Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session B2: Focus Session: Beyond Graphene - Transport & Heterostructures |
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Sponsoring Units: DMP Chair: David Cobden, University of Washington Room: 001B |
Monday, March 2, 2015 11:15AM - 11:27AM |
B2.00001: Improved Performance in MoS$_{2}$ Field-Effect Transistors Contacted by Highly Doped Graphene Electrodes and Passivated by Hexagonal Boron Nitride Meeghage Perera, Hsun-Jen Chuang, Zhixian Zhou A major challenge for developing semiconducting transition-metal-dichalcogenide (TMD)-based electronic devices is that TMDs tend to form a substantial tunneling or Schottky barrier (SB) with most metals commonly used for making electrical contacts, while low resistance Ohmic contacts are needed for exploring intrinsic transport properties of the channel material, and performance limits of realistic devices. We have fabricated low-resistance contacts to MoS$_{2}$ field-effect transistors by using graphene as work function tunable electrode material. To minimize the Schottky barrier height at the MoS$_{2}$/graphene junction and the contact resistance, both electrostatic and surface charge transfer doping methods were used to selectively dope the graphene electrodes. Substantial improvement of device performance was observed in devices with highly n-doped graphene electrodes. Four-probe electrical transport measurement was performed on MoS$_{2}$ devices with the active channel stacked between atomically flat hexagonal boron nitride (hBN) to further investigate the intrinsic performance limit of MoS$_{2}$ as a channel material. [Preview Abstract] |
Monday, March 2, 2015 11:27AM - 11:39AM |
B2.00002: Electrical and optical properties of chemically doped $p$-type MoS$_{2}$ Joonki Suh, Tae-Eon Park, Der-Yuh Lin, Sefaattin Tongay, Junqiao Wu Molybdenum disulfide is a model example of two-dimensional semiconductors, holding promise for applications in optoelectronic devices and field-effect transistors. So far, however, its practical use has been exclusively restricted to native, n-type doping. Here we experimentally demonstrate stable $p$-type conduction in molybdenum disulfide substitutionally doped with niobium. This chemical doping leads to a degenerate hole density of $\sim$ 1.8 $\times$ 10$^{14}$ cm$^{\mathrm{-2}}$ and enables gate-tunable van der Waals $p$-$n$ homo-junctions. Also, the $p$-type monolayer molybdenum disulfide exhibits a greatly enhanced and broadened photoluminescence compared to that acquired from undoped monolayers. Our study demonstrates the stable $p$-type doping in molybdenum disulfide, and also reveals an effective way to tailor optical and electrical properties of two-dimensional semiconductors with extrinsic dopants. [Preview Abstract] |
Monday, March 2, 2015 11:39AM - 11:51AM |
B2.00003: Electrostatic control of polarity of $\alpha $-MoTe$_{2}$ transistors with dual top gates Shu Nakaharai, Mahito Yamamoto, Keiji Ueno, Yen-Fu Lin, Song-Lin Li, Kazuhito Tsukagoshi Transition metal dichalcogenides have been expected for future applications in nanoelectronics due to their unique features of the atomically-thin structure. Using semiconducting $\alpha $-molybdenum ditelluride ($\alpha $-MoTe$_{2})$, we realized field effect transistors (FETs) in which the polarity (n- or p-type) can be electrostatically controlled without impurity doping. The fabricated device had a pair of top gates (aluminum electrode on silicon dioxide) attached in series with a gap length of 100 nm in between. We experimentally performed transistor operations in both n-FET and p-FET modes in a single device by changing the voltage applied to one of the two top gates, which determined the transistor polarity, and sweeping the bias of the other gate. The demonstrated reversibility of the transistor polarity will contribute to the renovated architecture of logic circuits with lower numbers of transistors and hence the lower power consumption than the conventional technology. [Preview Abstract] |
Monday, March 2, 2015 11:51AM - 12:03PM |
B2.00004: Field-Effect Transistors Based on Few-Layered Ambipolar MoSe$_{2}$ and $\alpha $-MoTe$_{2}$ Daniel Rhodes, Nihar Pradhan, Simin Feng, Byoung-Hee Moon, Yan Xin, Sharhriar Memaran, Muhandis Siddiq, Lakshmi Bhaskaran, Stephen Hill, Humberto Terrones, Mauricio Terrones, Ajayan Pulickel, Luis Balicas We report a room temperature study on the electrical responses of field-effect transistors (FETs) based on few-layered MoSe$_{2}$ and MoTe$_{2}$, grown by chemical vapor transport, mechanically exfoliated onto SiO$_{2}$. MoSe$_{2}$ FETs electrically contacted with Ti display ambipolar behavior with current on/off ratios up to 10$^{6}$ for both hole and electron channels. For both channels the Hall effect indicates Hall mobilities $\mu_{\mathrm{H}} \quad \simeq $ 250 cm$^{2}$/(Vs), which are comparable to the corresponding field-effect mobilities, $\mu_{\mathrm{FE}} \quad \sim $ 175 cm$^{2}$/(Vs), evaluated through two-terminal field-effect configuration. MoTe$_{2}$ field-effect transistors are observed to be hole-doped, displaying on/off ratios of $\sim$ 10$^{6}$ and subthreshold swings of $\sim $140 mV per decade. Our results suggest that MoSe$_{2}$ is a good candidate for single atomic layer p$-$n junctions and for low-power, complementary logic applications, with MoTe$_{2}$ having similar properties. However, in MoTe$_{2}$ we observe a field-effect mobility of only $\mu_{\mathrm{FE}} \quad \sim $ 20 cm$^{2}$/(Vs) in a bilayer device and $\sim$ 27 cm$^{2}$/(Vs) in seven layers. [Preview Abstract] |
Monday, March 2, 2015 12:03PM - 12:15PM |
B2.00005: ABSTRACT WITHDRAWN |
Monday, March 2, 2015 12:15PM - 12:27PM |
B2.00006: Conduction and Valence Band Offsets in WSe2-Graphene Heterostructures Kyounghwan Kim, Stefano Larentis, Babak Fallahazad, Kayoung Lee, Jiamin Xue, David Dillen, Chris Corbet, Emanuel Tutuc We investigate the electron transport in graphene-WSe$_{2}$ heterostructures realized using a layer-by-layer transfer. Lateral electron transport shows ambipolar behavior characteristic of graphene, with a marked saturation at high positive (negative) gate bias, associated with the population of the conduction (valence) band in WSe$_{2}$. The graphene carrier density dependence on gate bias was extracted from magneto-transport measurements. Using WSe$_{2}$ as a top dielectric in dual-gate graphene field-effect transistors, we determine the WSe$_{2}$ dielectric constant along the c-axis. By combining the graphene density dependence on gate bias in back-gated graphene-WSe$_{2}$ heterostructures with the WSe$_{2}$ dielectric constant, we determine the offset between the graphene charge neutrality point and the WSe$_{2}$ conduction and valence bands. [Preview Abstract] |
Monday, March 2, 2015 12:27PM - 12:39PM |
B2.00007: Strain induced change in electronic and thermoelectric properties in few layers of MoS$_{2}$ Tribhuwan Pandey, Swastibrata Bhattacharyya, Abhishek K. Singh The sensitivity of the electronic and thermoelectric properties of MoS$_{2}$ on application of strain can open up a variety of applications in the emerging area of straintronics. Using first principles calculations, we investigate the effect of normal compressive (NC), bi-axial compressive (BC), and bi-axial tensile (BT) strain on the electronic properties of few layered MoS$_{2}$. Regardless of the manner of strain, a reversible semiconductor-to-metal transition is observed in this material. We further show that under NC strain, the inter-layer interactions between Mo-$d_{z^2}$ and S-$p_z$ causes the S-M transition, whereas under BC and BT strain it is caused by the strong hybridization of the intra-layer Mo-$d_{x^2-y^2}$ and S-$p_x$ orbitals and Mo-$d_{z^2}$ and S-$p_{z}$ orbitals, respectively. We also study number of layer ($n$L)and stain dependent transport properties using Boltzmann transport theory. Our study reveals that the 3L and 2L-MoS$_{2}$ emerge as the most efficient thermoelectric materials under NC and BT strain, respectively. The concept proposed from our study can also be extended to other semiconducting TMDs owing to similar crystal structure and electronic properties. [Preview Abstract] |
Monday, March 2, 2015 12:39PM - 12:51PM |
B2.00008: Tuning the Schottky barrier heights at MoS$_{2}$$\mid$metal contacts: a first-principles study Mojtaba Farmanbar, Geert Brocks The nature of the Schottky barrier at metal contacts with the two-dimensional semiconductor MoS$_{2}$ is controversial. Using first-principles DFT calculations we show that the Schottky barrier height (SBH) for high work function ($>$ 4.7 eV) metals typically obeys the Schottky-Mott limit, provided that a potential step that arises at the metal-MoS$_{2}$ interface is taken into account. It suggests that selecting a metal with an appropriate work function may reduce the SBH to zero. However, we find that for low work function metals the Fermi level is pinned below the conduction band edge of MoS$_{2}$ , leading to SBHs of 0.1-0.3 eV. We attribute the pinning to the metal-MoS$_{2}$ interaction at the interface perturbing the electronic structure of MoS$_{2}$ , and causing a broadening of the MoS2 conduction band edge. Inserting a monolayer of boron nitride (BN) between the metal surface and the MoS$_{2}$ layer disrupts this interaction. In addition the BN layer effectively decreases the metal work function, thereby enabling a line-up of the Fermi level with the MoS$_{2}$ conduction band with a vanishing SBH. [Preview Abstract] |
Monday, March 2, 2015 12:51PM - 1:03PM |
B2.00009: Structural and transport properties of finite length grain boundaries in two-dimensional materials Yuanxi Wang, Vincent Crespi Grain boundaries in two dimensional materials such as graphene and monolayer transition metal dichalcogenides are unfortunate consequences of grains growing in different orientations, keeping the material away from a monocrystalline ground state. We show that when 2D materials are grown on substrates with gaussian curvatures, grain boundaries produced from self-intersections are in fact the ground state. They screen out the curvature imposed by the substrate and form finite length chain structures, terminating with cone and saddle shapes of partial disinclinations. The structural stability and transport properties of finite length grain boundaries are studied at the tight-binding level. Every dislocation in the grain boundary contributes an intrinsic pseudo-flux and induces loop currents during transport. We further show that transport properties depend sensitively on the number of dislocations in the grain boundary. [Preview Abstract] |
Monday, March 2, 2015 1:03PM - 1:15PM |
B2.00010: Intrinsic Electron and Hole Transport in Channel Passivated WSe$_{2}$ Field-Effect Transistors with Graphene Contacts Hsun jen Chuang, Nirmal Jeevi Ghimire, Jiaqiang Yan, David Mandru, Zhixian Zhou We report electrical transport measurement of high-quality WSe$_{2}$ field-effect transistors. As a nearly intrinsic semiconductor with a relatively large bandgap, WSe$_{2}$ tends to form substantial Schottky barriers with common contact metals for both electron and hole channels, which obstructs the charge injection especially at low temperatures. In this work, we use highly n- and p-doped graphene as an electrode material to form low resistance electrical contacts to the electron and hole channels, respectively. To minimize surface and interface scattering, hexagonal boron nitride was used to passivate both the top and bottom surfaces of the WSe$_{2}$ channel. Four-terminal transport measurement was carried out for a wide temperature range to understand the intrinsic transport properties of atomically thin WSe$_{2}$. Field-effect mobility and effective mobility for both electron and hole channels as well as their temperature dependence will be discussed. [Preview Abstract] |
Monday, March 2, 2015 1:15PM - 1:27PM |
B2.00011: The Effect of Substrate on the Electron Transport Properties of MoS$_{2}$ Field-Effect Transistors Bhim Chamlagain, Hsun-Jen Chuang, Meeghage Madusanka Perera, Zhixian Zhou Substrate plays an important role in the performance of field-effect transistors (FETs) with two-dimensional transition metal dichalcogenide (TMD) channels. In this work, we systematically study the transport properties of few-layer MoS$_{2}$ FETs consistently fabricated on various substrates including SiO$_{2}$, Al$_{2}$O$_{3}$, SiO$_{2}$ modified by octadecyltrimethoxysilane (OTMS) self-assembled monolayers (SAMs), and hexagonal boron nitride (hBN). Standard four-probe electrical transport measurement was carried out at temperatures ranging from 77 K to room temperature to understand the scattering mechanism. Surprisingly, the room temperature mobility extracted from devices on different substrates is nearly the same. In contrast, a substantially higher mobility is observed in MoS$_{2}$ devices on clean hBN substrates at low temperatures. The role of various sources of scattering originating from the substrate and the channel/substrate interface such as charged impurities, charge traps, surface roughness, and remote surface optical phonons will be discussed. [Preview Abstract] |
Monday, March 2, 2015 1:27PM - 1:39PM |
B2.00012: Enhanced mobility electrons at the monolayer / multilayer MoS$_2$ homo-interface Y. Jia, E.J. Lenferink, T. Stanev, N.P. Stern Energy band alignment at interface of heterostructures can give rise to non-trivial local electronic structure and charge states with low dimensionality. In transition metal dichalcogenides (TMDCs), the optical band gap depends on the number of 2D crystal layers, transitioning from 1.29 eV in bulk to 1.88 eV for a monolayer of MoS$_2$, for example, and providing the possibility to create unusual charge state at the monolayer/multilayer homo-interface. Here, we examine the boundaries between MoS$_2$ monolayers and multilayers using scanning photocurrent microscopy and gate-dependent transport. Enhanced photocurrent and conductance were observed at the 1D homo-interface, which can be explained as accumulated carriers in the bent-band region of the junction. Our heterojunction modeling suggests a high local carrier density and enhanced mobility at the homo-interface. Our work presents an opportunity to achieve a 1D electron state in a homojunction and a pathway to break the mobility limit of TMDC monolayer transistors. [Preview Abstract] |
Monday, March 2, 2015 1:39PM - 1:51PM |
B2.00013: High Powerfactor in single and few-layer MoS2 Ying Wang, Yu Ye, Kedar Hippalgaonkar, Yuan Wang, Xiang Zhang The thermoelectric effect enables conversion between thermal and electrical energy, and provides one way to extract energy from waste heat. The efficiency of a thermoelectric device can be defined by a dimensionless figure of merit given by \textit{ZT}$=S^{2}\sigma {\rm T}/\kappa .$ In order to achieve efficient thermoelectric devices, $S^{2}\sigma $ needs to be kept high by optimizing the interplay between the $S^{\mathrm{\thinspace }}$and $\sigma $. The thin layered transition-metal dichalcogenide semiconductor MoS$_{\mathrm{2}}$ has attracted great interest because of two dimensional density of states and relatively high mobility, which could give a large $S^{\mathrm{\thinspace }}$and $\sigma $. Here we study on pristine exfoliated 1L-, 2L- and 3L MoS$_{\mathrm{2}}$ samples by simultaneous measurement of the Seebeck coefficient($S)$ and two probe electrical conductivity using nano-fabricated heater and thermometer. It firstly shows that atomic thin MoS2 which has a large effective band masses ($m\ast )$ as well as high mobilies ($\mu )$, increases the powerfactor $S^{2}\sigma .$to as high as $8.5\,mWm^{-1}K^{-2}$at room temperature$_{\mathrm{\thinspace }}$(twice as high as commercially used Bi$_{\mathrm{2}}$Te$_{\mathrm{3}})$. Further, we show for the first time that the confined two-dimensional density of states of the conduction band can be studied in monolayer MoS$_{\mathrm{2}}$ by measuring the gate-dependent Seebeck voltage. [Preview Abstract] |
Monday, March 2, 2015 1:51PM - 2:03PM |
B2.00014: Carrier injection in van der Waals multilayer systems Marcelo Kuroda, Christopher Coger Carrier injection is critical for the use of two-dimensional material systems like transition metal dichalcogenides (TMD) or graphene in electronic devices. Here we use first principles calculations (within the density functional theory) to quantify and classify the contact formed between metals and TMD multilayer systems. In particular we study the cases of multilayer MoS$_2$ and MoSe$_2$ with different metal electrodes (e.g.~Pd, Au, Ti). We find different behaviors depending on the choice of metal, thickness and electric field. An analytical model is produced accounting for the Fermi level pinning and the layer quantum capacitance. This atomistic description also sheds light on the ambipolar behavior observed in these systems and the gate-dependent contact resistance in field effect transistors. [Preview Abstract] |
Monday, March 2, 2015 2:03PM - 2:15PM |
B2.00015: Ambipolar conduction in MoS$_2$/WSe$_2$ hetero-bilayers Hema Chandra Prakash Movva, Sangwoo Kang, Amritesh Rai, Sanjay Banerjee Recent interest in layered semiconductors, and the ability to assemble them into artificial heterostructures with atomically sharp interfaces has opened up new avenues for the design of future electronic devices. In this work, we fabricated vertical heterostructures of exfoliated monolayer MoS$_{2}$ and monolayer WSe${_2}$ using a facile flake pick-up-and-place technique, and studied their optical and electrical properties. Photoluminescence measurements showed evidence of indirect excitons at $\sim$ 1.55 eV, indicating a clean interface between the two layers. We observed back-gate tunable, layer-selective ambipolar conduction in field effect transistors (FETs) made using these hetero-bilayers, with e-transport occurring through the MoS$_{2}$, and h-transport through WSe$_{2}$. The addition of a top-gate using a thin hBN dielectric further enabled selective operation of the hetero-bilayer FET as an n-FET/p-FET depending on the back-gate bias. [Preview Abstract] |
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