Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session D51: Focus Session: Beyond Graphene Devices: Function, Fabrication, and Characterization I |
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Sponsoring Units: DMP Chair: Marc Bockrath, University of California, Riverside Room: Mile High Ballroom 1E |
Monday, March 3, 2014 2:30PM - 2:42PM |
D51.00001: High-Performance n-type and p-type WSe$_{2}$ Field Effect Transistors with Ionic-Liquid Gated Graphene Electrodes Hsun Jen Chuang, Xuebin Tan, Mark Ming-Cheng Cheng, Nirmal Jeevi Ghimire, Jiaqiang Yan, David Mandrus, Bhim Chamlagain, Meeghage Madusanka Perera, Zhixian Zhou We report the application of graphene as a work-function-tunable electrode material for few-layer WSe$_{2}$ field-effect transistors (FETs). By tuning the carrier density of graphene at the graphene/WSe$_{2}$ contacts using an extremely-large-capacitance ionic liquid gate, we have successfully achieved low resistance Ohmic contacts and high ON-current for both holes and electrons in WSe$_{2}$ FETs. The extrinsic electron and hole mobility values increase with decreasing temperature reaching $\approx $ 300 cm$^{2}$V$^{-1}$s$^{-1}$ at 77 K when the graphene contacts are highly n- and p-doped by large positive and negative ionic-liquid gate voltages, respectively, indicating that the intrinsic phonon-limited mobility is approached for both electrons and holes in graphene contacted few-layer WSe$_{2}$. We attribute the enhanced device performance to the drastic reduction of the Schottky barrier height via tuning the work function of graphene electrodes to align with the conduction and valence band edges of WSe$_{2}$ by an ionic liquid gate. This work was supported by NSF (DMR-1308436). [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D51.00002: Contacts and transport characteristics of few-layer transition metal dichalcogenides Junjie Wang, Jing Li, Jacob Shevrin, An Nguyen, Tom Mallouk, J. Zhu, Daniel Rhodes, Luis Balicas, K. Watanabe, T. Taniguchi Two-dimensional layered transition metal dichalcogenides (TMDs) are potentially useful for electronic and optoelectronic applications. However, the lack of reliable methods to make ohmic contacts has been a major challenge. This work addresses two aspects of this challenge, i.e. interface cleanness and conductivity of the material in the contact area. Using gentle Ar ion milling immediately before the deposition of metal electrodes, we can completely remove polymer residue from prior lithography without significantly damaging the few-layer TMD sheet. Gate stacks made of Au and HfO$_2$ films can inject carriers up to 3$\times$10$^{13}$ cm$^{-2}$. We make van der Pauw devices of few-layer ($<$ 5 L) TMD (MoS$_2$, WS$_2$, WSe$_2$) sheets using Ti/Au contacts with area $<$ 2 (um)$^2$ and observe contact resistance less than 10 k$\Omega$ at high carrier densities, where the sheet conductance is well above 2e$^2$/h. We eliminate hysteresis in the transfer curve of TMD devices by pulsing the gate voltage. Ambipolar conduction is observed in WSe$_2$ devices, with an on/off ratio exceeding 10$^6$ for both electrons and holes. WSe$_2$ devices supported on h-BN show field-effect (hole) mobility $>$ 100 cm$^2$/(Vs) at 300K. We discuss the effects of the various approaches taken above. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D51.00003: Effect of interfaces on electron transport properties of MoS2--Au Contacts Maral Aminpour, Prokop Hapala, Duy Le, Pavel Jelinek, Talat S. Rahman Single layer MoS$_{2}$ is a promising material for future electronic devices such as transistors since it has good transport characteristics with mobility greater than 200 cm$^{-1}$V$^{-1}$s$^{-1}$ and on-off current ratios up to 10$^{8}$ [1]. However, before MoS$_{2}$ can become a mainstream electronic material for the semiconductor industry, the design of low resistive metal-semiconductor junctions as contacts of the electronic devices needs to be addressed and studied systematically. We have examined the effect of Au contacts on the electronic transport properties of single layer MoS$_{2}$ using density functional theory in combination with the non-equilibrium Green's function method. The Schottky barrier between Au contact and MoS$_{2}$, transmission spectra, and I-V curves will be reported and discussed as a function of MoS$_{2}$ and Au interfaces of varying geometry. \\[4pt] [1] B. Radisavljevic et al., Nature Nanotechnology \textbf{6}, 147 - 150 (2011). [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:42PM |
D51.00004: to be determined by you Invited Speaker: Ali Javey |
Monday, March 3, 2014 3:42PM - 3:54PM |
D51.00005: Vertical Field-Effect Transistor Based on Graphene-Transition Metal Dichalcogenides Heterostructures Jatinder Kumar, Hui-Chun Chien, Matthew Z. Bellus, David L. Sicilian, Davis St. Aubin, Hsin-Ying Chiu The remarkable properties of graphene has made it possible to create transistors just few atoms thick. A new development was that the other two-dimensional materials can be stacked on it with atomic layer precision, creating numerous heterostructures on demand. Here, novel vertical field-effect transistor composed of graphene- transition metal dichalcogenides (TMDs) heterostructures is fabricated and characterized at various temperatures. Due to ultrathin nature of these transistors, they present the ultimate limit for electron transport in heterostructures. Tunneling and thermionic transport characteristics are studied among different graphene-TMDs heterostructures. Their electronic properties have been investigated and can be used in vast range of devices. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D51.00006: High performance MoS$_{2}$ Field-Effect Transitors in a simple design S. Velez, O. Txoperena, L. Pietrobon, F. Casanova, L.E. Hueso The discovery of graphene, with its rich and fascinating physical properties, has opened up a new world where 2D-layered materials is the platform for developing powerful devices. Molybdenum disulfide (MoS$_{2})$, a 2D material belonging to the family of transition metal dichalcogenides, has an intrinsic band-gap and strong spin-orbit coupling which would complement those applications pristine graphene cannot cover. In particular, MoS$_{2}$ has been shown to work well as a field-effect transistor (FET), to exhibit superconductivity and valley polarization, demonstrating its potential in spintronics, valleytronics, or for designing other novel devices. Here we will show high performance of FETs based on monolayer and a few layer MoS$_{2\, }$working with a simple design (Si/SiO2 back gate and two terminal configuration). The FETs show room temperature ON/OFF ratios exceeding 10$^{7}$ and with mobilities higher than 10 cm$^{2}$V$^{-1}$s$^{-1}$. These values are among the best previously reported ones in similar designs and support the viability of building up simpler but still powerful devices which would allow large scale fabrication suitable for nanoelectronics. Further investigations exploiting both spintronics and valleytronics of layered MoS$_{2}$ are the final goal of this work. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D51.00007: MoS$_{2}$ Field-effect Transistors with Graphene/Metal Hetero-contacts Yuchen Du, Lingming Yang, Jingyun Zhang, Nathan Conrad, Han Liu, Peide Ye MoS$_{2}$, as one of the mostly studied transition-metal dichalcogenides, has already revealed a series of new physics and potential device applications. However, the performance of the MoS$_{2}$ field-effect transistors is limited by the large contact resistance at metal/MoS$_{2}$ interface due to the non-negligible Schottky barrier. In this study, n-type few-layer MoS$_{2}$ field-effect transistors with graphene/Ti as the metal contacts have been fabricated showing more than 160 mA/mm drain current at 1 $\mu$m gate length and on-off current ratio of 10$^{7}$. Different metal contacts (Ti, Ni, Au, and Pd) from low work function to high work function metals on MoS$_{2}$/graphene hetero contacts have been performed and studied. Moreover, for the first time, 2D Fermi-level pinning concept is introduced to understand the band alignment of hetero-structured metal/graphene/MoS$_{2}$ or other 2D semiconductor interfaces. Temperature dependent, noise, and stress measurement results will also be presented. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D51.00008: Electrical transport measurements on monolayer and few-layer MoSe2 and WSe2 Zaiyao Fei, Joe Finney, Yun Ling, Serkan Kasirga, Xiaodong Xu, David Cobden The two-dimensional monolayer semiconductors WSe2 and MoSe2 have recently been shown to have excellent optical properties, but their intrinsic electrical properties, relevant to many device applications, remain undetermined. This is due to the difficulty of obtaining good contacts and applying a sufficient electric field to induce carriers, especially at lower temperatures. We have investigated a range of device geometries and contact techniques aimed at improving the situation. So far we have achieved ambipolar gating of the linear-response conductance persisting at temperatures down to 4 K with contact resistance for both carrier of around 50 kiloohm at room temperature. Four terminal Hall-bar measurements have been made to separate the contact resistance, sheet resistivity, carrier density and mobility. Methods are being explored to eliminate large intrinsic contact noise. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D51.00009: Improved Carrier Mobility in Few-Layer MoS$_{2}$ Field-Effect Transistors with Ionic-Liquid Gating Meeghage Perera, Ming-Wei Lin, Hsun-Jen Chuang, Bhim Chamlagain, Chongyu Wang, Xuebin Tan, Mark Cheng, David Tom\'anek, Zhixian Zhou We report the fabrication of ionic liquid (IL) gated field-effect transistors (FETs) consisting of bilayer and few-layer MoS$_{2}$. Our transport measurements indicate that the electron mobility $\mu \approx $60~cm$^{2}$V$^{-1}$s$^{-1}$ at 250~K in ionic liquid gated devices exceeds significantly that of comparable back-gated devices. IL-FETs display a mobility increase from $\approx $100 cm$^{2}$V$^{-1}$s$^{-1}$ at 180~K to $\approx $220~cm$^{2}$V$^{-1}$s$^{-1}$ at 77 K in good agreement with the true channel mobility determined from four-terminal measurements, ambipolar behavior with a high ON/OFF ratio \textgreater 10$^{7}$ (10$^{4})$ for electrons (holes), and a near ideal sub-threshold swing of $\approx $50 mV/dec at 250 K. We attribute the observed performance enhancement, specifically the increased carrier mobility that is limited by phonons, to the reduction of the Schottky barrier at the source and drain electrode by band bending caused by the ultrathin ionic-liquid dielectric layer. In addition, graphene contacted MoS$_{2}$ FETs with IL-gating will also be discussed. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D51.00010: Electronic properties of few-layer MoS2 under an external electrical field Jose Eduardo Padilha, Hartwin Peelaers, Anderson Janotti, Chris G. Van de Walle MoS$_2$ is a two-dimensional (2D) layered material with a band gap in the 1-2 eV range, depending on the number of layers, and with promising applications in nanoelectronics. Field-effect transistors based on MoS$_2$ have been fabricated, displaying room-temperature electron mobility of $~200 cm^{2}V^{-1}s^{-1}$ and high on/off ratios on the order of $10^{8}$. In these devices, the effect of the electric field across the MoS$_2$ layers is important for device operation, so understanding these effects will aid in improving device performance. Here we use first-principles calculations to determine the electronic properties of MoS$_2$ layers as a function of an electric field applied perpendicular to the layers, representing the effect of gate electrodes. In the absence of an external field, the valence and conduction bands of multilayer MoS$_2$ are degenerate. However, an applied external field generates a gradient potential inside the material, breaking the symmetry between the layers, lifting the degeneracies, and modifying the band gap. We will discuss the evolution of the band gap and the various minima in the conduction band as a function of the field intensity and the number of layers. Work supported by FAPESP and by NSF-IMI. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D51.00011: Photocurrent studies on multi-walled WS$_{2}$ nanotube devices John Mathew, Gobinath Jegannathan, Sameer Grover, Sudipta Dubey, Pratiksha Dongare, Mandar Deshmukh Multi-walled WS$_{2}$ nanotubes were used for transport and photo response studies. The nanotubes were structurally characterized by scanning electron microscopy, high resolution transmission electron microscopy and Raman spectroscopy. Nanotube devices in field effect transistor geometry were fabricated on Si/SiO$_{2}$ substrates using nano lithography techniques. I-V measurements of these devices were carried out in ambient conditions. A confocal microscope system was used to study the photo response of the devices to 633 nm and 532 nm laser wavelengths using lock-in technique. Photocurrent map of the devices was obtained and studied as a function of applied bias voltage and gate voltage. The devices showed non-linear increase in photocurrent with increasing bias voltage and light intensity. Further, heterostructure devices of graphene and WS$_{2}$ nanotubes were fabricated for enhanced field effect behavior. Results of the photo response studies of these devices will also be presented. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D51.00012: Flexible metallic nanowires with self-adaptive contacts to semiconducting transition-metal dichalcogenide monolayers Junhao Lin, Ovidiu Cretu, Wu Zhou, Kazu Suenaga, Dhiraj Prasai, Kirill Bolotin, Nguyen Cuong, Minoru Otani, Susumu Okada, Andrew Lupini, Juan Idrobo, Dave Caudel, Arnold Burger, Jiaqiang Yan, Nirmal Ghimire, David Mandrus, Stephen Pennycook, Sokrates Pantelides We report direct electron-beam sculpting of ultrathin nanowires connecting designated points within semiconducting transition-metal dichalcogenide (TMDC) monolayer. In-situ electrical measurements reveal the nanowires are intrinsically metallic. The nanowires remain conducting and maintain structural integrity as they undergo continuous electron-beam-induced rotations and flexing, indicating their self-adaptive connections to the monolayers. The observed mechanical behavior is explained by density-functional-theory calculations, which further predict that the metal-semiconductor contacts could be Ohmic to p-type TMDC monolayers. These metallic nanowires can, therefore, serve as interconnects in future flexible nano-circuits fabricated entirely within a monolayer. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D51.00013: Interface effect on multilayer tungsten disulfide device caused by substrate and water molecule Xue Liu, Yun Ling, Jin Hu, Chunlei Yue, Zhiqiang Mao, Jiang Wei We investigated field effect transistor (FET) device made of multilayered WS2 with Poly(methyl methacrylate) (PMMA) as the dielectric layer. The device was fabricated using shadow mask evaporation to improve contact. Comparing to the same FET with SiO2 dielectric layer, PMMA-WS2 device shows an excellent on-off ratio (up to 6 orders magnitude), an easily induced ambipolar behavior and a significantly reduced hysteresis at high gate voltage region during the gate sweep. Furthermore, we discovered that the water molecule absorbed onto the WS2 surface depletes the extra charge carriers at the neutrality point and transforms the device into insulating state at room temperature. In addition, we found that the effect caused by water absorption is reversible. [Preview Abstract] |
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