Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Z2: Focus Session: Beyond Graphene - Devices II |
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Sponsoring Units: DMP Chair: Tauno Palomaki, University of Washington Room: 001B |
Friday, March 6, 2015 11:15AM - 11:27AM |
Z2.00001: Doping Dependent Electronic Properties of Atomically Thin Two-dimensional Crystals Ting Cao, Zhenglu Li, Steven G. Louie Using first-principle calculations, we find that the electronic structure of atomically thin 2D crystals such as GaSe and NbSe2 can be modified significantly through doping, which can lead to major changes in their other properties. We elucidate the origins of these doping dependent effects, and connect our theoretical predictions to experimental measurements since high level of doping can be achieved in 2D materials. [Preview Abstract] |
Friday, March 6, 2015 11:27AM - 11:39AM |
Z2.00002: Ferroelectric Controlled Nanoscale MoS$_{2}$ Transistor Zhiyong Xiao, Jingfeng Song, Stephen Ducharme, Xia Hong We report the study of the device characteristics of MoS$_{2}$ field effect transistors with a SiO$_{2}$ backgate and a ferroelectric polymer top gate. We mechanically exfoliated MoS$_{2}$ flakes on 300 nm SiO$_{2}$ substrates. The thinner MoS$_{2}$ pieces were identified by Raman spectroscopy and atomic force microscopy (AFM), and flakes of 1 - 5 nm thick were fabricated into two point devices via e-beam lithography with Ti/Au (5nm/50nm) as the contact electrodes. We then deposited on the top of the device a ferroelectric polymer layer, 20-40 nm polycrystalline poly(vinylidene-fluoride-trifluorethylene) (PVDF-TrFE), using the Langmuir-Blodgett approach. At room temperature, we achieved a current modulation of a factor of 10$^{3}$ using the SiO$_{2}$ back gate. The field effect mobility of the devices is $\sim$ 20 cm$^{2}$V$^{-1}$s$^{-1}$. We then used conducting AFM to control the polarization of the top ferroelectric gate, and examined the SiO2-gated I-Vg characteristics in different polarization states of PVDF-TrFE. By switching the ferroelectric polarization, we induced a 30 V shift in $I$-$V_{\mathrm{g}}$. At fixed backgate voltage, we achieved a maximum switching ratio in the drain current of $\sim$ 15. [Preview Abstract] |
Friday, March 6, 2015 11:39AM - 11:51AM |
Z2.00003: Controlled n-doping of monolayer MoS2 by atomic hydrogen Jyoti Katoch, Tiancong Zhu, Hua Wen, Roland Kawakami Molybdenum Disulfide (MoS2) is a 2D layered material with potential applications in optoelectronics, electronics and spintronics. The injection of electrons in MoS2 is affected and limited by the Schottky barrier formed between metal contact and MoS2. There is a great deal of research interest to experimentally demonstrate ohmic contacts onto MoS2 in order to fully unravel the potential of this exciting material. We will present our results on controlled doping of single layer MoS2 surface by hydrogen to improve the contact resistance. We measured the transport properties of the MoS2 as a function of successive atomic hydrogen dosage at low temperature in ultra-high vacuum. Atomic hydrogen is generated using a hydrogen cracker. We observe that hydrogen adsorption results in negative doping due to charge donation by hydrogen to MoS2. Furthermore we will discuss the stability of atomic hydrogen on warming up the sample to room temperature. [Preview Abstract] |
Friday, March 6, 2015 11:51AM - 12:27PM |
Z2.00004: Structural design of 2D materials for electronic and optoelectronic applications Invited Speaker: Ju Li In accordance with Richard Feynman's 1959 statement, ``there's plenty of room at the bottom,'' we explore the structural design space of 2D materials for electronic and optoelectronic applications. Homogenous and inhomogeneous elastic strain [\textit{Nature Photonics} \textbf{6} (2012) 866; \textit{MRS Bulletin} \textbf{39} (2014) 108], bending [\textit{ACS Nano} \textbf{5} (2011) 3475], interlayer twist and slip [\textit{Nano Letters} \textbf{14} (2014) 5350] lead to tunable, low-energy artificial atoms, artificial superlattices and pseudoheterostructures that can regulate quasiparticle motion (excitons, electrons). The amenability of 2D materials for mechanical manipulations, combined with lithographic patterning and annealing [\textit{Nanoscale} \textbf{4} (2012) 4883; \textit{PNAS} \textbf{106} (2009) 10103], could lead to new topological physics [\textit{Science} \textbf{346} (2014) 1344] and device designs. [Preview Abstract] |
Friday, March 6, 2015 12:27PM - 12:39PM |
Z2.00005: Thickness-dependent Electrical and Thermoelectric Transport in few-layer MoS$_{2}$ Morteza Kayyalha, Li Shi, Yong Chen Layered semiconducting Transition Metal Dichalcogenides such as MoS$_{2}$ have recently gained a lot of attention as promising 2D materials for electronic and optoelectronic device applications. Here, we present a systematic study of thickness-dependent electrical and thermoelectric transport in few-layer MoS$_{2}$. MoS$_{2}$ flakes with various thicknesses ranging from 1-23 layers are prepared using the standard scotch-taped exfoliation technique and are then transferred onto a SiO$_{2}$/Si substrate. Electrical and thermoelectric measurements are carried out using AC and DC techniques with samples in vacuum. We observe five-fold enhancement in the electrical conductivity of two-layer MoS$_{2}$ compared to the bulk. However, the thermopower (TEP) exhibits less change except for monolayer where TEP is twice smaller. We also observe six times larger power factor in two-layer MoS$_{2}$ compared to the bulk. Additionally, we used a back gate to modulate the Fermi energy inside MoS$_{2}$ where an enhancement in TEP is observed close to the off state. Our results give insight into future prospects of MoS$_{2}$-based devices in thermoelectric applications. [Preview Abstract] |
Friday, March 6, 2015 12:39PM - 12:51PM |
Z2.00006: Effects of Contact Geometry on Measurements of Contact and Film Resistance in Thin MoS$_{2}$ Films Jinsong Xu, Yunqiu Luo, Roland Kawakami, Jonathan Pelz There is great interest in MoS$_{2}$ films and other 2D materials for fundamental studies and possible applications. However, the critical contacts to these films are not well understood, in part because of the strong band bending, depletion, and lateral Schottky barriers (SB). We report measurements and finite-element modeling of Au contacts on few-layer MoS$_{2}$ films on SiO$_{2}$/Si substrates, and find that certain common contact geometries produce mixing of contact and film resistance and highly/completely-misleading results in 3- and 4-probe measurements, and that lateral depletion can produce errors in TLM measurements. However, with appropriate contact geometry and comparison to modeling, contact and film resistances can be independently monitored. Both are found to have strong back-gate and temperature dependence, with small ($\sim$ 10 -- 30 meV) but different activation energies (AEs) near 100 -- 300 K. The contact AE is similar to several previous reports. The activated film resistance has not been previously reported, though evidence can be seen some previous published data. Possible origins of these temperature dependences will be discussed, as well as implications for determining contact SB heights. [Preview Abstract] |
Friday, March 6, 2015 12:51PM - 1:03PM |
Z2.00007: Junction properties of MoS2 between 1T and 2H phases Junsuk Kim, Jeasu Kim, Byounghee Moon, Hamza Gul, Jungho Kim, Young Hee Lee, Seong Chu Lim The Molybdenum disulfide (MoS2) with band gap of 1.8eV shows strong Fermi level pinning that hinders the modulation of Schottky barrier height using metals of different work function. In this reason, only n-type behaviors have been exhibited from MoS2 FET. However, different from 2H phase MoS2, exhibiting semiconducting characteristics, 1T phase MoS2 is metallic. Thus, the junction of semiconducting and metallic MoS2 is possible if 2H phase MoS2 is locally transformed into 1T phase. In this study, we prepare MoS2 in-plane junction of 1T and 2H phase by locally intercalating Li ions into MoS2 multilayer that is confirmed using Raman and PL spectroscopes and will discuss the electrical properties of the junction of two different phases of MoS2. [Preview Abstract] |
Friday, March 6, 2015 1:03PM - 1:15PM |
Z2.00008: Electrical edge contacts to monolayer MoS$_{2}$ Byoung Hee Moon, Gang Hee Han, Hyun Kim, Homin Choi, Youngjo Jin, Hye Yun Jeong, Seongchu Lim, Young Hee Lee Charge injection through the metal contacts to nanostructures has been the interesting issue for the physical mechanism involved as well as the device applications. In this talk, we discuss the effects of electrical edge contact to CVD grown monolayer MoS$_{2}$. The edge contacts are achieved by sandwiching MoS$_{2}$ with hexagonal boron nitrides (BN) [1]. It appears to show that charge injection by thermionic emission is suppressed, while tunneling effects take over. \\[4pt] [1] L. Wang, \textit{et al}. science 342, 614 (2013). [Preview Abstract] |
Friday, March 6, 2015 1:15PM - 1:27PM |
Z2.00009: MoS$_{2}$ Transistors Operating at Gigahertz Frequencies Daria Krasnozhon, Dominik Lembke, Clemens Nyffeler, Yusuf Leblebici, Andras Kis The presence of a direct band gap and an ultrathin form factor has caused a considerable interest in 2D semiconductors from TMD family with MoS$_{2}$ being the most studied representative of this family of materials. While diverse electronic elements, integrated circuits and optoelectronic devices have been demonstrated using ultrathin MoS$_{2}$ and related materials, very little is known about their performance at high frequencies. We fabricated top-gated MoS$_{2}$ transistors operating in the gigahertz range of frequencies. The presence of a band gap also gives rise to current saturation, allowing voltage gain higher than 1. The RF transistors are fabricated from exfoliated MoS$_{2}$ with different layer thickness. All our devices presented transconductance typical of n-type materials with on-state current reaching 300 $\mu$A/$\mu$m for $V_{\mathrm{ds}} = $ 2 V and gate voltage $V_{\mathrm{tg}} = $ 10 V in the case of monolayer MoS$_{2}$. The current gain of the MoS$_{2}$ FETs decreases with increasing frequency and shows the typical 1/$f $dependence. In conclusion, we studied top-gated MoS$_{2}$ transistors with a 240 nm gate length. Our MoS$_{2}$ RF-FETs show an intrinsic transconductance higher than 50 uS/um and a drain-source current saturation with a voltage gain higher than 1. Our devices show cut-off frequencies in the GHz range and are able not only to amplify current in this frequency range but also power and voltage, with the maximum operating frequency $f_{\mathrm{max}}= $ 8.2 GHz. [Preview Abstract] |
Friday, March 6, 2015 1:27PM - 1:39PM |
Z2.00010: Experimental evidence for inhomogeneous charge transport in MoS$_{2}$ nanoflakes Chi-Te Liang, Shun-Tsung Lo, Oleh Klochan, C.-H. Liu, W.-H. Wang, Alex R. Hamilton We study electron transport in a monolayer MoS$_{2}$ nanoflake over a wide range of density, temperature, and electric bias. We find that the transport is best described by a percolating picture in which the disorder breaks translational invariance, breaking the MoS$_{2}$ system up into a series of puddles, rather than previous pictures in which the disorder is treated as uniform and homogeneous. Our work provides insight to a unified picture of charge transport in monolayer MoS$_{2}$ nanoflakes and contributes to the development of next-generation MoS$_{2}$-based devices. [Preview Abstract] |
Friday, March 6, 2015 1:39PM - 1:51PM |
Z2.00011: First Principles Study of Contact Resistance across Nickel-Treated-Graphene-MoS$_{2}$ Interfaces W.S. Leong, X. Luo, Y. Li, K.H. Khoo, S.Y. Quek, John T.L. Thong Recently, Ni-treated-graphene electrodes were fabricated on MoS$_{2}$ using a dry transfer technique and metal-catalyzed graphene treatment process, yielding contact resistances as low as 200 $\Omega \mu $m and a substantial contact enhancement of $\sim$ 2 orders of magnitude relative to Ni-MoS$_{2}$ interfaces. By performing a Schottky barrier height (SBH) analysis on Ni-MoS$_{2}$ and Ni-graphene-MoS$_{2}$ interfaces using first-principles DFT calculations, we have found that the smaller contact resistance in Ni-treated-graphene-MoS$_{2}$ can be attributed to the smaller SBH of Ni-graphene-MoS$_{2}$ contacts. This reduction in SBH can in turn be related to the lower work function of Ni-graphene electrodes relative to Ni. The effect of Ni treatment further reduces the contact resistance due to stronger coupling between Ni and graphene edges. [Preview Abstract] |
Friday, March 6, 2015 1:51PM - 2:03PM |
Z2.00012: Theoretical study of quantum transport in two dimensional MoS2 Indra Yudhistira, Hennrik Schmidt, Leiqiang Chu, Goki Eda, Shaffique Adam In this work, we calculate the quantum correction to the conductivity for disordered electrons in a two-dimensional transition metal dichalcogenide and show that it exhibits crossover from the symplectic to orthogonal universality classes. Due to the strong spin-orbit coupling, this system displays weak anti-localization behaviour in a strong magnetic field, while symmetry breaking introduces a crossover parameter, causing weak localization in a weak magnetic field. Temperature and carrier density dependence of coherence length and crossover parameter are discussed making comparisons with available experimental data. [Preview Abstract] |
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