Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session Y33: 2D Semiconductor Electronic Devices |
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Sponsoring Units: DMP Room: 296 |
Friday, March 17, 2017 11:15AM - 11:27AM |
Y33.00001: Fabrication of Ohmic Contacts to Atomically Thin Transition Metal Dichalcogenides by Work Function Engineering Kateryna Pistunova, Luis Jauregui, Andrew Joe, Eshaan Patheria, Philip Kim Semiconducting transition metal dichalcogenides (TMDCs) are 2-dimensional semiconductors that can exhibit unique quantum transport phenomena, such as spin-valley coupling and valley Hall effect. However, most metal contacts to TMDCs form a large Shottky barrier due to Fermi level pinning and large work function mismatch, making it challenging to study the intrinsic transport properties at low temperatures. Here, work function engineering by controlling the thickness of metal film and surface morphology, we demonstrate to achieve highly transparent electric contacts to atomically thin TMDCs. We use x-ray photoelectron spectroscopy (XPS) to determine the work function of thin metal films that match the conduction/valence band edge of TMDCs to reduce the work function mismatch. We also utilize a top-gated device geometry with metal contacts underneath the TMDC flake together with metal work function tuning based on XPS to achieve Ohmic contacts to single and bilayer TMDCs. Gate dependent quantum oscillations in conductance were observed in TMDC materials and we will discuss photo response of magneto transport in the quantum limit. [Preview Abstract] |
Friday, March 17, 2017 11:27AM - 11:39AM |
Y33.00002: First-principles study of Schottky and Ohmic contacts at metal/two-dimensional semiconductor interface. Yanwen Chen, Yuanchang Li, Jian Wu, Wenhui Duan The quality of electrical contact is as important as the semiconductor itself to the performance of the entire devices, which becomes particularly crucial in the two-dimensional (2D) cases. Using first-principles calculations, we investigate the interfacial properties of single-layer titanium trisulfide (TiS$_{\mathrm{3}})$ and metal contacts in detail, including the geometry, bonding, electronic structure, charge transfer and local potential. We observe that the contacts with Au(111), Ag(111), Al(111), and Cu(111) are of Schottky type with the barriers of 2.15, 1.67, 1.55, and 0.84 eV while that with Sc(111) is of a low-resistance Ohmic type, originating from the strong hybridization between TiS$_{\mathrm{3}}$ and Sc(111). In comparison with calculated results of other five typical 2D semiconductors (namely, graphene, MoS$_{\mathrm{2}}$, WS$_{\mathrm{2}}$, MoSe$_{\mathrm{2}}$ and black phosphorene), we reveal an interesting dependence of the contact type on the separation $d$ between metal and 2D semiconductor. [Preview Abstract] |
Friday, March 17, 2017 11:39AM - 11:51AM |
Y33.00003: Ab Initio Simulation of Metal Contacts to 2D Semiconductors with Electron-Phonon Interactions Wushi Dong, Peter Littlewood Contact resistance could potentially limit the performance of aggressively scaled devices based on two-dimensional (2D) materials. In this work, we present full-band atomistic quantum transport simulations of metal contact to 2D semiconductors including electron-phonon scattering. A combined approach based on the Keldysh Non-equilibrium Green's Function (NEGF) formalism and a mean-field description of the electronic structure is used to calculate vertical transport in an extended device. Tight-binding parameters and electron-phonon coupling constants obtained through the maximally localized Wannier function technique enable us to model the transport at low computational costs. The long-range polar optical contribution is split from the short-range one in order to properly treat the divergence of the electron-phonon matrix elements at long-wavelength limit. Electron transport is found to happen mainly near the energy range where the bands of metal contact and 2D semiconductor cross. The dependence of contact resistance on the overlap length is also studied to determine the ultimate scalability. Our analysis of transport efficiencies provides the foundation and motivation for experimental works. [Preview Abstract] |
Friday, March 17, 2017 11:51AM - 12:03PM |
Y33.00004: Influence of Au Nanoparticles and Gas Adsorption on Transport Behaviors of MoS2 Layers Yunae Cho, Ahrum Sohn, Sujung Kim, Dong-Wook Kim, Byungjin Cho, Myung Gwan Hahm, Dong-Ho Kim Chemical vapor deposition (CVD) techniques can provide large-area wafer-scale MoS2 thin films, which are very useful for electronic and optoelectronic device applications. Control of the carrier concentration and doping type of MoS2 is crucial for its application in electronic and optoelectronic devices. The electrical properties of atomically thin MoS2 layers are very sensitive to ambient gas adsorption and coating of metal nanoparticles (NPs). In this work, we studied the relationship between the resistance (R) and surface work function (WF) of CVD-grown MoS2 layers with and without Au NPs while varying the gas (N2, O2, and H2/N2) environment. The ambient gas largely varied the WF but could not cause measurable R change for both the bare and NP-coated samples. Temperature-dependent transport suggests that variable range hopping is the dominant mechanism for electrical conduction in the MoS2 layers. The charges transferred from the gas adsorbates might be insufficient to change R and/or be trapped in the defect states. The smaller WF and larger localization length of the NP-coated sample, compared with the bare sample, suggest that more carriers and less defects enhanced the electrical conduction in MoS2. [Preview Abstract] |
Friday, March 17, 2017 12:03PM - 12:15PM |
Y33.00005: Abstract Withdrawn
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Friday, March 17, 2017 12:15PM - 12:27PM |
Y33.00006: Observing Ambipolar Behavior and Bandgap Engineering of MoS$_{\mathrm{2}}$ with Transport Measurements Rachael Morris, Cedric Wilson, Glen Hamblin, Ryuichi Tsuchikawa, Vikram V. Deshpande Molybdenum disulfide is a transition metal semiconductor with a relatively large bandgap about 1.8 eV. In MoS$_{\mathrm{2\thinspace }}$it is expected that the bandgap is layer dependent and changes with the application of strain. In this talk I will outline our attempt to make simple field effect transistors with thin MoS$_{\mathrm{2}}$ on flexible substrates. Our aim was to see the bandgap of MoS$_{\mathrm{2}}$ directly via transport measurements using electrolytic gating, then apply uniaxial strain to a single layer MoS$_{\mathrm{2}}$ device to see the bandgap change. This was to be one way of confirming theoretical expectations, as well as compare with experimental results already obtained through photoluminescence spectroscopy. Though we did not obtain our target result with this stage of the experiment, future experimental work is planned. I will discuss the experimental method, the challenges of obtaining data and the results we obtained. [Preview Abstract] |
Friday, March 17, 2017 12:27PM - 12:39PM |
Y33.00007: Low-Temperature Transport Measurements of Multilayer WSe$_{2}$ FETs L. J. Stanley, Dragana Popovi\'{c}, Hsun-Jen Chuang, Zhixian Zhou, Michael Koehler, Jiaqiang Yan, David Mandrus Transition metal dichalcogenides (TMDs) offer an exciting new opportunity to study transport in 2D and the universality of the 2D metal-insulator-transition (MIT), but high-resistance, non-ohmic contacts have been a major impediment. Recently, however, 2D/2D low-resistance ohmic contacts have been produced on ultrathin TMD samples [1] allowing for low-temperature characterization. We report studies of WSe$_2$ FET samples with $\sim 10$ layers each, patterned into Hall bars, and with a favorably high mobility of up to $\sim 0.2$~m$^2/$Vs at $\sim 5$~K. Two-terminal and four-terminal conductivity ($\sigma$) measurements were performed at temperatures $0.25\leq T$(K)$<200$. Our results show that contacts remain ohmic down to 0.25~K over the entire useful range of back-gate voltages $V_{bg}$. Furthermore, the measurements reveal a change in the sign of $ d\sigma/dT $ and the form of $\sigma(T)$ with $V_{bg}$, strongly suggesting the existence of a 2D MIT in this system. [1] Hsun-Jen~Chuang \textit{et al.}, Nano Lett. \textbf{16}, 1896 (2016). [Preview Abstract] |
Friday, March 17, 2017 12:39PM - 12:51PM |
Y33.00008: High-Field Transport and Velocity Saturation in Synthetic Monolayer MoS$_{\mathrm{2}}$ Kirby Smithe, Saurabh Suryavanshi, Chris English, Eric Pop Understanding drift velocity saturation of charge carriers at high lateral fields is essential for practical applications of two-dimensional materials. Here, we measure drift velocity saturation in monolayer MoS$_{\mathrm{2}}$ grown by chemical vapor deposition as a function of temperature for the first time. We select few-$\mu $m long devices such that contact resistance is negligible, and bias the device in strong accumulation to maintain uniform lateral electric field. The drift velocity is extracted from the measured current and the calculated charge density. Though data were taken from 80 to 300 K ambient, devices self-heat by up to 150 K at high bias. Thus, we capture the temperature \quad dependence of drift velocity by fitting the data with a model including the population of dominant phonons. The raw data reveal that the drift velocity saturates at lateral fields \textgreater 4 V/$\mu $m, up to 1.2x10$^{\mathrm{6}}$ cm/s (2x10$^{\mathrm{6}}$ cm/s) at 300 K (80 K) ambient. However, our model suggests that the true saturation velocity would be 50{\%} to 60{\%} higher if self-heating were eliminated, i.e. 2x10$^{\mathrm{6}}$ cm/s (3x10$^{\mathrm{6}}$ cm/s) at 300 K (80 K). These results shed light into high-field transport of monolayer 2D materials, revealing the strong role of electron-phonon scattering and of self-heating in terms of limiting current flow. [Preview Abstract] |
Friday, March 17, 2017 12:51PM - 1:03PM |
Y33.00009: Electric Field Effect in Multilayer Cr$_{\mathrm{2}}$Ge$_{\mathrm{2}}$Te$_{\mathrm{6}}$: a Ferromagnetic Two-Dimensional Material Wenyu Xing, Yangyang Chen, Patrick M. Odenthal, Xiao Zhang, Wei Yuan, Tang Su, Qi Song, Tianyu Wang, Jiangnan Zhong, Shuang Jia, X. C. Xie, Yan Li, Wei Han Two-dimensional (2D) ferromagnetic materials are very attractive for their magnetic properties and future spintronics devices applications. In this talk, I will present our experimental results on the preparation and characterization of a 2D ferromagnetic material: Cr$_{\mathrm{2}}$Ge$_{\mathrm{2}}$Te$_{\mathrm{6}}$ (CGT). The bulk crystal CGT is synthesized by flux method, and it is a ferromagnetic insulator with the Curie temperature of \textasciitilde 65 K. Owing to the van der Waals nature of the crystal structure, we are able to prepare CGT flakes with thicknesses down to a few nanometers using the mechanical exfoliation method. Then the electrical measurements are performed by fabricating nano devices on a series of CGT flakes with different thicknesses. There, we have observed that the intrinsically insulating 2D CGT flakes could be tuned to be metallic using the electric field effect. Furthermore, the ferromagnetic properties of the 2D CGT flakes are characterized by Kerr rotation and anomalous Hall measurements. [Preview Abstract] |
Friday, March 17, 2017 1:03PM - 1:15PM |
Y33.00010: High Performance WSe2 Field-Effect Transistors via Controlled Formation of In-Plane Heterojunctions. Bilu Liu, Yuqiang Ma, Chongwu Zhou Monolayer WSe2 is a 2D semiconductor with a direct bandgap, and is promising for electronics and optoelectronics. Low field effect mobility is the main constraint preventing WSe2 from becoming a competing channel material for field-effect transistors (FETs). Here, we report that controlled heating in air significantly improves device performance of WSe2 FETs. After heating at optimized conditions, chemical vapor deposition grown monolayer WSe2 FETs showed average FET mobility of 31 cm2\textbullet V$-$1\textbullet s$-$1 and on/off current ratios up to 5×108. For few-layer WSe2 FETs, after the same treatment, we achieved a high mobility up to 92 cm2\textbullet V$-$1\textbullet s$-$1. The underlying chemical processes involved during air heating and the formation of in-plane heterojunctions of WSe2 and WO3-x, which is believed to be the reason for the improved FET performance, were studied in detail. We further demonstrated that by combining air heating method developed here with supporting 2D materials on BN substrate, we achieved a noteworthy field effect mobility of 83 cm2\textbullet V$-$1\textbullet s$-$1 for monolayer WSe2 FETs. This work is a step towards controlled modification of the properties of WSe2 and potentially other TMDCs, and may greatly improve device performance for future applications of 2D materials in electronics.. [Preview Abstract] |
Friday, March 17, 2017 1:15PM - 1:27PM |
Y33.00011: Electrical transport and structural characterization of epitaxial monolayer MoS$_{2}$ /n- and p-doped GaN vertical lattice-matched heterojunctions D Ruzmetov, T O'Regan, K Zhang, A Herzing, A. Mazzoni, M. Chin, S. Huang, Z. Zhang, R. Burke, M. Neupane, AG Birdwell, P. Shah, F. Crowne, A Kolmakov, B. LeRoy, J. Robinson, A. Davydov, T. Ivanov We investigate vertical semiconductor junctions consisting of monolayer MoS$_{2}$ that is epitaxially grown on n- and p-doped GaN crystals. Such a junction represents a building block for 2D/3D vertical semiconductor heterostructures. Epitaxial, lattice-matched growth of MoS$_{2}$ on GaN is important to ensure high quality interfaces that are crucial for the efficient vertical transport. The MoS$_{2}$/GaN junctions were characterized with cross-sectional and planar scanning transmission electron microscopy (STEM), scanning tunneling microscopy, and atomic force microscopy. The MoS$_{2}$/GaN lattice mismatch is measured to be near 1{\%} using STEM. The electrical transport in the out-of-plane direction across the MoS$_{2}$/GaN junctions was measured using conductive atomic force microscopy and mechanical nano-probes inside a scanning electron microscope. Nano-disc metal contacts to MoS$_{2}$ were fabricated by e-beam lithography and evaporation. The current-voltage curves of the vertical MoS$_{2}$/GaN junctions exhibit rectification with opposite polarities for n-doped and p-doped GaN. The metal contact determines the general features of the current-voltage curves, and the MoS$_{2}$ monolayer modifies the electrical transport across the contact/GaN interface. [Preview Abstract] |
Friday, March 17, 2017 1:27PM - 1:39PM |
Y33.00012: Ambipolar behavior and thermoelectric properties of WS2 nanotubes Yohei Yomogida, Hideki Kawai, Mitsunari Sugahara, Ryotaro Okada, Kazuhiro Yanagi WS2 nanotubes are rolled multi-walled nanotubes made by a layered material, tungsten disulfides Since the discovery by Tenne et al in 1992, various physical properties have been revealed. Theoretical studies have suggested their distinct electronic properties from those of two dimensional sheet, such as one-dimensional electronic strucutures with sharp van Hove singularities and chiralitiy depended electronic structures. Their fibril structures enable us to make their random network films, however, the films are not conducting, and thus have not been used for electronic applications. Here we demonstrate that carrier injections on the WS2 networks by an electrolyte gating approach could make the networks as a semiconducting channel. We clarified the Raman characteristics of WS2 nanotubes networks under electrolyte gating, and confirmed capability of electron and hole injections. We revealed ambipolar behaviors of the WS2 nanotube networks in field effect transistor setups with electrolyte gating. In additio, we demosntrate N-type and P-type control of thermoelectric properties of WS2 nanotubes by electrolyte gating.The power factor of the WS2 nanotubes almost approached to that of the single crystalline WS2 flakes, suggesting good potential for thermoelectric applications.. [Preview Abstract] |
Friday, March 17, 2017 1:39PM - 1:51PM |
Y33.00013: Characterizing resistivity anisotropy in black phosphorous flakes with 5-point method Lintao Peng, Spencer A. Wells, Christopher R. Ryder, Mark C. Hersam, Matthew Grayson Black phosphorous (BP) thin flakes have been recently studied as a promising candidate material for electronics and optoelectronics. The anisotropy caused by the in-plain structural asymmetry plays a significant role in transistor performance optimization, and polarized optical detection. Here, we present a simple method to characterize the electrical transport anisotropy of BP devices of arbitrary shape. It requires 5 or more contacts at the periphery and consists of a sequence of van der Pauw measurments with various pairs of current and voltage terminals. Identifying the sample shape and contact geometry with a conformal map, we can uniquely define the two components of the resistivity tensor as well as the in-plane a- and c- axis directions. Raman spectroscopy confirmed the axis orientation as determined by our method. Further temperature dependence and gate dependence were measured to illustrate consistency of this method. Theoretically, this method could be applied to identify anisotropic resistivity components of any other low-symmetry 2D materials. [Preview Abstract] |
Friday, March 17, 2017 1:51PM - 2:03PM |
Y33.00014: Transient Hall Effect Measurement in Black Phosphorus Jiajun Luo, Lintao Peng, Spencer A. Wells, Christopher R. Ryder, Mark C. Hersam, Matthew Grayson Black phosphorus exfoliated flakes exhibit a slow conductivity transient after a sudden change of the gate voltage. To separate transients in carrier density from those in mobility in the overall conductivity transient, a traditional Hall effect method requires either a Hall bar geometry that is difficult to fabricate out of a single flake, or van der Pauw geometry with a sequence of measurements first at +B and again at -B to eliminate the B-symmetric offset signal. Unfortunately, the latter requires that the exact same transient response can be perfectly repeated. In this work, we introduce a heterodyne circuit for measuring Hall effect in a single transient signal. The heterodyne circuit performs an electrical analog of the alternating B-field, using an ac modulation of the Onsager-Casimir relations to eliminate the offset signal in the transverse Hall resistance. Thus the carrier density transient can be accurately measured with sub-second time resolution. The mobility transient response can be extracted by additionally measuring the sheet conductivity transient. Transient Hall effect measurements in black phosphorus flakes showed a stretched exponential relaxation of carrier density and an approximately constant mobility. [Preview Abstract] |
Friday, March 17, 2017 2:03PM - 2:15PM |
Y33.00015: Graphene Hall sensor fabricated by CMOS-compatible processes and its performance prediction without magnetic field Joonggyu Kim, Min-Kyu Joo, Ji-Hoon Park, Van Luan Nguyen, Ki Kang Kim, Young Hee Lee, Dongseok Suh Graphene has been considered as an optimal material for a magnetic sensor because of its high carrier mobility, atomic thickness, and easy modulation of carrier concentration. Although numerous prototype device structures have revealed their advanced performances utilizing these unique and beneficial features, device fabrications using mechanically exfoliated graphene material and electron beam lithography method limit its feasibility as an industrial-level magnetic sensor. Here, we demonstrate a large-area and photolithographically patterned graphene Hall element (GHE) array based on heterostructure of 2-D materials grown by chemical vapor deposition (CVD) techniques [1]. Outstanding performance of GHE was achieved with a bottom hexagonal-BN ($h$-BN) layer, and the device showed maximum current relative sensitivity (S$_{\mathrm{I}})$ of 1,986 V/AT and minimum magnetic resolution of 0.5 mG/Hz$^{\mathrm{0.5}}$ at f $=$ 300 Hz. Besides, from the deep understanding of various parameters such as the shift of charge neutrality point and transconductance dependent S$_{\mathrm{I}}$, an analytical model is proposed for the prediction of sensor performance without magnetic field. [1] \textit{ACS Nano}, \textbf{10}, 9, 8803-8811 (2016) [Preview Abstract] |
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