threshold voltage shifting by irradiated infrared light decreases from 238mV to 214 mV as the thickness of SiO2 increases inducting the capacitance reduction.3

Figure S5. Temporal responses of the device a) under 520 nm light and b) 655 nm light at aVD of 0.1 V and a VG of -0.5 V. The normalized drain current is plotted as a function ofmeasurement time durations and the rising and decaying time values can be extracted between10% and 90% of the increasing and decreasing drain current, respectively. The responses to thevisible light are similar to the results of previous studies: the rising time is 15.67 s for λ 520nm and 28.67 s for λ 655 nm, and the decay time is 52.91 s for λ 520 nm and 61.33 s for λ 655 nm.Figure S6. Basic synaptic behaviors of Ge-gated MoS2 phototransistor. a) Synaptic currentstriggered by a single gate pulse. b) Spike-timing-dependent plasticity (STDP) behavior.Figure S6a shows the decay of synaptic currents triggered by a single gate pulse of -30 V inamplitude and 50 ms in duration. Figure S6b shows the STDP behavior of the device. The draincurrent change ( Δ I/I0) measured at VD 0.5 V after presynaptic and postsynaptic spikes areapplied with a time difference, Δt tpost - tpre, where tpost and tpre are the time when thepresynaptic and postsynaptic pulses spike, respectively. If the presynaptic spike occurs before thepostsgynaptic spike (Δt 0), the device undergoes a potentiation. And, if the postsynaptic spikeoccurs before the presynaptic spike (Δt 0), the device undergoes a depression.4

Figure S7. Schematics showing the fabrication process for the MoS2/SiO2/Gephototransistor. To use p-Ge for the back-gate electrode, the conventional gate oxide, SiO2, wasdeposited on the Ge substrate via PE-CVD. The few-layer MoS2 was exfoliated using PDMStape via the micromechanical exfoliation method, because PDMS tape is effective for obtaininglarge and uniform flakes.Figure S8. ID0.5–VG characteristics of the device for extracting the threshold voltage. Thethreshold voltage was calculated using the equation ID K(VG ― VTH)2, where ID is the draincurrent, K is a constant, VG is the applied gate voltage, and VTH is the threshold voltage. Thus, inthe square root of the drain-current curve as a function of applied gate voltage, the VTH is derivedby the x-intercept points of the tangential lines in the linear region of the curves.5