Kristof Tahy

Broadband graphene terahertz modulators enabled by intraband transitions.

Terahertz technology promises myriad applications including imaging, spectroscopy and communications. However, one major bottleneck at present for advancing this field is the lack of efficient devices to manipulate the terahertz electromagnetic waves. Here we demonstrate that exceptionally efficient broadband modulation of terahertz waves at room temperature can be realized using graphene with extremely low intrinsic signal attenuation. We experimentally achieved more than 2.5 times superior modulation than prior broadband intensity modulators, which is also the first demonstrated graphene-based device enabled solely by intraband transitions. The unique advantages of graphene in comparison to conventional semiconductors are the ease of integration and the extraordinary transport properties of holes, which are as good as those of electrons owing to the symmetric conical band structure of graphene. Given recent progress in graphene-based terahertz emitters and detectors, graphene may offer some interesting solutions for terahertz technologies.

Transistors with chemically synthesized layered semiconductor WS2 exhibiting 105 room temperature modulation and ambipolar behavior

We report the realization of field-effect transistors (FETs) made with chemically synthesized multilayer crystal semiconductor WS2. The Schottky-barrier FETs demonstrate ambipolar behavior and a high (∼105×) on/off current ratio at room temperature with current saturation. The behavior is attributed to the presence of an energy bandgap in the ultrathin layered semiconductor crystal material. The FETs also show clear photo response to visible light. The promising electronic and optical characteristics of the devices combined with the chemical synthesis, and flexibility of layered semiconductor crystals such as WS2 make them attractive for future electronic and optical devices.

Thermally Limited Current Carrying Ability of Graphene Nanoribbons

We investigate high-field transport in graphene nanoribbons (GNRs) on SiO(2), up to breakdown. The maximum current density is limited by self-heating, but can reach >3 mA/μm for GNRs ~15 nm wide. Comparison with larger, micron-sized graphene devices reveals that narrow GNRs benefit from 3D heat spreading into the SiO(2), which enables their higher current density. GNRs also benefit from lateral heat flow to the contacts in short devices (<~0.3 μm), which allows extraction of a median GNR thermal conductivity (TC), ~80 W m(-1)K(-1) at 20 °C across our samples, dominated by phonons. The TC of GNRs is an order of magnitude lower than that of micron-sized graphene on SiO(2), suggesting strong roles of edge and defect scattering, and the importance of thermal dissipation in small GNR devices.

Graphene nanoribbon field-effect transistors on wafer-scale epitaxial graphene on SiC substrates a

We report the realization of top-gated graphene nanoribbon field effect transistors (GNRFETs) of ∼10 nm width on large-area epitaxial graphene exhibiting the opening of a band gap of ∼0.14 eV. Contrary to prior observations of disordered transport and severe edge-roughness effects of graphene nanoribbons (GNRs), the experimental results presented here clearly show that the transport mechanism in carefully fabricated GNRFETs is conventional band-transport at room temperature and inter-band tunneling at low temperature. The entire space of temperature, size, and geometry dependent transport properties and electrostatics of the GNRFETs are explained by a conventional thermionic emission and tunneling current model. Our combined experimental and modeling work proves that carefully fabricated narrow GNRs behave as conventional semiconductors and remain potential candidates for electronic switching devices.

Transport properties of graphene nanoribbon transistors on chemical-vapor-deposition grown wafer-scale graphene

Graphene nanoribbon (GNR) field-effect transistors (FETs) with widths down to 12 nm have been fabricated by electron beam lithography using a wafer-scale chemical vapor deposition (CVD) process to form the graphene. The GNR FETs show drain-current modulation of approximately 10 at 300 K, increasing to nearly 106 at 4 K. The strong temperature dependence of the minimum current indicates the opening of a bandgap for CVD-grown GNR-FETs. The extracted bandgap is estimated to be around 0.1 eV by differential conductance methods. This work highlights the development of CVD-grown large-area graphene and demonstrates the opening of a bandgap in nanoribbon transistors.

Quantum transport in graphene nanoribbons patterned by metal masks

Graphene nanoribbons (GNRs) were fabricated by metal mask lithography and plasma etching. GNRs with width ∼20 nm show field-effect conductance modulation of ∼12 at room temperature and >106 at 4.2 K. Conductance quantization due to quantum confinement in low field transport was observed. Landauer formula was utilized to fit the experimental data and excellent agreement was obtained. The extracted subband energy separation was found to deviate from the predicted values of perfect armchair GNRs. Transmission probability is much smaller than unity due to scattering by GNR edge/bulk disorder and impurities, indicating a mean free path ∼40 nm. High field family I-Vs exhibited current saturation tendency and current density as high as 2 A/mm has been measured at low temperature.

Fabrication of top-gated epitaxial graphene nanoribbon FETs using hydrogen-silsesquioxane

Top-gated epitaxial graphene nanoribbon (EGNR) field effect transistors (FETs) were fabricated on epitaxial graphene substrates which demonstrated the opening of a substantial bandgap. Hydrogen silsesquioxane (HSQ) was used for the patterning of 10 nm size linewidth as well as a seed layer for atomic layer deposition (ALD) of a high-k dielectric aluminum oxide (Al2O3). It is found that the resolution of the patterning is affected by the development temperature, electron beam dose, and substrate materials. The chosen gate stack of HSQ followed by Al2O3 ALD permits stable device performance and enables the demonstration of the EGNR-FET.

High field transport properties of 2D and nanoribbon graphene FETs

In the past we were able to drive back-gated 2D graphene transistors to saturation regime. Now we present the realization of these properties in double-gated graphene nanoribbon field effect transistors (GNR FETs). We were able to achieve Ion/Ioff ratio of 103 using either top or back-gates and we analyzed the high field characteristics of such devices.

First demonstration of two-dimensional WS 2 transistors exhibiting 10 5 room temperature modulation and ambipolar behavior

Two-dimensional (2D) WS2 transistors were fabricated and characterized for the first time from chemically-synthesized material. Raman measurements confirm the 2D crystal nature of the material, and the presence of a bandgap leads to high on/off current ratios and current saturation in the transistors at room temperature. In addition, the observed photoresponse of the 2D layered semiconductor can enable optical device applications.

Graphene nanoribbon FETs for digital electronics: experiment and modeling

One-dimensional nanostructures of graphene such as graphene nanoribbons (GNRs) can prove attractive for digital electronics in the form of interband tunneling transistors, as they are capable of high drive currents. Here, we report on the transport properties of p-n junctions formed in GNR field effect transistors (FETs). It is found that the current density in the devices is indeed high; in the 1–1.5 A/mm range have been measured, comparable to Si-MOSFETs and III-V Nitride HEMTs. The observed unique current–voltage characteristics of the double-gated GNR FETs having a lateral p-n junction as their channel is explained by a field-effect model. Due to the lack of sufficiently large bandgap in the 30 nm wide GNR, the device still cannot be turned off completely, but rectification is achieved. The results suggest that the fabrication of tunneling FETs made out of graphene is possible and their characteristics may meet the expectations. Copyright