Browsing by Author "Li, Ming-Yang"

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A Numerical Study of Si-TMD Contact with n/p Type Operation and Interface Barrier Reduction for Sub-5 nm Monolayer MoS2 FET
(2016-01-01)
Tang, Ying-Tsan
;
Li, Kai-Shin
;
Li, Lain-Jong
;
Li, Ming-Yang
;
Lin, Chang-Hsien
An atomic-scale numerical study of Si contact with transition metal dichalcogenides (TMD) semiconductor materials is proposed by first-principles simulation for the first time. The monolayer MoS2 channel can be operated as both of n- and p-type FET by properly doping Si S/D to adjust the TMD channel potential. The gradient MoSx junction of dichalcogenide vacancies enables Si-MoS2 contact resistance lower than 100 Omega-mu m for interface Schottky barrier height reduction. The compact Si-MoS2 interface study can potentially provide monolayer TMD contact design guideline for the sub-5 nm TMD FET fabrication technology.
Circular Dichroism Control of Tungsten Diselenide (WSe2) Atomic Layers with Plasmonic Metamolecules
(2018-05-09)
Lin, Hsiang-Ting
;
Chang, Chiao-Yun
;
Cheng, Pi-Ju
;
Li, Ming-Yang
;
Cheng, Chia-Chin
Controlling circularly polarized (CP) states of light is critical to the development of functional devices for key and emerging applications such as display technology and quantum communication, and the compact circular polarization-tunable photon source is one critical element to realize the applications in the chip-scale integrated system. The atomic layers of transition metal dichalcogenides (TMDCs) exhibit intrinsic CP emissions and are potential chiroptical materials for ultrathin CP photon sources. In this work, we demonstrated CP photon sources of TMDCs with device thicknesses approximately 50 nm. CP photoluminescence from the atomic layers of tungsten diselenide (WSe2) was precisely controlled with chiral metamolecules (MMs), and the optical chirality of WSe2 was enhanced more than 4 times by integrating with the MMs. Both the enhanced and reversed circular dichroisms had been achieved. Through integrations of the novel gain material and plasmonic structure which are both low-dimensional, a compact device capable of efficiently manipulating emissions of CP photon was realized. These ultrathin devices are suitable for important applications such as the optical information technology and chip-scale biosensing.
Circular Polarized Emission of Tungsten Diselenide (WSe2) Atomic Layers with Plasmonic Metasurface
(2018-01-01)
Lin, Hsiang-Ting
;
Chang, Chiao-Yun
;
Cheng, Pi-Ju
;
Li, Ming-Yang
;
Cheng, Chia-Chin
A circularly polarized light source with ultrathin geometry were demonstrated by integrating the WSe2 atomic layers with plasmonic chiral metasurfaces. The intrinsic chiral emission of WSe2 was manipulated by chiral metasurfaces to achieve emission circular dichroism more than 25%.
Demonstration of 40-nm Channel Length Top-Gate p-MOSFET of WS2 Channel Directly Grown on SiOx/Si Substrates Using Area-Selective CVD Technology
(2019-12-01)
Chung, Yun-Yan
;
Lu, Kuan-Cheng
;
Cheng, Chao-Ching
;
Li, Ming-Yang
;
Lin, Chao-Ting
For high-volume manufacturing of 2-D transistors, area-selective chemical reaction deposition (CVD) growth is able to provide good-quality 2-D layers and may be more effective than exfoliation from bulk crystals or wet/dry transfer of large-area as-grown 2-D layers. We have successfully grown continuous and uniform WS2 film comprising around seven layers by area-selective CVD approach using patterned tungsten source/drain metals as the seeds. The growth mechanism is inferred and supported by the transmission electron microscope (TEM) images, as well. The first top-gate MOSFETs of CVD-WS2 channels on SiOx/Si substrates are demonstrated to have good short channel electrical characteristics: ON-/OFF-ratio of 10(6), a subthreshold swing of 97 mV/decade, and nearly zero drain-induced barrier lowering (DIBL).
Effective N-methyl-2-pyrrolidone wet cleaning for fabricating high-performance monolayer MoS2 transistors
(2019-02-01)
Chen, Po-Chun
;
Lin, Chih-Pin
;
Hong, Chuan-Jie
;
Yang, Chih-Hao
;
Lin, Yun-Yan
Two-dimensional semiconductors, such as MoS2 are known to be highly susceptible to diverse molecular adsorbates on the surface during fabrication, which could adversely affect device performance. To ensure high device yield, uniformity and performance, the semiconductor industry has long employed wet chemical cleaning strategies to remove undesirable surface contaminations, adsorbates, and native oxides from the surface of Si wafers. A similarly effective surface cleaning technique for two-dimensional materials has not yet been fully developed. In this study, we propose a wet chemical cleaning strategy for MoS2 by using N-methyl-2-pyrrolidone. The cleaning process not only preserves the intrinsic properties of monolayer MoS2, but also significantly improves the performance of monolayer MoS2 field-effect-transistors. Superior device on current of 12 Am-1 for a channel length of 400 nm, contact resistance of 15 km, field-effect mobility of 15.5 cm(2)V(-1)s(-1), and the average on-off current ratio of 10(8) were successfully demonstrated
Epitaxial growth of a monolayer WSe2-MoS2 lateral p-n junction with an atomically sharp interface
(2015-07-31)
Li, Ming-Yang
;
Shi, Yumeng
;
Cheng, Chia-Chin
;
Lu, Li-Syuan
;
Lin, Yung-Chang
Two-dimensional transition metal dichalcogenides (TMDCs) such as molybdenum sulfide MoS2 and tungsten sulfide WSe2 have potential applications in electronics because they exhibit high on-off current ratios and distinctive electro-optical properties. Spatially connected TMDC lateral heterojunctions are key components for constructing monolayer p-n rectifying diodes, light-emitting diodes, photovoltaic devices, and bipolar junction transistors. However, such structures are not readily prepared via the layer-stacking techniques, and direct growth favors the thermodynamically preferred TMDC alloys. We report the two-step epitaxial growth of lateral WSe2-MoS2 heterojunction, where the edge of WSe2 induces the epitaxial MoS2 growth despite a large lattice mismatch. The epitaxial growth process offers a controllable method to obtain lateral heterojunction with an atomically sharp interface.
Evidence of indirect gap in monolayer WSe2
(2017-10-13)
Hsu, Wei-Ting
;
Lu, Li-Syuan
;
Wang, Dean
;
Huang, Jing-Kai
;
Li, Ming-Yang
Monolayer transition metal dichalcogenides, such as MoS2 and WSe2, have been known as direct gap semiconductors and emerged as new optically active materials for novel device applications. Here we reexamine their direct gap properties by investigating the strain effects on the photoluminescence of monolayer MoS2 and WSe2. Instead of applying stress, we investigate the strain effects by imaging the direct exciton populations in monolayer WSe2-MoS2 and MoSe2-WSe2 lateral heterojunctions with inherent strain inhomogeneity. We find that unstrained monolayer WSe2 is actually an indirect gap material, as manifested in the observed photoluminescence intensity-energy correlation, from which the difference between the direct and indirect optical gaps can be extracted by analyzing the exciton thermal populations. Our findings combined with the estimated exciton binding energy further indicate that monolayer WSe2 exhibits an indirect quasiparticle gap, which has to be reconsidered in further studies for its fundamental properties and device applications.
First demonstration of 40-nm channel length top-gate WS2 pFET using channel area-selective CVD growth directly on SiOx/Si substrate
(2019-01-01)
Cheng, Chao-Ching
;
Chung, Yun-Yan
;
Li, Ming-Yang
;
Lin, Chao-Ting
;
Li, Chi-Feng
Area-selective channel material growth for 2D transistors is more desirable for volume manufacturing than exfoliation or wet/dry transfer after large area growth. We demonstrate the first top-gate WS(2)p-channel field-effect transistors (p-FETs) fabricated on SiOx/Si substrate using channel area-selective CVD growth. Smooth and uniform WS2 comprising approximately 6 layers was formed by area-selective CVD growth in which a patterned tungsten-source/drain served as the seed for WS2 growth. For a 40 nm gate length transistor, the device has impressive electrical characteristics: on/off ratio of similar to 10(6), a S.S. of similar to 97 mV/dec.. and nearly zero DIBL.
Layer-Dependent and In-Plane Anisotropic Properties of Low-Temperature Synthesized Few-Layer PdSe2 Single Crystals
(2020-04-28)
Lu, Li-Syuan
;
Chen, Guan-Hao
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Cheng, Hui-Yu
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Chuu, Chih-Piao
;
Lu, Kuan-Cheng
Palladium diselenide (PdSe2), a peculiar noble metal dichalcogenide, has emerged as a new two-dimensional material with high predicted carrier mobility and a widely tunable band gap for device applications. The inherent in-plane anisotropy endowed by the pentagonal structure further renders PdSe2 promising for novel electronic, photonic, and thermoelectric applications. However, the direct synthesis of few-layer PdSe2 is still challenging and rarely reported. Here, we demonstrate that few-layer, single-crystal PdSe2 flakes can be synthesized at a relatively low growth temperature (300 degrees C) on sapphire substrates using low-pressure chemical vapor deposition (CVD). The well-defined rectangular domain shape and precisely determined layer number of the CVD-grown PdSe2 enable us to investigate their layer-dependent and in-plane anisotropic properties. The experimentally determined layer-dependent band gap shrinkage combined with first-principle calculations suggest that the interlayer interaction is weaker in few-layer PdSe2 in comparison with that in bulk crystals. Field-effect transistors based on the CVD-grown PdSe2 also show performances comparable to those based on exfoliated samples. The low-temperature synthesis method reported here provides a feasible approach to fabricate high-quality few-layer PdSe2 for device applications.
Ledge-directed epitaxy of continuously self-aligned single-crystalline nanoribbons of transition metal dichalcogenides
(1970-01-01)
Aljarb, Areej
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Fu, Jui-Han
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Hsu, Chih-Chan
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Chuu, Chih-Piao
;
Wan, Yi
Two-dimensional transition metal dichalcogenide nanoribbons are touted as the future extreme device downscaling for advanced logic and memory devices but remain a formidable synthetic challenge. Here, we demonstrate a ledge-directed epitaxy (LDE) of dense arrays of continuous, self-aligned, monolayer and single-crystalline MoS(2)nanoribbons on beta-gallium (iii) oxide (beta-Ga2O3) (100) substrates. LDE MoS(2)nanoribbons have spatial uniformity over a long range and transport characteristics on par with those seen in exfoliated benchmarks. Prototype MoS2-nanoribbon-based field-effect transistors exhibit high on/off ratios of 10(8)and an averaged room temperature electron mobility of 65 cm(2) V-1 s(-1). The MoS(2)nanoribbons can be readily transferred to arbitrary substrates while the underlying beta-Ga(2)O(3)can be reused after mechanical exfoliation. We further demonstrate LDE as a versatile epitaxy platform for the growth of p-type WSe(2)nanoribbons and lateral heterostructures made of p-WSe(2)and n-MoS(2)nanoribbons for futuristic electronics applications. Aligned arrays of single-crystalline monolayer TMD nanoribbons with high aspect ratios, as well as their lateral heterostructures, are realized, with the growth directed by the ledges on the beta-Ga(2)O(3)substrate. This approach provides an epitaxy platform for advanced electronics applications of TMD nanoribbons.
MOS2 U-shape MOSFET with 10 nm Channel Length and Poly-Si Source/Drain Serving as Seed for Full Wafer CVD MOS2 Availability
(2016)
Li, Kai-Shin
;
Wu, Bo-Wei
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Li, Lain-Jong
;
Li, Ming-Yang
;
Cheng, Chia-Chin Kevin
A U-shape MOS2 pMOSFET with 10nm channel and poly-Si source/drain is demonstrated. The fabrication process is simple. Because the Si S/D serves as the nucleation seed for CVD MOS2 deposition, thin MOS2 is well deposited in the channel region any where over the fully scale oxide coated Si wafer. This is a big step forward toward a low cost multi-layer stacked TMD IC technology.
Nonvolatile molecular memory with the multilevel states based on MoS2 nanochannel field effect transistor through tuning gate voltage to control molecular configurations
(2020-04-17)
Lan, Yann-Wen
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Hong, Chuan-Jie
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Chen, Po-Chun
;
Lin, Yun-Yan
;
Yang, Chih-Hao
A new flexible memory element is crucial for mobile and wearable electronics. A new concept for memory operation and innovative device structure with new materials is certainly required to address the bottleneck of memory applications now and in the future. We report a new nonvolatile molecular memory with a new operating mechanism based on two-dimensional (2D) material nanochannel field-effect transistors (FETs). The smallest channel length for our 2D material nanochannel FETs was approximately 30 nm. The modified molecular configuration for charge induced in the nanochannel of the MoS2 FET can be tuned by applying an up-gate voltage pulse, which can vary the channel conductance to exhibit memory states. Through controlling the amounts of triggered molecules through either different gate voltage pulses or gate duration time, multilevel states were obtained in the molecular memory. These new molecular memory transistors exhibited an erase/program ratio of more than three orders of current magnitude and high sensitivity, of a few picoamperes, at the current level. Reproducible operation and four-level states with stable retention and endurance were achieved. We believe this prototype device has potential for use in future memory devices.
Observation of Switchable Photoresponse of a Monolayer WSe2-MoS2 Lateral Heterostructure via Photocurrent Spectral Atomic Force Microscopic Imaging
(2016-06)
Son, Youngwoo
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Li, Ming-Yang
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Cheng, Chia-Chin
;
Wei, Kung-Hwa
;
Liu, Pingwei
In the pursuit of two-dimensional (2D) materials beyond graphene, enormous advances have been made in exploring the exciting and useful properties of transition metal dichalcogenides (TMDCs), such as a permanent band gap in the visible range and the transition from indirect to direct band gap due to 2D quantum confinement, and their potential for a wide range of device applications. In particular, recent success in the synthesis of seamless monolayer lateral heterostructures of different TMDCs via chemical vapor deposition methods has provided an effective solution to producing an in-plane pn junction, which is a critical component in electronic and optoelectronic device applications. However, spatial variation of the electronic and optoelectonic properties of the synthesized heterojunction crystals throughout the homogeneous as well as the lateral junction region and the charge carrier transport behavior at their nanoscale junctions with metals remain unaddressed. In this work, we use photocurrent spectral atomic force microscopy to image the current and photocurrent generated between a biased PtIr tip and a monolayer WSe2MoS2 lateral heterostructure. Current measurements in the dark in both forward and reverse bias reveal an opposite characteristic diode behavior for WSe2 and MoS2, owing to the formation of a Schottky barrier of dissimilar properties. Notably, by changing the polarity and magnitude of the tip voltage applied, pixels that show the photoresponse of the heterostructure are observed to be selectively switched on and off, allowing for the realization of a hyper-resolution array of the switchable photodiode pixels. This experimental approach has significant implications toward the development of novel optoelectronic technologies for regioselective photodetection and imaging at nanoscale resolutions. Comparative 2D Fourier analysis of physical height and current images shows high spatial frequency variations in substrate/MoS2 (or WSe2) contact that exceed the frequencies imposed by the underlying substrates. These results should provide important insights in the design and understanding of electronic and optoelectronic devices based on quantum confined atomically thin 2D lateral heterostructures.
Spectroscopic Signatures for Interlayer Coupling in MoS2-WSe2 van der Waals Stacking
(2014-09-01)
Chiu, Ming-Hui
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Li, Ming-Yang
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Zhang, Wengjing
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Hsu, Wei-Ting
;
Chang, Wen-Hao
Stacking of MoS2 and WSe2 monolayers is conducted by transferring triangular MoS2 monolayers on top of WSe2 monolayers, all grown by chemical vapor deposition (CVD). Raman spectroscopy and photoluminescence (PL) studies reveal that these mechanically stacked monolayers are not closely coupled, but after a thermal treatment at 300 degrees C, it is possible to produce van der Waals solids consisting of two interacting transition metal dichalcogenide (TMD) monolayers. The layer-number sensitive Raman out-of-plane mode A(1g)(2) for WSe2 (309 cm(-1)) is found sensitive to the coupling between two TMD monolayers. The presence of interlayer excitonic emissions and the changes in other intrinsic Raman modes such as E \'\' for MoS2 at 286 cm(-1) and A(1g)(2) for MoS2 at around 463 cm(-1) confirm the enhancement of the interlayer coupling.
TMD FinFET with 4 nm Thin Body and Back Gate Control for Future Low Power Technology
(2015)
Chen, Min-Cheng
;
Li, Kai-Shin
;
Li, Lain-Jong
;
Lu, Ang-Yu
;
Li, Ming-Yang
A 4 nm thin transition-metal dichalcogenide (TMD) body FinFET with back gate control is proposed and demonstrated for the first time. The TMD FinFET channel is deposited by CVD. Hydrogen plasma treatment of TMD is employed to lower the series resistance for the first time. The 2 nm thin back gate oxide enables 0.5 V of Vth shift with 1.2 V change in back bias for correcting device variations and dynamically configuring a device as a high performance or low leakage device. TMD can potentially provide sub-nm thin monolayer body needed for 2 nm node FinFET.