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Infrared Detectable MoS Phototransistor and Its Application to Artificial Multi-Level Optic-Neural Synapse Seung-Geun Kim, Seung-Hwan Kim, June Park, Gwang-Sik Kim, JaeHyeun Park, Krishna C. Saraswat, Jiyoung Kim, and Hyun-Yong Yu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b03683 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Infrared Detectable MoS2 Phototransistor and Its Application to Artificial Multi-Level Optic-Neural Synapse

Seung-Geun Kim†, Seung-Hwan Kim‡, June Park§, Gwang-Sik Kim‡, Jae-Hyeun Park‡, Krishna C. Saraswat∥, Jiyoung Kim⊥, and Hyun-Yong Yu†, ‡,* †Department

of 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, Seongbuk-gu, 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]

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ABSTRACT Layered two-dimensional (2D) materials have entered the spotlight as promising channel materials for future optoelectronic devices owing to their excellent electrical and optoelectronic properties. However, their limited photodetection range caused by their wide bandgap remains a principal challenge in 2D layered materials-based phototransistors. Here, we developed a germanium (Ge)-gated MoS2 phototransistor that can detect light in the region from visible to infrared (λ = 520–1550 nm) using a detection mechanism based on band bending modulation. In addition, the Ge-gated MoS2 phototransistor is proposed as a multi-level optic-neural synaptic device, which performs both optical-sensing and synaptic functions on one device and is operated in different current ranges according to the light conditions: dark, visible, and infrared. This study is expected to contribute to the development of 2D material-based phototransistors and synaptic devices in next-generation optoelectronics.

Keywords: two-dimensional materials, MoS2 phototransistor, germanium gate, wide detection range, optic-neural synapse

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The first two-dimensional (2D)-layered material-based photodetector was implemented by P. Avouris and F. Xia in 20091 with graphene as the channel material. Graphene is a promising material for various photodetectors, allowing them to interact with light over a broad bandwidth from terahertz to ultraviolet wavelengths owing to its zero bandgap.2-4 However, its zero bandgap leads to a short photocarrier lifetime and high dark current,2 which is unfavorable for use as an efficient photodetector. After the development of the graphene photodetector, extensive research on photodetectors using transition-metal dichalcogenides, such as molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten disulfide (WS2), tungsten diselenide (WSe2), rhenium disulfide (ReS2), and rhenium diselenide (ReSe2), has been conducted due to their potential applications in nanoelectronics and optoelectronics.5-14 However, these phototransistors are usually limited to a narrow visible spectral range due to their wide bandgap. In recent years, the development of photodetectors capable of detection in the infrared region has become necessary for many fields, such as autonomous driving, military systems, freespace communication, and robot engineering.15-18 To increase the detection range of photodetectors based on 2D layered materials, there are two promising approaches: channelmaterial substitution19-21 and heterostructure formation.22-26 Although black phosphorus (BP) is a suitable channel material for infrared photodetectors, there are serious problems with its practical utilization in devices. BP is highly hygroscopic and easily oxidized in air, which makes device fabrication more difficult.27 Furthermore, although the heterojunction photodetector has high performance and a wide detection range, it is difficult to align different 2D materials. Therefore, it is necessary to develop a photodetector that can solve these significant problems. 3


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Here, we demonstrate a germanium (Ge)-gated MoS2 phototransistor with a wide detection range using a band bending modulation-based detection mechanism. This phototransistor can detect light from the visible to infrared range and has a rapid temporal photo-response time of 0.1 ms under infrared light condition. Furthermore, because the proposed device can perform both optical-sensing and synaptic operations, it can function as a multi-level optic-neural synapse device that operates in dark, visible, and infrared conditions.

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RESULTS AND DISCUSSION Device scheme. A schematic of the device under light illumination is shown in Figure 1a. Instead of the silicon (Si) substrate commonly used in MoS2-based photodetectors, Ge is adopted as a back-side gate electrode of the phototransistor to make use of its narrow bandgap (~0.66 eV) for infrared light detection. A silicon dioxide (SiO2) layer with thickness of 50 nm was deposited on a p-type Ge (p-Ge) substrate (Na = 1 x 1016 cm-3) to form a gate oxide. Multilayer MoS2 flakes were transferred onto the SiO2/p-Ge substrate using a polydimethylsiloxane (PDMS)-based mechanical exfoliation method to obtain uniform and large-area MoS2 flakes. As shown in Figure S1, the thickness of the MoS2 flake was measured to be 10.87 nm by using atomic force microscopy (AFM), which corresponds to approximately 15 layers.6 Raman spectroscopy of the MoS2 flake excited by a 532-nm line revealed two conventional peaks (E12g and A1g) at 381.3 and 406.6 cm-1, indicating the presence of multi-layer MoS2, as shown in Figure S2.28 A titanium (Ti) layer with thickness of 80 nm was deposited as a source/drain contact metal. Figure 1b shows a top-view optical microscope image of the fabricated MoS2 phototransistor device. The Ge-gated MoS2 phototransistor was electrically characterized by sweeping the gate voltage (VG) with a constant drain voltage (VD). In the dark condition, where the device was not irradiated by a light source, the Ge-gated MoS2 phototransistor exhibited ntype transfer characteristics with a high on/off current ratio of ~106, as shown in Figure 1c, which is similar to that of typical MoS2 field-effect transistors (FETs) with a Si gate.6-10 In addition, the device exhibited a low threshold voltage (VTH) of -0.553 V, which was extracted from the square root of the drain-current (ID) curve as a function of the gate voltage, as shown in the inset of Figure 1c.

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Operating mechanism and optical characteristics. Figure 2 shows a band diagram of the channel/gate oxide/gate electrode and drain/channel/source. As shown at the top of Figure 2a, the MoS2 channel and Ge gate have different work functions, which led to initial band bending at the MoS2/SiO2 and SiO2/p-Ge interfaces under thermal equilibrium condition. When infrared light (λ = 1550 nm) was incident onto the device, photons could reach the Ge gate because the MoS2 and SiO2 layers acted as transparent windows for the infrared light owing to their wide bandgap values of 1.2 eV and 9.0 eV, respectively. When the infrared light reached the SiO2/pGe interface, it was absorbed by the Ge gate due to its narrow bandgap of 0.66 eV. Therefore, as shown at the top of Figure 2b, electron–hole pairs were generated only at the Ge-gate region, and the generated electron–hole pairs were separated because of the initial band bending in the gate. While the generated holes moved to the bulk of the Ge, the generated electrons accumulated at the SiO2/Ge interface. The accumulated electrons reduced the band bending at the gate/oxide interface, and then the energy band of the MoS2 channel was pulled up. Consequently, as shown at the bottom of Figure 2a and b, current conduction under infraredlight condition became more difficult than under dark condition, resulting in an increase of VTH. Therefore, ID decreased owing to the reduction of electron injection at the same applied gate voltage. However, when the Si was used as a gate material, it could not absorb the infrared light due to its wide bandgap of 1.12 eV, as shown at the top of Figure 2c. Thus, ID was not changed, because the band diagram of the drain/channel/source was the same under dark and infrared conditions, as shown at the bottom of Figure 2c. For the optical measurements, light with various wavelengths (λ = 520 and 655 nm, i.e., visible, and λ =1550 nm, i.e., infrared) was irradiated perpendicular to the MoS2 channel region, as shown in Figure 1a. Figure 3a shows the ID–VG characteristics of the Ge-gated MoS2 phototransistor with and without irradiation of 1550 nm infrared light. With irradiation, the MoS2 phototransistor exhibited a 6


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positive VTH shift from -0.553 to -0.312 V (ΔVTH = 0.241 V, inset in Figure 3a) and a reduction of ID by a factor of 35 at VG = -0.5 V and VD = -0.5 V. As shown in Figure 3b, the Si-gated MoS2 phototransistor had identical electrical characteristics to the Ge-gated MoS2 phototransistor in the dark; however, the device did not respond to infrared light, owing to the wide bandgap of the Si gate. This result is supporting evidence for the Ge-gated MoS2 phototransistor responding to infrared light due to the Ge gate. When visible light (λ = 520 and 655 nm) was applied to the Ge-gated MoS2 phototransistor, ID increased by a factor of >1000 at VG = -0.5 V and VD = -0.5 V, as shown in Figure S3. Unlike previously reported devices, the proposed phototransistor can have a multi-level drain current at the same applied gate voltage because the drain current is shifted in the opposite direction depending on the type of irradiated light, i.e., visible or infrared. The responsivities for different wavelengths of 520 nm, 655 nm and 1550 nm are calculated using the following equation: |𝑅| = |(𝐼𝑙𝑖𝑔ℎ𝑡 𝑜𝑛 ― 𝐼𝑙𝑖𝑔ℎ𝑡 𝑜𝑓𝑓)/𝑃𝑙𝑖𝑔ℎ𝑡| = |𝐼𝑝ℎ𝑜𝑡𝑜/𝑃𝑙𝑖𝑔ℎ𝑡|

(1)

where R is the photoresponsivity, Ilight on is the drain current under illumination of light, Ilight off is the drain current without light, and Plight is the light power density. The responsivity under infrared condition (1550 nm) reaches its highest absolute value of 0.55 A/W at the power density of 127 mW/cm2, gate bias of 0.8 V, and source-drain bias of 0.5 V. Under visible condition, the responsivity for the 520 nm wavelength light reaches its highest value of 1.73 A/W at the power density of 159 mW/cm2, gate bias of 0.1 V, and source-drain bias of 0.5 V. Furthermore, the responsivity for the 655 nm wavelength light reaches its highest value of 2.18 A/W at the power density of 159 mW/cm2, gate bias of 0.15 V, and source-drain bias of 0.5 V. As shown in Figure S4, the effects of gate oxide thickness on the optical performance of the device has been also demonstrated. As the thickness of the gate oxide increase, it was confirmed that an amount of threshold voltage shifting by irradiated infrared light decreases 7


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due to the capacitance reduction. This result gives compelling experimental evidence of the operation mechanism of the Ge-gated MoS2 phototransistor. Figure 4a and b show the temporal photo-response characteristics measured at a wavelength of 1550 nm. The normalized drain current is plotted as a function of measurement time durations and the rising and decay time values can be extracted between 10% and 90% of the increasing and decreasing drain current, respectively. A rising time of 0.1 ms and decay time of 45 ms were obtained for the Ge-gated MoS2 phototransistor with VD = 0.1 V and VG = -0.5 V under infrared-light condition. In contrast to the rapid temporal photo-response under infrared-light condition, a slow temporal photo-response was seen under visible-light condition. Under visible-light condition, the rising time was 15.67 s for λ = 520 nm and 28.67 s for λ = 655 nm, and the decay time was 52.91 s for λ = 520 nm and 61.33 s for λ = 655 nm, as shown in Figure S5a and b. The reason for the difference in response times is directly caused by the material and region in which the light is absorbed. Because the MoS2 channel absorbing visible light has slow minority hole trapping and de-trapping at the surface-absorbed molecules, and several defect states, the temporal response time of our device to visible light is measured extremely slow like several previous studies on MoS2 photodetectors.8,29-31 On the other hand, the temporal response of our device to infrared light is very fast due to the vertically formed Ge/SiO2 interface. The vertically formed interface makes the transition length very short for the photocarrier, which is generated in the Ge gate, and the photocarrier can be efficiently collected and separated at the interface junction.32 As a result, the temporal photo-response under infrared-light condition is faster than that under visible-light condition. A comparison of the critical performance indicators is provided in Table 1.8,9,12,13,22-25,29,33-36

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Application to multi-level optic-neural synapse. MoS2 FETs have been widely studied for electronic synapses in order to mimic neurotransmitter release dynamics in chemical synapses, which emulate chemical synaptic functions such as potentiation and depression.37-39 Interface traps between MoS2 and SiO2 can lead to positive and negative shifts of VTH via the trapping and de-trapping of electrons, respectively, under applied gate voltage pulses,40 as shown in Figure 5a. Therefore, the Ge-gated phototransistor can also be used as an electronic synapse and the basic synaptic behaviors of the device are shown in Figure S6a and b. Figure S6a shows the decay of synaptic current, which was obtained by applying a single gate voltage pulse of -30 V in amplitude and 50 ms in duration, and the retention time was more than 1000 s. In Figure S6b, the spike-time-dependent plasticity (STDP) behavior is also confirmed to verify that this device can be applied to a neural network. The drain current change (ΔI/I0) was determined according to the relative difference of spike timing between the presynaptic and postsynaptic spikes , as in biological synapses. The STDP behavior of the device is well fitted with exponential decay functions, verifying that STDP behaviors similar to those of biological synaptic systems can be achieved by the Ge-gated MoS2 phototransistor. In addition, the opticneural synaptic device, which performs both optical-sensing and synaptic operations, was recently developed to emulate the colored pattern recognition capability of the human vision system.41,42 Because the proposed Ge-gated MoS2 phototransistor is capable of both opticalsensing and synaptic operations, it can also be used as an optic-neural synapse device. For measurement of the optic-neural synapse behavior, the optical and electrical signals were inputted at the same time, as shown in Figure 5b. Figure 5c shows the input gate voltage pulse sequences used to mimic the excitatory and inhibitory synaptic transmissions. Negative and positive gate voltage pulses with amplitudes of -30 and 15 V, respectively, were applied, with an increasing number of pulses. These pulses were 10 and 50 ms apart, respectively, as shown 9


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in the inset. The adjacent pulse trains were separated by 10 s and the pulse sequence was applied to the back gate of our device. Figure 5d shows the results of synaptic measurements performed under different light conditions. We measured the drain-current values at VG = 0 to prevent additional electron trapping or de-trapping due to the applied gate voltage. Before the measurement, the positive gate voltage pulse was applied to fully filled interface traps with electrons. As shown in Figure 5d, the negative gate pluses increased the drain current due to the de-trapping of charges, whereas the positive gate pulses reduced the drain current due to the trapping of charges. The drain current was changed by factors of 5.38, 18.74, and 27.65 by the synaptic action under visible-light, dark, and infrared-light conditions, respectively. The large change provided more storage states and a clear distinction between adjacent states. In addition, the proposed device exhibited synapse operating characteristics in the nonoverlapping current range according to the optical conditions, because the directions of the ID change differed according to the type of irradiated light (visible or infrared). Thus, the device can have massive number of states using the various optical inputs and can function as a multilevel optic-neural synapse device. The results show that, the Ge-gated MoS2 phototransistor can achieve synaptic operation in a broad wavelength region from visible to infrared and that can perform both optical-sensing and synaptic operations in one device.

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CONCLUSION We developed a Ge-gated MoS2 phototransistor with broadband detection range. The suggested phototransistor can detect infrared light by using Ge as the gate material, which absorbs the infrared light and induces band shifting. In addition, the phototransistor detects visible light in the same manner as the typical MoS2 photodetectors. Therefore, the Ge-gated MoS2 phototransistor has a considerably wider detection range (from visible to infrared, i.e., 520–1550 nm) than the Si-gated MoS2 phototransistor. The detection mechanisms for visible and infrared light were successfully investigated. Regarding the temporal response, the proposed phototransistor exhibited an extremely fast rising time of 0.1 ms and a decay time of 45 ms for infrared light. The reason for the fast response time is the photoreaction occurs at the vertically formed Ge/SiO2 interface, which makes the transit length very short for the photocarrier. In the case of visible light, the device had similar optical properties comparable to those of conventional MoS2 photodetectors. In addition, the Ge-gated MoS2 phototransistor can perform both optical-sensing and synaptic operations and has a non-overlapping current range according to the optical conditions, i.e., dark, visible, or infrared. Therefore, the proposed device can be used as a multi-level optic-neural synapse device. Owing to these advantages, Ge-gated MoS2 phototransistors are expected to be used in next-generation 2D layered nanomaterial-based phototransistors and synaptic devices in the field of optoelectronics.

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METHODS Device fabrication. A p-type Ge substrate was purchased from Semiconductor Wafer Inc. The 50-nm-thick SiO2 layer as a gate oxide was deposited via plasma-enhanced chemical vapor deposition (PE-CVD), and then few-layer MoS2 was transferred onto the SiO2/Ge substrate via PDMS by using the micromechanical exfoliation method. After the MoS2 was transferred, the Au/Ti (40 nm /80 nm) contacts were fabricated using electron-beam evaporation and a mask aligner. Finally, the devices were annealed at 150 ℃ for 30 min to improve the contact quality (Figure S7).

Characterization of MoS2 flakes. Raman fourier transform infrared spectroscopy (LabRam ARAMIS IR2, Horiba Jobin Yvon) and AFM (XE-100, Park systems) were performed to confirm the existence of the MoS2 flakes and measure their thickness and number of layers, respectively. The Raman spectroscopy was performed with an excitation wavelength of 532 nm and a spatial resolution of 1 µm. The AFM was performed with lateral and vertical resolutions of 2–3 and 0.1 Å, respectively.

Electrical and optical characterization of the device. The electrical properties of the fabricated devices were measured using a Keithley 4200-SCS with laser irradiation at various wavelengths (520, 655, and 1550 nm) and without laser irradiation. The threshold voltages were calculated using the square root of the drain-current (ID) curve with respect to the gate voltage (VG). Details are shown in Figure S8.

Data availability. The data that support the findings of this study are available from the corresponding authors upon request. 12


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SUPPORTING INFORMATION The Supporting Information is available free of change on the ACS Publications website, which includes AFM images, Raman spectra, sample fabrication, method of extracting threshold voltage, and additional experimental data for comparison. ACKNOWLEDGEMENTS This research was supported by Nano-Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning under Grant 2016M3A7B4910426 and was supported in part by the Basic Science Research Program within the Ministry of Science, ICT, and Future Planning through the NRF of Korea under Grant 2017R1A2B4006460. AUTHOR CONTRIBUTIONS S.-G K. and H.-Y. Y. conceived and designed the experiments. S.-G K., G.-S. K., S.-H. K., and J. P. contributed to the experimental design and device fabrication. S.-G. K. fabricated the device and conducted the electrical and optical measurements. S.-H. K. and J. P. helped with the device fabrication. S.-G. K., G.-S. K., K. C. S. and J. K. analyzed the data. H.-Y. Y. supervised the research. All the authors discussed the results and commented on the manuscript. COMPETING FINANCIAL INTERESTS. The authors declare no competing financial interests.

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Figure 1. MoS2 phototransistor with a p-type Ge gate. a) Schematic illustration of the Gegated MoS2 phototransistor under incident light (520, 655, and 1550 nm). b) Optical microscopy image of the multi-layer MoS2 device with Au/Ti as source and drain electrodes. c) ID–VG characteristics of the MoS2 transistors. The inset shows the method used to extract the threshold voltage from the square root of the drain-current curve as a function of the gate voltage.

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Figure 2. Energy band diagram of the MoS2 phototransistor in the vertical direction (top panel) and in the horizontal direction (bottom panel) a) without irradiation using a Ge gate, b) with infrared light (1550 nm) using a Ge gate, and c) with infrared light (1550 nm) using a Si gate.

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Figure 3. Optical characterization of MoS2 phototransistors with and without 1550 nm infrared light. a) ID–VG characteristics of the Ge-gated MoS2 phototransistor. The inset shows the threshold-voltage shift according to the incident light. b) ID–VG characteristics of the Sigated MoS2 phototransistor.

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Figure 4. Temporal response of the device. a) Decaying and b) rising characteristics under 1550 nm light, measured in units of 0.1 ms.

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Table 1. Performance Comparison of Optoelectronic Parameters in Various Phototransistors wavelength [nm]

device visible condition infrared condition

520 ~ 1550

monolayer MoS2 monolayer MoS2 MoS2 p-n junction MoS2 with thickness modulation Perovskite/MoS2/APTES PbS/MoS2 Graphene/MoS2 WSe2/Graphene/MoS2 bilayer MoS2 Multi-layer MoS2 Multi-layer MoS2 PPh3/Multi-layer MoS2 MoS2 with triboelectronic nanogenerator

400 ~ 800 450 ~ 800 300 ~ 700 365 ~ 680 520 ~ 850 800 ~ 1340 400 ~ 1000 400 ~ 940 532 ~ 1070 380 ~ 800 450 ~ 850 520 ~ 980 365 ~ 780

Ge-gated MoS2

response time [ms] 15670 ~ 61330 0.1 ~ 45 4000 ~ 9000 50 100 ~ 200 300 ~ 1500 4000 ~ 11000 7.8 0.13 0.03~0.05 28500 ~ 404700 0.07 ~ 0.11 8310 ~ 14100 13400 ~ 34500

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responsivity [A/W] 2.18 - 0.55 880 7.5 x 10-3 5.07 2.12 x 104 4.5 x 104 3 104 5.2 0.57 0.1 6.13 x 105 727.87

reference this work 8 9 12 13 22 23 24 25 29 33 34 35 36

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Figure 5. Synaptic operation of Ge-gated MoS2 phototransistor. a) Band diagram showing the traps at the MoS2/SiO2 interface according to the applied gate pulse. b) Schematic of the multi-level optic-neural synapse device based on the Ge-gated MoS2 phototransistor. The light sources were lasers with wavelengths of 520 and 1550 nm. c) Positive and negative pulse sequences applied to the back gate for mimicking synaptic transmissions. d) Potentiation and depression curves obtained under different light conditions: visible (λ = 520 nm), dark, and infrared (λ = 1550 nm).

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Table of Contents

Title: Infrared Detectable MoS2 Phototransistor and Its Application to Artificial Multi-Level Optic-Neural Synapse Seung-Geun Kim, Seung-Hwan Kim, June Park, Gwang-Sik Kim, Jae-Hyeun Park, Krishna C. Saraswat, Jiyoung Kim, and Hyun-Yong Yu* Keyword: two-dimensional materials, MoS2 phototransistor, germanium gate, wide detection range, optic-neural synapse

A Ge-gated MoS2 phototransistor is investigated to achieve a wide detection range from visible to infrared (520-1550 nm) for various optoelectronic nanodevices. The proposed device can be used as a multi-level optic-neural synapse device, because it can perform both opticalsensing and synaptic operations with a non-overlapping current range according to the optical conditions.

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Infrared Detectable MoS2 Phototransistor and Its ... - ACS Publications

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