Current Status of Graphene Transistors

This paper reviews the current status of graphene transistors as potential supplement to silicon CMOS technology. A short overview of graphene manufacturing and metrology methods is followed by an introduction of macroscopic graphene field effect transistors (FETs). The absence of an energy band gap is shown to result in severe shortcomings for logic applications. Possibilities to engineer a band gap in graphene FETs including quantum confinement in graphene Nanoribbons (GNRs) and electrically or substrate induced asymmetry in double and multi layer graphene are discussed. Graphene FETs are shown to be of interest for analog radio frequency applications. Finally, novel switching mechanisms in graphene transistors are briefly introduced that could lead to future memory devices.Comment: 11 pages, 6 figure

Local gating of a graphene Hall bar by graphene side gates

We have investigated the magnetotransport properties of a single-layer graphene Hall bar with additional graphene side gates. The side gating in the absence of a magnetic field can be modeled by considering two parallel conducting channels within the Hall bar. This results in an average penetration depth of the side gate created field of approx. 90 nm. The side gates are also effective in the quantum Hall regime, and allow to modify the longitudinal and Hall resistances

Direct Graphene Growth on Insulator

Fabrication of graphene devices is often hindered by incompatibility between the silicon technology and the methods of graphene growth. Exfoliation from graphite yields excellent films but is good mainly for research. Graphene grown on metal has a technological potential but requires mechanical transfer. Growth by SiC decomposition requires a temperature budget exceeding the technological limits. These issues could be circumvented by growing graphene directly on insulator, implying Van der Waals growth. During growth, the insulator acts as a support defining the growth plane. In the device, it insulates graphene from the Si substrate. We demonstrate planar growth of graphene on mica surface. This was achieved by molecular beam deposition above 600{\deg}C. High resolution Raman scans illustrate the effect of growth parameters and substrate topography on the film perfection. Ab initio calculations suggest a growth model. Data analysis highlights the competition between nucleation at surface steps and flat surface. As a proof of concept, we show the evidence of electric field effect in a transistor with a directly grown channel.Comment: 13 pages, 6 figure

Bistability and oscillatory motion of natural nano-membranes appearing within monolayer graphene on silicon dioxide

The recently found material graphene is a truly two-dimensional crystal and exhibits, in addition, an extreme mechanical strength. This in combination with the high electron mobility favours graphene for electromechanical investigations down to the quantum limit. Here, we show that a monolayer of graphene on SiO2 provides natural, ultra-small membranes of diameters down to 3 nm, which are caused by the intrinsic rippling of the material. Some of these nano-membranes can be switched hysteretically between two vertical positions using the electric field of the tip of a scanning tunnelling microscope (STM). They can also be forced to oscillatory motion by a low frequency ac-field. Using the mechanical constants determined previously, we estimate a high resonance frequency up to 0.4 THz. This might be favorable for quantum-electromechanics and is prospective for single atom mass spectrometers.Comment: 9 pages, 4 figure

Extended papers selected from ESSDERC 2015

This special issue of Solid State Electronics includes 28 papers which have been carefully selected from the best presentations given at the 45th European Solid-State Device Research Conference (ESSDERC 2015) held from September 14–18, 2015 in Graz, Austria. These papers cover a wide range of topics related to the research on solid-state devices. These topics are used also to organize the conference submissions and presentations into 7 tracks: CMOS Processes, Devices and Integration; Opto-, Power- and Microwave Devices; Modeling & Simulation; Characterization, Reliability & Yield; Advanced & Emerging Memories; MEMS, Sensors & Display Technologies; Emerging Non-CMOS Devices & Technologies

Effect of disorder on a graphene p-n junction

We propose the theory of transport in a gate-tunable graphene p-n junction, in which the gradient of the carrier density is controlled by the gate voltage. Depending on this gradient and on the density of charged impurities, the junction resistance is dominated by either diffusive or ballistic contribution. We find the conditions for observing ballistic transport and show that in existing devices they are satisfied only marginally. We also simulate numerically the trajectories of charge carriers and illustrate challenges in realizing more delicate ballistic effects, such as Veselago lensing.Comment: (v2)Version accepted to Phys. Rev.

Growth-Induced Strain in Chemical Vapor Deposited Monolayer MoS2: Experimental and Theoretical Investigation

Monolayer molybdenum disulphide (MoS$_2$) is a promising two-dimensional (2D) material for nanoelectronic and optoelectronic applications. The large-area growth of MoS$_2$ has been demonstrated using chemical vapor deposition (CVD) in a wide range of deposition temperatures from 600 {\deg}C to 1000 {\deg}C. However, a direct comparison of growth parameters and resulting material properties has not been made so far. Here, we present a systematic experimental and theoretical investigation of optical properties of monolayer MoS$_2$ grown at different temperatures. Micro-Raman and photoluminescence (PL) studies reveal observable inhomogeneities in optical properties of the as-grown single crystalline grains of MoS$_2$. Close examination of the Raman and PL features clearly indicate that growth-induced strain is the main source of distinct optical properties. We carry out density functional theory calculations to describe the interaction of growing MoS$_2$ layers with the growth substrate as the origin of strain. Our work explains the variation of band gap energies of CVD-grown monolayer MoS$_2$, extracted using PL spectroscopy, as a function of deposition temperature. The methodology has general applicability to model and predict the influence of growth conditions on strain in 2D materials.Comment: 37 pages, 6 figures, 10 figures in supporting informatio

A Survey for Infall Motions toward Starless Cores. II. CS(2−1)CS (2-1) and N2H+(1−0)N_2H^+ (1-0) Mapping Observations

We present the results of an extensive mapping survey of 53 `starless' cores in the optically thick line of CS 2-1 and the optically thin lines of N2H+ 1-0 and C18O 1-0. The purpose of this survey was to search for signatures of extended inward motions. This study finds 10 `strong' and 9 `probable' infall candidates, based on $\delta V_{CS}$ analysis and on the spectral shapes of CS lines. From our analysis of the blue-skewed CS spectra and the $\delta V_{CS}$ parameter, we find typical infall radii of 0.06-0.14 pc. Also, using a simple two layer radiative transfer model to fit the profiles, we derive one-dimensional infall speeds, half of whose values lie in the range of 0.05-0.09 km s$^{-1}$. These values are similar to those found in L1544 by Tafalla et al., and this result confirms that infall speeds in starless cores are generally faster than expected from ambipolar diffusion in a strongly sub-critical core. In addition, the observed infall regions are too extended to be consistent with the `inside-out' collapse model applied to a very low-mass star. In the largest cores, the spatial extent of the CS spectra with infall asymmetry is larger than the extent of the $\rm N_2H^+$ core by a factor of 2-3. All these results suggest that extended inward motions are a common feature in starless cores, and that they could represent a necessary stage in the condensation of a star-forming dense core.Comment: Two tex files for manuscript and tables, and 38 figures. To appear in ApJ

Ultra Low Specific Contact Resistivity in Metal-Graphene Junctions via Atomic Orbital Engineering

A systematic investigation of graphene edge contacts is provided. Intentionally patterning monolayer graphene at the contact region creates well-defined edge contacts that lead to a 67% enhancement in current injection from a gold contact. Specific contact resistivity is reduced from 1372 {\Omega}m for a device with surface contacts to 456 {\Omega}m when contacts are patterned with holes. Electrostatic doping of the graphene further reduces contact resistivity from 519 {\Omega}m to 45 {\Omega}m, a substantial decrease of 91%. The experimental results are supported and understood via a multi-scale numerical model, based on density-functional-theory calculations and transport simulations. The data is analyzed with regards to the edge perimeter and hole-to-graphene ratio, which provides insights into optimized contact geometries. The current work thus indicates a reliable and reproducible approach for fabricating low resistance contacts in graphene devices. We provide a simple guideline for contact design that can be exploited to guide graphene and 2D material contact engineering.Comment: 26 page

Non-invasive Scanning Raman Spectroscopy and Tomography for Graphene Membrane Characterization

Graphene has extraordinary mechanical and electronic properties, making it a promising material for membrane based nanoelectromechanical systems (NEMS). Here, chemical-vapor-deposited graphene is transferred onto target substrates to suspend it over cavities and trenches for pressure-sensor applications. The development of such devices requires suitable metrology methods, i.e., large-scale characterization techniques, to confirm and analyze successful graphene transfer with intact suspended graphene membranes. We propose fast and noninvasive Raman spectroscopy mapping to distinguish between freestanding and substrate-supported graphene, utilizing the different strain and doping levels. The technique is expanded to combine two-dimensional area scans with cross-sectional Raman spectroscopy, resulting in three-dimensional Raman tomography of membrane-based graphene NEMS. The potential of Raman tomography for in-line monitoring is further demonstrated with a methodology for automated data analysis to spatially resolve the material composition in micrometer-scale integrated devices, including free-standing and substrate-supported graphene. Raman tomography may be applied to devices composed of other two-dimensional materials as well as silicon micro- and nanoelectromechanical systems.Comment: 23 pages, 5 figure