Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session L37: Devices from 2D Materials II - ElectronicsFocus
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Sponsoring Units: DMP Chair: Andrea Young, University of California Santa Barbara Room: LACC 411 |
Wednesday, March 7, 2018 11:15AM - 11:27AM |
L37.00001: Injection properties of MoS2 field effect transistors via selectively deposited thiol-molecules Kyungjune Cho, Jinsu Pak, Jae-Keun Kim, Jiwon Shin, Seungjun Chung, Takhee Lee Although two-dimensional (2D) molybdenum disulfide (MoS2) has gained huge attention due to its unique physical properties, the limited electrical contact to 2D semiconductors still impedes to realize high-performance MoS2-based devices. In this regard, many studies have been conducted to improve the injection properties by inserting tunneling layers, such as hexagonal boron nitride or graphene, between MoS2 and electrodes.[1,2] However, the reported strategies require relatively low-yield and time consuming transfer processes on MoS2 flakes. Here, we suggest a simple contact modification method, introducing chemically adsorbed thiol molecules as thin tunneling barriers between the metal electrodes and MoS2 channels. The directly deposited thiol-molecules via the vapor-deposition process introduce additional tunneling paths at the contact regions, improving the carrier injection properties with lower activation energies in MoS2 field-effect transistors. Additionally, by inserting thiol-molecules at the only one contact region, asymmetric carrier-injection was feasible depending on the temperature and gate bias. |
Wednesday, March 7, 2018 11:27AM - 11:39AM |
L37.00002: Fermi level de-pinning by using one-dimensional edge contact to MoS2 Changsik Kim, Inyong Moon, Kwangyoung Lee, Won Jong Yoo Electrical metal contacts to two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) are considered as a main obstacle to the device performance due to high contact resistance and uncontrollable Schottky barrier heights (SBH), indicating strong Fermi level pinning. Here, we fabricate one-dimensional edge contact to MoS2 covered with h-BN by using plasma etching technique. The pinning factor S for MoS2 contacted to metal via edge increases from 0.1 to 0.3, but still shows strong pinning due to large band gap of thin MoS2. However, edge contact to bulk MoS2 with high work function metal Pd shows ambipolar behavior, indicating clear de-pinning effect. Additionally, we compare transport mechanism between surface and edge contacts without h-BN. Based on these differences, we accomplished rectifying junction with heterogeneous contact structure. |
Wednesday, March 7, 2018 11:39AM - 11:51AM |
L37.00003: Negative Fermi-level Pinning Effect Observed in Metal/GaAs Junction with Graphene Insertion Layer Hoon Hahn Yoon, Wonho Song, Sungchul Jung, Junhyung Kim, Gahyun Choi, Kyuhyung Mo, Kibog Park We report the direct observation revealing that the electric dipole layer due to the chemical interaction at metal/graphene interface and the doping of graphene can induce the negative Fermi-level pinning effect in metal/graphene/n-GaAs(001) junction made on a GaAs substrate containing regions with low interface-trap density in combinational manners. The graphene insertion layer takes a role of diffusion barrier preventing the atomic intermixing at interface and preserving the low interface-trap density region. The change of electrostatic potential across the metal/graphene interface due to the chemical interaction dipole layer and the doping of graphene is found to cause the negative Fermi-level pinning effect, supported by the Schottky barrier decreasing as metal work-function increasing. The low Schottky barrier patches with very small total areal fraction are considered to serve as preferred paths for electron transport through metal/graphene/n-GaAs(001) junctions. This work provides an experimental method to form Schottky contacts (metal/GaAs) and Ohmic contacts (metal/graphene/GaAs) simultaneously on a GaAs substrate covered partially with graphene by using identical metal electrodes. |
Wednesday, March 7, 2018 11:51AM - 12:03PM |
L37.00004: Origins of interface traps in MoS2-based field-effect transistors Andrew O'Hara, Adithi Krishnaprasad, Hirokjyoti Kalita, Tania Roy, Sokrates Pantelides Field-effect transistors utilizing few-layer MoS2 as an n-type channel material were fabricated with both h-BN and Al2O3 as the gate dielectric layer. Whereas exfoliated h-BN is transferred directly to the MoS2, growth of Al2O3 via atomic layer deposition (ALD) must be preceded by a nucleation seed layer of either oxidized aluminum (AlOX) or silicon (SiOX). Measurements of the density of interface traps (Dit) show marked differences in the behavior of each of the three types of fabricated devices. While h-BN exhibits close to acceptable values of Dit, the aluminum and silicon based ALD seed layers exhibit significantly increased Dit and Fermi-level pinning, respectively. In order to gain insight into the source of interface traps, atomistic models of the three relevant interfaces were constructed. Using density functional theory, we investigate the role that near-interface defects in the dielectric layer, dangling bonds, and accidental dopants in the MoS2 from seed layer sputtering play in creating interfacial electron traps. These calculations are intended to provide guidance in materials selection and improvements to device processing during fabrication. |
Wednesday, March 7, 2018 12:03PM - 12:15PM |
L37.00005: Via Method for Lithography Free Contact and Preservation of 2D Materials Evan Telford, Avishai Benyamini, Daniel Rhodes, Da Wang, Younghun Jung, Amirali Zangiabadi, Kenji Watanabe, Takashi Taniguchi, Shuang Jia, Katayun Barmak, Abhay Narayan, Cory Dean, James Hone Atomically thin 2D materials span the common electronic material properties such as metals, semi-conductors, and insulators, and can manifest correlated phases such as superconductivity, charge density waves, and magnetism. An ongoing challenge in the field is to incorporate these 2D materials into multi-layer hetero-structures with robust electrical contacts while preventing disorder and degradation. In particular, preserving and studying air-sensitive 2D materials has presented a significant challenge since they readily oxidize under atmospheric conditions. We report a new technique for contacting 2D materials, in which metal ‘via’ contacts are pre-patterned into insulating h-BN layers, which are then placed onto the desired conducting 2D layer, avoiding direct lithographic patterning onto the 2D conductor. The metal contacts are planar with the bottom surface of the h-BN and form robust contacts to multiple 2D materials, as well as 1D carbon nanotubes. These structures protect air-sensitive 2D materials for months with no degradation in performance while providing electrical contact exceeding the best techniques previously demonstrated. This technique will provide the capability to produce ‘atomic PCBs’ that can form the basis of more complex multi-layer heterostructures. |
Wednesday, March 7, 2018 12:15PM - 12:27PM |
L37.00006: Low Temperature Electronic Transport in Field-effect Transistors based on 2D Layers of Copper Indium Selenide (CuIn7Se11) Prasanna Patil, Milinda Wasala, Sujoy Ghosh, Sidong Lei, Robert Vajtai, Pulickel Ajayan, Saikat Talapatra Ternary system of Copper Indium Selenide (CIS) in thin film form has been extensively used in optoelectronics industry in the past due to its exceptional optical properties. It is, therefore, expected that 2D layers of CIS could play a key role for developing next generation micro/nanoscale optoelectronics devices based on 2D materials. We will report on the fabrication of field-effect transistors (FETs) using few-layers of CuIn7Se11 flakes exfoliated from crystals grown using chemical vapor transport technique. Our transport measurements indicate n-type FET with electron field effect mobility, µFE ≈ 27 cm2 V-1 s-1 at room temperature when silicon dioxide (SiO2) is used as a back gate. Low temperature electronic transport measurements, in order to understand the nature of electron transport, performed on these devices will also be presented. Our results suggests the possibility of two-dimensional (2D) variable range hopping (VRH) mechanism in CuIn7Se11 FETs. Temperature dependence of several other key parameters associated with FETs such as field effect mobility, threshold voltage, subthreshold swing, interface trap density, Schottky barrier height etc. will presented and discussed. |
Wednesday, March 7, 2018 12:27PM - 1:03PM |
L37.00007: Lateral and Vertical Electronic Transport in 2D Layered Materials Invited Speaker: Joerg Appenzeller Electronic devices from two-dimensional (2D) systems have become the focus of research activities worldwide over the last couple of years. Since the discovery of graphene, many other 2D structures have been identified and studied with a particular focus on transition metal dichalcogenides (TMDs) like MoS2, WSe2, MoTe2, and black phosphorus (BP). Different from graphene, these materials exhibit a finite bandgap, which makes them highly relevant for electronic device applications. While most of the previous studies have focused on the lateral transport in these structures, most recently attention has been drawn to the transport between van der Waals layers for tunneling device applications and to create “atomically abrupt” p-n junctions. Here we will discuss experimental results on the topic of lateral and vertical electronic transport in a number of 2D materials and heterostructures from the same. The results can be summarized as follows: 1) Schottky barriers and their response to drain and gate voltages give rise to unique ambipolar device characteristics that can be evaluated in terms of bandgaps and Schottky barrier heights; 2) Large current rectification ratios observed in lateral/vertical 2D heterostructures are a result of the Schottky barrier response; 3) Vertical transport masses can be determined from a quantitative analysis of vertical transport through TMDs; and 4) An electric field induced reversible phase transition from a metallic to a semiconducting phase can be achieved in certain binary and ternary TMDs. |
Wednesday, March 7, 2018 1:03PM - 1:15PM |
L37.00008: Confinement and Waveguide Effects of Quantum Wires Formed in Graphene by Strain Dawei Zhai, Yong Wu, Cheng Pan, Bin Cheng, Takashi Taniguchi, Kenji Watanabe, Nancy Sandler, Marc Bockrath Confinement of electrons in graphene to make devices has proven to be a challenging task. Fortunately, the mechanical flexibility of graphene raises the possibility of using strain to alter its properties. Recent transport measurements on graphene nanowires created by linearly-shaped strained folds encapsulated by boron nitride reveal Coulomb blockade signatures, indicating charge confinement effects [1]. Here, we model the system within the continuum Dirac formalism with the effect of strain included as a pseudo-magnetic field. Apart from extended scattering states similar to those in pristine graphene, the fold supports different types of bound states distinguished by their distinctive dispersion, which can contribute to the confinement effect observed experimentally. Possible effects of the encapsulation process, which may modify the geometry of the folded structure, as well as the crystalling orientation of the fold are examined. Our results show that confinement is robust against variations in the geometry but vanishes continuously as the armchair orientation is reached. |
Wednesday, March 7, 2018 1:15PM - 1:27PM |
L37.00009: Transport Properties of Graphene Decorated by Li and K adatoms Sung Oh Woo, Shayan Hemmatiyan, Tyler Morrison, K.D.D Rathnayaka, Igor Lyuksyutov, Donald Naugle We report transport properties of the K and Li decorated graphene (n ~ 2×1012 cm-2). Upon deposition of K and Li atoms at T = 20 K, graphene is doped with electrons, and the charge carrier mobility is decreased. As temperature is increased, electrons donated to graphene and charged scatterers are reduced, and the mobility of the metal decorated graphene is increased. This differs from the typical temperature-dependent transport in undecorated graphene where the mobility decreases with increasing temperature. To investigate the kinetic behavior of adatoms on graphene, we estimate the hopping time of the Li and K adatoms on graphene based on the migration barrier by DFT calculations. The calculations reveal that these adatoms are mobile even at cryogenic temperatures and become more mobile with increasing temperature, allowing for cluster formation of adatoms. This indicates that the temperature-dependent transport properties of graphene decorated by Li and K adatoms is due to the decrease of charged scatterers by cluster formation of adatoms. |
Wednesday, March 7, 2018 1:27PM - 1:39PM |
L37.00010: First-principle simulation and design of graphene nanoribbon-based devices Wenchang Lu, Zhongcan Xiao, Chuanxu Ma, Jinsong Huang, Liangbo Liang, Kunlun Hong, An-Ping Li, Jerry Bernholc
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Wednesday, March 7, 2018 1:39PM - 1:51PM |
L37.00011: Top-Contact Self-Aligned Printing for High-Performance Carbon Nanotube Thin-Film Transistors with Sub-Micron Channel Length Fanqi Wu, Xuan Cao, Chongwu Zhou
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Wednesday, March 7, 2018 1:51PM - 2:03PM |
L37.00012: Correlated 2D Fermion Systems as Interconnects in CMOS circuits Pouya Dianat, Kiana Montazeri, Bahram Nabet A major bottleneck in current CMOS technology is the parasitic capacitance associated to interconnects. Correlated two-dimensional fermion systems (2DFS) may be incorporated into interconnect structures or replace them as a whole. Specifically, such a system benefits from a quantum capacitance due to the exchange-correlation energies, which can take a negative value. Furthermore, such lines can overcome Ohmic limitations of cupper lines by usage of charge density waves propagation, e.g. plasmaron that would replace conventional charge transport mechanisms. Here we present devices with embedded 2DFS attained at AlGaAs/GaAs interfaces, which are specially fostered for to be highly correlated. First, we define the design criteria to achieve a negative capacitance value that may be used along with cupper lines in a CMOS circuit to reduce line coupling, and therefore increase the packing factor in transistor placements; second, we exhibit a device that uses a 2DFS as interconnect between two metal terminals. This device defies the limitations of charge transport through launching plasmarons (electron-plasmon coupling) as a means for signal transfer between contacts. These concept devices offer intriguing solutions to the interconnect problem in modern CMOS circuits. |
Wednesday, March 7, 2018 2:03PM - 2:15PM |
L37.00013: Crystal-Field Engineering of Spin-Orbit Coupling in Graphene Heterostructures Tatiana Rappoport, Aires Ferreira, Tarik Cysne Spin-orbit interaction is an essential ingredient to the manifestation of interesting phenomena like spin-Hall effect and nontrivial topological phases. Graphene has an extremely weak spin-orbit coupling. However, different substrates are able to generate a variety of spin-orbit couplings by proximity effect, depending on the symmetries that are broken in graphene. |
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